ARTICLE IN PRESS
Journal of Crystal Growth 272 (2004) 333–340
www.elsevier.com/locate/jcrysgro
Dual wavelength InGaN/GaN multi-quantum well LEDs
grown by metalorganic vapor phase epitaxy
Y.D. Qi, H. Liang, W. Tang, Z.D. Lu, Kei May Lau
Photonics Tech. Center, Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong
Abstract
InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with different well widths and barrier
widths were grown on sapphire substrates using metalorganic vapor phase epitaxy (MOVPE). The designed emission
wavelengths were in the blue and green regions. The blue MQWs and green MQWs were deposited sequentially and the
growth parameters were separately optimized for single color blue and green emission LEDs. Room-temperature
photoluminescence (PL) spectra of the dual wavelength samples under high-intensity laser excitation showed dual
emission peaks. Electroluminescence (EL) of fabricated LEDs at the typical driving current of 20 mA was
predominantly single color, depending on the sequence and the number of blue and green MQWs. The second
emission could be enhanced with larger driving current and the relative intensity of the two emission peaks could thus
be tuned. Total optical power of more than 7 mW was obtained from an unpackaged 300 mm die when over driven.
r 2004 Elsevier B.V. All rights reserved.
PACS: 81.15.Gh; 85.60.Jb; 78.60.Fi; 71.55.Eq
Keywords: A3. Metalorganic vapor phase epitaxy; A3. Quantum wells; B1. Nitrides; B3. Light emitting diodes
1. Introduction
With major breakthroughs in material growth
and device fabrication technologies in the past
decade, light-emitting diodes (LEDs) and their
applications are undergoing revolutionary
Corresponding author. Tel.: +852-235-87049; fax: +852-
2358-1485.
E-mail address: eekmlau@ust.hk (K.M. Lau).
changes. The emergence of short wavelengths
violet, blue, and green LEDs using aluminum
indium gallium nitride (AlInGaN) [1], and long
wavelength yellow–red LEDs made of aluminum
indium gallium phosphide (AlInGaP) [2] made
outdoor full color displays and white light
illumination possible. One of the most important
light-emitting active layers, InGaN/GaN single or
multiple quantum wells, draw special attention
because of its unique material characteristics and
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2004.08.097
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Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
334
device performance. Despite the large density of
defects, primarily threading dislocation that exist
in the InGaN/GaN multiple quantum well
(MQW) region, the radiative recombination efficiency is surprisingly high and LEDs can achieve
an external efficiency as high as 20% [3–8].
As an alternative to the phosphor based white
LEDs in which single or tri-color phosphors are
pumped by InGaN/GaN MQW blue or ultraviolet
LED to generate white light [1,9], prototype
phosphor-free monolithic white LEDs have recently been proposed and reported. Basically, this
type of LEDs consist of a stack of InGaN/GaN
quantum wells (QWs) with designated emission
wavelength in different color regions. The two
types of QWs with different emitting colors are
either grown sequentially [10,11], or separated by a
tunnel junction [12,13]. Different emission colors
come from the InGaN/GaN QWs with different
well and barrier widths, and different indium
compositions. White light is expected from the
mixing of different generated colors. However, the
mixed emission from QWs of different sizes is
different from that of single emission wavelength
QWs and can be quite complex. In this work, we
designed and fabricated LEDs with dual emission
wavelengths in the blue and green regions that are
tunable by the drive current. Furthermore, we
investigated the mixed emission issue by studying
the growth and luminescence of various InGaN/
GaN MQWs with dual emission wavelength.
2. Experiments
All the LED samples, in this study, were grown
by MOVPE on c-plane sapphire substrates in an
Aixtron 2000HT system. First a 250 Å GaN
nucleation layer was deposited at 550 1C after the
substrate was annealed in an H2 environment at
1200 1C for 10 min. Then the temperature was
elevated to 1190 1C to grow a 2 mm Si-doped GaN
contact layer. The active region consisted of four
or five periods of InGaN/GaN QWs with emission
wavelengths in the blue and green regions. The
blue QWs and green QWs were deposited sequentially and the growth parameters were separately
optimized for single color blue and green LEDs.
