Ge-Si layered structures: Artificial crystals and complex cell ordered superlattices

We report the first successful synthesis of ordered GeSi superlattices grown on (001) Si substrates by molecular beam epitaxy. Two types of structures were prepared and characterized: superlattices with one-dimensional periodicity of one unit cell (GeGeSiSi...) and complex cell superlattices made up of sublayers of pure Si and alternating bilayers of Ge and Si. In the first case, the artificial stacking in the 001 direction results in a vertical array of alternating Ge and Si monolayers parallel to the (110) or (11̄0) planes. In spite of the lattice mismatch of 4.2%, Rutherford backscattering and channeling experiments indicate high quality crystallinity in both types of structures. Long-range order is deduced from the electron diffraction patterns that exhibit characteristic superlattice reflections and from high resolution lattice imaging. The precise deposition control on the scale of a fraction of a monolayer should allow band structure engineering in this and in other related systems and in turn tailoring of the transport and optical properties over a wide range.