Correlation between elastic and thermal properties of chalcogenide memories by surface brillouin scattering
Date
2019
Authors
Baloi, Mmapula
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Abstract
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.
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science.
June, 2019
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Citation
Baloi, Mmapula. (2019). Correlation between elastic and thermal properties of chalcogenide memories by surface brillouin scattering. University of the Witwatersrand, https://hdl.handle.net/10539/28073