Characterization of In2Se3 Crystalline Phase Changes as the Basis for Multilevel Phase Change Memory

Layered materials such as In2Se3 and MoTe2 have multiple crystal phases with differing electrical properties and can achieve reversible phase changes between them. Compared to transitions between amorphous and crystalline phase in conventional phase change memories (PCM), changes between crystalline phases are expected to have faster and low energy write operations for PCM even compared to interfacial PCM based Sb2Te3-GeTe superlattices due to the lower entropy of the crystalline-crystalline phase change. In2Se3 PCM have been demonstrated utilizing β and γ as the low and high resistance state respectively, however multilevel switching has not been extensively explored in part due to the stability of the crystalline phases at room temperature (RT).<br/><br/>In this work, we demonstrate the persistence of β phase In2Se3 in bulk phase at RT, previously only seen in single-crystal thin films and polycrystalline powders after annealing in an inert environment. Scanning tunneling microscopy (STM) is used to visualize the different phases as well as to measure the band gap with scanning tunneling spectroscopy (STS) after annealing at different temperatures and cooling to RT. After annealing α In2Se3 in UHV (&lt;10^-10 mbar) at 300C for 1 hour, β was found as confirmed by the shift in band gap from 1.27 eV to 0.52 eV. We also report in-situ Raman spectroscopy taken during heating and cooling of monolayer and effective bulk films of In2Se3 in both inert atmosphere and in air. We show evidence that upon bulk In2Se3 cooling to RT after heating in an inert atmosphere a mixture α and β phases results as compared to annealing in air where β reverts to α upon cooling.