Probing Nitride Thin Films in 3-Dimensions using a Variable Energy Electron Beam
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ABSTRACT In this paper we illustrate the application of electron beam techniques to the measurement of strain, defect and alloy concentrations in nitride thin films. We present brief comparative studies of CL spectra of AlGaN and InGaN epilayers and EBSD patterns obtained from two silicon-doped 3 µm thick GaN epilayers grown on an on-axis o (0001) _ sapphire substrate and a sapphire substrate misoriented by 10 toward the m-plane (10 1 0).
INTRODUCTION Thin films incorporating GaN, InGaN and AlGaN are presently arousing considerable excitement because of their suitability for UV and visible light emitting diodes and laser diodes. However, the films are of variable quality because of the lattice mismatch between them and presently used substrates (sapphire and α-SiC). We are presently using a number of electron beam analysis techniques namely cathodoluminescence (CL) imaging, CL spectroscopy and electron backscattered diffraction (EBSD), to investigate both the structural and optical properties of nitride films in 3-dimensions. Information in the 3rd dimension is extracted by acquiring data at different electron beam energies. Electron beams of energy between 1 and 20 keV are well matched to the length scales typical of nitride heterostructures. For example, a 2 keV beam deposits energy to a depth of ≈50 nm, a 10 keV electron beam deposits energy to a depth of ≈ 600 nm, while a 20 keV beam deposits energy to a depth of ≈ 2 µm in a GaN layer. In this paper we illustrate the application of electron beam techniques to the analysis of nitride thin films by presenting brief comparative studies of (i) CL spectra of AlGaN and InGaN epilayers and (ii) EBSD patterns obtained from two silicon-doped 3 µm thick GaN epilayers grown on an on-axis (0001) sapphire substrate _ and a sapphire substrate misoriented by 10o toward the m-plane (10 1 0).
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EXPERIMENTAL DETAILS All of the samples described in this paper were grown by metalorganic vapour phase epitaxy (MOVPE) on (0001) sapphire substrates (unless otherwise indicated). CL spectra were acquired using a home-built electron beam excitation system providing beams of energy up to 30 keV in a spot size of ≈ 200 µm at current densities up to 20 A cm-2 (but limited in the work to be described here to < 10 mA cm-2). In the present work the front faces of the samples were positioned normal to the exciting electron beam and CL detected from the sample edge. Samples were cooled to a temperature of approximately 25 K using a closed cycle helium cryorefrigerator. Spectra were acquired using an Oriel InstaSpecTM cooled 2-dimensional CCD array mounted at the output focal plane of a Chromex 0.5M monochromator. A Cambridge 600S scanning electron microscope has been adapted by the addition of a home-built CL detection system (incorporating an Oxford
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