Tunneling into and through Evaporated Semiconducting Films

If two metallic conductors are separated by a sufficiently thin, evaporated, semiconducting film, the observed conduction is mainly due to electron tunneling. We have experimentally demonstrated this fact by using superconducting electrodes. For some of the semiconductors we have used, i.e., CdS and PbS, the tunnel current is sensitive to light, and the resistance of the sample may change by as much as a factor of 1000. The observed resistive change is permanent for all practical purposes if the sample is kept at or below liquid nitrogen temperature. The original resistive state is recovered by cycling the sample to a temperature above 110 K. The energies of the longitudinal optical phonons in the semiconducting films can be obtained from small irregularities in the current-voltage characteristic of the samples; in practice, it is necessary to obtain the first or second derivative of the current-voltage characteristic to see this effect. By altering the samples such that the electrons tunnel into the semiconducting film rather than through the film, the effect caused by the phonons in the current-voltage characteristic is greatly enhanced.