Thin-film flip-chip UVB LEDs realized by electrochemical etching [original]

Appl. Phys. Lett. 116, 121101 (2020); https://doi.org/10.1063/1.5143297 116, 121101

Thin-film flip-chip UVB LEDs realized by

electrochemical etching

Cite as: Appl. Phys. Lett. 116, 121101 (2020); https://doi.org/10.1063/1.5143297

Submitted: 21 December 2019 . Accepted: 11 March 2020 . Published Online: 24 March 2020

Michael A. Bergmann , Johannes Enslin , Filip Hjort , Tim Wernicke , Michael Kneissl , and Åsa

Haglund

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Thin-film flip-chip UVB LEDs realized by

electrochemical etching

Cite as: Appl. Phys. Lett. 116, 121101 (2020); doi: 10.1063/1.5143297

Submitted: 21 December 2019 .Accepted: 11 March 2020 .

Published Online: 24 March 2020

Michael A. Bergmann,

1,a)

Johannes Enslin,

Filip Hjort,

Tim Wernicke,

Michael Kneissl,

and A

˚sa Haglund

AFFILIATIONS

Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden

Institute of Solid State Physics, Technische Universit€

at Berlin, 10623 Berlin, Germany

Author to whom correspondence should be addressed: michael.bergmann@chalmers.se

ABSTRACT

We demonstrate a thin-ﬁlm ﬂip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate

removal has been achieved by electrochemical etching of a sacriﬁcial Al0:37Ga0:63N layer. The electroluminescence spectrum of the TFFC

LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical

etching. This is achieved by a proper epitaxial design of the sacriﬁcial layer and the etch stop layers in relation to the LED structure and the

electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efﬁciency that limits

the power conversion efﬁciency in AlGaN-based LEDs.

C2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://

creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/1.5143297

Ultraviolet (UV) light sources are used in a number of applica-

tions such as water disinfection, UV curing, and phototherapy.

light-emitting diodes (LEDs) are expected to replace conventional UV

light sources like mercury gas discharge lamps due to their small form

factor, robustness, environmentally friendly materials, and selectable

emission wavelength.

However, the poor power conversion efﬁciency

of UV LEDs below 10% for wavelengths shorter than 350 nm, see Ref.

2, strongly limits their widespread use.

One of the major factors limiting the power conversion efﬁciency

is the light extraction efﬁciency (LEE),

which can be improved by a

thin-ﬁlm ﬂip-chip (TFFC) design. Aoshima et al.

have fabricated

TFFC LEDs emitting in the ultraviolet A (UVA) and achieved an

improvement in the LEE of 1:7compared to a ﬂip-chip (FC) design.

Sung et al.

demonstrated an improvement in the LEE for ultraviolet C

(UVC) LEDs of 1:31for a TFFC design compared to a FC design. In

both the cases, a major factor for the improvement was the surface

roughening to reduce total internal reﬂection, which was made possible

by removing the substrate. The inﬂuence of the surface roughening on

the LEE for TFFC LEDs emitting in the UVA and UVC has also been

reported by other groups.

5–8

Despite these demonstrations using laser lift-off (LLO) for the

fabrication of UV TFFC LEDs,

3–5,7,8

substrate removal remains chal-

lenging. LLO is a standard process for GaN-based LEDs emitting in

the blue,

but for AlGaN-based LEDs, the thermal decomposition of

AlGaN yields aluminum residues that are more rigid than Ga residues

as in the case of GaN.

In addition, the substrate removal by LLO can

lead to cracking of the strained epitaxial layers due to the thermal

shock that is enhanced since AlGaN requires higher temperature to

decompose.

10,11

To circumvent these issues, a GaN interlayer has been

used for LLO, which was completely removed afterwards using dry

etching to avoid optical absorption of the UV emission from the

LED.

The integration of such a non-lattice matched layer can lead to

dislocation generation, relaxation, and roughness of the layers grown

on top,

13,14

which strongly limits the aluminum contents and layer

thicknesses that can be used for the LED structure grown on top.

