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Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures
Thin Solid Films 520 (2011) 437–441
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures Ji Hye Kang, Jae Hyoung Ryu, Hyun Kyu Kim, Hee Yun Kim, Nam Han, Mi So Lee, Young Jae Park, Periyayya Uthirakumar, Volodymyr V. Lysak, Chang-Hee Hong ⁎ School of Semiconductor and Chemical Engineering, Chonbuk National University, Chonju 561-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 29 April 2010 Received in revised form 12 May 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: Light-emitting diodes Nanoporous oxide Indium tin oxide Gallium nitride Scanning electron microscopy
a b s t r a c t We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction GaN-based light-emitting diodes (LEDs) are promising devices for use in numerous applications including traffic signals, full color displays, solid state lighting, and backlighting in liquid crystal displays. However, to make such solid-state lighting systems practical, further improvements in the output efficiencies of GaN-based LEDs are required. The extraction efficiency still remains low because of the total reflection occurring at the surfaces of the LEDs [1]. Intensive study has led to significant improvements in the light output power, such as using a surface texturing technique to increase the critical angle of the light emitted from the device to the air. Several approaches such as indium tin oxide (ITO) surface texturing using polystyrene spheres [2], an anodic aluminum oxide template [3], and photolithography [4] have been used in attempts to improve the light extraction efficiencies of GaN-based LEDs. Surface texturing with these methods requires an inductively-coupled plasma (ICP) process and an additional patterning process to form the textured surface. However, the ICP process causes degradation of electrical properties due to plasma damage when exposed to the high density plasma. In a previous study, we examined the use of ITO nanospheres to create a textured ITO transparent layer with hemispherical shapes in order to enhance light extraction [5]. The hemispherical shapes were created by two independent step processes. First, the ITO nanosphere
⁎ Corresponding author. E-mail address: [email protected] (C.-H. Hong). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.05.051
was formed using a wet etching process with diluted HCl solution on an as-deposited ITO film. Then, a thin ITO film was evaporated onto the ITO nanosphere structures. In this study, we propose and demonstrate a simpler chemical route for the surface texturing of ITO films with nanoporous structure. The fabrication of ITO film with nanoporous structure using the wet etching process has not yet been reported. However, some researchers have reported on the formation of nanoporous ITO by glancing angle deposition [6] and reactive magnetron sputtering in direct current mode [7]. The later said techniques need specific equipment and hard to control the pores diameter. We were able to create a nanoporous structure on the surface of ITO films using a simple wet etching method. The porous shapes were easily achieved without using additional complex processes or equipment. GaN-based LEDs with nanoporous ITO films were then fabricated in an attempt to improve the light output power. We also examined the properties of nanoporous ITO films for various etch times, and the results are discussed in detail.
1.1. Experimental details In order to study the effects of the wet etch process on the optical and electrical properties of ITO films, a 300 nm-thick ITO film was evaporated onto sapphire using an electron beam evaporator. The asdeposited ITO films were then dipped into diluted HNO3 (2%) for 30, 50, 70, and 90 s. The etched ITO films were annealed at 600 °C for 30 s in air. The electrical and optical properties of the samples were then measured.
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J.H. Kang et al. / Thin Solid Films 520 (2011) 437–441
The GaN-based LEDs used in this study were grown on sapphire substrates using metal organic chemical vapor deposition. The GaN nucleation layer was deposited at a temperature of 560 °C, followed by a 2 μm-thick layer of undoped GaN epilayers deposited at 1100 °C, and a 2 μm-thick layer of Si-doped n-type GaN deposited at 1120 °C. After the growth of the n-type GaN template, we grew seven-period multi-quantum wells of InGaN/GaN pairs at 810 °C, which emit at approximately 460 nm. Next, a 110 nm-thick p-GaN layer was grown at 1050 °C. Finally, a 10 nm-thick highly doped p-GaN layer was grown at 1050 °C. After the growth processes, LED chips with dimensions of 315 × 315 μm 2 were made by mesa etching of the exposed n-type GaN using the ICP process. Then, a 300 nm-thick ITO layer was evaporated onto the p-type GaN surface to form a transparent ohmic contact layer using an electron beam evaporator. To form nanoporous ITO films, the GaN epilayers with an ITO layer on the p-GaN were dipped into diluted HNO3 (2%) for about 50 s (Sample A) and 70 s (Sample B). We note that an etch time of 50 and 70 s are considered to be the optimum conditions based on the experimental results. The conventional LEDs had a planar ITO transparent ohmic contact layer, and Sample A and Sample B had a nanoporous ITO layer as the transparent ohmic contact. In the typical LED process, each ITO film on the p-type GaN layer as the transparent ohmic contact layer was annealed at 600 °C for 30 s in air. Then, the Cr/Au layer was deposited, as metal contacts, on the n- and p-type layers. The surface morphology of the ITO layers was examined by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The FESEM was performed using a JSM-6400 SEM (JEOL, Japan), with an acceleration voltage of 15 kV. To investigate the transparency of the various ITO films, we recorded optical transmittance spectra using a Jasco V-570 UV–VIS-NIR spectrophotometer (JASCO, Japan). The sheet resistance was measured by four point probe system. The current– voltage (I–V) and light output–current (L–I) measurements were carried out using a probe station system. The light intensity of the LED was characterized using an OL 770 multichannel spectroradiometer (Optronic Laboratories, America), and the simulated data was evaluated using LightTools™ software (7.0, Optical Research Associates, America) for comparison between experimental data and simulated data.
