Local structure of [(GeTe)2/(Sb2Te3)m]n super-lattices by x-ray absorption spectroscopy

Abstract

Herein, the local structure of [(GeTe)2/(Sb2Te3)m]n chalcogenide super-lattices (SLs), which are at the basis of emerging interfacial Phase-Change Memory (iPCM), is studied by x-ray absoprtion spectroscopy at the Ge-K edge. The quantitative analysis of the first coordination shells reveals that the SLs possess a structure very similar to that of thin film of the canonical Ge2Sb2Te5 (GST225) phase-change alloy. By comparing experimental data with ab initio molecular dynamics simulations of the extended x-ray absorption fine structure spectra, we show that chemical disorder is mandatory in order to reproduce the experimental data in the full spectral range. As a result, we can unambiguously conclude that Ge/Sb intermixing resulting from inter-diffusion of the GeTe and Sb2Te3 layers within SLs is inherent to SLs and is not induced by sample preparation method nor by interaction with the electron beam of electron microscopes used in all the previous studies that were suggesting such a phenomenon. We further evidence that the short Ge-Te distance is the same in GeTe and GST225 films, as well as in SLs. The main difference is the impact of disorder in GST225 and SLs. Intermixing being definitively present in [(GeTe)2/(Sb2Te3)m]n SLs, this parameter must be considered in future models aiming at going further in the understanding and the development of iPCM technology. This seems mandatory in order to allow such technology to emerge in the near future on the non-volatile memory market.