An alternative to thermal decomposition of AlGaN for the sub-

strate removal is doping-selective electrochemical etching. This has been

used to laterally etch GaN and low Al composition AlGaN layers for the

fabrication of thin-ﬁlm LEDs emitting in the blue regime,

15–17

transfer

of III-nitride membranes,

18–21

and fabrication of photonic crystal struc-

tures.

Electrochemical etching has low requirements on the processing

equipment and can be accomplished using standard electroplating

tools. This is in contrast to photoelectrochemical etching that is based

on bandgap-selective absorption.

23,24

Therefore, photoelectrochemical

etching requires a more complex setup with a wavelength-speciﬁc

homogeneous illumination. Thus, for photoelectrochemical etching, the

Appl. Phys. Lett. 116, 121101 (2020); doi: 10.1063/1.5143297 116, 121101-1

CAuthor(s) 2020

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sacriﬁcial layer and device layers have to be designed with respect not

only to selective current ﬂows as in electrochemical etching but also to

selective optical absorption, which, in most cases, involves the use of

non-lattice matched sacriﬁcial layers.

Recently, we have demonstrated that electrochemical etching can

be applied to etch AlGaN with an Al content up to 50%.

This opens

up the possibility to incorporate a lattice matched sacriﬁcial AlGaN layer

that can be selectively removed to realize TFFC UV LEDs of high qual-

ity. In this work, we demonstrate a TFFC ultraviolet B (UVB) LED

using this method.

The epitaxial layer structure of the UVB LED including the layers

required for the electrochemical etching is shown in Fig. 1. The structure

was grown in a 3 200 close-coupled showerhead reactor using the

standard precursors trimethylaluminium (TMAl), trimethylgallium

(TMGa), trimethylindium (TMIn), and ammonia (NH

)withhydrogen

or nitrogen as the carrier gas. Silane (SiH

) was used as a n-dopant

source and cyclopentadienylmagnesium (Cp

Mg) as a p-dopant. First,

an AlN/AlGaN-superlattice for strain management was grown on an

AlN/sapphire substrate

provided by the Ferdinand-Braun-Institut.

This was followed by a 4 lm thick relaxed silicon doped

Al0:5Ga0:5N-layer with a Si concentration of 2 1018 cm3to complete

the quasi-substrate.

14,26

After that, a 130 nm thick Al0:37Ga0:63N:Si sacri-

ﬁcial layer with a Si-concentration of 2 1019 cm3was sandwiched

between two 240 nm thick Al0:5Ga0:5N:Si etch-stop layers with a

reduced Si-concentration of 0:51018 cm3. On top of that, the

Al0:5Ga0:5N:SicurrentspreadinglayerfortheLEDwasgrown.Thiswas

followed by a threefold InAlGaN MQW active region emitting at

311 nm, an Al0:75Ga0:25N electron blocking layer, and a Mg-doped

AlGaN/AlGaN superlattice with an average Al concentration of 50%

and a Mg concentration of 7 1019 cm3.Finally,thestructurewas

capped by a 20 nm thick GaN:Mg contact layer. The threading disloca-

tion density of the epitaxial layers is around 3 109cm2.

