IEEE ELECTRON DEVICE LETTERS, VOL. 27, NO. 5, MAY 2006
329
Temperature-Dependent Electroluminescence of
AlGaN-Based UV LEDs
X. A. Cao, S. F. LeBoeuf, and T. E. Stecher
Abstract—The electrical and optical characteristics of AlGaNbased ultraviolet (UV) light-emitting diodes (LEDs) (265–365 nm)
at elevated temperatures (25 ◦ C–175 ◦ C) were investigated, and
compared to those of InGaN-based visible LEDs (400–465 nm).
Strong carrier localization and localized-state emission were retained in the InGaN LEDs up to 175 ◦ C, leading to temperatureindependent emission intensity at low-energy tails. The deep-UV
LEDs, however, showed dominant band-edge emission, much
smaller alloy broadening, and weaker localization effects. The
optical power of the UV LEDs decreased much more rapidly with
increasing temperature. The characteristic temperature was in the
range of 31–73 K, and decreased with increasing Al content in
the active region. These findings implicate the lack of localization
effects in AlGaN alloys as one of the causal factors in the poor
thermal performance of the UV LEDs and suggest that increasing carrier-confining potentials will provide a critical means to
improve their radiative efficiencies.
Index Terms—Carrier confinement, electroluminescence (EL),
light-emitting diode (LED), localization effects.
I. I NTRODUCTION
A
lGaN-BASED ultraviolet (UV) light-emitting diodes
(LEDs) have received tremendous attention over the past
several years due to their promise for numerous applications
including biological-agent sensing, air and water purification,
and biomedical diagnosis. With recent progress in the epitaxy of
high-Al AlGaN alloys and heterostructures, efficient UV LEDs
with milliwatt power output and peak wavelengths as short as
250 nm have been demonstrated on sapphire substrates [1]–[8].
Compared to InGaN-based LEDs, AlGaN LEDs may be subject to more severe self-heating due to higher operation voltage
and lower radiative efficiency. The poor electrical conductivity
of AlGaN materials was found to exacerbate the current crowding and localized heating problems [8]. An efficient and robust
packaging scheme is needed to prevent premature saturation
of light output and ensure long-term stability. On the other
hand, it is desirable that the UV LEDs have high-temperature
tolerance, which would enable high-current operation, simplify
the thermal designs, and allow LED use in a wide range of
thermal environments. Little work has been done to understand
the optical characteristics of AlGaN-based LEDs at elevated temperatures. In this paper, we report on the temperature-
dependent performance of state-of-the-art AlGaN-based LEDs
with different Al contents. In contrast to dominant localizedstate emission in InGaN LEDs, the UV emission is mainly
caused by band-to-band transition, and much more sensitive to
the junction temperature due to smaller carrier-confining and
localization potentials.
II. E XPERIMENT
Four UV LEDs, each with a different peak emission, were
grown on sapphire using metal–organic chemical vapor deposition (MOCVD) by Sensor Electronic Technology [1]–[4].
The LED structure consisted of an n-AlGaN: Si cladding
layer, an AlGaN multiple-quantum-well (MQW) active layer,
a p-AlGaN: Mg cladding layer, and a p+ -GaN: Mg contact
layer, with different AlGaN compositions in the QW and
cladding layers. Further growth and structure details can be
found in [1]–[4]. The room-temperature (RT) peak wavelengths
of the LEDs at 10 mA were 365, 335, 280, and 265 nm,
corresponding to Al mole fractions in the range of 0.01–0.5
in the active regions. To relieve the large strain in high-Al
AlGaN layers, the deep-UV LEDs were grown on a highquality AlN template and AlN/AlGaN superlattice produced
by the migration-enhanced MOCVD (MEMOCVD) technique
[3], [4]. LED chips with an active area of 7.0 × 10−4 cm2
were fabricated and flipchip mounted directly on printed circuit
boards. The flipchip-on-board packaging, having a thermal
resistance of ∼ 5 K/W, remarkably improved heat removal from
the LED chips, allowing much higher operation currents. The
LEDs were then passively heated to temperatures ranging from
25 ◦ C to 175 ◦ C. The current–voltage (I–V ) characteristics and
electroluminescence (EL) under steady-state conditions were
measured using a Keithley 238 current source and an Ocean
Optics spectrometer. The LEDs were pulse pumped at currents
above 10 mA to ensure a negligible difference between the
junction temperature and ambient temperature. For comparison,
commercially available InGaN-based MQW LEDs with peak
emission at 400 and 465 nm were packaged and characterized
in the same way.
