Mechanisms of Charge Transport and Photoelectric Conversion in CdTe-Based X- and Gamma-Ray Detectors

Abstract

This chapter deals with (i) the charge transport mechanisms in X- and gamma-ray detec- tors both Ohmic and Schottky types based on CdTe and its alloys with an almost intrinsic conductivity (the peculiarities of the formation of self-compensated complexes due to the doping of Cd(Zn)Te crystals with elements of III or V groups (In, Cl) are taken into account); (ii) the reasons of insufficient energy resolution in the X- and gamma-ray spec - tra taken with the detectors under study; (iii) the quantitative model which describes the spectral distribution of the detection efficiency of Cd(Zn)Te crystals with Schottky diodes; (iv) a correlation between the concentration of uncompensated impurities in the Cd(Zn)Te crystals and collection efficiency of photogenerated charge carriers in the detec - tors with a Schottky contact; (v) the possibility of applications of CdTe thin films with a Schottky contact as an alternative to the existing X-rays image detectors based on a-Se. and τ po are the effective lifetimes of electrons and holes in the SCR, and the quantities n 1 = N c exp(− E t / kT ) and p 1 = N v exp[−( E g - E t )/ kT ] are determined by the depth of the generation-recombination level E t . The results of calculations of the I – V characteristic, by using formula (6) show that the model of generation-recombination processes in the SCR adequately describes not only the current dependence on the voltage, but also the temperature induced variations in the Ni/p-CdTe Schottky diode I – V characteristic: (1) The reverse current, which has a generation origin, cannot vary in a wide range of the material resistivity ρ since this current is governed by the carrier lifetime and by the thickness of the SCR, which have no direct relation with a value of ρ . (2) In the region of low forward biases, where the dependence I the current is governed by the same parameters and, therefore, is also only slightly ρ -dependent. (3) As ρ increases, the Fermi level recedes from the valence band; that is, ∆μ increases at the same time as φ 0 decreases. In this case, the part of the forward branch, where the forward current is propor tional to exp( qV /2 kT ), is increasingly restricted from above, as is observed in the experimental curves.