Comparison of SiNx-based Surface Passivation Between Germanium and Silicon

Germanium (Ge) has attracted much attention as a promising channel material in nanoscale metal-oxide-semiconductor devices and near-infrared sensing because of its high carrier mobilities and narrow bandgap, respectively. However, efficient passivation of Ge surfaces has remained challenging. Herein, silicon nitride (SiNx)-based passivation schemes on Ge surfaces are studied and the observations are compared to Si counterparts. These results show that instead of a high positive charge density (Q(tot)) that is found in SiNx-passivated Si samples, similar Ge samples contain a high amount of negative Q(tot) (in the range of 10(12 )cm(-2)). The maximum surface recombination velocity of the samples is shown to reduce by a factor of three in both Si and Ge samples by a post-deposition anneal at 400 degrees C. The SiNx-coated samples are capped with an atomic-layer-deposited aluminum oxide (Al2O3) layer, which reduces the midgap interface defect density (D-it) after annealing to 7 x 10(10) and 4 x 10(11) cm(-2) eV(-1) in Si and Ge, respectively. Interestingly, while the Al2O3 capping seems to have no impact on Q(tot) of the Si samples, it turns the stack virtually neutral (similar to-1.6 x 10(11) cm(-2)) on Ge. The presented SiNx-based passivation schemes are promising for optoelectronic devices, where a low D-it and/or a low charge are favored.