Document [original]

Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys. 53 (2020) 503001 (57pp) https://doi.org/10.1088/1361-6463/aba64c

Roadmap

The 2020 UV emitter roadmap

Hiroshi Amano1,2, Ram´

on Collazo3, Carlo De Santi4, Sven Einfeldt5, Mitsuru Funato6,

Johannes Glaab5, Sylvia Hagedorn5, Akira Hirano7, Hideki Hirayama8, Ryota Ishii6,

Yukio Kashima8,9, Yoichi Kawakami6, Ronny Kirste3, Michael Kneissl5,10, Robert

Martin11,27, Frank Mehnke12, Matteo Meneghini4, Abdallah Ougazzaden13, Peter J

Parbrook14, Siddharth Rajan15,16, Pramod Reddy17, Friedhard Römer18, Jan Ruschel5,

Biplab Sarkar3,19, Ferdinand Scholz20, Leo J Schowalter21,22, Philip Shields23,

Zlatko Sitar3, Luca Sulmoni10, Tao Wang24, Tim Wernicke10,27, Markus Weyers5,

Bernd Witzigmann18, Yuh-Renn Wu25, Thomas Wunderer26 and Yuewei Zhang15

1IMaSS, Nagoya University, Nagoya, 464-8603, Japan

2Nagoya University, Nagoya, 464-8601, Japan

3Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC,

27695, United States of America

4Department of Information Engineering, University of Padova, via Gradenigo 6/b 35131, Padova, Italy

5Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4 12489,

Berlin, Germany

6Department of Electronic Science and Engineering, Kyoto University, Katsura Campus, Nishikyo-ku,

Kyoto 615-8510, Japan

7UV Craftory Co., Ltd., Nagoya 464-0015, Japan

8RIKEN, 2-1 Hirosawa Wako, Saitama 351-0198, Japan

9Marubun Corporation, 8-1 Oodenma-cho, Nihonbashi, Chuo Ward, Tokyo 109-8577, Japan

10 Technische Universit¨

at Berlin, Institute of Solid State Physics, Hardenbergstr. 36, 10623 Berlin,

Germany

11 Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, United Kingdom

12 Georgia Institute of Technology, School of Electrical and Computer Engineering, 777 Atlantic Drive,

Atlanta, GA 30332, United States of America

13 Georgia Institute of Technology, School of Electrical and Computer Engineering, Georgia Tech-CNRS,

UMI 2958 57070, Metz, France

14 Tyndall National Institute and School of Engineering, University College Cork, Cork, Ireland

15 Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH 43210,

United States of America

16 Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210,

United States of America

17 Adroit Materials, Inc., 2054 Kildaire Farm Rd, Suite 205, Cary, NC 27518, United States of America

18 Department of Electrical Engineering/Computer Science and CINSaT, University of Kassel,

Wilhelmshoeher Allee 71 D-34121, Kassel, Germany

19 Department of Electronics and Communication Engineering, Indian Institute of Technology, Roorkee,

Uttarakhand 247667 India

20 Institute of Functional Nanosystems, Ulm University 89069, Ulm, Germany

21 Crystal IS Inc., 70 Cohoes Ave., Green Island, NY 12183, United States of America

22 Asahi Kasei Corporation, Fuji 416-8501, Japan

23 Department of Electrical and Electronic Engineering, University of Bath, Bath BA2 7AY, United

Kingdom

Original content from this work may be used under the terms

of the Creative Commons Attribution 4.0 licence. Any fur-

ther distribution of this work must maintain attribution to the author(s) and the

title of the work, journal citation and DOI.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

24 Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, United

Kingdom

25 National Taiwan University, Institute of Photonics and Optoelectronics and Department of Electrical

Engineering

26 Electronics Materials and Devices Laboratory, PARC, a Xerox Company, 3333 Coyote Hill Road, Palo

Alto, CA 94304, United States of America

27 Authors to whom any correspondence should be addressed.

E-mail: r.w[email protected] and [email protected]

Received 16 October 2019, revised 29 June 2020

Accepted for publication 15 July 2020

Published 15 September 2020

Abstract

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N

material system is best suited to produce LEDs and laser diodes from 400 nm down to

210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the

largest bandgap material AlN and a growth and fabrication technology compatible with the

current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer

from numerous challenges compared to their visible counterparts that become most obvious by

consideration of their light output power, operation voltage and long term stability. Most of

these challenges are related to the large bandgap of the materials. However, the development

since the first realization of UV electroluminescence in the 1970s shows that an improvement in

understanding and technology allows the performance of UV emitters to be pushed far beyond

the current state. One example is the very recent realization of edge emitting laser diodes

emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap

summarizes the current state of the art for the most important aspects of UV emitters, their

challenges and provides an outlook for future developments.

Keywords: ultraviolet, light emitting diodes, InGaN, UV-LED, AlGaN

(Some figures may appear in colour only in the online journal)

Contents

1. UV-LEDs: state-of-the-art and applications 4

2. AlN on sapphire 7

3. The growth of bulk AlN and fabrication of AlN wafers 9

4. Radiative recombination efficiency of AlGaN quantum wells: do we estimate it accurately in a proper way? 12

5. Point defects 14

6. Toward ohmic n-contacts on n-AlGaN with high Al mole fraction 17

7. Doping of AlGaN 19

8. Boron-containing (Al, Ga)N heterostructures 22

9. Development of UV-A LEDs 24

10. UVB-LEDs 26

11. UVC LEDs 28

12. UVC LEDs with emission below 250 nm 31

13. Light extraction efficiency of UVC LEDs 34

14. Nanostructuring for UV emitters 36

15. Simulation of UV-light emitting diodes and lasers 39

16. Reliability of UV LEDs 41

17. Tunnel junction-based UV LEDs 44

18. E-beam pumped emitters 47

19. UV laser diodes 50

References 52

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

List of terms and acronyms

AFM atomic force microscopy—section 3

CL cathodoluminescence—section 4

C-DLTS and

O-DLTS

capacitance-/optical-deep level transient

spectroscopy—section 16

DBR distributed Bragg reflector—section 7

DFB distributed feedback—section 19

DFT density functional theory—section 5

DUV deep ultra violet—section 4

EQE external quantum efficiency—section 1

EBL electron blocking layer—section 10

ELO epitaxial lateral overgrowth—section 2

ηrad radiative recombination efficiency—section 1

ηinj current injection efficiency—section 1

ηel electrical efficiency—section 1

FC flip chip—section 13

FDTD finite-difference time-domain—section 13

LEE ηext light extraction efficiency—section 1

IQE ηIQE internal quantum efficiency—section 1

HR-PhC highly reflective photonic crystal—section 13

HTA high temperature annealing (of AlN)—section 2

HVPE hydride vapour phase epitaxy—section 3

Ifforward current—section 13

LED light emitting diode—section 1

MBE molecular beam epitaxy—section 7

MOVPE metal-organic vapour phase epitaxy, same

method as MOCVD—section 2

MQW multi-quantum well—section 12

NRC nonradiative recombination centres—section 4

PL photoluminescence—section 4

pss patterned sapphire substrate—section 2

PVT physical vapour transport (growth of AlN)—section 3

PDD point defect density—section 11

QW quantum well—section 4

RMS

roughness

root mean square roughness—section 3

SIMS secondary ion mass spectrometry—section 5

SNOM scanning near-field optical microscopy—section 4

TDD threading dislocation density—section 2

TEM transmission electron microscopy—section 7

TJ tunnel junction—section 7

TE transverse electric—section 12

TM transverse magnetic—section 12

UV ultra violet

UVA 315–400 nm, according to ISO 21348

UVB 280–315 nm, according to ISO 21348

UVC 100–280 nm, according to ISO 21348

VCSEL vertical cavity surface emitting laser—section 7

Vfforward voltage—section 13

WPE wall plug efficiency—section 1

XRD x-ray diffraction—section 7

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

1. UV-LEDs: state-of-the-art and applications

Michael Kneissl1,2

1Ferdinand-Braun-Institut, Leibniz-Institut für

Höchstfrequenztechnik

2Technische Universit¨

at Berlin, Institute of Solid State

Physics

1.1. Status

Driven by the wide range of applications the development of

group III-nitride based ultraviolet emitters has gained consid-

erable momentum since the early 2000s [1,2]. In the UVA

spectral band (400–315 nm) key applications include UV cur-

ing of polymers, inks, coatings, resins, and adhesives [3]. UVA

emitters are also employed for sensing applications, e.g. the

detection of blankophores and fluorescent labels as well as

blood gas analysis and light therapy. Phototherapy, in particu-

lar the treatment of skin diseases like vitiligo and psoriasis, is

one of the key applications for LEDs in the UVB wavelength

range (315–280 nm). UVB-LEDs are also of great interest for

the curing of surfaces since the penetration depth of UVB

light in polymers is much smaller than that of UVA emit-

ters. In addition, plant growth lighting constitutes a large scale

application for UVB-LEDs. Here UVB-LEDs are employed

to enhance the production of secondary metabolites in veget-

ables, which carry a number of health benefits for the con-

sumer or to control the morphology and shapes of flowers [3].

Disinfection of drinking water and wastewater treatment are

certainly the highest volume applications for LEDs emitting

in the UVC spectral band (100–280 nm). In addition, ster-

ilization of medical equipment, home appliances (e.g. refri-

gerators, washing machines), and various surfaces (e.g. in the

food industry), would also benefit from UVC-LEDs. Sensing

applications require specific emission wavelengths, depending

on the absorption band of the gases or biomolecules of interest,

and therefore cover the entire UV spectral range. Figure 1

presents some of the key applications for UV-LEDs. As can

be seen, the wavelength and power requirements vary greatly

for these applications. Whereas sensing applications entail low

power but spectrally pure LEDs, for UV curing and disinfec-

tion LED modules delivering many Watts of UV light are cru-

cial.

Since UVA-LEDs with emission wavelength above 315 nm

have already reached excellent performance levels with out-

put powers of several watts per LED chip, UV curing has

evolved as the first large scale application for UV-LEDs. Com-

pared to conventional UV sources, like low and medium pres-

sure mercury lamps, UV-LEDs provide a number of advant-

ages. Besides a freely selectable single wavelength emission,

UV-LEDs require no warm-up time, are electronically dim-

mable, exhibit no forward heat radiation, are highly temperat-

ure stable, and exhibit long lifetimes. Due to their small form

factors, UV-LEDs provide a much greater design flexibility for

UV modules and are also environmentally friendly, since they

contain no mercury or produce ozone. Even though the output

Figure 1. Key applications for UVA, UVB, and UVC LEDs.

Reprinted by permission from Springer Nature Customer Service

Centre GmbH: Springer [3] (2016).

power levels of UVB- and UVC-LEDs are currently only in

the 1–100 mW range, these shorter wavelength UV-LEDs have

already found entry in first applications, especially in areas

were solution with conventional UV sources are impossible

or significantly more complex.

1.2. Current and future challenges

As can be seen from figure 2the external quantum effi-

ciencies (EQE) of UV-LEDs vary greatly with the emission

wavelength. Whereas UVA-LEDs with emission longer than

315 nm exhibit EQEs exceeding 50%, LEDs in the UVB and

UVC band lag significantly behind. Although record EQEs

of more than 20% for LEDs emitting near 275 nm have been

reported [4], the performance levels of commercial devices in

these wavelength bands are still in the single digit percentage

range.

In order to analyse the shortcomings it is useful to take

a look at the key parameters that describe the performance

of UV-LEDs, i.e. the external quantum efficiency (EQE) and

wall-plug efficiency (WPE). Both are related through the fol-

lowing equation

WPE =Pout

I·V=EQE ℏω

e·V=EQE ·ηel

where Pout, I, and Vrepresent the output power, operating

current, and operating voltage of the UV-LED, respectively,

ℏωthe emitted photon energy, and ηel the electrical efficiency

[3]. The EQE in turn is dependent on the radiative recombina-

tion efficiency ηrad, the carrier injection efficiency ηinj, and the

light extraction efficiency ηext. Accordingly, the EQE can be

described as

EQE =ηrad ·ηinj ·ηext =ηIQE ·ηext

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 2. External quantum efficiencies of UV-LEDs in the spectral range between 200 and 400 nm. Reprinted by permission from

Springer Nature Customer Service Centre GmbH: Springer [3] (2016).

where the product of the radiative recombination efficiency

ηrad and the carrier injection efficiency ηinj is typically referred

to as the internal quantum efficiency ηIQE. For example, record

EQE and WPE for commercially available 280 nm LEDs are

currently around 6.4% and 4.1% [5,6]. Although it is difficult

to precisely determine the different contributions to the overall

efficiencies, from the description of the material properties and

device structure one would estimate the radiative efficiency to

be around 40%, an extraction efficiency around 20%, and an

electrical efficiency of 64%. Consequently, progress in all of

the contributing factors will be required in order to improve

the EQE, wallplug efficiency, and output power of UV-LEDs.

1.3. Advances in science and technology to meet challenges

There are a number of different possible solutions in order to

improve each of the parameters that govern the overall UV-

LED performance, i.e. ηrad,ηinj, and ηext. For example, the

extraction efficiency ηext can be greatly enhanced by encap-

sulating the LED chips with a UV-transparent high refractive

index material. A key challenge here is to find suitable mater-

ials, that are UV-transparent and also do not degrade under

UV exposure. This is not a trivial task, but first candidates

have emerged, e.g. fluoropolymers like CYTOP® [7]. Other

approaches to enhance light extraction include the utilization

of UV-reflective metal contacts together with a UV-transparent

p-AlGaN heterostructure. However, finding suitable metal lay-

ers that are highly reflective in the UVB and UVC spec-

tral range and at the same time enable low resistance ohmic

contacts is not straightforward. Therefore, in many cases a

trade-off between enhancing light extraction and degrading

other parameters like operating voltages and electrical effi-

ciency has to be made. Another key parameter is the radiat-

ive recombination efficiency ηrad which is strongly affected

by the defect density in the semiconductor materials [8].

As most UV-LEDs are grown on sapphire substrates a large

number of threading dislocations are generated at the AlN/s-

apphire interface. Even with advanced growth technologies

in order to reduce the defect density, the threading disloca-

tion density (TDD) in AlGaN heterostructures is typically

between 1010 cm−2and 108cm−2[9,10], which corres-

ponds to IQEs ranging from a few percent to over 60% [8].

Therefore, reducing the TDD is paramount to improving the

radiative recombination efficiency and IQE of UV-LEDs. We

have just provided these examples to illustrate the issues and

refer to the discussion in the following articles describing in

more detail the recent developments in materials and device

technologies.

1.4. Concluding remarks

Even though the large scale application of UV-LEDs will

require substantial advances in the performance levels of

UVB- and UVC-LEDs, it is clear that in the not too distant

future group III-nitride based UV emitters will replace most

conventional UV sources like mercury lamps. UV-LED and

blue LED technologies share the basis of III-nitride material

and device technologies. Although the technological solutions

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

are very different there seem to be no fundamental roadblocks

that would inhibit a steady improvement in UV-LED perform-

ance. How quickly these advances will be realized, will also

depend on the magnitude of the research efforts in the field, but

based on the trajectory of the performance improvements of

the past decade one can anticipate that UVB and UVC LEDs

with WPE of more than 10% should become commercially

available by 2022 [2].

Acknowledgements

M K gratefully acknowledges support by the German Research

Foundation (DFG) within the Collaborative Research Cen-

ter ‘Semiconductor Nanophotonics’ (CRC 787) as well as

funding by the Federal Ministry of Education and Research

(BMBF) of Germany within the ‘Zwanzig20’ consortium

‘Advanced UV for Life’.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

2. AlN on sapphire

Sylvia Hagedorn and Markus Weyers

Ferdinand-Braun-Institut, Leibniz-Institut für Höchst-

frequenztechnik

2.1. Status

UV LEDs are nearly exclusively bottom emitters, since p-

GaN and contact materials are not transparent in the UV. Sap-

phire (or rather the pure corundum) is transparent and mass-

produced for GaN LEDs. For UV transparency the semicon-

ductor layer stacks have to start with AlN, which creates much

bigger challenges in MOVPE (metalorganic vapour phase epi-

taxy) growth than GaN. The target is to obtain a low density

of threading dislocations (TDD). Such dislocations form at the

sapphire—AlN hetero-interface by coalescence of small nuc-

lei and are non-radiative recombination pathways. A low TDD

thus is prerequisite for high internal quantum efficiency [11].

While growth of thick layers is effective in reducing the TDD

by mutual annihilation of dislocations, the high strain result-

ing from lattice mismatch and mismatch in the thermal expan-

sion coefficients leads to strong wafer bow or even cracking

of thicker layers. Strong bow makes processing of the wafers

difficult, crack formation renders them useless for devices.

Total internal reflection at the AlN-sapphire interface limits

light extraction, which is especially low at short wavelengths.

Patterned sapphire substrates (pss) are used for blue LEDs to

enhance light extraction andto reduce TDD (or the layer thick-

ness at which a low TDD is obtained). For UV LEDs this is still

being developed. While UV LEDs have made tremendous pro-

gress in recent years, there is still plenty of room to improve

their performance towards that achieved by blue LEDs which

already approach the theoretical limits. The AlN-sapphire buf-

fer layers are the basis for this further development. As yet,

there is no completely satisfying solution that is proven by

device performance and widely adopted.

2.2. Current and future challenges

Due to the high bond strength of AlN, high growth temperat-

ures are preferred. On the other hand, high temperatures in an

MOVPE reactor may result in surface reactions between the

sapphire substrate and the gas phase. This does not only con-

tain the injected process gases but also species stemming from

parasitic deposits within the reactor. As one result of such pre-

reactions, islands with N-polarity may form, which later are

buried by the matrix with Al-polarity aided by the formation

of AlON [12]. So mastering the growth start is a challenge

with strong impact on TDD. After coalescence of the initial

nuclei, the TDD needs to be further reduced. A rough growth

front with a high density of steps can help in this (by bending

dislocations away from the +c surface normal). However, for

the growth of the AlGaN device layers, high steps are detri-

mental since they can result in compositional modulation by

a higher Ga incorporation rate at such steps [13]. To reprodu-

cibly manage the transition to a rough growth front and back

Figure 3. Reduction of threading dislocation density with grown

AlN layer thickness. For planar growth, cracking limits the

thickness that can be achieved. Epitaxial lateral overgrowth (ELO)

patterning allows thicker layers and thus lower TDD but is not very

economic [10]. High temperature annealing (HTA) achieves low

TDD already for much thinner layers. (S Hagedorn et al,

Ferdinand-Braun-Institut, unpublished results.)

to a smooth final AlN surface is a challenge. The pss approach

for enhanced light extraction is hampered by the low surface

mobility of Al. Thus the patterns (holes in the c-plane surface

or pillars with c-plane top facet) need to be on the sub-micron

scale (nano-pss). Also, AlN tends to nucleate in misoriented

crystallites on non-c-plane sidewalls of such patterns. The c-

oriented grains are difficult to coalesce if they are bounded by

m-plane facets. So it remains a challenge to obtain smooth,

fully coalesced AlN buffers on such nano-pss. Wafer bow res-

ults in an inhomogeneous temperature over the wafer making

it a challenge to obtain homogeneous layer properties, espe-

cially when wafer size is being scaled up.

2.3. Advances in science and technology to meet challenges

There are a number of approaches towards reducing TDD.

These include specific growth schemes for example by mul-

tistep growth with variation of the growth temperature, the

ratio of TMAl and NH3flow, the total pressure or by pulsed

supply of the precursors. Although different growth recipes

have been published as being successful, discussion with many

groups active in the field indicates that reproducibility and

homogeneity are issues that are being faced (but usually not

discussed in scientific papers). A better understanding of the

reasons for these limitations is required to optimize the pro-

duction of AlN–sapphire templates.

A relatively new approach is the reduction of the TDD by

annealing of thin AlN layers at high temperature (HTA) [14].

This approach allows TDD in the range of 5 ×108cm−2

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

already for layer thicknesses of less than 1 µm reducing prob-

lems with wafer bow and cracks. It also works if sputtered

layers of initially low quality (high TDD) are subjected to

HTA (figure 3). In addition to being a relatively cheap pro-

cess, sputtering also removes the issue of chemical attack

on sapphire during heating up in an MOVPE environment.

While a proof of principle has been given [15], further stud-

ies of process reproducibility and resulting device lifetime

are necessary before this approach can be considered as

validated.

To address the issue of light extraction, growth on nano-pss

needs to be developed further [16]. This includes finding the

right patterns (which are different from those used for GaN

not only in pitch but also in shape). Also the patterning of sap-

phire on this nanoscale needs to be optimized towards homo-

geneous and reproducible shape. The right growth parameters

to obtain smooth and fully coalesced layers without misori-

ented crystallites need to be found. While the better under-

standing being developed in studies of planar growth provides

guidelines, the presence of different orientations of sapphire

and AlN will require modifications to the growth processes on

nano-pss.

While work currently is mainly on 2 inch sapphire sub-

strates, higher volumes will require the transition to larger

diameters. To cope with wafer bow this will probably require

thicker substrates which as a consequence may require another

round of growth process optimization.

2.4. Concluding remarks

While a low TDD and a smooth surface and not too big wafer

bow are necessary requirements for AlN templates for UV

LEDs, their suitability needs to be verified by growing device

layer sequences, processing devices and studying their per-

formance including reliability. Thus this topic will remain

on the agenda for the foreseeable future. A combination of

nano-patterning with defect reduction by HTA promises high

crystalline perfection, acceptable wafer bow and improved

light extraction and is worthwhile being studied more in detail

[17,18].

