EFFECT OF OXYGEN ADSORPTION ON ELECTRICAL RESISTANCE OF EVAPORATED CARBON FILMS

The effect of oxygen on the electrical resistance of evaporated carbon films has been studied over the range - 196°C to 450°C. At all temperatures over this range oxygen increased the resistance. The sensitivity of the resistance to oxygen at 22°C depended on the temperature to which the films had previously been heat treated. For heat treatment temperatures (HTT's) below 100°C oxygen had no effect on resistance at 22°C, but the oxygen sensitivity increased steadily for HTT's above this value. Increases in resistance in contact with oxygen over the range 22-203°C were completely reversed only by evacuation at the temperature to which films had previously been heat treated. The effect of oxygen over this range is attributed to an irreversible chemisorption. The activation energies of chemisorption at 22-203°C, calculated from plots of change in resistance with time, increased with extent of reaction from 6 to 14 kcal/mole. Elovich kinetics were found to apply to oxygen chemisorption over the range 22-203°C. Physical adsorption is indicated at - 196°C. Preliminary results showed that water vapour also increased film resistance at 22°C but that only some of the sites which chemisorbed oxygen were capable of chemisorbing water vapour. The temperature dependence of the resistance of films at temperatures below that at which they had been heat treated followed expressions of the type R = AT-B, where A and B are constants, and not the usual semiconductor equation R = Ro exp (ΔE/2kT). This confirms the results of Blue and Danielson(42) rather than those of Hirabayashi et al.(46) Because of their structure, purity and high surface to volume ratio films are particularly suited to a study of the effect of gas adsorption on the electronic properties of carbon. © 1968.