For the blue QWs, the well and barrier were grown
at a constant temperature of 780 1C. For the green
QWs, the well was grown at 760 1C while the
barrier was grown at 950 1C. A higher growth
temperature is preferred to grow the GaN barrier
of the green QWs in order to increase the residual
strain and suppress the defect formation in the
barrier layer. The topmost layer is a 0.1 mm Mgdoped GaN contact layer grown at 1010 1C. Lower
p-type GaN layer growth temperature was employed to prevent the formation of metallic indium
inclusions by the decomposition of high indium
percentage InGaN layer in the green QWs. Before
processing, the p-GaN contact layer was activated
in a rapid thermal annealing system at 950 1C for
about 30 s. The sample was then partially etched
until the n-GaN layer was exposed to form a
300 300 mm2 mesa. Optimized Al/Ti and Au/Ni
metallization layers were used, respectively, for
contacts on the n- and p-GaN layers. After ohmic
contact alloying, 2500 Å Au electrodes were
deposited for wire bonding.
Two batches, including four LED structures
were grown with the differences only in the
number and the sequence of blue and green
QWs. Table 1 lists the QW structures and the
Table 1
QW structure and the estimated indium composition, well and barrier thickness of four dual emission wavelength LED samples
Batch
1
2
Sample
A
B
C
D
QW structure (bottom+top)
3
3
3
3
green+1 blue
blue+1 green
green+2 blue
blue+2 green
Blue QW
Green QW
Well (Å)
Barrier (Å)
In. comp. (%)
Well (Å)
Barrier (Å)
In. comp. (%)
22
22
23
23
95
95
70
70
20
20
16
16
25
25
28
28
125
125
160
160
24
24
22
22
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Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
estimated indium composition, well and barrier
thickness of the four LEDs. LED A has one blue
QW above three green QWs, LED B has one green
QW above three blue QWs, LED C has two blue
QWs above three green QWs, and LED D has two
green QWs above three blue QWs. The separation
between the blue QW and the green QW is onethird the thickness of regular GaN barrier grown
at 780 1C and one-third the thickness of GaN
barrier grown during the temperature ramping to
950 1C.
3. Results and discussion
Fig. 1 is the high resolution X-ray diffraction
(HRXRD) o 2y scans for the (0 0 0 2) reflection
from two regular single color emission LEDs (left)
and four dual wavelength emission LEDs (right)
measured by a Philips MRD system. The dominant peak at the angle of 17.281 comes from the
diffraction of the thick n-type GaN layer under the
MQWs while the satellite peaks come from the
diffraction of the InGaN/GaN multi-layer structure. As shown in the left graph of Fig. 1, for single
color emission InGaN/GaN MQWs with identical
indium composition, well and barrier thickness for
all superlattice repeats, the spacing between the
two nearby satellite peaks are the same and their
shapes look similar. The satellite peak separation
is determined by the QW superlattice period, and
the position of the satellite peaks relative to the
main n-GaN diffraction peak is determined by the
indium composition. For the four dual- wavelength LED samples in this study, composed of
two types of InGaN/GaN QWs with different
superlattice periods and different indium compositions, the satellite peaks are averages of the
diffraction from two types of superlattice, as
shown in the right graph of Fig. 1. Because green
QWs usually have a larger period than that of blue
QWs, the satellite peak spacing is smaller and the
number of the satellite peaks that appear within a
certain scanning range is larger. Therefore, for
samples B and D, in which one or two green QWs
are grown on top of three blue QWs, the
perturbation from the diffraction of the green
QW on the sparser diffraction peaks from the blue
QWs made the final diffraction curves flatter. The
fact that high-order satellites are still discernable in
a 31 scan range shows good InGaN/GaN interfaces. XRD simulation based on the Philips X0 pert
Epitaxy and Smoothfit hardly yields reasonable
result because of the complexity of the diffraction
pattern. The estimated indium composition, well
and barrier thickness, as listed in Table 1, are
inferred from calibration growths before each
growth batch, assuming the QW growth rate does
0
2
0
-4
16.0
-3
-2
1
2
Green MQWs
Intensity (a.u.)
Intensity (a.u.)
Blue MQWs
-2
-5
B
C
3
16.5
17.0
A: 3G+1B (top)
B: 3B+1G (top)
C: 3G+2B (top)
D: 3B+2G (top)
A
1
-1
-1
335
17.5
ω-2θ
18.0
D
18.5
16.0
16.5
17.0
17.5
ω-2θ
18.0
18.5
Fig. 1. HRXRD o 2y scans for the (0 0 0 2) reflection from the single color emission blue and green LEDs (left), and from LEDs
consisting of both blue QWs and green QWs (right). The thicker curves in the spectra of single color emission blue and green LEDs are
the XRD simulation.
