Beta- and Gamma-Induced Conductivity in Semiconductors: Application to CdS Neutron Detectors

Abstract

Methods for determining the mobility-lifetime (μτ) product of high-resistivity n- or p-type semiconductors using induced conductivity resulting from high-energy beta and gamma irradiations are developed. The μτ product is determined, under conditions of steady-state excitation, from the measured induced conductivity and the calculated electron-hole generation rate g. The carrier free lifetime τ is determined from these results using the measured Hall mobility. For gamma irradiation g is determined from the exposure rate, density, mass energy-absorption coefficients, and radiation-ionization energy (ε). For beta irradiation gβ is determined for the case of ionization resulting from the beta decay of radioisotopes which are produced by neutron irradiation and which are randomly distributed throughout the semiconductor. gβ is determined from the radioactive decay rate, energy of the beta radiation, and ε. The absorbed dose rates resulting from these radiations are also determined. Gamma-induced conductivity experiments were performed using 60Co gamma irradiation on high-resistivity n-type CdS, CdSe, GaAs, and ZnS single crystals. The measured μτ products ranged from ∼10−5 to 25 cm2/V; τ varied from ∼10−8 to 10−1 sec. Some reduction in μτ resulting from high-temperature annealing of CdS and GaAs was also observed. Beta- and gamma-induced conductivity experiments (32P beta decay and 60Co gamma irradiation) were performed on neutron-irradiated CdS single crystals. Comparison of these results show that: (1) The μτ products determined at a given dose rate from the beta and gamma irradiations are in very good agreement (within 15%). Thus, the theories developed here for determining g and gβ are valid. (2) For samples which have considerable neutron damage, μτ is very dependent upon exposure rate (or absorbed dose rate); decreasing with increasing exposure rate. This is in contrast to samples which have minimal (or no) neutron damage, where very little dependence on exposure rate is observed. (3) The μτ product is also dependent upon the conductivity or Fermi level position. For irradiated high-resistivity crystals μτ is 1–6 cm2/V, and τ is of the order of tens of msec. However, for irradiated low-resistivity crystals μτ is larger by a factor of ∼102, and τ is of the order of seconds. The dependence of μτ on the excitation intensity, defect concentration, and Fermi level position in neutron-damaged samples results from sensitizing centers associated with neutron-induced defects. The concepts developed here have resulted in new techniques for measuring neutron fluences using electrical changes associated with induced radioactivity in semiconductors. Gamma-induced conductivity provides a valuable technique for ``calibrating'' these detectors (i.e., determining the μτ product).