Synthesis and screening of phase change chalcogenide thin film materials for data storage

A combinatorial synthetic methodology based on evaporation sources in ultra-high vacuum has been used to directly synthesise compositional gradient thin film libraries of the amorphous phases of GeSbTe alloys at room temperature over a wide compositional range. An optical screen is described which allows the rapid parallel mapping of the amorphous to crystalline phase transition temperature, and the optical contrast associated with the phase change, on such libraries. The results are shown to be consistent with the literature for compositions where published data are available along the Sb2Te3-GeTe tie line. The results reveal a minimum in the crystallisation temperature along the Sb2Te3-Ge2Te3 tie line, and the method is able to resolve subsequent cubic to hexagonal phase transitions in the GST crystalline phase. HT-XRD has been used to map the phases at sequentially higher temperatures and the results are reconciled with the literature and trends in crystallisation temperatures. The results clearly delineate compositions which crystallise to pure GST phases, and those which co-crystallise Te. High throughput measurement of the resistivity of the amorphous and crystalline phases has allowed the compositional and structural correlation of the resistivity contrast associated with the amorphous to crystalline transition which range from 5 to 8 orders of magnitude for the compositions investigated. The results are discussed in terms of the compromises in the selection of these materials for phase change memory applications, and the potential for further exploration through more detailed secondary screening of doped GST or similar classes of phase change materials designed for the demands of future memory devices