Ionization thresholds in silicon and germanium avalanche photodiodes under hydrostatic pressure and strain

Silicon has the lowest multiplication noise of any known semiconductor for avalanche photodiodes (APDs), due to the large ratio of the hole-initiated ionization threshold energy to the electron threshold energy ( approximately=1.5). In order to identify the bandstructure features responsible for this behavior and to investigate the expected noise performance of Si-Ge strained-layer APDs, the bandstructure dependence of threshold energies in Si and Ge was studied both experimentally and theoretically. In Si the lowest calculated thresholds are for electron-initiated umklapp processes with the ionizing electron relaxing into the X minima. The measured breakdown voltage of Si APDs decreased with pressure at a rate determined by the pressure dependence of the X minima. This provides experimental evidence that umklapp processes are indeed important in indirect bandgap semiconductors. In Ge, the Gamma , L, and X minima are close in energy and there are several possible ionization processes involving the different minima. The measured breakdown voltage increased with pressure up to approximately=5 kbar, and then decreased. This pressure is close to that at which the Gamma and X minima cross. Thus, by tuning the bandstructure, it is possible to select which process will dominate. The results show that strained layers fabricated in indirect bandgap semiconductors are potentially attractive for low-noise avalanche multiplication.