Enhanced performance of InGaN-based light emitting diodes through a special etch and regrown process in n-GaN layer Binglei Fu,1,2,* Junjie Kang,1,2 Tongbo Wei,1 Zhiqiang Liu,1 Zhe Liu,1 Naixin Liu,1 Zhuo Xiong,1 Zhi Li,1 Xuecheng Wei,1 Hongxi Lu, 1 Xiaoyan Yi,1 Jinmin Li,1 and Junxi Wang1 1
Research and Development Center for Semiconductor Lighting, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China 2 Binglei Fu and Junjie Kang contributed equally to this work * [email protected]
Abstract: We reported that the peak efficiency together with the efficiency droop in InGaN-based light emitting diodes could be effectively modified through a simple and low-cost etch-regrown process in n-GaN layer. The etched n-GaN template contained pyramid arrays with inclined side planes. The following lateral overgrowth process from the etched n-GaN template substantially reduced the edge dislocation density and residential compressive strain in epilayers. The efficiency droop of LED samples thus could be modified due to the reduced polarization field, resulting from the strain relaxation in epilayers. What is more, the peak efficiency and reverse current leakage were also modified due to the reduction of dislocations. ©2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (230.0250) Optoelectronics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photon. 3(4), 180–182 (2009). S. T. Tan, X. W. Sun, H. V. Demir, and S. P. DenBaars, “Advances in the LED materials and architectures for energy-saving solid-state lighting toward ‘lighting revolution’,” IEEE Photon. J. 4(2), 613–619 (2012). M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1028–1040 (2009). D. S. Meyaard, G. B. Lin, J. Cho, E. F. Schubert, H. Shim, S. H. Han, M. H. Kim, C. Sone, and Y. S. Kim, “Identifying the cause of the efficiency droop in GaInN light-emitting diodes by correlating the onset of high injection with the onset of the efficiency droop,” Appl. Phys. Lett. 102(25), 251114 (2013). D. S. Meyaard, G. B. Lin, Q. Shan, J. Cho, E. F. Schubert, H. Shim, M. H. Kim, and C. Sone, “Asymmetry of carrier transport leading to efficiency droop in GaInN based light-emitting diodes,” Appl. Phys. Lett. 99(25), 251115 (2011). Z. H. Zhang, Y. Ji, W. Liu, S. T. Tan, Z. Kyaw, Z. Ju, X. Zhang, N. Hasanov, S. Lu, Y. Zhang, B. Zhu, X. W. Sun, and H. V. Demir, “On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer,” Appl. Phys. Lett. 104(7), 073511 (2014). Z. H. Zhang, W. Liu, S. T. Tan, Z. Ju, Y. Ji, Z. Kyaw, X. Zhang, N. Hasanov, B. Zhu, S. Lu, Y. Zhang, X. W. Sun, and H. V. Demir, “On the mechanisms of InGaN electron cooler in InGaN/GaN light-emitting diodes,” Opt. Express 22(S3), A779–A789 (2014). K. T. Delaney, P. Rinke, and C. G. Van de Walle, “Auger recombination rates in nitrides from first principles,” Appl. Phys. Lett. 94(19), 191109 (2009). J. Iveland, L. Martinelli, J. Peretti, J. S. Speck, and C. Weisbuch, “Direct measurement of auger electrons emitted from a semiconductor light-emitting diode under electrical injection: identification of the dominant mechanism for efficiency droop,” Phys. Rev. Lett. 110(17), 177406 (2013). Y. Yang, X. A. Cao, and C. Yan, “Investigation of the nonthermal mechanism of efficiency rolloff in InGaN light-emitting diodes,” IEEE Trans. Electron Dev. 55(7), 1771–1775 (2008). M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M. H. Kim, S. Yoon, S. M. Lee, C. Sone, T. Sakong, and Y. Park, “Polarization-matched GaInN/AlGaInN multi-quantum-well light-emitting diodes with reduced efficiency droop,” Appl. Phys. Lett. 93(4), 041102 (2008). S. C. Ling, T. C. Lu, S. P. Chang, J. R. Chen, H. C. Kuo, and S. C. Wang, “Low efficiency droop in blue-green m-plane InGaN/GaN light emitting diodes,” Appl. Phys. Lett. 96(23), 231101 (2010). Y. K. Kuo, J. Y. Chang, M. C. Tsai, and S. H. Yen, “Advantages of blue InGaN multiple-quantum well lightemitting diodes with InGaN barriers,” Appl. Phys. Lett. 95(1), 011116 (2009).
