  Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and
germanium (a-Ge), are commonly believed to have localized electronic states at
the top of the valence band and the bottom of the conduction band. Electrical
conductivity is thought to be by the hopping mechanism through localized
states. The carrier mobility of these materials is usually very low, in the
order of ~10^-3 - 10^-2 cm^2/(Vs) at room temperature. In this study, we
present the Hall effect characterization of a-Ge prepared by self-ion
implantation of Ge ions. The a-Ge prepared by this method is highly homogenous
and has a mass density within 98.5% of the crystalline Ge. The material
exhibits an exceptionally high electrical conductivity and carrier mobility
(~100 cm^2/(Vs)) for an amorphous semiconductor. The temperature-dependent
resistivity of the material is very-well defined with two distinctive regions,
extrinsic and intrinsic conductivity, as in crystalline Ge. These results are
direct evidence for a largely-preserved band structure and non-localized states
of the valence band in a-Ge, as proposed by Tauc et al. from optical
characterization alone. This finding is not only significant for the
understanding of electrical conductivity in covalent disordered semiconductors,
but the exceptionally high mobility we have observed in amorphous Ge opens up
device applications not previously considered for amorphous semiconductors.
