Content uploaded by Saulius Miasojedovas
Author content
All content in this area was uploaded by Saulius Miasojedovas on Feb 10, 2014
Content may be subject to copyright.
Vol. 106 (2004) ACTA PHYSICA POLONICA A No. 2
Proceedings of the XXXIII International School of Semiconducting Compounds, Jaszowiec 2004
Luminescence in Highly Excited
InGaN/GaN Multiple Quantum Wells
Grown on GaN and Sapphire Substrates
S. Miasojedovasa,∗, S. Jurˇ
s˙
enasa, G. Kurilˇ
cika,
A. ˇ
Zukauskasa, V.Yu. Ivanovb, M. Godlewskib,c,
M. Leszczy´
nskid, P. Perlindand T. Suskid
aInstitute of Materials Science and Applied Research, Vilnius University
Saul˙etekio al. 9, building III, 10222 Vilnius, Lithuania
bInstitute of Physics, Polish Academy of Sciences
al. Lotnikow 32/46, 02-668 Warsaw, Poland
cDept. Math. and Natural Sci. College of Science
Cardinal S. Wyszy´nski Univ., Warsaw, Poland
dHigh Pressure Research Center, Polish Academy of Sciences
Warsaw, Poland
We report on high-excitation luminescence spectroscopy in
InxGa1−xN/GaN multiple quantum wells grown by MOCVD over
sapphire and bulk GaN substrates. High excitation conditions enabled us
to achieve a screening of the built-in field by free carriers. This allowed for
the evaluation of the influence of band potential fluctuations due to the
variation in In-content on efficiency of spontaneous and stimulated emission.
InGaN/GaN multiple quantum wells grown on bulk GaN substrate exhibit a
significantly lower stimulated emission threshold and thus enhanced lateral
emission. Transient and dynamic properties of luminescence indicate a
significant reduction in compositional disorder in homoepitaxially grown
structures.
PACS numbers: 78.45.+h, 78.47.+p, 78.67.De
∗corresponding author; e-mail: saulius.miasojedovas@ff.vu.lt
(273)
274 S. Miasojedovas et al.
1. Introduction
InGaN is the key material for production of light emitting diodes that operate
from the visible to UV spectral region [1, 2]. Despite the commercialization of these
devices, the luminescence efficiency suffers from a high density of dislocations due
to the lattice mismatch between the substrate and active layers. The dislocations
are also involved in the formation of compositional inhomogeneity [3]. Various
substrates were used to avoid excessive heating of structure during the operation
and penetration of dislocations into the active layer [4–6]. A significant reduction
in dislocation density can be obtained in multiple quantum well (MQW) structures
grown on GaN substrates [7].
Here we present results on high-excitation luminescence spectroscopy in
InGaN/GaN 20-period MQWs grown over sapphire and bulk GaN substrates.
High-power excitation conditions were close to those of semiconductor laser oper-
ation regime. The created carrier density was of the order of 1019 cm−3, which
enabled us to characterize the structures under conditions of screened built-in
electric field [8–10] and to analyse recombination dynamics within the tail of the
density of the states.
2. Experimental
Samples were grown on sapphire and bulk GaN substrates. Each structure
consists of 20 InGaN quantum wells separated by GaN barrier layers. GaN sub-
strates were prepared by high-pressure technique and the MQWs were grown by
metal organic chemical vapor deposition. Both samples contain a GaN buffer layer
of 0.55 µm thickness doped with silicon (approximately 1018 cm−3). MQWs con-
sisting of 3.5 nm InGaN quantum wells (approximately 7% of indium) and 8 nm
GaN barriers were deposited on the buffer layer. The structures were covered by a
18 nm GaN capping layer. The sample grown on sapphire substrate has the same
structure except that a nucleation layer of AlN was grown on the substrate.
The samples were excited by the third harmonic (photon energy hν =
3.49 eV) of the actively–passively mode-locked YAG:Nd3+ (yttrium aluminum gar-
net) laser (pulse duration of τ= 20 ps, repetition rate — 2.7 Hz, maximum pump
energy of the third harmonic — 25 µJ). The size of excitation spot was approxi-
mately 1.3 mm. Luminescence was collected in backward and lateral geometries
and dispersed by a 0.4 m grating monochromator. Toluene optical Kerr shutter
was used for temporal resolution (20 ps) of the luminescence. The experiments
were carried out at room temperature.