1.2. Results and discussion The as-deposited ITO film grown by electron beam evaporation was found to be polycrystalline. It had relatively weak binding energy at the grain boundaries. As a result, the etching process started from the grain boundary when the sample was immersed in weakly diluted HNO3. Fig. 1 illustrates nanopore formation for various etch times. In
the etching process, the reaction began at the In-O bond that was attacked by an HNO3 molecule, resulting in the subsequent formation of In(NO3)3, NO2, and O\H bonds. The reaction is described by the following reaction expression: In2O3 + 12HNO3 → 2In(NO3)3 + 6NO2 + 6H2O [8]. The average diameters of the pores were 44, 81, 104, and 132 nm for etch times of 30, 50, 70, and 90 s, respectively. The corresponding pore densities were 3 × 10 9, 5.1 × 10 9, 6.3 × 10 9, and 6.8 × 10 9 cm −2, respectively. These results indicate that the diameters and densities of the nanopores increased with etch times above 70 s due to cohesion between adjacent pores. However, the increase in pore density saturated. The thicknesses of the ITO films after etching were measured to be 262 and 225 nm for ITO films etched for 70 and 90 s, respectively, as shown in the inset of Fig. 1. The reduction in thickness was caused by an active etching reaction at the surface of the ITO films. Fig. 2(a) shows the transmittance spectra of the ITO films before and after etching for different etch times. We observed a systematic decrease in transmittance as the etch time increased. The decrease in transmittance of the etched ITO films can be explained by an increase in the surface roughness, which led to an increase in light scattering at the surface. As shown in Table I, the root mean square (rms) surface roughness of the wet-etched ITO films was higher than that of conventional ITO films. Thus, the transmittance decreased because the amount of light incident on the detector decreased with the increase of light scattering and scattered reflection from the nanoporous ITO films. Also, as the etch time increased, the sheet resistance increased from 34 to 113 Ω/sq, as shown in Fig. 2(b) and Table I. The increase in sheet resistance was caused by the increase of surface roughness and the decrease of ITO film thickness. The degradation of electrical properties and the increase of surface roughness of the ITO film can be explained by the decrease in mean free path lengths, which was caused by electron scattering [9]. Therefore, the degradation of electrical properties needs to be minimized and optimized in order to enhance the light output power. Based on the transmittance at a wavelength of 460 nm, the sheet resistance, and the rms roughness values given in Table I, we selected etch times of 50 s and 70 s for the fabrication of the LEDs. Fig. 3(a) shows the I–V characteristics of LEDs made with nanoporous ITO layers. For comparison, the I–V curve of a conventional LED is also shown. The forward voltages measured at 20 mA were 3.4, 3.4, and 3.5 V for the conventional LED, Sample A, and Sample B, respectively. The observed marginal increase in forward voltage for sample B is attributed to the increase in sheet resistance, as shown in Table I. The degradation of the layer's electrical properties was minimum when the etch time was short. Fig. 3(b) shows the L–I characteristics of the three different LEDs with and without a nanoporous ITO layer. The output power of Samples A and B was found to be 35% and 13% higher, respectively, than the conventional
Fig. 1. Scanning electron microscope (SEM) images of ITO films illustrating nanopore formation with respect to the etching time, and the respective cross section SEM images (insets).
