APPLIED PHYSICS LETTERS 91, 071901 共2007兲
231– 261 nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN
multilayer buffers grown by ammonia pulse-flow method on sapphire
Hideki Hirayamaa兲
RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako-shi,
Saitama 351-0198, Japan
Tohru Yatabe, Norimichi Noguchi, Tomoaki Ohashi, and Norihiko Kamata
Saitama Univensity, 255 Shimo-Okubo, Sakura-ku, Saitama 388-8570, Japan
共Received 18 June 2007; accepted 20 July 2007; published online 13 August 2007兲
The authors demonstrated AlGaN multiquantum-well 共MQW兲 deep-ultraviolet light-emitting diodes
共LEDs兲 with wavelengths in the range of 231– 261 nm, fabricated on low threading dislocation
density AlN buffers formed through an ammonia 共NH3兲 pulse-flow multilayer growth technique.
The authors obtained a single-peaked operation of the AlGaN-MQW LED with a wavelength of
231 nm, which is the shortest wavelength of AlGaN-based LED on sapphire. The maximum output
power and external quantum efficiency of the 261 and 231 nm LEDs were 1.65 mW and 0.23%
under room-temperature 共RT兲 continuous-wave 共cw兲 operation, and approximately 5 ␮W and
0.001% under RT pulsed operation, respectively. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2770662兴
High-brightness deep-ultraviolet 共UV兲 light-emitting diodes 共LEDs兲 or laser diodes 共LDs兲 with emission wavelengths in the range of 230– 350 nm have a wide range of
potential applications,1,2 such as in water purification, sterilization, medicine and biochemistry, white light illumination,
and light sources for high density optical recording. AlGaN
has a direct transition band gap between 3.4 and 6.2 eV and
also covers the spectrum obtained with conventional gas and
solid-state UV lasers. The main advantages of using AlGaNbased materials for UV light sources are that there is the
possibility of obtaining high-efficiency emission from quantum wells2 共QWs兲 and of producing both p- and n-type semiconductors in a wide band gap spectral region, that the nitrides are mechanically hard and the devices have long
lifetimes, and that the materials are free from harmful arsenic
or mercury.
Research into AlGaN-based UV LEDs for wavelengths
shorter than 360 nm, i.e., wavelengths between 330 and
355 nm,3–5 was initiated by several research groups between
1996 and 1999. The development of short-wavelength UV
LEDs is now becoming extremely competitive. In the U.S.,
the effort directed at deep-UV light sources has been coordinated by DARPA’s Semiconductor Ultraviolet Optical
Sources 共SUVOS兲 program. Innovations developed by Asif
Khan and co-workers at the University of South Carolina
共USC兲 have led to recent advances in deep-UV LEDs. His
group reported 270– 280 nm band AlGaN-based LEDs in
2002–20046,7 and 250– 270 nm band AlGaN-based LEDs in
2004.8,9 They achieved sub-300 nm LEDs with record power
and efficiency in 2004, i.e., a 280 nm, 5.2 mW LED with a
maximum external quantum efficiency 共EQE兲 of 0.94%,7 and
a 269 nm, 0.85 mW LED with a maximum EQE of 0.32%,9
both under RT cw operation. They set new records of efficiency in the short-wavelength range, i.e., EQE of approximately 2.8%, 0.6%, 0.4%, and 0.2%, for the LEDs with
Author to whom correspondence should be addressed; Fax: ⫹81-48-4621276; electronic mail: [email protected]
a兲
wavelengths of 280, 265, 255, and 244 nm, respectively, in
2006.10
The shortest wavelength nitride LED was reported by
NTT in 2006. They achieved 210 nm emission using an AlN
emitting layer.11 However, the EQE was quite low because
they did not obtain sufficient electron-hole confinement or
efficient carrier injection because of using the highest band
gap AlN for the emitting region.
We started our research into AlGaN-based deep-UV
LEDs in 1997. We reported 230 nm efficient photoluminescence 共PL兲 from AlGaN QWs,12 and 333 nm AlGaN-QW
UV LED on SiC in 1999.5 We have also developed quaternary InAlGaN-based high-brightness UV LEDs 共Refs. 2, 13,
and 14兲 and have achieved a high internal quantum efficiency of 47% for 338 nm quaternary InAlGaN QWs and
352 nm 7.4 mW dc operation of an InAlGaN-QW
UV LED.15
In this report, we fabricated a high-quality AlN buffer
suitable for deep-UV LEDs and demonstrate single-peaked
operations of AlGaN-QW LED with emission wavelengths
of 231– 261 nm. We also demonstrate cw milliwatt output
power of a 261 nm LED.
