CHIN. PHYS. LETT. Vol. 26, No. 8 (2009) 087802
Indium-Induced Effect on Polarized Electroluminescence from InGaN/GaN
MQWs Light Emitting Diodes *
RUAN Jun(阮军)1,2 , YU Tong-Jun(于彤军)1** , JIA Chuan-Yu(贾传宇)1 , TAO Ren-Chun(陶仁春)1 ,
WANG Zhan-Guo(王占国)2 , ZHANG Guo-Yi(张国义)1
1
State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871
2
Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 100083
(Received 8 April 2009)
Polarization-resolved edge-emitting electroluminescence (EL) studies of InGaN/GaN MQWs of wavelengths from
near-UV (390 nm) to blue (468 nm) light-emitting diodes (LEDs) are performed. Although the TE mode is
dominant in all the samples of InGaN/GaN MQW LEDs, an obvious difference of light polarization properties
is found in the InGaN/GaN MQW LEDs with different wavelengths. The polarization degree decreases from
52.4% to 26.9% when light wavelength increases. Analyses of band structures of InGaN/GaN quantum wells
and luminescence properties of quantum dots imply that quantum-dot-like behavior is the dominant reason for
the low luminescence polarization degree of blue LEDs, and the high luminescence polarization degree of UV
LEDs mainly comes from QW confinement and the strain effect. Therefore, indium induced carrier confinement
(quantum-dot-like behavior) might play a major role in the polarization degree change of InGaN/GaN MQW
LEDs from near violet to blue.
PACS: 78. 60. Fi, 78. 66. Fd
III-nitrides are promising materials for optoelectronic applications in ultraviolet and visible
ranges. InGaN/GaN multiple quantum well structures (MQWS) are the most commonly used structures as active layers of these optical devices. Great
efforts have been taken to improvement of the luminescence efficiency of LEDs. The indium induced carrier confinement (called quantum-dot-like behavior or
QD-like behavior) in InGaN/GaN MQWs grown by
MOCVD and the piezoelectricity-induced quantumconfined Stark effect (PQCSE) have been suggested
to be the main issues which influence internal quantum efficiencies.[1−9] Many researchers have reported
that the indium fluctuation gives rise to QD-like states
in active regions, which is highly beneficial to obtain high external quantum efficiency LEDs.[1−5] This
has motivated intensive investigations of QD behaviors and strain effect on the optical properties of InGaN/GaN MQWs. However, few reports have referred to the spontaneous luminescence polarization
of InGaN/GaN MQW LEDs. Recently, an intrinsic property of polarized selective emission has also
been suggested to influence the quantum efficiencies
of LEDs.[10] The biaxial strain effect on the modification of the valence band of GaN alloys has been
studied theoretically,[11] and artificial control of polarized optical gain by strain in GaAs-based QDs has
been reported.[12] The effects of indium-induced carrier confinement and strain on spontaneous luminescence polarization in InGaN/GaN MQWs are important but still lack understanding.
In this study, InGaN/GaN MQWs light-emitting
diodes (LEDs) of the wavelengths from 390 nm to
468 nm are fabricated. Edge-emitted electroluminescence (EL) is observed at room temperature and polarization degrees are measured for LEDs at different
wavelengths. The valence band structures and the optical gains of TE and TM emissions in InGaN/GaN
quantum wells are calculated based on the 𝐾 · 𝑃
perturbation approach. With analyses of theoretical results of transition rules and experimental results
of polarized electroluminescence from quantum wells,
indium-induced influence on spontaneous polarized luminescence processes is discussed.
All the samples of InGaN/GaN MQW LEDs in our
experiments were grown on (0002) oriented sapphire
substrates, with low-pressure metal-organic chemical vapor deposition (LP- MOCVD). The structures
of the LED wafers consisted of Si-doped GaN, InGaN/GaN (3 nm/10 nm) MQWs, an AlGaN electron
blocking layer, and Mg-doped GaN. Six wafers with
different indium compositions (from 4% to 18%) in
InGaN/GaN MQWs were used to fabricate the LED
chips. The chip size was 300 µm ×300 µm, manufactured by a standard LED process and mounted on
a heat sink. The emitting light peak wavelengths of
LEDs were from 390 nm to 468 nm.