Acknowledgements

Work at Ferdinand-Braun-Institut on AlN templates for UV

emitters is funded by the German Ministry of Research and

Education (BMBF) within the ‘Advanced UV for life’ con-

sortium and the German Research Foundation (DFG) within

the Collaborative Research Center 787 ‘Nanophotonics’. The

authors thank all coworkers at FBH and TU Berlin for their

input and the jointly obtained results.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

3. The growth of bulk AlN and fabrication of AlN

wafers

Ronny Kirste and Zlatko Sitar

North Carolina State University

3.1. Status

For most semiconductor technologies, like Si, Ge, III-

arsenides, single crystalline substrates obtained from bulk

growth provide for highest epitaxial layer quality and devices

with optimum efficiency and lifetime. With the progress in

hydride vapour phase epitaxy (HVPE) and ammonothermal

growth of GaN crystals, these substrates are also entering

InGaN and GaN device development. For Al-rich AlGaN epi-

taxial layers desired for UV laser diodes, LEDs, next genera-

tion power diodes, and sensors, single crystal AlN substrates

are fabricated either via HVPE or sublimation growth/physical

vapour transport (PVT) [11,19–21]. In PVT growth, an AlN-

powder source is sublimed at temperatures exceeding 2200 ◦C

in a nitrogen atmosphere. On the opposite side of the cru-

cible, a seed is positioned and heated to temperatures slightly

lower than the source. Due to the thermal gradient, the sub-

limed source molecules diffuse to the seed where they form

AlN via a reverse reaction. The structural quality of the grown

AlN crystals depends strongly on the type and quality of seed

crystals, which are needed to provide structural information

for the condensing Al and N atoms. Next to the control of

thermal fields in PVT growth the use of native AlN seeds is

essential to achieve high-quality crystalline boules and wafers.

These seeds are typically obtained by spontaneous nucleation

and growth of Lely-like, c-oriented platelets close to the equi-

librium conditions. The first results of bulk growth of AlN on

native seeds were presented in 2002; this process later became

a basis for the development of an iterative boule expansion

process and increase of the diameter of the native seeds over

time.

Over the past two decades, PVT growth of AlN has

been pursued by various laboratories around the world

[19,20,22,23]. Despite all these efforts, commercial availab-

ility of these substrates is still limited. Available PVT grown

substrates typically have a dislocation density <103cm−2and

are available in diameters up to 2”. Figure 4shows a typical

AlN boule (a) and a wafer cut from such a boule (b); an x-ray

topograph (c) from a high quality AlN wafer confirms the low

dislocation density that can be achieved.

AFM imaging of polished AlN wafers reveals c/2 steps and

an RMS roughness <0.1 nm. Using these AlN wafers, epitaxial

growth via metalorganic vapour phase epitaxy (MOVPE) has

been developed and doped and undoped AlGaN layers with up

to 40% Ga-content can be grown pseudomorphically [24,25].

Optically pumped UV lasers on AlN substrates have been

demonstrated by various groups and the lowest threshold of

6 kW cm−2was achieved [21,26,27].

3.2. Challenges and future development

While the crystal quality of PVT grown AlN substrates and

epitaxial films grown on them is excellent, a few challenges

need to be addresses in the near future before a widespread

commercial adoption can be anticipated.

First, availability of 2” AlN wafers was limited up until

recently. This hindered a possible entry even for fundamental

epitaxy and device growth for many research facilities since

many fabrication lines are designed for at least 2” wafers, with

many foundries even moving further to 4”. Although there is

no fundamental limit for large-size AlN boules, the expansion

process is relativelyslow and will require significant long-term

investment to achieve production of 4” or even larger boules.

Second, for UV light emitting diode (LED) applications,

AlN substrates need to be transparent in the targeted emission

window. PVT-grown AlN contains relatively high concentra-

tions of C, Si, and O, which results in reduced thermal con-

ductivity and various absorption centres that negatively impact

the performance of any LEDs grown on them by absorbing the

emitted UV light [28]. While the impact on UV laser diodes

is not as significant, any leakage of the optical mode into

the substrate will negatively impact the laser diode threshold.

The most impactful absorption centre for AlN is located at

4.7 eV or 265 nm, which is at the desired UVC LED emission

wavelength for disinfection applications. Absorption coeffi-

cients exceeding 1000 cm−1are commonly observed at this

wavelength. This absorption band has been associated with

the luminescence emissions around 2.7 eV and 3.9 eV. It has

recently been discussed that the absorption is related to the

CN−point defect and the emission is related to the transition

between the VNand CNpoint defects [29]. Thus, controlling

these two point defects is key to reducing the absorption coef-

ficient. Indeed, AlN epi-ready wafers have been deployed that

have reasonable absorption coefficients <100 cm−1, which

was achieved by compensation with Si (figure 5(a)) [28].

While this approach significantly reduces the 265 nm absorp-

tion, the growth conditions to achieve the desired transparency

may result in the formation of low angle grain boundaries in

the PVT AlN crystals, which reduces the quality of the sub-

strates up to the point that epitaxial layers show signs of relax-

ation and surface deterioration. Therefore, more efforts need

to be made to reduce carbon and other point defects in PVT-

grown AlN [30]. Alternatively, either the substrate could be

removed during the LED fabrication or HVPE could be used

to grow transparent AlN on PVT crystals to retain crystalline

quality and obtain low point defect concentrations. Since the

incorporation of C in HVPE and MOVPE can be controlled

more efficiently, such substrates are excellent for UV LED and

laser applications when the PVT seed is removed (figure 5(b)).

Third, conductive AlN substrates would be beneficial for

UV laser diodes as they would allow for vertical devices with

more homogenous carrier injection, simplified laser design,

and simpler fabrication. In addition, AlN and Al-rich AlGaN

could potentially be used for high power Schottky diodes due

to their high breakdown fields. AlN has a figure of merit that

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 4. (a) Photograph of an AlN boule grown via the PVT method (mm-grid); (b) image of a 1” AlN wafer after mechanical and

chemo-mechanical polishing; (c) x-ray topograph confirming that most of the wafer area is dislocation free.

Figure 5. (a) 265 nm absorption can be reduced by orders of magnitude by controlling the carbon concentration in AlN; (b) using PVT

substrates as seeds for HVPE and subsequent PVT seed removal results in transparency down to the fundamental bandedge.

is nearly 200 times higher than that of SiC and 35 times higher

than that of GaN. The main obstacle in using AlN substrates

for power devices is their lack of conductivity—all AlN sub-

strates are highly insulating. While the most promising n-

dopant in AlGaN is Si, it forms DX-centre and suffers from

high activation energy and compensation for Al-content >85%

[31]. Therefore, significant efforts are needed to better under-

stand the effect of compensation and DX-centre formation and

increase doping efficiency [24,32,33].

Fourth, the AlN surface and epitaxy of AlGaN films on

AlN substrates needs to be addressed beyond simple trial-and-

error approaches. This includes surface preparation, growth

initiation, control of the surface morphology during growth,

as well as engineering of possible relaxation schemes for

thick AlGaN films. So far, optically pumped laser structures

and fully fabricated laser diodes for electrical injection have

been successfully grown on AlN substrates by a few groups

[21,26]. While significant efforts have been made to explain

the epitaxial growth of AlGaN and the impact of various pro-

cess parameters on the growth in general terms to allow for

process transfer between different reactors, the reported res-

ults are not always optimal; in the worst case, epitaxy of

AlGaN on AlN may lead to re-nucleation and the associ-

ated high dislocation density, thus nullifying the use of AlN

substrates.

Lastly, it should be mentioned that, despite high current

prices, substrate cost is not a major challenge for future applic-

ation of AlN substrates. The price per device will significantly

drop for larger substrate areas (2”, 4”, …) and, even at the cur-

rent price levels, the substrate contributes only 10–20% of the

overall cost of a UV laser diode or LED. In addition, since the

input cost of AlN crystal growth is not significantly different

to that of SiC, the AlN substrate prices are expected to follow

those of SiC in high volume production.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

3.3. Concluding remarks

Device growth on native substrates is always more desirable

than the growth on non-native substrates, since it allows for

better crystal quality, resulting in more efficient and more

robust devices. Although commercially available AlN sub-

strates have improved greatly in quality and size over the

years, a widespread penetration has not been achieved. This

is mostly due to the challenges discussed above and lack of

a demonstration of a vastly superior device grown on AlN.

However, in order to advance AlN PVT and HVPE crys-

tal growth, and consequently also AlN-based device techno-

logy, significant investments need to be made not only in

further research and development of the AlN crystal growth,

but also in AlGaN epitaxy, property control, and develop-

ment of associated devices. Only the development of all these

components in unison will ultimately justify the AlN-based

technology.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

4. Radiative recombination efficiency of AlGaN

quantum wells: do we estimate it accurately in a

proper way?

Yoichi Kawakami, Mitsuru Funato and Ryota Ishii

Kyoto University

4.1. Status

Recent progress on the research of nitride semiconductors

has led to the realization of highly efficient blue LEDs, act-

ive layers of which are composed of InGaN-based quantum

wells (QWs). Currently, the improvement of efficiencies in

Al-rich AlGaN-QWs based LEDs is a challenging subject

for the development of deep ultraviolet (DUV) emitters. The

external quantum efficiency (EQE) is expressed by the product

of three terms, carrier injection efficiency, radiative recombin-

ation efficiency (ηrad) and light extraction efficiency. Although

EQEs are directly measurable values, it is difficult to resolve

them accurately into the three terms.

The ηrad has generally been estimated by the temperature

dependence of photoluminescence (PL) intensities. However,

it should be noted that these values are overestimated in many

cases if they were based on the assumption that the ηrad is

100% at cryogenic temperatures. Precise PL measurements

dependent on photo-excitation intensity [34], and rate equation

analysis on time-resolved PL data [35], have revealed the fail-

ure of such an assumption. Moreover, one should be aware that

ηrad is dependent on carrier density in the active layers. Car-

rier densities can be estimated precisely if the selective photo-

excitation is made in the PL measurement, but it is not the case

for the non-selective excitation.

In recent years, various experimental techniques have been

reported for the estimation of ηrad. One approach is to detect

the absolute emission intensities by means of the omnidirec-

tional PL measurements using an integration sphere [36].

The second approach is to detect the heat generation induced

by the non-radiative recombination processes, by employing

techniques such as transient lens spectroscopy [37] or photo-

acoustic spectroscopy [38]. In the former approach, estima-

tion not only of light extraction efficiency but also of photon-

recycling-effect is the key for the exact estimation of ηrad. In

the latter approach, the quantification of the heat signal is the

subject for the accuracy in determining ηrad.

4.2. Current and future challenges

If we aim at the improvement of ηrad, it is very important to

understand the carrier recombination processes as initially dis-

cussed in the Shockley–Read–Hall model. The carrier recom-

bination rate (R) can be divided into (1) nonradiative recom-

bination rates due to defects and/or dislocations (Rnr), (2) radi-

ative recombination rate (Rr) and (3) nonradiative recombina-

tion rate due to Auger process (Rau), so that R=Rnr +Rr+Rau.

In the so-called ABC model, the term of Rnr,Rrand Rau is

assumed to be proportional to injected carrier density (n), to

Figure 6. PL lifetimes of an AlGaN/AlN QW emitting at 245 nm

under various photo-excitation energy densities which are taken at

room temperature.

the square of n, and to the cube of n, respectively if the back-

ground carrier density in the active layers is negligibly small.

Consequently, Rcan be expressed as R=An +Bn2+Cn3,

and we can derive ηrad =Rr/R=Bn2/(An +Bn2+Cn3).

This model gives us apparently good results on the fit-

ting of optical powers as a function of injected currents,

by choosing proper parameters of A, B and C. Therefore,

a lot of research groups have reported their fitting results

based on this model. However, there has been no direct veri-

fication of such power laws in the ABC model, and there

exists the risk not only for misunderstanding the physics

of recombination processes but also for misinterpreting the

ηrad estimation.

We have performed photoluminescence (PL) measure-

ments on Al-rich AlGaN QWs in a wide range of photo-

excited carrier densities, and carrier recombination processes

have been analysed with a model based on rate equations.

This has led to the quantitative estimation of ηrad values.

Our finding was that radiative recombination processes are

exciton recombination under relatively small carrier densit-

ies which correspond to the operating conditions of LEDs

[35]. Since exciton recombination has to be treated as mono-

molecular process, its radiative recombination should show

single exponential decay which is independent on exciton

density. Moreover, the probability of carriers/excitons to be

captured by nonradiative recombination centres (NRCs) is not

constant but decreases with increasing carrier/exciton densit-

ies because of the effect of saturation induced by the filling of

NRCs. Figure 6shows the PL lifetimes of an AlGaN/AlN QW

emitting at 245 nm at room temperature which are increased

with increasing photo-excited carrier densities due probably

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

227

100 nm

Wavelength (nm)

noituloser laitapS

10 nm

1 µm

10 µm

100 µm

200220400

Macroscopy using refractive and reflective optics N.A. < 0.3

Microscopy with

refractive optics

N.A. <

0.9 (1.4)

Our target

Nanoscopy with

nearfield VIS optics

Ref. 39

206

Microscopy with

reflective optics N.A. < 0.5

250

300350

Nanoscopy with

nearfield DUV optics

Ref. 41 Ref. 42, 43

Figure 7. Current status and future prospect of PL-mapping

technology.

to the saturation of NRCs. These results indicate the failure of

the conventional ABC model.

4.3. Advances in science and technology to meet challenges

The visualization of radiative and nonradiative recombina-

tion centres is the key to reveal the recombination mechan-

isms in semiconductors. Cathodoluminescence (CL) is a use-

ful technique to map CL intensities in AlGaN-based DUV

emission. However, the spatial resolution is affected not only

by the spread of the incident electron beam in the sample

but also by the diffusion length of the excitons/carriers. Scan-

ning near-field optical microscopy (SNOM) has various mer-

its for assessing the detailed recombination dynamics. We can

choose the optical mode in the PL mapping with SNOM.

In the illumination collection mode (IC-mode), both photo-

excitation and the PL detection are performed through an

optical fibre probe with small aperture. In the IC-mode, the

spatial resolution is only limited by the diameter of aperture.

The best spatial resolution achieved is down to about 10 nm in

the visible spectral detection [39]. In the illumination mode

(I-mode), the optical excitation is made through an optical

fibre probe while the PL detection is made with a lens in the

far-field configuration. In this mode the spatial resolution may

be limited by the diffusion length like in the case of CL map-

ping. In the collection mode (C-mode), photo-excitation or

electrical injection is done on large areas while the detection

is through an optical fibre probe. Careful comparison of PL

mapping images taken at the same area in between IC-mode,

I mode and C-mode (we named it as multi-mode SNOM)

gives us useful information on the carrier/exciton diffusion

processes [40].

However, there have been existing technological diffi-

culties in developing the SNOM in the DUV region. The

shortest wavelength reported was 227 nm in the excitation,

and longer than 240 nm in the detection with a spatial res-

olution of about 100 nm [41]. We have recently achieved an

excitation with 206 nm, and the detection down to 220 nm

with a spatial resolution better than 100 nm [42,43]. As

illustrated in figure 7, one target is to reach the detec-

tion wavelength down to 200 nm with an improved spatial

resolution.

4.4. Concluding remarks

In this contribution, we have reviewed the issue in estimat-

ing ηrad of UV-emitting AlGaN QWs. The role of excitons in

the emission mechanism has already been reported for InGaN

QWs [44]. Since the exciton binding energies of AlGaN QWs

are much larger than those of InGaN QWs, one should bear

in mind the importance of excitons in discussing the recom-

bination model. As for the origins of NRCs in Al-rich AlGaN

QWs, point defects play more important role than in InGaN

based QWs [45]. Development of characterization techniques

such as DUV SNOM that visualizes the capture process of

excitons or carriers to such NRCs and establishment of estim-

ation scheme of ηrad may contribute to drastic improvement of

EQE of UV LEDs.

Acknowledgements

This work is partially supported by JSPS KAKENHI (Grant

Nos. JP15H05732, JP16H02332, and JP16H06426).

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

5. Point defects

Pramod Reddy1and Ram´

on Collazo2

1Adroit Materials Inc.

2North Carolina State University

5.1. Status

There exists a ‘point defect problem’ in Al rich AlxGa1−xN

(x> 0.5) that has presented a major challenge to implementing

AlGaN-based deep-UV optoelectronics. An increased incor-

poration of impurities and generation of native defects such

as vacancies has been observed with increase in Al content

(necessary for a technological push into deep-UV regimes)

and is likely a consequence of the dependence of the defect

formation energy on the Fermi energy whose range increases

from GaN (3.4 eV) to AlN (6.1 eV). The incorporated point

defects vary with the growth method, growth facet, dop-

ing type and concentration, and the growth environment, i.e.

temperature, metal/N chemical potentials, pressure, etc, and

accordingly impact various optical and electronic properties

of different regions in the optoelectronic device and hence

the technological feasibility of ultra-wide bandgap-based deep

UV emitters. Consequently, there are many point defect phe-

nomena that are of high technological interest and we men-

tion a few in the following: in metalorganic vapour phase epi-

taxy (MOVPE) grown AlGaN, when doped with Si (n-type)

at low to moderate levels (<1 ×1019 cm−3), and in unin-

tentionally doped regions, carbon and vacancy-oxygen com-

plexes are reported to be the major compensating impurit-

ies and are a major constraint in designing active regions in

optoelectronics including quantum wells [46,47]. At higher

concentrations, Si itself forms an acceptor complex with an

Al/Ga-vacancy resulting in a self-compensation mechanism

[48]. Further, at Al content >80%, Si relaxes from a donor type

III-substitutional site to an acceptor type DX configuration and

results in a monotonically increasing activation energy from

20 to 50 meV for x< 0.8 to around 300 meV for x=1 [31].

In p-type AlGaN, doping itself is less understood with reports

of both very low and very high dopant (Mg) activation ener-

gies [49,50]. Nevertheless, the hydrogen passivation and self-

compensation by nitrogen vacancies have also been reported

in Al-rich AlGaN similar to GaN. With recent availability of

single crystal AlN substrates and very low dislocation dens-

ity (<103cm−2) AlGaN epitaxy on them [24], interesting phe-

nomena have been observed. They include: the crucial balance

between carbon, silicon and oxygen impurities in PVT-grown

AlN single crystals whose ratios control the optical proper-

ties of the substrate [51], and the observed dependence of the

compensating point defect (vacancy-silicon complex) forma-

tion and hence electronic performance on the extended defect

(threading dislocation) density [24].

5.2. Current and future challenges

The various point defect phenomena mentioned in the previ-

ous section manifest very important technological challenges

to the implementation of Al-rich AlGaN-based optoelectron-

ics: The strong dependence of internal quantum efficiency

(IQE) and associated non-radiative recombination coefficient

in the quantum wells and active regions of UV emitters on

the impurity concentrations demonstrates that point defects

act as recombination centres thereby greatly reducing device

performance [52,53]. Hence, low doped or ‘intrinsic’ regions

such as quantum wells require low point defect density for

high IQEs. This is evidenced by the low IQE shown in fig-

ure 8(a) for AlGaN/AlN multi-quantum well structures at low

ammonia flows where incorporation of impurities such as car-

bon and oxygen is energetically favourable. Further, genera-

tion of native defects (VIII and VN) and formation of defect

complexes have also been demonstrated to be energetically

favourable under various growth conditions. The point defects

have been identified by density functional theory (DFT) ana-

lysis in corroboration with thermodynamic analysis and elec-

trical characterization, secondary ion mass spectrometry and

photoluminescence characterizations in III-nitrides especially

at high Al content [46,47,54]. At higher doping concentra-

tions (n- or p-type) required in charge injection layers, con-

ductivity exhibits a ‘knee behaviour’ due to self-compensation

(vacancy-silicon complexes in n-type and nitrogen vacancies

in p-type) [24]. Hence, an upper doping limit exists that lowers

the maximum conductivity resulting in increased ohmic losses

and large contact resistances. Finally, the relaxation of Si from

donor configuration to a DX centre configuration has pre-

cluded AlN and AlxGa1−xN (x> 0.8) from being employed

in deep-UV optoelectronics [31]. Consequently, DX control is

one of the most important technological challenges for imple-

menting LEDs and lasers operating at wavelengths <240 nm.

A summary of the Si behaviour in AlGaN is shown in figure

8(b) showing the ‘knee behaviour’ due to self-compensation

and the abrupt decrease in carrier concentrations for Al con-

tent >80%. So far, we have mentioned the point defect issues

within the active regions of the device structure. However,

point defects have also played a major role in substrate suit-

ability. Specifically, in the development of physical vapour

transport (PVT) AlN single crystals with dislocation densit-

ies <103cm−2which promised highly efficient AlGaN-based

optoelectronics, excessive carbon has led to absorption in the

deep-UV region and has posed the most significant challenge

to implementing UV emitters directly on PVT AlN substrates

[29].

5.3. Advances in science and technology to meet challenges

The difficulty in overcoming the point defect challenge arises

from the dependence of the type and concentration of these

defects on a large number of factors including growth tech-

nique, the precursors, growth mode, temperature, doping, etc.

