A theory is developed to explain the dependence of carrier transport in a thin semiconducting film on film thickness, magnetic field strength, and the dominant bulk scattering mechanism. This theory is based on the solution of the linearized Boltzmann equation in relaxation time form. The semiconductor is assumed to be bounded and nondegenerate with spherical energy surfaces and a scalar effective mass, It is also assumed to be flat banded with totally diffuse scattering at the surface. Classical Boltzmann statistics are used for equilibrium. The dependence of the relaxation time on the carrier energy is approximated by a power law equation. The principle improvement over similar theories is the treatment of the dependence of the relaxation time on carrier energy. The power law approximation for this dependence is valid for randomizing and elastic scattering mechanisms.

Made available in DSpace on 2015-03-03T11:52:23Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-09-24Bitstream added on 2015-03-03T12:07:20Z : No. of bitstreams: 1 000796019_20160925.pdf: 638112 bytes, checksum: 56a92de10406e2a851d2652673de6f2c (MD5) Bitstreams deleted on 2016-09-26T12:32:56Z: 000796019_20160925.pdf,. Added 1 bitstream(s) on 2016-09-26T12:33:33Z : No. of bitstreams: 1 000796019.pdf: 4045834 bytes, checksum: fc3a47f551bf88aeaaabd9c26bd1e28d (MD5) / A molécula de DNA imobilizada sobre um substrato metálico possui uma infinidade de aplicações: inspeção de alimentos, autenticação pessoal, combate ao bioterrorismo, taxonomia, entre outras. Contudo, nos dispositivos que utilizam as propriedades elétricas desta biomolécula, ainda existem muitas dúvidas, haja vista que nem mesmo a literatura específica chegou a um consenso se o DNA possui características de um material elétrico condutor, semicondutor, isolante ou ainda supercondutor. Neste contexto, a investigação da condução elétrica de fitas duplas de DNA imobilizadas em eletrodos de ouro e sua caracterização por medidas eletroquímicas é bastante atraente. O uso do azul de metileno (MB), um agente intercalante, e a modificação da biomolécula com um grupo ferroceno, com propriedades redox, foram os dois tipos de abordagens utilizados para estudar o comportamento elétrico de fitas duplas e curtas de DNA com conteúdo distinto de bases nitrogenadas: uma rica em guanina e citosina e outra em adenina e timina, denominadas de DNA condutor (ds-DNAc) e DNA isolante (ds-DNAi), respectivamente. Para a primeira abordagem, a caracterização voltamétrica e impedimétrica não permitiu uma diferenciação da resistência elétrica dos sistemas de ds-DNAc e ds-DNAi, pois a transferência eletrônica do MB para o DNA ocorria em paralelo ao processo redox do MB com a superfície do eletrodo, inviabilizando a análise. Deste modo, a modificação do DNA com uma molécula redox foi necessária. Nesta segunda abordagem foi possível diferenciar os sistemas de ds-DNA. Primeiramente, a monocamada de tiol foi caracterizada usando o 6-ferrocenil-hexano-1-tiol. A área do pico de oxidação no voltamograma forneceu um valor de densidade de moléculas igual a 7,5x1012 moléculas/cm2 e, após o tratamento dos espectros de capacitância eletroquímica, o perfil de densidade de estados foi obtido... / Immobilization of DNA on metallic substrate has an infinity of applications: food inspection, personal authentication, combating bioterrorism, taxonomy and so on. However, there are still some questions about devices based on the electrical properties of this biomolecules because specific literature did not reach consensus if the DNA possesses electric characteristics of conductor, semiconductor, insulator or superconductor. In this context, we were interested to investigate electrical conduction of double-strands DNA immobilized on gold surface and to analyze this system by electrochemical techniques. Two methodologies were used to distinguish electrical behavior of shorts double-strands DNAs with different content of nitrogen bases: one double-strand rich in guanine and cytosine (conductive DNA or ds-DNAc) and another rich in adenine and thymine (non-conductive DNA or ds-DNAi). Intercalating agent methylene blue (MB) was used to characterize the electrical resistance of ds-DNAc and ds-DNAi by cyclic voltammetry and electrochemical impedance spectroscopy. Nevertheless, this methodology was not good enough because the electronic transfer between MB and DNA occurs simultaneously to the MB redox process on electrode surface. Therefore, modification of DNA with a redox group was necessary. In the second approaches we used the ferrocene-modified DNA monolayer. This approach was effective to discern ds-DNAc and ds-DNAi electric properties. Firstly, 6-ferrocenyl-1-hexanethiol molecule was assembled on gold electrode to find out the response of thiol film on ds-DNA systems. Voltammogram curves showed a narrow and intense oxidation peak and the surface density of molecule was obtained by integrating curve (7.5x1012 molecules/cm2). After, the density of states (DOS) profile was calculated from capacitance spectra and the integrating curve results on 2.9x1013 states/cm2. The quotient DOS/surface density...

