In order to develop more compact optical fiber sensing systems, modal filtering can be performed in-line by using micro-optical devices. Two such devices are a laterally offset few-moded optical fiber mechanical splice and a modal conversion optical fiber coupler. A third device, the air-gap splice used with multimode fibers, can examine the modal content of an optical fiber. A basic theoretical understanding on how these devices operate is reviewed. A splice loss calculation for few-moded optical fibers is presented. Applications of the asymmetrical few-moded mechanical optical fiber splice, the modal conversion coupler, and the air-gap splice are discussed. / Master of Science

<p>Silicon nanostructures formed in silicon-rich silicon nitride (SRSN) and cerium and terbium doped silicon oxide thin films grown using different types of plasma-enhanced chemical vapour deposition have been studied through photoluminescence (PL) and synchrotron-based X-ray absorption spectroscopies to determine the effects of deposition and processing parameters on the luminescent and structural properties of these materials. The SRSN films exhibited bright PL attributed to quantum confinement effects in the silicon nanoclusters (Si-ncs) as well as radiative defects in the silicon nitride host matrix. The peak emission energy could be tuned from the near-infrared across the entire visible spectrum by controlling the film composition and the post-deposition annealing temperature and time to change the size of the Si-ncs. Preliminary experiments on cerium doped SRSN samples indicated that although the cerium ions coordinate in the optically active trivalent oxidation state, they were not effectively sensitized by Si-ncs in the films tested, most likely due to the nanoclusters having bandgap energies that were unsuitable for this purpose. In cerium and terbium co-doped silicon oxide films, cerium disilicate (Ce₂Si₂O₇) nanocrystallites were formed by annealing at temperatures of 900°C and higher. The A-Ce₂Si₂O₇, G-Ce₂Si₂O₇, and Ce₆[Si₄O₁₃][SiO₄]₂ phases of cerium disilicate were observed to form under different deposition and annealing conditions. All three phases exhibited extremely bright violet-blue PL and were found to efficiently sensitize green emission from co-dopant Tb³⁺ ions in the films. The Tb³⁺ luminescence predominantly corresponded to the ⁵D₄→⁷F_3–6 emission lines, although weak ⁵D₃→⁷F_2–6 emission lines were also observed in films containing relatively high concentrations of terbium indicating that the sensitization of Tb³⁺ ions occurred through the ⁵D₃, ⁵L₁₀, or ⁵D₂ energy levels.</p> / Doctor of Philosophy (PhD)

Heterojunctions of CuInSe2 (CIS) nanowires with cadmium sulfide (CdS) were fabricated demonstrating for the first time, vertically aligned nanowires of CIS in the conventional Mo/CIS/CdS stack. These devices were studied for their material and electrical characteristics to provide a better understanding of the transport phenomena governing the operation of heterojunctions involving CIS nanowires. Removal of several key bottlenecks was crucial in achieving this. For example, it was found that to fabricate alumina membranes on molybdenum substrates, a thin interlayer of tungsten had to be inserted. A qualitative model was proposed to explain the difficulty in fabricating anodized aluminum oxide (AAO) membranes directly on Mo. Experimental results were used to corroborate this model. Subsequently, a general procedure to use any material that can be deposited using sputtering or evaporation as a back contact for nanowires grown using AAO templates was developed. Experimental work to demonstrate this by transferring thin AAO templates onto flexible Polyimide (PI) substrates was performed. This pattern transfer approach opens doors for a wide variety of applications on almost any substrate. Any material that can be deposited by physical means can then be used as a back contact. Electron-beam induced deposition using a liquid precursor (LP-EBID) was used to selectively grow preconceived patterns of compound semiconductor (CdS) nanoparticles. Stoichiometric CdS nanoparticle patterns were grown successfully using this method. They were structurally and optically characterized indicating high purity deposits. This approach is promising because it marries the precision of e-beam lithography with the versatility of solution based deposition methods.

