IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 10, OCTOBER 2010
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Boosting Green GaInN/GaN Light-Emitting Diode
Performance by a GaInN Underlying Layer
Yong Xia, Wenting Hou, Liang Zhao, Mingwei Zhu, Theeradetch Detchprohm, and Christian Wetzel
Abstract—The light output of 530 nm green GaInN/GaN
light-emitting diodes on sapphire has been nearly doubled by
the insertion of a 130-nm GaInN underlayer (UL) between the
n-GaN electron injection layer and the quantum-well (QW) active
region. Under variation of the alloy composition, best results were
obtained for an x = 6.3% Ga1−x Inx N UL. By low-temperature
depth-resolved cathodoluminescence spectroscopy, an interplay of
the impurity-related donor-acceptor pair recombination, the UL,
and the QW emission has been observed. We propose that the
resonance and level alignments between the defect and UL levels
reroute excitation toward radiative recombination in the QWs.
Index Terms—Cathodoluminescence (CL), GaN, light-emitting
diode (LED), underlayer (UL).
I. I NTRODUCTION
period superlattice reduces the strain in the QW stack [6], while
Törmä et al. proposed that, in 440 nm LEDs, a 20 nm, x = 2%
UL acts to inject electrons into the QWs at a lower energy
than a GaN barrier would [7]. Here, we successfully expand
the concept to green emitting LEDs and find an explanation
that combines the aspects of the previous models, yet reveals
the UL’s particular suitability to the longer wavelength region
of green LEDs. In the electroluminescence (EL) of 530 nm
LEDs, we find a relevant 85% enhancement of the light output power (LOP) as measured in fabricated LED dies. In
order to reveal the mechanism of efficiency enhancement, we
characterize the luminescence properties by a depth-resolved
cathodoluminescence (CL) spectroscopy and a low-temperature
photoluminescence (PL).
S
OLID-STATE lighting thrives on the efficient energy conversion in group-III nitride light-emitting diodes (LEDs)
[1], [2]. The benefit of a narrow-band emission has widely been
employed in colored display lighting; yet, for full-spectrum
white light generation, usually, a combination of blue LED
and yellow phosphor is employed. Green-emitting LEDs have
yet to live up to their theoretical potential as the dominant
component in the red–green–blue combination white LED light
sources. Here, we propose to boost the green LED efficiency
in Ga1−y Iny N/GaN multiple-quantum-well (MQW) LEDs by
implementing a Ga1−x Inx N thin-film underlayer (UL) between
the n-GaN electron injection layer and the MQW active region
[3], [4].
The concept has been used in blue Ga1−y Iny N/GaN LEDs
[5], yet, its use in green LEDs requires new approaches and
offers the opportunity to elucidate the processes at work. Various mechanisms have been proposed to explain its benefits.
Akasaka et al. proposed that, in 395-nm LEDs, a 50-nm
thick x = 4% UL separates the active region from any centers
responsible for nonradiative recombination [4]. Niu et al. proposed that, in 464-nm LEDs, a 25-nm x = 9% UL with a shortManuscript received May 17, 2010; accepted July 5, 2010. Date of publication August 12, 2010; date of current version September 22, 2010. This work
was supported in part by the Department of Energy/National Energy Technology Laboratory Solid-State Lighting Contract of Directed Research under Grant
DE-FC26-06NT42860 and in part by the National Science Foundation Smart
Lighting Engineering Research Center under Grant EEC-0812056. The review
of this paper was arranged by Editor L. Lunardi.
Y. Xia, W. Hou, L. Zhao, T. Detchprohm, and C. Wetzel are with the Future
Chips Constellation and the Department of Physics, Applied Physics, and
Astronomy, Rensselaer Polytechnic Institute, Troy, NY 12180 USA (e-mail:
wetzel@rpi.edu).
