- Email: [email protected]
Characteristic enhancement of the blue LED chip by the growth and fabrication on patterned sapphire (0 0 0 1) substrate
ARTICLE IN PRESS Journal of Crystal Growth 312 (2010) 258–262
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Characteristic enhancement of the blue LED chip by the growth and fabrication on patterned sapphire (0 0 0 1) substrate Seong-Muk Jeong a, Suthan Kissinger a, Dong-Wook Kim a, Seung Jae Lee b, Jin-Soo Kim a, Haeng-Keun Ahn a, Cheul-Ro Lee a,n a
School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development (RCAMD), Chonbuk National University, Chonju 664-14, Chonbuk 561-756, South Korea b Korea Photonics Technology Institute, Cheomdan 4gil, Buk-Gu, Gwangju 500-779, South Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 April 2009 Received in revised form 14 October 2009 Accepted 22 October 2009 Communicated by R.M. Biefeld Available online 10 November 2009
Blue light-emitting diodes (LEDs) with an InGaN multi-quantum well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metal organic chemical vapor deposition (MOCVD). The electrical and optical properties of these LEDs were investigated. The crystal quality of epitaxial GaN film was improved by using the PSS structure. At 40 mA injection current, the peak wavelength and the full-width at half-maximum of the electroluminescence spectra of PSS were 461 and 24 nm, respectively. The electroluminescence intensity (EL) of LEDs grown on patterned substrate was 2.44 times greater than that of unpatterened sapphire substrate (UPSS). The operating voltage was measured about 3.1 V for the LED with a PSS structure. This significant increase resulted from the improvement of the epitaxial quality of the InGaN/GaN epilayers and the improvement of the light extraction efficiency through patterned sapphire substrates. & 2009 Elsevier B.V. All rights reserved.
PACS: 85.60.Jb 81.15.Gh 81.65.Cf 78. 6.Fi. Keywords: A3. MOCVD B1. GaN B1. Patterned sapphire substrate B3. Light-emitting diodes
1. Introduction Group III-nitride-based semiconductors have been recently used as high-brightness light emitting diodes (LEDs) in large full color outdoor displays, signal lights and high performance back light units in liquid crystal displays with emission in the near-UV, blue, and green spectral range [1]. InGaN/GaN LEDs have greater potential to replace the incandescent bulbs and fluorescent lamps to save energy [2]. Even though high brightness GaN-based LEDs are commercially available, it is very difficult to manufacture highly efficient LEDs. The main limitation on light output power is due to the low internal quantum efficiency and light extraction efficiency. The main reason for low internal quantum efficiency is due to the great number of threading dislocation (TD) densities that occur when group III-nitride alloys are grown on c-plane sapphire substrates [3]. The electrons injected into the active layer can leak through these TD, which creates a
n
Corresponding author. Tel.: + 82 63 270 2304; fax: + 82 63 270 2305. E-mail address: [email protected] (C.-R. Lee).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.10.049
diffusion pathway for metals. The LEDs grown on unpatterned sapphire substrates result in relatively high TD density of the order of 108–1010 cm 2[4]. The epitaxial lateral overgrowth (ELOG) method has reduced the TD density to the range of 106 cm 2[5]. To improve the LEDs performance, both high TD density and low light extraction efficiency must be considered. Using the ELOG technique, we can significantly reduce the TD density [6–7]. However, this method does not consider the light extraction efficiency and also the two-step growth procedure is time consuming. The ELOG technique typically requires multiple metal organic chemical vapor deposition (MOCVD) growths, and also often requires doping or induces contamination [8]. Recently, for improving the crystalline quality and light extraction efficiency of the LEDs, single growth technique with PSS has been extensively studied. On the other hand, GaN-based LEDs prepared on PSS have increased light extraction efficiency by scattering and also reduce the TD density [9–11]. In this study, we describe the fabrication, optical and electrical properties of InGaN/GaN LEDs on lens-shaped PSS by using MOCVD. The characteristic improvement of the LED fabricated on
ARTICLE IN PRESS S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
259
Fig. 2. Schematic diagram of GaN deposited on PSS.
quality of GaN epilayers was investigated by double crystal X-ray diffraction (DCXRD). The optoelectronic properties of the LEDs were measured using an optics LED characterization system with an integrated sphere detector. Fig. 1. SEM morphology of the lens-shaped array patterns.
lens-shaped PSS compared with unpatterened LED is depicted here. In addition, the structural analysis of these LEDs was observed by a scanning electron microscope and an atomic force microscope.
