Crystal structure assessment of nanostructured GeSbTe-based Phase Change Materials by Advanced Transmission Electron Microscopy techniques

Further improvement of phase change memory devices based on Ge-Sb-Te alloys imposes the reduction of the active cell dimensions to the nanoscale. Along with technological implementation, a deeper knowledge of the structure of the phases involved in the transition is an essential requirement for a successful downscale. Moreover, it has been demonstrated that the GeSbTe-based compounds, when nanostructured, can present a crystal structure different from bulk, and/or from expected, as it often happens in other material systems. In this work, we identify each crystal structure and its stacking sequence, by combining the direct observation by High Angle Annular Dark Field imaging and proper simulations. The High Angle Annular Dark Field Scanning TEM (HAADF- STEM) imaging technique is used to determine crystallographic structures as it is free from delocalization phenomena and predicts the chemical composition with very high sensitivity. This is due to the detector geometry allowing an incoherent image formation and a direct relationship between the intensity at each atomic column and its mean square atomic number (Z). Z contrast simulation has been performed by using STEM_CELL software[1], in the framework of the linear STEM image approximation [2]. We assessed an unpredicted stacking in Ge1Sb2Te4 and Ge2Sb2Te5 nanowires, with random mixed occupation of the Ge and Te atomic sites [3], and we identified a new polymorph in Sb2Te3 nanowires that can only be stabilized at the nanoscale [4]. 1.Grillo V, http: tem-s3.nano.cnr.it 2.Kirkland E, Adv Comp in El Microscopy, Springer (2010) 3.Rotunno E, Lazzarini, L.; Longo, M.; Grillo, V. Nanoscale 5, 1557 (2013) 4.Rotunno E, Longo M, Wiemer C, Fallica R, Campi D, Bernasconi M, Lupini AR, Pennycook SJ, Lazzarini L, submitted