Optical properties of cubic AlGaN
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Optical properties of cubic AlGaN Stéphane Fanget1, Catherine Bru-Chevallier1, Gérard Guillot1, Esteban Martinez-Guerrero2, Denis Jalabert2, Bruno Daudin2, Henri Mariette2, Le Si Dang2, Gabriel Ferro3 and Yves Monteil3 1 LPM (UMR CNRS 5511) INSA de Lyon –Bât Blaise Pascal, 69621 Villeurbanne Cedex France. 2 CEA DRFMC and UJF-LSP-38054 Grenoble Cedex 9 – France. 3 LMI (UMR CNRS 5615) UCB-Lyon1 Bât 731—69622 Villeurbanne Cedex France. ABSTRACT In this work we report optical characterization on several cubic c-AlGaN layers grown by MBE on SiC on Si pseudo-substrates, with different aluminum concentrations ranging from 0 to 70 %. Excitation power evolution of AlGaN photoluminescence (PL) spectra as well as reflectivity spectra allow to attribute PL peak to band gap recombination. PL energy dependence versus aluminum concentration is given. Reflectivity investigations are performed in the energy range between 1.5 eV and 4 eV on the samples. Theoretical calculations of multilayered structure reflectivity are fitted to experimental results, allowing an accurate determination of refractive index evolution versus Al concentration. From this analysis, qualitative information about interface roughness at AlGaN/SiC is also be derived.
INTRODUCTION At present, most studies on GaN and AlGaN alloys are devoted to the wurtzite (hexagonal) phase as the growth of pure zinc-blende (cubic) phase is more difficult due to its metastable nature. However optimization of Molecular Beam Epitaxy (MBE) growth conditions has led to improve the quality of cubic materials grown on 3C-SiC pseudo-substrates [1]. The c-AlGaN/GaN material system has an expected maximum band offset of 1.6 eV : so intersubband absorption in GaN quantum wells could therefore reach the 1.55 µm wavelength for telecom applications. Such intersubband transitions have already been reported in the hexagonal phase [2] but not yet in the cubic phase. Cubic phase allows to get rid of the huge piezoelectric and spontaneous electric fields occurring in the hexagonal phase of nitrides [3,4] and which may be detrimental to optoelectronic device performance. For the design of optoelectronic devices, optical constants for c-GaN and c-AlGaN need to be precisely known, but up to now, experimental determination of such constants for c-AlGaN ternary alloys still remain scarce in the literature. In this work we perform PL and reflectivity investigations on several layers with different aluminum concentration ranging from 0 to 70 %.
EXPERIMENTAL The samples used in this study consist of cubic AlGaN layers grown by rf-plasma assisted Molecular Beam Epitaxy on SiC/Si pseudosubstrates. The growth was controlled by Reflection High Energy Electron Diffraction (RHEED). The samples are composed of a c-AlGaN layer deposited on a 20 Ǻ AlN buffer grown on a SiC/Si pseudo-substrate. For A type samples the substrate consists of a thin (20 Ǻ) carburated Si layer on bulk silicon. For B type samples a thick I6.44.1
(3 µm) c-SiC layer is deposited on top of the thin Si carburated layer. All c-AlGaN lay
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