Browsing by Subject "Electronic transport"

Now showing 1 - 3 of 3
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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, Magnetic, and Dielectric Properties of Titanate Films and Heterostructures
    (2021-01) Yue, Jin
    Titanates are the earliest discovered perovskite oxides and have been studied extensively ever since. Owing to their versatile properties (ferroelectricity, superconductivity, ferromagnetism, just to name a few), perovskite titanates are not only scientifically interesting, but also technologically attractive in many aspects such as catalysis, memory, computing, energy storage, power harvesting, etc. Further excitement has been stimulated by the two-dimensional electron gas (2DEG) discovered at the SrTiO3/LaAlO3 interface, where both components are insulators in bulk.In this thesis work, we present a detailed study of electronic, magnetic, and dielectric properties of perovskite titanate thin films and heterostructures grown by the hybrid molecular beam epitaxy approach. We first start with a systematic study of the prototypical perovskite SrTiO3 (STO) with carrier densities ranging from 1017 to 1020 cm-3. Detailed transport measurements reveal that the electronic and structural instabilities of STO are intimately coupled, and, the superconducting dome of STO thin films is found to be dramatically different from that in bulk. Dielectric measurements of homoepitaxial STO thin films also manifest the strong influence of structural transitions. Then we turn to rare earth titanates, a model system for the study of strong electron-electron interactions. Two members with ferromagnetic ground states, YTiO3 and DyTiO3, are picked, and their growth, band structure, and magnetic properties are investigated. For YTiO3, the transport is found to be dominated by small hole polaron hopping and the Mott Hubbard gap is determined to be around 1.5 eV. A ferrimagnetic ground state has been identified in DyTiO3 films, and the magnetic properties turn out to be extremely sensitive to the cation stoichiometry. The final part of this thesis work is focused on heterostructures based on perovskite titanates. A hopping process is found to be responsible for the transport behavior of SrTiO3/Nd1-xTiO3/SrTiO3 heterostructures, and the detailed hopping mechanism varies according to the Nd vacancy concentration. We also demonstrate SrTiO3-based transistor devices and identify potential routes to improve the performance through dielectric analysis.
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    Synthesis, Characterization and Electronic Transport Properties of Thin Film Iron Pyrite for Photovoltaic Applications
    (2015-08) Zhang, Xin
    The pyrite form of FeS2 has long been recognized as an earth-abundant and non-toxic material with exceptional properties as a solar absorber for inexpensive photovoltaic devices. However, a significant research effort from the mid 1980’s achieved power conversion efficiencies of only less than 3 %. The reasons for such low efficiencies have not been fully elucidated yet, primarily because the electronic transport and doping mechanisms of pyrite are poorly understood. One classic example is well-known puzzle remaining in pyrite, where bulk single crystals are almost exclusively n-type based on Hall effect measurements, whereas polycrystalline thin films are typically deduced to be p-type, mostly from thermopower measurements. The fundamental reason(s) for this are not understood, and identifying the unintentional dopants in FeS2 remains an outstanding challenge. In this work we address, using ex situ sulfidation synthesis, this long-standing problem of understanding conduction mechanisms and doping in FeS2 films. This is done by systematically exploring the effects of film synthesis conditions on microstructure, surface morphology, chemical stoichiometry, electronic transport mechanisms, charge carrier mobility and charge density. More than a hundred of FeS2 thin films and synthetic crystals were probed in this study. In addition to conventional diffusive transport, hopping transport was also frequently observed in FeS2 thin films. This hopping transport was discovered to be caused by nanoscale inhomogeneity (e.g. nanoscale Fe or FeS clusters), which has been overlooked by the pyrite community until now. This hopping transport may explain the poor performance of some FeS2-based solar cells, since the carrier mobility and lifetime are significantly reduced in hopping. More importantly, accompanying the crossover from diffusive to hopping transport, we find significant suppression, and sign inversion from electron-like to hole-like, of Hall and themopower signals in FeS2 thin films. The results indicate that thin films with diffusive transport show n-type conduction, just like single crystals, which implies that the major n-type dopants may be the same for both FeS2 thin films and single crystals. As the transport crosses over to hopping, both Hall and thermopower measurements indicate sign inversions, which are not caused by real p-type doping, but are rather an artifact of hopping conduction. These findings provide the first potential resolution for the “doping puzzle” in FeS2, and emphasize that understanding the electronic transport mechanisms is mandatory for interpreting the sign of Hall and thermopower coefficients in FeS2. In the last part of this work, some preliminary results for identifying the unintentional dopant(s) in FeS2 are presented. The results suggest the major n-type dopants in FeS2 are unlikely to be metal impurities or oxygen. S vacancies are a genuine possibility however, although further study is still required to settle this issue. These findings answer several critical questions for understanding the electronic transport and doping mechanisms in pyrite FeS2 thin films. They also have important implications for FeS2 solar cell development, emphasizing the need for (a) nanoscale chemical homogeneity, (b) caution in interpreting carrier types and densities, and (c) doping control in pyrite FeS2 films.