The structural properties of Si0.7Ge0.3 films, which had been grown onto (001) Si substrates by means of molecular beam epitaxy, were determined. Various types of buffer layer were pre-deposited onto clean (001) Si substrates in order to relax the lattice mismatch between the uppermost Si0.7Ge0.3 layer and the Si substrate. The effect of the buffer layers upon the structural quality of the over-grown Si0.7Ge0.3 was investigated by using X-ray diffraction, X-ray photo-emission spectroscopy, photoluminescence and cross-sectional transmission electron microscopic techniques. It was found that the threading dislocation density in the alloy layer was markedly decreased by using buffer layers. Samples with step buffer layers relaxed the strain by introducing dislocations at the interfaces, some of which went through the alloy layer. On the other hand, samples with superlattice buffer layers relaxed strain by introducing dislocations, into the buffer layers, which terminated at the interface between the superlattice buffer layer and the uppermost alloy layer. The residual strain in Si0.7Ge0.3, on buffer layers that had been grown at 550C, was relaxed during annealing at 700C or growth at 700C.

T.Obata, K.Komeda, T.Nakao, H.Ueba, C.Tatsuyama: Journal of Applied Physics, 1997, 81[1], 199-204