Dependence of the Sn^0/2+ charge state on the Fermi level in semi-insulating CdTe

We explored the growth and characteristics of CdTe doped with Sn to heighten our understanding of the role of deep levels on electrical compensation and trapping. We demonstrated, for the first time, the strong dependence of the Sn_Cd charge state on the Fermi-level variation (2–3kT) in high-resistivity CdTe. The concentration of deep traps for electrons was determined by the number of doubly positively charged Sn²⁺ atoms. Thermoelectric-effect spectroscopy and photovoltage measurements revealed the conversion of the Sn_Cd defect from the electron Sn_Cd²⁺ trap to the hole Sn_Cd⁰ trap. The results agree well with the existence of a negative U-center in the Sn_Cd^0/2+ defect. We also showed that the neutral Sn defect is responsible for the near midgap C-band → bound hole radiative transitions band with a maximum at 0.76 eV.