Tunneling characteristics of amorphous Si barriers

Tunnel junctions using a barrier of amorphous silicon (a-Si) between normal and superconducting metals were studied at temperatures from 300 to 0.45 K. These junctions were reliably made by depositing on a 77 K glass substrate Ni or Au, a-Si 60 to 100 Å thick, and then Al. It was demonstrated that the dominant conduction process was elastic tunneling by the presence of structure caused by the superconducting energy gap of Al, and by comparing measurements of the voltage, temperature, and barrier thickness dependence of the conductance with theory. The effective barrier heights were grouped close to 2×10−2 eV. A semiquantitative argument suggests that the barriers controlling the elastic tunneling are much the same as those controlling the phonon-activated variable range tunneling at higher temperatures. Although a-Si barriers can be formed reliably and have low leakage, the low barrier height leads to large nonlinearity even at low voltages.