Impact ionization rate in ZnS.

The impact ionization rate and its orientation dependence in k space is calculated for ZnS. The numerical results indicate a strong correlation to the band structure. The use of a q-dependent screening function for the Coulomb interaction between conduction and valence electrons is found to be essential. A simple fit formula is presented for easy calculation of the energy dependent transition rate. PACS-No. 72.20.Ht, 72.20.Jv The impact ionization rate is of basic interest in understanding the electron transport in semiconductors at high electric field strengths. This microscopic quantity is necessary for calculating the probability of the electron to ionize which is described by the ionization coefficient α. Interesting features are the orientation dependence and the threshold behavior, which were studied for Si [1, 2, 3, 4] and GaAs [3, 5]. Recently, we have determined the ionization rate for GaAs and Si [3], following a scheme presented by Sano and Yoshii [2] for the evaluation of the impact ionization rate for Si using a very efficient deterministic numerical procedure. Their special integration scheme takes into account a rather high number of points in the Brillouin zone so that they were able to study effects of the anisotropy in k space as well as the threshold