2. Experimental GaN epilayers were deposited on the surface of patterned sapphire (0 0 0 1) substrate using MOCVD with a horizontal quartz reactor. Before growing GaN epilayers, the sapphire substrate was patterned by dry etching method to form lensshaped pattern array as shown in Fig. 1. We can observe that the lens-shaped pattern is uniformly arranged in equal size. The growth process of GaN on PSS is schematically depicted in Fig. 2. The sapphire substrate was etched using an inductively coupledplasma (ICP) reactive etcher of Cl2/Ar. Further, the PSS was heated to 1040 1C under a H2 ambient for 5 min and etched thermally. The lens height and diameter of the patterned sapphire are 1.5 and 5.8 mm, respectively. The distance between each lens is 3.7 mm. During MOCVD growth, trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) were used as gallium, indium, and nitrogen precursors. Biscyclopentadienyl magnesium (Cp2Mg) and disilane (Si2H6) were used as p- and n-type dopant sources, respectively. For the fabrication of blue LED structure, a 25 nm thick low temperature GaN nucleation layer was deposited on the PSS at a temperature of 535 1C, and then a high temperature 2 mm thick undoped GaN and a 4.5 mm thick Sidoped GaN template were grown at 1050 1C. Then InGaN/GaN supper lattices were grown at a temperature of 785 1C. After completing the growth of super lattices, five period multiquantum wells (MQWs) of InGaN/GaN pairs for 450 nm emission were grown at 835 1C, and also a 130 nm thick Mg-doped p-GaN was grown at 980 1C. Finally a Si-doped InGaN layer was grown at a temperature of 710 1C for a thickness of about 2 nm. The indium tin oxide (ITO) was deposited as a transparent conductive layer, and Cr–Au metals were deposited as the p- and n-type electrodes, respectively, by using an e-beam evaporator. The schematic diagram of the LEDs grown on PSS and UPSS is as shown in Fig. 3. The same LED structure has been grown on patterned and unpatterened sapphire substrates for comparison by the MOCVD system. The surface morphologies of the PSS and UPSS were observed with a field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The crystalline
3. Results and discussion Fig. 4 shows the SEM image of GaN epilayers grown on the lens-shaped PSS (0 0 0 1) for different growth times: (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min, (e) 60 min and (f) 80 min, respectively. It was observed that the GaN on PSS shows a hexagonal morphology growth for the lower growth time, and we also clearly noticed that many TDs were originated from the GaN/sapphire interface due to the large lattice mismatch between the GaN and sapphire. These TDs have been associated with nonradiative recombination center within the GaN layer [12] that appeared on the lens patterned region. At lower growth time, GaN grew as separate islands on the lens and trench regions as shown in Fig. 2(a). The islands started to coalesce at the trench region when the growth time is increased. As the growth time is further increased, the trench part grew separately and the TD density in the GaN epilayers was effectively reduced. Full coalescence takes place for the growth time of 80 min, leading to a smooth surface with less TD density [13]. Figs. 5(a) and (b) show the AFM images of the GaN grown on PSS and UPSS around the lens region, respectively. This clearly indicates that more TDs were observed for GaN on UPSS, but for PSS a smooth surface with fewer pits were observed. The etch-pit density (EPD) measurement reveals that the TDs propagate to the surface of GaN film, which originate at the GaN/sapphire interface due to the large lattice mismatch between GaN and sapphire substrates. The EPD value of GaN film grown on PSS was 3.8 108 cm 2, which was lower than that of GaN grown on UPSS (1.5 109 cm 2). This result demonstrates that in between the two lens regions, the dislocation was lower due to the full coalescence. However, more dislocations were observed over the lens region. The Si-doped GaN deposition was carried out on u-GaN/LTGaN/PSS and u-GaN/LT-GaN/UPSS epilayers. Before fabricating the LED device, the effects of patterned sapphire on the structural and optical quality of the Si-doped GaN epilayers deposited on the GaN were evaluated by DCXRD and PL, respectively. Fig. 6 shows the (0 0 0 2) X-ray diffraction peaks of the Si-doped GaN epilayers. From the DCXRD results, the full-width at half-maximum (FWHM) of GaN (0 0 0 2) peak is 50 and 57 arc-sec for Si-doped GaN/PSS and Si-doped GaN/UPSS, respectively. The FWHM of the n-GaN/PSS was smaller than Si-doped GaN/UPSS and also it was found to be narrower than n-GaN on the unpatterened sapphire. The strongest peak at 34.61 reveals that the GaN was highly oriented along the c-axis and is single crystalline in nature. The peak intensities of both the samples were almost same. The sharp and narrow peak intensity results from DCXRD for LED on PSS
ARTICLE IN PRESS 260
S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
Fig. 3. Schematic diagram of the epitaxial layers and LED structures fabricated on PSS and UPSS.