Orientador: Paulo Roberto Bueno / Co-orientador: Maria Célia Bertolini / Banca: Antonio Aparecido Pupim Ferreira / Banca: Carlos Frederico de Oliveira Graeff / Banca: Marcelo Mulato / Banca: Sergio Broschztain / Resumo: A molécula de DNA imobilizada sobre um substrato metálico possui uma infinidade de aplicações: inspeção de alimentos, autenticação pessoal, combate ao bioterrorismo, taxonomia, entre outras. Contudo, nos dispositivos que utilizam as propriedades elétricas desta biomolécula, ainda existem muitas dúvidas, haja vista que nem mesmo a literatura específica chegou a um consenso se o DNA possui características de um material elétrico condutor, semicondutor, isolante ou ainda supercondutor. Neste contexto, a investigação da condução elétrica de fitas duplas de DNA imobilizadas em eletrodos de ouro e sua caracterização por medidas eletroquímicas é bastante atraente. O uso do azul de metileno (MB), um agente intercalante, e a modificação da biomolécula com um grupo ferroceno, com propriedades redox, foram os dois tipos de abordagens utilizados para estudar o comportamento elétrico de fitas duplas e curtas de DNA com conteúdo distinto de bases nitrogenadas: uma rica em guanina e citosina e outra em adenina e timina, denominadas de DNA condutor (ds-DNAc) e DNA isolante (ds-DNAi), respectivamente. Para a primeira abordagem, a caracterização voltamétrica e impedimétrica não permitiu uma diferenciação da resistência elétrica dos sistemas de ds-DNAc e ds-DNAi, pois a transferência eletrônica do MB para o DNA ocorria em paralelo ao processo redox do MB com a superfície do eletrodo, inviabilizando a análise. Deste modo, a modificação do DNA com uma molécula redox foi necessária. Nesta segunda abordagem foi possível diferenciar os sistemas de ds-DNA. Primeiramente, a monocamada de tiol foi caracterizada usando o 6-ferrocenil-hexano-1-tiol. A área do pico de oxidação no voltamograma forneceu um valor de densidade de moléculas igual a 7,5x1012 moléculas/cm2 e, após o tratamento dos espectros de capacitância eletroquímica, o perfil de densidade de estados foi obtido... / Abstract: Immobilization of DNA on metallic substrate has an infinity of applications: food inspection, personal authentication, combating bioterrorism, taxonomy and so on. However, there are still some questions about devices based on the electrical properties of this biomolecules because specific literature did not reach consensus if the DNA possesses electric characteristics of conductor, semiconductor, insulator or superconductor. In this context, we were interested to investigate electrical conduction of double-strands DNA immobilized on gold surface and to analyze this system by electrochemical techniques. Two methodologies were used to distinguish electrical behavior of shorts double-strands DNAs with different content of nitrogen bases: one double-strand rich in guanine and cytosine (conductive DNA or ds-DNAc) and another rich in adenine and thymine (non-conductive DNA or ds-DNAi). Intercalating agent methylene blue (MB) was used to characterize the electrical resistance of ds-DNAc and ds-DNAi by cyclic voltammetry and electrochemical impedance spectroscopy. Nevertheless, this methodology was not good enough because the electronic transfer between MB and DNA occurs simultaneously to the MB redox process on electrode surface. Therefore, modification of DNA with a redox group was necessary. In the second approaches we used the ferrocene-modified DNA monolayer. This approach was effective to discern ds-DNAc and ds-DNAi electric properties. Firstly, 6-ferrocenyl-1-hexanethiol molecule was assembled on gold electrode to find out the response of thiol film on ds-DNA systems. Voltammogram curves showed a narrow and intense oxidation peak and the surface density of molecule was obtained by integrating curve (7.5x1012 molecules/cm2). After, the density of states (DOS) profile was calculated from capacitance spectra and the integrating curve results on 2.9x1013 states/cm2. The quotient DOS/surface density... / Doutor

