Crystalline-Crystalline Phase Transformation in Two-Dimensional In2Se3 Thin Layers

We report, for the first time, the fabrication of single-crystal In2Se3 thin layers using mechanical exfoliation and studies of crystalline-crystalline (alpha -> beta) phase transformations as well as the corresponding changes of the electrical properties in these thin layers. Particularly, using electron microscopy and correlative in situ micro-Raman and electrical measurements, we show that, in contrast to bulk single crystals, the beta phase can persist in single-crystal thin layers at room temperature (RT). The single-crystal nature of the layers before and after the phase transition allows for unambiguous determination of changes in the electrical resistivity. Specifically, the beta phase has an electrical resistivity about 1-2 orders of magnitude lower than the alpha phase. Furthermore, we find that the temperature of the alpha -> beta phase transformation increases by as much as 130 K with the layer thickness decreasing from similar to 87 nm to similar to 4 nm. These single-crystal thin layers are ideal for studying the scaling behavior of the phase transformations and associated changes of the electrical properties. For these In2Se3 thin layers, the accessibility of the beta phase at RT, with distinct electrical properties than the a phase, provides the basis for multilevel phase-change memories in a single material system.