Generation, transport, and trapping of excess charge carriers in Czochralski-grown sapphire

Abstract

The transient x-ray-induced conductivity in high-purity Union Carbide ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ single crystals is reported. The mobility lifetime product for electrons is much greater than that for holes and consequently the transport properties of the holes remain obscured. The trapping and detrapping rates for four different electron traps were determined from the data, including the trapping parameters for substitutional ${\mathrm{Cr}}^{3+}$. Even though the transport is dominated by trapping and detrapping, the high-mobility transport of electrons was observed in the first few nanoseconds; the mobility was found to be 3 \ifmmode\pm\else\textpm\fi{} 1 ${\mathrm{cm}}^{2}$/V sec independent of temperature between 100 and 350 K. The magnitude and temperature dependence of the intrinsic mobility are not in agreement with simple large-polaron theory. It was found that one of the deep traps (not ${\mathrm{Cr}}^{3+}$) could be filled with an accumulated dose of 2000 rad at 300 K, and that the filling led to an increase of up to fivefold in the lifetime.