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R&D: In Situ Atomic-Scale Observation of Transformation from Disordered to Ordered Layered Structures in Ge-Sb-Te Phase Change Memory Thin Films

Study provides new insights to understand phase transition mechanisms and to optimize microstructure in thin GST layers.

AMI: Acta Materialia has published an article written by Andriy Lotnyk, Ningbo University – Laboratory of Infrared Materials and Devices, Torben Dankwort, Kiel University, Marion Behrens, Leibniz Institute of Surface Engineering (IOM), Lennart Voß, Kiel University, Sonja Cremer, Leibniz Institute of Surface Engineering (IOM), and Lorenz Kienle, Kiel University.

Abstract: Ge-Sb-Te (GST) thin films are emerging materials for various non-volatile memory applications. The compounds contain a huge number of vacancies that play important roles in structural and electronic transitions. Control of disorder and understanding the structural transformations in GST thin films are of great importance for the design and reliability of phase change memory devices. In this work, in situ heating atomic-resolution transmission electron microscopy is used to observe structural transformations from vacancy disordered (fcc) to layered ordered (vacancy-ordered (vo)) and van der Waals-bonded trigonal (vdW t)) structures. Starting from 160°C, randomly distributed vacancies in fcc grains gradually accumulate into vacancy layers. Further heating to 220°C and 240°C results in the formation of vdW t-domains within the vo-GST grains and the formation of large <001>-oriented t-grains, in a good agreement with outcomes on in situ XRD heating of epitaxial vo-GST thin film. Moreover, vo and fcc GST structures can coexist with vdW t-structure in one grain. Full transformation to -oriented t-grains occurs at 280°C. Unlike other grains, the <001> t-grains show abnormal grain growth behaviour, while the electron beam irradiation accelerates the growth. Detailed characterizations of vo-grains reveal transient structures that indicate shear transformation mechanism and different layered defects as well as chemical changes during the transformations. Overall, this study provides new insights to understand the phase transition mechanisms and to optimize the microstructure in thin GST layers.