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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 1, JANUARY 1, 2013
GaN-Based LEDs With an HT-AlN Nucleation Layer Prepared on Patterned Sapphire Substrate Chung-Ying Chang, Shoou-Jinn Chang, Senior Member, IEEE, C. H. Liu, Shuguang Li, and Evan Chen
Abstract— We report the growth and fabrication of GaN-based light-emitting diodes (LEDs) with a high-temperature (HT) AlN nucleation on patterned sapphire substrate. It was found that the undercut sidewalls were only formed for the HT-AlN LED through defect selective etching. At 1-A current injection, the output power of the LED with HT-AlN nucleation was 12% higher than that of an LED with a conventional low temperature GaN nucleation layer.
to enhance the effect of defect selective etching. In this letter, we report the growth of GaN-based LEDs with a HTAlN nucleation layer on PSS. Detailed device fabrication procedures and the electro-optical properties of the fabricated LEDs will also be discussed.
Index Terms— GaN, high-temperature (HT)-AlN, lightemitting diodes (LEDs), nucleation, patterned sapphire substrate (PSS).
Samples used in this study were all grown by metalorganic chemical vapor deposition on c-plane cone-shaped PSS [10]–[12]. For the cone-shaped PSS, the diameter and height of the cones were 2.4 and 1.55 μm, respectively, while the spacing between the cones was 0.6 μm. We prepared both control and HT-AlN samples. The structure of the control sample consists of a 25 nm thick LT-GaN nucleation layer grown at 560 °C, a 2 μm thick undoped GaN layer grown at 1050 °C, a 2.5 μm thick Si-doped n-GaN layer grown at 1050 °C, a Sidoped strain releasing multiquantum well (SRMQW) grown at 900 °C [13], an undoped light emitting MQW active region grown at 770 °C, a 50 nm thick Mg-doped p-Al0.15 Ga0.85 N electron blocking layer grown at 1050 °C, a 0.22 μm thick Mg-doped p-GaN contact layer grown at 1050 °C and a Sidoped n+ -InGaN/GaN short period superlattice tunnel contact structure, (SPS). The SRMQW consists of 20 periods of 1.2 nm thick In0.04 Ga0.96 N well layers and 3 nm thick GaN barrier layers. The InGaN/GaN MQW active region consists of 8 periods of 3 nm thick In0.22 Ga0.78 N well layers and 8-nmthick GaN barrier layers. On the other hand, the SPS structure consists of four pairs of 5 Å thick In0.23Ga0.77 N layers and 5 Å thick GaN layers [14]. Instead of LT-GaN, a 10 nm thick HTAlN layer grown at 1050 °C was used for the HT-AlN sample. Other than the nucleation layer, the structure of the HT-AlN sample was identical to that of the control sample. X-ray diffraction (XRD) and photoluminescence (PL) were then used to evaluate the crystal quality of these two epitaxial layers. A field-emission scanning electron microscope (FESEM) was also used to characterize cross-sectional images of the cleaved samples. Standard procedures were used to fabricate the 1.143 mm × 1.143 mm LED chips. After scribing, chemical etching was performed by dipping the wafers into H3 PO4 for 10 min at 250 °C [7]. This chemical treatment also removes the debris produces during scribing [15]. Indium-tin-oxide (ITO) and Ti-Al-Ti-Au were subsequently deposited as p- and n-contacts, respectively. Current-voltage (I-V) characteristics of the fabricated devices were then measured at room temperature by an HP4156 semiconductor parameter analyzer. The chips were subsequently packaged into LED lamps. Intensity-current (L-I) characteristics of the
I. I NTRODUCTION
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HE III-NITRIDE semiconductors have become the most important material system for short wavelength light emitters. GaN-based light emitting diodes (LEDs), prepared on sapphire substrate, are now being extensively used [1]–[4]. It has been reported that one can enhance output power of GaN-based LEDs by using patterned sapphire substrate (PSS) [5], [6]. It was found that PSS can not only enhance light extraction efficiency (LEE) but can also enhance epilayer quality. For GaN grown on flat sapphire substrate, it is known that defect distribution is non-uniform along the vertical growth direction while the etching rate of GaN depends strongly on defect density. Knowing these properties, Kuo et al. used defect selective etching to form inclined undercut sidewall to enhance LEE for GaN-based LEDs prepared on flat sapphire substrate [7]. To achieve high quality GaN epitaxial layer on sapphire substrate, it is usually necessary to deposit a low-temperature (LT) grown GaN or AlN nucleation layer [8], [9]. Growth of GaN without the LT nucleation layer will often result in high defect density since high-temperature (HT) GaN growth occurs primarily via threedimensional island growth. The high defect density in the vicinity of epilayer/sapphire interface, however, could be used