III. R ESULTS AND D ISCUSSION
Manuscript received December 14, 2005; revised February 16, 2005. This
work was supported by the Department of Homeland Security through the
Interagency Technical Support Working Group (TSWG) under U.S. Army
RDECOM Acquisition Center Contract W91CRB-04-C-0063. The review of
this letter was arranged by Editor T. Mizutani.
The authors are with the GE Global Research Center, Niskayuna, NY 12309
USA (e-mail: cao@research.ge.com).
Digital Object Identifier 10.1109/LED.2006.873763
Fig. 1 compares the I–V curves of the 280- and 465-nm
LEDs at different temperatures. At low biases (< 1.5 V), the
blue LED has a thermally activated current component. At
moderate injection levels, the extracted ideality factor is 3.2,
and the slope of the I–V curves exhibits little temperature
sensitivity, indicating a significant contribution from carrier
0741-3106/$20.00 © 2006 IEEE
Authorized licensed use limited to: Boise State University. Downloaded on November 19, 2009 at 11:48 from IEEE Xplore. Restrictions apply.
330
IEEE ELECTRON DEVICE LETTERS, VOL. 27, NO. 5, MAY 2006
Fig. 1. Forward I–V
25 ◦ C–175 ◦ C.
characteristics of 280- and 465-nm LEDs at
Fig. 3. Temperature-dependent EL intensities of AlGaN and InGaN LEDs
measured at 100 mA. The RT peak wavelength of each LED is listed in the
legend.
well/barrier layers is ∼ 0.28 eV in the UV LED, compared to
∼ 0.71 eV in the blue LED. While the high-energy side of the
blue-LED spectra shows a gradual decrease in intensity, the
intensity and energy of the low-energy band, especially the tail,
are almost temperature independent. This behavior is attributed
to strong In segregation in InGaN, leading to the formation of
In-rich quantum-dot-like regions [10]. The low-energy EL in
the blue LED is dominated by emission at localized states in
these In-rich regions [11] and less affected by temperature.
In contrast, the low- and high-energy UV emission exhibits a
similar temperature dependence of light intensity and energy.
The red shift of the peak follows the temperature-dependent
bandgap shrinkage as described by Varshni’s equation
Fig. 2. Temperature-dependent EL spectra of 280- (left) and 465-nm (right)
LEDs at 10 mA.
Eg (T ) = Eg (0 K) + αT 2 /(T − β)
tunneling [9]. The deep-UV LED shows an evident tunneling
behavior at 0–4 V. At higher biases, the I–V characteristics are
dominated by series resistance, which increases with increasing
temperature, suggesting a reduced electrical conductivity of the
high-Al cladding layers at high temperatures.
The optical efficiency of all the LEDs under investigation
decreased as the temperature was raised. This can be attributed
to two major factors. First, the injected carriers have a higher
thermal energy, and therefore are more likely to escape from the
QWs and recombine nonradiatively in the barrier or cladding
layers. Second, the nonradiative recombination rate in the
MQWs increases rapidly with increasing temperature, leading
to a decrease in the radiative/nonradiative ratio. Fig. 2 illustrates
the evolution of the EL spectra of the 280- and 465-nm LEDs
with increasing temperature. As temperature is increased from
25 ◦ C to 175 ◦ C, the emission of the UV LED shows a
much sharper decrease, by a factor 48, compared to only a
42% decrease for the blue LED. This is due largely to the
shallower QWs in the UV LED. The band offset between the
with values α = 5.08 × 10−4 eV/K and β = 996 K [12]. These
behaviors suggest that the EL is dominated by band-to-band
transition in the AlGaN QWs [11]. As seen in Fig. 2, the
280-nm spectra, with a full-width at half-maximum of ∼ 10 nm,
show much smaller alloy broadening than the blue LED and
lower tail-state emission than deep-UV LEDs grown by conventional MOCVD [5]–[7]. This suggests that, by suppressing gasphase reaction and enhancing adatom surface migration, the
MEMOCVD technique produces high-quality AlGaN materials
with reduced band-tail states and alloy disorder [3], [4]. Carrier localization resulting from compositional inhomogeneities
plays a minor role in the UV light emission.
