The refractive indices of several AlxGa1−xN alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of AlxGa1−xN, the refractive index is given in form of a Sellmeier law.

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This paper reports the growth of GaN, (Al,Ga)N and AlN layers on (111)Si substrates by molecular beam epitaxy using ammonia. Using proper conditions, GaN layers with threading dislocation densities as low as 5 Â 10 9 cm -2 can be obtained on (111)Si. The structural and optical properties of GaN and (Al,Ga)N have been studied using electron microscopy, photoluminescence and reflectivity. In particular, the tensile strain has been assessed. Finally, a ten-period Al 0.2 Ga 0.8 N/AlN Bragg mirror has been grown, with a UV (340 nm) centered bandwidth of 35 nm and peak reflectivity of 78%.

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