Luminescence Efficiency of InGaN Multiple Quantum Well UV-LEDs
Chang-Chi Pan, Chia-Ming Lee, Jia-Wen Liu, Guan-Ting Chen, and Jen-Inn Chyi
Department of Electrical Engineering, National Central University
Chung-Li, Taiwan 32054, R.O.C.
Abstract
The electroluminescence efficiency of InGaN/GaN multiple quantum well ultraviolet
light-emitting diode (UV-LEDs) with emission wavelength of 400 nm has been investigated.
Based on the injection current-dependent characteristics of the diode and blue (470 nm) LEDs, it
can be concluded that carrier overflow is the dominant factor that degrades the external quantum
efficiency of UV-LED before thermal effect takes over. It is supported by the fact that the
luminescence efficiency of the UV LEDs is greatly improved by increasing the period of
quantum well from five to ten without replacing the GaN barrier to AlGaN.
1
High-brightness and high-efficiency GaN-based light-emitting diodes (LEDs) have
attracted great attention because of their vital roles in full-color display and solid-state
lighting.1 For the latter application, ultra-violet light-emitting diode (UV-LED), which pumps
red-green-blue phosphorus to generate white light, is one of the most promising candidates.
Compared to the more popular approach, which uses blue LED to pump YAG:Ce3+ and other
yellow inorganic or polymer phosphors, using UV LED to excite down-conversion phosphors
is expected to give better color rendering and power conversion efficiency. 2-4 However, UV
LEDs tend to be less efficient as the emission wavelength decreases.
The high efficiency radiative recombination in dislocated InGaN-based multiple
quantum wells (MQWs) has been attributed mainly to the excitons localized at In-rich
regions in the wells5-9 and a small In addition to the GaN active layer is beneficial to suppress
the nonradiative recombination process.10 In contrast to its blue/green counterparts, UV-LEDs
usually exhibit lower quantum efficiency. It was proposed that carrier overflow and less
localized states in the low indium-content active layer are the root causes of this
phenomenon.11 Based on a rate equation analysis, the densities of the localized states in the
green LEDs are more than two orders of magnitude higher than that in the UV-LEDs.
Temperature-dependent electroluminescence (EL) of InGaN/GaN multiple-quantum-well
light-emitting diodes has been investigated to illustrate the role of localization effects in
carrier capture and recombination.12 This implies that dislocation density in the active region
2
might play an important role in the internal quantum efficiency of low-indium content InGaN
quantum well.
In this work, we have conducted a series of measurements to investigate the
luminescence efficiency behaviors of UV-LEDs with emission wavelength of 400 nm.
Injection current-dependent characteristics are analyzed to clarify the dominant factors
affecting the drastically degraded luminescence efficiency of UV-LEDs. In order to clarify
the carrier radiative recombination behavior of 5 periods of quantum well UV-LEDs, the
devices were also compared with blue light-emitting diodes with 5 periods of
In0.23Ga0.77N/GaN quantum well, the detail growth conditions had been published
elsewhere.13 On the other aspect, we have optimized the structure of UV-LEDs with different
periods of quantum well. In our detail study, the optimized structure of UV-LEDs can
improve the issue of carrier overflow, which causes the external quantum efficiency degraded
gradually before thermal takes part.
The epilayers were grown on c-face sapphire substrates in a horizontal low-pressure
metal-organic chemical vapor deposition (MOCVD) reactor. The precursors were
trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylidium
(TMI), and ammonia (NH3). Diluted silane (SiH4) in H2 and bis(cyclopentadienyl)magnesium
(Cp2Mg) were used as the n-type and p-type doping sources, respectively. The layer structure
of the UV-LED, as shown in Fig.1, consists of a 25-nm-thick GaN nucleation layer, a 1.5
3
m-thick GaN buffer layer, a 1.5 m-thick n-type GaN:Si, a multiple-quantum-well active
layer, a 50 nm-thick p-type Al0.1Ga0.9N:Mg cladding layer, and a 0.1 m-thick p-type
GaN:Mg contact layer. In this work, devices with three types of active layers, i.e. 5, 10 and
15 periods of In0.5Ga0.95N (25 nm)/GaN (75 nm) multiple-quantum-well, were prepared for
comparison. The alloy composition and period thickness of the quantum well were
determined by x-ray rocking curve analysis. Ni/Au, Ti/Au and Ti/Al/Ti/Al were used as the
p-type transparent contact layer, p-type contact metal, and n-type contact metal, respectively.
