Post-Bombardment Conductivity in MgO Crystals

Abstract

Electrical conductivity following the termination of bombardment by 1.3-Mev electrons has been observed in MgO crystals. Depending upon the rate of decay, one of two different techniques is utilized to determine the time constants at various temperatures. Short time constants at high temperatures are measured oscillographically, whereas long time constants at low temperatures are determined by a dc method. In the former case, a single exponential function fits the crystal current vs time data, whereas in the latter, a departure occurs. This is accounted for by the theoretical analysis which provides a procedure for determining the relevant time constant. The post-bombardment time constant, $\tau$, changes with temperature in accordance with the relationship $\tau ={\tau }_0\exp \left(\frac{E}{\mathrm{kT}}\right)$ where ${\tau }_0$ is essentially constant, and $E=1.4\mathrm{}$ ev. The experimental results are interpreted in terms of a theoretical model involving shallow and deep traps. Thermal reemission of charge carriers from the former, constituting a single level, occurs following bombardment. A second level in addition to that at 1.4 ev was determined at 0.7 ev by an extension of the technique. The former very likely corresponds to the 2.1-ev optical transition observed previously, in which case the charge carriers involved in this thermal process are holes.