High light output intensity of titanium dioxide textured light-emitting diodes
K.C. Huang
a, W.H. Lan
b,*, K.F. Huang
a, J.C. Lin
c, Y.C. Cheng
d, W.J. Lin
d, S.M. Pan
ea
Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC
b
Department of Electrical Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROC
cInstitute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC dMaterials and Electro-optics Research Division, Chung-Shan Institute of Science and Technology, Taoyuan 325, Taiwan, ROC e
R&D Division, Formosa Epitaxy, Co., Ltd., Lung-Tan, TaoYung 325, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 6 January 2008 Accepted 29 February 2008 Available online 9 July 2008
The review of this paper was arranged by Prof. E. Calleja
Keywords:
Light-emitting diodes Titanium dioxide
External quantum efficiency
a b s t r a c t
Higher light output intensity and wider polar radiation pattern of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with a different nanoscale titanium dioxide (TiO2) textured densities film have been observed. The light output power values and external quantum efficiency of the conven-tional LEDs at an injection current of 20 mA are 6.34 mW and 11.7%, respectively. The light output power values and external quantum efficiency of the nanoscaled TiO2textured LEDs at an injection current of 20 mA are 7.55 mW and 14%, respectively. The light output intensity and power values of the nanoscaled TiO2textured LEDs is approximately 65% and 20% higher than that of the conventional LEDs, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Applications for the wide band gap nitrides includes emitters in the light-emitting diodes (LEDs), laser diodes (LDs), photoconduc-tive detectors and photovoltaic detectors in the blue-ultraviolet (UV) range of the light spectrum. Currently, GaN-based blue and green LEDs could be used in full color displays and traffic light lamps. The high brightness LEDs are necessary in these applica-tions. One important issue of LED is how to increase external quan-tum efficiency. Textured surface has been demonstrated that it is an effective technique to enhance external quantum efficiency and to reduce total internal reflection between the semiconductor and air[1]. Light escaped from the active layer of LEDs into air much more easily with a textured surface will be anticipated. Thus, the light output power values and external quantum efficiency of such LEDs with a textured surface is higher than that of conven-tional LEDs. It was studied that a high light output power values in LEDs was achieved by patterned sapphire substrate (PSS) tech-nique[2–7]and roughness surface[8,9]. On the other hand, surface roughness was defined by processes such as indium tin oxide (ITO) with a textured surface[10,11], etching thin film[12,13]and side-wall roughness[14]. However, such methods can also induce the output power values and external quantum efficiency although surface leakage current may be generated also by those processes.
In this study, we have obtained higher light output intensity and external quantum efficiency of InGaN/GaN multiple quantum well (MQW) LEDs using a different titanium dioxide (TiO2) textured
films on the top of p-GaN surface. The atomic force microscope (AFM) images, output power and current–voltage (I–V) character-istics and of InGaN/GaN MQW LEDs with a different TiO2textured
densities film will be discussed. 2. Experimental procedure
The InGaN/GaN MQW LEDs were grown by metal organic chem-ical vapor deposition (MOCVD). The LEDs structure consists of a low-temperature-GaN nucleation layer, a 2-lm-thick undoped GaN bulk layer, a 2-lm-thick Si-doped n+-GaN layer, five periods
of InGaN–GaN multiple quantum wells, and 0.2-lm-thick Mg-doped p+-GaN layer. Device patterns and mesa layer were defined
by standard photolithography and inductively coupled plasma reactive ion etching (ICP-RIE) techniques. The device chip size was 1.5 10 3cm2. Mixed TiO
2 solution (0.5 M) containing of
TiO2 powder (1 g) and isopropanol (25 ml) was prepared. The
nanoscaled TiO2 film with different densities was controlled by
an amount of mixed TiO2 solution and coated times. The
nano-scaled TiO2textured densities which were 1.7 l08cm 2(as
‘‘Sam-ple A”), 2.6 l08cm 2(as ‘‘Sample B”), 3.8 l08cm 2(as ‘‘Sample
C”) and 4.4 l08cm 2(as ‘‘Sample D”) were spun and coated on a
topp-GaN surface, and without a TiO2textured film (as
‘‘conven-tional LED”). The ITO film was evaporated on the p-GaN surface
0038-1101/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2008.02.007
*Corresponding author. Tel.: +886 7 5919 437; fax: +886 7 5919 374. E-mail address:[email protected](W.H. Lan).
Solid-State Electronics 52 (2008) 1154–1156
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and was subsequently alloyed at 600 °C in N2ambient. In this
pro-cess, the nanoscaled TiO2textured film is embedded between GaN
and ITO film. Cr–Pt–Au contacts were evaporated on the p- and n-GaN layer as bonding pad and n-type ohmic contacts. The Inn-GaN/ GaN MQW LEDs were characterized by I–V measurements per-formed with a Hewlett-Packard 4155 semiconductor analyzer. The light output power characteristics of bare chip LEDs were ana-lyzed with an integrated sphere detector.
