Thesis (M.S. in materials science and engineering)--Washington State University, December 2005. / Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Jack C. Lee. Vita. Includes bibliographical references.

A síntese, caracterização e propriedades de um novo compósito xerogel formado por óxido de vanádio e óxido de silício, com alta concentração de vanádio foi o objetivo deste trabalho. O compósito xerogel foi obtido mediante a formação de uma rede complexa envolvendo a condensação de polioxovanadato em meio aquoso com concomitante hidrólise e condensação de um alcóxido de silício. Conseqüentemente, este método possibilitou a obtenção de um material multicomponente homogêneo, no qual a rede Si O Si está interpenetrada com as cadeias poliméricas V-O- V e V-OH-V, promovendo uma solubilidade mútua, devido a formação de ligações cruzadas. Além disso, resultados experimentais apontam que, mesmo após a imobilização em matriz de sílica, a estrutura bi-dimensional, bem como as propriedades eletroquímicas do xerogel de pentóxido de vanádio são preservadas. A atividade catalítica do material obtido também foi avaliada na oxidação do cicloocteno e do estireno na fase líquida. O compósito xerogel V2O5/SiO2 obtido com tetraetiltrietóxisilano (TEOS), mediante catálise básica, o qual apresentou área superficial elevada (324 m2/g), e apresentou atividade catalítica em reações de oxidação do estireno e do cicloocteno na presença de PhIO como doador de oxigênio. Contudo, estes materiais apresentaram propriedades eletroquímicas inferiores as do xerogel de pentóxido de vanádio. Por outro lado, o comportamento eletroquímico óxido misto obtido com metiltrietóxisilano (MTES) é muito similar ao xerogel de V2O5; apresentando picos reversíveis ( par redox VV/IV: xe- + xLi+ + V2O5.nH2O ? LixV2O5.nH2O, em solução de LiClO4 0,1 mol.L -1 em acetonitrila). Além disso, a resposta eletroquímica á estável mesmo após sucessivos ciclos de oxidação e redução. Um aspecto interessante é que este compósito é formado por partículas esféricas de sílicas recobertas por xerogel de pentóxido de vanádio Neste contexto, pode-se afirmar que, o método empregado mostrou-se extremamente atrativo devido a sua simplicidade de realização, além de possibilitar um novo método de obtenção de materiais com potencial aplicação como dispositivos eletroquímicos, baterias, catalisadores e sensores químicos. / The synthesis, characterization and properties of new vanadium oxide silicon oxide composite xerogels with high vanadium content through formation of a complex network involving the condensation of polyoxovanadates in aqueous solution with concomitant hydrolysis and condensation of the silica alkoxide precursor have been the goal of this work. As a consequence, this procedure generated a homogeneous multicomponent material, in which Si-O-Si network is interpenetrated with V-V and V-OH-V polymeric chains, where a mutual \"solubility\" due to cross-links and entanglements was observed. In addition, the experimental data evidence that the vanadium pentoxide xerogel embedded in silica retains its bi-dimensional structure as well as its electrochemical properties. Besides, the catalytic activity of this material was evaluated in the oxidation of the cyclooctene and styrene in liquid phase. V2O5-SiO2 composite xerogels obtained from tetraethoxysilane under basic catalysis, present high surface area (324 m2/g) and have catalytic activities in alkene oxidation in the presence of PhIO as oxygen transfer agent and cyclooctene and styrene as substrates. However, these materials do not present a remarkable electrochemical property as evidenced by cyclic voltammetry. In contrast, the voltammetric behavior of the composites xerogel prepared with methyltriethoxysilane is quite similar to that found for V2O5 xerogel; showing reversible peaks (VV/IV redox pair: xe- + xLi+ + V2O5.nH2O LixV2O5.nH2O, in acetonitrile solutions containing 0.1 M LiClO4). Besides, the electrochemical response is stable under several successive redox cycles (over 50). An interesting feature is that is formed by silica spherical particles (4 to 8 micra) covered with V2O5 continuous polymeric network. Therefore, the synthetic approach applied in this study is extremely attractive due to its simplicity and can provide new strategies for tailoring new materials for electrochromic devices, batteries, catalysis and chemical sensing.

