Gwak, Yunki (2009-08). Thermal Transport Measurement of Silicon-Germanium Nanowires. Master's Thesis. Thesis uri icon

abstract

  • Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional thermoelectric figure of merit, ZT=S^2 o T/k, where S ,o , k and T are Seebeck coefficient, electrical conductivity, thermal conductivity and the absolute temperature respectively. These physical properties are interdependent. Therefore, making materials with high ZT is a very challenging task. However, nanoscale materials can overcome some of these limitations. When the size of nanomaterials is comparable to wavelength and mean free path of energy carriers, especially phonons, size effect contributes to the thermal conductivity reduction without bringing about major changes in the electrical conductivity and the Seebeck coefficient. Therefore, the figure of merit ZT can be manipulated. For example, the thermal conductivities of several silicon nanowires were more than two orders of magnitude lower than that of bulk silicon values due to the enhanced boundary scattering. Among the nanoscale semiconductor materials, Silicon-Germanium(SiGe) alloy nanowire is a promising candidate for thermoelectric materials The thermal conductivities of SiGe core-shell nanowires with core diameters of 96nm, 129nm and 177nm were measured using a batch fabricated micro device in a temperature range of 40K-450K. SiGe nanowires used in the experiment were synthesized via the Vapour-Liquid-Solid (VLS) growth method. The thermal conductivity data was compared with thermal conductivity of Si and Ge nanowires. The data was compared with SiGe alloy thin film, bulk SiGe, Si/SixGe1-x superlattice nanowire, Si/Si0.7Ge0.3 superlattice thin film and also with the thermal conductivity of Si0.5Ge0.5 calculated using the Einstein model. The thermal conductivities of these SiGe alloy nanowires observed in this work are ~20 times lower than Si nanowires, ~10 times lower than Ge nanowires, ~3-4 times lower than Si/SixGe1-x superlattice thin film, Si/SixGe1-x superlattice nanowire and about 3 time lower than bulk SiGe alloy. The low values of thermal conductivity are majorly due to the effect of alloy scattering, due to increased boundary scattering as a result of nanoscale diameters, and the interface diffuse scattering by core-shell effect. The influence of core-shell effect, alloy scattering and boundary scattering effect in reducing the thermal conductivity of these nanowires opens up opportunities for tuning thermoelectric properties which can pave way to thermoelectric materials with high figures of merit in the future.
  • Thermal properties of one dimensional nanostructures are of interest for
    thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional
    thermoelectric figure of merit, ZT=S^2 o T/k, where S ,o , k and T are Seebeck
    coefficient, electrical conductivity, thermal conductivity and the absolute temperature
    respectively. These physical properties are interdependent. Therefore, making materials
    with high ZT is a very challenging task. However, nanoscale materials can overcome some
    of these limitations. When the size of nanomaterials is comparable to wavelength and mean
    free path of energy carriers, especially phonons, size effect contributes to the thermal
    conductivity reduction without bringing about major changes in the electrical conductivity
    and the Seebeck coefficient. Therefore, the figure of merit ZT can be manipulated. For
    example, the thermal conductivities of several silicon nanowires were more than two orders
    of magnitude lower than that of bulk silicon values due to the enhanced boundary scattering.
    Among the nanoscale semiconductor materials, Silicon-Germanium(SiGe) alloy
    nanowire is a promising candidate for thermoelectric materials The thermal conductivities
    of SiGe core-shell nanowires with core diameters of 96nm, 129nm and 177nm were
    measured using a batch fabricated micro device in a temperature range of 40K-450K. SiGe nanowires used in the experiment were synthesized via the Vapour-Liquid-Solid (VLS)
    growth method. The thermal conductivity data was compared with thermal conductivity of
    Si and Ge nanowires. The data was compared with SiGe alloy thin film, bulk SiGe,
    Si/SixGe1-x superlattice nanowire, Si/Si0.7Ge0.3 superlattice thin film and also with the
    thermal conductivity of Si0.5Ge0.5 calculated using the Einstein model. The thermal
    conductivities of these SiGe alloy nanowires observed in this work are ~20 times lower
    than Si nanowires, ~10 times lower than Ge nanowires, ~3-4 times lower than Si/SixGe1-x
    superlattice thin film, Si/SixGe1-x superlattice nanowire and about 3 time lower than bulk
    SiGe alloy. The low values of thermal conductivity are majorly due to the effect of alloy
    scattering, due to increased boundary scattering as a result of nanoscale diameters, and the
    interface diffuse scattering by core-shell effect. The influence of core-shell effect, alloy
    scattering and boundary scattering effect in reducing the thermal conductivity of these
    nanowires opens up opportunities for tuning thermoelectric properties which can pave way
    to thermoelectric materials with high figures of merit in the future.

publication date

  • August 2009