Fig. 3 shows the temperature-dependent EL intensity of the
AlGaN and InGaN LEDs measured at 100 mA. With increasing
temperature, the optical power of the AlGaN LEDs degrades
more rapidly than that of the InGaN LEDs. The temperatureinduced intensity changes approximately follow [13]
I(T ) = I(0 K) exp(−T /Tc ).
Authorized licensed use limited to: Boise State University. Downloaded on November 19, 2009 at 11:48 from IEEE Xplore. Restrictions apply.
CAO et al.: TEMPERATURE-DEPENDENT ELECTROLUMINESCENCE OF AlGaN-BASED UV LEDs
331
The peak shift in the blue LED is less pronounced at elevated
temperatures, suggesting that thermal band filling at high temperatures dilutes the effect of band filling via carrier injection.
IV. C ONCLUSION
We have investigated the performance of AlGaN- and
InGaN-based MQW LEDs at elevated temperatures. Strong
localization effects in the InGaN LEDs markedly enhance
carrier confinement and radiative recombination efficiencies.
The emission intensity and energy of the low-energy band
were found to be insensitive to the junction temperature up to
175 ◦ C. The light output of the AlGaN deep-UV LEDs decreases much faster with increasing temperature due to the shallower QWs and the lack of localization effects. It is therefore
essential to increase carrier-confining potentials by designing
high-Al-content AlGaN barrier and cladding layers for better
thermal performance of deep-UV emitters.
Fig. 4. Current-dependent peak energies of 280- (solid lines) and 465-nm
(dotted lines) LEDs measured at 25 ◦ C, 100 ◦ C, and 175 ◦ C.
The characteristic temperature Tc of the AlGaN LEDs is in the
range of 31–73 K, and decreases with increasing Al content
in the QWs. The strong temperature dependence explains why
UV LEDs operate much more efficiently at pulse currents than
at continuous-wave (CW) currents, as reported in the literature,
and makes them unsuitable for applications requiring a small
power fluctuation. The InGaN LEDs have a much higher Tc
(170–270 K), which also appears to be a function of the In
content in the QWs. Tc is related to the activation energy of
nonradiative recombination centers and the energy required
to overcome the confining potentials. In the case of InGaN
LEDs, it is also associated with the localization energy. Note
that there is no essential link between Tc and the RT quantum
efficiency. For instance, the 280-nm LED is three times brighter
than the 335-nm LED at 25 ◦ C, but has a lower Tc . We have
also measured the temperature dependence of EL at currents
as low as 1 mA and found that Tc is nearly independent of
the injection current. The poor thermal stability of the UV
LEDs is therefore mainly a result of small carrier-confining and
localization potentials. This finding illustrates the importance
of increasing the Al contents in the cladding and barrier layers
for better confinement, and developing advanced packaging
schemes for more effective thermal management.
One characteristic feature of InGaN-based LED emission is
a large blueshift of the peak position with increasing forward
current, caused by band filling of localized states in the high
injection regime. Fig. 4 shows the EL peak energies of the
280- and 465-nm LEDs as a function of current measured at
25 ◦ C, 100 ◦ C, and 175 ◦ C. The emission energy of the 280-nm
LED is independent of injection current at all temperatures,
confirming negligible localization effects in the AlGaN material. As expected, the peak energy of the blue LED shows a considerable blue shift. The localization effects remain strong even
at 175 ◦ C, suggesting a localization energy > 39 meV. A previous study using time-resolved photoluminescence showed that
the localization energy in In0.2 GaN was ∼ 60 meV [14], corresponding to an indium compositional fluctuation of ∼ 0.03.
R EFERENCES
[1] J. P. Zhang, A. Chitnis, V. Adivarahan, S. Wu, V. Mandavilli,
R. Pachipulusu, M. Shatalov, G. Simin, J. W. Yang, and M. A. Khan,
“Milliwatt power deep ultraviolet light-emitting diodes over sapphire with
emission at 278 nm,” Appl. Phys. Lett., vol. 81, no. 26, pp. 4910–4912,
Dec. 2002.