M.Sc. / Thin film solar cells based on polycrystalline indium diselenide (CulnSe₂) and amorphous silicon (a-Si:H) are promising candidates for the efficient conversion of sunlight into useable, cheap electrical energy. However, typical device structures are rather complex and consists of semiconductor/metal contacts as well as complicated p - n and p - i - n heterojunctions. In this study, CulnSe₂ absorber layers with excellent material properties were prepared by the selenization of metallic alloys. The a-Si:H thin films were deposited by radio frequency (RF) glow discharge in vacuum. The polycrystalline and amorphous absorber layers were deposited on glass and flexible substrates. In each case, a systematic study was conducted in which all the relevant processing parameters were varied over a broad range. These studies indicated that the structural features of the substrate significantly influence the structural features of the semiconductor thin films. The flexible substrate (kapton) was characterized by the presence of ridges, which distorted the growth behavior of the films. Deposition of ln/Cu/ln metallic alloys onto Mo coated glass (kapton) resulted in discontinuous metallic alloys, which were characterized by the presence of separated elongated island structures. The structural features of the precursors were maintained in the absorber film after selenization in elemental Se vapor. The morphological features of the CulnSe₂ absorber films were also critically influenced by the reaction temperature And reaction period to Se. The structural features on a-Si:H was significantly influenced by the structural features of the particular substrate used. Atomic force microscopy (AFM) imaging in combination with statistical analysis revealed higher roughness values when the amorphous semiconductor materials were deposited onto kapton, which negatively impacts on the device properties of solar cell devices.

Philosophiae Doctor - PhD / In this thesis, intrinsic hydrogenated nanocrystalline silicon thin films for solar cells application have been deposited by means of the hot – wire chemical vapour deposition (HWCVD) technique and have been characterised for their performance. It is noticed that hydrogenated nanocrystalline silicon is similar in some aspects (mainly optical) to its counterpart amorphous silicon actually used as the intrinsic layer in the photovoltaic industry. Substantial differences between the two materials have been found however in their respective structural and electronic properties. We show that hydrogenated nanocrystalline silicon retains good absorption coefficients known for amorphous silicon in the visible region. The order improvement and a reduced content of the bonded hydrogen in the films are linked to their good stability. We argue that provided a moderate hydrogen dilution ratio in the monosilane gas and efficient process pressure in the deposition chamber, intrinsic hydrogenated nanocrystalline silicon with photosensitivity better than 102 and most importantly resistant to the Staebler Wronski effect (SWE) can be produced. This work explores the optical, structural and electronic properties of this promising material whose study – samples have been exclusively produced in the HWCVD reactors based in the Solar Cells laboratory of the Physics department at the University of the Western Cape. / South Africa

No description available.

In this thesis, I present studies in the field of terahertz (THz) spectroscopy. These studies are divided into three areas: Development of a narrowband THz source, the study of carrier transport in metal thin films, and the exploration of coherent dynamics of quasi-particles in semiconductor nanostructures with both broadband and narrowband THz sources. The narrowband THz source makes use of type II difference frequency generation (DFG) in a nonlinear crystal to generate THz waves. By using two linearly chirped, orthogonally polarized optical pulses to drive the DFG, we were able to produce a tunable source of strong, narrowband THz radiation. The broadband source makes use of optical rectification of an ultra-short optical pulse in a nonlinear crystal to generate a single-cycle THz pulse. Linear spectroscopic measurements were taken on NiTi-alloy thin films of various thicknesses and titanium concentrations with broadband THz pulses as well as THz power transmission measurements. By applying a combination of the Drude model and Fresnel thin-film coefficients, we were able to extract the DC resistivity of the NiTi-alloy thin films. Using the narrowband source of THz radiation, we explored the exciton dynamics of semiconductor quantum wells. These dynamics were made sense of by observing time-resolved transmission measurements and comparing them to theoretical calculations. By tuning the THz photon energy near exciton transition energies, we were able to observe extreme nonlinear optical transients including the onset of Rabi oscillations. Furthermore, we applied the broadband THz waves to quantum wells embedded in a microcavity, and time-resolved reflectivity measurements were taken. Many interesting nonlinear optical transients were observed, including interference effects between the modulated polariton states in the sample. / Graduation date: 2012