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1284
14. D. Zhu, A. N. Noemaun, M. F. Schubert, J. Cho, E. F. Schubert, M. H. Crawford, and D. D. Koleske, “Enhanced electron capture and symmetrized carrier distribution in GaInN light-emitting diodes having tailored barrier doping,” Appl. Phys. Lett. 96(12), 121110 (2010). 15. J. Piprek and Z. M. Simon Li, “Sensitivity analysis of electron leakage in III-nitride light-emitting diodes,” Appl. Phys. Lett. 102(13), 131103 (2013). 16. Z. H. Zhang, W. Liu, Z. Ju, S. T. Tan, Y. Ji, X. Zhang, L. Wang, Z. Kyaw, X. W. Sun, and H. V. Demir, “Polarization self-screening in [0001] oriented InGaN/GaN light-emitting diodes for improving the electron injection efficiency,” Appl. Phys. Lett. 104(25), 251108 (2014). 17. Z. H. Zhang, S. T. Tan, Z. G. Ju, W. Liu, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the effect of step-doped quantum barriers in InGaN/GaN light emitting diodes,” J. Disp. Technol. 9(4), 226–233 (2013). 18. Z. H. Zhang, W. Liu, Z. Ju, S. T. Tan, Y. Ji, Z. Kyaw, X. Zhang, L. Wang, X. W. Sun, and H. V. Demir, “Selfscreening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers,” Appl. Phys. Lett. 104(24), 243501 (2014). 19. R. A. Arif, Y. K. Ee, and N. Tansu, “Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes,” Appl. Phys. Lett. 91(9), 091110 (2007). 20. R. M. Farrell, P. S. Hsu, D. A. Haeger, K. Fujito, S. P. DenBaars, J. S. Speck, and S. Nakamura, “Lowthreshold-current-density AlGaN-cladding-free m -plane InGaN/GaN laser diodes,” Appl. Phys. Lett. 96(23), 231113 (2010). 21. Z. G. Ju, S. T. Tan, Z. H. Zhang, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer,” Appl. Phys. Lett. 100(12), 123503 (2012). 22. V. C. Su, P. H. Chen, R. M. Lin, M. L. Lee, Y. H. You, C. I. Ho, Y. C. Chen, W. F. Chen, and C. H. Kuan, “Suppressed quantum-confined Stark effect in InGaN-based LEDs with nano-sized patterned sapphire substrates,” Opt. Express 21(24), 30065–30073 (2013). 23. J. Kang, Z. Li, Z. Liu, H. Li, Y. Zhao, Y. Tian, P. Ma, X. Yi, and G. Wang, “Investigation of the wet-etching mechanism of Ga-polar AlGaN/GaN micro-pillars,” J. Cryst. Growth 386(0), 175–178 (2014). 24. K. Hiramatsu, K. Nishiyama, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Recent progress in selective area growth and epitaxial lateral overgrowth of III-nitrides: effects of reactor pressure in MOVPE growth,” Phys. Status Solidi (a) 176(1), 535–543 (1999). 25. K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO),” J. Cryst. Growth 221(1), 316–326 (2000). 26. C. Y. Cho, M. K. Kwon, I. K. Park, S. H. Hong, J. J. Kim, S. E. Park, S. T. Kim, and S. J. Park, “High-efficiency light-emitting diode with air voids embedded in lateral epitaxially overgrown GaN using a metal mask,” Opt. Express 19(S4), A943–A948 (2011). 27. A. Sakai, H. Sunakawa, and A. Usui, “Defect structure in selectively grown GaN films with low threading dislocation density,” Appl. Phys. Lett. 71(16), 2259–2261 (1997). 28. K. S. Lee, Isnaeni, Y.-S. Yoo, J.-H. Lee, Y.-C. Kim, and Y.-H. Cho, “Influence of defect reduction and strain relaxation on carrier dynamics in InGaN-based light-emitting diodes on cone-shaped patterned sapphire substrates,” J. Appl. Phys. 113(17), 173512 (2013). 29. R. Kirste, M. P. Hoffmann, J. Tweedie, Z. Bryan, G. Callsen, T. Kure, C. Nenstiel, M. R. Wagner, R. Collazo, A. Hoffmann, and Z. Sitar, “Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements,” J. Appl. Phys. 113(10), 103504 (2013). 30. V. Y. Davydov, N. S. Averkiev, I. N. Goncharuk, D. K. Nelson, I. P. Nikitina, A. S. Polkovnikov, A. N. Smirnov, M. A. Jacobson, and O. K. Semchinova, “Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H-SiC,” J. Appl. Phys. 82(10), 5097–5102 (1997). 31. J. H. Park, D. Y. Kim, S. Hwang, D. Meyaard, E. F. Schubert, Y. D. Han, J. W. Choi, J. Cho, and J. K. Kim, “Enhanced overall efficiency of GaInN-based light-emitting diodes with reduced efficiency droop by Alcomposition-graded AlGaN/GaN superlattice electron blocking layer,” Appl. Phys. Lett. 103(6), 061104 (2013). 32. C. Y. Hsu, W. H. Lan, and Y. S. Wu, “Effect of thermal annealing of Ni/Au ohmic contact on the leakage current of GaN based light emitting diodes,” Appl. Phys. Lett. 83(12), 2447–2449 (2003).
1. Introduction Due to the high efficiency, the feature of energy saving and being able to emit light from ultraviolet to green spectrum, the InGaN-based light emitting diodes (LEDs) have shown great potential in display and solid state lighting systems [1]. Further development of these technologies calls for LEDs devices with higher output power. However, with increased driven currents, there will be a substantial decrease for the efficiency of LEDs. This phenomenon, known as efficiency droop, is a severe limitation to further enhance the output power of LEDs [2, 3]. Many physical mechanisms have been suggested as explanations, including carrier leakage from active region [4–7], Auger recombination [8, 9] and carrier delocalization from In-rich regions at high currents [10]. It has been proposed that reducing the polarization field in InGaN multiple quantum wells (MQWs) is an effective way to realizing high efficiency, low droop LEDs [11, 12].
