Browsing by Subject "Thermopower"

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    Electronic transport in nanocrystalline germanium/hydrogenated amorphous silicon composite thin films
    (2015-02) Bodurtha, Kent Edward
    Recent interest in composite materials based on hydrogenated amorphous silicon (a-Si:H) stems in part from its potential for technical applications in thin film transistors and solar cells. Previous reports have shown promising results for films of a-Si:H with embedded silicon nanocrystals, with the goal of combining the low cost, large area benefits of hydrogenated amorphous silicon with the superior electronic characteristics of crystalline material. These materials are fabricated in a dual-chamber plasma-enhanced chemical vapor deposition system in which the nanocrystals are produced separately from the amorphous film, providing the flexibility to independently tune the growth parameters of each phase; however, electronic transport through these and other similar materials is not well understood. This thesis reports the synthesis and characterization of thin films composed of germanium nanocrystals embedded in a-Si:H. The results presented here describe detailed measurements of the conductivity, photoconductivity and thermopower which reveal a transition from conduction through the a-Si:H for samples with few germanium nanocrystals, to conduction through the nanocrystal phase as the germanium crystal fraction XGe is increased. These films display reduced photosensitivity as XGe is increased, but an unexpected increase in the dark conductivity is found in samples with XGe > 5% after long light exposures. Detailed studies of the conductivity temperature dependence in these samples exposes a subtle but consistent deviation from the standard Arrhenius expression; the same departure is found in samples of pure a-Si:H; a theoretical model is presented which accurately describes the actual conductivity temperature dependence.
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    Electronic transport properties of hydrogenated amorphous silicon-germanium thin films
    (2022-01) Stolik Valor, Lis
    Interest in amorphous semiconductors stems in part from their use in large-area thin-film applications, including photovoltaics, light-emitting diodes, thin film transistors, non-volatile memories and thermoelectrics. Furthermore, alloyed amorphous semiconductors have emerged as promising materials, as their optical bandgap can be easily engineered by controlling their chemical composition. Alloyed a-Si_{1-x}Ge_{x}:H thin film samples are fabricated in a dual-chamber plasma-enhanced chemical vapor deposition system, and a series of such films with Ge content raging from (0-100)% are obtained. The Ge content is determined through X-ray photoelectron spectroscopy and qualitatively corroborated through measurements of their Raman spectra. Measurements of their dark conductivity, photoconductivity, and thermopower reveal a dual-channel conduction through the dangling bond states. Alloys with concentrations of Ge below 20% exhibit anomalous hopping conduction, while the dark conductivity of alloys with higher Ge concentrations are best fit by a combination of anomalous hopping at high temperatures and power-law temperature dependence for the low to mid-ranges, characteristic of multi-phonon hopping transport. The samples' photoconductivies show evidence of high defect state densities in the mid-gap. Corresponding measurements of the thermopower find that conduction is n-type for the purely a-Si:H and a-Ge:H samples but that the Seebeck coefficient exhibits a transition from negative to positive values as a function of Ge content and temperature. A conduction model involving the parallel contributions of the two distinct conduction mechanisms is shown to describe both the conductivity and the thermopower data to a high degree of accuracy. The clear experimental evidence of hopping conduction reported here provides important information concerning the nature of electronic conduction in amorphous semiconductors, and suggests that the concept of a mobility edge, accepted for over four decades, may not be necessary to account for charge transport in certain amorphous semiconductors.