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.
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Though GaN CAVETs are promising new devices, they are expensive to develop due to new or exotic materials and processing steps. As a result, the accurate simulation of GaN CAVETs has become critical to the development of new devices. Using Silvaco Atlas 5.24.1.R, best practices were developed for GaN CAVET simulation by recreating the structure and results of the pGaN insulated gate CAVET presented in chapter 3 of [8].
From the results it was concluded that the best simulation setup for transfer characteristics, output characteristics, and breakdown included the following. For methods, the use of Gummel, Block, Newton, and Trap. For models, SRH, Fermi, Auger, and impact selb. For mobility, the use of GANSAT and manually specified saturation velocity and mobility (based on doping concentration). Additionally, parametric sweeps showed that, of those tested, critical CAVET parameters included channel mobility (and thus doping), channel thickness, Current Blocking Layer (CBL) doping, gate overlap, and aperture width in rectangular devices or diameter in cylindrical devices.
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.
First, in graphene quantum dot systems, conductance fluctuations are investigated from the respects of Fano resonances and quantum chaos. The conventional semi-classical theory of quantum chaotic scattering used in this field depends on an invariant classical phase-space structure. I show that for systems without an invariant classical phase-space structure, the quantum pointer states can still be used to explain the conductance fluctuations. Another finding is that the chaotic geometry is demonstrated to have similar effects as the disorders in transportations.
Second, in optomechanics systems, I find rich nonlinear dynamics. Using the semi-classical Langevin equations, I demonstrate a quasi-periodic motion is favorable for the quantum entanglement between the optical mode and mechanical mode. Then I use the quantum trajectory theory to provide a new resolution for the breakdown of the classical-quantum correspondences in the chaotic regions.
Third, I investigate the analogs of the electrical band structures and effects in the non-electrical systems. In the photonic systems, I use an array of waveguides to simulate the transport of the massive relativistic particle in a non-Hermitian scenario. A new form of Zitterbewegung is discovered as well as its analytical explanation. In mechanical systems, I use springs and mass points systems to achieve a three band degenerate band structure with a new pair of spatially separated edge states in the Dice lattice. A new semi-metal phase with the intrinsic valley-Hall effect is found.
At last, I investigate the nonlinear dynamics in the spintronics systems, in which the topological insulator couples with a magnetization. Rich nonlinear dynamics are discovered in this systems, especially the multi-stability states.
bandgap of 2.26 eV that is used in many applications in optoelectronic devices. Compared
to the two dimensional (2D) thin-film semiconductors, one-dimensional (1D)
nanowires can have different electronic properties for potential novel applications.
In this work, we present the study of ZnTe nanowires (NWs) that are synthesized
through a simple vapor-liquid-solid (VLS) method. By controlling the presence or
the absence of Au catalysts and controlling the growth parameters such as growth
temperature, various growth morphologies of ZnTe, such as thin films and nanowires
can be obtained. The characterization of the ZnTe nanostructures and films was
performed using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy
(EDX), high- resolution transmission electron microscope (HRTEM), X-ray
diffraction (XRD), photoluminescence (PL), Raman spectroscopy and light scattering
measurement. After confirming the crystal purity of ZnTe, two-terminal diodes and
three-terminal transistors were fabricated with both nanowire and planar nano-sheet
configurations, in order to correlate the nanostructure geometry to device performance
including field effect mobility, Schottky barrier characteristics, and turn-on
characteristics. Additionally, optoelectronic properties such as photoconductive gain
and responsivity were compared against morphology. Finally, ZnTe was explored in
conjunction with ZnO in order to form type-II band alignment in a core-shell nanostructure.
Various characterization techniques including scanning electron microscopy,
energy-dispersive X-ray spectroscopy , x-ray diffraction, Raman spectroscopy, UV-vis
reflectance spectra and photoluminescence were used to investigate the modification
of ZnO/ZnTe core/shell structure properties. In PL spectra, the eliminated PL intensity
of ZnO wires is primarily attributed to the efficient charge transfer process
occurring between ZnO and ZnTe, due to the band alignment in the core/shell structure. Moreover, the result of UV-vis reflectance spectra corresponds to the band
gap energy of ZnO and ZnTe, respectively, which confirm that the sample consists of
ZnO/ZnTe core/shell structure of good quality.
Unfortunately, commercial TCAD tools like Sentaurus and Silvaco Atlas are based on the effective mass approximation, while most 4H-SiC devices are not operated under low electric field, so the parabolic-like band approximation does not hold anymore. Hence, to get more accurate and reliable simulation results, full-band analysis is needed. The first step in the development of a full-band device simulator is the calculation of the band structure. In this work, the empirical pseudopotential method (EPM) is adopted. The next task in the sequence is the calculation of the scattering rates. Acoustic, non-polar optical phonon, polar optical phonon and Coulomb scattering are considered. Coulomb scattering is treated in real space using the particle-particle-particle-mesh (P3M) approach. The third task is coupling the bulk full-band solver with a 3D Poisson equation solver to generate a full-band device simulator.
For proof-of-concept of the methodology adopted here, a 3D resistor is simulated first. From the resistor simulations, the low-field electron mobility dependence upon Coulomb scattering in 4H-SiC devices is extracted. The simulated mobility results agree very well with available experimental data. Next, a 3D VDMOS is simulated. The nature of the physical processes occurring in both steady-state and transient conditions are revealed for the two generations of 3D VDMOS devices being considered in the study.
Due to its comprehensive nature, the developed tool serves as a basis for future investigation of 4H-SiC power devices.
Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process.
Highest efficiency single-junction silicon and bottom silicon sub-cell in silicon based tandems employ monocrystalline silicon wafer textured with random pyramids. Therefore, the light trapping performance of random pyramids in silicon solar cells is established. An accurate three-dimensional height map of random pyramids is captured and ray-traced to record the angular distribution of light inside the wafer which shows random pyramids trap light as well as Lambertian scatterer.
Second, the problem of front-surface reflectance common to all modules, planar solar cells and to silicon and perovskite based tandems is dealt. A nano-imprint lithography procedure is developed to fabricate polydimethylsiloxane (PDMS) scattering layer carrying random pyramids that effectively reduces the reflectance. Results show it increased the efficiency of planar semi-transparent perovskite solar cell by 10.6% relative.
Next a detailed assessment of light-management in practical two-terminal perovskite/silicon and perovskite/perovskite tandems is performed to quantify reflectance, parasitic and light-trapping losses. For this first a methodology based on spectroscopic ellipsometry is developed to characterize new absorber materials employed in tandems. Characterized materials include wide-bandgap (CH3NH3I3, CsyFA1-yPb(BrxI1-x)3) and low-bandgap (Cs0.05FA0.5MA0.45(Pb0.5Sn0.5)I3) perovskites and wide-bandgap CdTe alloys (CdZnSeTe). Using this information rigorous optical modeling of two-terminal perovskite/silicon and perovskite/perovskite tandems with varying light management schemes is performed. Thus providing a guideline for further development.
tromagnetics, semiconductors, chemical transport, and even chemical and material
electronic structure. However, few general or accurate methods have been developed
for quantum photonic devices. Here, a novel approach utilizing phase-space quantum
mechanics is developed to model photon transport in ring resonators, a form of en-
tangled pair source. The key features the model needs to illustrate are the emergence
of non-classicality and entanglement between photons due to nonlinear effects in the
ring. The quantum trajectory method is subsequently demonstrated on a sequence
of elementary models and multiple aspects of the ring resonator itself.