Engineering Interface-Dependent Photoconductivity in Ge 2 Sb 2 Te 5 Nanoscale Devices.

Phase change materials are increasingly being explored for photonics applications, ranging from high resolution displays to artificial retinas. Surprisingly, our understanding of the underlying mechanism of light-matter interaction in these materials has been limited to photothermal crystallization, because of its relevance in applications such as re-writable optical discs. Here we report a photoconductivity study of nanoscale thin films of phase change materials. We identify strong photoconductive behaviour in phase change materials, which we show to be a complex interplay of three independent mechanisms: photoconductive, photo induced-crystallization and photo-induced-thermoelectric effects. We find these effects also congruously contribute to a substantial photovoltaic effect, even in notionally symmetric devices. Notably, we show that device engineering plays a decisive role in determining the dominant mechanism; the contribution of the photothermal effects to the extractable photocurrent can be reduced to < 0.4 % by varying the electrodes and device geometry. We then show that the contribution of these individual effects to the photoresponse is phase-dependent with the amorphous state being more photoactive than the crystalline state and that a reversible change occurs in the charge transport from thermionic to tunnelling during phase transformation. Finally, we demonstrate photodetectors with an order of magnitude tuneability in photodetection responsivity and bandwidth using these materials. Our results provide insight to the photo-physics of phase change materials and highlight their potential in future opto-electronics.