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- Genre: Masters Thesis
Description
Microwave dielectrics are widely used to make resonators and filters in telecommunication systems. The production of thin films with high dielectric constant and low loss could potentially enable a marked reduction in the size of devices and systems. However, studies of these materials in thin film form are very sparse. In this research, experiments were carried out on practical high-performance dielectrics including ZrTiO4-ZnNb2O6 (ZTZN) and Ba(Co,Zn)1/3Nb2/3O3 (BCZN) with high dielectric constant and low loss tangent. Thin films were deposited by laser ablation on various substrates, with a systematical study of growth conditions like substrate temperature, oxygen pressure and annealing to optimize the film quality, and the compositional, microstructural, optical and electric properties were characterized. The deposited ZTZN films were randomly oriented polycrystalline on Si substrate and textured on MgO substrate with a tetragonal lattice change at elevated temperature. The BCZN films deposited on MgO substrate showed superior film quality relative to that on other substrates, which grow epitaxially with an orientation of (001) // MgO (001) and (100) // MgO (100) when substrate temperature was above 500 oC. In-situ annealing at growth temperature in 200 mTorr oxygen pressure was found to enhance the quality of the films, reducing the peak width of the X-ray Diffraction (XRD) rocking curve to 0.53o and the χmin of channeling Rutherford Backscattering Spectrometry (RBS) to 8.8% when grown at 800oC. Atomic Force Microscopy (AFM) was used to study the topography and found a monotonic decrease in the surface roughness when the growth temperature increased. Optical absorption and transmission measurements were used to determine the energy bandgap and the refractive index respectively. A low-frequency dielectric constant of 34 was measured using a planar interdigital measurement structure. The resistivity of the film is ~3×1010 ohm·cm at room temperature and has an activation energy of thermal activated current of 0.66 eV.
ContributorsLi, You (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Singh, Rakesh (Committee member) / Arizona State University (Publisher)
Created2013
Description
Multiple quantum well (MQW) structures have been employed in a variety of solid state devices. The InGaAs/GaAs material system is of special interest for many optoelectronic applications. This study examines epitaxial growth and defect creation in InGaAs/GaAs MQWs at its initial stage. Correlations between physical properties, crystal perfection of epitaxial structures, and growth conditions under which desired properties are achieved appear as highly important for the realization and final performance of semiconductor based devices.
Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature Tdep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response.
Molecular beam epitaxy was utilized to grow InGaAs/GaAs MQW structures with a variation in deposition temperature Tdep among the samples to change crystalline and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep=505°C was found for 20% In composition. The density of 60° primary and secondary dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection, as evaluated by lateral and vertical coherence lengths and diffuse scattering in reciprocal space maps. Likewise, the strength of non-radiative Shockley-Read-Hall recombination increased as deposition temperature was reduced. Elevated deposition temperature led to InGaAs decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering. High available thermal energy increased atomic surface diffusivity and resulted in growth surface instability against perturbations, manifesting in lateral layer thickness undulations. Carriers in structures grown at elevated temperature experience localization in local energy minima.InGaAs/GaAs MQW structures reveal correlation between their crystal quality and optical properties. It can be suggested that there is an optimal growth temperature range for each In composition with high crystal perfection and best physical response.