Developing specific experimental optimizations for each point

defect may not be feasible due to the high iterative costs and

the likely conflicting requirements of different point defects

and other aspects of crystal quality that affect device perform-

ance. A general solution would be a predictive point defect

control scheme that may be employed to determine the optimal

growth conditions within predefined constraints determined

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 8. (a) The IQE for AlGaN/AlN MQW structures on single

crystal AlN substrates as a function of excitation power density

shown for different point defect concentrations (here, higher

ammonia flow corresponds to lower non-radiative recombination

defect centre concentrations). (b) The dependence of carrier

concentration as a function of doping concentration for different Al

compositions in the alloy AlGaN. The black dashed line assumes

the carrier concentration to be same as silicon concentration.

by other factors including growth rates, crystal quality and

system limitations. Such a point defect scheme would employ

(a) DFT analysis in corroboration with characterization tech-

niques such as secondary ion mass spectroscopy (SIMS) and

photoluminescence (PL) to identify the point defect respons-

ible for a particular challenge and (b) dependence of ther-

modynamic and reaction kinetics describing the growth pro-

cess on practical growth knobs and (c) finally understanding

the relationship between the point defects and the thermody-

namics and reaction kinetics describing the growth process.

This would require a highly synergistic collaboration requir-

ing the development and extension of DFT from typical zero

kelvin and standard states to experimentally employed growth

temperatures and supersaturated states determined from the

developed thermodynamic and reaction models. In this dir-

ection, recent developments of a point defect control model

directly providing a quantitative relationship between point

defect formation energies and growth knobs has allowed for

Figure 9. A schematic of targeted and predictable point defect

control via chemical potential control and defect quasi Fermi level

control.

quantitative predictions of carbon as a function of growth con-

ditions [33]. Finally, using the dependence of point defects

on the chemical potentials (metal rich versus N rich), and

employing single crystal AlN substrates, record high IQEs

(>80%) and low lasing thresholds (<10 kW cm−2) have been

achieved [21,52]. An illustration is seen in figure 8(a) where

a shift towards N rich chemical potential via increased ammo-

nia flow results in lower non-radiative recombination centre

concentrations and a significant improvement in IQE. How-

ever, significant research efforts are necessary to generalize

the scheme to different materials and growth techniques by

including reaction kinetics and transients into the theoretical

models so that for a given growth technique and practical con-

straints, the optimal growth conditions for a given application

can be determined theoretically without the requirements for

expensive experimental iterations. Further, point defects form

when they are energetically favourable at equilibrium. Hence

non-equilibrium doping techniques such as implantation need

to be investigated in Al-rich AlGaN and AlN. Another solution

would be to develop a point defect control scheme that targets

a class of point defects having a common character where the

exact identification is unnecessary. In this direction, manipula-

tion of the Fermi level or electron chemical potential is another

useful tool to control the defect formation energy of a particu-

lar class of defects, i.e. charged compensating defects. Hence,

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

defect quasi Fermi level (dQFL) control-based point defect

reduction has been employed to improve the performance in

different material systems such as nitrides and arsenides, with

both n-type and p-type doping, with very different defect con-

figurations, but with one common characteristic, i.e. all the

defects were of compensating type and charged [55,56]. Here,

a significant advantage would be applying the point defect con-

trol scheme by external mechanism (in this case, an above

bandgap light source) without altering the growth conditions.

A summary of the developed point defect control frameworks

is shown in figure 9.

5.4. Concluding remarks

In conclusion, the ‘point defect problem’ in Al-rich

AlxGa1−xN (x> 0.5) requires a comprehensive solution for

high performance deep-UV optoelectronics. Although exper-

imental iterative approach towards low defect densities may

be feasible in the short term, a long-term solution requires

an understanding of point defect incorporation and ‘known

and controlled’ growth environments. In this direction, the

energy of formation of the defects may present a general solu-

tion where the reaction constituent chemical potentials and

electron chemical potential may be appropriately tuned within

predefined constraints to obtain minimum defect incorporation

and maximum efficiencies. This approach requires extending

the DFT analysis beyond the current 0 K and standard state

calculations towards practical growth environment to under-

stand point defect incorporation and developing and utilizing

developed thermodynamic and reaction kinetic models that

theoretically describe the growth environment which can then

be controlled by internal knobs (e.g. gas flows, temperature,

etc) and external knobs (e.g. illumination).

Acknowledgements

The authors acknowledge funding in part from NSF (ECCS-

1508854, ECCS-1610992, DMR-1508191, ECCS-1653383,

ECCS-1916800), ARO (W911NF-15-2-0068, W911NF-

16-C-0101), AFOSR (FA9550-17-1-0225), DOE (DE-

SC0011883), and ONRG NICOP (N62909-17-1-2004). We

also acknowledge fruitful discussions with Zlatko Sitar,

Douglas Irving, Jonathon Baker, Ronny Kirste, Seiji Mita,

James Tweedie and Shun Washiyama.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

6. Toward ohmic n-contacts on n-AlGaN with high

Al mole fraction

Luca Sulmoni

Technische Universit¨

at Berlin, Institute of Solid State Physics

6.1. Status

Ohmic contact formation on n-type III-nitrides is well estab-

lished, at least for n-GaN. The basic mechanism responsible

for ohmic behaviour involves N extraction by the contact metal

upon thermal annealing and the formation of a heavily n-doped

thin layer in the proximity of the interface [57,58]. In fact,

N vacancies act as donors in GaN thus enhancing the prob-

ability of tunnelling for carriers through the thinner Schottky

barrier [59,60]. Typically, the metal stack consists of four

metals divided in two bilayers [58]. The first bilayer is respons-

ible for the contact formation whereas the second one, usually

referred to as capping layer, prevents the oxidation and the out-

diffusion (‘ball up’) of the underlying metal stack. The most

widely employed contact scheme is Ti/Al/Ni/Au. In many pub-

lications, the onset of the ohmic behaviour was attributed to

the formation of a crystalline TiN layer (~5–20 nm thick)

at the metal/semiconductor interface, indicating the reaction

between Ti and GaN [57,58,60,61]. While investigating the

n-contact formation for higher Al mole fractions, the most

common approach has been to transfer the already established

metallization scheme for n-GaN to the n-AlGaN material sys-

tem. Unfortunately, Ti-based electrodes have shown to form

ohmic contacts with n-AlGaN only up to an AlN molar frac-

tion of 0.58 [62]. Typically, the best reported values of contact

resistivities are lower than 10−6Ωcm2on n-GaN. However,

due to the need of a transparent current spreading layer, UV

light emitters utilize n-AlxGa1−xN [63]. Up to an Al mole frac-

tion of x=0.7, contact resistivities on n-AlxGa1−xN are sim-

ilar to n-GaN but increase drastically to 1 Ωcm2on n-AlN

(figure 10) [64], which goes hand in hand with the observed

increase in n-sheet resistance [65]. In terms of wall-plug effi-

ciency (WPE), devices emitting in the UV still exhibit signi-

ficantly lower efficiencies than blue LEDs: >80% at 450 nm,

>4% at 280 nm and >0.01% at 230 nm [5,63,66]. As dis-

cussed in sections 10 and 11, the reduced external quantum

efficiency (EQE) at shorter wavelength is responsible to a sig-

nificant extent for this efficiency drop. Nevertheless, minimiz-

ing the contact resistivities of p- and n-layers in deep UV LEDs

and lasers is essential for improving WPE, reducing operating

voltages and avoiding resistive Joule heating.

6.2. Current and future challenges

The causes for the non-ohmic characteristics of Ti-based con-

tacts on n-AlGaN with high Al mole fractions are twofold.

Firstly, the Schottky barrier increases due to the lower elec-

tron affinity of AlGaN with increasing Al mole fraction as

well as the lack of appropriate low work-function metals [57].

The lack of metals with work-functions lower than 4 eV is a

fundamental physical limitation. Therefore, the main strategy

in order to achieve ohmic contacts on n-AlGaN is to reduce

Figure 10. Contact resistivity as a function of the Al mole fraction

for n-contacts on n-AlxGa1−xN in the entire alloy composition.

the Schottky barrier width and therefore enhance tunnelling

probabilities. Unfortunately, N extraction by Ti will become

less energetically favourable as the AlN bond in the n-AlGaN

layer is stronger than for GaN (lower formation enthalpy) [58].

During the annealing process, Ti substitutionally replaces Ga

in the alloy leading to the formation of voids, highly defect-

ive Al +Ti +N phases and TiN protrusion islands, prefer-

entially formed along dislocations, at the metal/nitride inter-

face thus hindering the development of an uniform contact

area [58,61]. Due to the higher resistivity of n-AlGaN layers

and of n-contacts in comparison to n-GaN, an interdigitated

n-electrode is usually implemented during the microfabrica-

tion process in order to improve the current uniformity in the

emitting area.

Nonetheless, a careful adjustment of the metal stack and

the thermal annealing conditions is not the only challenge in

order to form low resistance and ohmic n-contacts to n-AlGaN

at high Al mole fractions. Other important parameters play a

critical role, such as the doping concentration in the underly-

ing n-AlGaN layer and the dry etching conditions. Since UV

LEDs are fabricated on a non-conductive substrate such as

sapphire, the buried n-AlGaN contact layer must be exposed

using plasma etching. Plasma-induced damage often results

in an etched surface of poor quality thus undermining signi-

ficantly the optical and electronic properties of n-AlGaN. Cao

et al reported that when increasing the Al mole fraction the

plasma-induced defects act as deep-level centres, pinning the

Fermi level and increasing the Schottky barrier height [67]. It

is thus mandatory to carefully adjust the dry etching condi-

tions and to introduce an adequate surface treatment in order

to minimize the plasma damages and to assist the n-contact

formation.

6.3. Advances in science and technology to meet challenges

In order to reduce the aggressive Ti-GaN reaction occurring in

Ti-based n-contacts, different strategies have been suggested

such as an Al layer with an adequate thickness on top of Ti

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

(optimal Ti/Al ratio), the implementation of diffusion barriers

between Ti and AlGaN (e.g. Cr, Zr, Nb, …) or the substitution

of Ti by V. In the latter case, the generation of a heavily n-

doped thin layer at the AlGaN interface is driven by the Al in

the metal scheme while V acts as an indiffusion barrier [68].

Circumstantial evidence reported by several authors of the N

extraction from the n-AlGaN layer and the consequent low n-

contact resistance was the formation of a continuous thin AlN

layer (~2–5 nm thick) at the interface [62,68]. Nevertheless,

the driving force for the reaction metal/AlGaN weakens as

AlGaN becomes more energetically stable with the increase in

Al mole fraction: N atoms create a more stable compound with

Al than with Ga due to the lower formation enthalpy of AlN

(−318.1 kJ mol−1) in comparison to GaN (−110.9 kJ mol−1)

[58,61]. In fact, it was possible to obtain ohmic V-based n-

contacts only up to an AlN mole fraction of 0.7 [64]. The chal-

lenge is to find or develop a material (preferably highly con-

ductive as a metal) able to form more stable nitrides than TiN

or AlN on n-AlGaN layers with high Al content.

The high crystal quality with TDD lower than 104cm−2

of AlN bulk substrates is expected to improve the IQE of deep

UV devices (see section 3). Unfortunately, due to the high ion-

ization energy of donors in n-AlGaN layers with very high Al

mole fraction, conductive AlN bulk substrates are not expec-

ted to be utilized in near future. In fact, the practical use of

such substrates would be beneficial especially for UV lasers

to circumvent the need of exposing the buried n-AlGaN with

plasma etching as mentioned before (current fabrication tech-

niques typically involve substrate removal followed by a flip-

chip process) thus allowing for a more homogeneous vertical

carrier injection.

6.4. Concluding remarks

Ohmic n-contacts on wide-bandgap semiconductors such as

n-AlGaN with high Al content are challenging given sev-

eral material limitations. First of all, the self-compensation

of the Si dopant sets an upper limit on the amount of avail-

able donors in the n-AlGaN layer thus frustrating any effort in

thinning the depletion region of the Schottky metal/semicon-

ductor contact. Secondly, metals with sufficiently low work-

function are not available. Finally, the N vacancy formation

is hindered as the AlN bond in n-AlGaN becomes more and

more energetically stable with decreasing Ga content and

obstructs the formation of a heavily n-doped layer at the inter-

face upon electrode annealing. Therefore, breakthroughs in

terms of n-conductivity are expected to occur by new material

concepts.

Acknowledgements

Funding by the German Federal Ministry of Education and

Research (BMBF) within the ‘Advanced UV for Life’ project

is acknowledged.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

7. Doping of AlGaN

Biplab Sarkar1,2and Ram´

on Collazo2

1Indian Institute of Technology Roorkee

2North Carolina State University

7.1. Status

A high electrical conductivity in n- and p-type AlGaN is neces-

sary to minimize the resistive losses in the epitaxial layers

of UV light emitters. Furthermore, a high free carrier density

also leads to low-resistance contacts. Thus, effective doping of

AlGaN has been under serious investigation for more than two

decades. N-type dopants (Si, Ge, etc) can readily be incorpor-

ated in GaN and Al-rich AlGaN epitaxial layers during growth.

These dopants provide a relatively low ionization energy (EA)

and high free electron density (>1019 cm−3) in AlGaN films.

However, Mg is the only dopant that so far can be considered

viable for p-type GaN and AlGaN epitaxial films. Mg shows

a high EA(>120 meV) in metalorganic vapour phase epitaxy

(MOVPE) grown p-GaN films, resulting in a free hole dens-

ity that is only a fraction of the doping density. This becomes

worse as the Al mole fraction increases in the AlGaN films,

as EAis expected to increase monotonically to >500 meV for

AlN films. As mentioned in the previous chapter, increasing

the Al-mole fraction in AlGaN results in several point defects

and charge compensators. Accordingly, several methods have

been developed recently to achieve a high electrical conduct-

ivity in both n-type and p-type AlGaN epitaxial layers. This

chapter is dedicated to highlight the advances in the doping of

AlGaN to achieve a low EAand high electrical conductivity.

7.1.1. N-type doping of (Al)GaN. A low EAof Si donors

in (Al)GaN yields a free electron density in the range of

~1019 cm−3and conductivity of <10 mΩcm in n-AlXGa1−XN

films (x< 0.8) [73,74]. Availability of native substrates is a

bonus for achieving a higher conductivity in Al-rich AlGaN

films [75]. However, the donor EAundergoes a steady rise in

Al-rich AlGaN films when the Al mole fraction goes above

80%, as shown in figure 11. This sharp increase in EAis attrib-

uted to the formation of Si-DX centres acting as acceptor-

type compensating point defects. Furthermore, the Si doping

in AlGaN is also responsible for a ‘knee-behaviour’in resistiv-

ity with increase in the doping density [73,74]. While the low

Si doping density is responsible for the formation of carbon

and vacancy-oxygen complexes, a high Si doping leads to self-

compensation and vacancy related complexes (e.g. VIII +Si)

[48,76]. All these acceptor type point defects compensate the

free electron density in Al-rich AlGaN films.

Apart from Si, germanium and oxygen have also been

attempted to dope n-AlGaN films. Ge typically shows a donor

EAof ~30 meV in GaN, and is predicted to allow a doping

density beyond the Mott transition [77]. However, the onset of

DX centre formation is predicted to arise at an Al mole frac-

tion of 52%, much lower than the Si counterpart. Therefore,

a very high doping density can be achieved in AlXGa1−XN

films (x< 0.52) using Ge doping. Similarly, oxygen can also

Figure 11. Donor EAof Si in Al-rich AlGaN epitaxial films [74]. Si

donor EAin n-GaN is ~15 meV, which increases to ~50 meV in

n-AlXGa1−XN (x< 0.8) epitaxial films [73]. Thus, nearly full

ionization of free carriers at room temperature is typically observed

in n-AlXGa1−XN (x< 0.8) epitaxial films.

be incorporated in AlGaN epitaxial films, and gets incorpor-

ated during the growth due to the presence of residual oxygen

in the growth chamber. However, oxygen donors show a much

higher EAthan Si, and the onset of DX centres also occurs at

a lower Al mole fraction (~61% Al mole fraction) in AlGaN

[78]. All these issues make Si the preferred choice for n-type

doping of AlGaN epitaxial films.

7.1.2. P-type doping of AlGaN. Achieving a free hole density

of the order of 1018 cm−3in GaN and AlGaN films has been

a major challenge for decades. Mg is the only known viable

acceptor doping source that can occupy a substitutional site

in (Al)GaN epitaxial films. A free hole density in the order

of 1017 cm−3is achievable in p-GaN films grown on for-

eign substrates using high temperature growth techniques (e.g.

MOVPE). Although relatively lower temperature growth (e.g.

molecular beam epitaxy) results in a higher free hole density

due to lower hydrogen incorporation, the subsequent device

processing incurs very high temperature process steps raising

several reliability issues. Native AlN substrates offer a suit-

able alternative for high temperature p-GaN growth having

free hole density nearly an order of magnitude higher than the

foreign substrates [79]. The lower dislocation density of single

crystal AlN substrates minimizes the incorporation of donor-

type nitrogen vacancies in p-GaN. An EA< 100 meV is thus

achievable in p-GaN films grown on single crystal AlN sub-

strates using MOVPE.

For AlGaN, the Mg acceptor EAis believed to increase

monotonically from 120 to 200 meV in GaN to 500 meV

in AlN [49]. Alongside, compensating nitrogen vacancies are

also expected to exhibit a significant reduction in formation

energy in p-AlGaN w.r.t. increase in the Al mole fraction

[59]. However, recent studies reported a significantly lower

acceptor EAin Al-rich p-AlGaN films, as shown in figure 12.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 12. Recent reports on Mg-doping of AlGaN; (a) free hole density (p), and (b) acceptor EAfor bulk and superlattice p-AlGaN films.

Reproduced with permission from [75].

Kinoshita et al has reported an EAof <72 meV in bulk p-

Al0.7Ga0.3N films grown on sapphire substrate [49]. Similarly,

Chen et al reported a free hole density of ~4.75 ×1018 cm−3in

p-Al0.4Ga0.6N by using In-surfactants and a Mg delta-doping

scheme [80]. Along with improvement in bulk doping, altern-

ative approaches such as superlattice structures and polariz-

ation doping schemes have also been developed to achieve a

high free hole density in p-AlGaN (figure 12). Short-period

superlattice structures consisting of several thin p-AlGaN

films with alternating Al mole fractions allow periodic oscil-

lations of the valence band of p-AlGaN layers. This technique

results in regions where the Mg energy level is much closer

to the Fermi-level. The effective acceptor EAis thus reduced,

and a free hole density in the order of 1018 cm−3can be

achieved in p-AlGaN superlattice structures [81]. Similarly,

a graded p-AlGaN film (polarization doping scheme) results

in the presence of polarization induced 3D charges, and hence

the film is degenerate of free carriers. A free hole density of the

order of 1018 cm−3has also been reported from polarization

doped p-AlGaN films grown on foreign as well as native AlN

substrates [82].

7.2. Current and future challenges

Doping all III-nitride epitaxial films will remain a major

focus of the community for the coming years. Si doping of

AlGaN up to 70% Al mole fraction can be considered to

be matured enough. However, several scattering mechanisms

limit the carrier mobility thereby increasing the sheet res-

istance of the epitaxial films. Thus, a major effort will be

required to increase not only the free carrier density, but also

the carrier mobility in Al-rich AlGaN. Crystal growth tech-

nique has to be optimized to minimize the point defect incor-

poration and produce smooth high quality epitaxial films.

A lower dislocation density in the epitaxial films minim-

izes the point defect incorporation, and provides a better

internal quantum efficiency. Thus, AlGaN growth optimiz-

ation on low dislocation density templates will require a

major effort. Recent reports on AlGaN growth on native sub-

strates and high temperature annealed templates are promising

[75,83]. Moreover, contact mechanism in n-type Al-rich

AlGaN films is still unknown for Al-rich AlGaN films. Unlike

GaN and Ga-rich AlGaN films, obtaining a low resistance

ohmic contact is a challenge. Ti/Al based contacts typic-

ally show a non-linear current-voltage characteristic when

applied to Al-rich n-AlGaN films. Moreover, current conduc-

tion in Al-rich AlGaN films is dominated by different tun-

nelling mechanisms such as trap-assisted tunnelling. A major

effort will thus be required to understand the contact form-

ation mechanisms to enhance the contact performance and

mitigate the reliability issues of future III-nitride deep-UV

emitters.

Similarly, Mg doping of AlGaN will remain a major focus

for the coming years. The role of dislocation density, point

defects, vapour supersaturation, etc, are yet to be well under-

stood for p-(Al)GaN films. Alternative methods such as polar-

ization doping and superlattice structures will be required to

attain maturity before these technologies can be commercial-

ized. Apart from free carriers, contacting the p-side of the

III-nitride light emitters remains a major challenge. Stand-

ard Ni/Au contact metallization scheme to p-GaN results in

highly resistive ohmic contacts with a contact resistance of

>mid-10−3Ωcm2in most of the reported literature. Altern-

atively, the tunnel junction concept has been developed to

achieve low resistance ohmic contacts to the p-side of the III-

nitride light emitters. In this scheme, the contact is applied

to a highly doped n-GaN film grown on p-GaN. When bias

is applied, electrons are drawn from the valence band of p-

GaN into the n-GaN due to the inter-band tunnelling. Sim-

ilarly, free holes form the valence band of p-GaN are injec-

ted into the light emitter’s active region. Thus, low resist-

ance ohmic contacts can be formed in the p-side of the III-

nitride light emitter by contacting the highly doped n-GaN

instead of the p-GaN film. Note that an additional applied

voltage (hence power loss) is required for the tunnel junc-

tion to allow holes to be injected into the light emitter’s

active region. Moreover, growth of subsequent n-GaN films

on Mg-doped GaN at high temperatures result in ‘memory

effect’ where Mg incorporates into the n-GaN. Thus, excel-

lent doping control is mandatory to ensure a sharp and thin

depletion region across the highly doped tunnel junctions. All

these open challenges simply indicate the quantum of efforts

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

required to fully explore the potential of all III-nitride based

devices.