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1285
Polarization field induced quantum confined Stark effect (QCSE) would reduce the electron and hole wavefunction overlap in quantum wells (QWs), leading to the reduced overall recombination efficiency [13]. Also the efficiency droop is severe for LEDs with high polarization field in the MQWs, as the electrons would escape from the active region more easily under high polarization field [14–16]. The attempts to reduce polarization fields in QWs include special designs in quantum barriers (QBs) and QWs [17–19], nonpolar or semipolar QWs [12, 20], inserting a higher temperature interlayer after the growth of low temperature buffer layer [21] and nano-sized patterned sapphire substrates [22]. However, most of these technics are hindered by either the limited crystal quality in active layers or the complicated substrate manufacturing process. Simple techniques thus are highly desired to meet the growing requirement of LED industry. In this study, we reported that the efficiency droop of LED devices could be effectively reduced through a simple etch and regrown process in n-GaN layers. The reduced polarization field in MQWs which is resulted from the relaxation of the compressive strain in the regrown n-GaN layers and the following MQWs might be an explanation. The peak efficiency and reverse current leakage of LED devices are also modified with our method as a result of the reduction of dislocations. What is more, the etch-regrown process is easy to be accomplished in mass production as it is based on the mature technologies used in LED industry. 2. Experimental details The n-GaN templates used in our experiments were grown under 1050°C, after the deposition of the 30 nm GaN nucleation layer on the c-plane sapphire substrates with metal organic chemical vapor deposition (MOCVD) system. After that, a two-step etch process was used to form the n-GaN pyramid arrays from the planer n-GaN template. The first step is a traditional lithography process to form the 3 μm diameter and 1μm height GaN micro-pillars. The 45°tilted scanning electron microscope (SEM) image of the GaN micro-pillars is shown in Fig. 1(b). After the first step, the samples were etched by KOH in ethylene glycol solution at 150°C for 45 min to form the n-GaN pyramid arrays. Figure 1(c) shows the Plan-view SEM image of the n-GaN pyramid arrays. Detailed process procedures could be found in our previous work [23]. Each n-GaN pyramid contained six (11-212) planes which would promote the lateral growth mode in the following regrown process. The n-GaN template with surface pyramid arrays was then put into the MOCVD system for the 1 h regrown process to flatten the n-GaN surface. The growth temperature of regrown process was 1050°C and the growth rate was carefully modified to keep the thickness of regrown n-GaN layer identical with the unetched n-GaN template. The growth pressure of the regrown process is 200 Torr and the V/III is kept at 2700. After the regrown process, the n-GaN layer with the etch and regrown process (named etch-regrown n-GaN) and the n-GaN layer without this process (named reference n-GaN) was placed in the MOCVD system in a single run to finish the following MQWs and p-GaN layers for LED structure. The schematic illustrations of our LED structures with etch-regrown n-GaN layers could be seen in Fig. 1(a). All LEDs (1 × 1 mm2) were fabricated with a standard mesa structure. Cr/Pt/Au (70/40/1440 nm) were evaporated as both the p-type and n-type electrodes. The contacts were annealed in N2 at 250°C for 15 min under the atmosphere pressure.
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1286
Fig. 1. (a) The schematic illustrations of the LED structures with etch-regrown n-GaN layers; (b) 45°-tilted SEM image of the GaN pillars formed from the n-GaN template after the traditional lithography process; (c) Plan-view SEM image of the GaN pyramids formed from the GaN pillars after the wet etching process.
3. Results and discussion The detailed regrown process is examined by the atomic force microscopy (AFM) scans and shown in Figs. 2(a)–2(e). After a 4 minutes growth process, the initial pyramid shaped GaN arrays (Fig. 2(a)) were turned into a smooth cone structure (Fig. 2(b)). This is a combined effect of the lateral overgrowth from the (11-212) planes of the GaN pyramids and the vertical growth from the (0001) planes between the pyramids. As the height of GaN pyramids were not precisely controlled in our experiments, further growth of the GaN cones in Fig. 2(b) for 4 minutes lead to the surface morphology with staggered GaN disks. (Fig. 2(c)) After another 20 minutes growth, the staggered GaN disks disappeared and the surface of our samples were flatten. (Fig. 2(d)) The cross-sectional transmission electron microscopy (TEM) image of the etch-regrown n-GaN layers are shown in Fig. 2(e), the initial n-GaN pyramid arrays were completely coalesced after the regrown process and no voids were found in our etch-regrown n-GaN layers. The voids were often found in traditional lateral overgrowth GaN layers due to the uncompleted coalescence of GaN on top of the masks [24, 25] and may have an influence on the light extraction efficiency of LEDs [26].
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1287
Fig. 2. The 20 × 20 μm AFM scans of the etched pyramid shaped n-GaN surface (a) and the nGaN surface after regrown for 4 minutes (b), 8 minutes (c) and 28 minutes (d) from the pyramid shaped n-GaN layer. (d) the cross-sectional TEM image of the n-GaN layers after the etch-regrown process.
