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Enhancement of Light Output Power of InGaN/GaN Multiple Quantum Well Light-Emitting
Diodes by Titanium Dioxide Texturing
View the table of contents for this issue, or go to the journal homepage for more 2008 Jpn. J. Appl. Phys. 47 5438
(http://iopscience.iop.org/1347-4065/47/7R/5438)
Enhancement of Light Output Power of InGaN/GaN Multiple
Quantum Well Light-Emitting Diodes by Titanium Dioxide Texturing
Kuo-Chin HUANG, Wen-How LAN1, and Kai Feng HUANGDepartment of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China
1Department of Electrical Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, Republic of China
(Received July 13, 2007; accepted October 21, 2007; published online July 11, 2008)
The enhancement of external quantum efficiency in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with a titanium dioxide (TiO2) textured film has been observed. The output power values of conventional and TiO2textured LEDs at an injection current of 20 mA are 6.25 and 8 mW, respectively. The external quantum efficiencies of the conventional and TiO2textured LEDs at an injection current of 20 mA are 11.5 and 14.8%, respectively. The external quantum efficiency of the TiO2textured LEDs at an injection current of 20 mA is 28% higher than that of the conventional LEDs. A higher-output-power InGaN/GaN MQW LED has been obtained by coating with a TiO2textured film. [DOI:10.1143/JJAP.47.5438]
KEYWORDS: titanium dioxide (TiO2), textured, output power, external quantum efficiency
1. Introduction
Nitribased compound semiconductors have been de-veloped for use in high-performance optical devices such as light-emitting diodes (LEDs), laser diodes (LDs), photo-conductive detectors, and photovoltaic detectors operating in the blue-ultraviolet (UV) range of the light spectrum. GaN-based blue and green LEDs are used in full color displays and traffic light lamps. The output power of LEDs depends on the external quantum efficiency and is suppressed by the total internal reflection between the semiconductor and air.1)
The achievement of a high extraction efficiency is an important issue in this drive to improve the output power in LEDs. Previously, it was reported that a high brightness in LEDs was achieved using a patterned sapphire substrate (PSS) technique2–9) to improve output power. However,
such methods are complex and expensive. Recently, a high extraction efficiency in LEDs grown10) and processed11–13) with a textured surface to enhance light scattering has been reported. This may result in the induction of reverse leakage current and a higher operation voltage. In this work, we have developed a new approach to fabricate high-external-quantum-efficiency blue LEDs using a titanium dioxide (TiO2) textured film. The output power, external quantum
efficiency, and atomic force microscope (AFM) morphology of InGaN/GaN MQW LEDs with a TiO2 textured film will
be discussed.
2. Experimental Procedure
Samples were all grown on c-face (0001) sapphire substrates by metal organic chemical vapor deposition (MOCVD). The LED structure comprises a low-temper-ature-GaN buffer layer, a 2-mm-thick unintentionally doped GaN layer, a 2-mm-thick nþ-GaN:Si layer, five periods of
InGaN-GaN multiple quantum wells (MQWs), and a 0.2-mm-thick pþ-GaN:Mg layer. A detailed schematic of the
structure of the TiO2 textured InGaN/GaN MQW LEDs is
shown in Fig. 1. The device mesa layer was defined by ICP etching. The device size was 1:5 103cm2. Mixed
solu-tions containing TiO2 powder (1 g) and isopropanol (25 ml)
were prepared. The TiO2 mixed solutions were then spun
and coated onto the p-GaN surface. The TiO2 textured
density was about 2:6 108cm2. The indium tin oxide (ITO) was evaporated onto the p-GaN surface and covered the TiO2 textured film. Then, samples were subsequently
alloyed at 600C in N2 ambient. Cr–Pt–Au contacts were
evaporated onto the n-GaN layer as a bonding pad and n-type ohmic contacts. The InGaN/GaN MQW LEDs were characterized by current–voltage (I–V) measurements performed with a Hewlett-Packard 4156 semiconductor analyzer. The output power characteristics were analyzed with an integrated sphere detector.
3. Results and Discussion
Figures 2(a) and 2(b) show AFM images of conventional and TiO2 textured InGaN/GaN MQW LEDs. In Fig. 2(a),
the conventional InGaN/GaN MQW LED morphology is planar and smooth. The TiO2 textured InGaN/GaN MQW
LED is shown in Fig. 2(b). The thickness of the TiO2
textured film is about 500 nm. It is known that the external quantum efficiency of LEDs is limited by the total internal reflection at the semiconductor and air interface. The refractive index of TiO2 is approximately 2.114–16) and is
just between those of GaN and ITO, which are 2.4 and 1.9, respectively. In the process, the textured TiO2 film is
embedded between GaN and ITO films. The textured surface may reduce the internal reflection on the surface; thus, more light can be extracted. A higher external quantum efficiency
n+-GaN:Si Sapphire p+-GaN:Mg TiO2 textured Pad Pad MQWs
Fig. 1. Schematic structure of TiO2textured InGaN/GaN MQW LEDs.