ARTICLE IN PRESS
Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
not change much for sequential runs, which is
usually the case.
Time-of-flight secondary ion mass spectrometry
(TOF-SIMS) was used to examine the indium
composition inside the MQWs. Fig. 2 is the depth
profile down to 400 nm beneath the sample surface
for a typical InGaN/GaN MQW LED with two
blue QWs on top of three green QWs. Five indium
count maxima indicate that there are five InGaN
wells in the structure. In addition, the bottom
three wells have higher indium counts and larger
spacing, which means higher indium incorporation
and larger period. This is exactly what we expected
because the green QW was designed to have higher
indium composition and larger period than the
blue QW. From the indium distribution inside the
MQWs, we also found that indium count oscillates
with a larger amplitude in the green QWs than in
the blue QWs, which is attributed to the sharper
interface between GaN barrier and InGaN well in
the green QWs. The higher barrier growth
temperature in the green QWs enhances the
InGaN/GaN interfacial quality. Magnesium
(Mg) and gallium (Ga) distributions are also
shown in Fig. 2. On top of the MQW active
region is a 0.1 mm Mg-doped GaN contact layer.
Room temperature photoluminescence (PL)
characteristics of the four LED samples are shown
in Fig. 3. The excitation source is a 266 nm YAG
pulse laser using the forth harmonic generator.
Count (a.u.)
100000
10000
Ga
1000
In
Mg
100
10
1
0
50
100
150
200
250
300
350
400
Depth (nm)
Fig. 2. TOF-SIMS depth profile for a typical InGaN/GaN
MQW LED with two blue QWs on top of three green QWs.
Clearly the superlattice period and indium incorporation are
different between the blue and the green QWs.
A: 3G+1B(top)
PL Intensity (a.u.)
336
B: 3B+1G(top)
C: 3G+2B(top)
D: 3B+2G(top)
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 3. Room temperature PL spectra of dual emission
wavelength InGaN/GaN MQW LEDs. The fact that blue and
green luminescence peaks are discernable indicates good quality
for both blue QW and green QW.
The repetition rate of the pulse laser is 30 Hz and
in each 5 ns pulse duration the laser energy is
about 400 mJ. Considering the laser beam size, the
average excitation intensity is calculated to be
around 0.015 W/cm2. Under this laser excitation
density, light emissions from both blue QWs and
green QWs were observed for all LED samples.
Samples A and B (batch 1) have PL emission
wavelengths around 465 nm (blue) and 510 nm
(green), while samples C and D (batch 2) have PL
emission wavelengths around 450 nm (blue) and
510 nm (green). Because samples C and D have a
two/three structure (two blue/green on top and
three green/blue in the bottom), the PL emission
from the three bottom QWs is a little weaker due
to the laser absorption by the two QWs at the top.
In addition, in sample D the blue PL emission
from the three blue QWs at the bottom is the
weakest because it is partly reabsorbed by the two
top green QWs with smaller bandgap. In samples
A and B, there is only one top QW so the
absorption of the laser light or the light emitted
from the three QWs underneath is less. Thus, the
blue and green PL emissions have similar intensity.
The fact that both blue and green luminescence
could be detected with reasonable intensity under
the same laser excitation reaffirms the good QW
quality for all LED samples. Febry–Perot effect in
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Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
the room temperature PL spectra was observed for
all samples. However it is more significant in the
PL spectra of samples A and B, where there are
distinct multiple peaks, than in the PL spectra of
samples C and D, where there are only small
shoulders on the main emission peak. This is
attributed to the different roughness of the sample
surface between the two sample batches. Samples
A and B have smoother surface than samples C
and D because the latter have more QW layers.
Good Febry–Perot cavity cannot be formed for
optical interference because additional InGaN/
GaN layers will slightly deteriorate the surface
morphology of the final layer. It is not likely that
different surface roughness between the two
growth batches is caused by the drift of the growth
conditions because later-grown four and five QW
LED samples show similar PL features too.
Fig. 4 shows the room temperature electroluminescence (EL) spectra of the four LEDs with
different arrangements of blue and green QWs, at
20 mA drive current. In contrast to the two distinct
emission wavelengths in the PL measurement, the
EL characteristics at this typical driving current
range are significantly different. The EL is
predominantly single color, and which emission
wavelength is dominant depends on the number
and the sequence of the blue and the green MQWs.
For samples A and B (batch 1), the EL emission
EL Intensity(a.u.)
