AlGaN-based deep ultraviolet light-emitting diodes grown on nano

Journal of Crystal Growth 395 (2014) 9–13
Contents lists available at ScienceDirect
Journal of Crystal Growth
journal homepage: www.elsevier.com/locate/jcrysgro
AlGaN-based deep ultraviolet light-emitting diodes grown
on nano-patterned sapphire substrates with significant
improvement in internal quantum efficiency
Peng Dong a,n, Jianchang Yan a, Yun Zhang a, Junxi Wang a, Jianping Zeng a, Chong Geng b,
Peipei Cong a, Lili Sun a, Tongbo Wei a, Lixia Zhao a, Qingfeng Yan b, Chenguang He c,
Zhixin Qin c, Jinmin Li a
a
b
c
Research and Development Center for Semiconductor Lighting, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Department of Chemistry, Tsinghua University, Beijing 100084, China
State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
art ic l e i nf o
a b s t r a c t
Article history:
Received 20 October 2013
Received in revised form
17 January 2014
Accepted 25 February 2014
Communicated by R.M. Biefeld
Available online 3 March 2014
We report high-performance AlGaN-based deep ultraviolet light-emitting diodes grown on nanopatterned sapphire substrates (NPSS) using metal organic chemical vapor deposition. By nanoscale
epitaxial lateral overgrowth on NPSS, 4-μm AlN buffer layer has shown strain relaxation and a
coalescence thickness of only 2.5 μm. The full widths at half-maximum of X-ray diffraction (002) and
(102) ω-scan rocking curves of AlN on NPSS are only 69.4 and 319.1 arcsec. The threading dislocation
density in AlGaN-based multi-quantum wells, which are grown on this AlN/NPSS template with a lightemitting wavelength at 283 nm at room temperature, is reduced by 33% compared with that on flat
sapphire substrate indicated by atomic force microscopy measurements, and the internal quantum
efficiency increases from 30% to 43% revealed by temperature-dependent photoluminescent measurement.
& 2014 Elsevier B.V. All rights reserved.
Keywords:
A1. Defects
A3. Epitaxial lateral overgrowth
A3. Metalorganic chemical vapor deposition
B1. Nitrides
B2. Semiconducting aluminum compounds
1. Introduction
Because of their direct and wide band-gap, AlGaN alloys have
attracted considerable attention in fabricating light-emitting diodes
(LEDs) in deep ultraviolet (DUV) waveband [1], for disinfection,
sensing, water/air purification, bio-medical and non-line-of-sight
communication [2]. These applications require an emitting wavelength (λ) less than 300 nm, which means that the Al content is at
least 40% in LED's quantum wells and even higher in other layers.
However, the growth of high-aluminum (Al)-content AlGaN on
sapphire substrates is a big challenge due to the large lattice
mismatch and thermal expansion mismatch, as well as the low
surface mobility of aluminum species. Although an AlN buffer layer
has been widely adopted between flat sapphire substrates (FSS) and
AlGaN epi-layers for better material quality, the typical threading
dislocation density (TDD) in the AlGaN layers grown on the AlN/FSS
template is still as high as 1010–1011 cm 2 [3]. Threading dislocations in multi-quantum wells (MQWs) act as non-radiative
n
Corresponding author.
E-mail addresses: [email protected] (P. Dong), [email protected] (J. Yan),
[email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.jcrysgro.2014.02.039
0022-0248 & 2014 Elsevier B.V. All rights reserved.
recombination centers, thereby resulting in low internal quantum
efficiency (IQE) [4,5].
Various methods have been reported to suppress the TDD in
AlN and the upper epi-layers grown on sapphire substrates,
including migration-enhanced metal–organic chemical vapor
deposition (MEMOCVD) [6,7] and pulsed-flow multilayer AlN
buffers growth technique [8]. Particularly, epitaxial lateral overgrowth (ELO) techniques on micro-stripe patterned sapphire or
AlN/FSS template have significantly enhanced light-output power
(LOP) and reliability of DUV LEDs by reducing the TDD [9–12].
However, the major issue of the AlN ELO on micro-stripe patterned
sapphire is the large space between micro-patterns that needs a
coalescence thickness almost 10 μm for AlN and greatly increases
epitaxy time and cost.
2. Experimental
In this study, we fabricated nano-patterned sapphire substrates
(NPSS) by an optimized nanosphere lithography (NSL) process for
nanoscale ELO of AlN. Furthermore, we investigated the material
quality, stress state and TDD decrease of nanoscale ELO–AlN on
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P. Dong et al. / Journal of Crystal Growth 395 (2014) 9–13
NPSS as well as its effect on the quality and IQE of AlGaN
based MQWs.
Fig. 1(a) shows the schematic diagram of NSL for fabricating
NPSS. First, positive photo resist (PR) was spin-coated on a 2-in.