ContributorsKarow, Matthias (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Faleev, Nikolai N (Committee member) / Ning, Cun-Zheng (Committee member) / Arizona State University (Publisher)
Created2014
Description
In this work, I worked on the synthesis and characterization of nanowires and belts, grown using different materials, in Chemical Vapor Deposition (CVD) system with catalytic growth method. Through this thesis, I utilized the Photoluminescence (PL), Secondary Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) analyses to find out the properties of Erbium Chloride Silicate (ECS) and two segment CdS-CdSe samples. In the first part of my research, growth of very new material, Erbium Chloride Silicate (ECS), in form of core/shell Si/ECS and pure ECS nanowires, was demonstrated. This new material has very fascinating properties for new Si based photonic devices. The Erbium density in those nanowires is which is very high value compared to the other Erbium doped materials. It was shown that the luminescence peaks of ECS nanowires are very sharp and stronger than their counterparts. Furthermore, both PL and XRD peaks get sharper and stronger as growth temperature increases and this shows that crystalline quality of ECS nanowires gets better with higher temperature. In the second part, I did a very detail research for growing two segment axial nanowires or radial belts and report that the structure type mostly depends on the growth temperature. Since our final step is to create white light LEDs using single axial nanowires which have three different regions grown with distinct materials and give red, green and blue colors simultaneously, we worked on growing CdS-CdSe nanowires or belts for the first step of our aim. Those products were successfully grown and they gave two luminescence peaks with maximum 160 nm wavelength separation depending on the growth conditions. It was observed that products become more likely belt once the substrate temperature increases. Also, dominance between VLS and VS is very critical to determine the shape of the products and the substitution of CdS by CdSe is very effective; hence, CdSe growth time should be chosen accordingly. However, it was shown two segmented products can be synthesized by picking the right conditions and with very careful analyses. We also demonstrated that simultaneous two colors lasing from a single segmented belt structures is possible with strong enough-pumping-power.
ContributorsTurkdogan, Sunay (Author) / Ning, Cun-Zheng (Thesis advisor) / Tao, Meng (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2012
Description
The evolution of single hairpin vortices and multiple interacting hairpin vortices are studied in direct numerical simulations of channel flow at Re-tau=395. The purpose of this study is to observe the effects of increased Reynolds number and varying initial conditions on the growth of hairpins and the conditions under which single hairpins autogenerate hairpin packets. The hairpin vortices are believed to provide a unified picture of wall turbulence and play an important role in the production of Reynolds shear stress which is directly related to turbulent drag. The structures of the initial three-dimensional vortices are extracted from the two-point spatial correlation of the fully turbulent direct numerical simulation of the velocity field by linear stochastic estimation and embedded in a mean flow having the profile of the fully turbulent flow. The Reynolds number of the present simulation is more than twice that of the Re-tau=180 flow from earlier literature and the conditional events used to define the stochastically estimated single vortex initial conditions include a number of new types of events such as quasi-streamwise vorticity and Q4 events. The effects of parameters like strength, asymmetry and position are evaluated and compared with existing results in the literature. This study then attempts to answer questions concerning how vortex mergers produce larger scale structures, a process that may contribute to the growth of length scale with increasing distance from the wall in turbulent wall flows. Multiple vortex interactions are studied in detail.
ContributorsParthasarathy, Praveen Kumar (Author) / Adrian, Ronald (Thesis advisor) / Huang, Huei-Ping (Committee member) / Herrmann, Marcus (Committee member) / Arizona State University (Publisher)
Created2011
Description
The disordered nature of glass-forming melts results in two features for its dynamics i.e. non-Arrhenius and non-exponential behavior. Their macroscopic properties are studied through observing spatial heterogeneity of the molecular relaxation. Experiments performed in a low-frequency range tracks the flow of energy in time on slow degrees of freedom and transfer to the vibrational heat bath of the liquid, as is the case for microwave heating. High field measurements on supercooled liquids result in generation of fictive temperatures of the absorbing modes which eventually result in elevated true bath temperatures. The absorbed energy allows us to quantify the changes in the 'configurational', real sample, and electrode temperatures. The slow modes absorb energy on the structural relaxation time scale causing the increase of configurational temperature resulting in the rise of dielectric loss. Time-resolved high field dielectric relaxation experiments show the impact of 'configurational heating' for low frequencies of the electric field and samples that are thermally clamped to a thermostat. Relevant thermal behavior of monohydroxy alcohols is considerably different from the cases of simple non-associating liquids, due to their distinct origins of the prominent dielectric loss. Monohydroxy alcohols display very small changes due to observed nonthermal effects without increasing sample temperature. These changes have been reflected in polymers in our measurements.
ContributorsPathak, Ullas (Author) / Richert, Ranko (Thesis advisor) / Dai, Lenore (Thesis advisor) / Nielsen, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
Many physical phenomena and industrial applications involve multiphase fluid flows and hence it is of high importance to be able to simulate various aspects of these flows accurately. The Dynamic Contact Angles (DCA) and the contact lines at the wall boundaries are a couple of such important aspects. In the past few decades, many mathematical models were developed for predicting the contact angles of the inter-face with the wall boundary under various flow conditions. These models are used to incorporate the physics of DCA and contact line motion in numerical simulations using various interface capturing/tracking techniques. In the current thesis, a simple approach to incorporate the static and dynamic contact angle boundary conditions using the level set method is developed and implemented in multiphase CFD codes, LIT (Level set Interface Tracking) (Herrmann (2008)) and NGA (flow solver) (Desjardins et al (2008)). Various DCA models and associated boundary conditions are reviewed. In addition, numerical aspects such as the occurrence of a stress singularity at the contact lines and grid convergence of macroscopic interface shape are dealt with in the context of the level set approach.
ContributorsPendota, Premchand (Author) / Herrmann, Marcus (Thesis advisor) / Rykaczewski, Konrad (Committee member) / Chen, Kangping (Committee member) / Arizona State University (Publisher)
Created2015
Description
Self-heating degrades the performance of devices in advanced technology nodes. Understanding of self-heating effects is necessary to improve device performance. Heat generation in these devices occurs at nanometer scales but heat transfer is a microscopic phenomena. Hence a multi-scale modeling approach is required to study the self-heating effects. A state of the art Monte Carlo device simulator and the commercially available Giga 3D tool from Silvaco are used in our study to understand the self heating effects. The Monte Carlo device simulator solves the electrical transport and heat generation for nanometer length scales accurately while the Giga 3D tool solves for thermal transport over micrometer length scales. The approach used is to understand the self-heating effects in a test device structure, composed of a heater and a sensor, fabricated and characterized by IMEC. The heater is the Device Under Test(DUT) and the sensor is used as a probe. Therefore, the heater is biased in the saturation region and the sensor is biased in the sub-threshold regime. Both are planar MOSFETs of gate length equal to 22 nm. The simulated I-V characteristics of the sensor match with the experimental behavior at lower applied drain voltages but differ at higher applied biases.
The self-heating model assumes that the heat transport within the device follows Energy Balance model which may not be accurate. To properly study heat transport within the device, a state of the art Monte Carlo device simulator is necessary. In this regard, the Phonon Monte Carlo(PMC) simulator is developed. Phonons are treated as quasi particles that carry heat energy. Like electrons, phonons obey a corresponding Boltzmann Transport Equation(BTE) which can be used to study their transport. The direct solution of the BTE for phonons is possible, but it is difficult to incorporate all scattering mechanisms. In the Monte Carlo based solution method, it is easier to incorporate different relevant scattering mechanisms. Although the Monte Carlo method is computationally intensive, it provides good insight into the physical nature of the transport problem. Hence Monte Carlo based techniques are used in the present work for studying phonon transport. Monte Carlo simulations require calculating the scattering rates for different scattering processes. In the present work, scattering rates for three phonon interactions are calculated from different approaches presented in the literature. Optical phonons are also included in the transport problem. Finally, the temperature dependence of thermal conductivity for silicon is calculated in the range from 100K to 900K and is compared to available experimental data.
