In this paper, the RBS phenomenon is described and experimental results concerning Bragg reflectors and microcavities grown on silicon substrates by molecular beam epitaxy are presented. This spectroscopy is not sensitive to (i) the lateral fluctuation of the alloy composition (the analyzed surface is about 1 mm(2)), (ii) to the residual strain of the layer or to (iii) the internal electric field. The measured thicknesses and compositions of the layers are very close to the nominal values. The Al concentration have been confirmed by low temperature photoluminescence experiments and the thicknesses by scanning electron microscopy studies. The straggling effect has been highlighted and plotted as function of the penetration depth of the particles.