3. Results and discussion
Figure 1 depicts time-integrated luminescence spectra of the InGaN MQWs,
measured at 1.5 mJ/cm2excitation energy density and recorded in backward
Luminescence in Highly Excited InGaN/GaN .. . 275
Fig. 1. Time-integrated luminescence spectra of InGaN/GaN MQWs grown on GaN
(circles) and on sapphire (squares) substrates, measured at 1.5 mJ/cm2excitation energy
density and recorded in backward geometry.
geometry. Both spectra show one broad luminescence band originated from the
QWs region, peaked at 3.07 eV and at 3.1 eV for MQWs over sapphire and GaN
substrates, respectively. The only difference between the two structures is the
density of threading dislocations. The structure grown over bulk GaN is expected
to be of a higher quality and to possess a lower density of nonradiative traps.
However, the MQW grown over sapphire substrate shows more than twice intense
spontaneous emission as that grown over bulk GaN. In the structure grown over
sapphire, the broadening and red-shift of the luminescence band can be attributed
to smearing of the band edge which results in a more extended band tail.
Figure 2 shows time-resolved luminescence spectra of the two structures
observed in backward geometry for 1.5 mJ/cm2excitation energy density. The
Fig. 2. Time-resolved luminescence spectra measured at 1.5 mJ/cm2excitation energy
density and recorded in backward geometry for structures (a) grown on bulk GaN and
(b) grown on sapphire. Delay-time (in ps) is indicated on the left-hand side.
276 S. Miasojedovas et al.
structure grown over GaN shows a strong narrow emission band, which is typical
of amplified luminescence. Meanwhile, the structure grown over sapphire shows a
broad band of spontaneous emission. A higher threshold of stimulated emission
in the structure grown over sapphire can be attributed to a larger compositional
inhomogeneity [3] and thus to a lower density of states [9, 10].
A transient red-shift (of about 50 meV) in homoepitaxial sample is observed
on the initial stage (first 100 ps) of the relaxation. On this stage, an enhanced
carrier recombination rate that is slowing down with time was observed. Such be-
havior indicates a screened built-in field. The initial relaxation can be explained in
terms of carrier localization within random energy potential. After the excitation
pulse terminates, the carriers from higher energy states can be more efficiently cap-
tured by nonradiative traps or recombine radiatively via stimulated transitions.
The strong transient red-shift and the decrease in intensity are typical of lumi-
nescence in highly excited disordered systems [9]. After 100 ps, only spontaneous
emission is observed for both samples [10].
Fig. 3. Luminescence transients obtained of InGaN/GaN MQWs grown on GaN (open
circles) and on sapphire (solid circles) substrates, at the peak of spontaneous lumines-
cence band at 1.5 mJ/cm2.
Figure 3 shows the luminescence transients measured at the peak positions
of the corresponding time-integrated luminescence bands. Stimulated emission on
the initial stage of the transients rapidly diminishes the initially created carrier
density in MQWs grown over GaN. After exhaustion of stimulated emission, the
remaining carrier density is lower than that in MQWs on sapphire. This accounts
for a low efficiency of spontaneous luminescence of MQWs grown over GaN. On
the late relaxation stage, both structures show almost the same 330 ps decay-time,
which is due to nonradiative recombination of delocalized carriers.
Time-integrated luminescence dependence on excitation intensity was mea-
sured in InGaN/GaN MQWs using lateral geometry (not shown). At low exci-
Luminescence in Highly Excited InGaN/GaN .. . 277
tation intensities Ie<0.3 mJ/cm2, both samples showed one broad spontaneous
luminescence band situated at 3.05 eV and 2.97 eV for the structure grown on GaN
and on sapphire, respectively. With an increase in excitation density, a stimulated-
-emission peak appeared on the high-energy side of the spectrum. This peak was
positioned at 3.1 eV and 3.26 eV for the sample grown on GaN and sapphire,
respectively. This difference in energy indicates on enhanced fluctuations of the
band potential in the heteroepitaxial sample. Meanwhile, the threshold for stimu-
lated emission was lower for the structure grown over GaN (about ∼0.6 mJ/cm2).
The MQW structure grown on GaN also showed the lateral-luminescence efficiency
that was by two orders of magnitude higher in comparison with the sample grown
on sapphire substrate. This result also explains the reduction of the luminescence
intensity in backscattering measurements.
