Impact excitation and ionization

Abstract

Three impact processes relevant to high-field electroluminescence are considered, namely ionization across the band gap, impurity ionization and impurity excitation. The measured impact rate as a function of field is a convolution of the rate as a function of electron energy and of the distribution of electron energies. Experimental data for the three processes are discussed, with particular attention being given to ZnS:Mn and ZnSe:Mn. The data for electroluminescence in bulk crystals with Schottky contacts is presented in terms of quantum efficiency as a function of field. It is found that the rates of both ionization across the gap and excitation of manganese are a function of shallow donor density, and an explanation is advanced in terms of the band structure of ZnS and ZnSe. A maximum in the quantum efficiency of electroluminescence in ZnS:Mn and ZnSe:Mn Schottky diodes is reported, and its implication for device performance is pointed out.