J.H. Kang et al. / Thin Solid Films 520 (2011) 437–441
a 100
439
a 0.08
p-pad
nanoporous-ITO
60
40 Conventional ITO film 30s etched ITO film 50s ethed ITO film 70s etched ITO film 90s etched ITO film
20
0 300
400
500
600
700
n-pad
p-type GaN : Mg InGaN/GaN MQW
0.06
Current (A)
Transmittance (%)
80
n-type GaN : Si
0.04
Sapphire substrate
0.02 Conventional LED Sample A Sample B
1
800
2
3
4
5
6
Voltage (V)
Wavelength (nm)
b
b 35 30
Light intensity (arb.units.)
Sheet resistance (ohm/sq)
120
90
60
30
25 20 Conventional LED Sample A Sample B
15 10
at 0.5mA
5
30
Conventional LED
0
10
20
30
40
50
60
70
80
90
100
0
0
20
Etch time(s)
40
Sample A
60
Sample B
80
100
Current (mA)
Fig. 2. (a) Transmittance spectra of ITO films etched for different time durations, and (b) variation in sheet resistance of ITO films versus etching time.
Fig. 3. (a) Current–voltage (I–V) characteristics and (b) light output power–current (L–I) characteristics of different LEDs with planar and nanorough ITO film.
LEDs at 20 mA. The inset of Fig. 3 shows optical microscope photographs of a conventional LED, Sample A, and Sample B at an injection current of 0.5 mA. The images show that the emissions from Samples A and B were brighter than the emission from the conventional LED. In addition, current crowding occurred in Sample B around the p-pad due to the high resistivity of the ITO film etched for 70 s. The degradation of internal quantum efficiency could be due to the current crowding effect. Moreover, the electrical property has a connection with the wall-plug efficiency of the LEDs. We can calculate the wall-plug efficiency (output power/input power) using a simple calculation. The wall-plug efficiency of Samples A and B increased by 35% and 9.5% at an injection current of 20 mA compared to that of the conventional LED structure. Considering the electrical properties, the light extraction effect of Sample A was more suitable than that of Sample B. The light output power was quite
different for Samples A and B. We can expect that the enhancement of light output power is related to the size and density of pores and the electrical properties of the etched ITO film. We also performed a simulation to investigate the effect of different surface morphologies on light extraction efficiency. The optical simulation was carried out using LightTools commercial ray-tracer software for a conventional LED with a planar ITO layer, and Samples A and B with a nanoporous ITO layer. Over 100,000 rays were assumed to originate from the MQW region, and the size of the sample used in the simulation was 315 × 315 μm2. The nanoporous ITO film had smaller pore size structures than the emission wavelength of the LED. Since it is difficult to simulate such subwavelength structures using the ray tracing technique, we examined the output from the nanoporous ITO surface as a linear grating with 2π/λ vectors and different periods.
Table I Transmittance, sheet resistance, root-mean-square surface roughness, average pore diameter, average pore spacing, and grating period of conventional and etched ITO films. Samples
Transmittance (% at 460 nm)
Sheet resistance (Ω/sq)
Root-mean-square (nm)
Average pore diameter (nm)
Average pore spacing (nm)
Grating period (nm)
Conventional Etched for 30 s Etched for 50 s Etched for 70 s Etched for 90 s
90 89 85 79 72
34 42 62 89 113
2.9 5 12.5 16 17.2
– 44 81 104 132
– 160 100 80 60
– 204 181 184 192
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Fig. 4. LED extraction efficiency simulation: (a) ray traces for planar LED with a mirror under sapphire substrate, (b) close view in the center of the device, (c) ray traces for LED with nanoporous ITO film, and (d) close view in the center of the device showing partially reflected rays returning to the structure.
reflection. Blue dots in Fig. 5 show the measured results of Samples A and B. Theoretical results show similar efficiency at a similar grating period, but the real device efficiency of Sample B was decreased due to an increase in ITO sheet resistance at longer etching times. As shown in the inset of Fig. 3, current crowding was observed in Sample B. It confined current spreading in the structure and reduced a further decrease in the internal quantum efficiency. This is because the optical efficiency of the device is strongly dependent on the current density [11]. Thus, a more detailed optimization of the nanoporous structure is an essential step in increasing the light output power of GaN-based LEDs.
