Study of the charge collection efficiency of CdZnTe radiation detectors

The charge collection efficiency of CdZnTe radiation detectors with two different configurations: a Schottky diode detector and a resistive detector are compared. The average charge collection efficiencies for three different directions of irradiation (negative electrode, positive electrode and perpendicular to the electric field) are calculated. The mobility-lifetime product of the CdZnTe substrates is evaluated from the dependence of the measured spectra upon detector bias voltage. The measurement of the average charge collection efficiency is based on monitoring the shift of the peak channel with bias voltage in an experimental setup which is well calibrated. Two types of radiation are used: gamma photons from several radioactive sources and alpha particles from an ²⁴¹Am source. The models for the evaluation of mobility-lifetime product from the measured data for the two types of detector configurations as well as for the two types of radiation sources, are compared and discussed. The CdZnTe (Zn = 10%) substrates under study are obtained commercially and are grown by the high pressure Bridgeman method. The mobility-lifetime products and specific resistivity of the two types of detectors are evaluated and compared. A lower resistivity material has a narrower depletion region and behaves like a thinner detector thus exhibiting better collection efficiencies. Therefore, medium resistivity material which is completely inadequate for resistive detectors can still yield high performance Schottky detectors. The preferred direction of irradiation, i.e. from the negative electrode, is possible only in the case of n-type material which is reverse biased by negative voltages applied to the Schottky gate. The mobility-lifetime products that are derived on both the resistive detector (with specific resistivity of ∼1·10¹⁰ Ω·cm) and the Schottky diode (with specific resistivity of ∼1·10⁶ Ω·cm) are μ_nτ_n ≅ 4·10^-4 cm₂V^-1 and μ_pτ_p ≅ 8·10^-5 cm²V^-1.