Temperature Dependence of Hole Transport in Vitreous Selenium

Hole drift in vacuum-evaporated films of vitreous selenium was measured as a function of temperature (+ 35 to - 130 ^∘{}C) and applied electric field using a time-of-flight technique. Below - 70 ^∘{}C, a region not previously examined, the inverse hole transit time becomes a nonlinear function of the applied field. Therefore, a unique hole-drift mobility cannot be defined. In addition, the transient current below - 70 ^∘{}C shows increasing dispersion (normally characteristic of spatial spreading in the sheet of holes) with decreasing temperature. It is suggested that these effects are caused by an electric-field- and temperature-dependent release rate from shallow traps. Above - 70 ^∘{}C, well-defined electric-field-independent hole-drift mobilities are observed, and the values are in excellent agreement with previously reported results.