Appl. Phys. Lett. 118, 202101 (2021); https://doi.org/10.1063/5.0047021 118, 202101
© 2021 Author(s).
Origin of defect luminescence in ultraviolet
emitting AlGaN diode structures
Cite as: Appl. Phys. Lett. 118, 202101 (2021); https://doi.org/10.1063/5.0047021
Submitted: 09 February 2021 • Accepted: 03 May 2021 • Published Online: 17 May 2021
Martin Feneberg, Fátima Romero, Rüdiger Goldhahn, et al.
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Origin of defect luminescence in ultraviolet
emitting AlGaN diode structures
Cite as: Appl. Phys. Lett. 118, 202101 (2021); doi: 10.1063/5.0047021
Submitted: 9 February 2021 .Accepted: 3 May 2021 .
Published Online: 17 May 2021
Martin Feneberg,
1,a)
F
atima Romero,
1,b)
R€
udiger Goldhahn,
1
Tim Wernicke,
2
Christoph Reich,
2
Joachim Stellmach,
2
Frank Mehnke,
2
Arne Knauer,
3
Markus Weyers,
3
and Michael Kneissl
2,3
AFFILIATIONS
1
Institut f€
ur Physik, Otto-von-Guericke-Universit€
at Magdeburg, Universit€
atsplatz 2, 39106 Magdeburg, Germany
2
Institut f€
ur Festk€
orperphysik, Technische Universit€
at Berlin, Hardenbergstr. 30, 10623 Berlin, Germany
3
Ferdinand-Braun-Institut gGmbH, Leibniz-Institut f€
ur H€
ochstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany
a)
b)
Present address: Universidad Francisco de Vitoria, Carretera Pozuelo a Majadahonda, Km 1.800, 28223 Madrid, Spain.
ABSTRACT
Light emitting diode structures emitting in the ultraviolet spectral range are investigated. The samples exhibit defect luminescence bands.
Synchrotron-based photoluminescence excitation spectroscopy of the complicated multi-layer stacks is employed to assign the origin of the
observed defect luminescence to certain layers. In the case of quantum well structures emitting at 320 and 290 nm, the n-type contact
AlGaN:Si layer is found to be the origin of defect luminescence bands between 2.65 and 2.8 eV. For 230 nm emitters without such n-type
contact layer, the origin of a defect double structure at 2.8 and 3.6 eV can be assigned to the quantum wells.
#2021 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/10.1063/5.0047021
There is a growing demand for ultraviolet light emitting diodes
(UV-LEDs) for many different applications like surface polymeriza-
tion,
1
gas sensing,
2
or water disinfection.
3
Such UV-LEDs became
available recently based on wurtzite AlGaN. However, their wall-plug
efficiency is very low compared to their visible counterparts.
4,5
One of
the possible reasons for this striking difference in efficiency is
discussed to be defects in different functional layers of the UV-LED
structure.
6
This assumption seems reasonable because AlGaN as light-
emitting semiconductor material is less optimized compared to
InGaN. Some of these defects manifest itself in the form of a broad
unstructured defect luminescence band which is frequently observed
in UV-LED structures.
7,8
In this Letter, we contribute to the ongoing discussion by deter-
mining the layer from which the defect luminescence originates in
AlGaN UV-LED structures. Therefore, we performed photolumines-
cence excitation spectroscopy experiments on several UV-LED struc-
tures. Careful examination of our results shows that the broad band
peaking around 2.7eV stems from the n-type contact layer grown
below the quantum wells (QWs) of the UV-LEDs. This result proves
that material optimization even of contact layers might contribute to
advances in UV-LEDs.
We investigate samples designed for different emission wave-
lengths: 320, 290, and 230 nm. The selected samples were grown by
metal-organic vapor phase epitaxy with (0001) orientation on dif-
ferent buffer/substrate combinations, containing superlattices for
strain management,
9,10
AlN templates defined by epitaxial lateral
overgrowth (ELO) on structured sapphire,
11
and direct growth of
AlN on Al
2
O
3
.
12
Oursetofsamplesconsistsofthreesamplesemittingaround
320 nm (sample series A), one sample emitting at 290 nm (sample B)
and two emitting at 230 nm (series C). Samples A1 and A2 were grown
on a 1:3lm thick AlN template on sapphire with a dislocation density
estimated to be 5–7109cm2from omega rocking curve broadening
of the AlGaN (00.2) and (10.2) reflexes followed by an 80-period AlN/
GaN superlattice. Sample A3 was grown on an AlN ELO structure
with dislocation density of less than 2 109cm2. For all samples
of series A, the next layers are Al
0.35
Ga
0.65
N:Si with 6lm(1:5lm
in the case of A3), 3 QWs having nominal well and barrier compo-
sitions of In
0.02
Al
0.22
Ga
0.76
N/Al
0.30
Ga
0.70
N.
