Extended X-ray Absorption Fine Structure Studies of InGaN Epilayers
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Extended X-ray Absorption Fine Structure Studies of InGaN Epilayers V. Katchkanov1,2, K.P. O’Donnell1, J.F.W. Mosselmans2, S. Hernandez1, R.W. Martin1, Y. Nanishi3, M. Kurouchi3, I. Watson4, W. Van der Stricht5, E. Calleja6 1 Department of Physics, Strathclyde University, Scotland, U.K. 2 Synchrotron Radiation Department, CCLRC Daresbury Laboratory, Warrington, U.K. 3 Department of Photonics, School of Science and Engineering, Ritsumeikan University, Japan 4 Institute of Photonics, Strathclyde University, Scotland, U.K. 5 University of Ghent, Ghent, Belgium 6 Departamento de Ingeniería Electrónica, Universidad Politécnica de Madrid, Madrid, Spain
ABSTRACT The local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10% to 40% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13% to 96%. The In–N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate from average cation-cation distance predicted by Vegard’s law for MBE grown samples which indicates alloy compositional fluctuations. INTRODUCTION When X-rays have sufficient energy to eject a photoelectron, it is scattered by the surrounding atoms causing self-interference of the electronic wave at the central atom which leads to the modulation of X-ray absorption coefficient. Such modulation is called EXAFS. The chemical nature and coordination number of surrounding atoms and their radial separation from the central atom affect the modulation of the X-ray absorption coefficient, thus providing information about the local structure of the absorbing element on an atomic level [1]. EXAFS has been extensively applied to study local structure in various alloy systems such as InGaAs [2], ZnMnSe [3] etc. InGaN is a remarkable semiconductor alloy which offers the possibility of light emission covering the entire visible spectral region by changing the In content [4]. It is the material at the core of a novel and rapidly advancing lighting technology. However, one of the difficulties in growing InGaN is the limited solubility of In in GaN due to the large size mismatch between In and Ga atoms. It was theoretically calculated that InGaN is thermodynamically unstable at typical growth temperatures [5]. The issue of phase decomposition is particularly important since it is suggested that the high efficiency of InGaN luminescence is due to the formation of In-rich InGaN clusters [6, 7]. In this paper we report In local structure studies for InGaN epilayers wit
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