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 levels of energy flux necessary to induce an appreciable effect seems surprising given the high carrier concentration n of these ternary alloys (n>1020cm-3). 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 ≈105Ω at T≈4 K 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 103Ω, well below the range of disorder required for observing electron-glass effects. These two nonequilibrium 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 ΔG associated with PPC decays with time as a stretched exponential whereas a logarithmic relaxation law characterizes ΔG(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.