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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