Fig. 4. SEM micrographs of the GaN surface grown on the lens-shaped PSS for the growth time of (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min, (e) 60 min, and (f) 80 min.
Fig. 5. AFM images of GaN grown on (a) PSS and (b) UPSS.
ARTICLE IN PRESS S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
20000 16000 14000 12000 10000 8000 6000 4000 2000 -2000
0.06 0.04 0.02
0 34
35
0.00 0
2theta Fig. 6. DCXRD spectra of Si–GaN/u-GaN/LT-GaN /PSS and Si–GaN/u-GaN/LT-GaN/ UPSS.
25000
Luminous Intensity (mcd)
364 nm
10000 5000 0 300
3 Voltage (V)
4
5
LED/PSS LED/UPSS
400
20000 15000
2
500
nGaN PSS (FWHM : 60 meV) nGaN UPSS (FWHM : 77 meV)
365 nm
1
Fig. 8. Curves of current–voltage (I–V) characteristics of the PSS-LED (squares) and UPSS-LED (circles)
30000
PL intensity (arb.units)
LED/PSS LED/UPSS
0.08 Current (A)
Intensity (arb.units)
0.10
nGaN PSS (FWHM : 50 arc-sec) nGaN UPSS (FWHM : 57 arc-sec)
18000
261
300
200
100 350
400
450 500 550 Wavelength (nm)
600
650
Fig. 7. The room temperature PL spectra of Si–GaN/u-GaN/LT-GaN/PSS and Si–GaN/u–GaN/LT–GaN/PSS
0 0.00
0.02
0.04 0.06 Current (A)
0.08
0.10
Fig. 9. Luminous intensity of LEDs grown on PSS and UPSS, the inset shows the room temperature EL spectra of PSS-LED and UPSS-LED
indicate that the crystalline quality of PSS was improved when compared to UPSS. Fig. 7 shows the results of PL spectra of the Si-doped GaN on PSS and UPSS measured at room temperature (RT), respectively. A sharp and narrow PL spectrum was observed for GaN on PSS at 365 nm compared to GaN on UPSS. The emission peak intensity was high for GaN on PSS compared to UPSS. This increase in peak intensity indicates that the crystalline quality was improved significantly and the TD density was reduced for the GaN on PSS. This enhancement was due to the internal reflection on the lenses of the substrate. From the figure the FWHM of Si-doped PSS and UPSS were 60 and 77 meV, respectively. It clearly shows that the usage of patterned sapphire substrate can improve the light emission efficiency of blue LED. Fig. 8 displays the current–voltage (I–V) characteristics of PSS LED and UPSS LED with the same chip size as a function of forward driving current. The LEDs on PSS and UPSS have low operating voltages, and sharp turn-on voltages near 2.93 and 2.97 V, respectively. The forward voltage (VF) for the PSS and UPSS LED devices were 3.08 and 3.19 V at the forward injection current of 20 mA. The forward voltage of PSS LED was smaller than that of UPSS LED; also the forward I–V curve of LED grown on PSS did not
show any significant difference with that of UPSS. Almost identical I–V curves infer that the patterning on the GaN surface had very little impact on the I–V characteristics. The difference in the forward I–V curves of PSS and UPSS shows the improvement in the epitaxial film quality of PSS. Fig. 9 shows the luminous intensity variation for different currents. The luminous intensities of the PSS and UPSS at 40 mA applied current were 271.4 and 107.2 mcd, respectively. The luminous intensity of PSS LED was 2.53 times higher than that of UPSS LED at the injection current of 40 mA. This improvement in the luminous intensity of PSS LED was supplied to the enhancement of light extraction efficiency through lens patterned structures. The inset shows the room temperature EL spectra of the PSS and UPSS LEDs at an injection current of 20 mA. The EL peak positions of the PSS and UPSS LEDs appeared at 461 and 465 nm, respectively. A small blue shift of about 4 nm was observed for the PSS LED. It was found that the EL peak intensity of PSS LED was 2.44 times larger than that of the UPSS LED. It is well known that the MQW emission efficiency is related to the
ARTICLE IN PRESS 262
S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
leakage current through dislocations. Hence, such a significant enhancement in EL intensity was attributed to the reduction of TDs that induce non-radiative recombination centers using PSS.
