ETD Collection
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Item Correlation between elastic and thermal properties of chalcogenide memories by surface brillouin scattering(2019) Baloi, MmapulaPhase change materials based on Ge-Sb-Te are of technological importance due to their successful commercial application for optical recording. These alloys are also candidates for emerging phase change random access memory (PRAM) devices, which aims at bridging the memory gap as found in modern Von Neumann’s computer architecture. However, setbacks such as the high reset programming current and the amorphous resistance drift phenomenon hinder the successful implementation of this technology. Thus said, this study aims to understand the origin of thermal management in PRAM devices as associated with these problems, from a phonon point of view. Two technologically important chalcogenide alloys, Ge2Sb2Te5 and GeTe were studied using various material characterization techniques. The main aim of this study was to find the correlation between the elastic properties and the lattice thermal conductivity of the two alloys by using surface Brillouin scattering (SBS) as the main investigative tool. Thin films were prepared using radio frequency (RF) magnetron sputtering at room temperature. As-deposited, the films have a disordered structure. Crystallization was induced by thermal annealing in a furnace in an Argon atmosphere at various temperatures. The film’s thickness and mass density were scrutinized by X-ray reflectivity (XRR). Thickness reduction associated with density increase upon structural transformation was evident. This was one of the expected characteristics of PCMs. The Rutherford Backscattering spectrometry (RBS) probe of the tandem linear accelerator at iThemba LABS (Gauteng), was used to determine the atomic density and chemical composition, by irradiating the films with 3.6 MeV He2+ particles. Energy dispersive X-ray spectroscopy (EDS) coupled with field emission scanning electron microscopy (FESEM) was used as a complementary technique to RBS for verifying the chemical composition. The crystal structures of the annealed films were confirmed by grazing incidence X-ray diffraction (GIXRD). The optical and acoustic modes of the two materials were studied by micro-Raman spectroscopy and SBS. The optical vibration modes provided additional information about the chemical bonding present in different structural phases. SBS measurements allow one to derive phonon velocity dispersion curves which were fitted with the surface elastodynamic Green’s function for extrac tion of two independent elastic constants, C11 and C44, respectively. Subsequently, other engineering moduli were determined from the elastic constants, C11 and C44. In general, the results suggest progressive elastic stiffening between the amorphous and crystalline phases of Ge2Sb2Te5 and GeTe. This was also supported by the values of the longitudinal and transverse velocities. In general, both Ge2Sb2Te5 and GeTe exhibit low thermal conductivities in all structural phases. The low thermal conductivity of a−Ge2Sb2Te5 encourages good thermal management of PRAM devices during programming. The higher lattice thermal conductivity of a−GeTe compared to c−GeTe, could be the reason for its instability and hence the drift behaviour observed in PRAM applications. This is not good for data retention and the cyclability of the device. Thus new ways of reducing the lattice thermal conductivity must be employed and further studies on these properties must be done to provide more insight.Item Numerical simulation of structural, electronic and optical properties of transition metal chalcogenides(2017) Rugut, Elkana KipkogeiIntensive study on structural, electronic and optical properties of bulk transition metal dichalcogenides and dipnictogenides (MX2; where M = V, Nb and X = S, Se, Te, P) was undertaken. A relative stability test was done to determine the most stable ground state configuration via calculation of total ground state energy and volume which was fitted to the third order Birch-Murnaghan equation of state to extract lattice parameters. Cohesive energies of the above mentioned MX2 compounds and their elemental solids were then computed from which formation energies were acquired based on their respective equations of reaction between reactants and product. Its significance was to aid in determining if a material is energetically stable. Elastic constants were predicted from which mechanical properties i.e bulk, Young’s and shear moduli and consequently Poisson’s ratio were resolved by feeding the stiffness matrix onto online elastic tensor analysis tool which facilitated verification of their mechanical stability based on the well-known Born stability conditions which varies from one crystal system to another, at a later stage phonon dispersion curves were plotted after performing phonon calculation based on phonopy code to verify if the materials of concern are dynamically stable. After a material had fulfilled all the above stability tests, its structural study was initiated using various functionals. Functional that described best the structural properties of each individual compound considered was then applied in exploring its electronic and optical properties whose motivation was to find out the most stable phase as well as gauge if these materials could be used in various fields that suits their mechanical and optical properties. Furthermore, from carefully calculated optical spectra, plasma frequencies were analyzed which indicated the possibility of applying a material in plasmonic related fields. In addition to above, other factors to be considered when selecting a given electrode material that are crucial for optoelectronics are good chemical and thermal stabilities, high transparency and excellent conductivity.Item Computational study of chalcogenide based solar energy materials(2016) Dongho Nguimdo, Guy MoiseAmongst the major technological challenges of the twenty rst century is the harvesting of renewable energy sources. We studied the solar cell performance of the ternary compounds AgAlX2 (X = S, Se and Te) and AgInS2 as promising materials for meeting this challenge. Structural, electronic and optical properties of the compounds were investigated by means of the density functional theory and many body perturbation theory. Using cohesive energy and enthalpy, we found that among six potential phases of AgAlX2 and AgInS2, the chalcopyrite and the orthorhombic structures were very competitive as zero pressure phases. We predicted a low pressure-induced phase transition from the chalcopyrite phase to a rhombohedral phase. For the chalcopyrite phase, we found that the tetragonal distortion and anion displacement were the cause of the crystal eld splitting. The bandgaps from the general gradient approximation PBEsol were underestimated when compared to experiment and accurate bandgaps were obtained from the hybrid functioanl HSE06, the meta-general gradient approximation MBJ and GW approximation. Optical absorption from the Bethe-Selpeter equation indicated the presence of bound exciton in AgAlX2. We estimated the solar cell performance of the compounds using the Shockley and Queisser model and the spectroscopy limited maximum e ciency approach. We found that apart from AgAlS2, the estimated theoretical e ciency of the other compounds was greater that 13 %.