Electronic properties of germanium quantum films

The electronic band structures of hydrogen-terminated freestanding nanomete r size (0.5-2.4 nm) germanium films have been calculated from first princip les for three main low-index orientations. A quantum confinement-induced di rect band gap appears for (100) and (111) oriented films, whereas the (110) oriented films are characterized by an indirect band gap. For the same fil m thickness there is an anisotropy in the fundamental band gap E-g with res pect to the film orientation, e.g., E-g((111))<E-g((110))<E-g((100)). This is explained in terms of. electron effective mass anisotropy. The thickness dependence of the band gap is investigated and the results are fitted to a n empirical expression. We find that the band-gap blueshift is determined b y the quantum confinement in both valence and conduction bands.