7.3. Advances in science and technology to meet challenges

Recent advances in growth, characterization and measure-

ment techniques for III-nitride emitters have been a remark-

able achievement. Several research groups have developed

point defect control schemes (such as defect quasi fermi-level

control, chemical potential control, vapour supersaturation,

high quality templates, etc) resulting in very high free car-

rier density in Al-rich AlGaN epitaxial films. Formation of

DX centres in Si-doped Al-rich AlGaN films during growth

is a physics limited issue. However, recent reports indic-

ate that high energy implantation (a non-equilibrium pro-

cess) of Si into Al-rich AlGaN and AlN films may provide

pathway to mitigate the DX centre formation. A signific-

antly lower EAin ion-implanted n-AlN film grown on AlN

substrate has been demonstrated recently [84]. Therefore, a

paradigm shift of the doping scheme can be looked into to

mitigate some of the critical doping issues in Al-rich AlGaN

films.

Talking about the active region of the III-nitride light

emitters, nearly defect-free quantum wells providing >80%

internal quantum efficiency have been developed for both for-

eign and native substrates. Similarly, for laser diodes, doped

waveguides and reflecting mirrors has been optimized to

provide excellent wave confinement. However, to achieve las-

ing in deep-UV spectrum, resistive losses in the p-type clad-

ding region must be minimized. Bulk Mg-doping and alternate

doping schemes seems promising to provide low resistance p-

AlGaN cladding regions. So far, most of the III-nitride emit-

ters have been designed having p-GaN as the contact layer.

However, p-GaN absorbs the deep-UV light emitted by the act-

ive region of the emitter. Thus, recent reports on low EAand

high free hole density in Al-rich p-AlGaN seem promising to

directly apply contact metallization to the p-AlGaN cladding

layer. Moreover, standard Ni/Au contacts used in the p-side

of laser diode are also known to absorb the deep-UV light.

To overcome this absorption, reflective contact metallization

schemes to replace the Ni/Au contact have been developed.

Takano et al have even demonstrated improved light extraction

efficiency in deep-UV flip-chip LEDs (275 nm) by using trans-

parent p-AlGaN contact layer and Rh mirror contact [4]. All

these reports indicate the importance of developing cutting-

edge epitaxial film growth and contact metallization schemes

to realize highly efficient deep-UV emitters.

7.4. Concluding remarks

This chapter provides a brief review of current status of n-

and p-type doping in Al-rich AlGaN films for future highly

efficient III-nitride light emitters. Identification of different

point defects is necessary to understand the charge compensat-

ors responsible for compensating the free carriers in Al-rich

AlGaN films. Epitaxial films grown on low threading dislo-

cation density templates seems promising in many aspects.

Recent reports on Mg doping in Al-rich p-AlGaN films is very

promising for deep-UV emitters. With advancement in growth

techniques for both foreign and native substrates, high per-

formance Al-rich AlGaN based devices like LEDs and laser

diodes can be realized in the near future.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

8. Boron-containing (Al, Ga)N heterostructures

Ferdinand Scholz1and Abdallah Ougazzaden2

1Institute of Functional Nanosystems, Ulm University

2Georgia Institute of Technology, Georgia Tech-CNRS

8.1. Status

Boron (B) is the topmost and thus smallest element of the

3rd column of the periodic system of elements, being placed

right above aluminium (Al) and gallium (Ga). Hence, one can

expect a substantially smaller lattice constant and a larger band

gap of B-containing nitrides (e.g. Al1−xBxN) as compared to

their B-free equivalents (i.e. AlN in this example). Typically,

the lattice constant of such alloys follows Vegard’s law being

a linear relation between the composition xand the lattice

constants of the binary end members (here AlN and BN), if

both binaries crystallize in the same lattice type. However,

while AlN and its Ga- and In-containing alloys crystallize in

the hexagonal wurtzite structure, the thermodynamically most

stable structure of BN is a hexagonal layered structure sim-

ilar to graphite, whereas a wurtzitic phase can only be realized

under specific boundary conditions. Anyway, by considering

the data which are known for wurtzitic BN (w-BN), a variation

of the lattice constants of Al1−xBxN by about 10% when vary-

ing xcan be expected with B incorporation into AlN leading

to smaller lattice constants, while mixing Ga into AlN would

lead to larger lattice constants. Therefore, B is a prospective

candidate to manage lattice mismatch and strain in AlBGaN

heterostructures.

The bandgap of such alloys typically follows a quadratic

correlation described by a bowing parameter b besides the

bandgaps of the binary end members. The band gap of w-BN

is still heavily discussed (see e.g. [85,86]); it was calculated to

be about 6.8 eV with indirect character (figure 13), while the

direct band gap is estimated to be around 10 eV [85]. Simil-

arly, only sparse data are known about the bowing parameters.

Calculated bandgap data for AlBN presented in [85] may be

best described with a bowing parameter of 7 eV for the direct

bandgap and of 4 eV for the indirect transition.

Of course, essentially all other material characteristics are

also functions of the respective ternary or quaternary compos-

ition. Particularly the variation of the refractive index when

mixing B to Al(Ga)N alloys seems very promising for the real-

ization of Bragg mirrors made of virtually strain-free AlGaN-

AlBN superlattices [87]. Moreover, alloying with B creates

new degrees of freedom for managing both, the spontaneous

and the piezo-electric polarization in respective heterostruc-

tures [87–89]. Therefore, optical and electronic devices could

benefit from the development of this material system.

8.2. Current and future challenges

These simple considerations lead to very promising proper-

ties of B containing group III nitride heterostructures. How-

ever, a closer look reveals a lot of challenges. The domin-

ant problem is the controllable and reproducible synthesis of

Figure 13. Bandgap versus lattice constant of the ternary alloys

BGaN and AlBN. Notice the indirect band structure of w-BN. The

connecting lines are plotted as a best compromise of unfortunately

not yet fully consistent data from several sources. The direct band

gap of w-BN is taken as 10.2 eV [85], whereas bowing parameters

of 7 and 4 eV are assumed for the direct (full lines) and indirect

transitions (broken lines), respectively, for both B-containing

ternaries. For simplicity we assumed Vegard’s law for the lattice

constant. Any slight deviation (as sometimes discussed) would not

change the major message of this diagram.

such alloys in high crystalline quality—being a major reason

why their basic properties are still heavily debated. Calcu-

lations of their thermodynamic phase diagrams indicate that

the AlBGaN material system exhibits a substantial miscibility

gap at reasonable temperatures [90]. At temperatures around

1000 ◦C, only few percent of B are expected to be incor-

porated into Al(Ga)N, whereas full miscibility is only pre-

dicted for temperatures exceeding 9000 K [91]. On the other

hand, particularly for strain management, only small amounts

of B are needed: 6% of B lead to lattice matching between

AlBGaN quantum wells and AlN barriers for an emission

wavelength of 270 nm, whereas only 2% are required for a

strain-free AlxGa1−xN-AlyBzGa1−y−zN multi quantum well

structure with the same target wavelength and reasonable bar-

rier height (x~ 0.55, y~ 0.7).

Epitaxial growth results seem to confirm and even emphas-

ize these problems. Very high B contents, mostly in the range

of 10–15%, have been achieved by few groups by metalor-

ganic vapor phase epitaxial growth at comparably low tem-

perature of about 900–1000 ◦C (see e.g. [92,93]). Such lay-

ers and heterostructures exhibit fair x-ray diffraction results

and have been studied with respect to their refractive index

data in AlBN-AlN Bragg mirrors. Best reflectivities of only

about 80% at 311 nm have been measured [87], but no PL

is reported from such structures which may be taken as a

confirmation of their limited crystalline quality. When dir-

ectly compared to GaN, BGaN layers show reasonable lumin-

escence only for a B content below 1%, whereas larger B

concentrations dramatically reduce the emitted light intens-

ity (see e.g. [94]), although higher intensities are expected

in strain-optimized structures [95]. By careful optimization

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 14. Low-temperature PL (10 K) of a thin AlGaN layer and a

similar AlBGaN layer with about 1% B. Notice the comparable

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

of the growth procedure, strong PL intensities in AlBGaN

layers containing 1–2% of B similar as in respective B-free

AlGaN layers (figure 14) have been recently reported [96].

Transmission electron microscopy (TEM) studies confirm the

limited crystalline quality of essentially all AlBGaN layers

with boron concentrations larger than some tenths of a per-

cent. In consequence, many material properties of B contain-

ing nitrides are still questionable strongly limiting the consist-

ency of device simulation studies.

8.3. Advances in science and technology to meet challenges

The major challenge for benefitting from the above men-

tioned promising properties of B containing AlGaN struc-

tures is the suppression of defect formation typically accom-

panied with the incorporation of major B amounts, e.g. by

adopting nanoselective area growth [97]. To this end, a better

understanding of the characteristics of B on the growing semi-

conductor surface is mandatory. Only scarce, if any data are

available about its surface diffusion properties. Owing to the

strong B–N bond, very high growth temperatures are recom-

mended to enhance the B surface diffusion length. However,

this may lead to strong parasitic gas phase reactions between

the B precursor (typically tri-ethyl-boron TEB) and ammo-

nia (NH3). The mobility of B on the growing surface can

also be increased by modulating the precursor flows [93,98],

i.e. supply the group III and group V precursors sequen-

tially, which also minimizes the risk of gas phase pre-reactions

and hence may allow to use substantially higher growth tem-

peratures. This approach, currently studied by many groups,

needs further optimization. High resolution TEM may help

to get a better understanding about the interaction between

boron incorporation and defect generation. Moreover, some

groups report about the incorporation of parasitic impurities,

in particular oxygen and carbon, together with boron [99].

The reason for this behaviour is not yet well understood. The

fairly strong chemical binding of the ethyl groups to boron in

TEB may be responsible for the latter. Hence other B pre-

cursor molecules need to be investigated. Few studies have

been done with diborane (B2H6) and borazine (B3N3H6) as

well with tri-isopropyl-boron (TiPB), but here more detailed

studies are needed to evaluate their usefulness for the growth

of AlBGaN layers, particularly regarding the impurity prob-

lem. Such improved layer quality would also enable more

elaborate determination of their structural and optoelectronic

properties.

Obviously, the field is still at its infancy compared to the

much more developed AlGaN and GaInN fields. For instance,

successful p- or n-doping of BAlN and BGaN has not yet

been reported. Similarly, high performance optical or elec-

tronic devices comprising AlBN and BGaN have yet to be

demonstrated.

8.4. Concluding remarks

The addition of boron to the group III nitride alloy compon-

ents seems to open very promising solutions regarding strain,

polarization and refractive index management in Al(B)GaN

heterostructures. They may find applications in both optoelec-

tronic (deep UV LEDs, UV detectors, sensors, etc) as well as

in novel electronic devices (high electron mobility transist-

ors, Schottky diodes, etc). However, we are obviously faced

with very heavy problems regarding the epitaxial growth of B

containing layers as a consequence of the extraordinary chem-

ical properties of boron as a member of the second row of the

periodic system. Very thorough optimization of the epitaxial

growth procedure may lead to substantially better AlBGaN

layer qualities as compared to today’s situation, but it will

remain a major challenge to achieve device performance data,

e.g. UV emitters better than obtainable by relying on B-free

AlGaN structures.

Acknowledgements

We thank Oliver Rettig (Inst. of Functional Nanosystems,

Univ. of Ulm) and Suresh Sundaram (GT-Lorraine UMI

GT-CNRS, 2958 Metz, France) for their great help in pre-

paring this paper. Moreover, we gratefully acknowledge

the financial support of our AlBGaN research by the

Deutsche Forschungsgemeinschaft (Uni Ulm) and by the

French National Research Agency (ANR) to the UMI GT-

Lorraine as a part of the projects: GABORE (BLAN07-1-

203576), BATGAN (ANR-11-BS09-0038), VESUVE (ANR-

11-BS03-0012) and the GANEX Laboratory of Excellence

(Labex).

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

9. Development of UV-A LEDs

Peter J Parbrook1and Tao Wang2

1Tyndall National Institute and School of Engineering,

University College Cork

2University of Sheffield

9.1. Status

UVA covers the wavelength range 315–400 nm. Sources in

this region have application in UV curing, photolithography,

phototherapy (including melanoma treatment), security (bank-

note verification), dentistry, crime scene investigation and as

a pump source for general illumination [100]. As a result of

the wide range of applications, the UV market is dominated

by UVA, along with the growing UVB/UVC area mainly for

water purification [101]. The GaN bandgap (~362 nm) is in the

centre of this range, with longer/shorter wavelengths achieved

via InGaN, AlGaN (or InAlGaN) alloys. From a technolo-

gical perspective the bandgap of GaN splits UVA into two dis-

tinct parts. For longer (>365 nm) wavelengths, devices can use

adapted technology from that used in visible LEDs. Forshorter

wavelengths (<365 nm) there is a performance collapse, which

can be seen clearly in the device external quantum efficiency

(EQE) against wavelength plot shown in figure 2in section 1of

this Roadmap [11]. For most applications, the key parameter

is raw optical power produced at the target wavelength. This

correlates strongly to efficiency, due to the limits on the cur-

rent density and heating that a device can tolerate, and hence

the cost of ownership, running cost of the UV emission sys-

tem. Improvements will lead to improved take up in systems

using sources across this range.

Early reports of UVA-LEDs include: a 370 nm LED based

on an AlGaN/GaN double heterostructure (DH) in 1994 [102];

350 nm UV-LEDs, using GaN/AlGaN quantum wells (QWs)

for the active region (output power of 13 µW at 20 mA) in 1998

[103]; and the use of InGaN QWs for 380 nm was demon-

strated by Mukai and Nakamura [104]. A 1 mW 348 nm UV-

LED using AlInGaN quaternary as an active region at 50 mA

was reported in 2002 [105]. Using GaN as a substrate, the

crystal quality can be greatly improved, leading to 352 nm

UV-LEDs producing 0.55 mW at 20 mA (estimated internal

quantum efficiency (IQE) >80%) in 2001 [106]. These res-

ults are based on conventional GaN-on-sapphire technology,

or GaN substrates. More recently, there have been signific-

ant improvements in the EQE of near UV-LEDs (NUV-LEDs),

namely, the UVA-LEDs specifically in the spectral range from

365 to 400 nm, such as 30% EQE for 365 nm UVA-LEDs and

50% EQE for 385 nm UVA-LEDs. Properly packaged 365 nm

UVA-LEDs with an output power of up to 12 W have been

reported [100]. For 380 nm LEDs such technology can now

provide commercial devices with powers exceeding 1 W in a

suitably packaged envelope. However, this is not the case for

shorter wavelength devices.

Figure 15. Maximum IQE as a function of point defect density.

Reprinted with permission from [105], Copyright (2002), AIP

Publishing LLC.

9.2. Current and future challenges

The major challenge in all LEDs is to maximize wall plug

efficiency (light output over electrical input). This can be

defined in terms of the product of IQE, light extraction effi-

ciency (LEE)—giving EQE—and the electrical loss. As one

targets shorter wavelengths these elements all become more

challenging. LEE is the primary cause of the efficiency drop

for wavelengths <365 nm. In all LED design a primary goal

is to minimize all possible absorption losses in the device.

For LEDs grown on GaN operating >365 nm there is min-

imal optical absorption as GaN is transparent. However, for

wavelengths <365 nm optical absorption is harder to control

as one generally is required to use a GaN or AlN based tem-

plate layer to ensure high structural quality to ensure a high

IQE. If devices are grown on GaN many of the photons pro-

duced do not escape the diode, and LEE values of a few percent

are typical. The alternative is to apply a ‘GaN-free technology’

to prepare such LEDs on AlN templates. AlN, however, has a

~3% mismatch to GaN and UVA materials require high GaN

content alloys, leading to significant strain control issues. Fur-

thermore, convincing technologies to make a low resistance

ohmic p-contact directly to AlGaN (as opposed to GaN) means

that the vast majority of <360 nm LEDs still have absorption

(hence LEE) issues.

IQE is affected by a number of factors in this region.

For <360 nm point defects may become progressively more

important at shorter wavelength as shown in figure 15 [107].

Typically, the V/III ratio for AlGaN growth is much less than

(In)GaN to enhance atomic diffusion, potentially leading to

significant increase in vacancy point defect density. A fur-

ther issue is the control of the polarization fields across the

QWs. At longer wavelengths the need to include AlGaN to

prevent carrier overflow, and the strain consequences to an oth-

erwise GaN-based structure can lead to an IQE drop. Finally

for <365 nm LEDs n-AlGaN must be used, resulting in alloy

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 16. Improved electron–hole wavefunction overlap for InAlN-AlGaN versus AlGaN-AlGaN quantum well UVA LEDs (~340 nm).

scattering and consequently a large mobility drop [73], which

coupled with the challenges in making n-AlGaN thick (due to

strain and roughening) leads to additional voltage losses. This

is in addition to further challenges in optimizing the p- and

n-electrical contact design.

9.3. Advances in science and technology to meet challenges

For UVA LEDs >365 nm package design is likely to have the

biggest future impact on device performance, utilizing best

practice from those used in visible LEDs. This includes optim-

ization of light extraction and heat from the chip. Further, the

low In content in the QWs for these devices means that carri-

ers can potentially move to extended defects more easily than

in their visible counterparts, so the optimization of devices on

lower defect GaN templates will be beneficial. This needs to

be achieved either through obtaining best crystal quality cost

effectively on sapphire (or Si with a substrate removal step

and care to preventing cracking), or in finding routes to reuse

expensive high quality templates with device lift off.

For devices operating <360 nm routes to improve the LEE

are critical. Ideally this means a GaN-free growth process, or

route to removing GaN material in post growth fabrication. An

ideal would be use of thick, high quality AlGaN layers, which

is an area but this is not easily achievable as the strain leads

to cracking (GaN) or dislocation climb and layer roughening

(AlN). There is developing research using such approaches

(both for UVA and UVB LEDs) with improvements in emis-

sion quality and a more controlled roughening if a crystal-

lographic miscut is introduced. However, this does lead to a

step-bunched morphology with composition variations in the

AlGaN, and additionally quantum well thickness variations.

The impacts on this on device performance still require fur-

ther work [108]. Currently AlGaN on AlN is more prom-

ising, despite the issues of strain for the n-AlGaN buffer and

high Ga content AlGaN QWs (with polarization field control

issues) that this presents. Improved routes to allow smooth,

thick AlGaN buffer growth of high crystal quality, for example

using patterning to allow strain control, would revolutionize

the short wavelength UVA.

As for UVB and UVC LEDs, making direct contact to

AlGaN through doping and contact metal optimization is also

required. Such routes have demonstrated promise, but need

work to retain the benefits to LEE while minimizing elec-

trical loss [4]. The high spontaneous polarization in AlGaN

means that polarization fields are always an issue in UV LEDs.

Thus the potential of using semi- or non-polar materials, which

might allow thicker AlGaN as there is an easier route to strain

relaxation, is worth investigating. In theory InAlN-AlGaN

QWs can create polarization-free active regions on c-plane for

UVA, leading to improved electron–hole wavefuction overlap,

as shown in figure 16. However, while LED operation has been

demonstrated the challenges in material growth are high [109].

Small quantities of In in InAlGaN QWs may have benefit for

higher IQE, particularly in the 340–365 nm range.

9.4. Concluding remarks

In conclusion, UVA LEDs can be split into two distinct bands.

At longer wavelengths devices show high efficiency and can

generally use adapted forms of the mature technologies using

to make state-of-the-art blue LEDs. Devices with high powers

are commercially available and improvements in package and

defect reduction may lead to improved future performance.

For shorter wavelengths (315–365 nm) where GaN absorption

is a major problem compromising LEE in particular, develop-

ment of a true GaN-free technology for devices is critical if

UV LEDs to be fully exploited as sources in this range.

Acknowledgements

Peter Parbrook acknowledges Science Foundation Ireland sup-

port via 07/EN/E001A, 10/IN.1/I2993 and 12/RC/2276_P2.

Tao Wang is financially supported by the EPSRC Project No.

EP/M015181/1 and EP/M003132/1

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

10. UVB-LEDs

Tim Wernicke

Technische Universit¨

at Berlin, Institute of Solid State Physics

10.1. Status

For the UVB spectral range, 280–315 nm, the external

quantum efficiency (EQE) values achieved so far are lower

than those record efficiencies of LEDs emitting around 275 nm

and UVA LEDs (see section 1), i.e. there is a ‘UVB gap’ also

extending into the UVA region especially in the 305–330 nm

range with lower performance in comparison to shorter and

longer emission wavelengths. Nevertheless UVB LEDs emit-

ting at 305 nm are commercially available with an emission

power of 100 mW at a current of 350 mA (EQE 6%) with an

operation voltage of 7 V [110]. This EQE is similar to cur-

rently available LEDs emitting around 280 nm and 340 nm

[110]. This disparity in reported record values and available

products has technological and economic reasons. Research

efforts and number of publications for UVB-LEDs is much

smaller compared to the UVA and UVC spectral range, most

likely because there is currently no high volume application

(see section 1) and combing all possible approaches to increase

the light extraction efficiency (see section 13) have not been

attempted yet. These novel approaches have a high potential

for increasing the EQE of UVB LEDs as well but need to

be tested for their impact on degradation behaviour before a

general application. Nevertheless, UVB-LEDs have numer-

ous applications, e.g. phototherapy, plant growth lighting and

polymer curing (see section 1). Especially the first two require

a peak wavelength at about 310 nm as light below 300 nm is

harmful for these applications. From a technological point of

view, the UVB-LEDs profit from most technological advances

for UVC-LEDs but suffer from a lack of suitable substrates

as they cannot be grown on defect reduced AlN—the large

compressive strain leads to a formation of defects in UVB

emitting quantum wells (QWs) [111,112]. This can be mit-

igated by growing a relaxed Al0.5Ga0.5N buffer on AlN/sap-

phire template [113]. The relaxation process can be facilitated

by introducing an AlN/GaN superlattice as indicated in figure

17 [113] and is only possible with a sufficiently high threading

dislocation density (TDD) >3 ×109cm−2as the compressive

strain is released by tilt of edge type dislocations [113]. How-

ever, in this way a further reduction of the TDD in UVB LEDs

is not easily possible leading to a lower internal quantum effi-

ciency (IQE).

