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

We have observed new optical transitions at 0.76, 1.25, and 2.31 eV in an ordered Si–Ge superlattice. The superlattice structure consists of a sequence of four monolayers (ML) of Ge and 4 ML of Si repeated five times in the (001) crystal orientation. The strained-layer structure of 50 Å total thickness was produced by molecular-beam epitaxy on a Si substrate. Electroreflectance spectroscopy was used to measure seven band-to-band transitions in this structure, while only five 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.