Probing the Indium Mole Fraction in an InGaN Epilayer by Depth Resolved Cathodoluminescence
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ABSTRACT Cathodoluminescence spectroscopy is used to depth profile the indium mole fraction of a 0.4 ýtm InGaN epilayer grown by metalorganic chemical vapour deposition, and to probe an underlying 1.0 prm GaN epilayer and the sapphire substrate beneath. Spectral information is obtained by using a variable energy electron beam as the excitation source. Calibration of beam penetration is achieved using Monte Carlo simulations of electron beam trajectories. The indium mole fraction is found to decrease from a mean value of 27% at the surface of the InGaN layer to n 24% at its interface with the GaN layer. INTRODUCTION Shimuzu et al [1] and Hiramatsu et al [2] recently reported the "compositional pulling effect" in the MOCVD growth of InGaN films. That is, they observed that the indium mole fraction is low during the initial stages of InGaN growth on GaN epilayers, but increases with increasing film thickness. In the present work we have used a variable energy electron beam to probe the indium mole fraction as a function of depth in a single InGaN epilayer grown on GaN. A variable energy electron beam may be used to acquire, non-destructively, depth
information from solids by exploiting the fact that an electron penetrates to a depth which depends on its initial kinetic energy. The higher the energy of an electron, the deeper it will penetrate into the solid. Electron beams of between 1 and 30 keV in energy are well matched to the length scales typical of low-dimensional structures. For example, a 5 keV beam deposits most of its energy at a depth of about 50 nm, a 12 keV electron beam deposits most energy at a depth of - 200 nm, while a 30 keV beam deposits most of its energy at a depth of & 1 .tm in a typical semiconductor sample. In the present work, we derive spectral information from an analysis of the light, the cathodoluminescence (CL), which is emitted when a sample is bombarded by a beam of electrons. As the electron beam energy is progressively increased, CL is emitted from ever increasing depths of the sample. Depth-dependent changes in the sample's properties may therefore be deduced from a comparison of CL spectra. For the InGaN epilayer used in the present investigation, the CL band due to the alloy was observed to broaden and shift to higher energy with increasing penetration of the electron beam. This result implies a reduction of the indium mole fraction as a function of depth, which is consistent with the results reported in [1] and [2]. EXPERIMENT Nitride layers were grown by metalorganic chemical vapour deposition on sapphire (0001) substrates, in a vertical rotating disk reactor. The structure profiled in this work 715 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
comprised a 400 nm InGaN epilayer grown on an epilayer of GaN about 1 jtm thick, which in turn was grown on a 25 nm thick low temperature GaN buffer layer. The InGaN and GaN epilayers were grown at temperatures of 8500 C and 1050 0C respectively. A full account of the growth procedure has been presented elsewhere
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