X-ray Excited Optical Luminescence Studies of InGaN and Rare-Earth Doped GaN Epilayers
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X-ray Excited Optical Luminescence Studies of InGaN and Rare-Earth Doped GaN Epilayers V. Katchkanov1,2, J.F.W. Mosselmans2, K.P. O’Donnell1, N.R.J. Poolton2, S. Hernandez1 1 Department of Physics, Strathclyde University, John Anderson Building Glasgow, U.K. 2 Synchrotron Radiation Department, CCLRC Daresbury Laboratory, Warrington, U.K. ABSTRACT A successful attempt to use X-ray Excited Optical Luminescence (XEOL) for the detection of Extended X-ray Absorption Fine Structure (EXAFS) in III-nitrides is reported. The samples studied were InGaN and rare-earth (RE) doped GaN epilayers. For the first time Ga K-edge EXAFS oscillations were measured by monitoring the well-known “yellow” emission of GaN at 560 nm. The analysis of Optically Detected (OD) EXAFS data confirmed the expected local structure for Ga in GaN. The intensity oscillation of the “yellow” band when X-ray energy was scanned across the Ga K-edge indicates that core excitation of Ga atom has a high probability of transfer to defects responsible for “yellow” emission. INTRODUCTION Conventional X-ray Absorption Fine Structure (XAFS) analysis is a useful technique to obtain information about local structure in the neighbourhood of selected atomic species in a solid targeted by their characteristic X-ray absorption [1]. The main advantages of XAFS as a structural tool are: it is element-specific, so can provide information on the local structure of an element which is present in trace concentrations; and it is completely non-destructive. However, XAFS can only provide information about lattice location of target atoms in “majority” sites. Optical luminescence may also be emitted after excitation of core-levels by X-rays [2]. Since the structural environment of an emitting centre determines its fluorescence spectrum it is suggested that using XEOL for the collection of XAFS data may provide site specific information about optically active defects or impurities in semiconductors. The issue of site selectivity is important since it would offer an enormous advantage over other structural techniques. However, the complexity of cascade intra-atomic relaxation processes and possibility of energy migration may lead to excitation of optical emission of centres different from absorbing atoms [3]. In this paper we report results of OD XAFS studies of InGaN and rare-earth (RE) doped GaN samples carried out at the Daresbury synchrotron radiation source. EXPERIMENTAL DETAILS The samples studied, InGaN and RE doped GaN epilayers, had different structures. InGaN epilayers approximately 200–300 nm thick were grown on 1-2 µm thick GaN buffer layers on sapphire substrates by Metal-Organic Vapour Phase Epitaxy (MOCVD), while RE-doped layers approximately 1 µm thick were grown by Molecular Beam Epitaxy on an MOCVD GaN template. The thickness of the GaN template was ~ 1 µm. The structural and compositional properties of the samples are summarized in Table I and Table II.
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Figure 1. MOLES sample chamber.
Figure 2. XEOL spectrum of sample InGaN85 at 10 K. The ener
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