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Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN
surface
View the table of contents for this issue, or go to the journal homepage for more 2005 Nanotechnology 16 1844
(http://iopscience.iop.org/0957-4484/16/9/071)
Nanotechnology 16 (2005) 1844–1848 doi:10.1088/0957-4484/16/9/071
Enhanced light output of an InGaN/GaN
light emitting diode with a
nano-roughened p-GaN surface
Hung-Wen Huang
1,2, J T Chu
1, C C Kao
1, T H Hseuh
1,
T C Lu
1, H C Kuo
1, S C Wang
1,4and C C Yu
31Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung
University, Hsinchu 300, Taiwan, Republic of China
2TrueLight Corporation, Hsinchu 300, Taiwan, Republic of China 3Highlink Corporation, Hsinchu 300, Taiwan, Republic of China
E-mail:[email protected]
Received 2 April 2005, in final form 1 June 2005 Published 22 July 2005
Online atstacks.iop.org/Nano/16/1844
Abstract
This investigation describes the development of an InGaN/GaN light emitting diode (LED) with a nano-roughened top p-GaN surface using an Ni nano-mask and laser etching. The light output of the InGaN/GaN LED with a nano-roughened top p-GaN surface is 1.55 times that of a conventional LED, and the wall-plug efficiency is 68% higher at 20 mA. The series resistance of the InGaN/GaN LED was reduced by 32% by the increase in the contact area of the nano-roughened surface.
(Some figures in this article are in colour only in the electronic version)
GaN-based materials have attracted considerable interest in relation to their potential use in optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes (LDs) [1–4]. Recently, as the brightness of GaN-based LEDs has increased, applications such as displays, traffic signals, backlights for cell phones, exterior automotive lighting and printers have become possible. However, the internal quantum efficiency of GaN-based LEDs is about 70% at room temperature because of non-radiative defects. Furthermore, the external quantum efficiency of GaN-based LEDs is low because the refractive index of the nitride epitaxial layer differ greatly from that of the air. The refractive indices of GaN and air are 2.5 and 1.0, respectively. Thus, the critical angle at which light generated in the InGaN–GaN active region can escape is approximately
(θc = sin−1(nair/nGaN)) ∼ 23◦, which limits the external
quantum efficiency of conventional GaN-based LEDs to only a few per cent [5]. The light from LEDs can be enhanced either through the sample surface or through the side walls of the chip. Research into improving the light extraction efficiency (external quantum efficiency) and brightness in the LEDs [5–11] has been intense. Recently, Chang et al reported that cap layers grown at low temperature(800◦C) increased
4 Author to whom any correspondence should be addressed.
the power output by InGaN–GaN MQW LEDs by 10% [12]. Fujii reported an increase in the extraction efficiency of GaN-based light-emitting diodes by surface roughening [13]. These processes all allow the photons generated within the LEDs to find the escape cone, by multiple scattering from a rough surface. Huh et al reported that micro-roughening the top surface of an InGaN/GaN LED using metal clusters as a wet etching mask increased the wall-plug efficiency by 62% [5]. Huh et al showed a large improvement in the light output power, indicating that the use of metal clusters to fabricate a roughened p-GaN surface is an excellent means of making a high-power LED. This investigation reports the production of a GaN LED with a nano-roughened p-GaN surface using a self-assembled Ni metal cluster as the laser etching mask. The dimensions and density of the self-assembled Ni cluster can be controlled by rapid thermal annealing at temperatures from 750 to 850◦C, details of which have been recently reported [14]. As a result, the light output efficiency of the LED with a nano-roughened surface was significantly higher than that of a conventional LED without a roughened surface. Additionally, the current–voltage (I –V ) measurements demonstrate that the forward voltage of the LED with a nano-roughened surface was lower than that of a conventional LED from the same wafer with standard device processing.
Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN surface The GaN LED samples were grown by metal–organic
chemical vapour deposition (MOCVD) with a rotating-disc reactor (Emcore D75TM) on a c-axis sapphire (0001) substrate
at a growth pressure of 200 mbar. Trimethylgallium (TMG), trimethylaluminium (TMA) ammonia, CP2Mg and Si2H6were
used as sources of Ga, Al, N, Mg and Si. The LED structure includes a 30 nm thick GaN low-temperature buffer layer, a 4.0 µm thick highly conductive Si-doped GaN layer (grown at 1050◦C), an active region of undoped multiple quantum wells (MQWs) that includes 2/5 nm thick In0.21Ga0.79N/GaN with
five periods of multiple quantum wells (grown at 750◦C), a 50 nm thick Mg-doped AlGaN layer (grown at 1050◦C) and finally 0.1 µm thick Mg-doped GaN grown at 1050◦C. The top surface of LED, which is a p-GaN surface, was roughened by both the formation of an Ni nano-mask on a top p-GaN surface of an LED and by laser etching. The surface roughness of the LED cap layer was measured by tapping mode atomic force microscopy (Veeco).
Nano-roughened LEDs were formed by depositing Ni thin film with a thickness of 5 nm on a p-GaN surface by electron beam evaporation. Rapid thermal annealing (RTA) was then performed at 750◦C for 1 min to change the Ni layer to the metal Ni nano-mask on the top p-GaN surface. Then, a KrF excimer laser at wavelength of 248 nm with pulse width of 25 ns and the incident laser fluence was 300 mJ cm−2 was used to etch the p-GaN surface in air. In this process, the beam size 1 mm× 1 mm of the KrF laser was larger than the size of the LEDs. Therefore, the laser irradiation on the surface of p-GaN was uniform. Thus, the etching rate of the p-GaN layer in 300 mJ cm−2 was determined to be approximately 10 nm/pulse in air. Then, the nano-roughened LED sample was dipped into HCl solution for 5 min to remove the residual Ga, and Ga oxide on the p-GaN and dipped into a nitric acid solution (HNO3) for 5 min to remove the Ni
nano-mask from a nano-roughened LED after the laser etching process. Afterwards, the conventional LED and the LED with a nano-roughened surface were fabricated using the standard process (four mask steps) with a mesa area(300 × 300 µm2).
Firstly, the 0.5 µm SiO2was deposited onto the sample surface
by plasma enhanced chemical vapour deposition (PECVD). Photo-lithography was used to define the mesa pattern after wet etching of SiO2 by a buffer oxide etching solution.
The mesa etching was then performed with Cl2/Ar as the
etching gas in an ICP-RIE system (SAMCO ICP-RIE 101iPH) which the ICP source power and bias power were operated at 13.56 MHz. The metal contact layers, including transparent contact and pad layers, were patterned by a lift-off procedure and deposited onto samples by electron beam evaporation. Ni/Au (3/5 nm) was used for the transparent electrode and Ti/Al/Ni/Au (20/150/20/200 nm) was used for the n-type electrode. Finally, Ni/Au (20/150 nm) was deposited onto the p-type electrode.
Figures1(a)–(c) show the AFM images that describe the change of the surface morphology of the p-GaN surface during surface roughening. Figure1(a) shows that the conventional p-GaN cap has a root mean square (RMS) roughness of 0.7 nm, and a surface depth of approximately 2 nm. The surface of the conventional LED was smooth. Figure 1(b) shows a nano-mask AFM image RMS roughness of 7.4 nm before laser etching was performed. The self-assembled Ni
1 2 3 4 x 1.000 µm/div z 50.000 nm/div RMS roughness = 0.7 nm (a) RMS roughness = 7.4 nm (b) µm x 1.000 µm/div z 50.000 nm/div 1 2 3 4 µm RMS roughness = 5.8 nm (c) x 1.000 µm/div z 50.000 nm/div 1 2 3 4 µm
Figure 1. AFM images of the top surface morphology of an LED sample. (a) Conventional LED p-GaN surface image. (b) Ni nano-mask on p-GaN surface image. (c) Nano-roughened LED top p-GaN surface image.
mask dimension size and density were approximately 250 nm and 3× 109 cm−2, and the height of the Ni clusters was
approximately 30 nm when the original Ni thickness was 50 Å under RTA conditions of 750◦C for 1 min. Figure1(c) displays the AFM image that shows that RMS roughness of p-GaN surface increased drastically to 5.8 nm, and the surface depth was approximately 17 nm after laser etching and the removal of the Ni nano-mask.