The process ﬂow for the TFFC LEDs is summarized in

Figs. 2(a)–2(d),3(a),and3(b). The fabrication started with the deﬁni-

tion of the LED’s emission area. A circular mesa with a diameter of

80 lm was deﬁned by standard photolithography and chlorine-based

reactive ion etching into the n-doped Al0:5Ga0:5N current spreading

layer of the LED. An additional dry etching step into the bottom n



doped Al0:5Ga0:5N-layer, used as the etch stop layer, was performed to

expose the n-doped Al0:37Ga0:63N sacriﬁcial layer. This etch also

deﬁnedtheareatobelifted-off.Thecontactpadfortheelectrochemi-

cal etching consisting of Ti/Al/Ti/Au (20/80/40/100 nm) was deposited

on one end of the sample using electron-beam evaporation and

annealed for 1 min at 900 CinaN

atmosphere. A 50 nm Pd

p-contact was then evaporated on top of the double mesa and annealed

for 1 min at 550 CintheN

atmosphere. To protect the doped epitax-

ial LED layers from parasitic etching during the electrochemical pro-

cess, a SiO

layer was deposited on the sample using sputtering. In a

ﬁrst dry etching step, the SiO

layer was etched down beside the mesa

to 300 nm to obtain a thinner tether layer, while its full thickness of

1lm was kept on top of the mesa to create a larger distance between

the mesa top and the sample surface to facilitate bonding. In a second

dry etching step, the SiO

layer was opened up at three different loca-

tions: on top of the mesas to access the p-contact layer, at one point at

the mesa edge to expose the sacriﬁcial layer, and on the n-contact pad

used for electrochemical etching. Subsequently, a Ti/Au pad for bond-

ing and contacting was deposited on top of the mesa by tilted e-beam

evaporation. The device structure after the p-side processing is shown

in Fig. 2(a).

FIG. 1. Epitaxial structure of the UVB LED and the layers required for substrate

removal by electrochemical etching.

FIG. 2. (a) Sample structure after the p-side processing, (b) three-electrode setup

for electrochemical etching, (c) sacriﬁcial layer removal, and (d) current ﬂow during

electrochemical etching.

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Appl. Phys. Lett. 116, 121101 (2020); doi: 10.1063/1.5143297 116, 121101-2

CAuthor(s) 2020

To release the LEDs from the substrate, the sacriﬁcial layer was

laterally removed using electrochemical etching. In this step, the sam-

ple was immersed in 0.3 M nitric acid and electrically connected to a

Biologic potentiostat in a three-electrode setup, see Fig. 2(b). To pre-

vent parasitic etching, the electrical connection to the sample was

protected from the electrolyte. A graphite rod was used as the coun-

ter electrode to drive the required current through the sample in

order to achieve the set potential on the sample relative to the Ag/

AgCl reference electrode. The electrolyte was stirred with a magnetic

stir bar, and etching was conducted at room temperature with no

intentional illumination. An etching voltage of 25 V vs the Ag/AgCl

reference electrode was chosen to achieve complete removal of the

n-doped Al0:37Ga0:63N sacriﬁcial layer, as shown in Ref. 25, and still

to prevent etching of the etch stop and doped device layers. The

lower Al composition and higher doping of the sacriﬁcial layer rela-

tive to the etch block layers on both the sides conﬁne the etching to

only the sacriﬁcial layer. Because the sacriﬁcial layer is only exposed

to the electrolyte at one side of the mesa, the etching proceeds across

the mesa from one direction in one step, see Fig. 2(c). The SiO

side-

wall protection further ensures a good current ﬂow throughout the

full electrochemical etch process and constant etching conditions, see

Figs. 2(c) and 2(d). After the etching process, the sample was

immersed in de-ionized water for 1 min to dissolve acid residues and

then immersed in isopropanol to reduce any force on the membrane

when allowed to dry in air. The underetched LEDs were held in place

by the SiO

tethers. To ﬂip-chip bond the underetched thin-ﬁlm

LEDs, thermocompression bonding was used. Prior to the bonding,

the bonding metal surfaces on both the LED devices and the Si car-

rier chip with a Ti/Au (10/300 nm) bonding layer were cleaned by

UV-ozone. Subsequently, the chips were pressed together using an

in-house designed stainless-steel compression ﬁxture that was placed

in an oven at 300 C for 2 h, see Fig. 3(a). The bonding pressure on

the bond pad area was set to 50 MPa. To reduce air pockets, the

oven was evacuated to a pressure 200 mbar. During the bonding

process, the SiO

tethers break and the thin-ﬁlm LEDs are trans-

ferred to the Si carrier.

The device process was completed by the formation of an Al/Ti/

Au (100/20/30 nm) n-contact on top of the mesa as shown in the opti-

cal microscope image in Fig. 3(b). An annealing temperature of 550 C

for the n-contact was chosen to minimize the degradation of the

p-contact. The cross-sectional SEM image of the ﬁnal TFFC LED

achieved by a focused-ion-beam-cut shows well-deﬁned device layers

and indicates a smoothly etched surface as seen in Fig. 4.Moreover,

the device layers show no indication of being porosiﬁed by the electro-

chemical etch process. An improper combination of doping levels,

thicknesses, and Al compositions of the LED layers and etch stop layer

in relation to the sacriﬁcial layer and etch voltage would result in

undesired porosiﬁcation with pore diameters of 50 nm–100 nm,

which would clearly be visible in the cross-sectional SEM image. In

addition, time-resolved photoluminescence (PL) measurements

yielded similar PL decay times of 340630 ps at room temperature for

the quantum wells in the as-grown LEDs and in the TFFC LEDs, indi-

cating no degradation of the active region.

Figure 5 shows the electroluminescence spectrum at room tem-

perature of the TFFC LED at a current density of 0:8A=cm2with a

single emission peak at 311 nm. This corresponds well to the electrolu-

minescence peak wavelength of 312 nm of the as-grown LED structure

that was contacted using an In-dot, for which the emission was col-

lected through the sapphire substrate. The full width at half maximum

for the TFFC LED is 9 nm and 11 nm for the as-grown LED structure.

Figure 6 shows the voltage and optical output power as a function

of current density for a TFFC LED with a p-contact diameter of

68 lm. The electroluminescence was measured using a broad-area

UV-enhanced Si photodiode S2281 from Hamamatsu. The graph

shows a linear output power vs current density, as expected for an

LED. This proof-of-principle device is not optimized for high light

extraction efﬁciency, and therefore, the optical output power that is in

the low lW regime is stated in arbitrary units. A homogeneous cur-

rent injection across the p-contact area is assumed when calculating

the current density. However, near-ﬁeld imaging of the spontaneous

emission at a current of 0:2 mA [see Fig. 7(b)] shows that this is an

underestimation of the current density due to a non-homogeneous

FIG. 3. (a) Flip-chip bonding in the vacuum oven and (b) schematic and optical

microscope image of the thin-ﬂip ﬂip-chip LED before the n-contact annealing.

FIG. 4. Cross-sectional SEM image of the transferred LED structure.

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Appl. Phys. Lett. 116, 121101 (2020); doi: 10.1063/1.5143297 116, 121101-3

CAuthor(s) 2020

current injection caused by a degraded p-contact. The voltage vs cur-

rent density reveals a high differential resistance of the LED of about

63 kXat 3 A=cm2. This is attributed to a highly resistive n-contact as

the fabrication process was not optimized for the N-polar backside

with low n-type doping, additionally hindered by the limited annealing

temperature to avoid damaging of the LED and possible degradation

of the p-contact. A different process ﬂow, where the n-contact can be

annealed before the p-contact, could solve these issues.

In conclusion, we have demonstrated a thin-ﬁlm ﬂip-chip UVB

LED where the substrate removal was achieved by lateral electrochemi-

cal etching of an AlGaN sacriﬁcial layer. The LED structure was not

structurally or optically affected by the substrate removal technique,

through a proper design of the sacriﬁcial layer (doping level, thickness,

and Al composition) in relation to the etch stop layer and the device

layers, and choice of the etch voltage. The electroluminescence spec-

trum shows a single peak emission at 311 nm for the thin-ﬁlm ﬂip-

chip LED, which corresponds to the emission wavelength of the as-

grown LED structure, indicating no signiﬁcant strain being introduced

or released in the thin-ﬁlm ﬂip-chip process. The high series resistance

of the LED was attributed to high contact resistance caused by a non-

optimized annealing temperature and n-contacts on low-doped,

N-polar AlGaN. An alternative process ﬂow, where the n-contact is

annealed before the p-contact, should solve this issue. The developed

thin-ﬁlm process is not limited to UVB LEDs but can also be applied

to deep-UV LEDs and other devices such as UV vertical-cavity sur-

face-emitting lasers that beneﬁt from the integration of high-quality

III-nitride device layers with other structures such as dielectric distrib-

uted Bragg reﬂectors.

This work was performed in part at Myfab Chalmers. This

project was ﬁnancially supported by the Swedish Foundation for

Strategic Research, the Swedish Research Council (Project Nos.

2016-04686 and 2018-00295), and the Deutsche

Forschungsgemeinschaft (DFG) within the Collaborative Research

Center “Semiconductor Nanophotonics” (SFB 787).

REFERENCES

III-Nitride Ultraviolet Emitters, Springer Series in Materials Science Vol. 227,

edited by M. Kneissl and J. Rass (Springer International Publishing, 2016).

M. Kneissl, T.-Y. Seong, J. Han, and H. Amano, “The emergence and prospects

of deep-ultraviolet light-emitting diode technologies,” Nat. Photonics 13,

233–244 (2019).

H. Aoshima, K. Takeda, K. Takehara, S. Ito, M. Mori, M. Iwaya, T. Takeuchi,

S. Kamiyama, I. Akasaki, and H. Amano, “Laser lift-off of AlN/sapphire for

UV light-emitting diodes,” Phys. Status Solidi C 9, 753–756 (2012).

Y. J. Sung, M.-S. Kim, H. Kim, S. Choi, Y. H. Kim, M.-H. Jung, R.-J. Choi, Y.-

T. Moon, J.-T. Oh, H.-H. Jeong, and G. Y. Yeom, “Light extraction enhance-

ment of AlGaN-based vertical type deep-ultraviolet light-emitting-diodes by

using highly reﬂective ITO/Al electrode and surface roughening,” Opt. Express

27, 29930–29937 (2019).

C. E. Leez, B. S. Cheng, Y. C. Lee, H. C. Kuo, T. C. Lu, and S. C. Wang,

“Output power enhancement of vertical-injection ultraviolet light-emitting

diodes by GaN-free and surface roughness structures,” Electrochem. Solid-

State Lett. 12, H44–H46 (2009).

B. K. SaifAddin, A. Almogbel, C. J. Zollner, H. Foronda, A. Alyamani, A.

Albadri, M. Iza, S. Nakamura, S. P. DenBaars, and J. S. Speck, “Fabrication

technology for high light-extraction ultraviolet thin-ﬁlm ﬂip-chip (UV TFFC)

LEDs grown on SiC,” Semicond. Sci. Technol. 34, 035007 (2019).

L. Zhou, J. E. Epler, M. R. Krames, W. Goetz, M. Gherasimova, Z. Ren, J. Han,

M. Kneissl, and N. M. Johnson, “Vertical injection thin-ﬁlm AlGaN/AlGaN

multiple-quantum-well deep ultraviolet light-emitting diodes,” Appl. Phys.

Lett. 89, 241113 (2006).

M. Lachab, F. Asif, B. Zhang, I. Ahmad, A. Heidari, Q. Fareed, V. Adivarahan,

and A. Khan, “Enhancement of light extraction efﬁciency in sub-300 nm

nitride thin-ﬁlm ﬂip-chip light-emitting diodes,” Solid-State Electron. 89,

156–160 (2013).

M. K. Kelly, O. Ambacher, R. Dimitrov, R. Handschuh, and M. Stutzmann,

“Optical process for liftoff of group III-nitride ﬁlms,” Phys. Status Solidi A

159, R3–R4 (1997).

FIG. 5. Normalized electroluminescence spectrum of the as-grown epitaxy and the

TFFC LED at room temperature.

FIG. 6. P–I–V characteristics of the TFFC LED.

FIG. 7. (a) Optical microscope image of the TFFC LED and (b) spatially resolved

near-ﬁeld emission at a current of 0:2 mA. Electroluminescence intensity is on a lin-

ear scale.

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