4. Conclusions In this study, nitride-based blue LEDs grown by MOCVD on patterned and unpatterened sapphire substrates were investigated. The SEM micrograph image reveals a smooth GaN surface for the PSS surface with a growth time of 80 min. The narrow fullwidth at half-maximum of PSS from the DCXRD indicates the good quality of GaN on PSS compared to GaN on UPSS. According to the result of AFM, the average TD density can be apparently reduced from 1.5 109 to 3.8 108 cm 2, which reveals less dislocation on PSS compared to UPSS. The PL spectra clearly indicate good optical quality of GaN due to the reduction in TD density and the improvement in the light emission efficiency. The forward voltages of both samples were almost similar, but the difference of the forward I–V curves results from the improved crystalline quality. As much as 2.44 times of increased electroluminescence intensity of PSS LED was obtained at a forward current of 40 mA. This performance improvement was due to the better epitaxial quality of InGaN/GaN epilayers. The dislocation density would be reduced greatly and higher extraction efficiency could be achieved from lens-shaped PSS. Our work brings out GaN-based LED structure with better performance for the next generation of high-brightness blue LEDs using the PSS method.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) National Research Laboratory (NRL) Program grant funded by the Korea government (MEST) (no. ROA-2008000-20031-0), grant (no. R01-2006-000-10352-0) from the Basic Research Program of the Korea Science and Engineering Foundation, and the Post BK21 program of the Ministry of Education and Human Resources development. References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274. [2] Y. Narukawa, J. Narita, T. Sakamoto, K. Deguchi, T. Yamada, T. Muka, Jpn. J. Appl. Phys. 45 (2006) L1084. [3] F.A. Pounce, D.P Bour, Nature, London 386 (1997) 351. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 211. [5] T.S. Zheleva, O.H. Nam, M.D. Bremser, R.F. Davis, Appl. Phys. Lett. 71 (1997) 2472. [6] A. Sakai, H. Sunakawa, A. Usui, Appl. Phy. Lett. 71 (1997) 2259. [7] K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, T. Maeda, J. Cryst. Growth 221 (2000) 316. [8] O.H. Nam, T.S. Zheleva, M.D. Bremser, R.F. Davis, J. Electron. Mater. 27 (1998) 233. [9] M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, T. Mukai, Jpn. J. Appl. Phys. 41 (2002) L1431. [10] D.S. Wuu, W.K. Wang, W.C. Shih, R.H. Horng, C.E. Lee, W.Y. Lin, J.S. Fang, IEEE Photonics Technol. Lett. 17 (2005) 288. [11] Z.H. Feng, K.M. Lau, IEEE Photonics Technol. Lett. 17 (2005) 1812. [12] N. Kuwano, K. Horibuchi, K. Kagawa, S. Nishimoto, M. Sueyoshi, J. Cryst. Growth 237 (2002) 1047. [13] J.C Song, S.H Lee, I.H Lee, K.W Seol, S Kannappan, C.R Lee, J. Cryst. Growth 308 (2007) 321.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Characteristic enhancement of the blue LED chip by the growth and fabrication on patterned sapphire (0 0 0 1) substrate Seong-Muk Jeong a, Suthan Kissinger a, Dong-Wook Kim a, Seung Jae Lee b, Jin-Soo Kim a, Haeng-Keun Ahn a, Cheul-Ro Lee a,n a
School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development (RCAMD), Chonbuk National University, Chonju 664-14, Chonbuk 561-756, South Korea b Korea Photonics Technology Institute, Cheomdan 4gil, Buk-Gu, Gwangju 500-779, South Korea
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 April 2009 Received in revised form 14 October 2009 Accepted 22 October 2009 Communicated by R.M. Biefeld Available online 10 November 2009