2.0 1st order TR 1st order T
Extraction efficiency enhancement
Each simulated structure was designed by the grating periods and thickness of experimental samples. The grating periods were calculated using the sum of the average pore diameter and the spacing. The grating periods of Samples A and B were almost the same, as shown in Table I. Fig. 4 shows the results of ray-tracing simulated data of possible light paths from a dot-light source for the conventional LED and nanoporous ITO film LED. The dot-light source can fix the incident angle from the light source to the target surfaces. We now explain briefly the light scattering at each surface using the dot-light source. Fig. 4(a) shows the full structure of the conventional LED. Considering the disparate refractive indices of GaN (n= 2.5), ITO (n= 1.9), sapphire (n= 1.75), and air (n= 1), the internal light escaped with critical angles of 23°, 32°, 44°, and 50° at the GaN/air, ITO/air, GaN/sapphire, and GaN/ITO interfaces, respectively, as deduced from Snell's equation. Fig. 4(b) shows a closer view of the light paths at the center of the structure shown in Fig. 4(a). Generally, the incident light is reflected inside at each interface except for rays that strike the surface within the critical angle. The incident angle of the dot-light source was fixed at 30° in Fig. 4, which is smaller than the critical angle at each interface. Therefore, all incident rays were passed through the planar ITO film to air, as shown in Fig. 4(b). In contrast, inserting the grating layer allow changing an effective index effect on the first order of reflectivity. In that case, lights are partially scattered and reflected from the interface between the air and grating layer. The scattered and reflected lights then come back to the device with a redirected angle (Fig. 4(d)). These lights would increase the redirecting of light and improve the total extraction efficiency of the LED with a grating layer (Fig. 4(c)). Thus, we can state that the increase in light scattering is dominated by the nanoporous structure [10]. The enhancement of light extraction efficiency with various nanoporous ITO layers can be represented by a grating with different grating periods and transmission coefficients. Fig. 5 shows a comparison of the experimental power enhancement of Samples A and B with the simulated results for a linear grating with different periods. The changes of extraction efficiency for a grating with first-order transmission and first-order transmission and reflection ray tracings are shown by red and black curves, respectively. It can get a more accurate result for the scattering effect in simulation that was considering first-order transmission and
1.8
1.6
1.4
Sample A
12
Sample B 1.0 0.0
0.1
0.2
0.3
0.4
0.5
Grating period (µm) µ Fig. 5. Extraction efficiency enhancement versus grating period for two cases: grating with first-order transmission (red line), and first-order transmission and reflection. Blue points show enhancements for Sample A and Sample B devices.
J.H. Kang et al. / Thin Solid Films 520 (2011) 437–441
2. Conclusion We demonstrated a simple wet etching strategy for the surface texturing of ITO films. The wet etching strategy uses diluted HNO3 and forms a nanoporous morphology on the surface of ITO film. The effects of nanoporous ITO film on electrical and transmittance properties were studied as a function of etch time. For long etch times, the transmittance decreased and the sheet resistance increased. The forward voltage of Sample A (etched for 50 s) was almost the same as that of conventional LEDs at 20 mA, whereas it increased by 0.1 V for Sample B (etched for 70 s). The wall-plug efficiencies of Samples A and B at 20 mA were found to be 35% and 9.5% higher than the conventional LEDs, respectively, despite the decreased electrical properties. We attributed the enhancement of light extraction by the nanoporous structure to a reduction in Snell reflections. The grating period and electrical performance of the nanoporous-ITO film were the dominant factors responsible for the marked improvement in light extraction efficiency. Acknowledgment This research was financially supported by the Ministry of Knowledge Economy (MKE), the Korea Institute for Advancement in Technology (KIAT) through the Workforce Development Program in
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Strategic Technology, and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology of the Korean government (no. 2009-0094032). The authors gratefully acknowledge this support.
