Browsing by Subject "Hopping"

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    Electronic transport in mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films
    (2013-05) Wienkes, Lee Raymond
    Interest in mixed-phase silicon thin film materials, composed of an amorphous semiconductor matrix in which nanocrystalline inclusions are embedded, stems in part from potential technological applications, including photovoltaic and thin film transistor technologies. Conventional mixed-phase silicon films are produced in a single plasma reactor, where the conditions of the plasma must be precisely tuned, limiting the ability to adjust the film and nanoparticle parameters independently. The films presented in this thesis are deposited using a novel dual-plasma co-deposition approach in which the nanoparticles are produced separately in an upstream reactor and then injected into a secondary reactor where an amorphous silicon film is being grown. The degree of crystallinity and grain sizes of the films are evaluated using Raman spectroscopy and X-ray diffraction respectively. I describe detailed electronic measurements which reveal three distinct conduction mechanisms in n-type doped mixed-phase amorphous/nanocrystalline silicon thin films over a range of nanocrystallite concentrations and temperatures, covering the transition from fully amorphous to ~30% nanocrystalline. As the temperature is varied from 470 to 10 K, we observe activated conduction, multiphonon hopping (MPH) and Mott variable range hopping (VRH) as the nanocrystal content is increased. The transition from MPH to Mott-VRH hopping around 100K is ascribed to the freeze out of the phonon modes. A conduction model involving the parallel contributions of these three distinct conduction mechanisms is shown to describe both the conductivity and the reduced activation energy data to a high accuracy. Additional support is provided by measurements of thermal equilibration effects and noise spectroscopy, both done above room temperature (>300 K). This thesis provides a clear link between measurement and theory in these complex materials.
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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.