dc conductivity, optical absorption, and photoconductivity of amorphous arsenic telluride films

Abstract

Conductivity, optical absorption, and photoconductivity experiments on amorphous As2Te3 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 σ=σ0e-ΔEkT, with σ0=600 Ω-1 cm-1 and ΔE=0.4 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 cm2/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 cm2/V sec for the lower limit of the mobility of carriers. The photoconductivity data also show the presence of trapping states. © 1970 The American Physical Society.