The electrical efficiency ηel of UVB-LEDs, which

describes the losses by Joule heating at contacts and in the

semiconductor layers, still gives room for improvement: Even

comparably low operation voltages of 6 V at 100 mA [114] or

7 V at 350 mA [110] are several volts higher than the phys-

ical limit being defined by the bandgap in the quantum wells,

i.e. close to 4 V (see also section 1). In the UVA these issues

are solve with 365 nm LEDs exhibiting a very high electrical

efficiency and low voltage of 3.49 V at 20 mA [115] and

at 700 mA [110]. For UVB-LEDs ohmic n-contacts—Ti/Al-

based as well as V/Al-based—with resistivities of 10−5Ωcm2

(see section 6) and AlGaN layers with low resistivity have

been realized. Therefore whole process needs to be optim-

ized for lower voltages. Additionally, the operation voltage is

increased if transparent p-doped AlGaN layers are employed

[116,117].

The lifetime of UVB-LEDs can reach several thousand

hours ([114] and see section 16) and even though their visible

counterparts are much more stable, these values are sufficient

for first applications.

In order to develop high efficiency and high power UVB-

LEDs with EQEs exceeding 20% it is necessary to solve a

number of challenges.

10.2. Current and future challenges

One of the main challenges to achieve UVB-LEDs with a

higher light output power is reducing the threading disloca-

tion density to allow for higher IQE values. Even though

threading dislocations can be reduced by annihilation in a

thick Al0.5Ga0.5N buffer layer the lowest threading disloca-

tion density that could be achieved is still >109cm−2. To fur-

ther increase the internal quantum efficiency of UVB emitting

quantum wells, the relaxation process needs to be decoupled

from threading dislocations and/or the annihilation of disloca-

tions has to be enhanced. One approach is the structuring of

the AlN surface before AlGaN overgrowth leading to relax-

ation and effective dislocation annihilation and TDD of 2–

5×108cm−3depending on the aluminium content in the layer

[118]. A similar effect is achieved by introducing a 3D growth

of Al0.55Ga0.45N and subsequent coalescence achieving TDD

of 7–9 ×108cm-3 [112]. Another way to mitigate the effect of

threading dislocations is growing UV-LEDs on highly miscut

sapphire substrates of 1◦with a step bunched surface leading

to UVB-LEDs emitting at 300 nm with a record high EQE of

6% [114].

There are only few publications discussing the carrier injec-

tion efficiency ηinj mainly by a variation of the electron block-

ing layer (EBL) or by introducing an electron blocking het-

erostructure [111,119] and the exact value of the injection

efficiency is not clear, i.e. how much improvement of the

external quantum efficiency can be achieved by further optim-

izing these layers. However, high efficiency LEDs require an

almost perfect injection efficiency and the design of the elec-

tron blocking layer seems to have a crucial effect on the life-

time of the LEDs (see section 16).

The LEE has the highest potential for an EQE increase

as current UVB LEDs do not incorporate techniques for

increasing the LEE (see section 13). Successfully reducing the

absorption in layers [111] and at interfaces as well as reducing

the total internal reflection are the main tasks.

For the degradation of UVB LEDs the root cause seems to

be deep levels at the pn-junction and within the quantum wells

introduced by point defects leading to a reduction in output

power (see section 16). The main challenge is to reduce these

point defects by optimizing the growth parameters which will

not only lead to longer lifetimes but also higher IQE and CIE.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 17. (a) Schematic of a typical UVB-LED heterostructure. For coherent growth of the MQW emitting at 305 nm a relaxed

Al0.5Ga0.5N layer is grown (from Susilo et al, pssa). (b) XRD reciprocal space map around the AlN 105 reflection of a full UVB LED

illustrating the strain relaxation process. The AlN/GaN superlattice is grown coherently strained on the AlN/sapphire template (nearly the

same Qxvalue). The Al0.5Ga0.5N starts to grow first strained but then relaxes during further overgrowth (Qxshifting to lower values) thus

forming a metamorphic quasi-substrate that allows to grow EBL and buffer layer with the new lattice constant. Reprinted from [113],

10.3. Advances in science and technology to meet

challenges

To further increase the IQE, a further reduction of the

TDD down to 2 ×108cm−2would be desirable [8].

Here approaches involving lateral overgrowth or maybe even

AlGaN quasisubstrates prepared by hydride vapour phase epi-

taxy (HVPE) or achieving high quality AlGaN by high tem-

perature annealing (see section 2) might be a necessary tech-

nological development. For the material of the QW novel

indium or boron containing alloys might allow for a boost

in IQE. Indium containing quaternary InAlGaN or even tern-

ary InAlN might allow for an increased IQE by introducing

localization and maybe also a reduced point defect density,

however recent UVB LEDs with record high quantum effi-

ciency employ pure AlGaN quantum wells [114,117] sug-

gesting that TDD and point defect density are most important.

Boron containing quaternary AlBGaN MQWs would allow

to reduce the compressive strain (see section 8) and grow

coherently strained on AlN making use of optimized AlN buf-

fer layers or bulk substrates with much lower threading dis-

location densities. However, the prerequisite for a success-

ful implementation of such materials will be mastering their

growth.

The LEE will by strongly increased by increasing the alu-

minium content in the p-doped AlGaN layers so the emitted

photons are not absorbed in this layer [116,117], see also

section 13. In comparison to UVC-LEDs, the necessary alu-

minium content to achieve transparency is much lower leading

to a smaller increase of the operation voltage. However, very

high LEEs will only be achieved in combination with reflect-

ive contacts leading to a significant improvement of LEE and

EQE. Here the challenge is to find a highly reflective mater-

ial that forms an ohmic contacts to p-doped AlGaN or realize

photonic crystals (see section 13). The alternative approach

to realize transparent LEDs is employing tunnel junctions as

demonstrated for 326 nm LEDs (see section 17). To reduce

internal reflections the substrate removal by laser lift off [120]

or electrochemical etching [114] and/or roughening [121] will

increase the light extraction efficiency. In parallel the encap-

sulation of UVB-LEDs will positively affect the LEE for

transparent as well as non-transparent LEDs (see sections 11

and 13).

10.4. Concluding remarks

UVB LEDs will also in future exhibit lower external quantum

efficiencies than UVA and UVC LEDs due to the lack in suit-

able substrates with a low threading dislocation density. In

future several approaches might allow for an effective reduc-

tion in threading dislocation density even though most of them

require considerable development effort. A strong increase in

LEE can be expected for the near future as increasing the alu-

minium content on the p-side and incorporating tunnel junc-

tions lead to a lower additional voltage in comparison to UVC-

LEDs due to the smaller bandgap of the material allowing for

a high efficiency operation as soon as these techniques can be

implemented.

Acknowledgements

The author thanks Sven Einfeldt and Akira Hirano for fruit-

ful discussions. The author gratefully acknowledges support

by the German Research Foundation (DFG) within the Col-

laborative Research Center ‘Semiconductor Nanophotonics’

(CRC 787) and the German Federal Ministry of Education and

Research (BMBF) within the project ‘Advanced UV for Life’.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

11. UVC LEDs

Akira Hirano1and Hiroshi Amano2,3

1UV Craftory Co., Ltd.

2IMaSS, Nagoya University

3Nagoya University

11.1. Status

UVC LEDs with wavelengths between 250 and 280 nm are

expected to be used as an alternative to low-pressure mercury

lamps. In this section, UVC refers to this limited wavelength

range. The main application of UVC light sources is steril-

ization, and the wavelength giving the peak disinfection effi-

ciency is about 265 nm [122]. Low-pressure mercury lamps

with an intense emission at the sterilization line (253.7 nm)

have a high WPE of about 25% and a lifetime of 6000 h.

Although AlGaN-based deep UV-LEDs (λ< 300 nm) in UVB

have a potentiality of WPE of about 10% and the lifetime

over 10,000 h [122], WPE of 265 nm LEDs is significantly

less than 10%. The LED lifetime decreases as the wavelength

decreases from 280 nm [122,123]. With this background, at a

wavelength of 275 nm, which is slightly shorter than 280 nm,

higher initial EQEs over 10% were reported to be achiev-

able by using a p-reflective contact and unknown encapsula-

tion [124] and by integrating a p-reflective contact, patterned

sapphire substrate, and lens [4], respectively. However, reli-

ability tests have not yet been reported for the case of using

a p-AlGaN contact, and the devices with a p-reflective elec-

trode also exhibited an increased forward voltage (Vf). In

the case of using a p-GaN contact layer, EQEs of 4.9% at

270 nm with encapsulation [125] and 5.5–6.0% at 265 nm

with distinctive roughening [126] were obtained using an AlN

bulk substrate. Also, the results of reliability tests have been

reported for 265 nm UVC LEDs with a p-GaN contact layer

on sapphire [113,122]. Using a p-GaN layer, encapsulations

using a fluoro-polymer [127] and a sapphire lens [123] were

demonstrated to enhance the LEE 2.3-fold and 1.5-fold in

UVC, respectively, which do not adversely affect the lifetime

for UVC LEDs without increasing Vf. Figures 18(a) and (b)

show the EQEs for bare dies of DUV-LEDs with a p-GaN

layer grown on sapphire and the reported EQEs incorporat-

ing the techniques of enhancing LEE, respectively. Figure

18(b) includes the EQEs achievable from the results shown in

figure 18(a).

11.2. Current and future challenges

Generally, the near-future tasks are considered to be best dis-

cussed under the premise of using a p-GaN layer because the

LED lifetime is expected to be longer than that of a low-

pressure mercury lamp. An improvement in IQE by reducing

the TDD and point defect density (PDD) and an improvement

in LEE are considered to be priorities.

To reduce the TDD to near 108cm−2, a typical approach is

to use bulk AlN substrates with a TDD of less than 106cm−2.

Also, the TDD can be reduced to 5 ×108cm−2by using a

1.0◦-miscut sapphire substrate [113]. Figure 19(a) indicates

the further improvement of TDD by using 1.5◦-miscut sap-

phire [122], showing the possible improvement in IQE by

reducing the TDD.

The possibility of increasing the IQE by reducing the PDD

has been reported [52,53]. A moderately Si-doped quantum

well (QW) exhibited improved IQEs for AlGaN QWs with a

TDD of greater than 109cm−2near 250 nm [53]. Also, con-

trol of the PDD was shown by increasing the V/III ratio when

growing 258 nm QWs [52] using an AlN bulk substrate. These

results indicate that the IQE of UVC LEDs can be improved

by reducing the PDD.

For the LEE, it is necessary to discuss the packaging tech-

niques for UVC LEDs (λ< 250 nm). The molecular struc-

ture of the fluoro-resin used for encapsulating UVC-LEDs

(265 nm) has been clarified to be polymerized perfluoro(4-

vinyloxy-1-butene) with end groups of –CF3[127]. How-

ever, the low refractive index of 1.35 limits the improvement

in LEE. The refractive index of fluoro-resin is difficult to

increase. Thus, consideration of the shape effect is important

for increasing LEE. It is desirable to fabricate hemispherical

lenses on surface-mounted devices (SMDs) by forming a lens

array on a large AlN sheet (e.g. 4 ×4 inch2) followed by isol-

ation to make small SMDs (e.g. 3.5 ×3.5 mm2). Another suc-

cessful approach to improve LEE is to use sapphire lenses with

a refractive index of 1.8 [123].

11.3. Advances in science and technology to meet

challenges

The utilization of EL emission from QWs penetrating p-

contact layers is currently difficult in practice owing to

the lack of both a low-resistivity p-contact and a conduct-

ive p-AlGaN cladding layer. Aluminium (Al) is the only

metal with reflectivity in UVC. Furthermore, when an Al-

based contact on AlGaN is annealed, the Al-based electrode

loses its reflectivity in UVC. Thus, the development of an

effective reflective structure above EB layers is considered

to be a long-term challenge, which will be described in

sections 13 and 17.

To date, mass-produced DUV-LEDs on sapphire have been

grown on AlN with TDD of 2–5 ×108cm−2[113,124] or

greater. To reduce the TDD to near 108cm−2, studies on the

behaviour of AlN at around and above 1400 ◦C will also be

beneficial as well as fabricating and evaluating LEDs. Also,

AlN templates with a low TDD is considered to be useful

for determining the possible improvement in IQE because the

required TDD for fabricating DUV-LEDs are considered to be

between 108cm−2and 109cm−2[113]. The behaviour of AlN

is expected to be similar to that of GaN when the growth tem-

perature is increased.

A detailed study on the decrease in EQE during reliability

tests will be meaningful for achieving devices with compar-

able lifetime to those of low-pressure mercury lamps. In figure

19(b), the increased deterioration of the output of UVC-LEDs

compared with that of UVB-LEDs is shown. The reliability

test for 265 nm LEDs suggests two main mechanisms for the

initial and subsequent decrease in the EL output. The decrease

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 18. (a) EQEs from various groups for dies with p-GaN contact layer on sapphire. Coloured data are the results obtained at UV

Craftory. (b) Initial EQEs of UV-LEDs with roughening (+R), with encapsulation (+E), with a sapphire lens (+L), with a p-reflective

contact (+P), with an n-reflective electrode (+N), and with the use of a PSS (+PSS). The subscripts ‘S’ and ‘F’ after ‘+E’ indicate

unknown/or silicone and fluoro-resin, respectively. ‘A’ and ‘S’ denote sapphire and AlN bulk substrates, respectively. The regrowth of an

AlN layer with UVC transmittance by HVPE on an AlN substrate followed by removing the AlN bulk substrate is indicated as (+H). The

data in (b) are limited to those published later than 2013. Also, the calculated achievable EQEs are shown by hollow symbols. Our

calculated data for the case of fluoro-resin encapsulation from the data in (a) are indicated by hollow violet, and the solid violet is based on

the experiment. In the graphs, data for 280–310 nm UVB-LEDs show the lower EQEs for UVC (250–280 nm). The EQE in UVC is

considered to be suppressed by the decreases in IQE and CIE. Reproduced with permission from [122].

Figure 19. (a) Full width at half maximum (FWHM) of XRCs for AlN template and AlGaN layer on AlN template and (b) result of the

reliability test for 265 nm LEDs. In (a), the improvement in XRCs using sapphire substrates with high miscut angles is shown. Recently

reported FWHMs of XRCs obtained using a sputtered AlN film on sapphire followed by annealing have also been added (hollow symbols),

showing the possible improvement in IQEs, which is considered to be useful for examining the possible improvement. Reproduced from

in EL output for the UVC-LEDs was much larger than that

for the UVB-LEDs, although fatal device faults resulting in

no light or a short-circuit did not occur.

To improve the current injection efficiency (CIE), (Al)BN

is considered to be a promising material. The possibility of

using (Al)BN should be investigated, which is described in

section 8.

The other approaches to improving EQE are the use of r-

(10-12) and m-(10-10) planes, which have the possibility of

improving CIE and IQE.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

11.4. Concluding remarks

The initial EQEs of UVC-LEDs (265 nm) have been reported

to be about 2/3 of those for UVB-LEDs (280–300 nm). Fur-

ther reduction of the PDD and TDD is expected to increase

the IQEs of UVC-LEDs. Also, the improvement in LEE

by encapsulating dies was demonstrated. The utilization of

light towards the p-side without increasing Vfand decreas-

ing the lifetime of UVC-LEDs is also useful. The UVC-LEDs’

lifetime is expected to be much longer than 6000 h. To date,

a decrease in EL output of about 50% for 265 nm LEDs dur-

ing the reliability test was observed; thus, the deterioration of

UVC-LEDs must be reduced.

The production of low-pressure mercury lamps is similar to

that of fluorescent room lamps. To satisfy the Minamata Con-

vention on Mercury by replacing low-pressure mercury lamps,

an EL output of 100 mW for 2–3 US dollars must be targeted,

keeping the cost-feasibility including the packaging in mind.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

12. UVC LEDs with emission below 250 nm

Frank Mehnke1, Leo J Schowalter2,3and Tim Wernicke4

1Georgia Institute of Technology, School of Electrical and

Computer Engineering

2Crystal IS Inc.

3Asahi Kasei Corporation

4Technische Universit¨

at Berlin, Institute of Solid State

Physics

12.1. Status

Ultraviolet (UV) light emitting diodes (LEDs) in the UVC

spectral region below 250 nm are of great interest as they, for

example, enable in-situ sensing applications of diverse gases

and liquids in industrial, medical, and automotive sectors. This

includes the monitoring of nitrates in water, NOxand SOx

in gas emissions, DNA purity analysis, and high-performance

liquid chromatography.

Currently, such short wavelength LEDs are, in design and

technology, similar to LEDs emitting around 270 nm grown

on c-oriented AlN templates or substrates (see sections 2and

3) but contain much higher aluminium mole fractions in the

AlGaN/AlGaN multiple quantum wells (MQW) and n-side

current spreading layers. The understanding of the n-type dop-

ing mechanisms of these high aluminium mole fraction layers

enabled short wavelength LED emission down to the phys-

ical limit of AlN at 210 nm [128]. However, due to several

material and heterostructure design challenges the external

quantum efficiency (EQE) and wall plug efficiency (WPE)

of such short emitting devices is exponentially dropping with

decreasing emission wavelengths. As shown in figure 20 (left)

one order of magnitude in EQE is lost for every 8 nm in

reduced emission wavelength, i.e. 10−2at 242 nm, 10−3at

234 nm, 10−4at 226 nm, 10−5at 218 nm, and 10−6at 210 nm

[63,128–135]. This is attributed to a strongly decreasing radi-

ative recombination efficiency (ηrad), carrier injection effi-

ciency (ηinj), and light extraction efficiency (ηext) with decreas-

ing emission wavelength and directly translates in reduced

emission powers. In addition, the WPE is further reduced at

short wavelengths since making contacts to high aluminium

mole fraction AlGaN is more difficult (see section 6). Another

issue with these short wavelength devices is that their lifetime

is often much shorter than their longer wavelength counter-

parts. A successful commercial product will need to address

all of these issues.

Up to now there are only a few research reports on LEDs

emitting below 250 nm [63,128–135] and only a limited-

release of LEDs at 235 nm is commercially available (see fig-

ure 20, right) [136]. In order to open the market for LEDs emit-

tingbelow250nm which are considered to become muchmore

cost-effective, small, robust, and persistent in comparison to

discharge lamps both the efficiency and the emission power

need to be improved. This will allow for applications such as

gas sensing and enable new applications.

12.2. Current and future challenges

The main limiting mechanisms prohibiting higher emission

powers and efficiencies of AlGaN-based UV LEDs with emis-

sion below 250 nm are related to the low ηrad,ηinj, and the

low ηext and to high operation voltages (leading to a lim-

itation of the driving current and the WPE)—all of which

need to be addressed in future research. Firstly, ηrad within

the MQW active region is strongly connected to the thread-

ing dislocation density (TDD) favouring LEDs on low TDD

monocrystalline AlN substrates. However, ηrad might also be

reduced by an increasing point defect incorporation (see sec-

tion 5) at these high aluminium mole fractions which is yet

to be proven by measurements. Another potential issue is

that ηinj into the MQWs decreases with decreasing emission

wavelength as electron blocking becomes inefficient due to

the smaller conduction band offset from the MQW to the AlN

electron blocking layer (EBL). Furthermore, not only suppres-

sion of electron leakage but also supporting hole injection into

the MQWs is a major challenge to the heterostructure design

and the doping profile of the UV LEDs [134]. Finally, ηext

is reduced with decreasing wavelength due to the transition

of the optical polarization of the emitted light from domin-

ant transverse electric (TE) to dominant transverse magnetic

(TM) [137,138]. This is caused by valence band switching

at an emission wavelength of around 240 nm and results in

an emission pattern where photons tend to propagate paral-

lel to the c-oriented surface. Consequently, ηext through the

substrate backside is reduced. Additional challenges arise by

using monocrystalline AlN substrates as these may exhibit

sub-bandgap absorption due to impurity incorporation dur-

ing growth (see section 3) depending on the particular growth

method employed [139]. Furthermore, the poor p-type doping

of high aluminium mole fraction AlGaN necessitates the usage

of absorbing p-layers and p-contacts, which strongly reduces

ηext.

In order to further improve the WPE of UV LEDs with

emission below 250 nm lower operation voltages are needed.

In particular, a lower AlGaN:Si layer resistivity and n-contact

resistivity need to be achieved for high aluminium mole frac-

tion layers. Both exhibit physical limitations due to the large

bandgap of the material. The AlGaN:Si layer resistivity is

increasing with increasing aluminium mole fraction due to

increasing donor ionization energy, DX-centre formation, and

incorporation of compensating point defects (see section 7).

The n-contact resistivity is increasing with increasing alu-

minium mole fraction due to a transition from Ohmic to

Schottky behaviour due to the low electron affinity, a very

stable oxide, a high probability of forming deep levels during

plasma etching, and the difficulty in forming shallow donors

during the contact formation process (see section 6).