Blue light-emitting diodes (LEDs) with an InGaN multi-quantum well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metal organic chemical vapor deposition (MOCVD). The electrical and optical properties of these LEDs were investigated. The crystal quality of epitaxial GaN film was improved by using the PSS structure. At 40 mA injection current, the peak wavelength and the full-width at half-maximum of the electroluminescence spectra of PSS were 461 and 24 nm, respectively. The electroluminescence intensity (EL) of LEDs grown on patterned substrate was 2.44 times greater than that of unpatterened sapphire substrate (UPSS). The operating voltage was measured about 3.1 V for the LED with a PSS structure. This significant increase resulted from the improvement of the epitaxial quality of the InGaN/GaN epilayers and the improvement of the light extraction efficiency through patterned sapphire substrates. & 2009 Elsevier B.V. All rights reserved.
PACS: 85.60.Jb 81.15.Gh 81.65.Cf 78. 6.Fi. Keywords: A3. MOCVD B1. GaN B1. Patterned sapphire substrate B3. Light-emitting diodes
1. Introduction Group III-nitride-based semiconductors have been recently used as high-brightness light emitting diodes (LEDs) in large full color outdoor displays, signal lights and high performance back light units in liquid crystal displays with emission in the near-UV, blue, and green spectral range [1]. InGaN/GaN LEDs have greater potential to replace the incandescent bulbs and fluorescent lamps to save energy [2]. Even though high brightness GaN-based LEDs are commercially available, it is very difficult to manufacture highly efficient LEDs. The main limitation on light output power is due to the low internal quantum efficiency and light extraction efficiency. The main reason for low internal quantum efficiency is due to the great number of threading dislocation (TD) densities that occur when group III-nitride alloys are grown on c-plane sapphire substrates [3]. The electrons injected into the active layer can leak through these TD, which creates a
n
Corresponding author. Tel.: + 82 63 270 2304; fax: + 82 63 270 2305. E-mail address: [email protected] (C.-R. Lee).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.10.049
diffusion pathway for metals. The LEDs grown on unpatterned sapphire substrates result in relatively high TD density of the order of 108–1010 cm 2[4]. The epitaxial lateral overgrowth (ELOG) method has reduced the TD density to the range of 106 cm 2[5]. To improve the LEDs performance, both high TD density and low light extraction efficiency must be considered. Using the ELOG technique, we can significantly reduce the TD density [6–7]. However, this method does not consider the light extraction efficiency and also the two-step growth procedure is time consuming. The ELOG technique typically requires multiple metal organic chemical vapor deposition (MOCVD) growths, and also often requires doping or induces contamination [8]. Recently, for improving the crystalline quality and light extraction efficiency of the LEDs, single growth technique with PSS has been extensively studied. On the other hand, GaN-based LEDs prepared on PSS have increased light extraction efficiency by scattering and also reduce the TD density [9–11]. In this study, we describe the fabrication, optical and electrical properties of InGaN/GaN LEDs on lens-shaped PSS by using MOCVD. The characteristic improvement of the LED fabricated on
ARTICLE IN PRESS S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
259
Fig. 2. Schematic diagram of GaN deposited on PSS.
quality of GaN epilayers was investigated by double crystal X-ray diffraction (DCXRD). The optoelectronic properties of the LEDs were measured using an optics LED characterization system with an integrated sphere detector. Fig. 1. SEM morphology of the lens-shaped array patterns.
lens-shaped PSS compared with unpatterened LED is depicted here. In addition, the structural analysis of these LEDs was observed by a scanning electron microscope and an atomic force microscope.
