In our PL experiment, the exciting laser light (532 nm) is absorbed mainly in the n
+-Ge cap layer (Fig. 1). The peak position in luminescence of a doped semiconductor has to reflect the redshift from band gap narrowing and the blueshift (Burstein shift [
18]) from Fermi energy level rise above the corresponding band edge. The occupied states below the Fermi energy level cause a blueshift as the maximum of carriers shifts from near band edge at low doping to the Fermi energy at higher doping. This effect is very pronounced in direct semiconductors, where the Fermi level rise in the conduction band above effective density of states levels (typically 10
18 range). In indirect semiconductors, only the weak indirect emission is strongly influenced by this effect, the higher lying direct band is less influenced, in n
+-Ge the Fermi level touch the direct band at around 10
20 doping. Indeed the main PL- peak (Fig. 2) is at 0.73 eV that is the 70 meV narrowed direct transition from the n-doped top layer (in undoped, unstrained Ge the direct transition is expected at 0.80 eV). A much smaller peak appears at 0.80 eV that stems from photo-exited carriers diffusing into the intrinsic Ge layer below. Unexplained in this article is the low energy shoulder at around 0.7 eV, which could be related to dislocation levels [
14]. At the position of the indirect gap (0.664 meV at room temperature), only weak indications of optical transitions and their phonon replica are visible.