To grow high-quality heteroepitaxial layers, we propose a compliant silicon substrate consisting of a thin epitaxial silicon film on a high-density porous layer as a membrane and an expansive low-density porous layer as a mechanical damper which shields the overlying layers from the massive wafer. GeSi films over the critical thickness have been grown by molecular-beam epitaxy on these substrates. Transmission electron microscopy analysis shows that Ge0.2Si0.8 films have no dislocations owing to just elastic strain relaxation whereas plastic flow in the pseudomorphic films that are being grown on conventional Si substrates occurs with generation of dislocations in a regular manner. The experimental data on porous silicon structure are presented in some detail and are briefly discussed in connection with substrate compliance.

The physical system under study is a strained porous Si/Si/GeSi mesa stripe on a rigid Si substrate. We analyse the relaxation of elastic deformations in a pseudomorphic GeSi film using the finite element method. Calculations were carried out for stripes with a porosity of about 60% (the Young modulus is less than that of GeSi by a factor of ten). The widths of the stripe were 5 and 10 µm and larger, the heights were 1 and 5 µm. The calculations have proven that the relaxation is facilitated by the increase of the porous mesa height-to-width ratio (aspect ratio). However, the Si membrane (an ultra-thin Si film between the porous Si and GeSi films) decreases the value of elastic relaxation in the pseudomorphic layer. This effect is stronger for high aspect ratios. Contrary to the mesa stripe, an extensive porous Si layer cannot be a compliant substrate for the growth of a lattice-mismatched GeSi film. The residual elastic strains in GeSi films grown on porous mesa stripes appear to be much lower than those on non-porous mesas.

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