Time-of-flight drift mobility measurements on chlorine-doped amorphous selenium films

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Abstract
Time-of-flight drift mobility experiments have been carried out on pure and chlorine-doped amorphous selenium films. The temperature and field dependence of the drift mobility of both types of carriers have been investigated and the charge transport mechanism has been critically examined in terms of various shallow-trap-controlled transport processes, including hopping for the microscopic mobility. Addition of 30 and 40 wt ppm Cl to a-Se has been found to increase the hole drift mobility activation energy by approximately 0.16 eV. Two interpretations have been shown to be plausible: (i) Cl doping diminishes the existing charged centres (possibly Se1-) acting as shallow hole traps and/or changes the mode of microscopic transport, (ii) Cl introduces a new set of charged hole traps (probably Cl-) at approximately 0.45 eV above Ev. Furthermore, it is shown that at low temperatures the hole drift mobility may be described by an empirical expression similar to that due to Gill (1972, 1976).