The composition dependence of the optical properties of InN-rich InGaN grown by MBE
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The composition dependence of the optical properties of InN-rich InGaN grown by MBE R.W. Martin1, P.R. Edwards1, S. Hernandez1, K. Wang1, I. Fernandez-Torrente1, M. Kurouchi2, Y. Nanishi2, K.P. O’Donnell1 1 Department of Physics, Strathclyde University, Glasgow, G4 0NG, U.K. 2 Department of Photonics, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 5258577, Japan ABSTRACT Study of the relationship between the composition and optical energies of InxGa1-xN has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from InxGa1-xN epilayers covering the full range of composition (0 < x < 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 < x < 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed. INTRODUCTION Direct correlation of the composition and optical energies of relaxed InxGa1-xN has been the subject of many literature reports in the last decades, driven forward by an increasing range of important applications covering the full spectral range from UV to NIR. A large body of data covers relatively InN-poor (x < 0.4) samples grown by Metal-Organic Vapour Phase Epitaxy (MOVPE) [1,2] and there are also increasing numbers of reports concerning InN and InN-rich InGaN grown by Molecular Beam Epitaxy (MBE) [3-6]. Here attention is concentrated on a wide-ranging set of MBE-grown InN-rich layers (0.6 < x < 1.0) in order to make comparison with sets of MOVPE and MBE InGaN epilayers with InN fractions up to 0.4 measured using identical techniques. Wavelength dispersive X-ray (WDX) analysis is employed for the accurate measurement of the InN fraction and stoichiometry, and the results correlated with photoluminescence (PL) or cathodoluminescence (CL) spectra. The CL is measured simultaneously with the WDX, and near-identical sample volumes are interrogated in the two cases. PL and optical absorption have much larger footprints. This allows careful investigation of the composition dependence of the optical properties of InGaN along with that of differences between samples produced by MOVPE and MBE. The comparison illustrates possible dangers in combining data from samples grown by the two different methods. Measurement of the composition of InGaN layers relies upon a variety
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