There has been unprecedented growth in wireless technologies in recent years; wireless devices such as cellular telephones and wireless local area network (WLAN) transceivers are becoming ubiquitous. It is now common for a single hardware device, such as a cellular telephone, to be capable of multi-band operation. Implementing a dedicated radio frequency (RF) front-end for each frequency band increases the component count and therefore the cost of the device. Consequently, there is now a requirement to design RF and microwave circuits that can be reconfigured to operate at different frequency bands, as opposed to switching between several fixed-frequency circuits. Barium strontium titanate (BST) thin films show great promise for application in reconfigurable microwave circuits. The material has a high dielectric constant which can be controlled by the application of a quasi-static electric field, combined with relatively low losses at microwave frequencies. Tunable microwave components based on BST-thin films have the potential to replace several fixed components, thereby achieving useful size and cost reductions. This thesis is concerned with the growth and microwave circuit applications of BST thin films on c- and r-plane sapphire substrates. Sapphire is an ideal substrate for microwave integrated circuit fabrication due to its low cost and low loss. Electronically tunable capacitors (varactors) were fabricated by patterning interdigital electrode structures on top of the BST films. High capacitance tunabilities of 56% and 64% were achieved for the films grown on c-plane and r-plane sapphire, respectively, at 40 V bias. A novel electronically tunable 3 dB quadrature hybrid circuit was also developed. Prototypes of this circuit were initially implemented using commercial varactor diodes, in order to validate the design. An integrated version of the coupler was then fabricated using BST varactors on c-plane sapphire. The results achieved demonstrate the potential of sapphire-based BST thin films in practical microwave circuits.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Graduation date: 1994

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 195-202).

Electronic components, based on current semiconductor technologies and operating in radiation rich environments, suffer degradation of their performance as a result of radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation hard material, because of its wide bandgap and higher atomic displacement energies, for devices intended for radiation environment applications. However, the radiation tolerance and reliability of SiC-based devices needs to be understood by testing devices  under controlled radiation environments. These kinds of studies have been previously performed on diodes and MESFETs, but multilayer devices such as bipolar junction transistors (BJT) have not yet been studied. In this thesis, SiC material, BJTs fabricated from SiC, and various dielectrics for SiC passivation are studied by exposure to high energy ion beams with selected energies and fluences. The studies reveal that the implantation induced crystal damage in SiC material can be partly recovered at relatively low temperatures, for damag elevels much lower than needed for amorphization. The implantation experiments performed on BJTs in the bulk of devices show that the degradation in deviceperformance produced by low dose ion implantations can be recovered at 420 oC, however, higher doses produce more resistant damage. Ion induced damage at the interface of passivation layer and SiC in BJT has also been examined in this thesis. It is found that damaging of the interface by ionizing radiation reduces the current gain as well. However, for this type of damage, annealing at low temperatures further reduces the gain. Silicon dioxide (SiO2) is today the dielectric material most often used for gate dielectric or passivation layers, also for SiC. However, in this thesis several alternate passivation materials are investigated, such as, AlN, Al2O3 and Ta2O5. These materials are deposited by atomic layer deposition (ALD) both as single layers and in stacks, combining several different layers. Al2O3 is further investigated with respect to thermalstability and radiation hardness. It is observed that high temperature treatment of Al2O3 can substantially improve the performance of the dielectric film. A radiation hardness study furthermore reveals that Al2O3 is more resistant to ionizing radiation than currently used SiO2 and it is a suitable candidate for devices in radiation rich applications. / QC 20120117

Dielectric waveguides are becoming important for their numerous applications in integrated optics. The study of dielectric waveguides by analytical techniques is not sufficient for many variations in waveguide shape, anisotropy, and inhomogeneity commonly encountered in waveguide materials. This work studies the finite element method as an accurate tool for the numerical modeling of dielectric waveguides. Other commonly used numerical techniques are also considered. The implementation of the finite element method is discussed. The finite element technique is also modified to incorporate the lack of fixed-potential boundary conditions in dielectric waveguides. The results of the simulations are documented for several experimental and analytical test cases. Measurements were made on waveguides fabricated in-house using the plasma-enhanced chemical vapor deposition (PECVD) films of silicon oxynitride. The light source was a 6328 A helium-neon laser. The results of the finite element simulations are compared with the experimental results and with other previously documented numerical and analytical results from the literature. The finite element method developed here is shown to be in good agreement with these results and will be useful in solving for the modes of novel waveguide designs. / Graduation date: 1993

In this work, spin-on processes were used to improve front- and rear-side technologies of solar cells to increase efficiencies to >20 %. A limited source diffusion process was developed using phosphoric acid dopant solutions developed in-house. An optimal emitter was obtained to be used in conjunction with screen-printed contacts. This emitter was used to improve the efficiency of conventional full aluminum back surface field solar cells to 19.6 %. A streamlined process was then developed to fabricate high-efficiency dielectric rear passivated cells in a single high temperature step. This process combined the diffusion process described earlier with a spin-on dielectric for rear passivation to achieve solar cell efficiencies of ~20%. Several laser candidates were investigated to improve process reproducibility and throughput. Ultra-violet laser with nanosecond pulse width was identified as the optimal choice. Cell efficiencies of ~20% were reproduced using UV laser for ablation of rear dielectric. This cell design and process were transferred to low-cost low-lifetime commercial grade substrates after identifying the optimal substrate characteristics using modeling.

Polymer/ceramic nanocomposites benefit by combining high permittivities (r) of metal oxide nanoparticles with high dielectric strength and excellent solution-processability of polymeric hosts. Simple mixing of nanoparticles and polymer generally results in poor quality materials due mainly to the agglomeration of nanoparticles and poor miscibility of nanoparticles in host materials. Surface modification of metal oxide nanoparticles with phosphonic acid-based ligands was found to afford a robust surface modification and improve the processablity and the quality of nanocomposites. The use of phosphonic-acid modified barium titanate (BaTiO₃) nanoparticles in dielectric nanocomposites dramatically improved the stability of the nanoparticle dispersion and the quality of the nanocomposites. Surface modification of BaTiO₃ nanoparticles allowed high quality nanocomposite thin films in ferroelectric polymer hosts such as poly(vinylidene fluoride-co-hexafluoropropylene) with large volume fractions (up to 50 vol. %), which exhibited a remarkable combination of a high permittvity (35 at 1 kHz) and a high breakdown strength (210 V/µm) leading to a maximum energy storage density of 6.1 J/cm³. The effect of nanoparticle volume fractions on the dielectric properties of this nanocomposite system was investigated and compared with theoretical models. At high volume fraction of nanoparticles, the porosity of the nanocomposites was found to have important role in the dielectric performance. A combined effective medium theory and finite difference simulation was used to model the dielectric properties of high volume fraction dielectric nanocomposites with porosity. These results provide a guideline to optimize the volume fractions of nanoparticles for maximum energy density. Nanocomposites based on phosphonic acid-modified BaTiO₃ nanoparticles can also be used as printable high permittivity dielectrics in organic electronics. High volume fractions (up to 37 vol. %) of phosphonic acid-modified BaTiO₃ nanoparticles dispersed in cross-linked poly(4-vinylphenol) allowed solution-processable high permittivity thin films with a large capacitance density (~50 nF/cm²) and a low leakage current (10 8 A/cm²) suitable as a gate insulator in organic field-effect transistors (OFETs). Pentacene-based OFETs using these nanocomposites showed a low threshold voltage (< -2.0 V) and a large on/off current ratio (Ion/off 104 ~ 106) due to the high capacitance density and low leakage current of the gate insulator.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 111-119). Available also in a digital version from Dissertation Abstracts.