Monte Carlo study of electron initiated impact ionization in bulk zincblende and wurtzite phase ZnS

This paper presents a theoretical study of the high field electronic transport properties of the cubic and hexagonal phases of zinc sulfide (ZnS) using an ensemble Monte Carlo method. Essential features of the model are the inclusion of realistic energy band structures calculated from a local pseudopotential method and numerically calculated impact ionization transition rates. The polar optical phonon scattering rate has also been computed numerically from the band structure. The relevant transport quantities have been computed for field values between 100 kV/cm and 2 MV/cm. On the basis of these calculations it is predicted that the electron distribution is cooler and the average energy lower in the wurtzite phase than in the zincblende phase over the entire field range examined. The difference in average energy between the two phases becomes pronounced for field magnitudes above 1 MV/cm while it is smaller in the field range between 700 kV/cm and 1 MV/cm. As a result, the ionization coefficients are expected to be higher in the zincblende phase than in the wurtzite phase. This can be attributed to differences in the density of states between the two polytypes. The quantum yield has also been computed. It is found that even though the threshold for impact ionization is relatively hard in both polytypes, the threshold for the wurtzite phase is harder than the threshold for the zincblende phase.