Thickness effect on the band gap and optical properties of germanium thin films

The band gap and optical properties (dielectric functions and optical constants) of Ge thin films with various thicknesses below 50 nm, which were synthesized with electron beam evaporation technique, have been determined using spectroscopic ellipsometry and UV-visible spectrophotometry. The optical properties are well described with the Forouhi–Bloomer model. Both the band gap and optical properties show a strong dependence on the film thickness. For film thickness smaller than ∼10 nm, a band gap expansion is observed as compared to bulk crystalline Ge, which is attributed to the one-dimensional quantum confinement effect. However, a band gap reduction was observed for thickness larger than ∼10 nm, which is explained in terms of the amorphous effect in the Ge layers.