References [1] I. Schnitzer, E. Yablonovitch, C. Caneau, T.J. Gmitter, A. Scherer, Appl. Phys. Lett. 63 (1993) 2174. [2] T.S. Kim, S.M. Kim, Y.H. Jang, G.Y. Jung, Appl. Phys. Lett. 91 (2007) 171114. [3] T. Dai, B. Zhang, X.N. Kang, K. Bao, W.Z. Zhao, D.S. Xu, G.Y. Zhang, Z.Z. Gan, IEEE Photo. Tech. Lett. 20 (2008) 1974. [4] S.M. Pan, R.C. Tu, Y.M. Fan, R.C. Yeh, J.T. Hsu, IEEE Photonics Technol. Lett. 15 (2003) 649. [5] C.J. Huang, Y.K. Su, S.L. Wu, Mater. Chem. Phys. 84 (2004) 146. [6] P.C.P. Hrudey, M.A. Martinuk, M.A. Mossman, A.C. van Popta, M.J. Brett, J.S. Huizinga, L.A. Whitehead, Proc. SPIE 6645 (2007) 66450K. [7] I.A. Ryzhikov, A.A. Pukhov, A.S. Il'in, N.P. Glukhova, K.N. Afanasiev, A.S. Ryzhikov, Microelectron. Eng. 69 (2003) 270. [8] D.S. Liu, T.W. Lin, B.W. Huang, F.S. Juang, P.H. Lei, C.Z. Hu, Appl. Phys. Lett. 94 (2009) 143502. [9] W. Tang, K. Xu, P. Wang, X. Li, Microelectron. Eng. 66 (2003) 445. [10] D.S. Liu, T.W. Lin, B.W. Huang, F.S. Juang, P.H. Lei, C.Z. Hu, Appl. Phys. Lett. 94 (2009) 143502. [11] H.S. Kim, S.J. Park, H.S. Hwang, IEEE Trans. Electron Devices 48 (2001) 1065.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures Ji Hye Kang, Jae Hyoung Ryu, Hyun Kyu Kim, Hee Yun Kim, Nam Han, Mi So Lee, Young Jae Park, Periyayya Uthirakumar, Volodymyr V. Lysak, Chang-Hee Hong ⁎ School of Semiconductor and Chemical Engineering, Chonbuk National University, Chonju 561-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 29 April 2010 Received in revised form 12 May 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: Light-emitting diodes Nanoporous oxide Indium tin oxide Gallium nitride Scanning electron microscopy
a b s t r a c t We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface. © 2011 Elsevier B.V. All rights reserved.
1. Introduction GaN-based light-emitting diodes (LEDs) are promising devices for use in numerous applications including traffic signals, full color displays, solid state lighting, and backlighting in liquid crystal displays. However, to make such solid-state lighting systems practical, further improvements in the output efficiencies of GaN-based LEDs are required. The extraction efficiency still remains low because of the total reflection occurring at the surfaces of the LEDs [1]. Intensive study has led to significant improvements in the light output power, such as using a surface texturing technique to increase the critical angle of the light emitted from the device to the air. Several approaches such as indium tin oxide (ITO) surface texturing using polystyrene spheres [2], an anodic aluminum oxide template [3], and photolithography [4] have been used in attempts to improve the light extraction efficiencies of GaN-based LEDs. Surface texturing with these methods requires an inductively-coupled plasma (ICP) process and an additional patterning process to form the textured surface. However, the ICP process causes degradation of electrical properties due to plasma damage when exposed to the high density plasma. In a previous study, we examined the use of ITO nanospheres to create a textured ITO transparent layer with hemispherical shapes in order to enhance light extraction [5]. The hemispherical shapes were created by two independent step processes. First, the ITO nanosphere
⁎ Corresponding author. E-mail address: [email protected] (C.-H. Hong). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.05.051
was formed using a wet etching process with diluted HCl solution on an as-deposited ITO film. Then, a thin ITO film was evaporated onto the ITO nanosphere structures. In this study, we propose and demonstrate a simpler chemical route for the surface texturing of ITO films with nanoporous structure. The fabrication of ITO film with nanoporous structure using the wet etching process has not yet been reported. However, some researchers have reported on the formation of nanoporous ITO by glancing angle deposition [6] and reactive magnetron sputtering in direct current mode [7]. The later said techniques need specific equipment and hard to control the pores diameter. We were able to create a nanoporous structure on the surface of ITO films using a simple wet etching method. The porous shapes were easily achieved without using additional complex processes or equipment. GaN-based LEDs with nanoporous ITO films were then fabricated in an attempt to improve the light output power. We also examined the properties of nanoporous ITO films for various etch times, and the results are discussed in detail.
1.1. Experimental details In order to study the effects of the wet etch process on the optical and electrical properties of ITO films, a 300 nm-thick ITO film was evaporated onto sapphire using an electron beam evaporator. The asdeposited ITO films were then dipped into diluted HNO3 (2%) for 30, 50, 70, and 90 s. The etched ITO films were annealed at 600 °C for 30 s in air. The electrical and optical properties of the samples were then measured.