The structural quality of the etch-regrown and reference n-GaN layers were characterized and compared with the High-resolution X-ray diffraction (HRXRD) measurements. The Xray rocking curves measured on the symmetric (002) and asymmetric (102) planes are shown in Figs. 3(a) and 3(b). It is shown that the full width at half maximum (FWHM) values of the symmetric (002) planes for both n-GaN layers are almost the same (302.4 arcsec for etchregrown n-GaN and 296 arcsec for reference n-GaN), while those of the asymmetric (102) planes for etch-regrown n-GaN layers are clearly reduced compared with the reference n-GaN layers (476.3 arcsec for etch-regrown n-GaN and 596.5 arcsec for the reference n-GaN). The FWHM of the X-ray rocking curves on the symmetric (002) planes are related to the screw and mixed type dislocations; whereas the FWHM of the X-ray rocking curves on the asymmetric (102) planes are related to the edge dislocations. From the XRD results, we found that the edge dislocations in the etch-regrown n-GaN layers were clearly reduced compared with reference n-GaN layers. The reduction of edge dislocations can be explained by the dislocation propagation resulting from the lateral overgrowth of (11-212) planes which the dislocations intersect [27]. The lateral overgrowth process is often accompanied with the reduction of compressive strain in GaN layers [22, 28]. In order to investigate the strain state in our samples, Raman measurements were performed on 20 different positions of the wafer and the peak position of the E2 (high) mode are shown in Fig. 3(c). From 1 to 20, the measurement position moves from the orientation flat to the other side of the wafer. The typical Raman spectra of etchregrown and reference n-GaN layers are displayed in Fig. 3(d). The position of E2 (high) mode shown in the picture is closely related to the strain and could be used as a metric for the strain state in GaN layers. The position of the E2 (high) peak of the relaxed GaN layers is 567 cm−1 [29] and shown in Fig. 3(d) as the blue dashed line. Both n-GaN layers are under compressive strain as can be seen from the picture. This compressive strain results from the large lattice mismatch of the GaN epilayers and the sapphire substrates. However, the position of E2 (high) peak for etch-regrown n-GaN is shifted from 569.94 cm−1 to 569.71 cm−1 compared with the reference sample, indicating the relaxation of compressive strain. Using the method depicted by Davydov et. al [30], the calculated residual compressive strain is 1.34 × 10−3 for reference and 1.24 × 10−3 for etch-regrown sample. The value is comparable to the previous reported strain relaxation for GaN layers grown on nano patterned sapphire
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1288
substrates, which reduced the QCSE in QWs [22]. However, our results are little smaller as the size of etched pyramids and space between them are not modified.
Fig. 3. (a) Symmetric (002) and (b) asymmetric (102) X-ray rocking curves of the etchregrown and reference n-GaN layers. (c) the peak position of the E2 (high) mode of the Raman spectra for the etch-regrown and reference n-GaN layers. From 1 to 20, the measurement position moves from the orientation flat to the other side of the wafer. (d) the typical room temperature Raman spectra of the etch-regrown and reference n-GaN layers.
In order to study the polarization field induced QCSE, the excitation power dependent photoluminescence (PL) measurement was carried out in Figs. 4(a) and 4(b). Two characterizations should be noticed from the pictures, the shorter PL peak wavelength and smaller wavelength shift with excitation power for LEDs grown with etch-regrown n-GaN layers. First, the PL peak wavelength of LEDs grown with etch-regrown n-GaN layers are reduced from 460 nm to 450 nm compared with the reference LEDs. As the LED structures were grown in the same run, the indium composition and quantum well width are the same in both structures. This consumption is also confirmed by the HRXRD ω-2θ scans which are not shown here. Thus, the shorter peak wavelength of the PL spectra for LEDs with etch-regrown n-GaN layers indicated the reduced QCSE in active region. Second, the wavelength shift with excitation power increased from 0.4 mW to 40 mW for LEDs with etch-regrown n-GaN layers is 0.5 nm, which is much smaller than that of LEDs with reference n-GaN layers (4.9 nm as shown in the picture). The smaller blue shift of PL peak is another evidence for the reduced QCSE in active region. The reduced QCSE indicates the reduction of the polarization field in active region of LED with etch-regrown n-GaN layers which is induced by the relaxed strain in epilayers.
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1289
Fig. 4. The excitation dependent PL spectra measured at room temperature of LED samples with etch-regrown n-GaN (a) and reference n-GaN (b).
The electrical properties of LEDs with and without etch-regrown n-GaN layers are shown in Figs. 5(a) and 5(b). From Fig. 5(a), the efficiency droop in LED samples with etch-regrown n-GaN layers is clearly modified compared with reference sample. As many previous works has proved [12, 22], we believe that the reduced polarization field in active region will lead to the reduced efficiency droop in LED samples with etch-regrown n-GaN layers. Note that the peak efficiency of LED samples with etch-regrown n-GaN layers is also increased, which may be a combined effect of the reduced QCSE in QWs and the elimination of threading edge dislocations penetrated from n-GaN to MQWs. Figure 5(b) is the forward I-V curves for both samples, from inset is the reverse I-V curves. The forward operation voltage for LEDs with etch-regrown n-GaN layers is clearly reduced compared with the reference sample, attributing to the reduced sheet charges which result from the polarization field in LED structures. The large triangular barriers induced by sheet charges in the active region would impede carrier flow and result in the higher operation voltage [31] The reverse current leakage is also modified due to the reduced threading dislocations in LED structures with etch-regrown nGaN layers. The threading dislocations often act as current leakage path as previous work reported [32].
Fig. 5. (a) The EQE and output power as a function of forward current for LEDs with reference and etch-regrown n-GaN. (b) the forward I-V curves of LEDs with reference and etch-regrown n-GaN. Inset shows the corresponding reverse I-V characteristics.
4. Conclusions In conclusion, this paper reports a novel etch and regrown process in n-GaN layers to suppress the efficiency droop effect for LEDs. The etch and regrown process in n-GaN layers relaxed the compressive strain in epilayers. Thus the polarization filed in active layers could
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1290
be reduced and lead to the reduced efficiency droop. What is more, the lateral overgrowth process initialed from the (11-212) planes of the etched n-GaN pyramid effectively reduced the threading edge dislocations. The peak efficiency and the reverse current leakage for LED samples are clearly modified as a result. Acknowledgments This work was financially supported by the National High Technology Research and Development Program of China No. 2013AA03A101 and the National Natural Science Foundation of China No. 61306051. The authors also thank Miss Yanhong Liu for the language assistance.
