New optical transitions in Ge-Si ordered atomic-layer structures.

We have observed new optical transitions at 0.76 eV, 1.25 eV and 2.31 eV in an ordered Si-Ge superlattice. The superlattice structure consists of a sequence of 4 monolayers of Ge and 4 monolayers of Si repeated five times in the (001) crystal orientation. The strained layer structure of 50angstroms total thickness was produced by molecular beam epitaxy on a Si substrate. Electro-reflectance spectroscopy was used to measure 6 band- to-band transitions in this structure, while only 5 transitions are expected for a Ge-Si random alloy. Analysis of our measurements indicate that a new electronic band-structure is created for the layered structure as a consequence of its tetragonal unit cell.