438
J.H. Kang et al. / Thin Solid Films 520 (2011) 437–441
The GaN-based LEDs used in this study were grown on sapphire substrates using metal organic chemical vapor deposition. The GaN nucleation layer was deposited at a temperature of 560 °C, followed by a 2 μm-thick layer of undoped GaN epilayers deposited at 1100 °C, and a 2 μm-thick layer of Si-doped n-type GaN deposited at 1120 °C. After the growth of the n-type GaN template, we grew seven-period multi-quantum wells of InGaN/GaN pairs at 810 °C, which emit at approximately 460 nm. Next, a 110 nm-thick p-GaN layer was grown at 1050 °C. Finally, a 10 nm-thick highly doped p-GaN layer was grown at 1050 °C. After the growth processes, LED chips with dimensions of 315 × 315 μm 2 were made by mesa etching of the exposed n-type GaN using the ICP process. Then, a 300 nm-thick ITO layer was evaporated onto the p-type GaN surface to form a transparent ohmic contact layer using an electron beam evaporator. To form nanoporous ITO films, the GaN epilayers with an ITO layer on the p-GaN were dipped into diluted HNO3 (2%) for about 50 s (Sample A) and 70 s (Sample B). We note that an etch time of 50 and 70 s are considered to be the optimum conditions based on the experimental results. The conventional LEDs had a planar ITO transparent ohmic contact layer, and Sample A and Sample B had a nanoporous ITO layer as the transparent ohmic contact. In the typical LED process, each ITO film on the p-type GaN layer as the transparent ohmic contact layer was annealed at 600 °C for 30 s in air. Then, the Cr/Au layer was deposited, as metal contacts, on the n- and p-type layers. The surface morphology of the ITO layers was examined by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The FESEM was performed using a JSM-6400 SEM (JEOL, Japan), with an acceleration voltage of 15 kV. To investigate the transparency of the various ITO films, we recorded optical transmittance spectra using a Jasco V-570 UV–VIS-NIR spectrophotometer (JASCO, Japan). The sheet resistance was measured by four point probe system. The current– voltage (I–V) and light output–current (L–I) measurements were carried out using a probe station system. The light intensity of the LED was characterized using an OL 770 multichannel spectroradiometer (Optronic Laboratories, America), and the simulated data was evaluated using LightTools™ software (7.0, Optical Research Associates, America) for comparison between experimental data and simulated data.
1.2. Results and discussion The as-deposited ITO film grown by electron beam evaporation was found to be polycrystalline. It had relatively weak binding energy at the grain boundaries. As a result, the etching process started from the grain boundary when the sample was immersed in weakly diluted HNO3. Fig. 1 illustrates nanopore formation for various etch times. In
the etching process, the reaction began at the In-O bond that was attacked by an HNO3 molecule, resulting in the subsequent formation of In(NO3)3, NO2, and O\H bonds. The reaction is described by the following reaction expression: In2O3 + 12HNO3 → 2In(NO3)3 + 6NO2 + 6H2O [8]. The average diameters of the pores were 44, 81, 104, and 132 nm for etch times of 30, 50, 70, and 90 s, respectively. The corresponding pore densities were 3 × 10 9, 5.1 × 10 9, 6.3 × 10 9, and 6.8 × 10 9 cm −2, respectively. These results indicate that the diameters and densities of the nanopores increased with etch times above 70 s due to cohesion between adjacent pores. However, the increase in pore density saturated. The thicknesses of the ITO films after etching were measured to be 262 and 225 nm for ITO films etched for 70 and 90 s, respectively, as shown in the inset of Fig. 1. The reduction in thickness was caused by an active etching reaction at the surface of the ITO films. Fig. 2(a) shows the transmittance spectra of the ITO films before and after etching for different etch times. We observed a systematic decrease in transmittance as the etch time increased. The decrease in transmittance of the etched ITO films can be explained by an increase in the surface roughness, which led to an increase in light scattering at the surface. As shown in Table I, the root mean square (rms) surface roughness of the wet-etched ITO films was higher than that of conventional ITO films. Thus, the transmittance decreased because the amount of light incident on the detector decreased with the increase of light scattering and scattered reflection from the nanoporous ITO films. Also, as the etch time increased, the sheet resistance increased from 34 to 113 Ω/sq, as shown in Fig. 2(b) and Table I. The increase in sheet resistance was caused by the increase of surface roughness and the decrease of ITO film thickness. The degradation of electrical properties and the increase of surface roughness of the ITO film can be explained by the decrease in mean free path lengths, which was caused by electron scattering [9]. Therefore, the degradation of electrical properties needs to be minimized and optimized in order to enhance the light output power. Based on the transmittance at a wavelength of 460 nm, the sheet resistance, and the rms roughness values given in Table I, we selected etch times of 50 s and 70 s for the fabrication of the LEDs. Fig. 3(a) shows the I–V characteristics of LEDs made with nanoporous ITO layers. For comparison, the I–V curve of a conventional LED is also shown. The forward voltages measured at 20 mA were 3.4, 3.4, and 3.5 V for the conventional LED, Sample A, and Sample B, respectively. The observed marginal increase in forward voltage for sample B is attributed to the increase in sheet resistance, as shown in Table I. The degradation of the layer's electrical properties was minimum when the etch time was short. Fig. 3(b) shows the L–I characteristics of the three different LEDs with and without a nanoporous ITO layer. The output power of Samples A and B was found to be 35% and 13% higher, respectively, than the conventional
Fig. 1. Scanning electron microscope (SEM) images of ITO films illustrating nanopore formation with respect to the etching time, and the respective cross section SEM images (insets).
