Electron tunneling into amorphous germanium

A detailed study was undertaken of electron tunneling into amorphous Ge. At low temperature, $T\ensuremath{\lesssim}100$ K, the high-field conductivity of the $a\ensuremath{-}\mathrm{G}\mathrm{e}$ film is similar to the tunneling conductance. For $a\ensuremath{-}\mathrm{G}\mathrm{e}$ thicknesses $t\ensuremath{\lesssim}500$ \AA{} the conductance of $a\ensuremath{-}\mathrm{G}\mathrm{e}$ blends smoothly with the tunneling conductance. This makes separation of tunneling conductance from the bulk $a\ensuremath{-}\mathrm{G}\mathrm{e}$ conductance difficult at low temperatures for junctions with thick $a\ensuremath{-}\mathrm{G}\mathrm{e}$ layers. The relation $\ensuremath{\sigma}={\ensuremath{\sigma}}_{0}\mathrm{exp}[\ensuremath{-}{(\frac{{T}_{1}}{T})}^{\frac{1}{4}}]$ holds well for these junctions at zero bias for temperatures not showing bulk $a\ensuremath{-}\mathrm{G}\mathrm{e}$ effects. In junctions with sufficiently thin $a\ensuremath{-}\mathrm{G}\mathrm{e}$ layers, $t\ensuremath{\approx}100$ \AA{}, the bulk $a\ensuremath{-}\mathrm{G}\mathrm{e}$ does not seriously modify the conductance away from zero bias. A series of junctions formed on the same oxide with $t\ensuremath{\le}80$ \AA{} of $a\ensuremath{-}\mathrm{G}\mathrm{e}$ 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 $a\ensuremath{-}\mathrm{G}\mathrm{e}$, but does place an upper limit on the possible tunneling range, varying from 28 \AA{} at 300 K to 50 \AA{} at 4.2 K. Tunneling thus only probes the surface layers of $a\ensuremath{-}\mathrm{G}\mathrm{e}$ and does not reflect the bulk properties. Capacitance studies of the junctions indicate the presence of a high density of interface states, ${N}_{s}\ensuremath{\ge}2.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{eV}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}2}$. Superconductive tunneling confirms that tunneling is the dominant conduction mode for these junctions.