Polycyclic aromatic hydrocarbons composed of benzenoid rings fused in a linear fashion comprise the class of compounds known as acenes. The structures containing three to six ring fusions are brightly colored and possess band gaps and charge transport efficiencies sufficient for semiconductor applications. These molecules have been investigated throughout the past several decades to assess their optoelectronic properties. The absorption, emission and charge transport properties of this series of molecules has been studied extensively to elucidate structure-property relationships. A wide variety of analogous molecules, incorporating heterocycles in place of benzenoid rings, demonstrate similar properties to the parent compounds and have likewise been investigated. Functionalization of acene compounds by placement of groups around the molecule affects the way in which molecules interact in the solid state, in addition to the energetics of the molecule. The use of electron donating or electron withdrawing groups affects the frontier molecular orbitals and thus affects the optical and electronic gaps of the molecules. The use of bulky side groups such as alkylsilylethynyl groups allows for crystal engineering of molecular aggregates, and changing the volume and dimensions of the alkylsilyl groups affects the intermolecular interactions and thus changes the packing motif. In chapter 2, a series of tetracene and pentacene molecules with strongly electron withdrawing groups is described. The investigation focuses on the change in energetics of the frontier molecular orbitals between the base acene and the nitrile and dicyanovinyl derivatives as well as the differences between the pentacene and tetracene molecules. The differences in close packing motifs through use of bulky alkylsilylethynyl groups is also discussed in relation to electron acceptor material design and bulk heterojunction organic photovoltaic characteristics. Chapter 3 focuses on molecular acceptor and donor molecules for bulk heterojunction organic photovoltaics based on anthrathiophene and benzo[1,2-b:4,5-b’]dithiophene central units like literature molecules containing fluorene and dithieno[2,3-b:2’,3’-d]silole cores. The synthetic strategies of developing reduced symmetry benzo[1,2-b:4,5-b’]dithiophene to study the effect of substitution around the central unit is also described. The optical and electronic properties of the donors and acceptors are described along with the performance and characteristics of devices employing these molecules. The final two data chapters focus on new nitrogen containing polycyclic hydrocarbons containing indolizine and (2.2.2) cyclazine units. The optical, electronic and other physical properties of these molecules are explored, in addition to the synthetic strategies for incorporating the indolizine and cyclazine units. By use of alkylsilylethynyl groups, crystal engineering was investigated for the benzo[2,3-b:5,6-b’]diindolizine chromophore described in chapter 4 to target the 2-D “brick-work” packing motif for application in field effect transistor devices. Optical and electronic properties of the cyclazine end-capped acene molecules described in chapter 5 were investigated and described in relation to the base acene molecules. In both cases, density functional theory calculations were conducted to better understand unexpected optical properties of these molecules, which are like the linear acene series despite the non-linear attachment.

With the rapid progress of nanotechnologies, there are two developing trends for the next generation of sensors: miniaturization and multi-functionality. Device miniaturization requires less power consumption, or even self-powered system. Multi-functional devices are usually based on multi-property coupling effects. Piezoelectric semiconductors have been considered to be potential candidates for self-powered/multi-functional devices due to their piezotronic coupling effect. In this dissertation, ZnO and CdSe nanowire arrays have been synthesized as the piezoelectric semiconductor materials to develop the following self-powered/multi-functional sensors: (1) self-powered gas sensors of ZnO/SnO2, ZnO/In2O3, ZnO/WO3 and CdSe nanowire arrays have been assembled. All these gas sensors are capable of detecting oxidizing gas and reducing gas without any external power supply owing to piezotronic effect which can convert mechanical energies to electrical energy to power the sensors; (2) a self-powered ZnO/ZnSe core/shell nanowire array photodetector has been fabricated. This photodetector is able to detect the entire range of the visible spectrum as well as UV light because of its type II heterostructure. The absolute sensitivity and the percentage change in responsivity of the photodetector were significantly enhanced resulting from the piezo-phototronic effect. The photodetector also exhibited self-powered photodetection behavior; (3) three dimensional nanowire arrays, such as ZnO and ZnO/Co3O4, have been synthesized to investigate piezo-magnetotronic and piezo-photo-magnetotronic effects. Under magnetic field, the magnetic-induced current of ZnO nanowire array decreased as magnetic field increased, and the current difference was magnified by one order of magnitude caused by piezo-magnetotronic effect through applying a stress. In contrast, under UV light illumination, the current response increased with an increment of magnetic field. The current difference was enhanced by at least two orders of magnitude attributed to piezo-photo-magnetotronic effect. Furthermore, ZnO/Co3O4 core/shell structure was employed to further improve the magnetic-induced current difference. This phenomenon projects a potential for multi-functional piezo-magnetotronic and piezo-photo-magnetotronic device development.

