Radiation-induced conductivity ofAl2O3: Experiment and theory

Abstract

The steady-state radiation-induced conductivity (RIC) has been measured in single-crystal ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ samples maintained at elevated temperatures during continuous irradiation with 1.5-MeV electrons. As the temperature increases from 300 to 1300 K there are regions in which the RIC increases rapidly, with activation energies between 0.6 and 4.3 eV, and regions in which it slowly decreases with an activation energy of approximately 0.1 eV. A theoretical model is presented in which the rapid increases observed in the RIC are correlated with the thermal detrapping of electrons. In undoped Meller and Linde samples the RIC is controlled by trapping and detrapping from high concentrations of shallow (0.57 and 0.72 eV) electron traps. In 0.004- and 0.03-wt.%-${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$-doped samples there is a sufficient concentration of 1.2-eV ${\mathrm{Cr}}^{3+}$ electron traps to prevent the RIC from increasing at low temperatures. To account for the decreases in the RIC, additional rate equations describing the thermal quenching of the conductivity by hole release are considered. The RIC data of the undoped samples indicate that hole release from 0.77-eV $V_{\mathrm{OH}}^{-}$ centers may be thermally quenching the conductivity. The results of an isochronal annealing study of the ${\mathrm{Cr}}^{4+}$ EPR signal intensity in a 0.004-wt.%-${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$-doped sample and an undoped Linde sample are correlated with thermally-stimulated-current (TSC) measurements to show that electron release from the 0.72-eV trap at low temperatures is followed by hole release from the 0.77-eV $V_{\mathrm{OH}}^{-}$ centers at higher temperatures, in agreement with the assumptions of the thermal-quenching model. The RIC data of a 0.004-wt.%-${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$-doped sample indicate that the bulk electron-hole recombination rate at room temperature is less than 9 × ${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^3$/sec (electron-capture cross section <7 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$) assuming an electron mobility of 1 ${\mathrm{cm}}^2$/V sec. Since this recombination rate is significantly lower than the Langevin rate of 2 × ${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$/sec for diffusion controlled bulk recombination, it is likely that bulk electron-hole recombination occurs predominantly at repulsive hole centers such as $V_{\mathrm{OH}}^{-}$ centers. There are several mechanisms other than thermal quenching which could account for the weak decreases in the RIC. These are the LO-phonon scattering of conduction electrons ("large-polaron" mobility) and bulk electron-hole recombination occurring through multiphonon emission.