(001) sapphire substrate with a 200-nm-thick SiO2 film predeposited by plasma-enhanced chemical vapor deposition. The
wafer was then dip-coated with a highly ordered self-assembled
monolayer of polystyrene (PS) nanospheres with uniform diameters of 600 nm [13] and followed by flood UV-exposure. After
PS nanospheres were removed by DI water, the PR was developed
to form nano-holes. The pattern was transferred to the SiO2 film by
inductively coupled plasma (ICP) etching. Finally, the sapphire
substrate was etched for 10 min in a mixture of H2SO4 and H3PO4
solution (H2SO4:H3PO4 ¼3:1) at 280 1C, and the SiO2 mask was
removed by HF. Top-view SEM images of the developed PR and the
fabricated NPSS are shown in Fig. 1(b) and (c), respectively. The
patterns on the NPSS surface are concave triangle cones caused by
the anisotropic etching of the sapphire crystal. The period of
patterns is 900 nm that is determined by the PS diameter. The
depth of cones and the width of the unetched regions are
approximately 250 nm and 400 nm, respectively.
A home-made low-pressure metal–organic chemical vapor
deposition (LP–MOCVD) system with a vertical shower-head
reactor was used to process epitaxial growth. Trimethylaluminum
(TMAl), trimethylgallium (TMGa) and ammonia (NH3) were aluminum, gallium and nitrogen sources, respectively. The AlN
template growth on the NPSS started with a 25-nm AlN buffer
layer that was grown at 550 1C, with a V/III ratio of 3000, a TMAl
flux of 7.5 μmol/min and a growth rate of 12 nm/min. Then the
growth temperature rose to 1200 1C to finish the whole 4-μm AlN
growth using a V/III ratio of 1000, a TMAl flux of 40 μmol/min and
a growth rate of 1 μm/h. The reactor pressure during AlN growth
was kept at 50 Torr.
3. Results and discussion
A cross-sectional SEM image of 4-μm AlN on NPSS is shown in
Fig. 2(a). Thanks to the nano-scale substrate patterns and the AlN
lateral growth, AlN completely coalesces after 2.5-μm growth,
which is much shorter than the reported coalescence thickness
of nearly 10 μm [9–12,14,15]. Fig. 2(b) presents a 5 5 μm2 atomic
force microscopy (AFM) image of the surface morphology of the
AlN on NPSS, demonstrating an atomically flat surface with a stepflow growth mode and a root-mean-square (RMS) roughness of
0.19 nm.
Fig. 3(a) shows the typical X-ray rocking curves (XRCs) of 4-μm
AlN film on NPSS. The full width at half-maximum (FWHM) values
of (002) and (102) reflections are 69.4 and 319.1 arcsec, respectively. The corresponding screw and edge dislocation densities are
calculated to be 1.0 107 cm 2 and 1.2 109 cm 2 by the method
reported in Ref. [16]. The AlN material quality is much better than
a 1-μm AlN grown on FSS, which has (002) and (102) XRCs of 126
and 573 arcsec FWHM (data not shown here). Fig. 3(b) shows the
Raman spectrum of the E2(high) phonon mode for AlN template
layer grown on NPSS and FSS. The Raman shift peaks of E2(high)
phonon mode for AlN template layer grown on NPSS and FSS are
located at 658.7 and 660.2 cm 1, respectively. The vertical arrow
indicates the stress-free frequency of 657.4 cm 1 [17]. The Raman
shift peaks both appear on the higher frequency side, showing
residual compressive stresses, while the peak of AlN on NPSS is
more close to stress-free frequency, indicating strain relaxation.
Fig. 1. (a) Schematic diagram of the nanosphere lithography (NSL) for fabricating nano-patterned sapphire substrate (NPSS). SEM images of the patterned photoresist (PR) (b) and
wet-etched NPSS (c).
Fig. 2. (a) A cross-sectional SEM image of AlN grown on NPSS. (b) An AFM image of the AlN grown on NPSS (5 5 μm2).
P. Dong et al. / Journal of Crystal Growth 395 (2014) 9–13
The corresponding in-plane compressive stress (s) for AlN template layer on NPSS is therefore decreased according to the
following expression [18]:
ωE2 ðhighÞ ω0 ¼ C s
where C is the biaxial strain coefficient, ωE2 ðhighÞ and ω0 are the
Raman shift peaks for E2(high) mode of the AlN grown on NPSS
and the stress-free AlN, respectively. The reduced stress has
prevented the epilayers to suffer from cracking, which has been
11
indicated by our cross-sectional transmission electron microscope
(TEM) measurements (not shown here), and can also improve the
stress state of its upper layers.
An AlGaN-based MQWs structure was regrown on the AlN/NPSS
template (denoted as Structure A), consisting of 20 pairs of AlN/
AlGaN superlattices (SLs), a 3.5-μm-thick Si-doped Al0.55Ga0.45N
layer, five 3-nm-thick un-doped Al0.4Ga0.6N quantum wells
sandwiched by 12-nm-thick Si-doped Al0.5Ga0.5N barriers and a
Mg-doped Al0.65Ga0.35N electron blocking layer. For comparison, the
Fig. 3. (a) X-ray rocking curves (XRCs) of (002) and (102) diffractions for the AlN films grown on the NPSS and (b) Raman spectrum for the AlN films grown on the NPSS and
flat sapphire substrate (FSS).