Previous work done at Cal Poly has shown that thin-film nickel-titanium (NiTi) can be easily sputtered onto silicon wafers and annealed to create a crystallized shape memory alloy (SMA) film. Initial work on creating devices yielded cantilevers that were highly warped due to thin-film stress created during the sputtering process. The objective of this work was to create a thin-film NiTi SMA device that could be better characterized. A membrane was selected due to the simplicity of fabrication and testing which would also oppose the thin-film stress due to the increase in attachment points to the substrate. Silicon wafers were etched through the majority of the thickness (~75%) creating square etch pits of varying sizes varying from 1294 µm to 4394 µm. The wafers were then sputtered with an approximate NiTi film of 5 µm followed by a thin chromium film. The chromium film would act as a diffusion barrier and prevent oxygen from diffusing into the NiTi and reacting with the titanium and forming titanium dioxide. These wafers were then annealed in a custom built vacuum annealing chamber at 550 °C for 1 hour with a pressure around 77 kPa. The chromium was then etched away followed by the remaining silicon. This left a thin membrane of shape memory NiTi which was packaged in order for characterization. The devices were glued to an aluminum substrate using polydimethylsiloxane (PDMS) and sealed with a small Tygon tube leading to the sealed chamber. This packaged device was then able to be pressurized using a nitrogen tank and the resulting NiTi membrane deflection was measured using a profilometer. Due to the differences in elastic moduli of the room temperature phase (martensite) and the high temperature phase (austenite) a difference of deflection was expected. The austenite finish (Af) temperature of bulk NiTi films was found to be around 60 °C so the devices were tested at both room temperature and at 60 °C. After testing seven separate devices of varying sizes, a regression model was used to analyze the final data. It was found that pressure, membrane size and theoretical versus actual deflection all affected the maximum deflection, but temperature did not. Higher pressures and larger membranes led to higher deflections as membrane deflection models from fundamental principles indicated. Some devices showed inferior performance when compared to the model due to incomplete silicon etching which caused lower deflection due to the much higher modulus of the remaining silicon. Thickness could also limit the amount of deflection measured with a thicker film leading to less deflection, but this is likely not the case due to the high uniformity of the sputtering system. Other devices showed superior performance over the model most likely due to either local delamination or lateral silicon etching. Both these would create a membrane that was larger than expected leading to a higher deflection. Unforutnaly, differential scanning calorimetry (DSC) analysis showed no shape memory behavior on a test wafer which was anneald at 550 ˚C for 1 hour. A design of experiments was conducted in order to find a heat treatment that would anneal the NiTi film and ensure that shape memory behavior could be obtained. An annealing at 650 °C for 1 hour showed a sharper and clearer Af phase transformation at around the target temperature of 60 °C. Annealing a full wafer at this temperature and time also showed that no delamination would occur which has also been linked to nonideal behavior of the NiTi membranes which has also been linked to meaningful behavior of the NiTi membranes.

