  Transport measurements made on films of thallium-oxide (n-type semiconductor)
are presented and discussed. The focus in this work is on the
strongly-localized regime where charge transport is by variable-range-hopping.
It is demonstrated that, at liquid-helium temperatures, these films exhibit all
the characteristic features of intrinsic electron-glasses. These include a slow
(logarithmic in time) conductance-relaxation that may be induced by any of the
following protocols: Quench-cooling from high temperatures, sudden change of
gate-voltage, exposure to infrared radiation, and stressing the system with a
non-Ohmic field. The microstructure of the films are characterized by electron
microscopy and their carrier-concentration are measured by Hall effect.
Field-effect experiments reveal a memory-dip that has a width compatible with
the carrier-concentration of the system as compared with previously studied
electron-glasses. It is observed that the common ingredient in all the systems
that exhibit electron-glass effects is high carrier-concentration suggesting
that their localized sites may be multi-occupied even when deep into the
insulating regime. That lightly-doped semiconductors do not show intrinsic
electron-glass effects is consistent with this empirical observation. The
connection between the memory-dip and the Coulomb-gap is discussed in light of
these findings.
