Intrinsic electron-glass effects in strongly localized Tl ²O3_-x 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 multioccupied 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.