dc Conductivity, Optical Absorption, and Photoconductivity of Amorphous Arsenic Telluride Films

Conductivity, optical absorption, and photoconductivity experiments on amorphous ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ films are described. The films were prepared by condensation of the vapor on a cooled substrate. Over the temperature range from about 200 to 400°K, the conductivity is of the form $\sigma ={\sigma }_0{e}^{-\frac{\Delta E}{\mathrm{kT}}}$, with ${\sigma }_0=600\mathrm{}$ ${\mathrm{\Omega }}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $\Delta E=0.4\mathrm{}$ eV. This result is interpreted in terms of a "conductivity gap" of 0.8 eV. An analysis of the preexponental factor yields an average mobility between about 0.3 and 5 ${\mathrm{cm}}^2$/V sec, suggesting that the "mobility edges" which are responsible for the conductivity gap are on the borderline between localized and delocalized states. An "optical gap" is derived from a treatment of the optical-absorption data which suggests that for energies well above the conductivity gap the density of valence and conduction-band states have a parabolic dependence on energy. The optical gap of 0.95-0.98 eV is the energy difference between the edges of these parabolic states. Photoconductivity measurements indicate that the room-temperature life-time is less than 50 nsec. These measurements provide an estimate of 0.3 ${\mathrm{cm}}^2$/V sec for the lower limit of the mobility of carriers. The photoconductivity data also show the presence of trapping states.