Impact of Nitridation on Structural and Optical Properties of Epitaxial GaN Films Grown on M-Plane Sapphire by PAMBE
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Impact of Nitridation on Structural and Optical Properties of Epitaxial GaN Films Grown on M-Plane Sapphire by PAMBE Shruti Mukundan1, Lokesh Mohan1, Greeshma Chandan1, Basanta Roul1, 2, S.B.Krupanidhi1,* 1 2
Materials Research Centre, Indian Institute of Science, Bangalore, India. Central Research Laboratory, Bharat Electronics, Bangalore, India
E-mail: [email protected]
ABSTRACT GaN epilayers were grown on m-plane (10-10) sapphire substrates using plasma assisted molecular beam epitaxy. Impact of nitridation on structural and optical properties of GaN film was investigated. The film grown on a nitridated surface resulted in a nonpolar (1010) orientation while without nitridation caused a semipolar (11-22) orientation. The high resolution X-ray diffraction studies confirmed the orientation of the GaN films. X-ray rocking curve showed better crystallinity of semipolar as compared to nonpolar GaN. Atomic force microscopy showed smoother films in case of nonpolar GaN which might be in account of the nitridation treatment. Room temperature photoluminescence study showed nonpolar GaN to have higher value of compressive strain as compared to semipolar GaN film, which was further confirmed by room temperature Raman spectroscopy. Despite the fact that it is difficult to obtain high-quality nonpolar material due to the planar anisotropic nature of the growth mode, we hereby report the development of non-polar GaN of usable quality, on an m-plane sapphire, involving controlled steps of nitridation. INTRODUCTION The wide band gap semiconductor GaN has been intensively studied owing to its remarkable properties like direct bandgap, high critical electric field and low intrinsic carrier concentration [1-3]. Growing GaN on nonpolar orientation such as (10-10) m-plane or (1120) a-plane, results in avoiding the polarization-related effects. Devices grown on these orientations are receiving a lot of focus due to enhanced behaviour. However, morphology and crystallinity of GaN grown on polar surfaces has been found to be always better compared to nonpolar surfaces [4,5]. The difficulties in obtaining high-quality nonpolar material lie in the planar anisotropic nature of the growth mode, which is a consequence of both the lattice mismatch and the adatom diffusion length [6]. Growth of nonpolar m-GaN on m-sapphire by various techniques like HVPE [7], MOCVD [8] and on m-sapphire substrates with slight c-axis miscut [9,10] has been reported earlier but not much study was evident by MBE. Seo et. al. did extensive investigation of the relation between growth parameters and preferred crystallographic orientations of GaN layers grown on m-plane sapphire grown by HVPE [11]. Vennegues et.al. reported that the preferred orientation strongly depended on the
nitridation conditions. The effect of nitridation was not fully understood, however, was conjectured that either surface energy or elastic energy played a major role depending on the nitridation condition [12]. In the present work, we report successful growth of (10-10) mGaN film gr
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