Reasons of low charge collection efficiency in CdTe-based X/γ-ray detectors with ohmic contacts

Quantitative assessments of the possibility to improve the energy resolution of detectors based on CdTe crystals with two Ohmic contacts by varying the crystal thickness, applied voltage, temperature and lifetimes of charge carriers are presented. It is shown that a super-linear increase in the conductivity of Pt/CdTe/Pt detectors at larger voltages is caused by the space charge limited current, and the excess component over the linearly approximated current is virtually temperature independent. Due to such behavior of the excess current, cooling of the detector does not give the desired result. The calculations show that the effect of charge carrier recombination at the crystal surfaces can be ignored when V > 9-10 V, while trapping carriers in the crystal bulk for thick CdTe detectors can significantly degrade the charge collection efficiency. Thinning the crystal and lowering the applied voltage to maintain the same current value leads to considerable improving of the charge collection efficiency. In the case of the detector thickness of 0.25 mm, the collection efficiency is higher than 97-98% in the photon energy range of 10-1000 keV. The collection efficiency could exceed 99% if to perfect the crystal growth technology and thereby increase the charge carrier lifetime by one order of magnitude as compared to that in the currently used CdTe crystals (2-3 mu s).