E-mail address: [email protected]
Japanese Journal of Applied Physics Vol. 47, No. 7, 2008, pp. 5438–5440
#2008 The Japan Society of Applied Physics
for InGaN/GaN MQW LEDs induced by coating with a TiO2 textured film can be expected. The I–V characteristics
of the TiO2textured InGaN/GaN MQW LEDs are shown in
Fig. 3. In the forward-biased region, the curves are match quite well for samples with and without TiO2. With the
nanoscaled TiO2film, a higher conductivity can be observed
owing to the enhancement of the charge transport behavior on the nanoscale between different materials.17)In addition,
the conductivity of the nanoscaled TiO2film increases under
photon illumination.18) The nanoscaled TiO
2 film showed
well behavior in conductivity and no significant current difference in the forward-biased region. In the reverse-biased region, the current is the same within 3 V. When biased at 5 V, the sample with TiO2 shows weaker leakage current
characteristics. According to data of GaN-based devices grown on a sapphire substrate using epitaxial lateral over-grown (ELOG) techniques, dielectric mask may suppress threading dislocation and decrease leakage current.19,20) In
this work, the partially TiO2 textured film on top of the
p-GaN surface may effectively cover and suppress threading dislocation or decrease the number of defects due to the insulating material.
The output power values and external quantum
efficien-cies of the conventional and TiO2 textured InGaN/GaN
MQW LEDs are shown in Fig. 4. The output power values of the conventional and TiO2 textured InGaN/GaN MQW
LEDs at an injection current of 20 mA are 6.25 and 8 mW, respectively. The external quantum efficiencies of the conventional and TiO2 textured InGaN/GaN MQW LEDs
at a wavelength of 460 nm are 11.5 and 14.8%, respectively. Therefore, the external quantum efficiency of the TiO2
textured InGaN/GaN MQW LEDs is 28% higher than that of the conventional InGaN/GaN MQW LEDs. A higher output power in InGaN/GaN MQW LEDs can be obtained by coating with a textured TiO2 film on top p-GaN surface.
The thermal effects and output power saturation phenomen-on for the InGaN/GaN MQW LEDs are obvious in Fig. 4. It was known that the saturated output power of LEDs increases as the applied injection current increases owing to less heat forming in devices.21) The saturated injection
currents of the conventional and textured LEDs were 200 and 280 mA, respectively. We found out that applying a higher injection current leads to a higher saturated output power in TiO2textured InGaN/GaN MQW LEDs. Less heat
will thus generate in the TiO2 textured InGaN/GaN MQW
LEDs.
(a) (b)
Fig. 2. AFM images of (a) conventional LED (b) TiO2textured InGaN/GaN MQW LED.
-5 -4 -3 -2 -1 0 1 2 3 1x10-13 1x10-11 1x10-9 1x10-7 1x10-5 1x10-3 1x10-1 Log ( I ) (A) Bias (V) Conventional LEDs Textured LEDs
Fig. 3. Current–voltage characteristics of TiO2 textured InGaN/GaN
MQW LEDs. 0 50 100 150 200 250 300 350 400 0 10 20 30 40 50 Output power (mW)
Injection current (mA)
Conventional LEDs Textured LEDs 0 5 10 15 20
External quantum efficiency (%)
Fig. 4. Output power and external quantum efficiency values of conven-tional and TiO2textured InGaN/GaN MQW LEDs.
Jpn. J. Appl. Phys., Vol. 47, No. 7 (2008) K.-C. HUANGet al.
The polar radiation pattern of the InGaN/GaN MQW LEDs coated with a TiO2textured film is wider than that of
the conventional LEDs, as shown in Fig. 5. In general, the output power and external quantum efficiency values of LEDs is limited by the total internal reflection between the semiconductor and air interface for a flat surface.1) A
reduction of the total internal reflection effect and an increase in the level of surface light scattering induced by forming an appropriate TiO2 textured densities at a top
p-GaN surface can be anticipated. Light escapes from the LEDs into air much more easily with a textured surface. The higher output power of the InGaN/GaN MQW LEDs with the wider polar radiation pattern induced by coating with an appropriately textured TiO2film was observed. This result is
in good agreement with those of other reports on the PSS technique22,23)and epitaxial growth surface roughness.24)
Figure 6 shows the results of room temperature reliability tests on the conventional and textured LEDs burned and measured at injection currents of 50 and 20 mA, respective-ly. The decay of normalized light intensity for the textured LEDs was 6% less than that for the conventional LEDs (12%) after 840 h. This result can be attributed to the less heat generated in the TiO2 textured InGaN/GaN MQW
LEDs and the better I–V characteristics of such LEDs.
4. Conclusions
The improvement of external quantum efficiency of InGaN/GaN MQW LEDs coated with a TiO2 textured film
has been studied. The output power values of the conven-tional and TiO2 textured InGaN/GaN MQW LEDs at an
injection current of 20 mA are 6.25 and 8 mW, respectively. The external quantum efficiencies of the conventional and TiO2 textured InGaN/GaN MQW LEDs at an injection
current of 20 mA are 11.5 and 14.8%, respectively. The external quantum efficiency of the TiO2 textured InGaN/
GaN MQW LEDs at an injection current of 20 mA is 28% higher than that of the conventional InGaN/GaN MQW LEDs. A higher output power and wider polar radiation pattern in InGaN/GaN MQW LEDs are effectively obtained by coating with a TiO2 textured film.
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Fig. 5. Polar radiation patterns of InGaN/GaN MQW LEDs with textured TiO2 film. The dotted line indicates the pattern of the TiO2 textured
LEDs. 0 200 400 600 800 88 90 92 94 96 98 100
Normalized light intensity (%)
Time (h)
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Jpn. J. Appl. Phys., Vol. 47, No. 7 (2008) K.-C. HUANGet al.