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1291
Research and Development Center for Semiconductor Lighting, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China 2 Binglei Fu and Junjie Kang contributed equally to this work * [email protected]
Abstract: We reported that the peak efficiency together with the efficiency droop in InGaN-based light emitting diodes could be effectively modified through a simple and low-cost etch-regrown process in n-GaN layer. The etched n-GaN template contained pyramid arrays with inclined side planes. The following lateral overgrowth process from the etched n-GaN template substantially reduced the edge dislocation density and residential compressive strain in epilayers. The efficiency droop of LED samples thus could be modified due to the reduced polarization field, resulting from the strain relaxation in epilayers. What is more, the peak efficiency and reverse current leakage were also modified due to the reduction of dislocations. ©2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (230.0250) Optoelectronics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nat. Photon. 3(4), 180–182 (2009). S. T. Tan, X. W. Sun, H. V. Demir, and S. P. DenBaars, “Advances in the LED materials and architectures for energy-saving solid-state lighting toward ‘lighting revolution’,” IEEE Photon. J. 4(2), 613–619 (2012). M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1028–1040 (2009). D. S. Meyaard, G. B. Lin, J. Cho, E. F. Schubert, H. Shim, S. H. Han, M. H. Kim, C. Sone, and Y. S. Kim, “Identifying the cause of the efficiency droop in GaInN light-emitting diodes by correlating the onset of high injection with the onset of the efficiency droop,” Appl. Phys. Lett. 102(25), 251114 (2013). D. S. Meyaard, G. B. Lin, Q. Shan, J. Cho, E. F. Schubert, H. Shim, M. H. Kim, and C. Sone, “Asymmetry of carrier transport leading to efficiency droop in GaInN based light-emitting diodes,” Appl. Phys. Lett. 99(25), 251115 (2011). Z. H. Zhang, Y. Ji, W. Liu, S. T. Tan, Z. Kyaw, Z. Ju, X. Zhang, N. Hasanov, S. Lu, Y. Zhang, B. Zhu, X. W. Sun, and H. V. Demir, “On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer,” Appl. Phys. Lett. 104(7), 073511 (2014). Z. H. Zhang, W. Liu, S. T. Tan, Z. Ju, Y. Ji, Z. Kyaw, X. Zhang, N. Hasanov, B. Zhu, S. Lu, Y. Zhang, X. W. Sun, and H. V. Demir, “On the mechanisms of InGaN electron cooler in InGaN/GaN light-emitting diodes,” Opt. Express 22(S3), A779–A789 (2014). K. T. Delaney, P. Rinke, and C. G. Van de Walle, “Auger recombination rates in nitrides from first principles,” Appl. Phys. Lett. 94(19), 191109 (2009). J. Iveland, L. Martinelli, J. Peretti, J. S. Speck, and C. Weisbuch, “Direct measurement of auger electrons emitted from a semiconductor light-emitting diode under electrical injection: identification of the dominant mechanism for efficiency droop,” Phys. Rev. Lett. 110(17), 177406 (2013). Y. Yang, X. A. Cao, and C. Yan, “Investigation of the nonthermal mechanism of efficiency rolloff in InGaN light-emitting diodes,” IEEE Trans. Electron Dev. 55(7), 1771–1775 (2008). M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M. H. Kim, S. Yoon, S. M. Lee, C. Sone, T. Sakong, and Y. Park, “Polarization-matched GaInN/AlGaInN multi-quantum-well light-emitting diodes with reduced efficiency droop,” Appl. Phys. Lett. 93(4), 041102 (2008). S. C. Ling, T. C. Lu, S. P. Chang, J. R. Chen, H. C. Kuo, and S. C. Wang, “Low efficiency droop in blue-green m-plane InGaN/GaN light emitting diodes,” Appl. Phys. Lett. 96(23), 231101 (2010). Y. K. Kuo, J. Y. Chang, M. C. Tsai, and S. H. Yen, “Advantages of blue InGaN multiple-quantum well lightemitting diodes with InGaN barriers,” Appl. Phys. Lett. 95(1), 011116 (2009).
