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Title: Optical and electronic properties of defects and dopants in oxide semiconductors
Author: Vasheghani Farahani, Sepehr
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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Interest in semiconductor materials has continually grown over the past 60 years due to their potential use in electronic and optoelectronic device structures. Oxide semiconductors are a particular class of materials that also combine conductivity with optical transparency, properties not usually found in the same material. These transparent conducting oxides (TCOs) have been among the first oxide materials to benefit from the availability of improved epitaxial growth techniques, although perovskite oxides and heterostructures have also proved to be opening a new era of high mobility structures based on oxide materials. Optical and electronic properties of binary oxides, specifically, high quality CdO and SnO2 epi- taxial films have been investigated in this thesis. The main band structure quantities, the band gap and band edge effective mass of CdO has long been a subject of controversy due to the degeneracy of this material. The lowest carrier concentration for an as-grown CdO film is 1-2×1019 cm−3. This brings about further difficulties in determining the optoelectronic properties due to conduction band filling and many body effects. The effective mass value is of particular importance in carrier mobility studies. Simulation and analysis of data collected from Hall effect, mid- and near-infrared reflectance measurements together with optical absorption spectroscopy enabled the band gap and band edge effective mass values to be determined at room temperature. Variations of the band gap, band edge effective mass, high frequency dielectric constant and the Fermi level with temperature and carrier concentration, and taking into ac- count the non-parabolicity of the conduction band, the Burstein-Moss shift and band gap renormalization, revealed the 0 K band gap and band edge effective mass values of 2.31 eV and 0.266m0 at the limit of zero carrier concentration in CdO. With the emergence of sophisticated growth techniques (MBE), high quality growth has become a key property in semiconductor research as it enables further investigation into the intrinsic characteristics of these materials. Carrier mobilities in high quality SnO2(101) films grown on r-plane sapphire by molecular beam epitaxy were studied. Transmission electron microscopy revealed a high density of dislocations at the interface due to the large lattice mismatch of -11.3%, along the < 101 > direction, between the films and the substrate, with an exponential decrease towards the surface of the films. Carrier mobility modelling proved to be impossible if a constant density of threading dislocations was assumed, however, by introducing a layer-by-layer model for the simulation of the mobility as a function of carrier concentration, the donor nature of dislocations in epitaxial SnO2 films was revealed. The deformation potential produced by the presence of these defects has been shown to be the dominant scattering mechanism for carrier concentrations above the Mott transition level of SnO2. Finally, the surface electronic structure of antimony-doped SnO2 films has been studied by the Hall effect, infrared reflectance, X-ray photoemission spectroscopy and electrochemical capacitance-voltage measurements. The bulk Fermi level was determined by carrier statistics calculations and used to obtain the degree of surface band bending. Modelling the surface energy bands through the capacitance-voltage spectra, revealed that SnO2 has downward band bending and surface electron accumulation. The respective variations were attained as a function of depth and composition of the samples.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics