Reasons of low charge collection efficiency in CdTe-based x/γ detectors with ohmic contacts
2013
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 μs)
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