Neste trabalho foram fabricados e caracterizados eletricamente capacitores MOS com óxido de silício ultrafino (2,6 nm) com porta de silício policristalino (Si-poli) P+ e N+. Os capacitores MOS com porta de Si-poli dopados com boro tiveram a estrutura Si-poli/SiO2/Si previamente implantada com nitrogênio nas doses de 1.10\'POT.13\', 1.10\'POT.14\', 1.10\'POT.15\' e 5.10\'POT.15\' at.cm-², com o pico da concentração de nitrogênio próximo à interface SiO2/Si. Os capacitores MOS foram fabricados sobre lâminas de silício do tipo p que passaram por uma limpeza química préoxidação tipo RCA mais imersão final em solução diluída em HF. Na seqüência, as lâminas foram oxidadas em um ambiente de O2 (1,5 l/min) + N2/H2 (2l/min; 10 %) que proporcionou óxidos de silício com excelentes características elétricas. Para a fabricação dos capacitores MOS com porta de Si-poli P+, utilizou-se SOG de boro seguido por difusão térmica sobre camada de Si-poli (340 nm). Após testes com receitas de difusão a 950, 1000, 1050 e 1100 °C todas padronizadas por um tempo de 30 min optamos por realizar a difusão a 1050 °C por 30 min, pois essa receita proporcionou concentração de boro superior a 1.10\'POT.20\' at.cm-³ e segregação desprezível do boro em direção ao substrato de Si. A dopagem dos capacitores MOS com porta de Si-poli N+ foi realizada por aplicação do SOG de fósforo seguido por difusão a 1050 °C por 30 min. Os resultados indicaram segregação do boro desprezível para o Si, baixa densidade de estados de interface (< 1.10\'POT.11\' eV-¹ cm-²) e no aumento do campo elétrico de ruptura (de 14 MV/cm para 21 MV/cm) com o aumento da dose de nitrogênio (de 1.10\'POT.13\' a 5.10\'POT.15\' at/cm²). Embora ocorresse uma maior dispersão e um aumento desfavorável da tensão de banda plana com o aumento da dose de nitrogênio, os valores 1.10\'POT.15\' e 5.10\'POT.15\' at.cm-² resultaram em capacitores MOS com tensão de faixa plana próxima ao parâmetro diferença de função trabalho (\'fi\' MS) significando densidade efetiva de cargas no dielétrico de porta inferior à cerca de 1.10\'POT.11\' cm-². / In this work we manufactured and electrically characterized MOS capacitors with ultrathin silicon oxides (2,6 nm) and polysilicon gate (Si-poli), P+ or N+. P+ - doped polysilicon gate MOS capacitors (Si-poli/SiO2/Si structure) were previously implanted with nitrogen using doses of 1.10\'POT.13\', 1.10\'POT.14\', 1.10\'POT.15\' and 5.10\'POT.15\' at.cm-², and implantation peak centered close to the SiO2/Si interface before boron doping. The MOS capacitors were fabricated on p-type silicon wafers, which were submitted to RCA - based cleaning procedure and a final dip in diluted HF solution. Following, the wafers were oxidize in ultrapure O2 (1,5 l/min) + N2/H2 (2l/min; 10 %) having, as a result, silicon gate oxides with excellent electrical characteristics. To obtain P+ polysilicon, it Spin On Glass (SOG) of boron the wafers was annealed at 950, 1000, 1050 or 1100 °C during 30 min. We have chosen a diffusion recipe of 1050 °C during 30 min to obtain volumetric concentration of boron higher than 1.10\'POT.20\' cm-³ and no boron segregation to the silicon. N+ polysilicon was also obtained using phosphorus SOG and diffusion at 1050 °C during 30 min. As a result, besides no boron segregation to Si, the interface states density was low (< 1.10\'POT.11\' eV-¹cm-²) and the breakdown field of the gate oxides increased (from 14 MV/cm to 21 MV/cm) by increasing the nitrogen doses (from 1.10\'POT.13\' to 5.10\'POT.15\' at/cm²). Although a larger dispersion and increasing of the flat-band voltage have occurred as the nitrogen dose was increased, values of 1.10\'POT.15\' and 5.10\'POT.15\' at.cm-² induced flat band voltage close to the parameter workfunction difference (\'fi\'MS) which meant effective charge density in the gate dielectrics lower than about 1.10\'POT.11\' cm-².

