Properties of amorphous germanium tunnel barriers

Abstract

The properties of tunnel barriers made with amorphous Ge (a-Ge) deposited at approximately 80 K were studied in Al/a-Ge/Al tunnel junctions and also in junctions where one electrode was Ni or Fe. The conduction process was shown to be tunneling for barriers less than about 100 A at liquid He temperature and consistent with Mott variable-range hopping for higher temperatures and thicknesses. Measurements were made of current density J and dynamic conductance dJ/dV as a function of voltage V, thickness s, and temperature T. The measurements were compared with available theoretical expressions for rectangular tunnel barriers based on the WKB approximation. The applicability of these expressions for barrier heights less than 100 meV was examined and a modified equation for J(V) was derived which eliminated assumptions which are inaccurate for such low barriers. The measurements were also compared to this modified equation and to numerical solutions. Values for the effective tunnel barrier height ranging from 20 to 80 meV were obtained. Theoretical expressions for J(V) could be fitted to the measurements fairly well, but not perfectly; for J(T) the fit was poor. Values of s obtained using the modified expression for J(V) tended to be 10%–20% less than those measured by a quartz-crystal thickness gauge using the bulk crystal density. The conductance peaks corresponding to the peaks in the superconducting density of states were considerably broadened over Al/Al2O3/Al junctions either because of depairing of the Al films in contact with a-Ge or from an inelastic process in the barrier. No spin polarization of the tunnel currents was observed when one of the electrodes was Ni or Fe. Some measurements were made of a-Ge barriers treated with glow discharges in N2, O2, and H2. The properties of a-Ge were very similar to those previously found for a-Si. Evidently the basic conduction process in these junctions is tunneling, but the simple tunneling model cannot entirely explain the results. Various proposals to account for these divergences from the simple tunneling model are discussed.