13
Samples A1 and
A3 have 2nm (sample A2 4nm) thick QWs. The topmost layer
consists of 25 nm of Al
0.38
Ga
0.62
N. The Al
0.35
Ga
0.65
N:Si layers
of all three samples have the identical lattice parameters of
a¼ð3:1575 60:0007Þ˚
Aandc¼ð5:1184 60:0003Þ˚
Aaccording
to reciprocal space maps. This corresponds to compressive strain
of xx ¼0:013. The layers above are pseudomorphic to this layer.
The samples are sketched in Fig. 1.
Appl. Phys. Lett. 118, 202101 (2021); doi: 10.1063/5.0047021 118, 202101-1
#2021 Author(s).
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Sample B is very similar to sample A1. The superlattice thickness
was changed to 80 nm, followed by a 4:5lmthickAl
0.47
Ga
0.53
N:Si
contact layer. QW/barrier composition is Al
0.40
Ga
0.60
N/Al
0.48
Ga
0.52
N,
and the top layer consists of Al
0.6
Ga
0.4
N. The strain of the
Al
0.47
Ga
0.53
N:Si contact layer is known to be slightly compressive on
the order of 0:025 <
xx <0:001 while the layers above have the
same lattice parameters.
10
Sample C1, however, bases on a simpler lay-
out. On a sapphire substrate, 1:3lm of AlN was deposited followed by
10 Al
0.9
Ga
0.1
N QWs sandwiched between AlN barrier layers. Sample
C2 has the same layer structure but is based again on an AlN ELO
buffer identical to sample A3. Both have in-plane lattice parameters of
relaxed AlN of 3:112 ˚
A.
All samples were investigated by photoluminescence (PL) using
193 nm excimer ArF
laser radiation with an excitation density of the
order of 1 kW=cm2. Furthermore, we investigated them by radiation
from a synchrotron with much lower excitation density of the order of
1W=cm2in a near-normal incidence geometry. More details about
the experiments can be found in Ref. 14 and references therein. All
spectra shown in this Letter were recorded at T¼10 K.
First, we discuss the PL results from sample A1 which are shown
in Fig. 2. We find a signal at 4.28 eV which is identified as lumines-
cence from the Al
0.35
Ga
0.65
N n-type layer from its energy position,
15
which does neither match the Al
0.38
Ga
0.62
N contact layer nor the
Al
0.30
Ga
0.70
N quantum barriers. At 3:9 eV a double peak is observed
from the QWs. The energy distance between both contributions is
around 65 meV not matching a possible explanation as a phonon rep-
lica. We assign the low energy contribution to indium-rich regions
within the QWs as described in detail below.
The PL signal of sample A1 additionally shows a broad unstruc-
tured defect luminescence centered at around 2.65 eV. This lumines-
cence band is in the focus of our investigation as we are interested in
its origin. All signal bands are visible by both excitation using the ArF
laser and synchrotron radiation with the same wavelength (Fig. 2).
The signal at 4.28 eV attributed to the Al
0.35
Ga
0.65
Nn-typelayeris
strongly reduced in intensity at the synchrotron due to lower excita-
tion density. The low energy shoulder of the QW emission is, however,
strongly enhanced in relative signal strength, most likely due to a cor-
responding low density of states.
16
PLE spectra employing the defect
luminescence and the QW emission as monitor lines are presented in
Fig. 2 as well. Several photon energy edges are observed in PLE mark-
ing different channels to pump the two luminescence bands
investigated.
The intensity of the QW emission at 3.94 eV increases signifi-
cantly at excitation energies of 4.13 eV and again at 4.44 eV (red
curve). These energies are in good agreement with absorption onsets
in Al
x
Ga
1x
Nlayers.
17
Here, one has to keep in mind that the highest
valence band has C7symmetry for x>0.05.
17
Absorption processes
observed in our (0001) oriented samples are, therefore, most likely
related to the next valence band having C9symmetry.
17
The step at
4.44 eV is due to the top 25 nm thick Al
0.38
Ga
0.62
N layer, while that at
4.13 eV belongs to the quantum well’s barriers, consisting of
Al
0.30
Ga
0.70
N. In fact, the experimental energies hint toward slightly
higher aluminum concentrations of x¼0.43 and x¼0.32,
respectively.
