Coexistence of Electron-Glass Phase and Persistent Photoconductivity in GeSbTe Compounds

It is demonstrated that persistent-photoconductivity (PPC), well-studied in lightly-doped semiconductors, is observable in GeSbTe compounds using infrared excitation at cryogenic temperatures. The low level of energy-flux necessary to induce an appreciable effect seems surprising given the high carrier-concentration n of these ternary alloys. On the other hand, their high density of carriers makes GeSbTe films favorable candidates for exhibiting intrinsic electron-glass effects with long relaxation times. These are indeed observed in GeSbTe thin-films that are Anderson-localized. In particular, a memory-dip is observed in samples with sheet resistances larger than app. 100 kOhms at T=4K with similar characteristics as in other systems that exhibit intrinsic electron-glass effects. Persistent-photoconductivity however is observable in GeSbTe films even for sheet resistances of the order of 1 kOhm, well below the range of disorder required for observing electron-glass effects. These two non-equilibrium phenomena, PPC and electron-glass, are shown to be of different nature in terms of other aspects as well. In particular, their relaxation dynamics is qualitatively different; the excess conductance dG/G associated with PPC decays with time as a stretched exponential whereas a logarithmic relaxation law characterizes dG(t) of all electron-glasses studied to date. Surprisingly, the magnitude of the memory-dip is enhanced when the system is in the PPC state. This counter-intuitive result may be related to the compositional disorder in these materials extending over mesoscopic scales. Evidence in support of this scenario is presented and discussed.