THE INFLUENCE OF RESIDUAL-GAS ON THE PROPERTIES OF EVAPORATED SELENIUM FILMS

The photoconductivity of vacuum-evaporated vitreous selenium films has been found to depend strongly on residual gases which exist in the vacuum chamber. Specific effects depend on whether these gases are the result of increased system outgassing or purge gas leaked into the evaporation chamber. Films that were evaporated in the presence of 6*10-3 Torr of argon, oxygen, air or water-vapour-saturated air had greatly enhanced photoconductivity for superband gap radiation under positive bias. In contrast, increasing the vacuum pressure by partially closing a valve to reduce the pumping speed resulted in a decreased response. The use of a strong reducing agent, such as hydrogen, also caused a reduction in photoconductivity. Films produced from selenium containing 7% tellurium were less sensitive to residual gas. The photoconductivity measured under negative front-surface bias showed a more complicated dependence on evaporation conditions. The enhanced photoconductivity seen in the selenium films was usually correlated with an increased crystallinity, as determined by X-ray diffraction. The data are interpreted in terms of density changes in midgap traps resulting from changes in the concentration of chemically active gaseous impurities.