Intrinsic electron-glass effects in strongly-localized thallium-oxide films

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.