ASU Electronic Theses and Dissertations
This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.
In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.
Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.
As part of MS Thesis we developed electrostatic simulator for DG SOI devices by implementing the self consistent full band Schrodinger Poisson solver. We used the Empirical Pseudopotential method in conjunction with supercell approach to solve the Schrodinger Equation. EPM was chosen because it has few empirical parameters which give us good accuracy for experimental results. Also EPM is computationally less expensive as compared to the atomistic methods like DFT(Density functional theory) and NEGF (Non-equilibrium Green's function). In our workwe considered two crystallographic orientations of Si,namely [100] and [110].
Alloyed Ti/Al/Ni/Au contact and non-alloyed Al/Au contact were developed to form low-resistivity contacts to n-GaN and their stability at high temperature were studied. The alloyed Ti/Al/Ni/Au contact offered a specific contact resistivity (ρc) of 6×10-6 Ω·cm2 at room temperature measured the same as the temperature increased to 400°C. No significant change in ρc was observed after the contacts being subjected to 400°C, 450°C, 500°C, 550°C, and 600°C, respectively, for at least 4 hours in air. Since several device technology prefer non-alloyed contacts Al/Au metal stack was applied to form the contacts to n-type GaN. An initial ρc of 3×10-4 Ω·cm2, measured after deposition, was observed to continuously reduce under thermal stress at 400°C, 450°C, 500°C, 550°C, and 600°C, respectively, finally stabilizing at 5×10-6 Ω·cm2. Both the alloyed and non-alloyed metal contacts showed exceptional capability of stable operation at temperature as high as 600°C in air with low resistivity ~10-6 Ω·cm2, with ρc lowering for the non-alloyed contacts with high temperatures.
The p-GaN contacts showed remarkably superior ohmic behavior at elevated temperatures. Both ρc and sheet resistance (Rsh) of p-GaN decreased by a factor of 10 as the ambient temperature increased from room temperature to 390°C. The annealed Ni/Au contact showed ρc of 2×10-3 Ω·cm2 at room temperature, reduced to 1.6×10-4 Ω·cm2 at 390°C. No degradation was observed after the contacts being subjected to 450°C in air for 48 hours. Indium Tin Oxide (ITO) contacts, which has been widely used as current spreading layer in GaN-base optoelectronic devices, measured an initial ρc [the resistivity of the ITO/p-GaN interface, since the metal/ITO ρc is negligible] of 1×10-2 Ω·cm2 at room temperature. No degradation was observed after the contact being subjected to 450°C in air for 8 hours.
Accelerated life testing (ALT) was performed to further evaluate the contacts stability at high temperatures quantitatively. The ALT results showed that the annealed Ni/Au to p-GaN contacts is more stable in nitrogen ambient, with a lifetime of 2,628 hours at 450°C which is approximately 12 times longer than that at 450°C in air.
Despite an error based on a false assumption in the 1950’s, which had severely underestimated the efficiency of magneto-dielectric antennas, recently demonstrated magnetic-antennas have been shown to exhibit extraordinary efficiency in conformal applications. Whereas conventional metal-and-dielectric antennas carrying radiating electric currents suffer a significant disadvantage when placed conformal to the conducting surface of a platform, because they induce opposing image currents in the surface, magnetic-antennas carrying magnetic radiating currents have no such limitation. Their magnetic currents produce co-linear image currents in electrically conducting surfaces.
However, the permeable antennas built to date have not yet attained the wide bandwidth expected because the magnetic-flux-channels carrying the wave have not been designed to guide the wave near the speed of light at all frequencies. Instead, they tend to lose the wave by a leaky fast-wave mechanism at low frequencies or they over-bind a slow-wave at high frequencies. In this dissertation, we have studied magnetic antennas in detail and presented the design approach and apparatus required to implement a flux-channel carrying the magnetic current wave near the speed of light over a very broad frequency range which also makes the design of a frequency independent antenna (spiral) possible. We will learn how to construct extremely thin conformal antennas, frequency-independent permeable antennas, and even micron-sized antennas that can be embedded inside the brain without damaging the tissue.
Despite the promising potential of GaP/Si heterojunction solar cells, there are two main obstacles to realize high performance photovoltaic devices from this structure. First, the growth of the polar material (GaP) on the non-polar material (Si) is a challenge in how to suppress the formation of structural defects, such as anti-phase domains (APD). Further, it is widely observed that the minority-carrier lifetime of the Si substrates is significantly decreased during epitaxially growth of GaP on Si.
In this dissertation, two different GaP growth methods were compared and analyzed, including migration-enhanced epitaxy (MEE) and traditional molecular beam epitaxy (MBE). High quality GaP can be realized on precisely oriented (001) Si substrates by MBE growth, and the investigation of structural defect creation in the GaP/Si epitaxial structures was conducted using high resolution X-ray diffraction (HRXRD) and high resolution transmission electron microscopy (HRTEM).
The mechanisms responsible for lifetime degradation were further investigated, and it was found that external fast diffusors are the origin for the degradation. Two practical approaches including the use of both a SiNx diffusion barrier layer and P-diffused layers, to suppress the Si minority-carrier lifetime degradation during GaP epitaxial growth on Si by MBE were proposed. To achieve high performance of GaP/Si solar cells, different GaP/Si structures were designed, fabricated and compared, including GaP as a hetero-emitter, GaP as a heterojunction on the rear side, inserting passivation membrane layers at the GaP/Si interface, and GaP/wet-oxide functioning as a passivation contact. A designed of a-Si free carrier-selective contact MoOx/Si/GaP solar cells demonstrated 14.1% power conversion efficiency.