Efficient optical-to-electrical conversion is a fundamental requirement of a range of silicon devices such as those which employ photodetection, solid-state-imaging and photovoltaic power generation. This thesis investigates the effects of using indium, a deep-level acceptor in silicon, as a dopant for thin film single crystalline silicon solar cells and UV-A detectors. Indium acts as a p-type dopant in silicon and has been proposed previously as a substitutional lattice defect that would enable sub-band gap transitions as described by the so-called impurity photovoltaic (IPV) effect. The physical mechanisms responsible for operation of the devices presented in this work are described. Models for electrical performance, optical absorbance and device fabrication are used as methods to interpret data and optimize device parameters. Specifically, a two-diode model is used to account for the electrical loss mechanisms within a device, while modeling optical absorption by a multilayer structure consisting of Silicon-On-Insulator (SOI) is approached using a novel multi-wavelength numerical model that describes the reflections and transmissions at each of the device’s layers. Additionally, Technology Computer Aided Design (TCAD) simulations were used to optimize the critical fabrication parameters associated with the ion implantation and thermal annealing techniques used during the device fabrication process. Selected from multiple devices fabricated during the course of this work, the most efficient solar cells in SOI (2.5 μm thick active layer) exhibited a maximum conversion efficiency of 4.74 % for indium-doped and 4.16 % for boron-doped layers. The most efficient UV-A detector fabricated in SOI (100 nm thick) exhibited a maximum responsivity to 365 nm light of 20 mA/W for indium-doped and 16 mA/W for boron-doped devices. In both types of devices, indium doping consistently resulted in a relative increase in efficiency when compared to equivalently fabricated, boron doped devices, despite experimental carrier decay measurements confirming the action of the indium as a recombination centre. External and internal quantum efficiency measurements confirm a relative enhancement in absorption, for solar cells and detectors doped with indium, which is correlated with the p-type dopant concentration and the ratio of n-type to p-type concentrations. The origin of the enhancement is postulated to be caused by a relaxation of the momentum-space restrictions associated with undoped silicon, a postulate supported by previously reported absorption data. This thesis presents the first comprehensive data from indium doped silicon devices designed for optical-to-electrical conversion. The implications for a range of widely deployed devices may be significant. / Thesis / Doctor of Philosophy (PhD)

Thin film uniform and exponential distributed RC networks were fabricated and the network responses were investigated. It was necessary to take dielectric loss into consideration in the theoretical analysis before a close agreement could be obtained between the theoretical and experimental network responses. / Thesis / Master of Engineering (ME)

This thesis aims at investigating the feasibility of realizing antireflection (AR) and high-reflection (HR) to the semiconductor waveguide end facet using monolithically integratable deep-etching technology to replace the conventional thin film dielectric coating counterpart. Conventional AR coating and HR coatings are the building blocks of semiconductor optical amplifier and semiconductor lasers. In this thesis, the AR coating and HR coating are first studied systematically and comprehensively using two computational electromagnetics approaches: plane wave transmission matrix method (TMM) and finite difference time domain (FDTD) method. The comparison of the results from the two approaches are made and discussed. A few concepts are clarified based on the different treatment between the AR coatings for bulk optics and those for semiconductor waveguide laser structure. The second part uses the same two numerical tools and more importantly, the knowledge gained from the first part to analyze and design deep-etched waveguide gratings for the advantage of ease of monolithic integration. A variational correction to the TMM is provided in order to consider effect of the finite etching depth also in the plane wave model. Specially, a new idea of achieving AR using deep-etched waveguide gratings is proposed and analyzed comprehensively. A preliminary design is obtained by TMM optimization and FDTD verifications, which provides a minimum power reflectivity in the order of 10-5 and a bandwidth of 45nm for the power reflectivity less than 10-3. In order to eliminate the nonphysical reflections from the boundary, the perfectly matched layer (PML) absorbing condition is employed and pre-tested for antireflection analysis. The effects of etching depth and number of etching grooves are specifically analyzed for the performance of proposed structures. Numerical results obtained by FDTD method indicate a promising potential for this alternative technologies. / Thesis / Master of Engineering (ME)

We explore a specific small geometry containing a single thin bounded grain on a substrate with a hole at its center. By employing a mathematical model based on surface diffusion, no flux boundary conditions, and prescribed contact angles, we study the evolution of the hole as well as the exterior surface of the grain, based on energetic considerations and dynamic simulations. Our results regarding the formation and evolution of holes in thin films in small geometries shed light on various nonlinear phenomena associated with wetting and dewetting.

