Gallium-related defect centers in molecular-beam-epitaxy-grown ZnSe films: Influence of electric field on thermal emission of electrons

We report a study of the Poole-Frenkel (PF) effect in ZnSe. Our results show that ignoring the PF effect in deep-level transient spectroscopy (DLTS) leads to a significant scatter in the activation-energy data reported in the literature for deep defects in this material. The Ga-doped ZnSe films used in this study were grown on (100) GaAs by molecular-beam epitaxy. Our DLTS results show the presence of two prominent electron traps at depths E1=0.27 and E2=0.400.48 eV. The E2 trap exhibits a strong PF effect, indicating a donorlike character. Thermal emission from the E1 trap is independent of the electric field. By taking into account the PF effect, we are able to explain the carrier-concentration dependence of the activation energy of trap E2. The capture process for the E1 trap is thermally activated with a thermal-energy barrier of 0.096 eV. The existence of a thermal barrier for the carrier capture leads to persistent photoconductivity observed below T=90 K. In order to describe the properties of the Ga-related traps in ZnSe:Ga films, we propose a consistent defect model involving complexes of Ga atoms with zinc vacancies in the next-nearest-neighbor positions (GaZn-VZn). The model attributes the E2 level to a donorlike state, and the E1 level to an acceptorlike state of GaZn-VZn defect complex.