Fig. 4. AFM images of the AlGaN based MQWs of Structure A (a) , Structure B (b). (c) Cross-sectional high-resolution TEM image of the AlGaN based MQWs of Structure A.
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P. Dong et al. / Journal of Crystal Growth 395 (2014) 9–13
Fig. 5. Temperature-dependent PL spectra for Structure A (a), Structure B (b). (c) Arrhenius plot of the integrated PL spectra for Structure A and Structure B.
same structure was also grown on AlN/FSS template with 1-μmthick AlN (denoted as Structure B). We also prepared samples with
AlN/AlGaN SLs and 3.5-μm-thick Al0.55Ga0.45N layer grown on NPSS
and FSS without MQWs for material quality comparison in XRD, the
FWHM values of (002) and (102) reflections are 190 and 513 arcsec
for AlGaN on NPSS, and those values for AlGaN on FSS are 228 and
631 arcsec.
The surface morphology of Structures A and B was characterized by AFM measurements over an area of 1 1 μm2. The results
are shown in Fig. 4(a) and (b), both demonstrating a step-flow
growth mode, but structure A has a more atomically flat surface
than structure B. The RMS roughness of the two samples is
found to be 0.14 nm and 0.23 nm, respectively. By counting pits
in the AFM figures [19], the TDDs are estimated to be about
2.0 109 cm 2 and 3.0 109 cm 2, respectively, for structures
A and B. These results qualitatively demonstrate improvement in
material quality for AlGaN-based MQWs on NPSS compared to that
on FSS. Additionally, that is very close to the TD densities
estimated from cross-sectional TEM measurement reported previously [20]. Fig. 5(c) shows a cross-sectional high-resolution TEM
image of Al0.5Ga0.5N/ Al0.4Ga0.6N MQWs of structure A. The image
was taken with diffraction vector g¼[0002] near the [112̄ 0] zone.
The thicknesses of the quantum wells and the quantum barriers
are indicated to be 3 nm and 12 nm, respectively. Moreover,
MQWs in the image display decent periodicity and uniformity, as
well as atomically abrupt and smooth interfaces between barriers
and wells.
Temperature-dependent photoluminescence (PL) measurements were performed on structures A and B to compare their
IQE. The samples were placed inside a closed-cycle refrigerator,
and the temperature ranged from 10 K to 300 K. A 4th harmonic of
Q-switched YAG:Nd laser (λ¼ 266 nm, pulse width ¼ 7 ns) was
used for excitation, and an Ocean Optics USB2000þ VIS-NIR fiber
optic spectrometer recorded the PL spectra. Fig. 5(a) and (b) shows
the normalized temperature-dependent PL spectra of structures
A and B under identical excitation condition, respectively, both
demonstrating a single peak around 283 nm at 300 K. The peak
red-shift is low for structure A than that of structure B with
temperature increasing from 10 K to 300 K. Fig. 5(c) shows the
normalized integrated PL intensities as a function of inverse
temperature in an Arrhenius plot. Assuming that IQE equals to
100% for both AlGaN based MQWs at 10 K, their IQE at 300 K (¼ PL
intensity at 300 K/PL intensity at 10 K) are 43% and 30%, respectively, for structures A and B, demonstrating about 43% enhancement for structure A. We believe that the significant improvement
in IQE can be partly attributed to the decrease in TDs, which act as
non-radiative centers. Nanoscale ELO on NPSS decreases TD
densities in AlN template and upper epilayers, such as n-AlGaN
and MQWs. In addition, the less in-plane compressive stress
defined by Raman analysis will decrease the strain-induced piezoelectric polarization in AlGaN-based MQWs, which can be indicated
by the decreased red-shift. Thus, the recombination efficiency will
also be increased.
4. Conclusions
We have demonstrated AlGaN-based DUV LEDs fabricated on
an AlN/NPSS template with significant IQE improvement. The NPSS
is prepared by NSL and wet etching. The AlN layer has shown
strain relaxation and a coalescence thickness of only 2.5 μm by
nanoscale ELO on NPSS. Narrow XRC FWHMs and smooth surface
both demonstrated high material quality of AlN on NPSS. When
grown on the AlN/NPSS template, the Al0.5Ga0.5N/Al0.4Ga0.6N
MQWs showed better surface morphology, decreased straininduced piezoelectric polarization and decreased TD density,
contributing to the significant improvement in IQE from 30% to
43% observed in temperature-dependent PL measurement.
Acknowledgments
This work was supported by the National Natural Sciences
Foundation of China under Grant nos. 61376090, 61376047,
61006038, 61204053 and 51102226, by the National High Technology Program of China under Grant nos. 2014AA032608 and
2011AA03A111, by the Program of Science and Technology of
Beijing under Grant no. D12110300140000 and the National
1000 Young Talents Program.
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