The self-heating model assumes that the heat transport within the device follows Energy Balance model which may not be accurate. To properly study heat transport within the device, a state of the art Monte Carlo device simulator is necessary. In this regard, the Phonon Monte Carlo(PMC) simulator is developed. Phonons are treated as quasi particles that carry heat energy. Like electrons, phonons obey a corresponding Boltzmann Transport Equation(BTE) which can be used to study their transport. The direct solution of the BTE for phonons is possible, but it is difficult to incorporate all scattering mechanisms. In the Monte Carlo based solution method, it is easier to incorporate different relevant scattering mechanisms. Although the Monte Carlo method is computationally intensive, it provides good insight into the physical nature of the transport problem. Hence Monte Carlo based techniques are used in the present work for studying phonon transport. Monte Carlo simulations require calculating the scattering rates for different scattering processes. In the present work, scattering rates for three phonon interactions are calculated from different approaches presented in the literature. Optical phonons are also included in the transport problem. Finally, the temperature dependence of thermal conductivity for silicon is calculated in the range from 100K to 900K and is compared to available experimental data.
ContributorsShaik, Abdul Rawoof (Author) / Vasileska, Dragica (Thesis advisor) / Ferry, David (Committee member) / Goodnick, Stephan (Committee member) / Arizona State University (Publisher)
Created2016
Description
A basic theory and terminology that comprehensively applies to all different types
of contacts in silicon solar cells has, thus far, been elusive. While the well established diode model has been applied to many of the complex contacts, the theory is not adequate to intuitively describe the characteristics of novel contacts. This thesis shows that the many desirable characteristics of contacts that are discussed in the literature—carrier selectivity, passivation, and low majority carrier conductance, key among them—originate from the resistance to electrons and holes in the contact. These principles are applied to describe a few popular contact technologies in order to pave the path to envisioning novel contacts. Metrics for contact performance is introduced to quantify each of the above characteristics using the two carrier resistances. The the validity of the proposed metrics is explored using extensive PC-1D simulations.
of contacts in silicon solar cells has, thus far, been elusive. While the well established diode model has been applied to many of the complex contacts, the theory is not adequate to intuitively describe the characteristics of novel contacts. This thesis shows that the many desirable characteristics of contacts that are discussed in the literature—carrier selectivity, passivation, and low majority carrier conductance, key among them—originate from the resistance to electrons and holes in the contact. These principles are applied to describe a few popular contact technologies in order to pave the path to envisioning novel contacts. Metrics for contact performance is introduced to quantify each of the above characteristics using the two carrier resistances. The the validity of the proposed metrics is explored using extensive PC-1D simulations.
ContributorsKoswatta, Priyaranga L (Author) / Holman, Zachary C (Thesis advisor) / King, Richard (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2016
Description
The study of subwavelength behavior of light and nanoscale lasing has broad
potential applications in various forms of computation i.e. optical and quantum, as well as
in energy engineering. Although this field has been under active research, there has been
little work done on describing the behaviors of threshold and saturation. Particularly, how
the gain-molecule behavior affects the lasing behavior has yet to be investigated.
In this work, the interaction of surface-plasmon-polaritons (SPPs) and molecules is
observed in lasing. Various phenomenologies are observed related to the appearance of the
threshold and saturation regions. The lasing profile, as a visual delimiter of lasing threshold
and saturation, is introduced and used to study various parametrical dependencies of lasing,
including the number-density of molecules, the molecular thickness and the frequency
detuning between the molecular transition frequency and the SPP resonant frequency. The
molecular population distributions are studied in terminal and dynamical methods and are
found to contain unexpected and theoretically challenging properties. Using an average
dynamical analysis, the simulated spontaneous emission cascade can be clearly seen.
Finally, theoretical derivations of simple 1D strands of dipoles are presented in both
the exact and mean-field approximation, within the density matrix formalism. Some
preliminary findings are presented, detailing the observed behaviors of some simple
systems.
potential applications in various forms of computation i.e. optical and quantum, as well as
in energy engineering. Although this field has been under active research, there has been
little work done on describing the behaviors of threshold and saturation. Particularly, how
the gain-molecule behavior affects the lasing behavior has yet to be investigated.
