This thesis relates a technique for increasing the optical strength of NaCl and KCl single-crystal samples. The 1.06-μm pulsed laser damage thresholds were increased by factors as large as 4.6 for a bulk NaCl single-crystal sample. The bulk laser damage breakdown threshold (LDBT) of the crystal was measured prior to and after heat treatment at 800*C using a Nd:YAG laser operating at 1.06 μm. Bulk and surface LDBTs were also studied on samples annealed at 400° C. These samples showed differences in damage morphology on both cleaved and polished surfaces, and the cleaved surfaces had improved damage thresholds. However, neither the polished surfaces nor the bulk showed improved threshold at the lower annealing temperature.

This dissertation includes two parts. In the first part the study is focused on the fabrication of multifunctional thin films for photovoltaic applications. There is no doubt about the importance of transforming world reliance from traditional energy resources, mainly fossil fuel, into renewable energies. Photovoltaic section still owns very small portion of the production, despite its fast growth and vast research investments. New methods and concepts are proposed in order to improve the efficiency of traditional solar cells or introduce new platforms. Recently, ferroelectric photovoltaics have gained interest among researchers. First objective in application of ferroelectric material is to utilize its large electric field as a replacement for or improvement of built-in electric field in semiconductor p-n junctions which is responsible for the separation of generated electron-hole pairs. Increase in built in electric field will increase open-circuit voltage of the solar cell. In this regard, thin films of ferroelectric hafnium dioxide doped with silicon have been fabricated using physical vapor deposition techniques. Scanning probe microscopy techniques (PFM and KPFM) have been employed to analyze ferroelectric response and surface potential of the sample. The effects of poling direction of the ferroelectric film on the surface potential and current-voltage characteristics of the cell have been investigated. The results showed that the direction of poling affects photoresponse of the cell and based on the direction it can either improved or diminished. In the second part of this work, epitaxial thin films have been synthesized with physical vapor deposition techniques such as sputtering and electron beam evaporation for the ultimate goal of producing multifunctional three-dimensional structures. Three-dimensional structures have been used for applications such as magnetic sensors, filters, micro-robots and can be used for modification of the surface of solar cells in order to improve light absorption and efficiency. One of the important techniques for producing 3-D structures is using origami techniques. The effectiveness of this technique depends on the control of parameters which define direction of bending and rolling of the film or curvature of the structure based on the residual stress in the structure after film’s release and on the quality and uniformity of the film. In epitaxially grown films, the magnitude and direction of the stress are optimized, so the control over direction of rolling or bending of the film can be controlled more accurately. For this purpose, deposition conditions for epitaxy of Zn, Fe, Ru, Ti, NaCl and Cr on Si, Al2O3 or MgO substrates have been investigated and optimized. Crystallinity, composition and morphology of the films were characterized using reflective high energy diffraction (RHEED), Auger electron spectroscopy (AES), energy dispersive X-ray (EDX), and scanning electron microscopy (SEM).

Nanostructured materials have received a surge of interest in recent years since it has become apparent that reducing the size of a material often leads to heightened mechanical behavior. From a fundamental standpoint, this stems from the confinement of dislocations. Applications in microelectromechanical devices, lithium ion batteries, gas sensing and catalysis are realized by combining the improvements in mechanical behavior from material size reduction with the heightened chemical activity offered by materials with a high surface-area-to-volume ratio. In this study, films of nanoporous Si-Mg were produced through magnetron sputtering, followed by dealloying using an environmentally benign process with distilled water. The film composition and structure was characterized both at the surface and throughout the film thickness, while the mechanical behavior was explored with nanoindentation. Dispenser cathodes operate via thermionic emission and are an important area of interest in vacuum electron devices. While scientists have known for many years what elemental constituents are used to manufacture dispenser cathodes of excellent emission behavior, a fundamental understanding has yet to be realized. In this study, components of a scandate cathode that exhibited excellent emission behavior were characterized and used to inform the study of model thin films. Isolating relevant components of the scandate cathode for careful study could help inform future breakthroughs in understanding the working mechanism(s) of the scandate cathode. The structure, composition and electronic behavior of model W-Al alloy films were characterized experimentally and compared to computation. Moreover, a unique vacuum chamber was designed to activate modern thermionic dispenser cathodes, observe residual gas species present, and measure the work function through various state-of-the-art techniques.

A process for micromachining of micro-mirror devices from silicon-on-insulator wafers was proposed and implemented. Test methods and force applicators for these devices were developed. Following successful fabrication of these devices, a novel process for fabrication of devices out of the plane of the silicon wafer was proposed, so that the devices could be actuated electrostatically. In particular, the process makes use of thick photoresist layers as a sacrificial mold into which an amorphous nickel-phosphorous alloy may be deposited. Ideal design of the electrostatically actuated micro-mirrors was investigated, and a final design was selected and modeled using FEA software, which found that serpentine-hinged devices require approximately 33% of the actuation force of their straight-beamed counterparts. An aqueous electroless plating solution composed of nickel acetate, sodium hypophosphite, citric acid, ammonium acetate, and Triton X-100 in was developed for use with the process, and bath operating parameters of 85°C and 4.5 pH were determined. However, this electroless solution failed to deposit in the presence of the photoresist. Several mechanisms proposed for deposition failure included leaching of organic solvents from the photoresist, oxidation of the nickel-titanium seed layer on which the deposition was intended to occur, and nonlinear diffusion of dissolved oxygen in the solution.

