Effect of annealing in zinc vapors on charge trapping properties of ZnSe, ZnSe(Te) and ZnSe(Al) scintillation crystals: Revealing the mechanisms by DFT computational studies

Abstract The mechanisms of changes of ZnSe crystals conduction due to their doping with tellurium or aluminum ions as well as due to annealing in zinc vapors are analyzed in experimental and computational studies. The electrical resistivity of undoped, Te- and Al-doped ZnSe crystals were measured before and after annealing in zinc vapors. Doping with tellurium or aluminum leads to a decrease of electrical resistivity (increase of conductivity) of ZnSe crystals, and this decrease is stronger in case of Al-doped samples. After annealing, a decrease in resistivity by 5–8 orders of magnitude in zinc vapors was observed for all samples. The electronic structures of ZnSe crystals with various point defects were calculated within the DFT approximation using the plane-wave pseudopotential method. The native vacancies VZn and VSe, substitution defects AlZn, OSe and TeSe, interstitial impurities Zni, as well as some double and triple combinations of these defects were considered in calculations. The ionization energies of the defects were calculated using the transitive level (TL) formalism. It was found that the increase of conductivity in ZnSe → ZnSe(Te) → ZnSe(Al) sequence of samples arises primarily due to formation of hole trap centers on TeSe and AlZn and also due to higher ionization energies of such centers on AlZn in comparison with TeSe. The increase of conductivity of the samples after annealing in zinc vapors arises due to formation of the deep hole traps on interstitial defects Zni, and also due to annihilation of zinc vacancies. Annihilation of VZn changes the trapping properties of the system of native and doping-related point defects (deep electron traps disappear or become shallow, hole traps are formed) and this effect provides an additional mechanism of conductivity increase due to annealing.