1,721,207 research outputs found
Optical and electronic properties of defects and dopants in oxide semiconductors
Full text linkInterest 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 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
Anticrossing semiconducting band gap in nominally semimetallic InAs/GaSb superlattices
No full textWhile (InAs)n/(GaSb)n (001) superlattices are semiconducting for nnc the InAs electron level eInAs is below the GaSb hole level hGaSb, so the system is converted to a nominal semimetal. At nonzero in-plane wave vectors (k|| [not equal] 0), however, the wave functions eInAs and hGaSb have the same symmetry, so they anticross. This opens up a "hybridization gap" at some k||=k*||. Using a pseudopotential plane-wave approach as well as a (pseudopotential fit) eight-band k·p approach, we predict the hybridization gap and its properties such as wave-function localization and out-of-plane dispersion. We find that recent model calculations underestimate this gap severely
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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