This thesis describes the design, fabrication, and testing process for a microfluidic phosphate colorimeter utilized for water quality analysis. The device can be powered by, and interfaced for data collection with, a common cell phone or laptop to dramatically reduce costs. Unlike commercially available colorimeters, this device does not require the user to measure or mix sample and reagent. A disposable poly(dimethylsiloxane) (PDMS) microfluid chip, powered by an absorption pumping mechanism, was used to draw water samples, mix the sample at a specific ratio with a molybdovanadate reagent, and load both fluids into an onboard cuvette for colorimetric analysis. A series of capillary retention valves, channels, and diffusion pumping surfaces passively controls the microfluidic chip so that no user input is required. The microfluidic chip was fabricated using a modified SU-8 soft lithography process to produce a 1.67mm light absorbance pathlength for optimal Beer Lambert Law color absorbance. Preliminary calibration curves for the device produced from standard phosphate solutions indicate a range of detection between 5 to 30mg/L for reactive orthophosphate with a linearity of R²=91.3% and precision of 2.6ppm. The performance of the PDMS absorption driven pumping process was investigated using flow image analysis and indicates an effective pumping rate up to approximately 7µL/min to load a 36µL sample.

A piezoelectric microspeaker device that could be used in a variety of acoustic applications was designed and fabricated using a thin film ZnO layer that was reactively DC sputtered onto a single crystalline n-type silicon substrate. When tested the microspeaker did not produce sound due to complications in the etching process, the thickness of the diaphragms, and clamping effects. Instead, a characterization approach was taken and the structural, optical, electrical, and piezoelectric properties of the ZnO were investigated. Scanning electron microscopy, x-ray diffraction, and atomic force microscopy were utilized to discover the ZnO’s structural properties. Using the XRD and SEM, the as-sputtered ZnO films were found to have highly c-axis oriented columnar crystals. Optical properties were determined from the reflectance spectrums obtained from a Filmetrics F20 reflectometer and were used to determine the film thickness, the optical constants, and the optical band gap of the ZnO thin films. Using a four-point probe, the as-sputtered ZnO films were found to be highly resistive and insulative, mainly due to voided growth boundaries between the crystals. To improve electrical conductivity and piezoelectric response, ZnO samples were annealed at varying temperatures in a nitrogen environment. The annealing process successfully increased the electrical conductivity and piezoelectric properties of the films. The local piezoelectric properties of the ZnO were discovered with an Asylum MFP-3D and a piezoresponse force microscopy (PFM) technique called DART-PFM. The ZnO films that were sputtered with 70 watts and an argon to oxygen gas ratio of 2:1 were found to have the highest d33 piezoelectric coefficients. The ZnO sample that was annealed at 600°C for 30 minutes had the highest overall d33 value of 4.0 pm/V, which means that the 600°C annealed ZnO films would have the best chance of making a functional microspeaker.

This thesis devoted to the experimental studies of yellow and blue luminescence (YL and BL relatively) bands in Gallium Nitride samples doped with C and Si. The band BLC was at first observed in the steady-state photoluminescence spectrum under high excitation intensities and discerned from BL1 and BL2 bands appearing in the same region of the spectrum. Using the time-resolved photoluminescence spectrum, we were able to determine the shape of the BLC and its position at 2.87 eV. Internal quantum efficiency of the YL band was estimated to be 90\%. The hole capture coefficient of the BLC related state was determined as 7 10-10 cm3/s. Properties of BLC were investigated. The YL and BLC bands are attributed to electron transitions via the (0/-) and (+/0) transition levels of the CN defect.

Graduation date: 1993

Not available

Nanoscale metal thin film dewetting via laser treatment is studied in this dissertation. The purpose is to understand: 1) the spatial and temporal nature of intrinsic instabilities; and 2) mass transportation involved in dewetting pattern evolution in metal thin films as well as in lithographically patterned nanostructures; and finally 3) to explore advanced control of metallic nanostructure fabrication via the confluence of top down nanolithography and pulsed laser induced dewetting. This study includes three sections. In first section, thin film Cu-Ni alloys ranging from 2-8nm were synthesized and laser irradiated. The evolution of the spinodal dewetting process is investigated as a function of the thin film composition which ultimately dictates the size distribution and spacing of the nanoparticles, and the optical measurements of the copper rich alloy nanoparticles revealed characteristic plasmonic peaks. In section two, the dewetting behavior of nanolithographically patterned copper rings on Silicon substrate was studied. The self assembly of the rings into ordered nanoparticle/nanodrop arrays was accomplished via nanosecond pulsed laser heating. The resultant length scale of the 13nm and 7nm thick copper rings was correlated to the competition between transport and instabilities time scales during the liquid lifetime of the melted copper rings. To explore the influence of different substrates with different surface energy, the pulsed laser heated assembly of lithographically patterned copper rings on SiO2 substrate was studied in the last section. The correlated transport and instabilities show modified timescales. It is demonstrated again that the original geometry dictates the instability pathway, which for narrow rings obeys the Rayleigh-Plateau instability and for wider rings are influenced by the thin film instability.