M. Zhu is with Applied Materials, Inc., Santa Clara, CA 95054 USA.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2010.2061233
II. E XPERIMENTAL
A set of four different Ga1−y Iny N/GaN MQW LED structures were grown on a c-plane sapphire in a vertical flow
metal–organic vapor phase expitaxy [8]. Trimethylgallium,
trimethylindium, and ammonia were used as gallium, indium,
and nitrogen precursors, respectively. Silane and biscycolpentadienyl magnesium were used as the n-type and p-type dopant
sources. The active region contains eight periods of QWs
(3 nm thick) and barriers (∼23 nm thick). A nominally undoped
130 nm thick Ga1−x Inx N UL was grown before the QWs.
Structures contain ULs of different compositions: xUL = 0%,
3.8%, 6.3%, and 8.8%, as determined by a high resolution
ω−2θ X-ray diffraction. Here, xUL = 0% is a shorthand for
omitting the UL. The respective sample serves as a reference
of a conventional sample structure. For the UL and the QW
growth, a particular effort was paid to avoid inhomogeneities
of alloy composition and thickness as detailed elsewhere [9],
[10]. The MQW structures are topped off with a 20 nm
p-AlGaN electron-blocking layer and a 200 nm p-GaN layer.
Si doping levels of 3 × 1018 cm−3 and Mg doping levels of
2−5 × 1019 cm−3 are typically achieved. To minimize process
fluctuations, the samples have been grown in consecutive epitaxy runs using a proven base recipe that delivered reproducible
results before and after the growth sequence. For spectroscopic
analysis, the CL, at variable low temperature, was collected in
a scanning electron microscope. For a depth-resolved analysis,
the acceleration voltage was swept from 3 to 25 kV. Using the
Kanaya–-Okayama relation [11] and the calibration runs, we
correlate this to a mean excitation depth of 0.1–1.0 μm. To
maintain a constant carrier generation rate, the beam power
was kept at 750 nW by reducing the beam current. For EL,
epi wafers have been processed to (700 μm)2 LED dies with
0018-9383/$26.00 © 2010 IEEE
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Fig. 1. Reciprocal space mapping in X-ray diffraction around the (10–15)
maximum of GaN for the three samples with different UL compositions xUL .
Horizontal maxima for the UL and the MQW are ideally matched with that of
GaN. No in-plane relaxation can be observed.
Fig. 2. (a) EL spectra at 100 mA peaking at 530 nm. (b) (Left axis) LOP
of the green LED dies at 20 and 100 mA as a function of InN fraction in the
Ga1−x Inx N UL. (Right axis) IQE at room temperature. Both reach a clear
maximum in the xUL = 6.3% sample.
a mesa area of (600 μm)2 . The LOP of the bare unencapsulated
die was measured by a power-calibrated spectrometer inside a
10-in integrating sphere. All data is representative for the 1-in
center portion of the 2-in wafer structures with a variation of
not more than 5%.
III. R ESULTS AND D ISCUSSION
Reciprocal space mapping around the (10–15) diffraction
maximum of GaN is shown in Fig. 1 for the samples with
xUL = 3.8%, 6.3%, and 8.8%. The horizontal axis corresponds
to the reciprocal a-lattice constant, while the vertical axis
corresponds to the reciprocal c-lattice constant. In all three
samples, the maximum corresponding to the GaInN UL is in
very close alignment with the reciprocal a-lattice constant of
the GaN maximum. This proves that the biaxial strain is not
relaxed within the GaInN layer to a degree more than 3%.
The EL spectra of all LED structures show a single peak
around 530 nm at a current of 100 mA [see Fig. 2(a)]. Peak
wavelengths are 534 nm (xUL = 0%), 536 nm (xUL = 3.8%),
529 nm (xUL = 6.3%), and 529 nm (xUL = 8.8%). Line
widths are 38 nm, 40 nm, 36 nm, and 42 nm full-width at halfmaximum (FWHM), respectively. Apparently, the insertion of
Fig. 3. Low-temperature CL spectra of all four LED structures with an
acceleration voltage for the maximum excitation in the UL. The UL peak shows
a strong variation with the InN fraction, and the DAP recombination seems to
interact with the UL emission.
the UL with different alloy compositions has no significant
effect on the peak wavelength and the line widths of the QWs.