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1284
14. D. Zhu, A. N. Noemaun, M. F. Schubert, J. Cho, E. F. Schubert, M. H. Crawford, and D. D. Koleske, “Enhanced electron capture and symmetrized carrier distribution in GaInN light-emitting diodes having tailored barrier doping,” Appl. Phys. Lett. 96(12), 121110 (2010). 15. J. Piprek and Z. M. Simon Li, “Sensitivity analysis of electron leakage in III-nitride light-emitting diodes,” Appl. Phys. Lett. 102(13), 131103 (2013). 16. Z. H. Zhang, W. Liu, Z. Ju, S. T. Tan, Y. Ji, X. Zhang, L. Wang, Z. Kyaw, X. W. Sun, and H. V. Demir, “Polarization self-screening in [0001] oriented InGaN/GaN light-emitting diodes for improving the electron injection efficiency,” Appl. Phys. Lett. 104(25), 251108 (2014). 17. Z. H. Zhang, S. T. Tan, Z. G. Ju, W. Liu, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the effect of step-doped quantum barriers in InGaN/GaN light emitting diodes,” J. Disp. Technol. 9(4), 226–233 (2013). 18. Z. H. Zhang, W. Liu, Z. Ju, S. T. Tan, Y. Ji, Z. Kyaw, X. Zhang, L. Wang, X. W. Sun, and H. V. Demir, “Selfscreening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers,” Appl. Phys. Lett. 104(24), 243501 (2014). 19. R. A. Arif, Y. K. Ee, and N. Tansu, “Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes,” Appl. Phys. Lett. 91(9), 091110 (2007). 20. R. M. Farrell, P. S. Hsu, D. A. Haeger, K. Fujito, S. P. DenBaars, J. S. Speck, and S. Nakamura, “Lowthreshold-current-density AlGaN-cladding-free m -plane InGaN/GaN laser diodes,” Appl. Phys. Lett. 96(23), 231113 (2010). 21. Z. G. Ju, S. T. Tan, Z. H. Zhang, Y. Ji, Z. Kyaw, Y. Dikme, X. W. Sun, and H. V. Demir, “On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer,” Appl. Phys. Lett. 100(12), 123503 (2012). 22. V. C. Su, P. H. Chen, R. M. Lin, M. L. Lee, Y. H. You, C. I. Ho, Y. C. Chen, W. F. Chen, and C. H. Kuan, “Suppressed quantum-confined Stark effect in InGaN-based LEDs with nano-sized patterned sapphire substrates,” Opt. Express 21(24), 30065–30073 (2013). 23. J. Kang, Z. Li, Z. Liu, H. Li, Y. Zhao, Y. Tian, P. Ma, X. Yi, and G. Wang, “Investigation of the wet-etching mechanism of Ga-polar AlGaN/GaN micro-pillars,” J. Cryst. Growth 386(0), 175–178 (2014). 24. K. Hiramatsu, K. Nishiyama, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Recent progress in selective area growth and epitaxial lateral overgrowth of III-nitrides: effects of reactor pressure in MOVPE growth,” Phys. Status Solidi (a) 176(1), 535–543 (1999). 25. K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, and T. Maeda, “Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO),” J. Cryst. Growth 221(1), 316–326 (2000). 26. C. Y. Cho, M. K. Kwon, I. K. Park, S. H. Hong, J. J. Kim, S. E. Park, S. T. Kim, and S. J. Park, “High-efficiency light-emitting diode with air voids embedded in lateral epitaxially overgrown GaN using a metal mask,” Opt. Express 19(S4), A943–A948 (2011). 27. A. Sakai, H. Sunakawa, and A. Usui, “Defect structure in selectively grown GaN films with low threading dislocation density,” Appl. Phys. Lett. 71(16), 2259–2261 (1997). 28. K. S. Lee, Isnaeni, Y.-S. Yoo, J.-H. Lee, Y.-C. Kim, and Y.-H. Cho, “Influence of defect reduction and strain relaxation on carrier dynamics in InGaN-based light-emitting diodes on cone-shaped patterned sapphire substrates,” J. Appl. Phys. 113(17), 173512 (2013). 29. R. Kirste, M. P. Hoffmann, J. Tweedie, Z. Bryan, G. Callsen, T. Kure, C. Nenstiel, M. R. Wagner, R. Collazo, A. Hoffmann, and Z. Sitar, “Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements,” J. Appl. Phys. 113(10), 103504 (2013). 30. V. Y. Davydov, N. S. Averkiev, I. N. Goncharuk, D. K. Nelson, I. P. Nikitina, A. S. Polkovnikov, A. N. Smirnov, M. A. Jacobson, and O. K. Semchinova, “Raman and photoluminescence studies of biaxial strain in GaN epitaxial layers grown on 6H-SiC,” J. Appl. Phys. 82(10), 5097–5102 (1997). 31. J. H. Park, D. Y. Kim, S. Hwang, D. Meyaard, E. F. Schubert, Y. D. Han, J. W. Choi, J. Cho, and J. K. Kim, “Enhanced overall efficiency of GaInN-based light-emitting diodes with reduced efficiency droop by Alcomposition-graded AlGaN/GaN superlattice electron blocking layer,” Appl. Phys. Lett. 103(6), 061104 (2013). 32. C. Y. Hsu, W. H. Lan, and Y. S. Wu, “Effect of thermal annealing of Ni/Au ohmic contact on the leakage current of GaN based light emitting diodes,” Appl. Phys. Lett. 83(12), 2447–2449 (2003).
1. Introduction Due to the high efficiency, the feature of energy saving and being able to emit light from ultraviolet to green spectrum, the InGaN-based light emitting diodes (LEDs) have shown great potential in display and solid state lighting systems [1]. Further development of these technologies calls for LEDs devices with higher output power. However, with increased driven currents, there will be a substantial decrease for the efficiency of LEDs. This phenomenon, known as efficiency droop, is a severe limitation to further enhance the output power of LEDs [2, 3]. Many physical mechanisms have been suggested as explanations, including carrier leakage from active region [4–7], Auger recombination [8, 9] and carrier delocalization from In-rich regions at high currents [10]. It has been proposed that reducing the polarization field in InGaN multiple quantum wells (MQWs) is an effective way to realizing high efficiency, low droop LEDs [11, 12].
