Characterization of Schottky-diode X/γ-ray detectors based on CdTe crystals with different uncompensated impurity concentrations

The Schottky-diode X/.. -ray detectors based on semi-insulating p-like CdTe single crystals with four values of uncompensated impurity concentration (N approximate to 2 x 10(12) cm(-3), 2 x 10(11) cm(-3), 1 x 10(11) cm(-3), 4 x 10(10) cm(-3)) were developed and characterized by I-V measurements at low reverse bias (0.005-100 V) and detection of Cs-137 and Am-241 radioisotope emission spectra. All the detectors were fabricated by the same technique using vacuum evaporation of a Cr electrode (rectifying contact) and chemical deposition of an Au electrode (Ohmic contact) onto the B- and A-face of CdTe(111) crystals, respectively, pre-treated with Ar-ion bombardment. The change of the charge transport mechanism from over-barrier (thermionic) to generation current along with the analysis of the temperature dependence of the differential resistivity evidenced a high concentration of deep level impurities near the middle of the semiconductor bandgap. The detection efficiency eta of the detectors was calculated using the corrected expressions for the coordinate dependence of the electrical field strength in a diode which took into account the fact that the depletion region width exceeded the CdTe crystal thickness at higher bias voltages. It was shown that the dependence eta(N) decreased with increasing N without a maximum corresponding to an optimal value N-opt as reported earlier. The Cr/CdTe/Au Schottkydiode detectors, fabricated using CdTe with the lowest N, demonstrated the highest energy resolution (0.6% @662 keV and 7.5% @59.5 keV) and it degraded when crystals with higher N were used.