This, furthermore, is an evidence that the UL does not substantially relax the strain. The presence and the composition of the
UL, however, has a strong impact on the LOP [see Fig. 2(b)]:
Both, for the 20 mA and 100 mA operations, the LOP increases
with xUL and shows a maximum for xUL = 6.3%, before it
drops in the xUL = 8.8% sample. The maximum is 4.9 mW
at 100 mA for xUL = 6.3%, while the reference sample, xUL =
0%, shows an LOP of 2.7 mW. This is a significant enhancement of 85% due to the insertion of the UL.
We estimate the upper limit of the internal quantum efficiency (IQE) by the ratio of the peak-integrated PL intensity
at room temperature and that at 4.2 K. At room temperature,
the PL is highest in the xUL = 3.8% sample when excited at
325 nm above the GaN band gap. When excited with 20 mW
at 405 nm directly in the active region, the xUL = 6.3% sample has the strongest PL. We choose the latter excitation to
avoid absorption in the p-layers. Possible resonance-enhanced
absorption is unlikely to affect the result due to the small energy
variation of the transition energies. Under the assumption of the
negligible non-radiative recombination at low temperature, an
IQE value at room temperature as high as 66% is found in the
xUL = 6.3% sample versus 32% without the UL [see Fig. 2(b)].
There is a high chance that the absolute measurements of
efficiency will reveal values that could be lower than those
upper limit values; their relative increase, however, is quite
meaningful. This trend in IQE closely follows that of the LOP
in the EL, revealing a maximum at xUL = 6.3%.
To elucidate the mechanism of efficiency enhancement, a
CL spectroscopy at 77 K and an acceleration voltage of
11 kV, equivalent to a predominant excitation in the UL,
of the four LED structures is shown in Fig. 3. Besides the
dominant QW emission (530 nm), a donor-acceptor-pair (DAP)
recombination (380 nm) with a phonon replica (390 nm) and
a near-band-edge emission (NBE) of GaN (363 nm) are seen
in the reference sample (xUL = 0%). At 370 nm, we identify the corresponding free electron-to-acceptor transition as
a shoulder to the DAP transition. Three samples show strong
emission at 388 nm (xUL = 3.8%), 410 nm (xUL = 6.3%), and
XIA et al.: BOOSTING GREEN GaInN/GaN LED PERFORMANCE BY A GaInN UNDERLYING LAYER
Fig. 4. (a) Peak energies of relevant transitions at 77 K as a function of
InN fraction in the UL. (b) The same energies separated into VBE and CBE
contributions. In both cases, near-crossing points are seen. (a) UL transition
energy crosses the phonon replica of the DAP transition. (b) UL VBE crosses
with the acceptor level.
435 nm (xUL = 8.8%), which we assign to the UL. All identified energies are summarized in Fig. 4, versus the InN-fraction
of the UL. While Fig. 4(a) lists the transition energies directly,
Fig. 4(b) separates them into the valence band edge (VBE)
and conduction-band edge (CBE) contributions, according to
the relative band offset between GaN and InN [13]. Included
are the effective mass donor (Don), assumed at 30 meV, and
the resulting acceptor (Acc) levels as concluded from the peak
positions. Also shown are the anticipated absorption edges (abs)
for electrons (e) and holes (h) based on the QW-emission (em)
peak and extrapolated data on the reported Stokes’ shift in
similar structures [13].
A peculiarity is seen for the sample with xUL = 3.8% [see
Figs. 3 and 4(a)]. Here, the pattern typical for the DAP recombination appears distorted by the UL emission. The 370-nm
shoulder of the free electron-to-acceptor transition can still be
seen, but the DAP transition expected at 380 nm is missing.