3. Results and discussion
The schematic drawings of the conventional and nanoscaled TiO2 textured InGaN/GaN MQW LEDs structures are shown in
Fig. 1a and b, respectively. AFM images of the nanoscaled TiO2film
with different densities film are shown in Fig. 2a–d. The nano-scaled TiO2textured size is about 500 nm. The reverse leakage
cur-rent at an applied bias of 1 V and operation bias voltage at an injection current of 20 mA are subject to nanoscaled TiO2textured
densities as shown inFig. 3. A reduction of reverse leakage current and an increase of operation bias voltage with different nanoscaled TiO2textured densities are observed owing to a change in injection
area, i.e. the leakage current density remains constant.
Fig. 4shows the light output intensity of the conventional LEDs and the nanoscaled TiO2 textured LEDs with different densities.
The light output intensity of the conventional LEDs, Sample A, Sample B, Sample C and Sample D by driven at an injection current of 20 mA are approximately 11.9, 14.6, 16, 19.6 and 20 mcd,
n-GaN:Si
Sapphire
p-GaN:Mg
Pad Pad ITO MQWn-GaN:Si
Sapphire
p-GaN:Mg
TiO2 Pad Pad MQWa
b
Fig. 1. Structure of (a) the conventional and (b) nanoscaled TiO2textured InGaN/
GaN MQW LEDs.
Fig. 2. AFM images of nanoscaled TiO2textured InGaN/GaN MQW LEDs with different densities: (a) sample A, (b) sample B, (c) sample C, and (d) sample D.
0 1 2 3 4 5 0.90 0.95 1.00 1.05 1.10
Reverse biased current
Operation voltage (V)
Textured density ( x108 cm-2 )
Reverse biased current (nA)
3.36 3.40 3.44 3.48 3.52 3.56 3.60 Operation voltage
Fig. 3. The reverse biased current (at voltage 1 V) and operation voltage (at injec-tion current 20 mA) with different nanoscaled TiO2textured densities.
respectively. The light output intensity of the nanoscaled TiO2
tex-tured LEDs (Sample D) is about 65% higher than that of the conven-tional LEDs. This result indicates the higher light output intensity is attributed to a reduction of the total internal reflection effect and an increase in the level of surface light scattering induced by form-ing an appropriate TiO2textured densities at a top p-GaN surface.
The light output power characteristics of bare chip LEDs were mea-sured by an integrated sphere detector and are shown inFig. 5. The light output power values and external quantum efficiency of the conventional LEDs at an injection current of 20 mA are approxi-mately 6.34 mW and 11.7%, respectively. The light output power
values and external quantum efficiency of the nanoscaled TiO2
tex-tured LEDs (Sample D) at an injection current of 20 mA are approx-imately 7.55 mW and 14%, respectively. The external quantum efficiency of the nanoscaled TiO2 textured LEDs (Sample D) is
approximately 20% higher than that of the conventional LEDs. Polar radiation pattern of the conventional LEDs and nanoscaled TiO2textured LEDs with different densities are shown inFig. 6. The
polar radiation pattern of the nanoscaled TiO2 textured LEDs is
wider than that of the conventional LEDs. In general, the light out-put power values and external quantum efficiency of LEDs are lim-ited by the total internal reflection between the semiconductor and air interface for a flat surface. Wider polar radiation pattern in LEDs may implicate a reduction of the total internal reflection and thus increase the surface light extraction efficiency by the forming of a appropriate TiO2 textured film at a top p-GaN surface. The light
extraction from the LED surface into air is much more easily with this textured surface and thus, higher light output power behavior can be characterized.
4. Conclusion
Higher light output intensity and wider polar radiation pattern of InGaN/GaN MQW LEDs with different nanoscaled TiO2textured
density films have been fabricated. The light output power values of the conventional and nanoscaled TiO2textured LEDs at an
injec-tion current of 20 mA are 6.34 and 7.55 mW, respectively. The external quantum efficiency of the conventional and nanoscaled TiO2textured LEDs at an injection current of 20 mA are 11.7 mW
and 14%, respectively. The light output intensity and power values of nanoscaled TiO2textured LEDs are approximately 65% and 20%
higher than that of the conventional LEDs, respectively. References
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Light output intensity (mcd)
Injection current (mA)
Conventional LED Sample A Sample B Sample C Sample D
Fig. 4. Light output intensity of the conventional and different nanoscaled TiO2
textured densities LEDs.
0 20 40 60 80 100 0 5 10 15 20 25
External quantum efficiency (%)
Light output power (mW)
Injection current (mA)
6 8 10 12 14 16 18 Conventional LED
nanoscaled TiO2 textured LED
Fig. 5. Light output power values and external quantum efficiency of the conven-tional and different nanoscaled TiO2textured densities LEDs.
0 30 60 90 120 150 180 Conventional LED Sample A Sample B Sample C Sample D
Fig. 6. Polar radiation pattern of the conventional and different nanoscaled TiO2
textured densities LEDs.