The device size is 300m x 300 m. The optical output power of the LEDs under direct
current injection was measured by an integrating sphere equipped with a calibrated silicon
detector. The output powers obtained by dc and pulsed measurements were correlated,
assuming there is no thermal effect at low current regime.
Fig. 2 shows the output power and external quantum efficiency at various dc injection
densities for an UV LED and a blue LED, both having 5 periods of InGaN/GaN quantum
well in the active region. In contrast to its blue counterpart, the UV LED exhibits much lower
output power and external quantum efficiency. The striking difference between the two is the
behavior of the external quantum efficiency at low current level. The external quantum
efficiency of UV LED, just like conventional GaAs-based LEDs, starts from a low value and
reach a maximum as the injection is increased while that of blue LED begins with the highest
efficiency and decreases with increasing current. Same behavior has also been observed in
4
our green LEDs with higher indium content in the quantum wells. It is commonly attributed
to the high-density In-rich regions, i.e. localized states, caused by In composition fluctuation
or aggregation in InGaN. Whereas, UV LED, which has less In content and poorer carrier
confinement, suffers from carrier overflow and non-radiative recombination through
threading dislocations. It is therefore expected that GaN-based UV LEDs behave more like
GaAs-based LEDs, which are sensitive to the defect density of material. For in-depth
investigation on the efficiency of these diodes at high current injection, pulsed measurement
is necessary so that thermal effect can be separated.
Fig. 3 shows the electroluminescence characteristics of the UV LED under dc and
pulsed injection, including the integrated intensity, luminescence efficiency, and the peak
wavelength at different current densities. Obviously, current-induced thermal effect plays a
significant role in luminescence efficiency. As the duty cycle is reduced from 100 % to 10 %
or 1 %, the luminescence efficiency can reach higher value and maintain the same efficiency
at higher current densities until thermal effect comes in. The effect of thermal is clearly
reflected in the red-shifted of luminescence wavelength of the dc-driven device. Same
measurements were also carried out on the blue LED as shown in Fig. 4. It again depicts the
current-induced thermal effect on luminescence efficiency. Significant red-shifted of
luminescence wavelength occurs for the dc-driven device. It is also worthy to note that the
luminescence wavelength of blue LED is blue-shifted with current density at very low current
5
density regime, where thermal effect can be ignored. This injection current-dependent
wavelength shift can be well correlated to the behavior of the luminescence efficiency,
implying that the dominant emission mechanism is different for UV and blue LEDs in low
current regime, <50 A/cm2 in this case. In contrast, the efficiency for both diodes exhibits the
same behavior, i.e. maintains at a constant but different value at higher injection current
density. This implies that the dominant emission mechanism might be the same while the
utilization efficiency of the injected carriers for luminescence is different. We ascribe it to the
difference in the carrier confinement, namely carrier overflow, for these two quantum well
LEDs.
. Since the luminescence efficiency of UV LED does not degrade with increasing
injection current as shown in the pulsed measurements where thermal effect can be ignored,
LEDs with more periods of quantum well for radiative recombination are designed and
fabricated. Fig. 6 (a) and (b) show the output power and external quantum efficiency of the
UV-LEDs with 5, 10 and 15 periods of quantum well under dc injection. The significant
improvement on the light output power and luminescence by increasing the period of
quantum well from 5 to 10 indicates that the number of the injected carriers captured by
quantum well is increased. The ratio of the carriers inside and outside of the well is increased
as well. The fact that further increase in the period of quantum well does not result in
proportional improvement means that the injected carriers are exhausted mostly in the first 10
6
quantum well. It can therefore be concluded that carrier overflow in UV LED is a decisive
factor in luminescence efficiency due to its shallow potential well and high density of
dislocations in the material. This is in good agreement with the rapid degradation in
luminescence efficiency of the dc-driven UV LED shown in Fig. 3 (b), where thermal effect
greatly reduces the population of carriers in the well and facilitates non-radiative
recombination in other regions. As evidenced by the results obtained from pulsed
measurements shown in Fig. 6 (c), the maximum luminescence efficiency stays up to a
current density of 350 A/cm2 if thermal issue is taken care of.
