Temperature independent drift mobility along the molecular direction ofAs2S3

Abstract

Electron drift mobilities have been observed in ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ crystals along the molecular direction by transient photoconductivity techniques. The mobility is 1.0 ${\mathrm{cm}}^2$/V sec, independent of temperature from 207 to 465 K. This result is consistent with a trend among inorganic and organic molecular crystals, all of which have drift mobilities on the order of 1.0 ${\mathrm{cm}}^2$/V sec (within an order of magnitude) and weak temperature dependence. It is argued that the observed temperature dependence of the mobility appears to be inconsistent with the temperature dependence predicted by narrow-band theory. This result is in agreement with, and more general than, standard arguments used against band theory in molecular crystals which depend on a comparison between the intermolecular spacing and the mean free path based on calculated bandwidths, and reinforces the view that the charge carriers should be regarded as localized.