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1285
Polarization field induced quantum confined Stark effect (QCSE) would reduce the electron and hole wavefunction overlap in quantum wells (QWs), leading to the reduced overall recombination efficiency [13]. Also the efficiency droop is severe for LEDs with high polarization field in the MQWs, as the electrons would escape from the active region more easily under high polarization field [14–16]. The attempts to reduce polarization fields in QWs include special designs in quantum barriers (QBs) and QWs [17–19], nonpolar or semipolar QWs [12, 20], inserting a higher temperature interlayer after the growth of low temperature buffer layer [21] and nano-sized patterned sapphire substrates [22]. However, most of these technics are hindered by either the limited crystal quality in active layers or the complicated substrate manufacturing process. Simple techniques thus are highly desired to meet the growing requirement of LED industry. In this study, we reported that the efficiency droop of LED devices could be effectively reduced through a simple etch and regrown process in n-GaN layers. The reduced polarization field in MQWs which is resulted from the relaxation of the compressive strain in the regrown n-GaN layers and the following MQWs might be an explanation. The peak efficiency and reverse current leakage of LED devices are also modified with our method as a result of the reduction of dislocations. What is more, the etch-regrown process is easy to be accomplished in mass production as it is based on the mature technologies used in LED industry. 2. Experimental details The n-GaN templates used in our experiments were grown under 1050°C, after the deposition of the 30 nm GaN nucleation layer on the c-plane sapphire substrates with metal organic chemical vapor deposition (MOCVD) system. After that, a two-step etch process was used to form the n-GaN pyramid arrays from the planer n-GaN template. The first step is a traditional lithography process to form the 3 μm diameter and 1μm height GaN micro-pillars. The 45°tilted scanning electron microscope (SEM) image of the GaN micro-pillars is shown in Fig. 1(b). After the first step, the samples were etched by KOH in ethylene glycol solution at 150°C for 45 min to form the n-GaN pyramid arrays. Figure 1(c) shows the Plan-view SEM image of the n-GaN pyramid arrays. Detailed process procedures could be found in our previous work [23]. Each n-GaN pyramid contained six (11-212) planes which would promote the lateral growth mode in the following regrown process. The n-GaN template with surface pyramid arrays was then put into the MOCVD system for the 1 h regrown process to flatten the n-GaN surface. The growth temperature of regrown process was 1050°C and the growth rate was carefully modified to keep the thickness of regrown n-GaN layer identical with the unetched n-GaN template. The growth pressure of the regrown process is 200 Torr and the V/III is kept at 2700. After the regrown process, the n-GaN layer with the etch and regrown process (named etch-regrown n-GaN) and the n-GaN layer without this process (named reference n-GaN) was placed in the MOCVD system in a single run to finish the following MQWs and p-GaN layers for LED structure. The schematic illustrations of our LED structures with etch-regrown n-GaN layers could be seen in Fig. 1(a). All LEDs (1 × 1 mm2) were fabricated with a standard mesa structure. Cr/Pt/Au (70/40/1440 nm) were evaporated as both the p-type and n-type electrodes. The contacts were annealed in N2 at 250°C for 15 min under the atmosphere pressure.
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1286
Fig. 1. (a) The schematic illustrations of the LED structures with etch-regrown n-GaN layers; (b) 45°-tilted SEM image of the GaN pillars formed from the n-GaN template after the traditional lithography process; (c) Plan-view SEM image of the GaN pyramids formed from the GaN pillars after the wet etching process.
3. Results and discussion The detailed regrown process is examined by the atomic force microscopy (AFM) scans and shown in Figs. 2(a)–2(e). After a 4 minutes growth process, the initial pyramid shaped GaN arrays (Fig. 2(a)) were turned into a smooth cone structure (Fig. 2(b)). This is a combined effect of the lateral overgrowth from the (11-212) planes of the GaN pyramids and the vertical growth from the (0001) planes between the pyramids. As the height of GaN pyramids were not precisely controlled in our experiments, further growth of the GaN cones in Fig. 2(b) for 4 minutes lead to the surface morphology with staggered GaN disks. (Fig. 2(c)) After another 20 minutes growth, the staggered GaN disks disappeared and the surface of our samples were flatten. (Fig. 2(d)) The cross-sectional transmission electron microscopy (TEM) image of the etch-regrown n-GaN layers are shown in Fig. 2(e), the initial n-GaN pyramid arrays were completely coalesced after the regrown process and no voids were found in our etch-regrown n-GaN layers. The voids were often found in traditional lateral overgrowth GaN layers due to the uncompleted coalescence of GaN on top of the masks [24, 25] and may have an influence on the light extraction efficiency of LEDs [26].
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1287
Fig. 2. The 20 × 20 μm AFM scans of the etched pyramid shaped n-GaN surface (a) and the nGaN surface after regrown for 4 minutes (b), 8 minutes (c) and 28 minutes (d) from the pyramid shaped n-GaN layer. (d) the cross-sectional TEM image of the n-GaN layers after the etch-regrown process.
The structural quality of the etch-regrown and reference n-GaN layers were characterized and compared with the High-resolution X-ray diffraction (HRXRD) measurements. The Xray rocking curves measured on the symmetric (002) and asymmetric (102) planes are shown in Figs. 3(a) and 3(b). It is shown that the full width at half maximum (FWHM) values of the symmetric (002) planes for both n-GaN layers are almost the same (302.4 arcsec for etchregrown n-GaN and 296 arcsec for reference n-GaN), while those of the asymmetric (102) planes for etch-regrown n-GaN layers are clearly reduced compared with the reference n-GaN layers (476.3 arcsec for etch-regrown n-GaN and 596.5 arcsec for the reference n-GaN). The FWHM of the X-ray rocking curves on the symmetric (002) planes are related to the screw and mixed type dislocations; whereas the FWHM of the X-ray rocking curves on the asymmetric (102) planes are related to the edge dislocations. From the XRD results, we found that the edge dislocations in the etch-regrown n-GaN layers were clearly reduced compared with reference n-GaN layers. The reduction of edge dislocations can be explained by the dislocation propagation resulting from the lateral overgrowth of (11-212) planes which the dislocations intersect [27]. The lateral overgrowth process is often accompanied with the reduction of compressive strain in GaN layers [22, 28]. In order to investigate the strain state in our samples, Raman measurements were performed on 20 different positions of the wafer and the peak position of the E2 (high) mode are shown in Fig. 3(c). From 1 to 20, the measurement position moves from the orientation flat to the other side of the wafer. The typical Raman spectra of etchregrown and reference n-GaN layers are displayed in Fig. 3(d). The position of E2 (high) mode shown in the picture is closely related to the strain and could be used as a metric for the strain state in GaN layers. The position of the E2 (high) peak of the relaxed GaN layers is 567 cm−1 [29] and shown in Fig. 3(d) as the blue dashed line. Both n-GaN layers are under compressive strain as can be seen from the picture. This compressive strain results from the large lattice mismatch of the GaN epilayers and the sapphire substrates. However, the position of E2 (high) peak for etch-regrown n-GaN is shifted from 569.94 cm−1 to 569.71 cm−1 compared with the reference sample, indicating the relaxation of compressive strain. Using the method depicted by Davydov et. al [30], the calculated residual compressive strain is 1.34 × 10−3 for reference and 1.24 × 10−3 for etch-regrown sample. The value is comparable to the previous reported strain relaxation for GaN layers grown on nano patterned sapphire
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1288
substrates, which reduced the QCSE in QWs [22]. However, our results are little smaller as the size of etched pyramids and space between them are not modified.