In conclusion, we have investigated the injection current-dependent light output
characteristics of UV LEDs driven at dc and pulsed conditions. It is shown that the behavior
of luminescence efficiency at low current density for UV and blue LEDs is different due to
the different emission mechanisms in the quantum well. While at high current density, the
luminescence efficiency is dominated by carrier overflow and thermal effects, especially for
UV LED, which has shallow quantum well. Increasing period of quantum well from 5 to 10
is found to be effective in the utilization of the injected carriers for radiative recombination
and hence improve the luminescence efficiency markedly for the UV-LEDs.
Acknowledgments
The authors would like to thank Ms. I. -L Chen for her assistance in chip processing and
7
material characterization. This work was partially supported by TEKCORE Co.
8
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Figure Captions
Fig. 1. Schematic layer structure of the UV LED.
Fig. 2. (a) Output power and (b) external quantum efficiency of the UV and blue
light-emitting diodes (LEDs) with 5 periods of quantum well under various dc current
injection.
Fig. 3. (a) EL integrated intensity, (b) efficiency, and (c) wavelength of the UV LED under
various dc and pulsed injection levels.
Fig. 4. (a) EL integrated intensity, (b) efficiency, and (c) wavelength of the blue LED under
various dc and pulsed injection levels.
Fig. 5. (a) Output power and (b) efficiency under dc injection, and (c) efficiency under pulsed
injection (duty cycle of 1 %) of the UV LEDs with 5, 10, and 15 periods of quantum
wells.
10
Ti/Au
Ni/Au
p-GaN
p-AlGaN
InGaN/GaN:Si
MQW
n+-GaN
undoped GaN
(0001) sapphire substrate
FIG. 1.
11
Ti/Al/Ti/Au
Output Power ( mW )
20
 = 400 nm
 = 473 nm
15
10
5
(a)
External Quantum Efficiency ( % )
0
14
12
10
8
6
4
(b)
2
0
0
20
40
60
80
100 120 140 160 180 200
2
Injected Current Density ( A/cm )
FIG. 2.
12
25
 = 400 nm, UV-LED, MQW x 5
duty cycle 1%
duty cycle 10%
dc
EL Intensity ( a.u. )
20
15
10
5
( a )
0
( frequency = 1K Hz )
1.6
Efficiency ( a.u. )
1.4
1.2
1.0
0.8
( b )
0.6
Wavelength ( nm )
404
402
400
398
( c )
0
50
100
150
200
250
300
2
Injected Current Density ( A/cm )
FIG. 3.
13
350
400
EL intensity ( a.u. )
120
 = 470 nm, blue-LED, MQW x 5
duty cycle 1%
duty cycle 10%
dc
90
60
30
(a)
0
( frequency = 1K Hz )
Efficiency ( a.u. )
12
9
6
3
(b)
Wavelength ( nm )
0
485
480
475
470
(c)
465
460
0
50
100
150
200
250
300
2
Injection Current Density ( A/cm )
FIG. 4.
14
350
400
Output Power ( mW )
16
MQW x 5
MQW x 10
MQW x 15
12
8
4
(a)
0
Efficiency ( % )
5
4
3
2
(b)
1
Efficiency ( a.u. )
5
4
3
2
1
0
duty cycle 1%
( frequency = 1K Hz )
0
50
100
150
200
250
(c)
300
2
Injection Current Density ( A/cm )
FIG. 5.
15
350
400