Interplay Between Electronic and Structural Properties of Epitaxial and Amorphous Ge-Rich GST Alloys and Heterostructures

The impressive amount of data and information generated by modern electronic systems still requires large numbers of fast, cheap, and power-efficient embedded non-volatile memories and processing devices. Modern cars represent an important example of modern electronic system where sensors monitor every aspect of the vehicle as well as the street environment, resulting in a huge amount of collected and analyzed data. In this context, Phase change materials (PCM) based on chalcogenide alloys are a valid technology for the realization of non-volatile memories and are currently attracting much interest also as suitable materials for in-memory computing applications, where a reproducible set of multilevel resistance values is required [1]. Among the PCM most technologically relevant, the ternary (GeTe)_m(Sb₂Te₃)_n alloys (GST) are the most widely used, commonly in the composition Ge₂Sb₂Te₂(GST 225). Due to its great performances in terms of crystallization speed GST225 is in fact the prototype alloy chosen for most practical uses. However, in view of neuromorphic applications aiming at the emulation of synaptic behavior, improved material properties such as wide programming windows, low resistance drift and, particularly for automotive applications, high crystallization temperature T_x are still mandatory. For this reason, thermally stable PCM have been recently investigated [2], among which Ge-rich Ge-Sb-Te alloys (GGST) were found to be promising candidates for the realization of non-volatile memories working at high temperatures. Furthermore, the combination of different PCM is a valid strategy to tailor the material properties for the realization of embedded devices with optimized performances. Here, we present a photoemission study of amorphous GGST-based bilayer heterostructures grown by Physical Vapor Deposition (PVD), and epitaxial GGST samples grown by Molecular Beam Epitaxy (MBE). Sb₂Te₃/GGST and GST225/GGST heterostructures are obtained by successive partial depositions of amorphous GGST of increasing thickness by co-evaporation in ultra-high vacuum of the constitutive elements from solid source Knudsen cells. The evolution of the electronic properties of the heterostructures are studied <i>in-situ</i> during the interface formation by a combination of X-ray and Ultraviolet photoemission spectroscopies (XPS and UPS) and <i>ex-situ</i> X-ray diffraction (XRD) and Raman spectroscopy. Information on composition and intermixing of the alloys across the heterostructures and the final stable composition of thick GGST layers are obtained and discussed. Valence bands measured by UPS are compared to the theoretical density of states calculated by DFT. Also, the effects of thermal annealing at increasing temperatures are discussed with particular focus on the role of the interface on the crystallization of the GGST second layer.<br/>Molecular Beam Epitaxial GGST alloys along the pseudo-binary line are also characterized by XPS and UPS and compared to a standard GST225 sample. Results are interpreted based on the current structural models of trigonal GST and their evolution after subsequent annealing steps is discussed.<br/><br/>[1] A. Sebastian et al., <i>Nat. Nanotech.</i> 15, 529 (2020)<br/>[2] C. Chèze et al. Nanomaterials, 12, 1007 (2022)