Vasheghani Farahani, Sepehr (2013) Optical and electronic properties of defects and dopants in oxide semiconductors. PhD thesis, University of Warwick.

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Abstract

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.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Metal oxide semiconductors, Semiconductor doping, Semiconductors -- Defects, Oxides -- Optical properties, Oxides -- Electric properties
Official Date:	May 2013
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	McConville, C. F. (Chris F.)
Extent:	xii, 136 leaves : illustrations.
Language:	eng

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