17
In contrast, the broad unstructured defect emission at 2.65 eV is
most efficiently pumped if the excitation energy is>4.30 eV (black
curve), corresponding to the n-contact layer having nominally
x¼0.35 again assuming absorption processes related to the C9valence
band. This energy position is in line with an actual aluminum concen-
tration of x¼0.38; however, it must be clearly less than that of the cap
layer. Therefore, this intensity step can be assigned to the 6 lmthick
n-type Al
0.35
Ga
0.65
N (nominal composition) contact layer below the
active region.
Now we compare the excitation and emission spectra of sample
A2 (Fig. 3) with the same layer structure as sample A1 but thicker
QWs. We find a similar luminescence spectrum but shifted to lower
photon energies. However, the low energy shoulder found in the PL
spectrum of sample A1 seems to be separated by 300 meV from the
main peak in sample A2 making separate PLE experiments for both
contributions feasible. Intensity steps at virtually identical energy posi-
tions of 4.38 and 4.00 eV are detected. We again identify both intensity
steps with the 25 nm thick Al
0.38
Ga
0.62
N layer on top and the quantum
well’s barriers, respectively. In fact, 4.38 eV hints more toward
x¼0.42. For sample A2 the barrier absorption onset is found at lower
energy compared to sample A1 being in agreement with a higher net
built-in electric field due to the thicker QWs.
18
Finally, the 3.44 eV
FIG. 1. Schematic of samples A1, A2, and A3. Quantum well thickness is 2 nm for
A1 and A3, 4 nm for A2. Nominal quantum well/barrier compositions are
In
0.02
Al
0.22
Ga
0.76
N/Al
0.30
Ga
0.70
N. The AlGaN layer below the active region is n-type.
FIG. 2. Photoluminescence (PL) spectra of sample A1, excited by the ArF
laser
(grey) and synchrotron radiation (blue curve), both at 193 nm. Photoluminescence
excitation (PLE) is monitored once at the broad unstructured defect luminescence
at 2.65 eV (black) and once at the quantum well luminescence at 3.94 eV (red
curve), both positions are marked by vertical arrows.
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Appl. Phys. Lett. 118, 202101 (2021); doi: 10.1063/5.0047021 118, 202101-2
#2021 Author(s).
emission is efficiently excited for photon energies >3.77 eV marking
the absorption edge of the QWs.
From the PLE spectra, it is obvious that both emission contribu-
tions at 3.75 and 3.44 eV are from the same layer. We therefore assign
the 3.44 eV emission to indium-rich regions of the QWs.
19,20
Please
note that the low energy contribution is only prominent when exciting
by synchrotron light, i.e., low excitation density. These excitation con-
ditions also lead to a red shift of the QW emission as expected.
21,22
The defect emission of sample A2, monitored at 2.70 eV, is effi-
ciently excited for photon energies >4.22 eV. Eventually, a second
intensity step at 4.31 eV can be seen as well. Corresponding aluminum
concentrations are x¼0.35 and 0.39, respectively. We therefore
assign this emission again to the thick n-type Al
0.35
Ga
0.65
Ncontact
layer below the active region in agreement with our findings from
sample A1.
To unambiguously make clear that the defect luminescence
around 2.7 eV is not from the superlattice (SL) below the n-AlGaN
contact layer (Fig. 1), we further compare results from sample A3
whose active layer is identical to that of sample A1 but the active struc-
ture is grown on epitaxially lateral overgrown AlN on patterned sap-
phire with only a 1:5lmthickAl
0.35
Ga
0.65
N contact layer without
superlattice. Results are presented in Fig. 4.
The QW emission, monitored at 3.81eV, is connected with two
excitation edges at 4.00 and 4.26 eV. The higher energy step is assigned
to the nominal Al
0.38
Ga
0.62
N cap layer, hinting toward x¼0.37, while
the lower one is most likely due to absorption in the quantum barriers.
The broad defect emission is detected at 2.5eV and can be efficiently
excited for energies >4.3 eV which is very similar to the case of sample
A1 and thus identified with an origin in the same layer, the n-type
Al
0.35
Ga
0.65
N contact layer. Therefore, we identify the defect lumines-
cence visible in PL spectra of the three k320 nm QW structures as
originating from the n-type contact layer in all cases.
Now we expand our investigation to shorter wavelength emitters.
The results for sample B (k290 nm) show two distinct defect
luminescence bands (Fig. 5).Thebandcenteredat2.8eVyieldsaPLE
edge at 4.69 eV corresponding to x¼0.50 which is in good agreement
with the nominal n-type contact layer composition of Al
0.47
Ga
0.53
N.
We argue that this defect band is the one related to the 2:5 eV band
discussed in samples of series A because it is shifted to slightly higher
energy.
In contrast, the second defect band visible around 3.6 eV remains
mysterious. We find two PLE steps at 4.50 and 4.73 eV; however, these
energies correspond to Al concentrations of 0.43 and 0.52, respectively.
