Comparison of SiN_x‐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 (SiN_x)‐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 SiN_x‐passivated Si samples, similar Ge samples contain a high amount of negative Q_tot (in the range of 10¹² 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 °C. The SiN_x‐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 × 10¹⁰ and 4 × 10¹¹ 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 (∼−1.6 × 10¹¹ cm⁻²) on Ge. The presented SiN_x‐based passivation schemes are promising for optoelectronic devices, where a low D_it and/or a low charge are favored.