In order to realize high-brightness deep-UV LEDs, it is
important to develop a high-quality AlGaN / AlN template.
Several methods of fabricating high-quality buffers for the
realization of UV-emitting devices have been reported, for
example, the use of AlN / AlGaN superlattices 共SLs兲 grown
with alternate gas feeding,6 an AlGaN buffer deposited by
epitaxial lateral overgrowth,16 and a combination of
GaN / AlN SLs and AlGaN produced by alternate source
feeding epitaxy on SiC.17 It is required to satisfy several
conditions in order to fabricate high-quality AlGaN / AlN
templates that are applicable to deep-UV emitters, i.e., low
threading dislocation density 共TDD兲, crack free, atomically
flat surface and stable Al 共+c兲 polarity. To satisfy all of these
conditions, we proposed the ammonia 共NH3兲 pulse-flow
multilayer 共ML兲 AlN growth method.
Figure 1 shows 共a兲 the gas flow sequence used for NH3
pulse-flow growth and 共b兲 schematic layer structure of the
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91, 071901-1
© 2007 American Institute of Physics
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FIG. 1. 共a兲 Gas flow sequence used in NH3 pulse-flow growth. 共b兲 Schematic layer structure of multilayer 共ML兲-AlN buffer on sapphire.
ML-AlN buffer. Samples were grown on sapphire 共0001兲
substrates by low-pressure metal-organic chemical vapor
deposition. As group III precursors, trimethylaluminum
共TMAl兲 and trimethylgallium 共TMGa兲 were used with H2
carrier gas. First, an AlN nucleation layer and a burying AlN
layer were deposited, both by NH3 pulse-flow growth. TMAl
flow was continuous during the NH3 pulse-flow sequence, as
shown in Fig. 1共a兲. Low-TDD AlN can be achieved by the
coalescence process of AlN nucleation layer. After the
growth of the first AlN layer, the surface is still rough because of the low growth rate of the pulse-flow mode. We
introduced the high-growth-rate continuous-flow mode in order to reduce the surface roughness. By repeating the pulseand continuous-flow modes, we obtained a crack-free, thick
AlN layer with an atomically flat surface. NH3 pulse-flow
growth is effective for obtaining high-quality AlN because of
the enhancement of precursor migration. Furthermore, it is
effective for obtaining the stable Al 共+c兲 polarity that is necessary for realizing atomically flat surfaces because of the
Al-rich growth condition. We controlled the growth pressure,
temperature, and V / III ratio to be between 76 and 200 Torr,
1200– 1300 ° C, and 64–1060, respectively, for the growth of
ML-AlN. The growth rates in the pulse- and continuous-flow
FIG. 3. Cross-sectional TEM image of AlGaN-MQW deep-UV LED with
emission wavelength of 255 nm.
modes were approximately 0.6 and 6 ␮m / h, respectively.
The total thickness of ML-AlN was 3.3 ␮m. The full width
at half maximum of the x-ray 共102兲 ␻-scan rocking curve of
the AlGaN layer on AlN buffer was reduced from
2160 to 488 arc sec by introducing ML-AlN. The edge- and
screw-type dislocation densities of AlGaN layer on ML-AlN
were 3.2⫻ 109 and 3.5⫻ 108 cm−2, respectively, as observed
from the cross sectional transmission electron microscope
共TEM兲 image. We confirmed the flat surface by observation
in the step-flow growth mode using an atomic force microscope 共AFM兲. The rms value obtained from the AFM image
was 0.16 nm.