Both these compositions are not intentionally introduced into our
samples. Moreover, an absorption contribution around 4 eV remains
FIG. 3. Photoluminescence (PL) spectra of sample A2, excited by the ArF
laser
(grey) and synchrotron radiation (blue curve), both at 193 nm. Photoluminescence
excitation (PLE) is monitored at the broad unstructured defect luminescence at
2.70 eV (black), and at the quantum well luminescence bands at 3.75 eV (red
curve) and at 3.44 eV (green curve). The positions are marked by vertical arrows.
FIG. 4. Photoluminescence (PL) spectra of sample A3, excited by the ArF
laser
(grey) and synchrotron radiation (blue curve), both at 193 nm. Photoluminescence
excitation (PLE) is monitored at the broad unstructured defect luminescence at
2.5 eV (black), and at the quantum well luminescence band at 3.85 eV (red curve).
The positions are marked by vertical arrows.
FIG. 5. Photoluminescence (PL) spectra of sample B, excited by synchrotron radia-
tion (blue curve) at 193 nm. Photoluminescence excitation (PLE) is monitored at the
broad unstructured defect luminescence bands at 2.8 (black) and 3.6 eV (red), and
at the quantum well luminescence band at 4.42 eV (green curve). The positions are
marked by vertical arrows.
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Appl. Phys. Lett. 118, 202101 (2021); doi: 10.1063/5.0047021 118, 202101-3
#2021 Author(s).
unclear and so this band resists our attempts to unambiguously assign
it to certain layers. Finally, the QW luminescence monitored at
4.42 eV yields a clear but unstructured PLE signal not allowing for fur-
ther detailed analysis.
Finally, we are looking at the results of two samples of series C,
which emit around k¼230 nm. These samples are fundamentally dif-
ferent from series A and B as there exists no n-type contact layer.
Nevertheless, we find defect luminescence also in these structures.
Interestingly, the defect luminescence is found at an energy of 2:8eV
(Ref. 23) (and a weak shoulder at 3.6 eV at least in sample C1) despite
the fact that the layer consists of AlN. For PLE spectra, different longpass
filters with cutoff wavelengths at 360 and 280 nm, respectively, were
used to suppress second order contributions. The QW luminescence of
samples C1 and C2 (Fig. 6) yields two visible PLE steps in sample C1 at
5.86 and 6.16 eV. We identify them as QW absorption and AlN buffer
or quantum barrier absorption, respectively, as for this emission
wavelength the crystal field splitting causes a strong reduction of TM
polarized emission intensity in contrast to the absorption edge of the
TE-polarized PLE excitation.
12
The energy difference between QW lumi-
nescence and absorption is in agreement with our earlier results.
24
The same result is found in sample C2; however, due to lower sig-
nal to noise ratio, only the first PLE step is clearly visible, here
5:9 eV. Both defect luminescence contributions investigated (3.6 eV
in sample C1 and 2.75 eV in sample C2) yield only one clear PLE step
which seems to be identical to the QW absorption. The strongly
decreasing PLE efficiency for increasing photon energies >6.4 eV fur-
ther corroborates this interpretation because such light penetrates only
a few nm into the sample. Because photon energies above 5:9eV
already suffice to pump the defect luminescence, we conclude that the
defect luminescence in series C originates from the QWs rather than
from the AlN buffer or barrier layers.
According to photoluminescence data of undoped
25
and Si doped
AlGaN layers,
26
this defect luminescence was observed before at simi-
lar energy positions taking into account the layer composition. There,
an assignment to cation vacancy complexes was put forward.
In summary, our synchrotron-based PLE study of defect lumi-
nescence in AlGaN UV LED structures revealed that the dominating
defect luminescence band, that is even visible under 193 nm excitation
with a low penetration depth, originates in the n-type AlGaN contact
layer below the active region if such a layer is present. Only for hetero-
structures emitting at extremely short wavelengths, we find defect
luminescence from the QWs themselves. All these findings allow fur-
ther optimization of the relevant defect-containing layers to increase
the efficiency of future UV-LEDs.
A part of this research was carried out at the light source
DORIS III at DESY. DESY is a member of the Helmholtz
Association (HGF). We would like to thank A. Kotlov for excellent
assistance in using beamline I at DESY. This work was partially
supported by the Federal Ministry of Education and Research
(BMBF), under Contract No. 13N9933 and Berlin WideBase
initiative under Contract No. 03WKBT01D and the German
Research Council within the Collaborative Research Center 787.
DATA AVAILABILITY
The data that supports the findings of this study are available
within the article.
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