Silicon photovoltaic (PV) is a promising solution for energy shortage and environmental pollution. We are experiencing an era when PV is exponentially increasing. Global cumulative installation had reached 380 GW in 2017. Among which, silicon-based PV productions share more than 90% market. Performance of the first two-generation commercial popular silicon solar cells - Al-BSF and PERC - are limited by metal/Si contacts, where interface defects significantly reduce the open-circuit voltage. In this context, full-area passivation concepts are proposed for c-Si solar cells, with expectation to enhance the efficiency via reducing carrier recombination loss at the contact regions. In this thesis, poly silicon on oxide (POLO) passivating contact is developed for high efficiency c-Si solar cells. We unveiled the working mechanisms of POLO cells and then optimized the device performance based on our conclusion. We use boiling nitric acid to oxidize c-Si surface, which is of significance to determine the POLO working mechanisms. Phosphorus and boron doped silicon films are deposited by plasma enhanced vapor deposition (PECVD) or low-pressure vapor deposition (LPCVD) followed by high temperature (>800°C) annealing. SiOx structural evolution process under different annealing temperature was observed and the corresponding effects on passivation have been elucidated. The carrier transport mechanisms in the POLO contact annealed at high temperature, e.g. 800°C  900°C, were explored. We unveil that carrier transport in POLO structure is a combination of tunneling and pinhole transport, but dominant at varied temperature regions. Phosphorus-doped n-type POLO contact is optimized by several parameters, such as doping concentration, film thickness, annealing temperature, film deposition temperature, film relaxation time during annealing process, etc. We successfully obtained minority carrier lifetime over 10ms and contact resistivity lower than 30 mΩ·cm2. Boron-doped p-type POLO contact is also optimized by changing the doping concentration and annealing temperature. Finally, further hydrogen passivation is applied to enhance p-type POLO contact passivation, achieving an iVoc>690 mV, J0 <5 fA/cm2 and contact resistivity 1.3 mΩ·cm2. With the optimized n-type and p-type POLO contacts, an efficiency over 18% is achieved on n-type c-Si solar cells with a flat front surface.

Supplying the global energy demand through renewable sources has never been as accessible as it is now thanks to developments in technology and infrastructure that have enabled low-cost energy production from sources like wind, solar, and hydroelectric power. However, the challenge of integrating variable renewable energy generators into existing grid infrastructure has driven the demand for efficient and inexpensive energy storage technologies to buffer these intermittent energy supplies. Using electrochemical devices like fuel cells and electrolyzers is an attractive approach for both the long- and short-term storage of energy, where excess energy is used to drive the conversion of low energy reactants into high energy, storable fuels which can be consumed when energy supply is low. These devices rely on highly active electrocatalysts in order to drive these reactions efficiently. However, a major challenge for these technologies lies in developing catalysts at commercial scale without compromising their selectivity or lifetime. Several degradation mechanisms like catalyst particle detachment, dissolution, or surface poisoning by undesired species can quickly diminish the activity and selectivity of a given catalyst, and drive up the costs of electrochemical storage systems. Thus, developing catalysts that balance stability, activity, and selectivity is crucial to improve the economic viability of these energy storage devices. One approach towards mitigating the issues of catalyst stability and activity is through adhering a semi-permeable oxide membrane onto the catalyst surface, creating a structure known as an oxide encapsulated electrocatalyst (OEC). These architectures have previously been shown to improve reaction selectivity, poisoning resistance and nanoparticle stability by improving the adhesion of catalyst nanoparticles, preventing poisoning species from reaching the buried catalytic interface, and controlling the local concentrations of reactants as a means of shifting reaction kinetics. Though earlier studies of OECs have demonstrated a wide array of beneficial properties that encapsulated catalyst architectures offer, they have often been based on highly heterogeneous electrodes and been evaluated across a wide range of conditions, which complicates the identification of the mechanisms that underlie these improvements. Currently, little is understood about the governing mechanisms that influence how oxide overlayers interact with – and ultimately affect – the catalyst surface, as well as alter the reactions occurring at the buried interface. Design rules that relate OEC structure to catalytic performance have the potential to greatly accelerate the understanding and development of such architectures, and would allow for more rational, targeted design of OEC structure in a way that would accelerate their application to new electrocatalytic systems.The aim of this dissertation is therefore to systematically investigate the design space of OEC architectures by using well-defined, model planar