Real-space imaging of atomic arrangement and vacancy layers ordering in laser crystallised Ge₂Sb₂Te₅ phase change thin films

In this work, the local structure of metastable Ge₂Sb₂Te₅ (GST) phase change thin films crystallised by laser irradiation of amorphous GST films is studied by state-of-the-art aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM). By analysing simulated and experimental atomic-resolution Cs-corrected STEM images, a structure model for metastable GST is proposed. The GST lattice is described by a distorted rock-salt like structure with an ordered Te sublattice at the 4(a) site and a disordered sublattice of Ge, Sb and vacancies at the 4(b) site, where only a distorted octahedral atomic arrangement of Ge and Sb atoms exists without layered ordering of intrinsic vacancies. Additionally, no evidence for the presence of either tetrahedral Ge atoms in the GST lattice or an amorphous component at the grain boundaries is found. Moreover, a formation of vacancy layers in metastable GST in <111> direction under the influence of focused electron beam irradiation is observed. These vacancy layers vanish during repeated scanning of the electron beam over these defects. The gained outcomes of this study shed new insight into understanding the atomic arrangement and phase change mechanism in GST thin films as well as the control of disorder in phase change materials.