Determination of the atomic stacking sequence of Ge-Sb-Te nanowires by HAADF STEM
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Determination of the atomic stacking sequence of Ge-Sb-Te nanowires by HAADF STEM Laura Lazzarini1, Enzo Rotunno1, Vincenzo Grillo2, 1, Massimo Longo3 1
CNR-IMEM, Parco area delle Scienze 37/A, 43124 Parma, Italy. CNR-Center S3 Nano, via Campi 213/A, 41125 Modena, Italy. 3 CNR-IMM, MDM Lab, via C. Olivetti 2, 20864 Agrate Brianza, Italy. 2
ABSTRACT The reduction of the active cell size to the nanoscale is crucial for the improvement of the phase change memory devices (PCM) based on Ge-Sb-Te (GST) alloys. The self-assembly of Au catalyzed Ge1Sb2Te4 (GST-124) nanowires (NWs) has been achieved by metal organic chemical vapor deposition. The atomic arrangement of the NWs has been investigated and the stacking sequence has been identified, by combining the direct observation by High Angle Annular Dark Field (HAADF) imaging and simulations. It has been assessed that Ge and Sb atoms can randomly occupy the same sites in the crystal lattice, despite the adverse predictions of the theoretical models elaborated for the bulk material.
INTRODUCTION Ternary compounds based on the GST alloy are among the best active materials for the PCM devices, that rely on the fast and reversible phase transition between the amorphous and crystalline state, induced by suitable current pulses [1]. The necessity to increase the storage capacity and to cut the power consumption and costs, demands the device downscaling. In this respect, a promising option is to realize PCM nanocells with GST NWs, that is the topic of a significant amount of papers [2-7]. A successful downscale is based on the understanding of the phase transition mechanism as well, and in turn, on the knowledge of the structure of the phases involved. To this aim, many studies propose several different atomic arrangements either for the amorphous [8-10] or crystalline phases of the possible GST compounds. The bulk GST compounds exhibit two crystalline polymorphs, that very easily transform one into the other [11]: a metastable phase with rock-salt structure and a stable ground state phase at a slightly lower energy, having hexagonal/rhombohedral structure, P-3m1 or R-3m. However, while the space groups are assessed, the atomic arrangement of bulk crystalline GST compounds, in particular the determination of the Ge and Sb atom positions, is still debated in the literature [12, 13], due to the large number of atoms in the unit cell. Moreover, while the relative stability of the many proposed structures are well discussed in the literature for the bulk material [14,15], no experimental and theoretical data are available about the NWs. In this work, we study the atomic arrangement of the GST-124 compound by the comparison of HAADF-STEM images and proper simulations, that allows to identify, for the first time, the exact stacking sequence in each NW.
EXPERIMENTAL DETAILS The GST NWs analysed in this work were self-assembled by Metal Organic Chemical Vapour Deposition (MOCVD), via Vapour-Liquid-Solid (VLS) mechanism assisted by Au catalyst nano-islands, thermally evaporated onto 4
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