Initial lifetime tests on sub-250 nm LEDs have suggested

severely reduced lifetimes in comparison to longer wavelength

UVC LEDs. In order to improve the lifetime of these device,

the underlying mechanisms will need to be understood. How-

ever, an analysis of the degradation causes as well as a

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 20. (Left) External quantum efficiency of state-of-the-art UVC LEDs indicating the exponential reduction for emission wavelengths

below 250 nm [63,128–135]. The wall plug efficiency of these short wavelength devices fall off even more sharply. (Right) Image of

Optan™ 235 which is in limited release from Crystal IS [136]. The diode is 3.5 ×3.5 mm2in size with a 0.8 ×0.8 mm2size chip and is

packaged in standard Optan™ SMD package. It is designed to be run at 20 mA at a forward voltage of 6 V with powers binned from 50 µW

to greater than 500 µW. These devices are rated to have a lifetime over 1000 h.

variation of growth and fabrication processes parameters has

only recently been initiated [140].

12.3. Advances in science and technology to meet

challenges

In the future, higher emission powers with better WPE and

lifetime are desired. Longer lifetimes can be expected with

future heuristic and analytical studies as well as an improved

point defect control. Additionally, lower TDD may also lead

to longer lifetimes. However, there is only a limited amount of

data currently available comparing devices grown on AlN tem-

plates with devices grown on monocrystalline AlN substrates.

A reduction of the forward voltage requires further develop-

ment of highly conductive n-AlGaN layers by an improved

control of the point defect density as well as low contact res-

istivity n- and p-electrodes especially by improved doping,

novel contact metal stacks and improved surface treatments.

Future analysis of the LEDs will give insight to which por-

tion the drop in EQE is related to ηrad,ηinj, and ηext, respect-

ively, leading to a focus in the respective LED development.

In order to improve ηrad, point defects resulting in nonradiative

recombination in the active region can still result from the epi-

taxial growth conditions and further improvement is needed

in this area. This measure will only be effective for growth on

low TDD AlN, e.g. by growing LEDs pseudomorphically on

monocrystalline AlN substrates with a TDD in the 104cm−2

range [129]. The improvement of ηinj by a reduction of elec-

tron leakage needs to be obtained by clever heterostructure

design, as the height of the EBL cannot be increased bey-

ond the AlN bandgap and no larger bandgap semiconductor

material is currently known. This may be addressed by mul-

tiple quantum barrier EBLs [131] which can provide a lar-

ger conduction band offset in comparison to AlN and allow

for hole tunnelling due to quantum interference. Advanced

device simulations can provide the required design inform-

ation, however, these calculations rely on several material

parameters which are not well known and need to be fur-

ther investigated. Furthermore, supporting the hole generation

and injection needs to be mastered by heterostructure design

and doping profile, e.g. the development of p-type polariz-

ation doping using aluminium mole fraction graded layers.

It is also possible that new alloys such as AlBN (see sec-

tion 8) may offer an advantageous band alignment allowing

for improved ηinj. In any case, clever device designs will be

needed to maximize both ηrad and ηinj simultaneously since

improvement in one can also be made at the expense of the

other. ηext may be improved by shifting the optical polariz-

ation towards TE by tailoring the design of the MQW act-

ive region. The use of thin QWs and high aluminium mole

fraction barriers has been demonstrated to increase TE emis-

sion at a given wavelength [137,138]. Alternatively, growth

of semi- and nonpolar MQWs might boost ηext as the rota-

tion of the crystal axis also leads to a rotation of the emis-

sion profiles [128], although currently the growth techno-

logy of such devices is much more immature compared to c-

oriented ones. Nevertheless, the light extraction through the

substrate can be improved for TE but especially for TM polar-

ized light by one or multiple of the following approaches: (a)

substrate removal, (b) substrate or bottom layer roughening,

epitaxial lateral overgrowth, patterned substrates, or photonic

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

crystals (see sections 13 and 14), as well as (d) encapsu-

lation. A big boost in ηext could be gained by the use of

transparent p-doped short period superlattices combined with

reflective p-electrodes especially if the penalty of higher for-

ward voltage could be avoided. Also, new approaches such as

p-Si nanomembranes [133] acting as reflective contact yielded

excellent results and might push ηext and ηinj to higher levels.

Another promising approach to overcome the transparency-

conductivity dilemma is the implementation of p-(i)-n tunnel

junctions (see section 17). These offer the opportunity of using

highly conductive and transparent AlGaN:Si layers and n-

electrodes, which overcomes several but not all challenges of

p-AlGaN.

12.4. Concluding remarks

Improving the WPE, EQE and emission power of UV LEDs

with emission below 250 nm cannot be performed by small

single changes in the heterostructure design but requires man-

ifold optimization and tradeoffs. These LEDs will benefit from

technological improvements for longer wavelength LEDs,

however, they will need additional development effort to solve

the specific challenges, additionally hampered by the smaller

expected market size. The exponential decrease of the EQE

with decreasing emission wavelength is fundamental and will

persist in spite of strong improvements for each wavelength.

Additionally, applications based on these LEDs can only be

realized when the lifetimes of the devices can be guaranteed.

Major breakthroughs in device performance of UV LEDs

emitting below 250 nm may be expected to occur by new

design concepts. For example, the development of tunnel junc-

tions or p-Si nanomembranes can resolve the transparency-

conductivity dilemma, pushing the device performance of UV

LEDs with emission below 250 nm to higher levels.

Acknowledgements

Funding by the German Federal Ministry of Education and

Research (BMBF) within the ‘Advanced UV for Life’ pro-

ject and by the Deutsche Forschungsgemeinschaft (DFG)

within the Collaborative Research Center ‘Semiconductor

Nanophotonics’ (SFB 787) is acknowledged. Crystal IS is

a wholly owned subsidiary of the Asahi Kasei Corporation

which provided funding for the ‘235 nm LED’ project.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

13. Light extraction efficiency of UVC LEDs

Hideki Hirayama1and Yukio Kashima1,2

1RIKEN

2Marubun Company

13.1. Status

High output power 265–280 nm commercially available UVC

LEDs have flip-chip (FC) geometry, and UVC light is extrac-

ted through the sapphire or AlN single crystal substrate. The

performance of a UVC LED with wavelength at 280 nm

with 50 mW output power at a driving forward voltage (Vf)

of 5.5–7 V and a driving forward current (If) of 350 mA

exhibits an electrical power-to-light conversion efficiency

(wall-plug efficiency; WPE) of about 2–3%. The detailed

breakdown of the total WPE [2] is estimated to be approx-

imately 60% for internal quantum efficiency (IQE) includ-

ing injection efficiency (ηinj), 80% for electrical efficiency

(ηel), and about 6–8% for light extraction efficiency (LEE)

[2,141].

For a deep-UV (DUV) LED, the fact that the LEE is par-

ticularly small compared to that of a blue LED is a very big

problem. The main reasons for low LEE in a DUV LED are

that DUV light is heavily absorbed by the p-GaN contact layer,

and total internal reflection occurs at the interface between

the LED body and air. Therefore, improving LEE is the most

important subject and greatly contributes to the improvement

of output power and WPE of DUV LEDs.

The LEE of a current DUV LED was calculated by the com-

bination of the ray-tracing method and the finite-difference

time-domain (FDTD) method [141]. Figure 21 shows the cal-

culation models of flip-chip (FC) DUV-LEDs with Al reflector

mounted on ceramic package for (a) a usual LED and (b)

an LED with lens. The assumed layer structure for a cur-

rent LED on the sapphire substrate consists of a 4-µm-thick

AlN, a 2 µm thick n-AlGaN buffer layer with emitting and

electron blocking layers, a 1 µm thick p-GaN contact layer

and Ni/Au p-type electrode with a reflectivity of 30%. The

polarization of the emitted light determining the radiation

profile and LEE is influenced by the aluminium contents of

well and barriers and the strain state of the material and

dominated by transverse electric polarization for emission

wavelengths >240 nm [142]. The reflectivity of the Al-coated

reflector of the package is 90%. Using these conditions, the

calculation result of LEE for a 280 nm UVC LED is 6.3%

[141], which is in good agreement with the above estimated

value.

13.2. Current and future challenges

In order to achieve a significant increase of LEE, it is

effective to introduce a p-AlGaN transparent contact layer

which is completely transparent to a wavelength of 270 nm

instead of a p-GaN contact layer [4,141,143,144]. It

is also important to introduce a highly-reflective p-type

electrode [4,143], e.g. p-type silicon [133], Pd(1 nm)/Al

[145], Ni/Mg or rhodium [146] electrodes with reflectivity

of >70%, instead of a Ni/Au electrode with reflectivity of

30% [141]. By introducing them, LEE improves more than

twice and a calculated value of 13.1% was obtained [141].

However, since the hole concentration in the p-AlGaN con-

tact layer is 1 ×1014 cm−3or less and forward voltage

(Vf) rises by several volts [4], WPE cannot be greatly

improved.

In order to improve LEE while maintaining WPE, thin p-

GaN layers can be introduced. However, the LEE enhance-

ment drops quickly already for thin GaN layers of 10–

20 nm [115]. Therefore we propose forming a highly reflect-

ive photonic crystal (HR-PhC) in the p-GaN contact layer

[141]. Figure 22 shows (a) a cross section of the schematic

structure and (b) corresponding electric-field (E-field) map-

pings calculated by FDTD analysis and (c) the calculated res-

ults of LEE as a function of the distance from the quantum

well (QW) emitting layer to a hole array PhC for 280 nm

AlGaN UVC LEDs with HR-PhC [141]. In addition, as a

comparison, we also conducted the LEE enhancement sim-

ulation with a p-GaN-free structure, i.e. using a model of p-

AlGaN transparent contact layer with highly-reflective elec-

trode [141]. In this case the radiation from the QW emitting

layer does not penetrate into the PhC layer and reflected by

the PhC, as confirmed from the cross-sectional E-field map-

pings shown in figure 22(b). In figure 22(c), we can confirm

that with and without p-GaN the LEE is strongly enhanced

especially when the vertical field resonant condition is satis-

fied. We found that the maximum increases in LEE enhanced

by introducing the HR-PhC are by 2.8 and 1.8 times, respect-

ively, for the p-GaN and p-AlGaN contact layer UVC LEDs

[141]. The maximum LEE of the p-GaN and p-AlGaN contact

layer 280 nm UVC LED with PhC were 18.0% and 23.8%,

respectively.

13.3. Advances in science and technology to meet

challenges

As described above, when DUV light is emitted from the LED

body into air, the DUV light is totally internally reflected at

interfaces, and absorbed by a contact layer. Therefore, in order

to increase the LEE of a DUV LED, it is quite important to use

a low-absorption structure, e.g. by using a HR-PhC p-layer.

When the absorption is low, the LEE is further enhanced by

redirecting light into the light extraction cone within the semi-

conductor by p-side PhC [141], nanopatterned sapphire [4],

structured AlN/sapphire [142,147]. The total internal reflec-

tion at the substrate/air interface can be alleviated by directly

bonding a hemispherical lens [123], encapsulation [4,7] or

hybrid nanostructuring [148]. In the model shown in figure

22(b), a quartz hemispherical lens is assumed to be directly

bonded to the back surface of the sapphire substrate. The

calculation results of LEEs of the 280 nm UVC LEDs with

quartz hemispherical lenses are 13.1% and 28.3%, respect-

ively, for the p-GaN and p-AlGaN contact layer cases which is

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 21. Calculation models of flip-chip (FC) DUV LEDs with Al reflector mounted on ceramic package for (a) a usual LED and (b) a

LED with lens used for the analysis by the combination of ray-tracing and finite-difference time-domain (FDTD) methods.

Figure 22. (a) Cross section of schematic structure and (b) corresponding electric-field (E-field) mappings calculated by FDTD analysis

and (c) the calculated results of LEE as a function of the distance from the quantum well (QW) emitting layer to the PhC for 280 nm AlGaN

UVC LEDs with HR-PhC.

a strong enhancement compared to 6.3% and 13.1%, respect-

ively. Indeed, a record high EQE of 20% was demonstrated

for a 275 nm LED with transparent p-AlGaN layer, reflect-

ive Rh contact and hemispherical encapsulation [4]. The main

advancements which will lead to a widespread implementation

of these light extraction techniques are reduction of the addi-

tional Voltage by improving the conductivity of p-AlGaN (see

section 7), demonstrating the reliability of such LEDs (meth-

ods described in section 16) and implementing low cost fab-

rication techniques.

13.4. Concluding remarks

To date, the efficiency and the output-power of an AlGaN

DUV-LEDs are significantly limited by a low LEE, which is

reduced by a strong absorption of a p-GaN contact layer. The

LEE would be dramatically increased in near future by intro-

ducing an absorption-free contact layer, a highly-reflective p-

type electrode, a reflective PhC fabricated on p-contact layer,

by bonding lenses, by fabricating a vertical LED structure,

and/or by combining these effects.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

14. Nanostructuring for UV emitters

Philip Shields1and Robert Martin2

1University of Bath

2University of Strathclyde

14.1. Status

Reports of using nanostructures in UV-emitting devices have

increased sharply over the past 5 years as they can potentially

solve the existing obstacles for obtaining efficient nitride-

based UV-LEDs [148–155]. They follow a longer and more

substantial effort in the use of nanostructures to aid visible

LEDs. We describe below some examples of how existing uses

of nanostructures have overcome the following key issues: the

influence of defects on the internal quantum efficiency (IQE);

the poor light extraction efficiency (LEE) in planar UV-devices

due to the emission of predominantly TM-polarized light from

AlN-rich AlGaN and the high absorption in conventionally-

used layers; and the difficulty in achieving efficient p-doping

in AlGaN materials.

Firstly, annealing AlN nanostuctures at high temperatures

has been shown to improve the IQE in subsequent c-plane

QWs by dramatically reducing the number of edge dislo-

cations [150]. Laterally growing from these or other nano-

patterned templates, such as sapphire or silicon, alongside the

use of maskless techniques, permits the use of traditional epi-

taxial lateral overgrowth defect reduction techniques whilst

coping with the poor mobility of Al species in the growth

environment that traditionally leads to delayed coalescence

and poor quality surfaces [151,152].

Secondly, the presence of scattering interfaces in nano-

ELOG structures also improves the LEE, which is especially

important for AlN-rich AlGaN materials for which the valence

band ordering enhances TM-polarization [153]. The nanoscale

features can also promote strain relaxation, allowing strain

engineering of the valence band to influence the dominant

emission polarization as well as a potential improvement of

the crystal quality.

Thirdly, the ability to manipulate strain and polarization in

nanostructures has been employed to significantly increase the

incorporation of the Mg p-dopant and enhance hole mobilit-

ies compared with Al(Ga)N planar films. For example, fig-

ure 23 shows nitrogen polar AlN nanowires grown directly

on Si, which demonstrated increases in IQE and reductions in

turn on voltage [154]. Highly conductive p-AlGaN can avoid

the use of absorbing p-GaN, as in conventional UV LEDs,

and increase both the electrical efficiency and reduce optical

losses.

Finally, the nanostructuring of additional materials such as

Al can lead to enhanced UV emission through the coupling

of surface plasmons with the emitting dipoles [155]. Photonic

crystal effects have also been incorporated into the nanopat-

terning of AlN surfaces to increase LEE in deep-UV LEDs,

using nanoimprint lithography [148].

14.2. Current and future challenges

These examples show some different ways that nanostruc-

turing can improve UV-emitting devices. However, simul-

taneously improving the IQE, LEE and electrical efficiency

without introducing further detrimental effects remains a chal-

lenge. Following work in the visible regime, an ultimate

goal might be the use of a regular and organized array of

dislocation-free nanostructures as a scaffold for high-aspect-

ratio core–shell nanorods arranged to ensure maximum light

scattering from both TE and TM emission. Enhanced IQE due

to the high crystal quality would be combined with strain-

relaxed material for improved dopant incorporation.

Achieving this requires solving a number of challenges,

with the bottom-up growth of AlGaN materials difficult due

to the low mobility of Al species. Core–shell structures have

traditionally required MOVPE growth, whereas most reports

of AlGaN nanostructure growth have relied on MBE, using

UV-absorbing GaN pedestals as nucleation seeds. Controlling

the AlGaN alloy composition and dopant incorporation across

the various crystal facets is an additional challenge. Further-

more, most lighting applications require a large number of

parallel-driven nanorods and need a 3D contacting architec-

ture. Any filling materials need to be non-absorbing, which

becomes more challenging for emission further into the UV,

as does achieving sufficient heat extraction and conductivity.

Whilst the maturity of UV LEDs lags behind visible LEDs,

they are sufficiently well-developed to be commercially avail-

able. 3D nanoscale LEDs will need to demonstrate unequivoc-

ally that their advantages can justify their more complex cre-

ation to have a chance to compete.

Alternatively, nanostructuring could be directed simply at

improving IQE or LEE in planar devices. However, recovering

a planar surface from nanostructured material requires care-

fully controlled coalescence to ensure a net reduction of exten-

ded defects and complete surface recovery to step-flow growth

since AlGaN QWs are sensitive to any underlying roughness.

A further challenge will be to simultaneously optimize both

IQE and LEE as the ideal pattern for one is likely to com-

promise the other. One way of disentangling the effects is to

improve LEE by employing nanostructuring after growing the

active layers. Again, the benefits must outweigh the increased

complexity of fabrication.

14.3. Advances in science and technology to meet

challenges

To fully realize the potential benefits of nanostructuring for

Al(Ga)N UV-emitters, various advances are needed. These

include further steps towards achieving defect-free, bottom-up

growth of AlGaN nanowires with controlled size, spacing and

height, without relying on GaN seeding. As well as improv-

ing MBE approaches in this respect, a key advance will be

expansion out to MOVPE, which will also allow easier scale-

up. Further work should build on the use of polarization and

strain relaxation to enhance doping. Hybrid approaches may

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 23. Electrical and electroluminescence results from an AlN nanowire LED. Reprinted by permission from Springer Nature

Customer Service Centre GmbH: Science Reports [154] (2015).

Figure 24. Cross-section SEM images of AlN nanorod arrays (a) after etching and (b) after subsequent MOVPE regrowth. Reprinted with

permission from [156]. Copyright (2018) American Chemical Society.

be significant, for example building on the idea of the lat-

eral re-growth of AlGaN on etched AlN nanorods reported

by Coulon et al ([156], figure 24). In this case, advances

are required to increase the parameter space to grow high-

quality non-polar material and improve precursor efficiency.

Device designs and process flows optimized for performance

and manufacture are required taking into account the specific

vulnerabilities of electrical leakage and thermal management.

where nanopatterned layers are to be used to create planar tem-

plates with reduced dislocation densities or for device struc-

tures, it is important to be able to balance dislocation bend-

ing with dislocation creation during coalescence of the neigh-

bouring growth fronts. A deep understanding of the disloca-

tion reduction process is required to achieve the best material

quality for optimal LEE, additionally without having to resort

to long growth runs.

Exploiting gratings or photonic crystal effects could help

master the specific challenges in UV emitters but require

the development of reliable techniques to manufacture the

small nanostructure periodicities and the low edge rough-

nesses needed for the ultrashort wavelengths. Use of electron

beam lithography is unlikely to be scalable, so large-area nan-

opatterning techniques such as nanoimprint and displacement

Talbot lithography will likely become significant [157,158].

Use of flip-chip processing to minimize optical absorption

and improve thermal efficiency can be beneficially combined

with use of nanopatterned material or arrays of nanowires

grown on large area Si for commercial scale-up. Other prom-

ising directions are extending nano-patterning to substrates

beyond sapphire or growing Al(Ga)N on layered transition

metal dichalcogenides, such as MoS2, on sapphire with a view

to overcoming challenges associated with lattice and thermal

expansion mismatches [149].

Effort should be applied to further exploit plasmonic effects

as they have been shown to be effective in enhancing light

emission. Advances in modelling and fabrication are required

to optimize the positions of metal nanoparticles to enhance

TE-emission and LEE, even to positions within the active

regions themselves. Whilst Al is typically used in the UV,

advantage could be obtained by using other non-oxidizing

materials such as Rh.

There will also be challenges to overcome for advanced

characterization methods, such as mapping of dopant incor-

poration in core–shell nanorods, perhaps using optical signa-

tures to aid their rapid development. Developments with tech-

niques such as cathodoluminesence and atom probe tomo-

graphy for nanopatterned UV materials will accelerate pro-

gress.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

14.4. Concluding remarks

There are a number of reports that have signalled the prom-

ise of nanostructuring to potentially solve longstanding issues

in planar UV emitters. Whilst the field is in an early stage,

it has accelerated at a phenomenal rate and it will not be a

surprise if many of the challenges are surmounted in the next

few years. Particular game changers would be: the success-

ful bottom-up growth of regular arrays of non-absorbing and

dislocation-free AlGaN nanostructures, a detailed understand-

ing of the effect of polarity and strain on enhanced Mg incor-

poration for different AlGaN compositions, the convergence

on an optimized 3D device architecture, and a routine capab-

ility to pattern materials with sub-100 nm resolution. Follow-

ing that, the question will be whether nanostructured materi-

als can be made in the multi-wafer growth reactors for com-

mercial manufacture with sufficient yield to challenge conven-

tional planar devices. This may be the toughest challenge of

them all.