J.H. Kang et al. / Thin Solid Films 520 (2011) 437–441
a 100
439
a 0.08
p-pad
nanoporous-ITO
60
40 Conventional ITO film 30s etched ITO film 50s ethed ITO film 70s etched ITO film 90s etched ITO film
20
0 300
400
500
600
700
n-pad
p-type GaN : Mg InGaN/GaN MQW
0.06
Current (A)
Transmittance (%)
80
n-type GaN : Si
0.04
Sapphire substrate
0.02 Conventional LED Sample A Sample B
1
800
2
3
4
5
6
Voltage (V)
Wavelength (nm)
b
b 35 30
Light intensity (arb.units.)
Sheet resistance (ohm/sq)
120
90
60
30
25 20 Conventional LED Sample A Sample B
15 10
at 0.5mA
5
30
Conventional LED
0
10
20
30
40
50
60
70
80
90
100
0
0
20
Etch time(s)
40
Sample A
60
Sample B
80
100
Current (mA)
Fig. 2. (a) Transmittance spectra of ITO films etched for different time durations, and (b) variation in sheet resistance of ITO films versus etching time.
Fig. 3. (a) Current–voltage (I–V) characteristics and (b) light output power–current (L–I) characteristics of different LEDs with planar and nanorough ITO film.
LEDs at 20 mA. The inset of Fig. 3 shows optical microscope photographs of a conventional LED, Sample A, and Sample B at an injection current of 0.5 mA. The images show that the emissions from Samples A and B were brighter than the emission from the conventional LED. In addition, current crowding occurred in Sample B around the p-pad due to the high resistivity of the ITO film etched for 70 s. The degradation of internal quantum efficiency could be due to the current crowding effect. Moreover, the electrical property has a connection with the wall-plug efficiency of the LEDs. We can calculate the wall-plug efficiency (output power/input power) using a simple calculation. The wall-plug efficiency of Samples A and B increased by 35% and 9.5% at an injection current of 20 mA compared to that of the conventional LED structure. Considering the electrical properties, the light extraction effect of Sample A was more suitable than that of Sample B. The light output power was quite
different for Samples A and B. We can expect that the enhancement of light output power is related to the size and density of pores and the electrical properties of the etched ITO film. We also performed a simulation to investigate the effect of different surface morphologies on light extraction efficiency. The optical simulation was carried out using LightTools commercial ray-tracer software for a conventional LED with a planar ITO layer, and Samples A and B with a nanoporous ITO layer. Over 100,000 rays were assumed to originate from the MQW region, and the size of the sample used in the simulation was 315 × 315 μm2. The nanoporous ITO film had smaller pore size structures than the emission wavelength of the LED. Since it is difficult to simulate such subwavelength structures using the ray tracing technique, we examined the output from the nanoporous ITO surface as a linear grating with 2π/λ vectors and different periods.
Table I Transmittance, sheet resistance, root-mean-square surface roughness, average pore diameter, average pore spacing, and grating period of conventional and etched ITO films. Samples
Transmittance (% at 460 nm)
Sheet resistance (Ω/sq)
Root-mean-square (nm)
Average pore diameter (nm)
Average pore spacing (nm)
Grating period (nm)
Conventional Etched for 30 s Etched for 50 s Etched for 70 s Etched for 90 s
90 89 85 79 72
34 42 62 89 113
2.9 5 12.5 16 17.2
– 44 81 104 132
– 160 100 80 60
– 204 181 184 192
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Fig. 4. LED extraction efficiency simulation: (a) ray traces for planar LED with a mirror under sapphire substrate, (b) close view in the center of the device, (c) ray traces for LED with nanoporous ITO film, and (d) close view in the center of the device showing partially reflected rays returning to the structure.
reflection. Blue dots in Fig. 5 show the measured results of Samples A and B. Theoretical results show similar efficiency at a similar grating period, but the real device efficiency of Sample B was decreased due to an increase in ITO sheet resistance at longer etching times. As shown in the inset of Fig. 3, current crowding was observed in Sample B. It confined current spreading in the structure and reduced a further decrease in the internal quantum efficiency. This is because the optical efficiency of the device is strongly dependent on the current density [11]. Thus, a more detailed optimization of the nanoporous structure is an essential step in increasing the light output power of GaN-based LEDs.