Electronic applications have expanded to encompass a variety of materials. In particular, allotropes of carbon interest researchers for their electronic applications. Knowledge of carbon allotropes and their applications has expanded significantly since the discovery of C60 Buckminsterfullerene in 1985, the discovery of multi- and single-walled carbon nanotubes in the early 1990s, and the isolation of graphene in 2004. Single-walled carbon nanotubes (SWNTs) have the potential to bring next-generation electronic devices to fruition. Such devices could be flexible, conformable, and inexpensive. SWNT-based electronics are promising for chemical and biological sensing applications, for example, where high carrier mobilities are unnecessary, and material conformity and inexpensive processing are significant advantages. Considerable progress has been made in separating semiconducting SWNTs from metallic SWNTs, enabling SWNT incorporation into semiconducting electronic technologies. Selective sorting of semiconducting SWNTs using π-conjugated polymers is an effective and efficient technique to enrich large quantities of ultra-pure semiconducting SWNTs. Following semiconducting enrichment, SWNTs can be incorporated into electronic devices. This thesis focuses on the enrichment of semiconducting SWNTs via conjugated polymer extraction and incorporating the resulting polymer-SWNT dispersions into thin-film transistors (TFTs). Novel copolymers were investigated for their capacity to selectively sort and disperse large-diameter sc-SWNTs synthesized using the plasma torch technique. Absorption and Raman spectroscopy were employed to monitor the efficacy of the conjugated polymer extraction procedure. Following enrichment, the polymer-SWNT dispersions were incorporated into TFTs. The interaction between the conjugated polymer and the SWNT and the conjugated polymer and dielectric was an essential component of TFT optimization. Furthermore, the procedure of sorting and dispersing sc-SWNTs is investigated for its effect on TFT performance and was another component of TFT optimization. TFTs were electrically characterized in terms of carrier mobility, threshold voltage, hysteresis, and current on/off ratio. The film morphology of the SWNT TFTs was also investigated. Atomic force microscopy and Raman mapping were used to provide insight into the nanometre and micrometre scale film morphology, respectively.

This work describes the techniques of assembling colloidal gold on flexible polymeric substrates from solution. The process takes advantage of the strong affinity of gold to thiol and amino groups. Polymeric substrates were modified with silanes having these functional groups prior to Au attachment or in the case of poly(urethane urea) (PUU), no surface functionalization was required. This polymer has terminal amine and N-H groups on the polymer chain, which can act as coordination points for gold. Immersion in the colloidal gold solution led to the formation of a monolayer. Increased coverage was obtained by two methods. The first was a reduction or "seeding" process, where Au was reduced onto the attached particles on the surface. The second was using different linker molecules and creating a multilayered film by a layer-by-layer assembly. Three linker molecules of different lengths were used. Films fabricated using the smallest molecule had the least resistance whereas films fabricated with the longest molecule were not conductive. The resistance of these films may be varied easily by heating. Heating the films at temperatures as low as 120 °C caused a dramatic decrease in the resistance of over six orders in magnitude. Successful attachment of gold to PUU with very good adhesion properties was also demonstrated. The attachment of gold was stable in different solvents. Upon stretching the PUU-Au films, it was observed that there is a reversible resistance increase with strain and at a certain strain, the film becomes non-conductive. This sharp transition from conductive to insulating has potential applications in flexible switches and sensors. A hysteresis in the strain-resistance curves, analogous to the hysteresis in the stress-strain curves of the polymer was also observed. Using PUU as an adhesive agent, gold electrodes were successfully assembled on Nafion-based polymer transducers. These materials showed comparable actuation behavior to the electrodes made by the Pt-reduction method, with the added advantage of the ability to form patterned electrodes for distributed transducers. Patterning techniques were developed to form colloid-polymer multilayers for use in photonic crystal materials using selective deposition on patterned silane monolayers. Patterns of gold electrodes were also made on flexible polymers using a photoresist-based method. / Ph. D.

Well-defined central functionalized asymmetric tri-block copolymers (CFABC) were designed as a new type of polymer brush surface modifier, with a short central functionalized block that can form chemical bonds with a suitable substrate surface. A combination of sequential living anionic polymerization and polymer modification reactions were used for the synthesis of two CFABCs: PS-b-poly(4-hydroxystyrene)-b-PMMA and PS-b-poly(4-urethanopropyl triethoxysilylstyrene)-b-PMMA. GPC and NMR characterization indicated that the block copolymers possessed controlled molecular weights and narrow molecular weight distributions. CFABC polymer brushes were successfully prepared by chemically grafting PS-b-poly(4-urethanopropyl triethoxysilylstyrene)-b-PMMA onto silicon wafer surfaces. AFM, XPS and ellipsometry were used to confirm the CFABC polymer brush structures and thickness. The surface properties of CFABC polymer brush modified silicon wafer substrates subjected to different environmental parameters were studied. Reversibly switchable surface energies were observed when the polymer brush modified surfaces were exposed to solvents with different polarities. The phenomenon was attributed to the chain configuration auto-adjustment in the polymer brush systems. The same mechanism was also used to explain the enhanced adhesion capability between the modified surfaces and different polymer materials (PS and PMMA). Phase behaviors of polymer thin films on unmodified and CFABC polymer brush modified silicon wafer surfaces were also studied. For thin films of polymer blends, PS blend PS-co-PMMA, the effects of film thickness, chemical composition and temperature on the phase separation mechanism were investigated. The phase behavior in thin films of triblock copolymers with or without central functionalities were compared to reveal the role of the central functionalized groups in controlling film structures. Finally, the presence of CFABC polymer brush at the interface between PS-b-PMMA diblock copolymer thin film and silicon wafer substrate was found to decrease the characteristic lamellar thickness in the thin film. A mechanism of tilted chain configurations in the thin film due to the interactions with the CFABC polymer brushes was proposed. / Ph. D.

