Dependence of the Sn 0/2+ charge state on the Fermi level in semi-insulating CdTe
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J. Franca) and H. Elhadidy Charles University, Faculty of Mathematics and Physics, Institute of Physics, Prague CZ 121 16, Czech Republic
A. Fauler and M. Fiederle Materialforschungszentrum, Freiburg D-79104, Germany
R.B. James Energy, Environment and National Security Directorate, Brookhaven National Laboratory, Upton, New York 11973 (Received 19 July 2007; accepted 7 August 2007)
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 SnCd 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 SnCd defect from the electron SnCd2+ trap to the hole SnCd0 trap. The results agree well with the existence of a negative U-center in the SnCd0/2+ 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.
I. INTRODUCTION
Identifying electronic deep levels and their charge states in CdTe and CdZnTe is of fundamental and practical interest because they greatly affect charge collection in x-ray and gamma-ray detectors and photorefractive devices.1,2 Deep-level traps maintain the insulating behavior of the material by stabilizing the Fermi level against slight spatial variations in the concentrations of shallow-level traps. Thus, the constructive action of deep levels in compensation is to buffer the inhomogeneous distribution of shallow defects. In this role, deep levels are transformed from the neutral state to either positively or negatively charged states, thereby increasing carrier trapping that, in turn, lowers the sensitivity and energy resolution of x-ray detectors.1 However, it is vital to find a compromise in this compensation process to ensure reaching a high level of compensation at lower trapping rates. To achieve this, precise control is needed of the concentration and charge states of deep levels. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0404 J. Mater. Res., Vol. 22, No. 11, Nov 2007
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Technologically and practically, it is more convenient to optimize this compensation by incorporating foreign impurities rather than via manipulating native defects. The concentrations, energy levels in the band gap, and the charge states of foreign atoms with deep levels can be measured. Native defects reportedly also introduce midgap levels in the band gap of CdTe (e.g., cadmium vacancies, VCd, or tellurium antisites, TeCd, or their complexes, such as VCd–TeCd).3–5 However, measuring their concentration and the energy levels is difficult. This dearth of reliable information about native defects is a great obstacle
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