The UL peak appears at 388 nm, and the phonon replica at
400 nm appears to follow the UL recombination band instead
of the DAP transition. A possible reason is the energetic
near-coincidence of the DAP transition and the UL band-edge
transition.
Of even higher interest is the structure with xUL = 6.3%.
Fig. 5 shows the CL peak intensities as a function of
the effective excitation depth, together with the layers indicated as designed. We distinguish the GaN NBE emission
(362 nm), the UL emission (410 nm), the MQW emission
(532 nm), and the DAP transitions (380 nm). Starting from
the sample surface, the first intensity rise is seen in the QW
emission at a depth of 0.1 μm. At this depth, the excitation reaches the QWs. At ∼ 0.30 μm, the emission reaches a
maximum. This marks the center of the MQWs, which, per
design, should lie at 0.32 μm. The DAP transition peaks at
∼ 0.23 μm, which corresponds to the interface of the p-GaN
layer and the MQW region per design. At higher excitation
depth, the DAP transition drops in intensity, while the UL
emission rises to a maximum at 0.40 μm, corresponding to
the UL per design. At even larger depth, the GaN NBE rises,
and the DAP transition again follows the suit at similar rate.
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Fig. 5. Depth profiling of the CL peak intensities in the LED with x = 6.3%
UL at low temperature. The layers, as designed, are indicated. DAP transitions
are highest when the QW reaches its maximum. For larger depth, the QW
peak seems to follow the UL intensity, indicating a possible excitation-transfer
process.
The excitation here is predominant within the n-layers of the
structure. Notice that, while the DAP transition exhibits a clear
maximum at the depths of the p-GaN layer interface to the
MQW region, the QW emission does not drop off any faster
with depth than the UL emission.
This close correlation of the UL and QW emissions and the
anticorrelation of the UL and DAP emission intensities in the
xUL = 6.3% structure, as well as the energetic interaction of
the UL and DAP transitions in the xUL = 3.8% structure, can
be explained by an excitation–transfer mechanism between the
UL and the QWs. Apparently, whenever the UL is excited,
while it produces a strong CL, it also transfers excitation to
the QWs and enhances that recombination. On the other hand,
the GaN DAP luminescence is reduced whenever the excitation
reaches the UL.
While the DAP luminescence can only be observed at low
temperature in the CL, the relevant transitions are still active at
room temperature but are dominated by nonradiative recombination through the same defects. The DAP luminescence, therefore, indicates the existence of a major shunt-recombination
process that likely limits the overall device performance. The
ability to suppress the GaN DAP luminescence by a resonance
with an x = 3.8% UL suggests the possibility of recovering the
excitation from such DAP recombination and redirecting it to
the radiative centers of the QWs. This is a likely reason for
the LOP gains in the x = 3.8% UL structure over the reference
device.
The observation that the LOP gains are even higher in
the x = 6.3% UL sample can be explained by the following additional considerations. The proposed excitation transfer
from the DAP to the UL should gain further if the respective
levels line up, particularly those for the low-mobility holes.
According to the established relative band offsets between GaN
and InN, only some 40% appear on the valence band side.
The VBE of the Ga1−x Inx N UL should, therefore, vary with
ΔEv = 0.40(EGaN − EUL ) and amount to ΔEv = 160 meV
in the x = 6.3% UL sample. This value, indeed, is close to or
just below the acceptor level participating in the DAP transition observed near EGaN − EDAP − EDon = 130 meV. Here,
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 10, OCTOBER 2010
ED = 30 meV is the effective mass value for a donor [see
Fig. 4(b)]. Therefore, it appears possible that the holes initially
poised for the radiative and predominantly nonradiative recombination through DAP processes may be rerouted to the UL and
the QW as part of a resonant excitation transfer.
The same analysis for the x = 8.8% UL sample leads to
ΔEv = 220 meV, which lies an entire LO phonon energy
(90 meV) below the suspected acceptor level and may not provide an attractive route for holes to transfer to. This is in line with
a significantly lower LOP observed in such a sample in the EL.