B: 3B+1G(top)
A: 3G+1B(top)
C: 3G+2B(top)
D: 3B+2G(top)
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 4. EL spectra of LEDs consisting both blue and green
QWs driven at 20 mA. The emission is predominantly single
color. The underline in the graph legends indicates the
dominant emission color.
337
from the bottom three QWs is the dominant.
When adding one more QW in addition to the
existing single top QW, the emission is one way
around. For samples C and D (batch 2), the EL
emission from the top two QWs dominates. There
is competition between the blue and the green
QWs in thermalization of the injected carriers
from the barriers into the well for radiative
recombinations. The physical mechanisms controlling injected carrier distribution between QWs
with different indium composition, different well
and barrier thickness has not been quantitatively
determined. What we observed can be explained
qualitatively as follows. Because holes have
smaller mobility than electrons, electrons travel
more easily across the MQW regions, so it is the
transport of holes that determines the radiative
recombinations. Since the thickness of the barrier
between the two types of QW is thinner than the
normal barrier, if there is only one QW (either
green or blue) at the top, the holes will have higher
probability to diffuse or tunnel into the next QW
(blue or green) where they recombine radiatively
with electrons of higher density. So the emission
from the bottom three QWs will be dominant. If
we have two QWs on the top, most holes will stay
in these two wells because of the short mean free
path. So the emission from the top two QWs will
be dominant. Without thinning of the sapphire
substrate and special heat sinking in performing
the measurements, at the typical test current of
20 mA, the on-wafer measured output power is
0.8, 1.1, 1.7 and 0.9 mW for samples A (green
dominant), B (blue dominant), C (blue dominant)
and D (green dominant), respectively. The power
output of these four LEDs is consistent with our
typical results around 1.5 and 1.0 mW for five
MQW single color blue and green LEDs. In the
EL spectrum of sample B, the small shoulder in the
green region is likely a result of light pumping on
the top single green QW by the dominant blue
emission from the three blue QWs at the bottom.
The pumping effect in sample B is more significant
at larger driving current, as shown in Fig. 5. For
all four LED samples, the power output saturates
when the driving current is around 200 mA
because of the lack of quick heat dissipation in
the EL measurement setup. In addition, the second
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Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
338
100
140
3G+1B(top)
EL Intensity (a.u.)
EL Intensity (a.u.)
60
50 mA
40
100 mA
200 mA
250 mA
20
3B+1G(top)
120
80
100
80
50 mA
100 mA
60
200 mA
40
250 mA
20
0
350
400
(A)
450
500
550
600
0
350
650
550
600
650
100
EL Intensity (a.u.)
EL Intensity (a.u.)
500
3B+2G(top)
3G+2B(top)
300
250
200
150
50 mA
100 mA
100
200 mA
400
450
500
550
80
60
50 mA
100 mA
40
200 mA
250 mA
20
250 mA
50
(C)
450
Wavelength (nm)
120
350
0
350
400
(B)
Wavelength (nm)
600
0
350
650
Wavelength (nm)
(D)
400
450
500
550
600
650
Wavelength (nm)
Fig. 5. EL spectra under large continuous current driving of dual emission wavelength LEDs. Only LEDs consisting one green QW on
top of three blue QW has a strong second emission peak. The underline in the legends indicates the dominant emission color at 20 mA
forward current.
emission peak becomes more distinct under large
forward driving for samples B and C. For sample
B, the absorption of the emitted blue light from the
bottom wells by the top single green QW is very
large and the reemitted green light will eventually
overshadow the blue light when driven very hard.
For sample C, under large driving current the
chance of three green QWs at the bottom catching
carriers increases because there are huge amounts
of injected carriers in the active region. Even at
large forward current, there is no distinct second
emission peak in the EL emission of samples A
and D. For sample A, even though there could be
weak emission from the single blue QW at the top
at large current, it will be covered by the strong
emission from the three green QWs at the bottom
because the green emission has a very large
linewidth. For sample D, at large driving current,
the emission from the three blue QWs at the
bottom will be absorbed by the two green QWs at
the top, which will make the blue emission even
more difficult to distinguish from the broad
dominant green emission peak.