The edge-emitted EL spectra were measured by a
system including a rotational stage, a Glan–Thomson
prism and a monochromator at room temperature.
Among three coordinates of 𝑥, 𝑦 and 𝑧, the 𝑧 axis
was along [0001] (𝑐 axis), 𝑥 and 𝑦 were in the plane
of InGaN/GaN MQWs. The light was emitted from
the edge orientated [11̄00] of the sample and passed
* Supported by the National Natural Science Foundation of China under Grant No 60676032, and the National Key Basic
Research Special Foundation of China under Grant No TG2007CB307004.
** To whom correspondence should be addressed. Email: tongjun@pku.edu.cn
c 2009 Chinese Physical Society and IOP Publishing Ltd
○
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CHIN. PHYS. LETT. Vol. 26, No. 8 (2009) 087802
through a pinhole in front of a Glan–Thomson prism
and a monochromator, and finally was received by
a detector. The prism could be rotated with respect to the 𝑦 axis for resolving the lights of the
TE (𝐸‖𝑋) and TM (𝐸‖𝑋) polarizations. An unpolarized light source was applied for calibrating the
measurement results of polarization dependence of the
equipment. The degree of polarization is defined as
𝑃 = (𝑟 − 1)/(𝑟 + 1), where 𝑟 = 𝐼TE /𝐼TM .
Figures 1(a) and 1(b) show the polarized edgeemitting electroluminescence spectra of near-UV
(395 nm) and blue (458 nm) LEDs. Although TE polarized emissions were dominant in all our samples
of InGaN/GaN MQW LEDs, an obvious difference
of light polarization properties was found in the InGaN/GaN MQW LEDs with different wavelengths.
As shown in Fig. 2, the polarization degree decreases
when light wavelength increases in the range from
395 nm to 468 nm. The extremely near-UV LED of
395 nm exhibits a ratio of 3.2 between the EL integrated intensities with polarizations 𝐸⊥𝐶 (TE) and
𝐸‖𝐶 (TM), corresponding to a high EL polarization
degree up to 52.4%, while the blue LED of 468 nm has
a small ratio of 𝑟 = 1.7 and 𝑝 = 26.9%, respectively.
The polarization degrees of both near-UV LEDs and
blue LEDs are almost fixed when injection currents are
changed within low current injection levels. A small
energy peak position shift is also observed between the
TE and TM emissions.
as HH (heavy hole), LH (light hole), and CH (crystalfield split-off hole), respectively. In the wurtzite crystals, the polarization selection rules of the space group
4
𝐶6𝑣
permit C-HH and C-LH transitions which are TE
polarized at 𝑘 = 0. C-CH transition is permitted for
TM polarized light.[13] In the quantum well structure,
the valence band splits into subbands, and the optical
transition processes should be modified by the valence
band mixing effect.[14] It is considered that the valence
band wavefunctions’ symmetries are very important
for the polarization of transitions and the obvious difference between the wavefunctions at 𝑘𝑡 ̸= 0 and those
at 𝑘𝑡 = 0 should be included for more precise description of transition selection rules.[15]
Fig. 2. Polarization degree as a function of EL peak wavelength of InGaN/GaN LEDs.
Fig. 1.
Edge-emitting EL signals from (a) near-UV
(395 nm) and (b) blue (455 nm) LEDs at 20 mA.
The luminescence processes happen according to
the transition selection rules. In bulk GaN, the three
valence bands from top to bottom are usually labeled
corresponding with their zone-center wavefunctions,
We calculated the band structure and wavefunctions of InGaN/GaN QW in the vicinity of Γ point
(𝑘𝑡 = 0–0.2 Å−1 ) with a 6 × 6 Hamiltonian for a 3nm InGaN well layer and GaN barrier based on the
𝐾 · 𝑃 perturbation approach and finite difference approximation. The 6 × 6 effective-mass Hamiltonian
for (0001) WZ crystals was derived and diagonalized by Chuang and Chang.[16] The strain and straininduced piezoelectric field were included in our model.