Gallium Nitride (GaN) high electron mobility transistors (HEMTs) are becoming increasingly popular in power amplifier systems as an alternative to bulkier vacuum tube technologies. GaN offers advantages over other III-V semiconductor heterostructures such as a large bandgap energy, a low dielectric constant, and a high critical breakdown field. The aforementioned qualities make GaN a prime candidate for high-power and radiation-hardened applications using a smaller form-factor. Several different types of semiconductor substrates have been considered for their thermal properties and cost-effectiveness, and Silicon (Si) has been of increasing interest due to a balance between both factors. In this thesis, the DC, RF, and thermal characteristics of GaN HEMTs grown on Si-substrates will be investigated through a series of accelerated lifetime experiments. A figure of merit known as the critical voltage is explored and used as the primary means by which the GaN-on-Si devices are electrically strained. The critical voltage is defined as the specific voltage bias by which a sudden change in device performance is experienced due to a deformation of the target GaN HEMT’s epitaxial structure. Literature on the topic details the inevitable formation of pits and cracks localized under the drain-side of the gate contact that promote electrical degradation of the devices via the inverse piezoelectric effect. Characteristic changes in device performance due to high field strain are recorded and physical mechanisms behind observed degraded performance are investigated. The study assesses the performance of roughly 60 GaN-on-Si HEMTs in four experimental settings. The first experiment investigates the critical voltage of the device in the off-state mode of operation and explores device recovery post-stress. The second experiment analyzes alterations in DC and RF performance under varying thermal loads and tracks the dependence of the critical voltage on temperature. The third experiment examines electron trapping within the HEMTs as well as detrapping methodologies. The final experiment links the changes in RF performance induced by high field strain to the small-signal parameters of the HEMT. Findings from the research conclude the existence of process-dependent defects that originate during the growth process and lead to inherent electron traps in unstressed devices. Electron detrapping due to high electric field stress applied to the HEMTs was observed, potentially localized within the AlGaN layer or GaN buffer of the HEMT. The electron detrapping in turn contributed to drain current recovery and increased unilateral performance of the transistor in the RF regime. Thermal experiments resulted in a positive shift in critical voltage, which enhanced gate leakage current at lower gate voltage drives.

Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) kesterite compound has attracted much attention in the last years as a new abundant, low cost, and environmentally benign material with desirable optoelectronic properties for Photovoltaic (PV) thin film solar cell applications. Among various synthesis routes for CZTS thin films, sol-gel processing is one of the most attractive routes to obtain CZTS films with superior quality and low cost. In this study, sol-gel sulfurization process parameters for CZTS thin films were systematically investigated to identify the proper process window. In addition, temperature dependent Raman spectroscopy was employed to monitor the CZTS sulfurization process in real time and gain fundamental information about the phase formation and degradation mechanisms of CZTS under the relevant processing conditions. It was found that CZTS thin films with different Cu stoichiometry can be prepared using parts-per-million (ppm) level of hydrogen sulfide (H2S) gas as opposed to high percentage level of H2S (e.g., ≥ 5%) in all previous studies. Samples sulfurized at lower temperatures of ~350°C and 125°C revealed the formation of CZTS phase as confirmed by XRD, Raman micro-spectroscopy, and sheet resistance measurement. Local EDS analysis indicates that CZTS films prepared at those low temperatures have a near-stoichiometric composition and are sometimes accompanied by the formation of Cu2-xS phase(s). Also, stoichiometric and Cu-rich precursor solutions tend to yield CZTS samples with better crystallinity and superior optical properties compared with the Cu-deficient solution. Moreover, in situ Raman monitoring of phase formation of CZTS material was carried out from room temperature up to 350°C in a 100 ppm H2S+4%H2+N2 gas mixture. The results showed that CZTS phase formed in about 30 min via a direct reaction between the metal oxide precursor film and the H2S-H2 gas mixture at an intermediate temperature of 350°C and remained stable upon extended exposure. In comparison, at a lower temperature (170°C), the oxide precursor film had to be reduced first (e.g., in 4% H2/N2 forming gas) and then the CZTS phase emerged. However, continued sulfurization at a lower temperature (e.g., 170°C) led to the disintegration of CZTS and the formation of CuS impurity, which remains stable upon cooling the sample down to room temperature. Furthermore, results of in situ Raman monitoring of CZTS films in an oxygen-rich atmosphere at elevated temperatures up to 600°C suggested that CZTS oxidizes first at ~400°C to form tin oxide (SnO2) and binary sulfides of mainly copper sulfide (Cu2-xS) and zinc sulfide (ZnS). Then, at temperatures higher than 400°C, the remaining sulfides oxidize to form zinc oxide (ZnO). The outcomes of the current study set the directions for optimizing the CZTS film structure and stoichiometry toward developing low cost and high-performance CZTS solar cells in future.

