Nonequilibrium transport and Electron-Glass effects in thin GexTe films

We report on results of nonequilibrium transport measurements made on thin films of germanium-telluride (Ge_xTe) at cryogenic temperatures. Owing to a rather large deviation from stoichiometry (app. 10% of Ge vacancies), these films exhibit p-type conductivity with carrier-concentration N>10^20cm^(-3) and can be made either in the diffusive or strongly-localized regime by a judicious choice of preparation and post-treatment conditions. In both regimes the system shows persistent photoconductivity following excitation by a brief exposure to infrared radiation. Persistent photoconductivity is also observed in GexTe samples alloyed with Mn. However, in both Ge_xTe and GeMn_xTe_y the effect is much weaker than that observable in GeSb_xTe_y alloys suggesting that antimony plays an important role in the phenomenon. Structural studies of these films reveal an unusual degree of texture that is rarely realized in strongly-disordered systems with high carrier-concentrations. Anderson-localized samples of Ge_xTe exhibit non-ergodic transport which are characteristic of intrinsic electron-glasses, including a well developed memory-dip and slow relaxation of the excess conductance created in the excited state. These results support the conjecture that electron-glass effects with inherently long relaxation times is a generic property of all Anderson-localized systems with large carrier-concentration.