electrocatalysts in order to draw clear relationships between the structure, composition, and chemical/physical properties of OECs and the resulting effects they have on electrocatalytic performance. Using planar Pt catalysts encapsulated by a thin, highly tunable carbon-modified silicon oxide (SiOₓCy) overlayers, properties like overlayer thickness, carbon concentration, and density can be specifically adjusted during the room temperature photochemical synthesis procedures used for overlayer fabrication. Similarly, changing the composition of the underlying Pt catalyst while keeping overlayer properties constant can provide insights into how catalyst and overlayer materials interact with and influence the structure of one another. Rigorous materials characterization like X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), ellipsometry, and scanning electron microscopy (SEM) coupled with electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy relates observations in the physical and chemical properties of OECs directly to the electrochemical performance of various probe reactions. In Chapter 3, carbon-free, SiO₂-like overlayers of uniform thicknesses were synthesized using a room temperature, Ultraviolet (UV)-ozone photochemical process that allowed for specific control over the resulting overlayer thicknesses, which ranged between 1.8 nm and 18.0 nm. Two different compositions of the planar catalyst substrate were investigated at all thicknesses. The first catalyst investigated was a 50 nm thick, uniform layer of polycrystalline Pt that displayed bulk properties. The second, thinner catalyst substrate was only 3 nm thick, and contained trace quantities of oxophilic Ti species at the buried interface, which migrated to the surface during electrode fabrication. Ultimately it was found that electrodes based on ultrathin, Ti-doped Pt possesses thinner Pt oxide (PtOₓ) interlayers, while exhibiting reduced permeability for Cu²⁺ and H⁺ compared to the bulk Pt species. Thin layer Pt electrodes also demonstrated enhanced retention of the SiOₓ overlayer during stability testing in 0.5 M H₂SO₄, credited in part to the differences in PtOₓ concentration and structure that form at the buried interface as a result of trace Ti concentrations. These observations lead to the study presented in Chapter 4, which sought to assess the impact of chemical and physical overlayer properties on resulting electrochemistry. Compositions of the SiOₓCy overlayer were altered by restricting the exposure of electrodes to the photochemical UV-ozone curing step during synthesis, which was responsible for removing carbonaceous groups in the overlayer’s precursor. Limiting the length of this step between 15 minutes and 120 minutes yielded overlayer with residual carbon concentrations ranging between 30% and 4%, respectively, and demonstrated markedly different physical and chemical properties that impacted species transport through the overlayer. Specifically, the less dense, carbon-rich SiOₓCy layers restricted the flux of H⁺ to the Pt interface during the hydrogen evolution reaction (HER) under transport limited conditions, but displayed high permeability towards dissolved oxygen species for the oxygen reduction reaction (ORR). By contrast, the denser, carbon free SiOx layers blocked oxygen transport almost entirely, but showed limiting current densities for HER that were comparable to an unencapsulated surface. This is believed to occur from the differing transport mechanisms for H⁺ and O₂ through SiOₓ, where the former diffuses through a Grotthuss-type transport mechanism, and the latter through a solution-diffusion mechanism. The high density SiOₓ layers therefore constrain the flux of O₂ due to its lower free volume compared to the carbon rich overlayers, but has a higher concentration of silanol carrier groups that promote H⁺ transport. These results demonstrated the impact that overlayer compositions can have on modulating the local concentrations of reactants, and motivated the further study of OECs on alcohol oxidation reactions (AORs) in Chapter 5. Using the same approach to control overlayer composition detailed above, SiOₓCy overlayers deposited on Pt thin film electrodes were fabricated and their catalytic performance towards the oxidation of carbon monoxide, formic acid, and C₁-C₄ alcohols were assessed. All SiOₓCy - encapsulated electrodes decreased the overpotentials required to oxidize and remove Pt-bound CO species – a poisoning intermediate for a number of AORs, with the largest reductions seen for the carbon poor, SiO₂-like overlayers through a possible Si-OH mediated removal step. Unexpectedly though, electrodes that had the largest reductions in CO oxidation overpotentials showed the least enhancement for AOR activity for all encapsulated samples. These observations suggest that a different rate determining step may be governing the overall reaction rate on encapsulated electrodes over the potential ranges investigated - most likely bond scission of C-H bonds and/or oxidation of formate-based intermediates. Finally, Chapter 6 presents results obtained from state-of-the-art operando ambient pressure X-ray photoelectron spectroscopy (APXPS) studies, which were used to investigate the behavior of SiOₓ overlayers and ions in solution to understand local interactions and electronic effects that arise under wetted, electrochemical operating conditions. It was found that the choice