Fig. 3. (a) Symmetric (002) and (b) asymmetric (102) X-ray rocking curves of the etchregrown and reference n-GaN layers. (c) the peak position of the E2 (high) mode of the Raman spectra for the etch-regrown and reference n-GaN layers. From 1 to 20, the measurement position moves from the orientation flat to the other side of the wafer. (d) the typical room temperature Raman spectra of the etch-regrown and reference n-GaN layers.
In order to study the polarization field induced QCSE, the excitation power dependent photoluminescence (PL) measurement was carried out in Figs. 4(a) and 4(b). Two characterizations should be noticed from the pictures, the shorter PL peak wavelength and smaller wavelength shift with excitation power for LEDs grown with etch-regrown n-GaN layers. First, the PL peak wavelength of LEDs grown with etch-regrown n-GaN layers are reduced from 460 nm to 450 nm compared with the reference LEDs. As the LED structures were grown in the same run, the indium composition and quantum well width are the same in both structures. This consumption is also confirmed by the HRXRD ω-2θ scans which are not shown here. Thus, the shorter peak wavelength of the PL spectra for LEDs with etch-regrown n-GaN layers indicated the reduced QCSE in active region. Second, the wavelength shift with excitation power increased from 0.4 mW to 40 mW for LEDs with etch-regrown n-GaN layers is 0.5 nm, which is much smaller than that of LEDs with reference n-GaN layers (4.9 nm as shown in the picture). The smaller blue shift of PL peak is another evidence for the reduced QCSE in active region. The reduced QCSE indicates the reduction of the polarization field in active region of LED with etch-regrown n-GaN layers which is induced by the relaxed strain in epilayers.
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1289
Fig. 4. The excitation dependent PL spectra measured at room temperature of LED samples with etch-regrown n-GaN (a) and reference n-GaN (b).
The electrical properties of LEDs with and without etch-regrown n-GaN layers are shown in Figs. 5(a) and 5(b). From Fig. 5(a), the efficiency droop in LED samples with etch-regrown n-GaN layers is clearly modified compared with reference sample. As many previous works has proved [12, 22], we believe that the reduced polarization field in active region will lead to the reduced efficiency droop in LED samples with etch-regrown n-GaN layers. Note that the peak efficiency of LED samples with etch-regrown n-GaN layers is also increased, which may be a combined effect of the reduced QCSE in QWs and the elimination of threading edge dislocations penetrated from n-GaN to MQWs. Figure 5(b) is the forward I-V curves for both samples, from inset is the reverse I-V curves. The forward operation voltage for LEDs with etch-regrown n-GaN layers is clearly reduced compared with the reference sample, attributing to the reduced sheet charges which result from the polarization field in LED structures. The large triangular barriers induced by sheet charges in the active region would impede carrier flow and result in the higher operation voltage [31] The reverse current leakage is also modified due to the reduced threading dislocations in LED structures with etch-regrown nGaN layers. The threading dislocations often act as current leakage path as previous work reported [32].
Fig. 5. (a) The EQE and output power as a function of forward current for LEDs with reference and etch-regrown n-GaN. (b) the forward I-V curves of LEDs with reference and etch-regrown n-GaN. Inset shows the corresponding reverse I-V characteristics.
4. Conclusions In conclusion, this paper reports a novel etch and regrown process in n-GaN layers to suppress the efficiency droop effect for LEDs. The etch and regrown process in n-GaN layers relaxed the compressive strain in epilayers. Thus the polarization filed in active layers could
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1290
be reduced and lead to the reduced efficiency droop. What is more, the lateral overgrowth process initialed from the (11-212) planes of the etched n-GaN pyramid effectively reduced the threading edge dislocations. The peak efficiency and the reverse current leakage for LED samples are clearly modified as a result. Acknowledgments This work was financially supported by the National High Technology Research and Development Program of China No. 2013AA03A101 and the National Natural Science Foundation of China No. 61306051. The authors also thank Miss Yanhong Liu for the language assistance.
#214577 - $15.00 USD Received 23 Jun 2014; revised 9 Jul 2014; accepted 11 Jul 2014; published 7 Aug 2014 (C) 2014 OSA 25 August 2014 | Vol. 22, No. S5 | DOI:10.1364/OE.22.0A1284 | OPTICS EXPRESS A1291
