Electron and Hole Drift Mobilities in Vitreous Selenium

Abstract

The drift mobilities for electrons and holes in vitreous Se were measured as a function of temperature and as a function of the applied electric field. The drift mobilities exhibited a temperature dependence not previously observed. The electron drift mobility exhibited an exponential temperature dependence up to the glass transition temperature ($T_g=305\mathrm{}$°K). At this point, a sharp break from the exponential temperature dependence was observed. Below 305°K, the electron drift mobility exhibited an exponential temperature dependence which is characteristic of a trap-limited drift process. In this temperature range, the computed mobility-controlling electron trap densities and the measured activation energies depended on the temperature of the substrate at which the selenium films were prepared. It is shown that as the substrate temperature is increased, the activation energy for the mobility increases, and the density of the traps which control the mobility decreases. The hole drift mobility exhibited an exponential temperature dependence at low temperatures, but a gradual deviation from the exponential temperature dependence at a temperature of about 260°K was observed. This indicates that the microscopic hole mobility ${\mu }_{0p}$ has a low value. The microscopic hole mobility, evaluated at 293°K, yielded an average value of ${\mu }_{0p}=0.34\mathrm{}\pm 0.05$ ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$. In the low-temperature range the hole trap densities and the activation energies were computed. These quantities revealed the same dependence on the substrate preparation temperature as the corresponding quantities for electron transport. It is postulated that the mobility-controlling traps are caused by the disorder in the amorphous selenium. The electron and hole drift-mobility values at room temperature agreed with previous measurements.