Structural analysis of ELO-GaN grown on sapphire using the x-ray micro-beam of an 8-GeV storage ring
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0892-FF22-01.1
Structural analysis of ELO-GaN grown on sapphire using the x-ray micro-beam of an 8-GeV storage ring Takao Miyajima, Shingo Takeda1, Hideaki Kurihara1, Kyoko Watanabe1, Madomi Kato1, Nobuhide Hara1, Yoshiyuki Tsusaka1, Junji Matsui1,Yoshihiro Kudo2, Shigetaka Tomiya2, Shu Goto3, Masao Ikeda3 and Hironobu Narui Materials Laboratories, Sony Corporation, 4-14-1 Asahi, Atsugi, Kanagawa 243-0014, Japan 1 Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan 2 Materials Analysis Lab., Advanced Design Technology Ctr., Sony Corp. 3 Shiroishi Laser Ctr., Semiconductor Laser Div., Photonic Device & Module Business Gp., Sony Corp., 3-53-2 Shiratori, Shiroishi, Miyagi 989-0734, Japan
ABSTRACT We investigated the micrometer-scale structure of epitaxially laterally overgrown GaN (ELO-GaN) on a sapphire without a SiO2 mask using the 2x4 µm2 micro-beam x-ray of an 8-GeV storage ring. The GaN (0 0 0 12) rocking curve of a wing region had a sharp peak with a FWHM of 46 arcsec, while that of a seed region had several broad peaks. This peak-narrowing indicates that the density of threading dislocations (TD) is reduced and the grain size is extended in the wing region by the lateral growth. The observed grain size in the wing region was 10 µm x 5 µm. The mapping of the rocking curves shows that the wing regions hang down from the seed region at a tilting angle of 81.5 arcsec, although there is no SiO2 mask which is a cause of c-axis tilting in ELO-GaN. After comparing the results of ELO-GaN with and without coalescence, we conclude that the small c-axis tilting is caused by the compressive stress induced by the difference in the thermal expansion coefficient of the GaN and the sapphire substrate. Because a shrinkage of grain size was observed after the coalescence of the wing regions, it is thought the coalescence is one of the causes of the generation of the few threading dislocations in the wing regions. We believe that the reduction of the threading dislocations and the expansion of grain size in the wing regions are important for the further improvement of the device characteristics of GaN-based LDs grown on ELO-GaN. INTRODUCTION By growing a GaN-based laser diode (LD) on epitaxially laterally overgrown GaN (ELO-GaN) without a SiO2 mask [1, 2], Tojyo et al. demonstrated a practical lifetime of 15,000 hours with an output power of 30 mW at a temperature of 60 ºC [3]. ELO-GaN strongly contributed to this performance by reducing the threading-dislocation (TD) density to 106 cm2 and providing a smooth cleaved facet. It is therefore useful to characterize ELO-GaN in detail to improve the properties of GaN-based LDs. TEM [4] and standard x-ray diffraction (XRD) measurements [5] have been previously reported, but these measurements only clarified the structure at a nm- and mm-scale, respectively. Then, we started to analyze the micrometer structure in ELO-GaN using a micro-beam x-ray of synchrotron radiation [6], which was not previously available for lack of a
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