A new method of manufacturing heat flux sensors was developed using a combination of copper etching and stencil printing nickel/silver conductive ink thermocouple materials onto a thin-film polyimide Kapton® substrate. The semi-automated production capabilities of this manufacturing process significantly decrease the cost of producing thin-film heat flux sensors while still maintaining acceptable performance characteristics. Material testing was performed to first determine the most appropriate materials as well as the theoretical sensitivity and time response of the final sensor. Seebeck coefficient of a thermocouple formed using the combination of EMS CI-1001 silver and EMS CI-5001 nickel ink was measured to be 18.3 ± 0.9 uV/ deg C. Calibrations were then performed on a sample of sensors produced using the novel manufacturing process to verify theoretical values for both sensitivity and time response. The printed heat flux sensor (PHFS) made using this process has a nominal voltage output sensitivity of 4.10 ± 0.23 mV/(W/cm2) and first order time constant response time of 0.592 ± 0.026 seconds. Lastly, a cost analysis was performed to estimate that the final cost to produce the PHFS is approximately $7.73 per sensor. This cost is significantly lower than commercially available sensors which range from $210 upwards to $3000. / Master of Science

The present study focuses on local heat transfer characteristics on the tip and near-tip regions of a turbine blade with a flat tip, tested under transonic conditions in a stationary, 2-D linear cascade consisting of seven blades, the three center blades having a variable tip clearance gap. The effects of tip clearance and exit Mach number on heat transfer distribution were investigated on the tip surface using a transient infrared thermography technique. In addition, thin film gages were used to study similar effects on the near-tip regions at 94% based on engine blade span of the pressure and suction sides. The experiments were conducted at the Virginia Tech transonic blow-down wind tunnel facility with a seven-blade linear cascade. Surface oil flow visualizations on the blade tip region were carried-out to shed some light on the leakage flow structure. Experiments were performed at three exit Mach numbers of 0.7, 0.85, and 1.05 for two different tip clearances of 0.9% and 1.8% based on engine blade span. The exit Mach numbers tested correspond to exit Reynolds numbers of 7.6 x 105, 9.0 x 105, and 1.1 x 106 based on blade true chord. The tests were performed with a freestream turbulence intensity of 12%. Results at 0.85 exit Mach showed that an increase in the tip gap clearance translates into a 12% increase in the heat transfer coefficients on the blade tip surface. Similarly, at 0.9% tip clearance, an increase in exit Mach number from 0.85 to 1.05 also led to a 24% increase in heat transfer on the tip. High heat transfer was obtained at the leading edge area of the blade tip, and an increase in the tip clearance gap and exit Mach number augmented this leading edge heat transfer. At 94% of engine blade span on the suction side near the tip, a peak in heat transfer was observed in all test cases at an s/C of 0.66 due to the onset of a downstream leakage vortex. At the design condition, this peak represents an increase of a factor of 2.5 from the immediate preceding s/C location. An increase in both the tip gap and exit Mach number resulted in an increase, followed by a decrease in the near-tip suction side heat transfer. On the near-tip pressure side, a slight increase in heat transfer was observed with increased tip gap and exit Mach number. In general, the suction side heat transfer is greater than the pressure side heat transfer as a result of the suction side leakage vortices. / Master of Science

The thesis explores the proposal that macroscopic fluid convection in thin-film processors may be adequately represented by simple linear deterministic models. In addition, it examines the suggestion that the models themselves provide a useful tool in the search for a generalizable 'intrinsic' process heat transfer film coefficient, i.e., one that includes the effects of axial dispersion of the process fluid. Such a parameter would be helpful in the design and scale up of thin-film equipment. The following approach was used to investigate this proposal: first, experimental fluid residence time distributions were obtained t over a range of operating conditions, using an industrial pilot plant thin -film processor. The experimental data were used to select an appropriate linear fluid flow model for the process. The model parameters were evaluated over this range using frequency response techniques. These models were subsequently incorporated into a numerical heat transfer simulation of the thin -film processor. Careful matching of the pilot plant transient temperature responses to those predicted by the simulation yielded the sought after intrinsic (dispersion corrected) heat transfer film coefficients for the processor. / Master of Science