2. Experimental GaN epilayers were deposited on the surface of patterned sapphire (0 0 0 1) substrate using MOCVD with a horizontal quartz reactor. Before growing GaN epilayers, the sapphire substrate was patterned by dry etching method to form lensshaped pattern array as shown in Fig. 1. We can observe that the lens-shaped pattern is uniformly arranged in equal size. The growth process of GaN on PSS is schematically depicted in Fig. 2. The sapphire substrate was etched using an inductively coupledplasma (ICP) reactive etcher of Cl2/Ar. Further, the PSS was heated to 1040 1C under a H2 ambient for 5 min and etched thermally. The lens height and diameter of the patterned sapphire are 1.5 and 5.8 mm, respectively. The distance between each lens is 3.7 mm. During MOCVD growth, trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) were used as gallium, indium, and nitrogen precursors. Biscyclopentadienyl magnesium (Cp2Mg) and disilane (Si2H6) were used as p- and n-type dopant sources, respectively. For the fabrication of blue LED structure, a 25 nm thick low temperature GaN nucleation layer was deposited on the PSS at a temperature of 535 1C, and then a high temperature 2 mm thick undoped GaN and a 4.5 mm thick Sidoped GaN template were grown at 1050 1C. Then InGaN/GaN supper lattices were grown at a temperature of 785 1C. After completing the growth of super lattices, five period multiquantum wells (MQWs) of InGaN/GaN pairs for 450 nm emission were grown at 835 1C, and also a 130 nm thick Mg-doped p-GaN was grown at 980 1C. Finally a Si-doped InGaN layer was grown at a temperature of 710 1C for a thickness of about 2 nm. The indium tin oxide (ITO) was deposited as a transparent conductive layer, and Cr–Au metals were deposited as the p- and n-type electrodes, respectively, by using an e-beam evaporator. The schematic diagram of the LEDs grown on PSS and UPSS is as shown in Fig. 3. The same LED structure has been grown on patterned and unpatterened sapphire substrates for comparison by the MOCVD system. The surface morphologies of the PSS and UPSS were observed with a field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The crystalline
3. Results and discussion Fig. 4 shows the SEM image of GaN epilayers grown on the lens-shaped PSS (0 0 0 1) for different growth times: (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min, (e) 60 min and (f) 80 min, respectively. It was observed that the GaN on PSS shows a hexagonal morphology growth for the lower growth time, and we also clearly noticed that many TDs were originated from the GaN/sapphire interface due to the large lattice mismatch between the GaN and sapphire. These TDs have been associated with nonradiative recombination center within the GaN layer [12] that appeared on the lens patterned region. At lower growth time, GaN grew as separate islands on the lens and trench regions as shown in Fig. 2(a). The islands started to coalesce at the trench region when the growth time is increased. As the growth time is further increased, the trench part grew separately and the TD density in the GaN epilayers was effectively reduced. Full coalescence takes place for the growth time of 80 min, leading to a smooth surface with less TD density [13]. Figs. 5(a) and (b) show the AFM images of the GaN grown on PSS and UPSS around the lens region, respectively. This clearly indicates that more TDs were observed for GaN on UPSS, but for PSS a smooth surface with fewer pits were observed. The etch-pit density (EPD) measurement reveals that the TDs propagate to the surface of GaN film, which originate at the GaN/sapphire interface due to the large lattice mismatch between GaN and sapphire substrates. The EPD value of GaN film grown on PSS was 3.8 108 cm 2, which was lower than that of GaN grown on UPSS (1.5 109 cm 2). This result demonstrates that in between the two lens regions, the dislocation was lower due to the full coalescence. However, more dislocations were observed over the lens region. The Si-doped GaN deposition was carried out on u-GaN/LTGaN/PSS and u-GaN/LT-GaN/UPSS epilayers. Before fabricating the LED device, the effects of patterned sapphire on the structural and optical quality of the Si-doped GaN epilayers deposited on the GaN were evaluated by DCXRD and PL, respectively. Fig. 6 shows the (0 0 0 2) X-ray diffraction peaks of the Si-doped GaN epilayers. From the DCXRD results, the full-width at half-maximum (FWHM) of GaN (0 0 0 2) peak is 50 and 57 arc-sec for Si-doped GaN/PSS and Si-doped GaN/UPSS, respectively. The FWHM of the n-GaN/PSS was smaller than Si-doped GaN/UPSS and also it was found to be narrower than n-GaN on the unpatterened sapphire. The strongest peak at 34.61 reveals that the GaN was highly oriented along the c-axis and is single crystalline in nature. The peak intensities of both the samples were almost same. The sharp and narrow peak intensity results from DCXRD for LED on PSS
ARTICLE IN PRESS 260
S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
Fig. 3. Schematic diagram of the epitaxial layers and LED structures fabricated on PSS and UPSS.
