Thesis (Ph. D.)--University of South Florida, 2003. / Includes vita. Title from PDF of title page. Document formatted into pages; contains 204 pages. Includes bibliographical references.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Melkote, Shreyes; Committee Co-Chair: Danyluk, Steven; Committee Member: Graham, Samuel. Part of the SMARTech Electronic Thesis and Dissertation Collection.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

This dissertation has focused on the study of the ITO/organic heterojunction and the chemistries therein, it proposes appropriate strategies that enhance the interfacial physical and electronic properties for charge injection with application to organic thin-layer devices. We focused on four major aspects of this work: i) To characterize the ITO surface and chemistries that may be pertinent to interaction with adjacent organic layers in a device configuration. This developed a working model of surface and provided a foundation for modification strategies. Characterization of the electronic properties of the surface indicate less than 5% of the geometrical surface is responsible for the bulk of current flow while the rest is electrically inactive. ii) To determine the extent to which these chemistries are variable and propose circumstances where compositional changes can occur. It is shown that the surface chemistry of ITO is heterogeneous and possible very dynamic with respect to the surrounding environment. iii) To propose a strategy for modification of the interface. Modification of ITO surfaces by small molecules containing carboxylic acid functionalities is investigated. Enhancements in the electron transfer rate coefficient were realized after modification of the ITO electrode. The enhancements are found to stem from a light etching mechanism. Additionally, an elecro-catalytic effect was observed with some of the modifiers. iv) Apply these modifications to organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs). Enhancements seen in solution electrochemical experiments are indicative of the enhancements seen for solid state devices. Modifications resulted in substantially lower leakage currents (3 orders of magnitude in some cases) as well as nearly doubling the efficiency.An additional chapter describes the creation and characterization of electrochemically grown polymer nano-structures based on blazed angle diffraction gratings. The discussion details the micro-contact printing process and the electro-catalytic growth of the conductive polymers PANI and PEDOT to form diffraction grating structures in their own right. The resulting diffraction efficiency of these structures is shown to be sensitive to environmental conditions outlining possible uses as chemical sensors. This is demonstrated by utilizing these structures as working pH and potentiometric sensors based on the changing diffraction efficiency.

We have observed the electronic transitions within the ('4)T(,1g) ground state of Co('2+) in MgO by Raman spectroscopy. We identified the (GAMMA)(,6g) (--->) (GAMMA)(,8g)('1) transition at 305 cm('-1) and the (GAMMA)(,6g )(--->)(, )(GAMMA)(,8g)('2) transition at 930 cm('-1); the (GAMMA)(,6g )(--->)(, )(GAMMA)(,7g) transition is not seen and is lost in the two-phonon band. We also observed an impurity induced vibration mode at 280 cm('-1), in resonance with the acoustic phonon branches at the surface of the Brillouin zone. The position of the electronic levels cannot be explained by crystal field theory, with the spin-orbit interaction to second-order included. We have calculated the effect of a dynamical Jahn-Teller interaction and have shown, for the first time, the necessity to include the coupling to the T(,2g) mode, in addition to the coupling to the E(,g) mode, although Co('2+) in MgO is a weak Jahn-Teller system. / We also used Raman spectroscopy to observe the vibrational modes of (beta)-In(,2)Se(,2). We observed, for the first time, the layer to layer shear mode(E(,2g)('2)), at 19 cm('-1). The position of this peak and the others, at 42(E(,1g)('1)), 117(A(,1g)('1)), 179(E(,2g)('1)), 181(E(,1g)('2)) and 231 cm('-1)(A(,1g)('2)), follows the trend of the vibrational spectra of (beta)-Ga(,2)S(,2) and of (beta)-Ga(,2)Se(,2) {65}. Using the linear chain model, we calculated the shear force constants and deduced the position of the doublet (E(,2g)('1) and E(,1g)('2)) with very good agreement with the experimental values.

Ingots containing single crystals were grown from melts of Cu, In and Se in either stoichiometric proportions (CuInSe2) or with an excess of Se (CuInSe2.2). In addition, either sodium selenide (Na 2Se) or elemental sodium (Na0) was introduced to both sets of compositions in concentrations ranging from 0 to 3 at. %. The starting constituents were placed in quartz ampoules, which were evacuated and sealed before undergoing a vertical-Bridgman growth procedure. Analysis of deposits seen on the ampoule walls and on the ingot surface after growth revealed the presence of Na, as well as various forms of the other starting elements; however, no Na was found within the crystals. Electrical measurements revealed trends in the thermoelectric power of the ingots to correspond with additions of Na, as well as the presence of excess Se. A sign conversion from p- to n-type was confirmed with addition of sodium to stoichiometric CuInSe2. A suggested mechanism used to explain the effects of Na on the material, based on these experimental observations, is presented.

No description available.

InP obtained by metal-organic vapor phase epitaxy, with properties similar to GaAs, shows mobilities approaching the theoretical maxima at low temperatures. However, the corresponding values remain abnormally low at room temperature where a pronounced electronic excitation to the conduction band is observed simultaneously. This reduction of the mobility is attributed to the presence of deep centers that are electrically inactive at low temperatures but become excited when the temperature increases. A model based on an iterative solution to the Boltzmann equation and accounting for the usual scattering mechanisms, including inelastic interactions, is able to explain the data perfectly and shows that a very high mobility at low temperature is not a sufficient measure of the purity for this material. The binding energy of the deep centers depends on the organo-metalic source used for the growth. This links the solution of this problem to the purification of the chemicals. Depletion effects at the interfaces did not appear to be significant. (Abstract shortened by UMI.)

The growth, electronic structure and doping of the semiconductor InN has been explored and analysed. InN thin films were grown by plasma assisted molecular beam epitaxy. The significance of the relative fluxes, substrate temperature and buffer layers was explored and related to the electrical and structural properties of the films. An exploration of the effect of active nitrogen species on InN films found that excited molecular nitrogen was preferred for growth over atomic and ionic species. An optimised recipe for InN was developed incorporating all explored parameters. The bandgap of InN was explored using the techniques of optical absorption, photoluminescence and photoconductivity. All three techniques identified a feature near 0.67 eV as the only dominant and reproducible optical feature measurable from InN thin films. No evidence for any optical features above 1 eV was discovered. The effect of the Burstein-Moss effect is discussed and the debate over the relative impact of the effect is related to problems with precisely measuring electron concentrations. Photoluminescence from mixed phase InN films containing significant zincblende content is presented, with tentative evidence presented for a zincblende band gap near 0.61 eV. Native defects within InN were studied by near edge X-ray absorption fine structure spectroscopy. Nitrogen related defects were found to be unlikely candidates for the high as-grown n-type conductivity. The most likely candidate remains nitrogen vacancies. Ion implantation was shown to cause substantial damage to the InN lattice, which could not be fully repaired through annealing. The limitation on annealing temperatures may limit the use of implantation as a processing tool for InN. Mg was shown to exhibit great promise as a potential p-type dopant. Photoluminescence from Mg doped films was found to quench at high Mg concentrations, consistent with a depletion region near the surface. The potential dilute magnetic semiconductor In1-xCrxN was explored. All of the In1-xCrxN films were found to be ferromagnetic at room temperature and exhibited saturated magnetic moments of up to 0.7 emu/g. An interesting correlation between background electron concentration and remnant moment is presented and the consequences of theoretical exchange models discussed. The bandgap of chromium nitride was also investigated and found to be an indirect gap of 0.7 eV.