PHOTOCONDUCTIVITY OF THIN-FILM FULLERENES - EFFECT OF OXYGEN AND THERMAL ANNEALING

The photoconductive response of both oxygen-free and oxygenated thin f ullerene films is compared and contrasted. Exposure to oxygen results in a significant decrease in the magnitude of the photoconductivity. T he qualitative features of the spectral dependence of the photoconduct ivity in oxygen-free and oxygenated films are the same, however, and d ifferences in the temperature dependence are small. It is concluded th at the influence of oxygen is not the source of subgap responses and a nomalous features in the temperature dependence. In addition, the feat ures of the temperature dependence are not directly related to the str uctural phase transition, Upon heat treatment of the oxygen-free films under vacuum, both the photoconductive response at subgap photon ener gies, and the anomalous features in the temperature dependence are sub stantially reduced. The treatment is interpreted as a thermal annealin g process. In oxygenated films heat treatment results in an overall in crease in the photoconductive response by an order of magnitude, a res ult of a partial outheating of the absorbed oxygen.