The material parameters used for GaN and InN were
taken from Ref. [17]. The linear interpolation formula
is used for most of the material parameters for the
ternary compound In𝑥 Ga1−𝑥 N except for the band
gap. The momentum matrix element 𝑀𝑛𝑚 for transitions between the 𝑛th conduction subband state 𝜓𝑛𝐶
𝑉
and the 𝑚th valence subband state 𝜓𝑚
is evaluated
𝑉
𝐶
from (𝜓𝑚 /𝑝/𝜓𝑛 ), where 𝑝 is the momentum operator. Then, the momentum matrix elements for TE
and TM polarizations can be obtained by applying
polarization vectors 𝑒ˆ = 𝑥
ˆ and 𝑒ˆ = 𝑧ˆ to 𝑀𝑛𝑚 , as
𝑥
ˆ · 𝑀𝑛𝑚 = 𝑦ˆ · 𝑀𝑛𝑚 for TE mode and 𝑧ˆ · 𝑀𝑛𝑚 for TM
mode, respectively. The polarized
spontaneous
opti⃒2
∑︀ ∑︀ ∑︀ ⃒
𝑣
cal gain 𝐺𝑒𝑆𝑃 = 𝐶 · 𝑛 𝑚 𝑖 ⃒𝑒ˆ · 𝑀𝑛𝑚 ⃒ 𝑓𝑛𝑐 [1 − 𝑓𝑚
]·
𝑘𝑡 · ∆𝑘𝑡 can be evaluated for TE polarization and TM
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CHIN. PHYS. LETT. Vol. 26, No. 8 (2009) 087802
polarization:[15]
𝐺TE
SP
∑︁ ∑︁ ∑︁ ⃒⃒ ∫︁
1
⃒ 𝑔𝑚
(𝑧) · 𝜙𝑛 (𝑧)𝑑𝑧
=𝐶 ·
⃒
𝑛
𝑚
𝑖
· (⟨𝑆, 𝜂|𝑃𝑥 |1⟩)
⃒
∫︁
(︀
)︀⃒2
2
+ 𝑔𝑚 (𝑧) · 𝜙𝑛 (𝑧)𝑑𝑧 ⟨𝑆, 𝜂|𝑃𝑥 |2⟩ ⃒⃒
𝐺TM
SP
𝑣
· 𝑓𝑛𝑐 [1 − 𝑓𝑚
]𝑘𝑡 ∆𝑘𝑡 ,
⃒
∑︁ ∑︁ ∑︁ ⃒ ∫︁
3
⃒ 𝑔𝑚
(𝑧) · 𝜙𝑛 (𝑧)𝑑𝑧
=𝐶 ·
⃒
𝑛
𝑚
𝑖
⃒
)︀⃒2 𝑐
𝑣
]𝑘𝑡 ∆𝑘𝑡 ,
· ⟨𝑆, 𝜂|𝑃𝑧 |3⟩ ⃒⃒ 𝑓𝑛 [1 − 𝑓𝑚
(︀
(1)
𝑣
where 𝑓𝑛𝑐 (𝑘𝑥 , 𝑘𝑦 ) and 𝑓𝑚
(𝑘𝑥 , 𝑘𝑦 ) are the Femi–Dirac
distributions for electrons in the conduction and valence subbands, 𝑘𝑡 is the transverse wavevector, and
𝑗
𝑔𝑚
(𝑗 = 1, 2, 3) are the envelope functions of the 𝑚th
valence subbands and correspond to the relative oscillator strength (ROS) of bases |1⟩|2⟩ and |3⟩. The
|1⟩ and |2⟩ bases only have |𝑋⟩ and |𝑌 ⟩ components,
while the |3⟩ base only has a |𝑍⟩ component.
and CH valence subbands.[15] The TM transition
rate decreases because the symmetry properties of
the valence wavefunction change, so that the relative
strength of the Z-like state in LH decreases when the
compressive strain becomes larger. The strain effects
on properties of valence band wavefunctions and their
relative oscillator strengths (ROS) in InGaN/GaN
QW are similar to those in wurtzite GaN [18] and
InGaN.[19] However, the strain in InGaN well layers
should be larger when indium composition is greater.
It is clear that the decrease of polarization degree in
high indium content InGaN/GaN QWs in our experiments could not be a result of the increase of strain.
