Cadmium selenide films deposited at high substrate temperature as tunneling barriers between tin electrodes

Abstract

Very thin films of CdSe have been prepared as tunneling barriers in Sn–CdSe–Sn tunnel junctions. The CdSe barriers were prepared at a substrate temperature of 115°C. High substrate temperatures were used since under such conditions dielectric films of CdSe can be produced. Conductance measurements on these junctions at 300°K as well as 77°K showed that the conductance depended exponentially on the thickness of the CdSe barrier within the thickness range studied, 50–100 Å. This result is consistent with tunneling theory. In addition, at 1.4°K a sudden jump in current occurred at a voltage corresponding to the Sn superconducting energy gap. The magnitude of the current jump agrees with the theory of quasi-particle tunneling. The shape of the I-V curve as well as its weak dependence on temperature also indicate that tunneling is the conduction mechanism in these junctions. Since thin CdSe films are usually not continuous, it is necessary to oxidize those areas of the Sn electrode where no CdSe is present. A method is given for determining the pinhole area. From a knowledge of the pinhole area, one can estimate the fraction of the total current which flows through the CdSe. Our results indicate that essentially all the current flows through the CdSe. This conclusion is supported by the pronounced longitudinal optical (LO) phonon structure in the tunneling data. The LO phonon structure is characteristic of the CdSe barrier. Typical changes of 20% in conductance are observed as a result of excitation of LO phonons in the CdSe layer by the tunneling electrons. Measurement of the conductance as a function of the CdSe thickness show that the conductance of the tunnel junctions can be controlled in a fairly reproducible manner by varying the thickness of the CdSe layer.