Ab initio investigation of disordered crystalline phase-change materials

Tuning the degree of disorder in chalcogenide phase change materials is vitally important for applications such as data storage and brain-like computing. The different degree of disorder between the amorphous phase and the crystalline phase of PCMs directly leads to a big contrast in electronic properties which can be used to store digital information. Under the external electric pulse, the prototypical PCM Ge-Sb-Te alloys rapidly crystallize to form a metastable rocksalt-like crystal. In this phase, a huge amount of stoichiometric vacancies is present in one of the two sublattices and the electronic states near the Fermi level are localized inside low-probability configurations consisting of vacancy clusters. Here, ab initio simulations and transport experiments are combined to extend these concepts from Ge-Sb-Te alloy to the binary Sb${}_2$Te${}_3$ compound. Then, we systematically investigate the structural stability and the electronic properties of the rocksalt phase of IVV${}_2$VI${}_4$ ternary compounds and V${}_2$VI${}_3$ binary compounds. We show that the structural stability of this phase is jointly determined mainly by the relative size of the cation atoms and the degree of sp${}^3$ hybridization. Through high-throughput calculations, nearly 50 stable alloys have been screened out. Furthermore, most of these compounds display Anderson localization. We show that, besides vacancy disorder, structural deviations from the perfect rocksalt lattice and difference in electronegativity of the constituent elements also affect the localization properties. Finally, we study the structure and electronic properties of realistic, large-scale models of GeSb${}_2$Te${}_4$ obtained by ab initio crystallization of the amorphous structure and subsequent annealing. Our simulations confirm that vacancy clusters are the main source of disorder affecting the tail of the valence band relevant to transport. Furthermore, they reveal that other type of extended and point defects lead to Anderson localization in the conduction band. These include antisite defects and Sb-rich regions, such as Sb chains. Besides providing a comprehensive description of disorder in GeSb${}_2$Te${}_4$, these findings are highly relevant to disordered rocksalt-like chalcogenides that exhibit n-type self-doping.