Manuscript received September 3, 2012; revised November 12, 2012; accepted November 13, 2012. Date of publication November 29, 2012; date of current version December 20, 2012. This work was supported in part by the Advanced Optoelectronic Technology Center, NCKU, and in part by the Research Center for Energy Technology and Strategy, NCKU under projects from the Ministry of Education, Taiwan. C.-Y. Chang and S.-J. Chang are with the Institute of Microelectronics and Department of Electrical Engineering, Advanced Optoelectronic Technology Center, Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan (e-mail:
[email protected];
[email protected]). C. H. Liu is with the Department of Electronics, Nan Jeon Institute of Technology, Tainan 73746, Taiwan (e-mail:
[email protected]). S. Li is with the College of Science, China University of Petroleum (East China), Shandong 266555, China (e-mail:
[email protected]). E. Chen is with the Epistar Corporation, Southern Taiwan Science Park, Tainan 744, Taiwan (email:
[email protected]). Digital Object Identifier 10.1109/LPT.2012.2228637
II. E XPERIMENTS
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CHANG et al.: GaN-BASED LEDs WITH AN HT-AlN NUCLEATION LAYER
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III. R ESULTS AND D ISCUSSION Fig. 1 shows room temperature PL spectra measured from these two samples. Compared with the control sample, it was found that the PL peak position red-shifted for the HT-AlN sample. This suggests that more indium were incorporated in the HT-AlN sample. The full-width-half-maximum (FWHM) of the PL peaks were 12.84 and 13.40 nm for the control and HT-AlN samples, respectively. Fig. 2 shows XRD rocking curves measured from the (102) plane of these two samples. It can be seen that XRD FWHMs for the control and HT-AlN samples were 252.6 and 282.4 arcsec, respectively. The larger PL FWHM and XRD FWHM both suggest that the crystal quality of the HT-AlN sample was inferior to that of the control sample. This can be attributed to the use of the HT-AlN nucleation layer, which results in higher defect density in the epilayer. It should be noted that the 282.4 arcsec XRD FWHM indicates that the crystal quality of the HT-AlN sample was still reasonably good. Fig. 3(a) and 3(b) show cross-sectional FESEM images of the control and HT-AlN samples, respectively, after scribing and chemical etching. It was found that no scribing induced debris could be observed in both samples due to the effective hot phosphoric acid etching. It was also found that oblique sidewalls were formed for the HT-AlN sample, as shown in Fig. 3(b). In contrast, no such undercut sidewalls were formed for the control sample, as shown in Fig. 3(a). To clarify the discrepancy, we grew an undoped GaN layer for 1 hour with either a LT-GaN nucleation layer or a HT-AlN nucleation
layer on cone-shaped PSS. Figs. 4(a) and 4(b) show crosssectional FESEM images of the undoped GaN layer with LT-GaN and HT-AlN nucleation layers, respectively. It can be seen that the speed of lateral growth was lower for the sample with HT-AlN nucleation layer, as compared to that of the sample with LT-GaN nucleation layer. The FESEM image shown in Fig. 4(b) also suggests higher growth rate along the C-axis direction and inclined film growth on the cone-shaped dot surface for the HT-AlN sample. This should results in numerous grain boundaries near the cone-shaped dot surface due to the coalescence of different GaN facets. With higher lateral growth speed and inclined film on dot surface, we thus achieved inferior crystal quality and high indium incorporation in the MQW active region for the sample with HT-AlN nucleation layer. On the other hand, the low lateral growth speed could result in better crystal quality, particular near the epilayer/PSS interface. This could result in more uniform distribution of defects along the vertical growth direction. As a result, the defect selective etching used for GaN-based LEDs prepared on flat sapphire substrate to form undercut sidewalls [7] will not be very useful for the sample prepared with conventional LT-GaN nucleation layer on PSS. Thus, we observed vertical side-walls from the sample with LT-GaN nucleation layer, as shown in Fig. 3(a). Fig. 5 shows L-I-V characteristics measured from the two fabricated LEDs. It should be noted that these values were the average values measured from 500 fabricated chips.