The I –V characteristics of the conventional and nano-roughened LEDs were also measured. Figure 2(a) plots the I –V characteristics of conventional and nano-roughened 1845
0 20 40 60 80 100 0 1 2 3 4 5 6 7 8 Current ( mA ) (a) (c) (b) -5.0 -2.5 0.0 2.5 5.0 -3.0x10-3 -2.0x10-3 -1.0x10-3 0.0 1.0x10-3 2.0x10-3 3.0x10-3 Current ( A ) Voltage ( V ) Voltage ( V ) -10 -5 0 5 10 15 20 25 30 35 40 45 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 conventional p-GaN nano-roughened p-GaN conventional p-GaN nano-roughened p-GaN conventional LED nano-roughened LED To
tal Resistance (ohm)
Pad distance ( µm )
Figure 2. (a) I –V forward curves of conventional and nano-roughened LEDs fabricated in this investigation. (b) I –V characteristics of the nano-roughened and conventional p-GaN measured by the circular transmission line method (CTLM). (c) Total resistance(Y ) measured by CTLM is linearly related to the pad distance(X).
LEDs. The forward voltages of the conventional and nano-roughened LEDs were 3.54 and 3.27 V at a driving current of 20 mA, respectively. Furthermore, the dynamic resistance
(R = dV/dI) of the nano-roughened LED (27 ) was 32%
lower than that of the conventional LED(40 ). A 1 µm thick p-GaN sample (hole carrier concentration of 5.0 × 1017cm−3
and hole mobility of 10.2 cm2V−1s−1) and both nano-roughened and conventional p-GaN samples were deposited with the same metallization of Ni (20 nm)/Au (150 nm) and rapid thermal annealing (RTA) 450◦C for 1 min, to investigate the electric characteristics of the Ohmic contact of nano-roughened and conventional LEDs on the p-GaN layer. Figure2(b) plots the I –V characteristics of the nano-roughened and conventional p-GaN as measured by the circular transmission line method (CTLM). The nano-roughened p-GaN sample has better linearity than the conventional p-p-GaN sample. The total resistance (RT) measured by CTLM is
linearly related to the pad distance(X), as shown in figure2(c). The specific contact resistance ρc was determined from the
linear equation RT= 2Rc+ρs/2π R × X, and the relationship
between the specific resistance and the sheet resistance was
ρc = ρs× L2T, where Rc is the contact resistance; ρs is the
sheet resistance, and LTis the transfer length. The data were
fitted to obtain a lower specific contact resistance value of 2.5 × 10−4 cm2 for the nano-roughened p-GaN than the
value 7.2 × 10−3 cm2for the conventional p-GaN sample.
The results indicated that nano-roughening facilitates p-type contact, resulting in an Ohmic contact with a low specific resistance.
Electroluminescence (EL) was measured by injecting a continuous current into a device at room temperature. The light output was detected using a calibrated large-area Si photodiode placed 5 mm from the top of the device. This detecting condition covers almost all of the power emitted from the LEDs. Figures 3(a) and (b) plot the spectra and intensity–current (L–I ) characteristics of conventional and roughened LEDs. The EL intensity of the nano-roughened LED exceeds that observed from the conventional LED (as shown in figure 3(a)). At an injection current of 20 mA, all of the MQW emission peaks of these two were
Enhanced light output of an InGaN/GaN light emitting diode with a nano-roughened p-GaN surface 400 420 440 460 480 500 520 0 1000 2000 3000 4000 5000 6000
EL intensity ( arb. units )
Wavelength ( nm ) Conventional LED nano-roughened LED (a) (b) (c) (d) 0 20 40 60 80 100 0 5 10 15 20 25 30
Light output power ( mW )
Current ( mA ) Conventional LED
nano-roughened LED
Figure 3. (a) Room temperature EL spectrum of conventional and nano-roughened LED at a current of 20 mA. (b) Light output
power–current (L–I ) characteristics of conventional and nano-roughened LEDs. (c) Photons of conventional LED and (d) nano-roughened LED at a dc injection current of 20 mA.
at approximately 452 nm and the light output power of the conventional and nano-roughened LEDs were approximately 5.3 and 8.3 mW, respectively (as shown in figure 3(b)). Restated, nano-roughening the p-GaN surface increased the output power of the InGaN–GaN MQW LEDs by a factor of 1.55, indicating that the LED with the nano-roughened surface had larger light extraction efficiency. The wall-plug efficiency (output power/input power) was also calculated: The nano-roughened LEDs (12.6%) was 68% higher than that of the conventional LED (7.5%) at an injection current of 20 mA, because of enhanced light output power and a lower forward voltage. The intensity distributions of conventional and nano-roughened LEDs were measured to investigate further the influence of surface roughness on the light output performance of an LED. Figures3(c) and (d) show the light output of conventional and nano-roughened LEDs when a 20 mA dc current is injected into these two devices. Intensity distributions are also shown. The EL intensities observed from the nano-roughened LED clearly exceeded those from the conventional LED at the same injection current, especially on the LED top surface.