2.0 1st order TR 1st order T
Extraction efficiency enhancement
Each simulated structure was designed by the grating periods and thickness of experimental samples. The grating periods were calculated using the sum of the average pore diameter and the spacing. The grating periods of Samples A and B were almost the same, as shown in Table I. Fig. 4 shows the results of ray-tracing simulated data of possible light paths from a dot-light source for the conventional LED and nanoporous ITO film LED. The dot-light source can fix the incident angle from the light source to the target surfaces. We now explain briefly the light scattering at each surface using the dot-light source. Fig. 4(a) shows the full structure of the conventional LED. Considering the disparate refractive indices of GaN (n= 2.5), ITO (n= 1.9), sapphire (n= 1.75), and air (n= 1), the internal light escaped with critical angles of 23°, 32°, 44°, and 50° at the GaN/air, ITO/air, GaN/sapphire, and GaN/ITO interfaces, respectively, as deduced from Snell's equation. Fig. 4(b) shows a closer view of the light paths at the center of the structure shown in Fig. 4(a). Generally, the incident light is reflected inside at each interface except for rays that strike the surface within the critical angle. The incident angle of the dot-light source was fixed at 30° in Fig. 4, which is smaller than the critical angle at each interface. Therefore, all incident rays were passed through the planar ITO film to air, as shown in Fig. 4(b). In contrast, inserting the grating layer allow changing an effective index effect on the first order of reflectivity. In that case, lights are partially scattered and reflected from the interface between the air and grating layer. The scattered and reflected lights then come back to the device with a redirected angle (Fig. 4(d)). These lights would increase the redirecting of light and improve the total extraction efficiency of the LED with a grating layer (Fig. 4(c)). Thus, we can state that the increase in light scattering is dominated by the nanoporous structure [10]. The enhancement of light extraction efficiency with various nanoporous ITO layers can be represented by a grating with different grating periods and transmission coefficients. Fig. 5 shows a comparison of the experimental power enhancement of Samples A and B with the simulated results for a linear grating with different periods. The changes of extraction efficiency for a grating with first-order transmission and first-order transmission and reflection ray tracings are shown by red and black curves, respectively. It can get a more accurate result for the scattering effect in simulation that was considering first-order transmission and
1.8
1.6
1.4
Sample A
12
Sample B 1.0 0.0
0.1
0.2
0.3
0.4
0.5
Grating period (µm) µ Fig. 5. Extraction efficiency enhancement versus grating period for two cases: grating with first-order transmission (red line), and first-order transmission and reflection. Blue points show enhancements for Sample A and Sample B devices.
J.H. Kang et al. / Thin Solid Films 520 (2011) 437–441
2. Conclusion We demonstrated a simple wet etching strategy for the surface texturing of ITO films. The wet etching strategy uses diluted HNO3 and forms a nanoporous morphology on the surface of ITO film. The effects of nanoporous ITO film on electrical and transmittance properties were studied as a function of etch time. For long etch times, the transmittance decreased and the sheet resistance increased. The forward voltage of Sample A (etched for 50 s) was almost the same as that of conventional LEDs at 20 mA, whereas it increased by 0.1 V for Sample B (etched for 70 s). The wall-plug efficiencies of Samples A and B at 20 mA were found to be 35% and 9.5% higher than the conventional LEDs, respectively, despite the decreased electrical properties. We attributed the enhancement of light extraction by the nanoporous structure to a reduction in Snell reflections. The grating period and electrical performance of the nanoporous-ITO film were the dominant factors responsible for the marked improvement in light extraction efficiency. Acknowledgment This research was financially supported by the Ministry of Knowledge Economy (MKE), the Korea Institute for Advancement in Technology (KIAT) through the Workforce Development Program in
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Strategic Technology, and the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology of the Korean government (no. 2009-0094032). The authors gratefully acknowledge this support.
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