of electrolyte had a clear impact on the overlayer’s response to different applied potentials. Si 1s spectra of the SiOₓ overlayer taken in K₂SO₄ electrolytes showed a slight positive correlation with applied potential that signified a weak electronic interaction between the SiOₓ and the underlying Pt. However, when the anion was switched to Cl⁻, clear, non-linear correlations between the Si 1s binding energy and potential emerged, suggesting a major change in the local chemical and electronic conditions within the overlayer. Analyzing ion concentrations also showed that overlayers demonstrate different distributions and ion rejection properties based on an ion’s valence and size. The mechanism through which these changes manifest is quite complex, as the layers themselves can introduce numerous perturbations in the system by disrupting the electrochemical double layer, introducing steric confinement at the buried interface, or promoting different reaction pathways. Although continued work will be necessary to better de-convolute these effects and develop optimized, concise design rules, the studies presented in this thesis illustrate the unique opportunity that the application of OECs has towards the future customization of electrocatalysts for a wide range of chemistries and applications.

In this thesis, group IV material oxides for silicon photonics applications were deposited using novel deposition techniques. Erbium and terbium doped silicon oxide thin films were deposited through a novel hybrid radio frequency (RF) magnetron sputtering source in an electron cyclotron resonance (ECR)-plasma enhanced chemical vapour deposition (PECVD) reactor chamber. This approach contrasts with traditional doping methods which use metal-organic precursors to introduce rare-earth dopant species into the host matrix. The effects of sputtering power applied to the rare-earth target and system plasma pressure on the thin film properties were investigated. It was found that the sputtering power strongly influences the rare-earth incorporation, and a wide range of control over the doping level can be achieved. The effect of sputtering power on the refractive index, stoichiometry, and film density were also investigated. Doped thin films deposited with this technique showed low as-deposited hydrogen concentrations. In the case of terbium doped silicon oxide (SiOx), photoluminescence (PL) studies were conducted finding bright emission due to 5D4 → 7F5 transitions visible with the naked eye in films annealed above 1150 °C. Further investigation found that silicon nanostructures formed at the high annealing temperatures and were likely sensitizing the Tb3+ ions. These results demonstrate that hybrid sputtering in ECR-PECVD can be an effective tool for integrating optically active rare-earth dopants into silicon-based thin films. Using alternating current (AC) plasma assisted reactive magnetron sputtering (PARMS), low optical loss germanium oxide (GeO2) thin films were also produced. The films were fabricated at low temperature and high deposition rates of 6–38 nm/min on silicon and thermally oxidized silicon substrates. Prism coupling measurements demonstrated losses of 0.1 dB/cm at wavelengths ranging from 638 to 980 nm attributed to good uniformity and low surface roughness demonstrated through atomic force microscopy (AFM) measurements. The thin films materials developed here are highly promising for their applications in silicon photonics devices, including light sources and amplifiers. / Thesis / Candidate in Philosophy

This thesis examines melting in superheated silicon films in contact with SiO₂ under pulsed laser irradiation. An excimer-laser pulse was employed to induce heating of the film by irradiating the film through the transparent fused-quartz substrate such that most of the beam energy was deposited near the bottom Si-SiO₂ interface. Melting dynamics were probed via in situ transient reflectance measurements. The temperature profile was estimated computationally by incorporating temperature- and phase-dependent physical parameters and the time-dependent intensity profile of the incident excimer-laser beam obtained from the experiments. The results indicate that a significant degree of superheating occurred in the subsurface region of the film. Surface-initiated melting was observed in spite of the internal heating scheme, which resulted in the film being substantially hotter at and near the bottom Si-SiO₂ interface. By considering that the surface melts at the equilibrium melting point, the solid-phase-only heat-flow analysis estimates that the bottom Si-SiO₂ interface can be superheated by at least 220K during excimer-laser irradiation. It was found that at higher laser fluences (i.e., at higher temperatures), melting can be triggered internally. At heating rates of 10¹⁰ K/s, melting was observed to initiate at or near the (100)-oriented Si-SiO₂ interface at temperatures estimated to be over 300K above the equilibrium melting point. Based on theoretical considerations, it was deduced that melting in the superheated solid initiated via a nucleation and growth process. Nucleation rates were estimated from the experimental data using Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis. Interpretation of the results using classical nucleation theory suggests that nucleation of the liquid phase occurred via the heterogeneous mechanism along the Si-SiO₂ interface.

