Anomalous Composition Dependence of Optical Energies of MBE-grown InGaN
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Anomalous Composition Dependence of Optical Energies of MBE-grown InGaN I. Fernandez-Torrente1*, D. Amabile1, R.W. Martin1, K.P. O’Donnell+1, J.F.W.Mosselmans2, E. Calleja3 and F.B. Naranjo3 1 University of Strathclyde, Department of Physics, Glasgow, G4 0NG, Scotland, UK. 2 CLRC Daresbury Laboratories, Synchroton Radiation Department, Warrington WA4 4AD, England, UK, 3 ETSI Telecomunicación, Universidad Politécnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain. ABSTRACT A direct comparison of the optical energies of MBE- and MOVPE-grown In xGa1-xN epilayers of similar InN content is performed for the first time. The InN fraction in the 7 MBE samples examined ranged from x ~ 0.11 to x ~ 0.35 while the range in available MOVPE epilayers is [0, 0.4]. Wavelength Dispersive X-ray (WDX) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were used to measure composition and local structure (alloy character) of the samples. Cathodoluminescence (CL) spectroscopy in situ, ex situ photoluminescence (PL) mapping and large-area optical absorption spectroscopy were used to measure various optical energies. The composition dependence of the optical energies is determined by the growth method. The absorption bandgap and luminescence peak energies vary linearly with x for both growth methods, suggesting a near-zero value of the bowing parameter. But the energy intercept at zero InN content in MOVPE samples is close to the wurtzite-GaN bandgap of 3.4 eV at room temperature, as expected, while the equivalent for MBE samples falls near 3.2 eV. INTRODUCTION Two main methods are used for InxGa1-xN growth: Metalorganic Vapour Phase Epitaxy (MOVPE), carried out at relatively high temperatures, has proved to be very successful in the growth of materials for commercial light-emitting devices with low InN content (x0.5). It is generally assumed, and seems to go without saying, that materials made by different growth methods should not vary in their fundamental properties. Here, we present evidence that appears to challenge this assumption. We report the optical properties of thick InGaN epilayers, grown by MBE. A comparison with MOVPE layers of similar composition reveals systematic differences in both the PL/CL peak energies and the band gap energy, defined by fitting a sigmoidal function to the step observed in the optical absorption spectrum of such layers, as described in [1]. Moreover, the MBE layers are found not to exhibit the optical energy scalings previously described for MOVPE layers [1,2]. These observations are relevant in the context of recent discussions concerning the value of the fundamental band gap of InN [3,4,5]. In particular they render unsafe the practice of combining data from differently grown sample sets in order to calculate a ‘bowing parameter’ for the plot of band gap vs composition. The results presented here show no experimental evidence for a bowing parameter different from zero. In this perhaps entirely unphysical circumstance, the band gap of InN can be estimated from the wel
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