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Under 350 mA current injection, it was found that forward voltages were 3.36 and 3.29 V for the control and HT-AlN LEDs. It was also found that turn-on voltages @ 10 μA were 2.34 and 2.33 V while the voltages @ 1 μA were 2.16 and 2.17 V for the control and HT-AlN LEDs, respectively. Although the crystal quality of the HT-AlN sample was inferior to that of control sample, the lower 350 mA forward voltage and the almost identical voltages measured @ 10 μA and @ 1 μA suggest that electrical properties of the HT-AlN LED were still good. It was also found that the output power measured from the HT-AlN LED was larger than that of the control LED. With 350 mA injection current, it was found that output powers were 356 mw for the control and 385 mW for the HT-AlN LEDs. As we increased the injection current to 1 A, it was found that output powers increased to 812 mW and 904 mW for the control and HT-AlN LEDs, respectively. In other words, we can achieve 12% enhancement in LED output power from the HT-AlN LED. The higher output power can be attributed to the formation of undercut sidewalls and thus the enhanced LEE. The inset in Fig. 5 shows the distributions of EL peak wavelength and output power. These data were measured from the 500 control LEDs and 500 HT-AlN LEDs with 350 mA current injection. It can be seen that deviations of EL peak wavelength and output power were reasonably small. Fig. 6 shows the light output pattern of the fabricated LEDs with 350 mA DC current injection. It can be seen that the electroluminescence (EL) intensity observed from the HT-AlN LED was higher than that observed from the control LED. It should be noted that the EL enhancement was larger
in the near horizontal directions (i.e. 45° ≤ θ ≤ 90° or 270° ≤ θ ≤ 315°) as compared to that in the near vertical directions (i.e. 0° ≤ θ ≤ 45° or 315° ≤ θ ≤ 360°). Such a larger enhancement can again be attributed to the formation of undercut sidewalls so that photons could have a larger probability to be emitted from the device in the near horizontal directions. IV. C ONCLUSION In summary, we report the growth and fabrication of GaN-based LEDs with either a LT-GaN nucleation layer or a HT-AlN nucleation on cone-shaped PSS. After hot phosphoric acid etching, it was found that vertical sidewalls and oblique sidewalls were formed for the LEDs prepared on PSS with a conventional LT-GaN nucleation layer and with a HT-AlN nucleation layer, respectively. It was also found that the defect selective etching induced oblique sidewalls could effectively enhance output power for the LEDs with a HT-AlN nucleation layer prepared on PSS. R EFERENCES [1] S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High-power InGaN single-quantum-well-structure blue and violet light-emitting diodes,” Appl. Phys. Lett., vol. 67, no. 13, pp. 1868–1870, 1995. [2] S. J. Chang, W. C. Lai, Y. K. Su, J. F. Chen, C. H. Liu, and U. H. Liaw, “InGaN-GaN multiquantum-well blue and green light-emitting diodes,” IEEE J. Sel. Topics Quantum Electron., vol. 8, no. 2, pp. 278–283, Mar./Apr. 2002. [3] Y. J. Lee, et al., “Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates,” IEEE Photon. Technol. Lett., vol. 18, no. 10, pp. 1152–1154, May 15, 2006. [4] S. J. Chang, et al., “Nitride-based light emitting diodes with indium tin oxide electrode patterned by imprint lithography,” Appl. Phys. Lett., vol. 91, no. 1, pp. 013504-1–013504-3, 2007. [5] S. J. Chang, et al., “Nitride-based LEDs fabricated on patterned sapphire substrates,” Solid-State Electron., vol. 47, no. 9, pp. 1539–1542, 2003. [6] K. Tadatomo, et al., “High output power InGaN ultraviolet light-emitting diodes fabricated on patterned substrates using metalorganic vapor phase epitaxy,” Phys. Status Solidi A, vol. 188, no. 1, pp. 121–125, 2001. [7] D. S. Kuo, S. J. Chang, T. K. Ko, C. F. Shen, S. J. Hon, and S. C. Hung, “Nitride-based LEDs with phosphoric acid etched undercut sidewalls,” IEEE Photon. Technol. Lett., vol. 21, no. 8, pp. 510–512, Apr. 15, 2009. [8] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, and N. Sawaki, “Effects of AlN buffer layer on crystallographic structure and on electrical and optical properties of GaN and Ga1−x Alx N (0 < X 0.4) films grown on sapphire substrate by MOVPE,” J. Cryst. Growth, vol. 98, nos. 1–2, pp. 209–219, 1989. [9] X. H. Wu, et al., “Nucleation layer evolution in metal-organic chemical vapor deposition grown GaN,” Appl. Phys. Lett., vol. 68, no. 10, pp. 1371–1373, 1996. [10] C. S. Chang, et al., “Nitride-based LEDs with textured side walls,” IEEE Photon. Technol. Lett., vol. 16, no. 3, pp. 750–752, Mar. 2004. [11] S. J. Chang, et al., “Nitride-based LEDs with p-InGaN capping layer,” IEEE Trans. Electron. Devices, vol. 50, no. 12, pp. 2567–2570, Dec. 2003. [12] S. J. Chang, et al., “400-nm InGaN-GaN and InGaN-AlGaN multiquantum well light-emitting diodes,” IEEE J. Sel. Topics Quantum Electron., vol. 8, no. 4, pp. 744–748, Jul./Aug. 2002. [13] C. Y. Chang, S. J. Chang, C. H. Liu, S. G. Li, and T. K. Lin, “GaN-based LEDs with double strain releasing MQWs and Si delta-doping layers,” IEEE Photon. Technol. Lett., vol. 24, no. 20, pp. 1809–1811, Oct. 15, 2012. [14] S. J. Chang, et al., “Highly reliable nitride-based LEDs with SPS+ITO upper contacts,” IEEE J. Quantum Electron., vol. 39, no. 11, pp. 1439–1443, Nov. 2003. [15] S. J. Chang, D. S. Kuo, K. T. Lam, K. H. Wen, T. K. Ko, and S. J. Hon, “GaN-based LEDs with sapphire debris removed by phosphoric etching,” IEEE Trans. Compon. Packag. Manuf. Technol., vol. 2, no. 2, pp. 349–353, Feb. 2012.