Fig. 4. SEM micrographs of the GaN surface grown on the lens-shaped PSS for the growth time of (a) 10 min, (b) 20 min, (c) 30 min, (d) 40 min, (e) 60 min, and (f) 80 min.
Fig. 5. AFM images of GaN grown on (a) PSS and (b) UPSS.
ARTICLE IN PRESS S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
20000 16000 14000 12000 10000 8000 6000 4000 2000 -2000
0.06 0.04 0.02
0 34
35
0.00 0
2theta Fig. 6. DCXRD spectra of Si–GaN/u-GaN/LT-GaN /PSS and Si–GaN/u-GaN/LT-GaN/ UPSS.
25000
Luminous Intensity (mcd)
364 nm
10000 5000 0 300
3 Voltage (V)
4
5
LED/PSS LED/UPSS
400
20000 15000
2
500
nGaN PSS (FWHM : 60 meV) nGaN UPSS (FWHM : 77 meV)
365 nm
1
Fig. 8. Curves of current–voltage (I–V) characteristics of the PSS-LED (squares) and UPSS-LED (circles)
30000
PL intensity (arb.units)
LED/PSS LED/UPSS
0.08 Current (A)
Intensity (arb.units)
0.10
nGaN PSS (FWHM : 50 arc-sec) nGaN UPSS (FWHM : 57 arc-sec)
18000
261
300
200
100 350
400
450 500 550 Wavelength (nm)
600
650
Fig. 7. The room temperature PL spectra of Si–GaN/u-GaN/LT-GaN/PSS and Si–GaN/u–GaN/LT–GaN/PSS
0 0.00
0.02
0.04 0.06 Current (A)
0.08
0.10
Fig. 9. Luminous intensity of LEDs grown on PSS and UPSS, the inset shows the room temperature EL spectra of PSS-LED and UPSS-LED
indicate that the crystalline quality of PSS was improved when compared to UPSS. Fig. 7 shows the results of PL spectra of the Si-doped GaN on PSS and UPSS measured at room temperature (RT), respectively. A sharp and narrow PL spectrum was observed for GaN on PSS at 365 nm compared to GaN on UPSS. The emission peak intensity was high for GaN on PSS compared to UPSS. This increase in peak intensity indicates that the crystalline quality was improved significantly and the TD density was reduced for the GaN on PSS. This enhancement was due to the internal reflection on the lenses of the substrate. From the figure the FWHM of Si-doped PSS and UPSS were 60 and 77 meV, respectively. It clearly shows that the usage of patterned sapphire substrate can improve the light emission efficiency of blue LED. Fig. 8 displays the current–voltage (I–V) characteristics of PSS LED and UPSS LED with the same chip size as a function of forward driving current. The LEDs on PSS and UPSS have low operating voltages, and sharp turn-on voltages near 2.93 and 2.97 V, respectively. The forward voltage (VF) for the PSS and UPSS LED devices were 3.08 and 3.19 V at the forward injection current of 20 mA. The forward voltage of PSS LED was smaller than that of UPSS LED; also the forward I–V curve of LED grown on PSS did not
show any significant difference with that of UPSS. Almost identical I–V curves infer that the patterning on the GaN surface had very little impact on the I–V characteristics. The difference in the forward I–V curves of PSS and UPSS shows the improvement in the epitaxial film quality of PSS. Fig. 9 shows the luminous intensity variation for different currents. The luminous intensities of the PSS and UPSS at 40 mA applied current were 271.4 and 107.2 mcd, respectively. The luminous intensity of PSS LED was 2.53 times higher than that of UPSS LED at the injection current of 40 mA. This improvement in the luminous intensity of PSS LED was supplied to the enhancement of light extraction efficiency through lens patterned structures. The inset shows the room temperature EL spectra of the PSS and UPSS LEDs at an injection current of 20 mA. The EL peak positions of the PSS and UPSS LEDs appeared at 461 and 465 nm, respectively. A small blue shift of about 4 nm was observed for the PSS LED. It was found that the EL peak intensity of PSS LED was 2.44 times larger than that of the UPSS LED. It is well known that the MQW emission efficiency is related to the
ARTICLE IN PRESS 262
S.-M. Jeong et al. / Journal of Crystal Growth 312 (2010) 258–262
leakage current through dislocations. Hence, such a significant enhancement in EL intensity was attributed to the reduction of TDs that induce non-radiative recombination centers using PSS.
