ETD Collection

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    Correlation between elastic and thermal properties of chalcogenide memories by surface brillouin scattering
    (2019) Baloi, Mmapula
    Phase 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.
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    Surface brillouin scattering study of radio frequency sputtered hard thin films
    (2018) Kuria, Jonah Muiru
    Surface Brillouin Scattering (SBS) is used to investigate the elastic properties of NbN, TaN, and ZrN thin films deposited on Si substrates using rf magnetron sputtering. Influence of sputter power and hence the ad-atom energy on the elastic properties of NbN has been established for the films deposited at sputter powers ranging from 75 W - 250 W. TaN and ZrN thin films have been deposited at sputter powers of 150 W and 200 W respectively. For each sputter power, a sample set of eight films of thickness ranging from 50 nm - 800 nm has been used for the investigations. From the measured SBS spectra, the SAW phase velocities were calculated and used to obtain the velocity dispersion curves for all the sample sets. The surface elastodynamic Green’s technique was used to simulate Brillouin spectra of the films. Theoretical SAW phase velocities and hence the velocity dispersion curves have been obtained from the simulations. In NbN and TaN films, the dominant RSAW is observed in all the sample sets with higher order peaks (Sezawa modes) emerging from film thicknesses of 150 nm above. For the ZrN thin films on Si, no guided modes are observed. Increasing film thickness for NbN and TaN leads to a stiffening of the acoustic modes. Elastic constants for the NbN films and TaN films have been determined from the velocity dispersion modes. A least square fitting procedure has been used to optimise the elastic constants obtained from the Green’s function approach. The uncertainties of the elastic constants have also been calculated from the same procedure. Picosecond ultrasonic technique has been used as a complementary technique for the extraction of elastic constants of ZrN films.
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    Determination of elastic constants of transition metal oxide based thin films using surface brillouin scattering
    (2016-09-19) Ayele, Fekadu Hailu
    Bismuth ferrite BiFeO3 is a transition metal oxide that exhibits both antiferromagnetic and ferroelectric orderings and is termed a magnetoelectric multiferroic. These functional properties make it crucial for applications in various nanoelectronic devices and sensors. However, the integration of BiFeO3 in devices requires the scaling down of bulk BiFeO3 to nano dimensional length scales in thin lm format. For this purpose, the elements of the elastic constant tensor of BiF eO3 thin lms are requisite, especially in multilayered or single layer-on-substrate device con gurations. It is thus essential that mechanical properties of BiFeO3 thin lms be established due to their size and growth mode dependence. Therefore, the study aims to determine the propagation of the surface acoustic waves and the elastic constants of BiFeO3 BFO thin lms in order to tailor the mechanical properties for device applications. In this approach the e ect of morphology and microstructure on the elastic constants has been investigated.