Structural Characterization of GaN Epilayers Grown on Patterned Sapphire Substrates
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Structural Characterization of GaN Epilayers Grown on Patterned Sapphire Substrates
Chang-Soo KIM*, Ji-Hyun MOON, Sang-Jun LEE, Sam-Kyu NOH, Je Won KIM 1, Kyuhan LEE 1 , Yong Dae CHOI 2 and Jay P. SONG 3 Materials Evaluation Center, Korea Research Institute of Standards and Science(KRISS), Taejon, Korea 305-600 1 Photonic Device Group, Samsung Electro-Mechanics, Suwon, Korea 442-743 2 Department of Optical and Electronic Physics, Mokwon University, Taejon, Korea 302-729 3 SongJee Industrial Corporation, Sungnam, Korea 462-819
ABSTRACT The structural properties of GaN epitaxial layers grown on patterned sapphire substrates by MOCVD have been investigated using HRXRD(high-resolution X-ray diffraction), GIXRD(grazing incidence X-ray diffraction) and PL(photoluminescence). For X-ray characterizations rocking curves for GaN (10·5), (00·2), (11·4) and (11·0) reflections for which incidence angles of X-rays are 32.0º, 17.3º, 11.0º and 0.34º, respectively, were measured. For (10·5), (00·2) and (11·4) reflections FWHMs of the rocking curves for a patterned substrate were broader than those for a unpatterned substrate, for (11·0) reflection, however, FWHM for a patterned substrate was much narrower than that for a unpatterned substrate. The normalized FWHM for all reflections decreases as the incidence angle of X-ray decreases. The results indicate that the crystalline quality in the surface region of the epilayer on a patterned substrate was especially improved because the penetration depth of X-ray depends on the incidence angle. The intensity of PL peak of the epilayer for a patterned substrate increased compared to that for a unpatterned substrate, and the increase in PL intensity is attributed to the reduction in dislocation density at the surface region revealed the by X-ray results.
INTRODUCTION High-performance optical devices such as light-emitting diodes (LEDs) and laser diodes (LDs), and high-power transistors have been developed using III-V nitrides such as GaN, InN and AlN due to their wide band gaps, thermal stability and strong bond strength [1,2]. For a GaN epilayer, heteroepitaxial growth on a sapphire substrate has commonly been employed. It has ______________________ *
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been very difficult to grow a high quality GaN film with a smooth surface because of the large lattice and thermal mismatches between GaN and sapphire. The InGaN layer grown on GaN experiences a compressive biaxial strain. As the thickness of the strained layer exceeds a critical value, the accumulated elastic energy is relieved by the formation of misfit dislocations at the interface [3]. In order to reduce the dislocation density in GaN on sapphire, great efforts have been made by using growth methods of lateral epitaxial overgrowth(LEO) [4] and Pendeoepitaxy, [5] and it has been revealed that the methods were effective in reducing the dislocation density. Recently, it has been reported that the dislocation density can also be reduced by growing GaN on patterned sapphire substra
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