It has been found that non-magnetic organic semiconductors can show some magnetic responses in low magnetic field (<100 >mT). When applying magnetic field, the electroluminescence, electrical current, photocurrent, and photoluminescence could change with magnetic field, which are called magnetic field effects. Magnetic field effects are generated through spin-dependent process affected by the internal magnetic interaction. In nonmagnetic materials, hyperfine interaction has been supposed to dominantly affect the spin-dependent process recently. But the conclusion was made in weak spin-orbital coupling organic semiconductor. The hyperfine interaction might not be the main reason responsible for magnetic field effects in strong spin-orbital coupling materials. Therefore, the study of magnetic field effects in strong spin-orbital coupling organic semiconductor is important to get a whole view of the origin of the magnetic field effects in nonmagnetic organic semiconductors. This dissertation will clarify the generation mechanism of magnetic field effect in nonmagnetic organic semiconductors and further explore how the strong spin-orbital coupling affecting the magnetic field effect. It has been found the intermolecular excited states are important inter-median for magnetic field effects. The change of intersystem crossing at intermolecular excited states will change the singlet/triplet ratio and further generate magnetic field effects through different recombination and dissociation properties of singlet and triplet intermolecular excited states. Both the energy transfer effect coupled spin orbital coupling and energy transfer effect free spin orbital-coupling are discussed in the dissertation. The tuning of the magnetic field effect by adjusting the spin-orbital coupling is also established through distance effect and interface effect. It has been found that changing inter-molecular spin-orbital coupling is a critical factor to generate magnetic field effects in organic semiconductors. And the sensitivity of different magnetic field effects to strong spin-orbital coupling strength is depending on the final product. The internal magnetic interaction can be hyperfine interaction, spin orbital coupling and spin-spin interaction between electrons. The hyperfine interaction and spin orbital coupling are important in nonmagnetic organic semiconductors. But the electron spin-spin interaction is important in magnetic organic semiconductors. The magnetocurrent for magnetic and nonmagnetic organic semiconductors at different temperature has been compared.

This work focuses on discovery and development of novel binary halide scintillation materials for radiation detection applications. A complete laboratory for raw materials handling, ampoule preparation, material rapid synthesis screening, single crystal growth, sample cutting, polishing and packaging of hygroscopic halide scintillation materials has been established. Ce3+ and Eu2+ activated scintillators in three binary systems: Alkali Halide – Rare Earth Halide (AX–REX3), Alkali Halide – Alkaline Earth Halide (AX–AEX2) and Alkalin Earth Halide – Rare Earth Halide (AEX2–REX3) were systematically studied. Candidates for new scintillation materials in the three systems were selected based on a set of selection rules. A total of 42 Ce3+ or Eu2+ activated binary halide scintillation material candidates were synthesized and characterized. Among all synthesized candidates, 10 - 15 candidate materials were found to be highly promising in terms of high scintillation light output, fast scintillation decay or desirable emission wavelength. Three most promising candidates, Cs3EuI5, CsGd2Cl7:Ce3+ and CsSrI3:Eu2+ were selected for single crystal growth and further evaluation. Technologies for single crystal growth of hygroscopic halide scintillation materials were developed. Detailed design of experimental apparatuses was discussed. Single crystals were successfully grown via Bridgman or Vertical Gradient Freeze techniques. Study on optical and scintillation properties was performed. Possibility of using CsGd2Cl7:Ce3+ as a neutron detector was confirmed. CsSrI3:Eu2+ shows extraordinary scintillation light output (73,000 ph/MeV), excellent energy resolution (3.9%) and ease for crystal growth. A scaled-up crystal growth was carried out. A bulk crystal of 1” diameter CsSrI3:Eu2+ was successfully grown. Energy level structure and charge carrier traps in CsSrI3:Eu2+ were investigated. Potential of CsSrI3:Eu2+ in various radiation detection applications were evaluated.