[Some symbols cannot be rendered in the following metadata – please see the PDF file for an accurate version of the Abstract] ¶ Recombination within the bulk and at the surfaces of crystalline silicon has been investigated in this thesis. Special attention has been paid to the surface passivation achievable with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to their potential for widespread use in silicon solar cells. The passivation obtained with thermally grown silicon oxide (SiO2) layers has also been extensively investigated for comparison. ¶ Injection-level dependent lifetime measurements have been used throughout this thesis to quantify the different recombination rates in silicon. New techniques for interpreting the effective lifetime in terms of device characteristics have been introduced, based on the physical concept of a net photogeneration rate. The converse relationships for determining the effective lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have also been introduced, thus establishing the equivalency of the photoconductance and voltage techniques, both quasi-static and transient, by allowing similar possibilities for all of them. ¶ The rate of intrinsic recombination in silicon is of fundamental importance. It has been investigated as a function of injection level for both n-type and p-type silicon, for dopant densities up to ~5x1016cm-3. Record high effective lifetimes, up to 32ms for high resistivity silicon, have been measured. Importantly, the wafers where commercially sourced and had undergone significant high temperature processing. A new, general parameterisation has been proposed for the rate of band-to-band Auger recombination in crystalline silicon, which accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this new parameterisation, with the effects of photon recycling included. ¶ Surface recombination processes in silicon solar cells are becoming progressively more important as industry drives towards thinner substrates and higher cell efficiencies. The surface recombination properties of well-passivating SiN films on p-type and n-type silicon have been comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined. The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric in composition, rather than being silicon rich, a property which is attributed to the use of dilute silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface being determined by a high fixed charge density within the SiN film (even under illumination), has been proposed to explain the injection-level dependent Seff for a variety of differently doped wafers. The passivation obtained with the optimised SiN films has been compared to that obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown that the optimised SiN films show little absorption of UV photons from the solar spectrum and can be easily patterned by photolithography and wet chemical etching. ¶ The recombination properties of n+ and p+ emitters passivated with optimised SiN films and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances. Both planar and random pyramid textured surfaces were studied for n+ emitters, where the optimised SiN films were again found to be stoichiometric in composition. The optimised SiN films provided good passivation of the heavily doped n+-Si/SiN interface, with the surface recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of electrically active phosphorus atoms increased from 7.5x1018cm-3 to 1.8x1020cm-3. The optimised SiN films also provided reasonable passivation of industrial n+ emitters formed in a belt-line furnace. It was found that the surface recombination properties of SiN passivated p+ emitters was poor and was worst for sheet resistances of ~150./ . The hypothesis that recombination at the Si/SiN interface is determined by a high fixed charge density within the SiN films was extended to explain this dependence on sheet resistance. The efficiency potential of SiN passivated n+p cells has been investigated, with a sheet resistance of 80-100./ and a base resistivity of 1-2.cm found to be optimal. Open-circuit voltages of 670-680mV and efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells respectively. The recombination properties measured for emitters passivated with SiO2, both n+ and p+, were consistent with other studies and found to be superior to those obtained with SiN passivation. ¶ Stoichiometric SiN films were used to passivate the front and rear surfaces of various solar cell structures. Simplified PERC cells fabricated on 0.3.cm p-type silicon, with either a planar or random pyramid textured front surface, produced high Voc’s of 665-670mV and conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20.cm) demonstrated Voc’s up to 675mV, the highest ever reported for an all-SiN passivated cell, and excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2.cm multicrystalline silicon have also demonstrated very high Voc’s of 655-659mV and conversion efficiencies up to 17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency measurements confirmed the excellent passivation achieved with the optimised stoichiometric SiN films on n+ emitters, while long-wavelength measurements show that there is a loss of short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from the rear contact dots. An alternative is to have locally diffused p+ regions under the rear contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN passivation elsewhere, without using photolithography, has been demonstrated.