The main objective of this research is to fabricate and characterize an optically integrated interferometric sensor on standard silicon and silicon CMOS circuitry. An optical sensor system of this nature would provide the high sensitivity and immunity to electromagnetic interference found in interferometric based sensors in a lightweight, compact package capable of being deployed in a multitude of situations inappropriate for standard sensor configurations. There are several challenges involved in implementing this system. These include the development of a suitable optical emitter for the sensor system, the interface between the various optically embedded components, and the compatibility of the Si CMOS with heterogeneous integration techniques. The research reported outlines a process for integrating an integrated sensor on Si CMOS circuitry using CMOS compatible materials, integration techniques, and emitter components.

A new approach to the discovery of high absorbing semiconductors for solar cells was taken by working under a set of design principles and taking a systemic methodology. Three transition metal chalcogenides at varying states of development were evaluated within this framework. Iron pyrite (FeS���) is well known to demonstrate excellent absorption, but the coexistence with metallic iron sulfides was found to disrupt its semiconducting properties. Manganese diselenide (MnSe���), a material heavily researched for its magnetic properties, is proposed as a high absorbing alternative to iron pyrite that lacks destructive impurity phases. For the first time, a MnSe��� thin film was synthesized and the optical properties were characterized. Finally, CuTaS���, a known but never characterized material, is also proposed as a high absorbing semiconductor based on the design principles and experimental results. / Graduation date: 2013

Nanostructured ZnO films were obtained via thermal oxidation of thin films formed with metallic Zn-nanoparticle dispersions. Commercial zinc nanoparticles used for this work were characterized by microscopic and thermal analysis methods to analyze the Zn-ZnO core shell structure, surface morphology and oxidation characteristics. These dispersions were spin-coated on SiO₂/Si substrates and then annealed in air between 100 and 600 °C. Significant nanostructural changes were observed for the resulting films, particularly those from larger Zn nanoparticles. These nanostructures, including nanoneedles and nanorods, were likely formed due to fracturing of ZnO outer shell due to differential thermal expansion between the Zn core and the ZnO shell. At temperatures above 227 °C, the metallic Zn has a high vapor pressure leading to high mass transport through these defects. Ultimately the Zn vapor rapidly oxidizes in air to form the ZnO nanostructures. We have found that the resulting films annealed above 400 °C had high electrical resistivity. The zinc nanoparticles were incorporated into zinc indium oxide solution and spin-coated to form thin film transistor (TFT) test structures to evaluate the potential of forming nanostructured field effect sensors using simple solution processing. The functionalization of zinc tin oxide (ZTO) films with self-assembled monolayers (SAMs) of n-hexylphosphonic acid (n-HPA) was investigated. The n-HPA modified ZTO surfaces were characterized using contact angle measurement, x-ray photoelectron spectroscopy (XPS) and electrical measurements. High contact angles were obtained suggesting high surface coverage of n-HPA on the ZTO films, which was also confirmed using XPS. The impact of n-HPA functionalization on the stability of ZTO TFTs was investigated. The n-HPA functionalized ZTO TFTs were either measured directly after drying or after post-annealing at 140 °C for 48 hours in flowing nitrogen. Their electrical characteristics were compared with that of non-functionalized ZTO reference TFTs fabricated using identical conditions. We found that the non-functionalized devices had a significant turn-on voltage (V[subscript ON]) shift of ~0.9 V and ~1.5 V for the non-annealed and the post-annealed conditions under positive gate bias stress for 10,000 seconds. The n-HPA modified devices showed very minimal shift in V[subscript ON] (0.1 V), regardless of post-thermal treatment. The VON instabilities were attributed to the interaction of species from the ambient atmosphere with the exposed ZTO back channel during gate voltage stress. These species can either accept or donate electrons resulting in changes in the channel conductance with respect to the applied stress. / Graduation date: 2012 / Access restricted to the OSU Community at author's request from Jan. 6, 2012 - Jan. 6, 2013