Wafer-scale growth of highly textured TiTe2/Ge2Sb2Te5 heterostructure thin films

Phase-change materials (PCM) are leading candidates for memory and neuromorphic computing applications. By alternately growing PCM and confinement nanolayers, phase-change heterostructures (PCHs), in particular, TiTe2/Sb2Te3, have been developed to improve the programming consistency of memory devices. However, the crystallization temperature of Sb2Te3 is too low for practical use in non-volatile devices. In this work, we report the fabrication of highly-textured TiTe2/Ge2Sb2Te5 (GST) heterostructure thin films, leveraging the higher crystallization temperature of GST. We deposit the PCH on standard silicon substrates using GST as the seed layer, and determine that the minimum GST thickness required for epitaxial-like growth of PCH thin films is approximately 2 nm. Through atomic-scale structural characterization, we show that the resulting heterostructures consist of atomically flat GST and TiTe2 nanolayers with sharp interfaces. Although we apply annealing temperatures up to 400 C, obtaining the ordered hexagonal phase of GST remains challenging. This is because the formation of hexagonal GST requires a continuous vacancy ordering process, but the limited length scale in the vertical direction and the presence of TiTe2 nanolayers hinder vacancy migration. Finally, we outline the key steps for wafer-scale synthesis of TiTe2/GST and other PCHs.