Recently, active matrix flat-panel displays are widely used in consumer electronic products. With increasing popularity of flat-panel displays, market competition becomes more intense and demands for high performance flat-panel displays are increasing. Low-temperature polysilicon (LTPS) with higher mobility, as well as drive current can integrate electric circuit, such as controllers and memory on glass substrate of display to achieve the purpose of system on panel (SOP). Thus, flat-panel displays can be more compact, while reducing reliability issues and lowering production costs. In this dissertation, we studied the nonvolatile memory for system on panel applications and reducing cost of memory by increasing the memory density or reducing the processing steps. Therefore, we proposed several modes of operation in nonvolatile memory. First, we use channel hot-electron (CHE) to inject electrons into the nitride layer that¡¦s above source or drain sides of SONOS thin film transistor (TFT). Thus, we can increase the memory density by storing two-bit state in a memory cell. In this study, the two-bit memory effect is clearly observed for devices with a shorter gate length after CHE programming; however, the two-bit memory effect is absent in devices with a longer gate length. The gate-length-dependent two-bit memory effect is related to the location of injected electrons in the nitride layer. When electrons are injected into the nitride layer above the channel, they can create an additional energy barrier in the channel thus increasing the threshold voltage of the device to perform the programming operations. However, if electrons are injected into the depletion region at the P-N junction between the drain and the channel, the energy barrier induced by electrons is not significant when exchanging the source and drain electrodes to measure the memory status, and the program effect is not as significant. When the channel length is shorten, the built-in potential between the source and the channel can be decreased, the energy barrier caused by programmed electrons can affect electrons in the channel and increase the threshold voltage. Therefore, the two-bit memory effect can be seen in devices with the shorter gate length after CHE programming. Secondly, we stored charges in the body of the thin film transistor to make the conventional thin-film transistors become a non-volatile memory. This method does not need a floating gate or a tunneling oxide in the memory cell; therefore the memory cost can be reduced. In this study, we used trap-assisted band-to-band thermionic field emission enhanced by self-heating in TFT to produce electron-hole pairs. The hole will be separated by a vertical field under the gate and be injected into the body of TFT to complete the programming operation. The erasing operation is performed by applying a lateral electric field between the source/drain to remove holes in the body of TFT. Thirdly, we proposed an edge-FN tunneling method to allow SONOS TFT possess not only a pixel switch but also a two-bit nonvolatile memory function in a display panel, thus causing the memory density to increase. In this study, we used a channel FN tunneling to program the SONOS TFT. Because the electric field in the gate-to-drain overlap region is larger than that in the channel region, it will cause a smoother electron injection into the nitride layer inside of the gate-to-drain overlap region, which also increases the gate-induced drain leakage (GIDL) current. The edge-FN tunneling method is used to erase electrons in the gate-to-drain overlap region, by doing so, the GIDL current has decreased. The memory status at the source/drain side is determined by the corresponding GIDL current of the SONOS TFT. Fourthly, we stored electrons in the nitride layer at source, channel, and drain regions of SONOS TFT to make sure that TFT possess a three-bit memory effect in a unitary cell, which also allows the memory density to increase significantly. In this study, programming and erasing operations in the source/drain region are performed by channel hot-electron injection and edge-FN tunneling method, while that in the channel region are accomplished by channel FN tunneling. The memory status in the source/drain is determined by the corresponding GIDL current, while that in the channel region by threshold voltage of the device The memory density for the device operated by proposed method can be further increased. In addition, if we store a number of N different types of electrons in those three regions mentioned above, there are N3 status can be stored in a memory cell. The memory density can beyond conventional multi-level-cell (MLC) flash memory. Two-bit memory effect per cell in a MLC flash memory can be achieved by storing four quantitative electrons in the floating gate of the memory device. If we store four quantitative electrons in the nitride layer at source, channel, and drain regions of SONOS TFT, we can obtain 64 memory states or 6-bit memory effect in a memory cell. Thus, the proposed concept is promising to storage the messages in a memory cell beyond four-bit.