Relaxed Si0.7Ge0.3 buffer layers for high-mobility devices

The minimum epitaxial layer thickness required to produce relaxed, thermally stable, Si0.7Ge0.3 buffer layer structures for high electron- and hole-mobility devices has been determined, using high resolution x-ray diffraction. A 1.4-μm-thick layer, step graded to x=0.35, is sufficiently thick so that the residual strain in a uniform composition Si0.33Ge0.67 layer grown on top of it is essentially independent of thickness or growth temperature of the layer. Such structures are stable when annealed at 750 °C.