It has been noticed that the indium inhomogeneity
in InGaN/GaN QWs has been proved to contribute
much to the high luminescence efficiency, although
some researchers have shown that the spatial fluctuations of InGaN well layers are also important for
the luminescence processes.[20] The indium inhomogeneity in InGaN well layers was also observed in our
samples. In Fig. 4, transmission electron microscopy
(TEM) images show an obvious difference between InGaN/GaN MQWs with indium compositions around
4% and 14%. A uniform InGaN well layer could be
seen in near-UV MQWs (indium 4%, Fig. 4(a)), however, a laterally inhomogeneous InGaN well layer is
obviously shown in the blue MQWs (indium 14%,
Fig. 4(b)).
(a) InGaN/GaN MQWs
In 4%
20 nm
(b)
InGaN/GaN MQWs
In 14%
20 nm
Fig. 4. TEM images of (a) In0.04 Ga0.96 N/GaN and (b)
In0.14 Ga0.86 N/GaN MQWs.
Fig. 3. Optical gain spectra of (a) In0.02 Ga0.96 N/GaN
MQWs and (b) In0.15 Ga0.85 N/GaN MQWs.
The calculated optical gain spectra of
In0.02 Ga0.98 N/GaN and In0.15 Ga0.85 N/GaN quantum
wells are shown in Figs. 3(a) and 3(b), respectively.
It is found that TE polarization is dominant in the
samples with different indium compositions, but transition rates of TM emissions are different, compared
to the TE transition rates in InGaN/GaN QW. TM
emission is weaker in In0.15 Ga0.85 N/GaN QW than
In0.02 Ga0.98 N/GaN QW. This means that the light
emission polarization degree is greater in quantum
wells with larger indium contents. The emissions of
TM polarization are considered to be related to transitions from the conduction band to LH (𝑘𝑡 ̸= 0)
Such indium lateral inhomogeneity is considered
to be related to the decrease of emission polarization. Over the one-dimensional confinement of quantum wells, laterally inhomogeneous indium induces a
local three-dimensional confinement of carriers, which
is called QD-like behavior. QD-like behavior may be
presented as an ultimate limit of size quantization and
results in the strong possible modification of electronic
properties as compared to quantum wells. Therefore, both QW and QD luminescence processes in
InGaN-based quantum well structures should be considered. The indium inhomogeneity influences the luminescence process in InGaN/GaN QWs, not only its
intensity but the polarization properties as well. Theoretical research results of GaN-based hexagonal QDs
reveals that the HH and LH practically do not split
inside the QDs, although a small splitting is found
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CHIN. PHYS. LETT. Vol. 26, No. 8 (2009) 087802
outside.[21] The hole wavefunctions of QDs become extended in the direction of growth (0001) as compared
with that of QW. The optical selection rules of QD
describe that the C-HH transition will mostly occur
as the light is TE polarized, but the C-LH transition
involves the TM component.[12] Due to the degeneracy of the HH and LH bands, the QD luminescence
should present a low degree of polarization.[22,23]
According to the above discussion on polarized luminescence, it seems that the QW luminescence mode
is dominant in the case of near-UV LEDs of small indium component in the quantum well, leading to a
high degree of polarization up to 52.4%. However,
for blue LEDs, due to the obvious lateral inhomogeneity in InGaN well layers, QD-like behavior is enhanced, giving rise to an increasing 𝐸‖𝐶 EL signal
of low polarization degree such as 26.9%. Therefore,
the indium-induced effect on luminescence polarization might play a major role in polarization degree
changes of InGaN/GaN MQW LEDs of wavelengths
from near UV to blue. Further research on selective
transitions of GaN-based QD should be very important for polarization control of light-emitting devices.
In summary, polarized edge emitting EL of InGaN/GaN MQWs operating in the wavelength ranging from 395 nm to 468 nm has been performed. An
obvious decrease of the polarization degrees from
52.4% to 26.9% is observed when luminescence wavelength increases from 395 nm to 468 nm. A mixed effect of compressive strain and QD behavior has been
suggested to be responsible for the observed results.
Compressive strain in the InGaN/GaN MQWs favors
TE polarization, whereas indium-induced QD behavior gives rise to an increasing TM component. Analysis of band structures of InGaN/GaN quantum wells
and quantum dots imply that quantum-dot-like behavior is the dominant reason for the low luminescence
polarization degree of blue LEDs. Therefore, it could
be concluded that the indium-induced effect may play
a major role in the variation of luminescence polar-
ization degree of InGaN/GaN MQWs from violet to
blue.
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