The thermal effect can be seen after thinning of
one of the samples. After sample B was thinned to
80 mm and diced into 300 300 mm2 dies, the dual
emission effect became more prominent, as shown
in Fig. 6 (left). The emission wavelength can be
tuned from the blue color region into the green
color region by increasing the driving current to
100 mA and higher. Although the maximum
power output before and after wafer lapping and
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Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
8
80
110mA
B: 3B+1G(top)
70
120mA
After lapping
and dicing
60
50
40
7
Power Output (mW)
EL Intensity (a.u)
90
130mA
140mA
150mA
200mA
30
20
6
5
4
3
B:3B+1G(top)
After lapping and dicing
Before lapping and dicing
2
1
10
0
350
339
400
450
500
550
Wavelength (nm)
600
650
0
0
50
100
150
200
250
Driving current
Fig. 6. After the sample thinning and dicing, the dual emission effect become more prominent for LED B. The emission wavelength
can be tuned from blue color region into the green color region by increasing the driving current to 100 mA and higher (left), and the
maximum power output is around 7 mW (right).
device dicing is both around 7 mW, the required
driving current after lapping and dicing is much
lower, as shown in Fig. 6 (right). If the LED is
driven even harder, the power output starts to
decrease because of too much heat generation.
the green color region by increasing the driving
current.
Acknowledgments
4. Summary
We studied the electrical and optical properties
of four dual emission wavelength InGaN/GaN
MQW LEDs. HRXRD, TOF-SIMS and room
temperature PL revealed good QW quality.
Although PL showed the designed dual emission
wavelengths in the blue and green regions, EL at
typical driving showed only one predominantly
emission depending on the sequence and the
number of the blue and the green QWs. We
believed the competition of the carrier capture
between the two types of QWs played the deciding
role in the EL emission mechanism. At larger
driving current, the LED composed of one green
QW above three blue QW showed the expected
second emission wavelength with reasonable intensity, which was attributed to the optical
pumping on the top single green QW. After wafer
thinning and device dicing, the emission wavelength is easier to tune from blue color region into
This work was supported by a Grant (ITS239/
00) from the Innovation & Technology Commission of Hong Kong and a CERG Grant
(HKUST6235/01E) from the Research Grant
Council of Hong Kong.
References
[1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer,
Berlin, 1997.
[2] F.A. Kish, F.M. Steranka, D.C. Defevere, D.A. Vanderwater, K.G. Park, C.P. Kuo, T.D. Osentowski, M.J.
Peanasky, J.G. Yu, R.M. Fletcher, D.A. Steigerwald,
M.G. Craford, V.M. Robbins, Appl. Phys. Lett. 64 (1994)
2839.
[3] T. Muka, S. Nagahama, T. Yanamoto, M. Sano, Phys.
Stat. Sol. (a) 192 (2002) 261.
[4] S. Chichibu, K. Wada, S. Nakamura, Appl. Phys. Lett. 71
(1997) 2346.
[5] M.G. Cheong, C. Liu, H.W. Choi, B.K. Lee, E.K. Suh,
H.J. Lee, J. App. Phys. 93 (2003) 4691.
[6] H.K. Cho, J.Y. Lee, C.S. Kim, G.M. Yang, N. Sharma, C.
Humphreys, J. Crystal Growth 231 (2001) 466.
ARTICLE IN PRESS
340
Y.D. Qi et al. / Journal of Crystal Growth 272 (2004) 333–340
[7] Y.C. Cheng, C.H. Tseng, C. Hsu, K.J. Ma, S.W. Feng,
E.C. Lin, C.C. Yang, J.I. Chyi, J. Electron. Mater. 32
(2003) 375.
[8] T.L. Song, S.J. Chua, E.A. Fitzgerald, P. Chen, S.
Tripathy, Appl. Phys. Lett. 83 (2003) 1545.
[9] U. Kaufmann, M. Kunzer, K. Kohler, H. Obloh, W.
Pletschen, P. Schlotter, R. Schmidt, A. Ellens,
W. Rossner, M. Kobush, Phys. Stat. Sol. (a) 188 (2001)
143.
[10] M. Yamada, Y. Narukawa, T. Mukai, Jpn. J. Appl. Phys.
41 (2002) L246.
[11] B. Damilano, N. Grandjean, C. Pernot, J. Massies, Jpn. J.
Appl. Phys. 40 (2001) L918.
[12] I. Ozden, E. Makarona, A.V. Nurmikko, T. Takeuchi, M.
Krames, Appl. Phys. Lett. 79 (2001) 2532.
[13] C.H. Chen, S.J. Chang, Y.K. Su, J.K. Sheu, J.F. Chen,
C.H. Kou, Y.C. Lin, IEEE Photon. Technol. Lett. 14
(2002) 908.