The pressure dependence of the electronic properties of orthorhombic sulphur and vitreous selenium.

Drift mobility techniques have been used to measure the transport of excess electrons and holes in orthorhombic S and vitreous Se under hydrostatic pressures p of up to 6 kbar in the temperature range from -50°C to 80°C. In the S crystals, where electrons propagate by an intermolecular hopping mechanism, a strong pressure dependence was found for the electron mobility, [special characters omitted] exp (p/3.15 kbar) the activation energy, however, remained unchanged. It is concluded, on the basis of small polaron theory, that the observed effect is caused by an increase of the intermolecular resonance integral J. In fact, a LCAO calculation of the intermolecular overlap integral as a function of pressure predicts changes in J of the right order of magnitude. At low temperatures the hole mobility depends on the pressure as uh exp (p/3.15 kbar). If the temperature is increased, however, a progressively weaker dependence is found. This can be described phenomenologically by taking the trap density to be dependent on the pressure as [special characters omitted] exp (-P/5 kbar). In vitreous Se no pressure dependence of the mobilities could be detected and the implications of this result on the conduction mechanism are discussed. It is also suggested that the pronounced pressure dependence of the steady state dark- and photo-conductivity in this material is primarily due to the effect of pressure on the injecting properties of the contacts.