Figure 4 depicts time-resolved lateral-luminescence spectra of InGaN/GaN
MQW structures (the spectra are arbitrary shifted along the vertical axis). On
the initial stage (−20 ps — 0 ps), the sample grown on bulk GaN substrate shows
one narrow band of stimulated emission, which is situated at 3.12 eV. Owing to
lower joint density of states, stimulated emission starts from 0 ps delay time in
the heteroepitaxial sample. The band of stimulated emission decays rapidly with
the time constant below our temporal resolution (τ < 20 ps). After 70 ps, only
the spontaneous luminescence peak is observed at 3.07 eV (the peak is red-shifted
due to the reabsorption) (Fig. 4a). Meanwhile, the structure grown on sapphire
substrate (Fig. 4b) shows a remarkably blue-shifted (at about 3.25 eV) stimulated-
-emission band of lower intensity.
Fig. 4. Time-resolved luminescence spectra of lateral emission of InGaN/GaN MQW
structures grown (a) on GaN and (b) on sapphire (the spectra are arbitrary shifted along
the vertical axis).
278 S. Miasojedovas et al.
Results on laterally amplified luminescence evidently show that the het-
eroepitaxially grown structure can be characterized by a significantly higher varia-
tion in the In content, which results in a smeared density of states. The stimulated
and spontaneous emission bands are separated in time and energy scales. Spon-
taneous emission occurs within the tail of the density of states on the late stage
of relaxation, while the stimulated emission occurs close to mobility edge, imme-
diately after short-pulse excitation. Compositional disorder results in a decrease
in the effective density of states, which leads to a higher lasing threshold and a
blue-shift of stimulated emission. The resulting energy separation between the
spontaneous and stimulated emission bands is about 20 meV for the homoepi-
taxially grown sample and about 200 meV for the heteroepitaxially grown one.
The only difference between two samples is the density of threading dislocations,
which is significantly lower for homoepitaxially grown structures. Thus thread-
ing dislocations can play a key role in producing compositional inhomogeneity in
InGaN/GaN MQWs. Moreover, owing to threading dislocations and composi-
tional disorder, the structure grown on sapphire can have larger diffractive losses,
which is evidenced by the more than two orders of magnitude lower efficiency of
laterally amplified emission observed at high excitation.
4. Conclusions
InGaN/GaN MQWs grown on GaN and sapphire substrates were charac-
terized by luminescence spectroscopy under high excitation conditions, close to
those of semiconductor laser operation regime. Transient and dynamic properties
of luminescence indicate a significant reduction in compositional disorder in the
homoepitaxially grown structures. This results in a significantly lower threshold of
stimulated emission and thus in an enhanced intensity of the lateral luminescence
for InGaN/GaN MQWs grown over bulk GaN substrate.
Acknowledgments
The research at Vilnius University was partially supported by the Lithua-
nian State Science and Education Foundation under COST529 program and Eu-
ropean Commission supported SELITEC center Contract No. G5MA-CT-2002-
04047. A.ˇ
Z. acknowledges the Lithuanian Ministry of Education and Science for
his fellowship.
References
[1] S. Nakamura, S.F. Chichibu, Introduction to Nitride Semiconductor Blue Lasers
and Light Emitting Diodes, Taylor & Francis, London 2000.
[2] A. ˇ
Zukauskas, M.S. Shur, R. Gaska, Introduction to Solid-State Lighting, Wiley,
New York 2002.
Luminescence in Highly Excited InGaN/GaN .. . 279
[3] H. Sato, T. Sugahara, Y. Naoi, S. Sakai, Jpn. J. Appl. Phys. 37, 2013 (1998).
[4] W.S. Wong, M. Kneissl, P. Mei, D.W. Treat, M. Teepe, N.M. Johnson, Appl.
Phys. Lett. 78, 1198 (2001).
[5] D.G. Zhao, S.J. Xu, M.H. Xie, S.Y. Tong, H. Yang, Appl. Phys. Lett. 83, 677
(2003).
[6] T. Egawa, H. Ohmura, H. Ishikawa, T. Jimbo, Appl. Phys. Lett. 81, 292 (2002).
[7] A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, H. Zhang, P. Kung, M. Razeghi,
S.K. Lee, J.Y. Han, Appl. Phys. Lett. 81, 2151 (2002).
[8] E. Kuokstis, J.W. Yang, G. Simin, M.A. Khan, R. Gaska, M.S. Shur, Appl. Phys.
Lett. 80, 977 (2002).
[9] K. Omae, Y. Kawakami, S. Fujita, Y. Narukawa, T. Mukai, Phys. Rev. B 68,
085303 (2003).
[10] S. Miasojedovas, S. Jurˇs˙enas, G. Kurilˇcik, A. ˇ
Zukauskas, Y.-C. Cheng, T.-Y. Tang,
C.C. Yang, C.-T. Kuo, J.-S. Tsang, Phys. Status Solidi C 0, 2610 (2003).
