Compensation and trapping in CdZnTe radiation detectors studied by thermoelectric emission spectroscopy, thermally stimulated conductivity, and current-voltage measurements

The thermal ionization energies of traps and their types, whether electron or hole traps, were measured in commercial CdZnTe crystals for radiation detectors. The measurements were done between 20 and 400K using thermoelectric emission spectroscopy (TEES) and thermally stimulated conductivity (TSC). For reliable results, indium ohmic contacts had to be used instead of gold Schottky contacts. For filling of the traps, photoexcitation was done at zero bias, at 20K, and at wavelengths which gave the maximum bulk photoexcitation. In agreement with theory, the TSC current was found to be on the order of times or even larger than the TEES current, where V is the applied bias in TSC and ΔT is the applied temperature difference in TEES. Large concentrations of hole traps at 0.1 and 0.6 eV were observed and a smaller concentration of electron traps at 0.4 eV was seen. The deep traps cause compensation in the material, which is desirable, but they also cause carrier trapping that degrades the spectral response of radiation detectors made from the material.