Electron tunneling into amorphous germanium

A detailed study was undertaken of electron tunneling into amorphous Ge. At low temperature, T100 K, the high-field conductivity of the a-Ge film is similar to the tunneling conductance. For a-Ge thicknesses t500 {} the conductance of a-Ge blends smoothly with the tunneling conductance. This makes separation of tunneling conductance from the bulk a-Ge conductance difficult at low temperatures for junctions with thick a-Ge layers. The relation σ=σ₀exp[-(T₁T)1/4] holds well for these junctions at zero bias for temperatures not showing bulk a-Ge effects. In junctions with sufficiently thin a-Ge layers, t≈100 {}, the bulk a-Ge does not seriously modify the conductance away from zero bias. A series of junctions formed on the same oxide with t≤80 {} of a-Ge show an exponential drop in conductance with increasing t leveling off at temperature-dependent values. This is interpreted as incomplete surface coverage. This interpretation is independent of the details of the tunneling mechanism into a-Ge, but does place an upper limit on the possible tunneling range, varying from 28 {} at 300 K to 50 {} at 4.2 K. Tunneling thus only probes the surface layers of a-Ge and does not reflect the bulk properties. Capacitance studies of the junctions indicate the presence of a high density of interface states, Nₛ≥2.5×10¹⁴ eV^-1 cm^-2. Superconductive tunneling confirms that tunneling is the dominant conduction mode for these junctions.