Conductivity, Hall Effect and Thermo-electric Power of Selenium Single Crystals

Single crystals of selenium have been grown from the vapour phase and their conductivity and thermo-electric power were measured over a range of temperatures. The Hall effect was determined for one crystal. The conductivity obeys a relatively simple exponential law σ = σ 0 exp (- e E / k T ), with e E = 0.13 eV. and the room temperature conductivity between 10 -5 and 0.5 × 10 -6 ohm -1 cm -1 . Both thermo-electric power and Hall effect rise slightly with increasing temperature, indicating a fall in the current carrier concentration. Values calculated from thermo-power and Hall effect agree reasonably well, and give about 10 14 positive holes per cm 3 . Because of an experimentally established correlation between thermo-power and conductivity, differences in conductivity between various crystals are ascribed to different hole concentrations. Deviations from Ohm's law, the rapid increase in mobility with temperature and its low absolute value (about 0.14 cm 2 /V.sec. at room temperature) all point to the existence of potential barriers within the crystals. It is shown that a range of barriers of different heights could explain the observed behaviour. The conductivity is anisotropic (ratio about 10: 1), whereas the thermo-power is not.