It should be noted that the above explanation should also
work for blue-emitting LEDs utilizing the UL concept, as long
as the wavelength is above 410 nm. The spectrally larger separation in the green structures, however, simplifies the spectroscopic analysis. The considered model relies on the assumption
of an efficient hole-hopping conduction through the acceptor
states to add a relevant contribution to the QW luminescence.
From this, we may conclude that the density of the residual
acceptor states is quite high and might indeed be furthered by
defect clusters along threading dislocations.
The observation on an enhancement of the hole transport
mechanism by a UL located at the electron-injection side of
the QWs is surprising. It suggests a more complex exchange
of excitation throughout the active region than described in a
picture of individual charges. A possible alternate explanation
is that of the excitonic or optical excitation transfer between
the QWs and the DAP states. It would also well explain other
observations like the clear preference of the lowest transition energy and the identical emission wavelength in top and
bottom views. Such a model would resemble the processes
in organic polymer LEDs. Such evidence for less frequently
considered higher order excitation–transfer mechanisms may
possibly widen the scope for future study to resolve the details
of the recombination in the GaInN/GaN QWs in general.
[3] J. K. Son, S. N. Lee, T. Sakong, H. S. Paek, O. Nam, Y. Park, J. S. Hwang,
J. Y. Kim, and Y. H. Cho, “Enhanced optical properties of InGaN MQWs
with InGaN underlying layers,” J. Cryst. Growth, vol. 287, no. 2, pp. 558–
561, Jan. 2006.
[4] T. Akasaka, H. Gotoh, T. Saito, and T. Makimoto, “High luminescent
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[5] M. Crawford, D. Koleske, N. Missert, M. Lee, M. Banas, D. Follstaedt,
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grown on InGaN underlayers,” presented at the 7th Int. Conf. of Nitride
Semiconductors (ICNS-7) Conf., 2007.
[6] N. H. Niu, H. B. Wang, J. P. Liu, N. X. Liu, Y. H. Xing, J. Han, J. Deng,
and G. D. Shen, “Improved quality of InGaN/GaN multiple quantum wells
by a strain relief layer,” J. Cryst. Growth, vol. 286, no. 2, pp. 209–212,
Jan. 2006.
[7] P. T. Torma, O. Svensk, M. Ali, S. Suihkonen, M. Sopanen,
M. A. Odnoblyudov, and V. E. Bougrov, “Effect of InGaN underneath
layer on MOVPE-grown InGaN/GaN blue LEDs,” J. Cryst. Growth,
vol. 310, no. 23, pp. 5162–5165, Nov. 2008.
[8] C. Wetzel, T. Salagaj, T. Detchprohm, P. Li, and J. S. Nelson,
“GaInN/GaN growth optimization for high-power green light-emitting
diodes,” Appl. Phys. Lett., vol. 85, no. 6, pp. 866–868, Aug. 2004.
[9] T. Detchprohm, Y. Xia, Y. Xi, M. Zhu, W. Zhao, Y. Li, E. F. Schubert,
L. Liu, D. Tsvetkov, D. Hanser, and C. Wetzel, “Dislocation analysis
in homoepitaxial GaInN/GaN light emitting diode growth,” J. Cryst.
Growth, vol. 298, pp. 272–275, Jan. 2006.
[10] T. Detchprohm, M. W. Zhu, Y. F. Li, Y. Xia, C. Wetzel, E. A. Preble,
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pp. R13 302–R13 305, 2000.
IV. C ONCLUSION
The light output performance of green LEDs has been enhanced at 85% by the use of a 130-nm Ga1−x Inx N, xUL =
6.3% thin-film UL inserted between the n-layer and the QW active region. By variation of the alloy composition, the best value
of xUL = 6.3% has been determined. By the correlation of the
depth-resolved CL at low temperature, an excitation-transfer
mechanism between the defect-bound recombination, the UL,
and the QWs has been proposed as the mechanism of this
enhancement. The process, apparently, has worked independent
of the mode of excitation, geminate photogeneration or separate
charge injection from the opposite ends of the structure. The
observations suggest a relevant optical or excitonic excitationtransfer mechanism within the active region of the device.
