Structure and optical properties of Ge-Si ordered superlattices

We report the synthesis, structural characterization, and optical studies of ultrathin Ge-Si superlattices,grown by molecular beam epitaxy, on (001) silicon substrates. Structures consist of alternating layers of pure Ge and Si, with layer thicknesses of 1, 2, 4, and 6 monolayers. Using high-resolution transmission electron microscopy, we provide direct observation of order in these pseudomorphic layered films. Systematic study of optical transitions by means of Schottky barrier electroreflectance reveals that each of the ordered structures displays a unique set of optical transitions. Of particular interest is the 4×4 structure which shows new, well defined optical transitions at 0.76, 1.25, and 2.31 eV. These transitions constitute the first observation of structurally induced optical transitions in Ge-Si and may make the 4×4 structure suitable for optoelectronic devices.