Research master thesis: Ab initio study of structure-bonding relation in GeSexTe1–x phase-change memory materials

Abstract

Phase-change materials (PCMs) have a unique property to undergo quick and reversible transformation between two phases – high resistivity amorphous and low resistivity crystalline phase. The pronounced electrical contrast of phases is used in phase change random access memories (PCRAMs), a promising technology for next-generation non-volatile memories. In PCRAMs, the information is encoded in the electronic structure – the amorphous and crystalline phases correspond to logical states 0 and 1, respectively. An important requirement for good device performance is the stability of the amorphous phase against a spontaneous crystallisation leading to data loss. Therefore, the PCM of choice must have a sufficiently high crystallisation temperature, Tc. It has been demonstrated that the Tc of the prototypical PCM GeTe can be increased by Se doping. Together with other enhanced properties, this makes GeSexTe1-x alloy family a prospective candidate for embedded memory applications. These alloys belong to the GeTe-GeSe pseudo-binary line, and since GeSe itself is not a PCM, a question arises: which Te/Se atomic ratio stands in the vicinity of PC and non-PC properties? Recent experiments show that only 8% at. of Te in the GeSe lattice gives rise to high electrical and optical contrast between the phases. The focus of this Master's thesis is a theoretical study of this unprecedented result. For this, Density Functional Theory (DFT) and Molecular Dynamics (MD) simulation methods are used to investigate the structure and properties of the GeTe-GeSe system. We demonstrate that the PC properties of GeSexTe1-x alloys arise from a unique bonding mechanism directly linked to the R3m space group of GeTe. Next, we show that the short-range order of the amorphous phase of all alloys is very similar to that of crystalline GeSe, shedding light on the origin of optical contrast between the two phases. In the end, we try to establish a mechanism of how a small amount of Te atoms in the lattice impose a change of space group of GeSe, turning it into a phase-change material.