4. Conclusions In this study, nitride-based blue LEDs grown by MOCVD on patterned and unpatterened sapphire substrates were investigated. The SEM micrograph image reveals a smooth GaN surface for the PSS surface with a growth time of 80 min. The narrow fullwidth at half-maximum of PSS from the DCXRD indicates the good quality of GaN on PSS compared to GaN on UPSS. According to the result of AFM, the average TD density can be apparently reduced from 1.5 109 to 3.8 108 cm 2, which reveals less dislocation on PSS compared to UPSS. The PL spectra clearly indicate good optical quality of GaN due to the reduction in TD density and the improvement in the light emission efficiency. The forward voltages of both samples were almost similar, but the difference of the forward I–V curves results from the improved crystalline quality. As much as 2.44 times of increased electroluminescence intensity of PSS LED was obtained at a forward current of 40 mA. This performance improvement was due to the better epitaxial quality of InGaN/GaN epilayers. The dislocation density would be reduced greatly and higher extraction efficiency could be achieved from lens-shaped PSS. Our work brings out GaN-based LED structure with better performance for the next generation of high-brightness blue LEDs using the PSS method.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) National Research Laboratory (NRL) Program grant funded by the Korea government (MEST) (no. ROA-2008000-20031-0), grant (no. R01-2006-000-10352-0) from the Basic Research Program of the Korea Science and Engineering Foundation, and the Post BK21 program of the Ministry of Education and Human Resources development. References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274. [2] Y. Narukawa, J. Narita, T. Sakamoto, K. Deguchi, T. Yamada, T. Muka, Jpn. J. Appl. Phys. 45 (2006) L1084. [3] F.A. Pounce, D.P Bour, Nature, London 386 (1997) 351. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 211. [5] T.S. Zheleva, O.H. Nam, M.D. Bremser, R.F. Davis, Appl. Phys. Lett. 71 (1997) 2472. [6] A. Sakai, H. Sunakawa, A. Usui, Appl. Phy. Lett. 71 (1997) 2259. [7] K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, T. Maeda, J. Cryst. Growth 221 (2000) 316. [8] O.H. Nam, T.S. Zheleva, M.D. Bremser, R.F. Davis, J. Electron. Mater. 27 (1998) 233. [9] M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, T. Mukai, Jpn. J. Appl. Phys. 41 (2002) L1431. [10] D.S. Wuu, W.K. Wang, W.C. Shih, R.H. Horng, C.E. Lee, W.Y. Lin, J.S. Fang, IEEE Photonics Technol. Lett. 17 (2005) 288. [11] Z.H. Feng, K.M. Lau, IEEE Photonics Technol. Lett. 17 (2005) 1812. [12] N. Kuwano, K. Horibuchi, K. Kagawa, S. Nishimoto, M. Sueyoshi, J. Cryst. Growth 237 (2002) 1047. [13] J.C Song, S.H Lee, I.H Lee, K.W Seol, S Kannappan, C.R Lee, J. Cryst. Growth 308 (2007) 321.