[2] V. Adivarahan, W. H. Sun, A. Chitnis, M. Shatalov, S. Wu, H. P. Maruska,
and M. A. Khan, “250 nm AlGaN light-emitting diodes,” Appl. Phys.
Lett., vol. 85, no. 12, pp. 2175–2177, Sep. 2004.
[3] J. P. Zhang, X. Hu, Y. Bilenko, J. Deng, A. Lunev, M. Shur, R. Gaska,
M. Shatalov, J. W. Yang, and M. A. Khan, “AlGaN-based 280 nm lightemitting diodes with continuous-wave power exceeding 1 mW at 25 mA,”
Appl. Phys. Lett., vol. 85, no. 23, pp. 5532–5534, Dec. 2004.
[4] R. Gaska, “Migration-enhanced MOCVD advances AlGaN performance,” Compound Semicond., vol. 4, pp. 27–30, 2005.
[5] M. Khizar, Z. Y. Fan, K. H. Kim, J. Y. Lin, and H. X. Jiang, “Nitride deepultraviolet light-emitting diodes with microlens array,” Appl. Phys. Lett.,
vol. 86, no. 17, pp. 173504–173506, Apr. 2005.
[6] K. Mayes, A. Yasan, R. McClintock, D. Shiell, S. R. Darvish, P. Kung,
and M. Razeghi, “High-power 280 nm AlGaN light-emitting diodes based
on an asymmetric single-quantum well,” Appl. Phys. Lett., vol. 84, no. 7,
pp. 1046–1048, Feb. 2004.
[7] A. A. Allerman, M. H. Crawford, A. J. Fischer, K. H. A. Bogart, S. R. Lee,
D. M. Follstaedt, P. P. Provencio, and D. D. Koleske, “Growth and design
of deep-UV (240–290 nm) light emitting diodes using AlGaN alloys,”
J. Cryst. Growth, vol. 272, no. 1–4, pp. 227–241, 2004.
[8] A. Chitnis, J. Sun, V. Mandavilli, R. Pachipulusu, S. Wu, M. Gaevski,
V. Adivarahan, J. P. Zhang, M. A. Khan, A. Sarua, and M. Kuball, “Selfheating effects at high pump currents in deep ultraviolet light-emitting
diodes at 324 nm,” Appl. Phys. Lett., vol. 81, no. 18, pp. 3491–3493,
Oct. 2002.
[9] X. A. Cao, E. B. Stokes, P. M. Sandvik, S. F. LeBoeuf, J. Kretchmer,
and D. Walker, “Diffusion and tunneling currents in GaN/InGaN multiple
quantum well light-emitting diodes,” IEEE Electron Device Lett., vol. 23,
no. 9, pp. 535–537, Sep. 2002.
[10] L. Nistor, H. Bender, A. Vantomme, M. F. Wu, J. V. Lauduyt, K. P.
O’Donnell, R. Martin, K. Jacobs, and I. Moerman, “Direct evidence of
spontaneous quantum dot formation in a thick InGaN epilayer,” Appl.
Phys. Lett., vol. 77, no. 4, pp. 507–509, Jul. 2000.
[11] X. A. Cao, S. F. Leboeuf, L. R. Rowland, and H. Liu, “Temperaturedependent electroluminescence in InGaN/GaN multiple-quantum-well
light-emitting diodes,” J. Electron. Mater., vol. 32, no. 5, pp. 316–321,
May 2003.
[12] B. Monemar, “Fundamental energy gap of GaN from photoluminescence
excitation spectra,” Phys. Rev. B, Condens. Matter, vol. 10, no. 2, pp. 676–
681, Jul. 1974.
[13] E. F. Schubert, Light-Emitting Diodes. Cambridge, U.K.: Cambridge
Univ. Press, 2003.
[14] M. Smith, G. D. Chen, J. Y. Lin, H. X. Jiang, M. A. Khan, and Q. Chen,
“Time-resolved photoluminescence studies of InGaN epilayers,” Appl.
Phys. Lett., vol. 69, no. 19, pp. 2837–2839, Nov. 1996.
Authorized licensed use limited to: Boise State University. Downloaded on November 19, 2009 at 11:48 from IEEE Xplore. Restrictions apply.