R EFERENCES
[1] I. Akasaki and H. Amano, “Crystal growth and conductivity control of
group III nitride semiconductors and their application to short wavelength
light emitters,” Jpn. J. Appl. Phys., vol. 36, no. 9A, pp. 5393–5408,
Sep. 1997.
[2] I. Akasaki and C. Wetzel, “Future challenges and directions for nitride
materials and light emitters,” Proc. IEEE, vol. 85, no. 11, pp. 1750–1751,
Nov. 1997.
Yong Xia received the B.S. degree in physics from
the University of Science and Technology of China,
Hefei, China, in 2000 and the Ph.D. degree in physics
from Rensselaer Polytechnic Institute, Troy, NY,
in 2009.
His main research interests are the spectroscopic
characterization and material epitaxy of group-III
nitride light emitters.
Wenting Hou received the B.S. degree in physics
from the University of Science and Technology
of China, Hefei, China, in 2007. She is currently
working toward the Ph.D. degree in physics with
Rensselaer Polytechnic Institute, Troy, NY, in the
area of processing of GaN-based green light-emitting
diodes and laser diodes.
XIA et al.: BOOSTING GREEN GaInN/GaN LED PERFORMANCE BY A GaInN UNDERLYING LAYER
Liang Zhao received the B.S. and M.S. degrees in
physics from Tsinghua University, Beijing, China, in
2004 and 2007, respectively.
He is currently a Research Assistant with
Rensselaer Polytechnic Institute, Troy, NY. His work
is mainly focused on the optical spectroscopic characterization of MOVPE-grown GaInN/GaN green
multiple-quantum-well LEDs.
Mingwei Zhu received the B.S. degree in physics
from the University of Science and Technology of
China, Hefei, China, in 2004 and the Ph.D. degree in
physics from Rensselaer Polytechnic Institute, Troy,
NY, in 2010. His Ph.D. thesis is on the MOVPE
growth and characterization of GaInN/GaN green
LEDs in polar and nonpolar orientations.
He is currently a Process Engineer with Applied
Materials, Inc., Santa Clara, CA.
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Theeradetch Detchprohm received the B.S., M.S.,
and Ph.D. degrees from Nagoya University, Nagoya,
Japan, in 1991, 1993, and 1996, respectively, all in
electrical and electronic engineering.
In 1998, he joined the High Tech Research Center,
Meijo University, Nagoya, as a Postdoctoral Researcher. In 2001, he joined the Research and Development Team, Uniroyal Optoelectronics, Tampa,
FL, where he developed and managed the production
of high-brightness light-emitting diodes. In 2004, he
joined the Department of Physics, Applied Physics,
and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, where he is currently a Research Associate Professor with the Future Chips Constellation.
He has authored or coauthored more than 125 papers published in scientific
journals. He has been working on epitaxy and fabrication of AlGaInN-based
semiconductor devices (particularly optoelectronic applications) since 1990.
Christian Wetzel received the Ph.D. degree in
physics from the Technical University Munich,
Munich, Germany, in 1993.
He was a Visiting Scientist with Lawrence
Berkeley National Laboratory in 1996. In 1997, he
joined the High Tech Research Center, Meijo University, Nagoya, Japan. In October 2000, he joined
Uniroyal Optoelectronics as a Senior Epi Scientist
and Green Project Manager. He has been the Wellfleet Career Development Constellation Professor,
Future Chips Constellation, and an Associate Professor of physics with Rensselaer Polytechnic Institute, Troy, NY, since 2004. His
research centers on the electronic band and defect structure of piezoelectric
group-III nitrides to realize new concepts of high-efficiency light-emitting
devices and solar cells.