Figures 2 and 3 show a schematic of the sample structure
and a cross-sectional TEM image, respectively, of an AlGaN
MQW deep-UV LED with emission wavelength of 255 nm
fabricated on a ML-AlN buffer. Table I shows the designed
Al compositions x in AlxGa1−xN wells, buffer and barrier
layers, and electron-blocking layers 共EBLs兲 used for
231– 261 nm AlGaN-MQW LEDs. We grew a 1.7-␮m-thick
Si-doped AlGaN-buffer layer, 3 layer MQW consisting of
2-nm-thick AlGaN wells and 4-nm-thick AlGaN barriers,
7-nm-thick undoped AlGaN barrier, 15-nm-thick Mg-doped
AlGaN EBL, 10-nm-thick Mg-doped AlGaN, and an approximately 10-nm-thick Mg-doped GaN contact layer on
the ML-AlN. We can confirm flat heterointerfaces of the
QWs from the cross-sectional TEM image. Ni/ Au electrodes
were used for both n-type and p-type electrodes. The size of
the p-type electrode was 300⫻ 300 ␮m2. The output power
radiated into the back of the LED was measured using a Si
TABLE I. Designed values of Al composition x of AlxGa1−xN wells, buffer
and barrier layers, and electron blocking layers 共EBLs兲 used for
231– 261 nm AlGaN-MQW LEDs.
Wavelength 共nm兲
Well
Buffer and barrier
EBL
231
237
248
255
261
0.76
0.72
0.64
0.60
0.55
0.85
0.83
0.78
0.75
0.72
0.97
0.97
0.96
0.95
0.94
FIG. 2. Schematic of structure of AlGaN-MQW deep-UV LED with emission wavelength of 255 nm.
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Hirayama et al.
FIG. 4. Electroluminescence 共EL兲 spectra of AlGaN-MQW LEDs with
wavelengths of 231, 237, 248, 255, and 261 nm measured at room temperature 共RT兲 with injection current of around 50 mA.
photodetector located behind the LED sample, which was
calibrated to measure luminous flux from LED sources using
an integrated-spheres system. The LEDs were measured with
a bare chip.
Figure 4 shows electroluminescence 共EL兲 spectra of the
fabricated AlGaN-MQW LEDs with wavelengths of 231 nm
under pulsed operation and 237, 248, 255, and 261 nm under
cw operations, all measured at room temperature 共RT兲 with
injection current of around 50 mA. Pulse width and repetition frequency were 10 ␮s and 10 kHz, respectively. Singlepeaked operations were obtained for every sample. The intensities of deep emission at around 310 nm for the 248 nm
LED and at 380– 480 nm for the 231 and 237 nm LEDs were
negligible, as seen in Fig. 4. The wavelength of 231 nm is
the shortest one to date for an AlGaN-based LED on
sapphire.
Figure 5 shows output power and EQE ␩ext as functions
of current for 共a兲 261 and 共b兲 237 nm AlGaN-MQW LEDs
measured under RT cw operation. The maximum output
power and EQE were 1.65, 0.56, 0.076, and 0.0011 mW and
0.23%, 0.09%, 0.013%, and 0.0004% for the LEDs with
wavelengths of 261, 255, 248, and 237 nm, respectively, under RT cw operation. The maximum output power and EQE
of the 231 nm UV-LED were approximately 5 ␮W and
0.001%, respectively, under RT pulsed current injection. We
achieved milliwatt output power of the 261 nm LED. We
confirmed that the EQEs of AlGaN-MQW LEDs were much
higher than that of AlN LED.11 The reason for the low EQE
for shorter wavelength is considered to be that most of the
injection electrons overflowed to the p-side electrode. EQE
can be significantly increased if the electron overflow can be
suppressed by realizing higher hole concentration in p-type
layers.
In summary, we demonstrated single-peaked operations
of AlGaN-QW deep-UV LEDs on sapphire with peak emission between 231 and 261 nm. We achieved a high-quality
AlGaN / AlN buffer suitable for use in deep-UV emitters, by
introducing a ML-AlN formed by the NH3 pulse-flow
method. The maximum output power and EQEs were 1.65,
0.56, 0.076, and 0.0011 mW and 0.23%, 0.09%, 0.013%,
and 0.0004% for the LEDs with wavelengths of 261, 255,
248, and 237 nm, respectively, under RT cw operation. The
maximum output power and EQE of the 231 nm UV LED
FIG. 5. Output power and EQE as functions of current for 共a兲 261 and 共b兲
237 nm AlGaN-MQW LEDs measured under RT cw operation.
were approximately 5 ␮W and 0.001%, respectively, under
RT pulsed operation.
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas No. 18069014 of The Ministry of
Education, Culture, Sports, Science and Technology
共MEXT兲.
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