In this work, the interaction of surface-plasmon-polaritons (SPPs) and molecules is
observed in lasing. Various phenomenologies are observed related to the appearance of the
threshold and saturation regions. The lasing profile, as a visual delimiter of lasing threshold
and saturation, is introduced and used to study various parametrical dependencies of lasing,
including the number-density of molecules, the molecular thickness and the frequency
detuning between the molecular transition frequency and the SPP resonant frequency. The
molecular population distributions are studied in terminal and dynamical methods and are
found to contain unexpected and theoretically challenging properties. Using an average
dynamical analysis, the simulated spontaneous emission cascade can be clearly seen.
Finally, theoretical derivations of simple 1D strands of dipoles are presented in both
the exact and mean-field approximation, within the density matrix formalism. Some
preliminary findings are presented, detailing the observed behaviors of some simple
systems.
ContributorsBrewer, Andre J (Author) / Sukharev, Maxim (Thesis advisor) / Rivera, Daniel E (Thesis advisor) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2017
Description
Light Emitting Diodes even with their longer life, robust build and low power consumption, they are still plagued by some problems the most significant of which are the current droop and thermal droop. Current droop causes a lowering in the Internal Quantum Efficiency with increased current injection while thermal droop lowers the whole Internal Quantum Efficiency curve with increase in temperature. The focus here was understanding effects of thermal droop and develop a method to control it.
Shockley Read Hall recombination plays a dominant role in the thermal droop effect when the current injection is low. Since the blue light emitting diode is based on Gallium Nitride, we need to take into consideration the effect of piezoelectric polarization in the quantum wells. The effects of the piezoelectric fields were studied based on the Gallium Nitride plane orientations. It was found in a Gallium Nitride light emitting diodes simulation study that more the number of quantum wells, lower would be the Radiative recombination rate. The problem of exacerbated spatial separation of electron hole wavefunctions in a thick single quantum well structure lead to the development of a dual well structure where one well assisted the other during high temperature operations. The Electron Blocking Layer was reduced in thickness and was made only 10 nm thick with a 5 nm Gallium Nitride buffer between it and the active region wells. The main reason for reducing the electron blocking layer thickness was to reduce the valance band offset and improve hole transport into the active region. Three different dual well designs were simulated of 3nm, 6nm and 9nm wide wells. The output parameters like the Power Spectral Density, Electron bound density, Light Output Power and Electron-Hole wavefunction overlaps were calculated. It was found that one of the wells acted as an assisting well where it had very little radiative recombination activity in it at room temperature.
As the temperature increased, it was observed that the electrons in the main well started to overflow out of it and into the assisting well where the radiative recombination rate increased significantly. This lead to a boost in Internal Quantum Efficiency.
Shockley Read Hall recombination plays a dominant role in the thermal droop effect when the current injection is low. Since the blue light emitting diode is based on Gallium Nitride, we need to take into consideration the effect of piezoelectric polarization in the quantum wells. The effects of the piezoelectric fields were studied based on the Gallium Nitride plane orientations. It was found in a Gallium Nitride light emitting diodes simulation study that more the number of quantum wells, lower would be the Radiative recombination rate. The problem of exacerbated spatial separation of electron hole wavefunctions in a thick single quantum well structure lead to the development of a dual well structure where one well assisted the other during high temperature operations. The Electron Blocking Layer was reduced in thickness and was made only 10 nm thick with a 5 nm Gallium Nitride buffer between it and the active region wells. The main reason for reducing the electron blocking layer thickness was to reduce the valance band offset and improve hole transport into the active region. Three different dual well designs were simulated of 3nm, 6nm and 9nm wide wells. The output parameters like the Power Spectral Density, Electron bound density, Light Output Power and Electron-Hole wavefunction overlaps were calculated. It was found that one of the wells acted as an assisting well where it had very little radiative recombination activity in it at room temperature.
As the temperature increased, it was observed that the electrons in the main well started to overflow out of it and into the assisting well where the radiative recombination rate increased significantly. This lead to a boost in Internal Quantum Efficiency.
ContributorsDas, Shiladitya (Author) / Zhao, Yuji (Thesis advisor) / Vasileska, Dragica (Committee member) / Ning, Cun-Zheng (Committee member) / Arizona State University (Publisher)
Created2017