Acknowledgements

The authors wish to acknowledge funding from the Engin-

eering and Physical Sciences Research Council (EPSRC),

UK, Grant No. EP/M015181/1, ‘Manufacturing of nano-

engineered III-nitride semiconductors’.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

15. Simulation of UV-light emitting diodes and

lasers

Bernd Witzigmann1, Friedhard Römer1and Yuh-Renn Wu2

1University of Kassel

2National Taiwan University

15.1. Status

Optoelectronic device simulation is a multi-scale, multi-

physics task. Optical, electronic, and thermal properties, and

their interaction govern the characteristics. Nano-scale regions

require ideally atomistic methods, but electrical contacts,

thermal reservoirs or optical resonators are often on the mac-

roscopic scale. For lasers and LEDs in the III-nitride sys-

tem, numerical simulations are routinely done nowadays as

a guideline to design the device specifications, which starts

from the epitaxial layers for optimum waveguiding and car-

rier injection (see figure 25), and carrier-photon interaction,

up to the chip processing for placement of contacts, or light

outcouplers (such as facet coatings or anti-reflection struc-

tures). A comprehensive, but probably still incomplete over-

view of the large variety of numerical codes can be found

in [159], and the main approaches are as follows. The most

widely used models are based on the drift-diffusion currents

in combination with continuity equations for electrons and

holes, and the electrostatic potential being evaluated by a

Poisson equation. This coupled nonlinear system can discret-

ized in up to three dimensions, and solved efficiently with

a Newton type iterative method. Besides the current-voltage

characteristics, one can study local current densities, recom-

bination rates or carrier mobilities. For quantized regions, a

single particle Schrödinger equation can be coupled to this sys-

tem, also with decent convergence behaviour. Recent works

on solving Schrödinger equations with the localization land-

scape theory to obtain the effective quantum potential provide

an efficient tool to combine the semi-classical theory with

quantum effects [160]. For non-equilibrium carrier transport,

the Monte-Carlo method has been applied to LEDs, solv-

ing the semi-classical Boltzmann equation. While includ-

ing non-equilibrium transport, it requires massive computa-

tional resources, and lacks a consistent inclusion of quantum

effects. A full non-equilibrium quantum mechanical simula-

tion has been realized with the non-equilibrium Green’s func-

tion method, however again with large computational cost, and

no model for non-radiative recombination [161].

The optical part requires numerical methods for solving

Maxwells equations, such as finite element, finite difference

methods or ray tracing approaches. Here, the challenge is

to include the interaction between the electromagnetics and

the electronics, which occurs via stimulated and spontaneous

emission, or absorption.

For application to device design and analysis, active

research on the models, equation frameworks and material

parameters goes along with the advancement of material syn-

thesis and technology capabilities. This is currently the case

Figure 25. Schematic of conduction and valence band in a

UV-LED. Carrier injection is influenced by tunnelling (T), capture

dopant activation in AlGaN alloys. The active region consists of

three quantum wells, alloy fluctuations will modulate the band

edges, but are not shown for clarity here.

for III-nitride based light lasers and LEDs emitting at ultravi-

olet wavelengths. For their InGaN based counterparts, simula-

tions, in combination with numerous experiments, have con-

tributed significantly to pushing the electro-optical efficien-

cies and wavelength coverage in the visible close to optimum

values. As example, numerical simulations of the quantum

confined Stark effect have clarified the impact of polarization

charges at interfaces. Also, the characteristics of devices on

semi- and non-polar crystal orientations have been studied in

great detail by microscopic simulations. With this knowledge,

the potential performance gains compared to polar devices

could be studied. The efficiency droop has been studied using

various models, which have helped identifying the role of leak-

age currents and Auger recombination [162].

15.2. Current and future challenges

Experimental data for III-nitride based UV LEDs currently

exhibit low quantum efficiencies, high operation voltages and

low optical extraction efficiencies. UV light emitters in the III-

nitride system differ from their counterparts emitting in the

visible range mostly by the extensive use of AlGaN and AlN

layers. This introduces alloy fluctuations in the entire device,

and a different set of impurities and defects. The peculiarities

of AlGaN with high aluminium mole fractions have not been

studied in detail, and the existing models calculating the laser

or LED performance have not been tested to large extent. The

major challenge for the simulation models will be to quantitat-

ively explain the efficiency characteristics, and come up with

proposals for high performance laser and LED structures. For

strain, continuum models might not be able to represent the

local strain due to alloy fluctuations properly. Impurity model-

ling in III-nitrides is another challenging field, as these mater-

ials show high luminescence in the presence of defects. Most

of the AlGaN related studies have been done for high electron

mobility transistors, and while models for bulk and interface

impurities are available in drift-diffusion based simulations, it

is not clear how valid they are for UV light emitters. For silicon

devices, the simulation of the process technology has resulted

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 26. (a) Calculated fluctuated potential Ec of UVC-LEDs by considering the random alloy fluctuation in EBL, QBs, QWs, and even

the injection layer. (b), (c) Hole density and radiative recombination, which is clearly affected by random alloy fluctuation. (d) Calculated

strain ϵxx in the QW. Due to the fluctuated composition and local strain relaxation, the compressive strain of QW has been relaxed at the

interface, which may not be good for TE emission [164].

in a more accurate representation of the structural properties

of the devices as input to the device simulation [163]. LED and

laser simulation in the III-nitride system would benefit greatly

from a process simulation, but is far from being established.

15.3. Advances in science and technology to meet

challenges

AlGaN crystallizes in the anisotropic wurtzite lattice and is

a direct semiconductor for all compositions. Calibrated sets

of parameters for modelling the electronic band structure and

the influence of strain with the kp-Schrödinger method exist

[164]. The luminescence polarization changes from perpen-

dicular to parallel to the c-lattice direction with increasing

aluminium content. In thin film structures grown along the

c-lattice direction TM-polarized radiation dominates above

5.2 eV. AlGaN exhibits spontaneous and piezoelectric polar-

ization in the c-direction leading to sheet charges at hetero

interfaces. The growth conditions of AlGaN material lead to

alloy fluctuations. Ternary AlGaN layers build not only the

active region, but also the carrier injection regions. The influ-

ence of local fluctuation on UVCLEDs has been analysed in

[164] with 3D Poisson, drift-diffusion and localized landscape

model [160] to account the effect of quantum potential. figure

26 shows the cross section view of band profile, hole density,

radiative recombination, and strain distribution. The prelim-

inary results show that IQE is strongly affected by disorder

as the blue LEDs. The poor hole injection due to low activ-

ated dopant and weaker blocking ability of the EBL caused by

the fluctuating potentials may lead to strong droop effect. Fur-

thermore, as shown in figures 26(b) and (d), Carriers are less

confined in the QW and extend into the QBs due to the alloy

potential fluctuations. The hole wave function extension into

the QBs will enhances TM emission as shown from a k.p sim-

ulation of wave-functions admixture, which should then lead

to poor light extraction. These preliminary results show that

future simulation model should takes this effect into account

for the whole structure, which leads an even higher computa-

tion burden if a full atomistic model is applied.

Carrier injection in principle suffers from the low hole

mobility in p-doped AlGaN alloys, and the high ionization

energy in excess of 0.5 eV for Mg acceptor atoms. Super-

lattices made of alternating thin layers with different band

gap have been devised for enhancing the hole density and

injection. Carriers in the superlattice are subject to quantiz-

ation and minibands evolve due to the periodicity. The min-

ibands and the polarization induced potential reduce the ion-

ization energy and thus increase the hole density. The model-

ling and design of superlattices requires a quantum mechanical

approach. Miniband dispersion relations have been calculated

with the Kronig–Penney model [166]. The non-equilibrium

Green’s function method enables superlattice transport mod-

elling.

15.4. Concluding remarks

While the basic equations for carrier transport and electromag-

netics have been established, accurate material parameters and

the impact of impurities still need improvement for UV light

emitters. Parameters can be determined either by experiment

or by ab initio simulations. A close collaboration between

the communities in materials characterization and materials

theory is beneficial in order to verify parameters and estab-

lish a general understanding of the device physics. Ab initio

simulations for determining material and model parameters

have given many valuable data for the device simulation com-

munity, such as Auger coefficients for GaN [167], or dopant

activation in AlGaN. In the future, advanced methods beyond

pure bulk computations will help understanding interfaces and

quantum wells. In the field of materials characterization, tech-

niques such as atomic probe tomography can resolve the struc-

tural and recombination properties of the device on an atomic

level [168]. This will lead to much improved understanding of

the optoelectronic properties.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

16. Reliability of UV LEDs

Carlo De Santi1, Matteo Meneghini1, Johannes Glaab2, Jan

Ruschel2and Sven Einfeldt2

1University of Padova

2Ferdinand-Braun-Institut,Leibniz-Institut für Höchstfrequenztechnik

16.1. Status

In order to enable a wide market acceptance of UV LEDs,

these devices need to be stable over typically several thou-

sand hours of operation. For the time being, the lifetime of

UV LEDs cannot yet keep up with their visible counterparts.

Nevertheless UVB and near UVC LEDs with L70 lifetimes

(time until 70% of the initial optical power is reached) in the

order of 10 000 h have been demonstrated on commercially

available devices. On the other hand the lifetime of state-of-

the-art deep UVC LEDs with emission below 250 nm is still

short [135]. Usually, during constant current operation, several

degradation effects can be observed in parallel, including (i) a

reduction in optical power, which is rapid within the first hun-

dred hours and slower for longer operation times (see figure

27(a)); (ii) a variation of the operating voltage (see figure 27(b)

for voltages >4 V); (iii) an increase in the leakage current (see

figure 27(b) for voltages <4 V); (iv) changes in the spectral

purity [169]; (v) catastrophic failure (devices suddenly stop

working) [170]. Furthermore, it was found that typically the

degradation is accelerated by the operation current [171,172]

(figure 27(a)) and temperature [171].

So far, the main focus of most of the degradation studies has

been on semiconductor issues of the UV LED chip. Here, the

discussed possible physical causes of the degradation include:

(i) The generation of defects driven by current and temperat-

ure; such defects form deep levels and behave as centres

for non-radiative Shockley–Read–Hall recombination. For

example, the degradation was correlated to an increased

density of midgap states in 308 nm UVB LEDs [173].

(ii) The migration of defects through the heterostructure [174].

Some authors reported on the migration of hydrogen [175]

(see figures 28(b) and (c)) or aluminium [176]. Dislo-

cations or V-pits and the electric field were proposed to

enhance the migration.

(iii) The pile-up of charges at heterointerfaces due to the gen-

eration and/or diffusion of defects that can affect the injec-

tion or escape efficiency of charge carriers [177].

(iv) Changes in the charge distribution within the doped

regions due to activation/compensation processes

[135,178].

16.2. Current and future challenges

To narrow down critical issues in the fabrication chain and

to localize the degradation effects in the device are import-

ant aspects of current and future research on the degrada-

tion of UV LEDs. Therefore, empirical studies of the impact

of heterostructure design (e.g. design of the electron block-

ing layer), epitaxial growth (e.g. variation of substrates), chip

Figure 27. (a) Optical power (normalized to the value at 0 h) as a

function of operation time of UVB LEDs operated at 50 mA and

100 mA, respectively, and 25 ◦C. The L70 values indicate the time

at which the optical power of the LEDs reached 70% of its initial

value. (b) Current-voltage characteristics of UVB LEDs measured

after different operation times over 1000 h of operation at 100 mA

and 20 ◦C.

design (e.g. different insulator materials) and of chip fabrica-

tion and mounting on degradation are ongoing with the goal

of enhancing device lifetimes.

Another essential aspect is to analyse the degradation

mechanisms and stress-induced changes in the devices in order

to understand the physics behind. Here the identification and

localization of defects causing deep levels in the semicon-

ductor are the key issue (see section 5). Important techniques

to study the density and the energy of these levels are the capa-

citance and optical deep level transient spectroscopy (C-DLTS

and O-DLTS, see figure 28(a)), deep level optical spectro-

scopy, photocurrent spectroscopy and analysis of the low fre-

quency noise. The structure of emerging defects, their changes

and motion can be studied by techniques as transmission elec-

tron microscopy, secondary ion mass spectrometry (SIMS),

cathodoluminescence, or electron beam induced current.

Considering the future use of UV LEDs, new aspects might

be important. For example for applications which require

the LED wavelength to stay in a certain spectral window,

operation-induced parasitic emission bands lowering the spec-

tral purity might be unacceptable, even if other electro-optical

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 28. (a) Variation in C-DLTS signal in a 308 nm LED stressed at 100 mA. (b) Optical power and drive voltage as a function of

operation time of one InAlGaN-based UVB LED operated at 100 mA and 20 ◦C. (c) Corresponding depth profiles of the LEDs shown in (b)

of the H-concentration (blue curves) at 0 h and 100 h of operation as determined by SIMS. The Al concentration profile (red curve,

normalized to the max. value) is for the calibration of the depth.

parameters stay constant. Also, it is worth studying approaches

to make the devices less sensitive to temperature or to switch-

ing. Certain applications may require studying the influence

of the environmental gas or of ionizing radiation. For the use

of UV LEDs in a harsh environment, packaging issues will

be critical. Therefore, the impact of the UV radiation and the

large amount of generated heat on the stability of encapsula-

tion materials and optical reflectors used in the package have

to be considered. Recently, a photon-induced generation of

point defects that takes place even without any applied bias

was reported for GaN-based devices [179] and could turn out

to be relevant also in AlGaN-based UV LEDs.

Future UV LEDs may make use of new device concepts

such as tunnel junctions (see section 17) or polarization p-type

doping which could result in unforeseen reliability issues.

16.3. Advances in science and technology to meet

challenges

To improve device reliability, advances in clarifying the nature

of the relevant defects and in understanding how they form and

change during device operation are needed. Despite first stud-

ies showing that the defects form deep level acceptors [173],

only little is known about the defect structure, i.e. whether it

is an isolated vacancy or impurity, a complex of point defects

and whether dislocations are involved. The charge state and

the density of the defects are unknown as well. Therefore, spa-

tially resolved measurement techniques, particularly lumines-

cence studies, are needed at different stages of the degradation

process. As the generation or activation of defects requires the

supply of energy [175], hot carriers could be involved [172].

The generation of hot carriers in blue LEDs has already been

verified and attributed to Auger recombination by Iveland et al

[180]. A similar study on UV LEDs is missing.

Moreover, additional work is needed to reduce the initial

concentration of point defects and to suppress their migration,

which could for instance be achieved by specific epitaxy tech-

niques or conditions and by optimizing post-growth processes

such as the activation of p-type conductivity. The introduction

of additional layers into the heterostructure acting as migration

barriers could be of interest as well.

Degradation could possibly be reduced through specific

device concepts. For example the temperature and current

density could be lowered and made more uniform by increas-

ing the size of the active area. Additionally, an enhanced hole

injection efficiency could reduce the excess of electrons with

respect to holes in the active region, which may trigger the

formation of defects if Auger recombination is involved.

Finally, it is mandatory to develop and improve the math-

ematical models that describe the performance of UV LEDs

[135,172,181]. For example, De Santi et al proposed a simple

model for the temperature dependence of the emission spec-

trum of UVB LEDs [181]. Such models can be used for the

analysis of the variation in performance at increasing stress

time, and can eventually be extended to predict the LED life-

times under different operation conditions.

16.4. Concluding remarks

Before LEDs become competitive in the market of UV light

sources, the origin of their degradation should be understood

to improve their lifetime. The reduction in optical power in

current UV LEDs is most likely dominated by a generation

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

of deep-level acceptor states in the active region of the semi-

conductor heterostructure, which form non-radiative recom-

bination centres. This process may be connected with a migra-

tion of point defects within the junction. Further studies are

needed to understand the involved physical mechanisms. Both

an empirical optimization of the device reliability and an ana-

lysis of the involved physical processes are required.

Acknowledgements

The authors would like to thank all colleagues at the University

of Padova, at the Ferdinand-Braun-Institut and at the Technis-

che Universit¨

at Berlin for their contributions in the study. This

work was partially supported by the German Federal Ministry

of Education and Research (BMBF) through the consortia pro-

ject ‘Advanced UV for Life’ and the Deutsche Forschungsge-

meinschaft through the Collaborative Research Center ‘Semi-

conductor NanoPhotonics’. At the University of Padova, this

work was supported in part by the INTERNET OF THINGS:

SVILUPPI METODOLOGICI, TECNOLOGICI E APPLIC-

ATIVI project, co-founded (2018–2022) by the Italian Min-

istry of Education, Universities and Research (MIUR) under

the aegis of the ‘Fondo per il finanziamento dei dipartimenti

universitari di eccellenza’ initiative (Law 232/2016).

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

17. Tunnel junction-based UV LEDs

Siddharth Rajan and Yuewei Zhang

The Ohio State University, Columbus, OH

17.1. Status

The major factors limiting UV LED efficiency are the poor

light extraction efficiency and low carrier injection efficiency.

Both of them are closely related to the low p-type AlGaN con-

ductivity and high p-type contact resistance. Heavily doped p-

GaN layers typically grown on top of p-AlGaN to enable hole

injection lead to significant absorption losses, and poor light

extraction efficiency. Alternate strategies involving p-type

metal contacts to p-AlGaN [4] lead to significant improve-

ment in the light extraction efficiency, but cause high electrical

losses due to substantial increase in the operation voltage.

Therefore, simultaneous improvement in the electrical effi-

ciency and light extraction efficiency is intrinsically limited

for the conventional device structures, and has remained a

major challenge for the UV LED community. Tunnel-injected

UV LED structures can enable non-equilibrium hole injec-

tion through transparent AlGaN layers, and enable better light

extraction, thus simultaneously enabling low absorption and

electrical loss.

Realizing efficient tunnel junctions (TJs) becomes more

challenging as the bandgap of the semiconductor is increased.

Recent work [182–184] has shown that efficient interband tun-

nelling hole injection is feasible using tunnel junction struc-

tures where the spontaneous and piezoelectric polarization

charges are used to create extreme electric fields [185]. These

polarization sheet charges can create band-bending across

nanometre-scale lengths, thereby reducing the tunnelling bar-

rier and increasing tunnelling probability (figure 29(a)). This,

and other innovations in semiconductor heterostructure design

[186] have enabled high tunnelling conductivity to be achieved

for bandgaps as high as 5.4 eV, with demonstrations of ultra

violet LEDs for wavelengths as low as 257 nm (figure 29(b))

in planar films [187], and 242 nm in nanowires [188]. The

availability of such tunnel junctions is especially import-

ant for shorter wavelength (high Al-content films) since the

thermally activated hole concentration decreases dramatically

with increasing Al content in the p-AlGaN layer.

Interband tunnel junctions have the potential to enable ultra

violet LEDs with power density and efficiency greatly exceed-

ing state-of-art devices today. They could enable more effi-

cient light extraction strategies for AlGaN-based UV LEDs by

taking advantage of the transparent top and bottom n-AlGaN

contacts, and the use of UV-reflective n-type contacts. Fur-

thermore, the low sheet resistance of n-AlGaN could enable

new light extraction strategies, similar to those used for vis-

ible LEDs. Finally, electrically injected lasers have been a

long-standing challenge [189]. While lasing at wavelength as

short as 271.8 nm under pulsed current was recently demon-

strated [190], the devices showed high threshold current and

poor emission power due to the deficit in hole injection.

Figure 29. (a) Schematic energy band diagram of a tunnel-injected

UV LED structure under forward bias. (b) Summary of the emission

spectra achieved in tunnel-injected UV LEDs. Reprinted with

There are several potential applications for tunnel junctions in

such devices, since they could enable efficient hole injection

as well as a much thinner p-AlGaN layer, thereby lowering

p-type conduction resistance, and reducing free carrier losses.

However, each of these areas require a concerted effort to

improve understanding of growth physics, transport, doping,

and optical characteristics.

17.2. Current and future challenges

Even though tunnelling hole injection has been demonstrated

to be feasible, several challenges remain to be addressed

before we can realize the potential of tunnel-injected UV

LEDs. While metalorganic vapour phase epitaxy (MOVPE)

is the most technologically mature growth technique for UV

LEDs, it brings some unique challenges for the growth and

fabrication of tunnel-injected UV LED structures. The exist-

ence of hydrogen atoms in the MOVPE chamber leads to pas-

sivation of the Mg acceptors. The n-type top contact layer in

the tunnel-injected UV LED structure impedes hydrogen dif-

fusion, making it difficult to achieve Mg activation. To circum-

vent this problem, lateral Mg activation from etched sidewalls

was developed for MOVPE-grown tunnel-injected blue LEDs

[191]. However, these tunnel junction based visible LEDs have

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 30. Summary of AlGaN tunnel junction (TJ) resistance and

the excess voltage drop measured for tunnel-injected UV LEDs.

Publishing LLC.

shown higher turn-on voltage and differential resistance when

compared to standard LEDs with metal ohmic contacts.

Another challenge comes with the growth of the tunnel

junction layer using MOVPE. The large difference in the

growth temperature between AlGaN and the thin InGaN layer

adopted in the tunnel junction structure could lead to decom-

position and intermixing of the InGaN layer, resulting in poor

tunnel junction performance. In comparison, molecular beam

epitaxy (MBE) growth does not require p-AlGaN activation

after growth, and provides much lower growth temperature

difference between AlGaN and InGaN layers. However, recent

work [192] demonstrates promising results from all-MOVPE

based LEDs that could suggest a future pathway for UV emit-

ters based on tunnel injection.

Low tunnel junction resistance values below

2×10−3Ωcm2have been achieved for ultra-wide bandgap

AlGaN with Al content up to 70%. However, a substantial

increase in the excess voltage drop is observed as the AlGaN

bandgap increases in the tunnel junction structure as shown

in figure 30. Even though polarization engineering has been

utilized to shrink the tunnel barrier through the insertion of an

ultra-thin InGaN layer between p+and n+-AlGaN layers, the

large conduction band and valence band offsets at the hetero-

interfaces lead to extended depletion barriers for interband

tunnelling [186]. Increasing the Al content in the AlGaN lay-

ers leads to further increase in depletion barriers and reduced

tunnelling probability. Therefore, a detailed understanding of

the mechanisms in the tunnel junction layer is necessary to

reduce the excess voltage drop. Furthermore, although tun-

nelling injection potentially reduces internal light absorption

loss, there remains a severe limitation on the light extraction

efficiency due to the lack of good encapsulants and reflective

metals in the UV wavelength range.

