Electron tunneling into amorphous germanium

A detailed study was undertaken of electron tunneling into amorphous Ge. At low temperature, $T\lesssim 100$ K, the high-field conductivity of the $a-\mathrm{G}\mathrm{e}$ film is similar to the tunneling conductance. For $a-\mathrm{G}\mathrm{e}$ thicknesses $t\lesssim 500$ Å the conductance of $a-\mathrm{G}\mathrm{e}$ blends smoothly with the tunneling conductance. This makes separation of tunneling conductance from the bulk $a-\mathrm{G}\mathrm{e}$ conductance difficult at low temperatures for junctions with thick $a-\mathrm{G}\mathrm{e}$ layers. The relation $\sigma ={\sigma }_0\exp [-{\left(\frac{T_1}{T}\right)}^{\frac{1}{4}}]$ holds well for these junctions at zero bias for temperatures not showing bulk $a-\mathrm{G}\mathrm{e}$ effects. In junctions with sufficiently thin $a-\mathrm{G}\mathrm{e}$ layers, $t\approx 100$ Å, the bulk $a-\mathrm{G}\mathrm{e}$ does not seriously modify the conductance away from zero bias. A series of junctions formed on the same oxide with $t\le 80$ Å of $a-\mathrm{G}\mathrm{e}$ 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 $a-\mathrm{G}\mathrm{e}$, but does place an upper limit on the possible tunneling range, varying from 28 Å at 300 K to 50 Å at 4.2 K. Tunneling thus only probes the surface layers of $a-\mathrm{G}\mathrm{e}$ and does not reflect the bulk properties. Capacitance studies of the junctions indicate the presence of a high density of interface states, $N_s\ge 2.5\times {10}^{14}$ ${\mathrm{eV}}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$. Superconductive tunneling confirms that tunneling is the dominant conduction mode for these junctions.