In-Based PCM Heterostructures—Electronic Properties at the Interface and Confinement of Thin Sb Layers

<br/>In the last years the widespread of internet and the consequent rise of the so called “Internet of Things” led to a significative increase of interconnection of people with each other and especially between objects that surround us. Among the “things” that have benefited the most out of such revolution, modern cars are by far one of the most representative examples: packed with sensors that monitor every aspect of the vehicle as well as the street environment, they constantly collect and analyzed large amounts of data in order to grant the safest and most comfortable driving experience. To achieve this goal, having a performing, embeddable and highly scalable device for the storage and processing of the information is paramount. Among possible different solution for this task, Phase change Random access memories (PCRAM) are by far one of the most promising candidates since they provide non-volatile device characterized by all the above-mentioned properties and can implement in-memory computing for optimization of AI algorithm.<br/>However, current PCRAM benchmark active materials, Ge₂Sb₂Te₅ (GST 225) and other alloys lying on the (GeTe)_m(Sb₂Te₃)_n pseudo-binary tie line, fail to meet the strict requirements for the implementation in a vehicle. In fact, despite their outstanding performances in devices working in ambient conditions, GST crystallization temperature is too low for the realization of memory devices for automotive applications, for which high stability at temperatures as high as 160°C is mandatory. To address this issue, In-based PCMs, characterized by inherently high crystallization temperature, could be solid candidates for this goal. Our work focused on two alloys, In-Sb-Te (IST) and In-Ge-Te (IGT), both characterized by high transition temperature [1, 2], which we studied as single films and combined each other in a IST/IGT heterostructure. Amorphous samples were grown at room temperature by thermal evaporation of the constituting elements from ultrapure sources on Si(111)/SiO_x substrates by means of Knudsen cells in Ultra-High Vacuum conditions (UHV). Performing <i>in-situ</i> X-ray and Ultraviolet photoemission spectroscopy (XPS & UPS) we characterized the electronic properties, valence band (VB) and core levels, of the as-grown samples and managed to track composition variations across the IGT/IST interface. Valence band structures of the alloys will be discussed and compared with published density of states computed by <i>ab initio</i> molecular dynamics simulation based on DFT. Unexpectedly, the photoemission study of this heterostructure showed that no Sb diffuses through as-grown IGT, leading to the formation of sharp interface completely lacking antimony. This is particularly relevant especially because of the renewed interesting towards pure ultrathin Sb [3], a newfound promising PCM itself. In light of this, we have grown Sb/IGT heterostructures employing IGT as confinement material to study the crystallization of pure Sb and, by means of <i>ex-situ </i>X-ray diffraction, we characterized Sb/IGT heterostructures featuring Sb layers of different thicknesses to observe the effect of this parameter on the transition temperature.<br/><b>REFERENCES</b><br/>1. Saxena N.<i> et al</i>, Sci. Rep. 9, 19251 (2019)<br/>2. Morikawa T. <i>et al</i>, "Doped In-Ge-Te Phase Change Memory Featuring Stable Operation and Good Data Retention," 2007 IEEE International Electron Devices Meeting, 2007, pp. 307-310<br/>3. Salinga M. <i>et al, </i>Nature Mater. <b>17</b>, 681–685 (2018)