17.3. Advances in science and technology to meet

challenges

To fully exploit the advantages of tunnel-injected UV LEDs,

fundamental scientific and engineering studies related to

growth science, carrier transport, and optics are needed. One

promising solution could be a hybrid growth method employ-

ing both MOVPE and MBE growth techniques. The epitaxial

nucleation, thick n-AlGaN bottom contact layer and the act-

ive region can be grown using MOVPE, while growth of the

tunnel junction layer and the n-AlGaN top contact layer can

be done using MBE to exploit the high doping concentrations

and the sharp heterointerfaces. This method would combine

the high internal quantum efficiency of MOVPE-grown act-

ive regions with high electrical injection efficiency of the tun-

nel junctions grown by MBE. Alternately, further investiga-

tion of MOVPE-based tunnel junction design and growth, and

study of p-AlGaN activation could lead to solutions that take

advantage of the relative maturity and performance of MOVPE

growth.

In either case, it is critical to develop a better under-

standing of interband and intra-band tunnelling phenomena

so that future designs can minimize voltage and resistance

loss in tunnel-injected UV LEDs. For example, the intro-

duction of high density polarization charges to reduce the

depletion barrier was shown to reduce resistance due to con-

duction and valence band mismatch in p-AlGaN/InGaN/n-

AlGaN tunnel junction structures [186]. Background com-

pensating charges, such as unintentional impurities or native

defects, were found to be responsible for the extended

depletion barriers in the tunnel junction layer. Therefore,

growth optimization to reduce the compensating charge dens-

ity could be critical for highly efficient interband tunnel

junctions. Further improvement in the tunnelling probab-

ility could be achieved by exploring the doping limit in

AlGaN layers and by shrinking the interband tunnelling bar-

rier.

The tunnel-injected UV LED structure minimizes the

internal light absorption loss, making it possible to substan-

tially improve the light extraction efficiency. Due to the high

photon energy emitted from the UV LEDs, it is challenging

to find transparent encapsulation materials with long oper-

ation lifetime. The tunnel-injected UV LED structure does

provide flexibility to explore novel designs for light extrac-

tion and device packaging that are free from the widely

used packaging materials. For example, tunnel junctions could

allow for introduction of high reflectivity metal contact layers

and specifically designed mesa structures to enable enhanced

light extraction efficiency for both TE and TM polarized

light.

17.4. Concluding remarks

Tunnel-injected UV LEDs employing interband tunnel junc-

tions for non-equilibrium hole injection hold great promise

for high performance UV emitters. AlGaN tunnel junctions

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

have been demonstrated for emission in AlGaN-based LEDs

in wavelengths down to the UVC spectrum. While tunnel-

injected UV LEDs emitting across the UV spectrum have been

demonstrated, several opportunities still exist for improving

the performance of the tunnel junctions, understanding the

fundamental semiconductor physics in these structures, and

for enhancing light extraction efficiency. A better understand-

ing of these aspects could enable disruptive advances in solid

state ultraviolet sources, and have significant impact on down-

stream applications.

Acknowledgements

We acknowledge funding from the National Science Found-

ation (Nos. ECCS-1408 416 and PFI AIR-TT 1 640 700) and

the OSU TCO Accelerator Award.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

18. E-beam pumped emitters

Thomas Wunderer

Palo Alto Research Center, Inc., Palo Alto, CA

18.1. Status

As we have seen in the previous sections, the challenges

of realizing useful p-type doping in the ultra-wide bandgap

AlGaN materials limit the performance of UV emitters. Using

high-energy electron beam excitation as an alternative pump

strategy for the generation of electron-hole pairs can cir-

cumvent many of the issues encountered in conventional

p–n-junction configurations. When using e-beam excitation,

electron-hole pairs are created through a sequence of scat-

tering events within the semiconductor material that obviate

the need for p-type doping. Thus, resistive electrical losses

from contact and sheet resistances become negligible. Also,

efficient and homogeneous carrier injection can be achieved

even with a wide active zone. There is no need for an electron

blocking layer or other means to address the asymmetry in the

electrical properties between electrons and holes that influence

the effectiveness of carrier injection. Furthermore, high light

extraction efficiency can be realized, with low optical absorp-

tion materials within the entire device heterostructure, because

no low bandgap materials are needed to improve the electrical

properties. All of these aspects make electron beam excitation

an attractive platform technology, in particular for emitters in

the mid-UV where p-type doping otherwise limits device per-

formance.

An e-beam pumped AlGaN chip producing spontaneous

emission in the mid-UV was reported by Oto et al in 2010

[193]. Although their claimed optical output power effi-

ciency may have been obscured by x-ray radiation, the proof-

of-concept was clearly demonstrated. Other teams followed

showing pure optical output powers as high as 230 mW

at λ=246 nm [194,195] and demonstrated the feasibil-

ity of implementing a compact form factor [196]. Record

high optical intra-chip powers of about 10 W at 246 nm

were shown with liquid-nitrogen cooled chips [197]. E-beam

pumped spontaneous emitter sources compare favourably with

state-of-the art efficiencies from conventional UV-LEDs (see

figure 31), and can deliver high optical output powers at the

same time.

Using e-beam excitation for lasers is also particularly

intriguing for the same reasons discussed above. Des-

pite the fact that laser emission in the deep-UV has still

to be demonstrated, longer-wavelength III-N lasers reveal

the great benefits of this compelling platform technology

(figure 32) [198–201].

18.2. Current and future challenges

E-beam pumped light emitters based on semiconductor mater-

ials share some common aspects with conventional LED

manufacturing, including epitaxial growth and device fab-

rication processes. Various device design features follow

identical optimization schemes as known for LED develop-

ment. These involve improving the well-known efficiency

contributions through optimization of material quality, het-

erostructure design and device architecture. The key dif-

ference to consider between e-beam pumped light emit-

ters and p-/n-junction devices is how electrons and holes

are created and injected into the active zone for photon

generation.

E-beam pumping involves the bombardment of the target

sample with high energy electrons (i.e. 5–30 keV) through

which electron-hole pairs are generated in an extended volume

that is dependent on the pump spot diameter and the chosen

electron energy as well as the target materials. This process

has an intrinsic energy efficiency limit of about 1/3. The car-

riers subsequently diffuse into the QWs where they recom-

bine with the generation of photons. Proper electrical ground-

ing removes net excess charge. Excess heating and possible

hazards from soft x-ray generation require careful design and

mounting schemes.

The operation of the electron beam requires a vacuum

enclosure, dictating materials and manufacturing choices that

are compatible with the mechanical, optical, electrical, and

thermal needs.

Challenges for laser operation are not only related to the

semiconductor gain chip, but are also convoluted with the

actual e-beam properties. Both vertical and edge type laser

configurations are feasible. Whereas it is straight-forward to

realize a circular e-beam pump spot, as is convenient for ver-

tical emitters, the short gain length makes essential the need for

highly reflective mirrors. However, as known for electrically

driven devices, epitaxial III-nitride DBRs are challenging to

fabricate. Dielectric DBRs on the other hand might fulfil the

optical requirements, but their robustness against extreme e-

beam bombardment might be compromised. Also, in the case

of vertical lasers, substrate removal might be required to min-

imize absorption losses in the optical cavity. Edge emitting

laser configurations are easier to fabricate. However, realizing

a narrow and long continuous e-beam excitation spot is non-

trivial, and its precise alignment with respect to the resonator

is important.

18.3. Advances in science and technology to meet

challenges

In recent years, much progress has been made to develop high

quality AlN substrates and templates that provide the base for

the growth of the device heterostructures. The structural mater-

ial quality of today’s UV emitters has approached a satisfying

level with radiative efficiencies exceeding 90% for the best

materials. These advancements have also allowed the success-

ful demonstration of low threshold optically pumped lasers all

the way down to emission wavelength of 237 nm. Challenges

related to efficient carrier injection even at high carrier densit-

ies as seen in conventional p/n-junction devices are addressed

by means of e-beam pumping. Light extraction is one of the

factors that still limits UV emitter performance today, espe-

cially for wavelength shorter than 240 nm. This is where

light polarization switches from TE to TM and photons are

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 31. Comparison of e-beam pumped spontaneous emission device efficiencies with state-of-the art conventional LED performance.

Collection of LED performance from M Kneissl at http://www.ifkp.tu-berlin.de/?id=agkneissl.

Figure 32. Laser spectrum from electron beam pumped GaN-based edge type laser. Inset: Far-field pattern of the laser emission made

visible by focusing onto a fluorescing screen. The spot with lower intensity originates from reflections of the light on the wafer.

trapped within the chip. Structuring the chip surface and rely-

ing on scattering effects have showed promise for enhanced

light extraction. However, the effective useable active area is

compromised. An alternative approach to overcome the chal-

lenges of photon extraction is to fabricate devices on non- or

semipolar AlN crystal orientations. For example, first MQW

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

heterostructures grown on (2–201) AlN revealed very prom-

ising results [202] and further advances could enable efficient

sub-240 nm emitters.

With respect to e-beam pumped edge type lasers further

development of a compact line-shaped e-beam pump spot with

high power density could significantly improve laser device

performance. A longer excited resonator length can provide

more gain per photon roundtrip and consequently reduce

the laser threshold. Also implementing mirror concepts and

laser cavities (for vertical emitting lasers) that do not degrade

with high-energy electron bombardment would be of interest.

For example, an e-beam pumped surface emitting laser with

external cavity (VECSEL) could provide high optical output

power with excellent beam properties and would circumvent

the issues discussed above.

Further advances on thermal management and imple-

mentation of ultra-compact components, including the

electron gun, vacuum housing, beam shaping, and drive

electronics are greatly desired. It is believed that these

technical challenges can be mastered and that the final

product could be as compact as a conventional light

bulb.

18.4. Concluding remarks

Electron-beam pumping of semiconductor materials can be

considered an enabling alternative excitation strategy for UV

optical emitters. High pump powers of up to 100 Watts and

high pump-power densities (e.g. ~1 MW cm−2) are access-

ible to achieve laser operation in both edge type and vertical

emitting laser configurations. Especially for the ultra-wide

bandgap AlGaN materials e-beam excitation is particularly

appealing, as no p-type doping is required in the device het-

erostructure. As a result, device efficiencies of e-beam pumped

spontaneous UV emitters have been shown to meet or exceed

state-of-the-art performance levels from conventional LEDs,

while delivering high optical output powers. Further advances

in miniaturization of various device components will realize a

platform technology for compact and efficient emitter systems

that can access a wide range of emission wavelengths.

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

19. UV laser diodes

Tim Wernicke1and Michael Kneissl1,2

1Technische Universit¨

at Berlin, Institute of Solid State

Physics

2Ferdinand-Braun-Institut, Leibniz Institut für

Höchstfrequenztechnik

19.1. Status

Although AlGaN-based ultraviolet light emitting diodes (UV-

LEDs) with emission wavelength as short as 211 nm have

been realized with emitters covering the entire composition

range (see section 12) the situation is quite different for

current-injection UV laser diodes (LDs). Due to the greater

complexity and increased requirements commercially avail-

able UV laser diodes are limited to emission wavelengths

above 370 nm [203] and the shortest emission wavelength

for a current-injection LD was for a long time 326 nm [203].

However recently edge emitting laser diodes emitting in the

UVC at 271.8 nm [190], and UVB spectral region at 298 nm

[204] were demonstrated. As shown in figure 33, lasers emit-

ting below 370 nm exhibit an increase in threshold current

density with shorter emission wavelength [205]. Laser emit-

ting around 400 nm exhibit threshold current densities of 1–

2 kA cm−2, while the thresholds of lasers emitting below

340 nm exceed 15 kA cm−2and can be only operated under

pulsed current conditions [203]. The AlGaN quantum well

(QW) based current-injection lasers in the UVB and UVC

wavelength region exhibit a laser threshold of 41.2 kA cm−2

[204], 25 kA cm−2[190], and 19.6 kA cm−2[206].

Optically pumped lasing from AlGaN heterostructures

with emission wavelengths from 310 nm down to 237 nm

[21,121,207,208] has been demonstrated and over the past

decade the threshold power densities have been significantly

reduced and are now reaching <10 kW cm−2for AlGaN mul-

tiple quantum well (MQW) structures grown on low defect

density bulk AlN substrates [21]. Investigations of the optical

gain in such AlGaN MQW heterostructures show a high mater-

ial gain [207], moderate optical waveguide losses even in

the presence of Mg-doped layers [209], and fairly large con-

finement factors [207]. Advanced laser concepts such as ver-

tical cavity surface emitting lasers (VCSELs) [209], distrib-

uted feedback (DFB) [211], and tapered lasers are currently

being explored for emitters in the wavelength range from

400 nm to 369 nm. For emitters in the UVB and UVC spectral

range research activities are predominantly focused on edge-

emitting lasers due to significant challenges in materials devel-

opment (e.g. Mg-doping of AlGaN alloys) and device fabric-

ation technologies (e.g. ohmic contacts).

19.2. Current and future challenges

The challenges for UV lasers are tremendous but depend

strongly on the emission wavelength and composition. In the

spectral range above 370 nm, low threshold density GaN-

based laser diodes operating in continuous-wave mode with

long lifetimes are already commercially available. Lasers

emitting at shorter wavelength require AlGaN or InAlGaN

active regions and confinement heterostructures leading to

increasing tensile strain with increasing aluminium mole frac-

tion when grown on low defect density bulk GaN substrates

[203]. In order to avoid the formation of cracks in the laser het-

erostructure the development of special strain relief techniques

is required. Furthermore, the performance characteristic of

these lasers also degrades much faster due to the increased

operating voltages and currents leading to Joule heating as

well as the higher defect densities in these materials. For UVB

lasers one of the biggest challenges is the lack of high qual-

ity AlGaN substrates with a lattice constant corresponding to

the medium AlGaN composition range. For example, growth

on bulk AlN substrates is not ideal since the strong compress-

ive strain in the AlGaN quantum well heterostructures leads to

relaxation and defect formation during growth [213] and con-

sequently a degradation of the optical properties. This might

be the main reason for higher lasing threshold of the demon-

strated UVB laser diode in comparison for UVC laser diodes

grown on bulk AlN [190,204]. On the upside, highly con-

ductive n-AlGaN layers and low resistance Ohmic n-contacts

have been demonstrated for emitters in this aluminium alloy

composition range as also described in sections 6and 7. Fur-

thermore, due to the only moderately higher Mg acceptor ion-

ization energies, low resistivity Mg-doped p-AlGaN layers

are also easier to realize [207]. UVC lasers with high struc-

tural quality can be realized on low defect density bulk AlN

[21,190,207,208]. Since these high aluminium mole frac-

tion AlGaN heterostructures can be grown pseudomorphic-

ally strained on bulk AlN [213], they exhibit excellent optical

properties as described in the previous paragraph. However,

efficient carrier injection and confinement as well as low res-

istivity p-AlGaN cladding layers with an aluminium content

exceeding 70% pose very difficult challenges for the realiza-

tion of AlGaN QW lasers emitting in the UVC. As described

in section 7, the ionization energy for Mg acceptors in AlGaN

alloys steadily increases with the aluminium content leading

to very low room temperature conductivities. Here polariz-

ation doped p-AlGaN layers [214] without Mg employed in

all UVB and UVC laser diodes so far [190,204] allow mod-

erate voltages of 13.8 V at the threshold current density of

25 kA cm−2[190].

Finally, designing current injection structures that allow

high current density operation [190,204] is a crucial challenge

to achieve lasing and find a suitable way for optimizing the

structures.

19.3. Advances in science and technology to meet

challenges

For laser diodes in the UVA spectral range emitting below

370 nm recent research developments including reducing

point defects in AlGaN alloys (section 5), improved doping

schemes (section 7) as well as low resistance Ohmic con-

tacts to AlGaN layers (section 6) should lead to a reduction

in operation voltages, cw operation, and improved laser diode

lifetimes. As is often the case, laser diode technologies will

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

Figure 33. (a) Previously published threshold current densities of current injection laser diodes [190] with data added from [190,204,206].

(b) Spectra of optically pumped lasers in the range of 237 nm to 292 nm. Reproduced from [207]. CC BY 4.0.

also profit from the progress made in the development of UV-

LEDs (section 9). However, the root causes for the strong

increase in threshold current density at shorter wavelengths

is not yet well-understood. Since the increased strain in the

AlGaN layers is certainly playing a role different approaches

for the growth of low defect density AlGaN heterostructures

are critical. This could be either achieved by strain relaxa-

tion which, however, needs to be attained without introducing

additional threading dislocations or by realizing low defect

density AlGaN templates allowing for the coherent growth

of a laser diode heterostructure with AlGaN cladding lay-

ers exceeding an Al mole fraction of 30%. Another option

is the incorporation of InAlN cladding layers for which n-

and p-doping was successfully demonstrated [212] and which

can be grown lattice-matched onto low defect density bulk

GaN substrates. For lasers in the UVB spectral range the cre-

ation of high quality heterostructures by using metamorphic

AlGaN buffer layers that promote strain relaxation (see sec-

tion 10 and [121,204]) are effective to reduce the TDD. In

the longer term Al0.5Ga0.5N quasi substrates grown, e.g. by

hydride vapour phase epitaxy (HVPE), could provide near

lattice-matchedtemplates with lowthreadingdislocation dens-

ities, smooth surfaces and vertical current-injection through a

conducting substrate.

Now, shortly after the first realization of UVC laser diodes

it is not clear which factors contribute to the lasing threshold.

However, carrier injection and optical confinement under the

condition that the diode can sustain high current densities to

reach the threshold are probably the most crucial contribu-

tions. Part of this is optimizing the polarization doping for effi-

cient hole injection transport [214].

Novel concepts, especially tunnel junctions will also be

investigated as these would allow the incorporation of low

resistance Si-doped AlGaN cladding layers on both sides of

the active region with only a thin Mg-doped tunnel hetero-

junction for hole injection. Here the key challenge is to min-

imize the excess voltage drop at the AlGaN tunnel-junction,

e.g. by using low-bandgap interlayers (see section 17). The

best growth technologies for deep UV tunnel-junction laser

diodes either by MOVPE or hybrid MOVPE/MBE as well

as the optimum design for a low optical loss of the lasing

mode at the tunnel-heterojunction are currently being invest-

igated. Another approach for carrier injection that is currently

investigated is the pumping by a high energy electron beam

with acceleration voltages of 5–10 kV (see section 18). Using

this approach, electron-hole pairs are excited in the laser het-

erostructure through e-beam irradiation and thus completely

avoiding all problems connected to Mg-doping and current-

injection. However, such a device requires a very different

laser heterostructure design as well as a detailed understand-

ing of the carrier dynamics in order to efficiently collect the

charge carriers in the AlGaN quantum wells. Another concern

is the long term stability under high density electron beam irra-

diation as well as the effects of Joule heating, since only 1/3

of the energy is converted to generate electron-hole pairs and

the rest is dissipated via various other channels.

Advanced laser concepts will be a topic of intense research

as soon as technological building blocks are present and they

will be first realized in the UVA region. For DFB, DBR

and tapered lasers the technological prerequisites exist and

advances will be achieved by optimizing the fabrication tech-

nology. Future breakthroughs for VCSELS in the UVA region

J. Phys. D: Appl. Phys. 53 (2020) 503001 Roadmap

will be achieved by improving the fabrication of high reflectiv-

ity mirrors, e.g. based electro chemical etching [210], and

reducing the optical losses in p-contact layers. An extension

towards shorter emission wavelength will then be possible by

improved epitaxial growth and fabrication technology in com-

bination with tunnel junctions or e-beam pumping.

19.4. Concluding remarks

Laser diodes emitting in the UV spectral range have so far

proven to be a much bigger challenge than UV LEDs. For the

recent realization of deep UV current-injection laser diodes

significant advances in material quality and device fabrica-

tion were required. With the recent improvements in materials

growth (e.g. bulk AlN substrates, polarization doping, minim-

izing point defects) and fabrication technologies (e.g. Ohmic

contact formation) initiated by the development of deep UV-

LEDs significant reduction of the lasing threshold of UVB and

UVC laser diodes can be expected.

Acknowledgements

We gratefully acknowledge support by the German Research

Foundation (DFG) within the Collaborative Research Center

‘Semiconductor Nanophotonics’ (CRC 787).

ORCID iDs

Carlo De Santi https://orcid.org/0000-0001-6064-077X

Johannes Glaab https://orcid.org/0000-0002-9252-8368

Robert Martin https://orcid.org/0000-0002-6119-764X

Frank Mehnke https://orcid.org/0000-0001-5406-0832

Peter J Parbrook https://orcid.org/0000-0003-3287-512X

Jan Ruschel https://orcid.org/0000-0001-6914-0958

Biplab Sarkar https://orcid.org/0000-0003-0074-0626

Leo J Schowalter https://orcid.org/0000-0002-4854-8521

Philip Shields https://orcid.org/0000-0003-0517-132X

Luca Sulmoni https://orcid.org/0000-0002-5341-7032

Tim Wernicke https://orcid.org/0000-0002-5472-8166

Yuh-Renn Wu https://orcid.org/0000-0002-1457-3681

Yuewei Zhang https://orcid.org/0000-0002-4192-1442

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