Investigation on the elastic properties and crystallization dynamics of phase-change memory material

In recent years, phase-change materials have attracted much attention in the area of nonvolatile memory applications due to their advantages, such as the scaling properties, small energy consumption and fast writing speed. Phase-change materials depend on the properties difference between its crystalline and amorphous phase to realize the data storage. An important group of phase-change materials is based on the ternary alloy Ge-Sb-Te (GST). The phase transition involves a significant volume change resulting in an internal stress at the interface between the crystalline and amorphous phases, with this problem becoming even more important as the device structure is scaled down to increase the data storage density. The published data on the elastic properties of GST compounds is limited, and there are considerable disagreements among different experimental sources. In this research project, a full set of elastic properties of GST compounds have been calculated and analysed theoretically using first-principle density functional theory (DFT). In this work, we derived the elastic constants from the relation between the applied strain perturbation and the resulting stress change, and calculated the elastic modulus (i.e. bulk,Young’s and biaxial modulus) for polycrystalline congregate according to Voigt-Reuss-Hill approximation. It was found that with the addition of Sb2Te3 into the GST chemical composition, the trigonal and rocksalt structures of GST compounds become unstable with the reduction of elastic properties. In the fabrication and operation of phase-change random access memory (PCRAM) devices, the as-deposition and pre-annealing temperatures are important parameters that affect the amorphization and crystallization processes. Based on the ab initio molecular dynamics (AIMD) simulation, this research effort is devoted to the investigate the structural differences of amorphous models after the as-deposition and after the preannealing treatment. The cluster of four-fold tetrahedral-Ge chains acts like an obstacle to retard the crystallization dynamics, and higher as-deposition and pre-annealing temperatures can reduce the density of tetrahedral-Ge belonging to the Ge-Ge chains, as well as a reduction of average chain length. Thus, by controlling the asdeposition and pre-annealing temperatures, it is possible to speed up the crystallization process through a reduction of the crystallization onset time. In summary, this research project has added the missing information on the elastic properties of GST compounds, shedding light on the atomic mechanism of phase transition between the rocksalt and trigonal phases. Furthermore, it is shown that tuning the as-deposition and pre-annealing temperatures may improve the switching speed of PCRAM devices.Master of Engineering (MSE