Electrical conductivity induced in MgO by relativistic electron bombardment

Abstract

Observations have been made, by three different techniques, of the conductivity induced in single crystals of MgO when they are irradiated by relativistic electrons that traverse the targets with small energy loss. Investigation of the relationship of the crystal current, immediately following cessation of the bombardment, to the total dosage showed that the initial post-bombardment conductivity builds up to a temperature-dependent saturation value. A model that ascribes the post-bombardment conductivity to charge carriers thermally emitted from a single trapping level, which predominates in a given temperature range, has been formulated. Measurements of the decay of post-bombardment conductivity in the range 70°C–130°C were in accord with theoretical predictions based upon an analysis of the kinetics of this process, taking into account both retrapping and monomolecular recombination processes. It was determined that the activation energy for the centers is 0.96 eV, and the attempt to escape frequency is 10 10 sec −1. The initial trap occupation for a constant bombardment was found to be not strongly dependent upon temperature over the range considered. It was also established that neither radiation-induced defects nor iron, a major impurity in MgO crystals, constitute the trapping centers and that their concentration could be increased by annealing the crystal. The post-bombardment thermally-stimulated conductivity was studied by a method similar to that utilized heretofore in photoconductivity experiments, and traps were detected at 0.96, 1.20, 1.32 and 1.69 eV. The density of the first is of the order 10 17 cm 3, but the trap concentrations were observed to vary appreciably among different specimens. Theoretical considerations predicting a dependence of yield (crystal current/primary current) on trap occupation, which is a function of temperature and bombardment, were confirmed by bombardment-induced conductivity measurements. The true mobility variation with temperature was determined under conditions of constant trap occupation (empty traps). The data, extending up to 600°K, agreed with theoretical calculations in which the mean scattering time is ( 1 τ 0 + 1 τ a + 1 τ i ) −1 where τ 0 is the relaxation time of the scattering of the carrier for the optical modes, τ a for the acoustica modes, and τ i for ionized imperfections. The temperature variations of mobility determined by post-bombardment and bombardment-induced conductivity measurements over the range 80°C–120°C were consistent. The mobility of charge carriers in MgO is about 10 cm 2⧸volt sec.