In summary, this investigation describes the improvement of an InGaN/GaN MQW light emitting diode by nano-roughening the p-GaN surface using Ni nano-mask and laser etching. The nano-roughened surface improved the escape probability of light output inside the LED structure, increasing by 55% the light output of InGaN/GaN LED at 20 mA. The operating voltage of the InGaN/GaN LED was reduced from 3.54 to 3.27 V at 20 mA and the series resistance was reduced by 32% by the increase in the contact area of the nano-roughened surface. The wall-plug efficiency of the InGaN/GaN LED was increased by 68% by nano-roughening the top p-GaN surface using the Ni nano-mask and laser etching.
Acknowledgments
The authors would like to thank F I Lai from National Chiao Tung University for useful discussion. This work was supported in part by the National Science Council of Republic of China (ROC) in Taiwan under contract No NSC 92-2215-E-009-015, NSC 92-2112-M-009-026 and by the Academic 1847
Excellence Program of the ROC Ministry of Education under contract No 88-FA06-AB.
References
[1] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, Sugimoto Y and Kiyoku H 1997 Room-temperature continuous-wave operation of InGaN multi-quantum-well-structure laser diodes with a long lifetime Appl. Phys. Lett. 70 868–70
[2] Nakamura S, Senoh M, Iwasa N and Nagahama S 1995 High-brighness InGaN blue, green and yellow
light-emitting diodes with quantum well structures Japan. J.
Appl. Phys. 34 L797–9
[3] Nakamura S, Mukai T and Senoh M 1994 Candela-class high-brighness InGaN/AlGaN double-heterostructure blue-light-emitting-diode Appl. Phys. Lett. 64 1687–9 [4] Mukai T and Nakamura S 1999 Ultraviolet InGaN and GaN
single-quantum-well-structure light-emitting diodes grown on epitaxially laterally overgrown GaN substrates Japan. J.
Appl. Phys. 38 5735–9
[5] Huh C, Lee K S, Kang E J and Park S J 2003 Improved light-output and electrical performance of InGaN-based light-emitting diode by microroughening of the p-GaN surface J. Appl. Phys. 93 9383–5
[6] Wierer J J et al 2001 High-power AlGaInN flip-chip light-emitting diodes Appl. Phys. Lett. 78 3379–81
[7] Jin S X, Li J, Lin J Y and Jiang H X 2000 InGaN/GaN quantum well interconnected microdisk light emitting diodes Appl. Phys. Lett. 77 3236–8
[8] Oder T N, Shakya J, Lin J Y and Jiang H X 2003 III-nitride photonic crystals Appl. Phys. Lett. 83 1231–3
[9] Oder T N, Kim K H, Lin J Y and Jiang H X 2004 III-nitride blue and ultraviolet photonic crystal light emitting diodes
Appl. Phys. Lett. 84 466–8
[10] Choi H W, Dawson M D, Edwards P R and Martin R W 2003 High extraction efficiency InGaN micro-ring light-emitting diodes Appl. Phys. Lett. 83 4483–5
[11] Fujii T, Gao Y, Sharma R, Hu E L, DenBaars S P and Nakamura S 2004 Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening
Appl. Phys. Lett. 84 855–7
[12] Chang S J, Wu L W, Su Y K, Hsu Y P, Lai W C, Tsai J M, Sheu J K and Lee C T 2004 Nitride-based LEDs with 800◦C grown p-AlInGaN–GaN double-cap layers IEEE
Photon. Technol. Lett. 16 1447–9
[13] Fujii T, Gao Y, Sharma R, Hu E L, DenBaars S P and Nakamura S 2004 Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening
Appl. Phys. Lett. 84 855–7
[14] Huang H W, Kao C C, Hsueh T H, Yu C C, Lin C F, Chu J T, Kuo H C and Wang S C 2004 Fabrication of GaN-based nanorod light emitting diodes using self-assemble nickel nano-mask and inductively coupled plasma reactive ion etching Mater. Sci. Eng. B 113/2 125–9
