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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Description
The challenging search for clean, reliable and environmentally friendly energy sources has fueled increased research in thermoelectric materials, which are capable of recovering waste heat. Among the state-of-the-art thermoelectric materials β-Zn4Sb3 is outstanding because of its ultra-low glass-like thermal conductivity. Attempts to explore ternary phases in the Zn-Sb-In system resulted in the discovery of the new intermetallic compounds, stable Zn5Sb4In2-δ (δ=0.15) and metastable Zn9Sb6In2. Millimeter-sized crystals were grown from molten metal fluxes, where indium metal was employed as a reactive flux medium.Zn5Sb4In2-δ and Zn9Sb6In2 crystallize in new structure types featuring complex framework and the presence of structural disorder (defects and split atomic positions). The structure and phase relations between ternary Zn5Sb4In2-δ, Zn9Sb6In2 and binary Zn4Sb3 are discussed. To establish and understand structure-property relationships, thermoelectric properties measurements were carried out. The measurements suggested that Zn5Sb4In2-δ and Zn9Sb6In2 are narrow band gap semiconductors, similar to β-Zn4Sb3. Also, the peculiar low thermal conductivity of Zn4Sb3 (1 W/mK) is preserved. In the investigated temperature range 10 to 350 K Zn5Sb4In2-δ displays higher thermoelectric figure of merits than Zn4Sb3, indicating a potential significance in thermoelectric applications. Finally, the glass-like thermal conductivities of binary and ternary antimonides with complex structures are compared and the mechanism behind their low thermal conductivities is briefly discussed.
ContributorsWu, Yang (Author) / Häussermann, Ulrich (Thesis advisor) / Seo, Dong (Committee member) / Petuskey, William T (Committee member) / Newman, Nathan (Committee member) / Arizona State University (Publisher)
Created2011
Description
This thesis focuses on the theoretical work done to determine thermodynamic properties of a chalcopyrite thin-film material for use as a photovoltaic material in a tandem device. The material of main focus here is ZnGeAs2, which was chosen for the relative abundance of constituents, favorable photovoltaic properties, and good lattice matching with ZnSnP2, the other component in this tandem device. This work is divided into two main chapters, which will cover: calculations and method to determine the formation energy and abundance of native point defects, and a model to calculate the vapor pressure over a ternary material from first-principles. The purpose of this work is to guide experimental work being done in tandem to synthesize ZnGeAs2 in thin-film form with high enough quality such that it can be used as a photovoltaic. Since properties of photovoltaic depend greatly on defect concentrations and film quality, a theoretical understanding of how laboratory conditions affect these properties is very valuable. The work done here is from first-principles and utilizes density functional theory using the local density approximation. Results from the native point defect study show that the zinc vacancy (VZn) and the germanium antisite (GeZn) are the more prominent defects; which most likely produce non-stoichiometric films. The vapor pressure model for a ternary system is validated using known vapor pressure for monatomic and binary test systems. With a valid ternary system vapor pressure model, results show there is a kinetic barrier to decomposition for ZnGeAs2.
ContributorsTucker, Jon R (Author) / Van Schilfgaarde, Mark (Thesis advisor) / Newman, Nathan (Committee member) / Adams, James (Committee member) / Arizona State University (Publisher)
Created2011
Description
The object of this body of work is to study the properties and suitability of zinc oxide thin films with a view to engineering them for optoelectronics applications, making them a cheap and effective alternative to indium tin oxide (ITO), the most used transparent conducting oxides in the industry. Initially, a study was undertaken to examine the behavior of silver contacts to ZnO and ITO during thermal processing, a step frequently used in materials processing in optoelectronics. The second study involved an attempt to improve the conductivity of ZnO films by inserting a thin copper layer between two ZnO layers. The Hall resistivity of the films was as low as 6.9×10-5 -cm with a carrier concentration of 1.2×1022 cm-3 at the optimum copper layer thickness. The physics of conduction in the films has been examined. In order to improve the average visible transmittance, we replaced the copper layer with gold. The films were then found to undergo a seven orders of magnitude drop in effective resistivity from 200 -cm to 5.2×10-5 -cm The films have an average transmittance between 75% and 85% depending upon the gold thickness, and a peak transmittance of up to 93%. The best Haacke figure of merit was 15.1×10-3 . Finally, to test the multilayer transparent electrodes on a device, ZnO/Au/ZnO (ZAZ) electrodes were evaluated as transparent electrodes for organic light-emitting devices (OLEDs). The electrodes exhibited substantially enhanced conductivity (about 8×10-5 -cm) over conventional indium tin oxide (ITO) electrodes (about 3.2×10-5 -cm). OLEDs fabricated with the ZAZ electrodes showed reduced leakage compared to control OLEDs on ITO and reduced ohmic losses at high current densities. At a luminance of 25000 cd/m2, the lum/W efficiency of the ZAZ electrode based device improved by 5% compared to the device on ITO. A normalized intensity graph of the colour output from the green OLEDs shows that ZAZ electrodes allow for a broader spectral output in the green wavelength region of peak photopic sensitivity compared to ITO. The results have implications for electrode choice in display technology.
ContributorsSivaramakrishnan, Karthik (Author) / Alford, Terry L. (Thesis advisor) / Schroder, Dieter K. (Committee member) / Newman, Nathan (Committee member) / Theodore, David N (Committee member) / Arizona State University (Publisher)
Created2010
Description
This thesis explores the possibility of fabricating superconducting tunnel junctions (STJ) using double angle evaporation using an E-beam system. The traditional method of making STJs use a shadow mask to deposit two films requires the breaking of the vacuum of the main chamber. This technique has given bad results and proven to be a tedious process. To improve on this technique, the E-beam system was modified by adding a load lock and transfer line to perform the multi-angle deposition and in situ oxidation in the load lock without breaking the vacuum of the main chamber. Bilayer photolithography process was used to prepare a pattern for double angle deposition for the STJ. The overlap length could be easily controlled by varying the deposition angles. The low-temperature resistivity measurement and scanning electron microscope (SEM) characterization showed that the deposited films were good. However, I-V measurement for tunnel junction did not give expected results for the quality of the fabricated STJs. The main objective of modifying the E-beam system for multiple angle deposition was achieved. It can be used for any application that requires angular deposition. The motivation for the project was to set up a system that can fabricate a device that can be used as a phonon spectrometer for phononic crystals. Future work will include improving the quality of the STJ and fabricating an STJs on both sides of a silicon substrate using a 4-angle deposition.
ContributorsRana, Ashish (Author) / Wang, Robert Y (Thesis advisor) / Newman, Nathan (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2019
Description
The use of nanoparticle-in-matrix composites is a common motif among a broad range of nanoscience applications and is of particular interest to the thermal sciences community. To explore this morphological theme, crystalline inorganic composites were synthesized by mixing colloidal CdSe nanocrystals and In2Se3 metal chalcogenide complex (MCC) precursor in hydrazine solvent and then thermally transform the MCC precursor into a crystalline In2Se3 matrix. The volume fraction of CdSe nanocrystals was varied from 0 to ~100% .Rich structural and chemical interactions between the CdSe nanocrystals and the In2Se3 matrix were observed. The average thermal conductivities of the 100% In2Se3 and ~100% CdSe composites are 0.32 and 0.53 W/m-K, respectively, which are remarkably low for inorganic crystalline materials. With the exception of the ~100% CdSe samples, the thermal conductivities of these nanocomposites are insensitive to CdSe volume fraction.This insensitivity is attributed to competing effects rise from structural morphology changes during composite formation.
Next, thermoelectric properties of metal chalcogenide thin films deposited from precursors using thiol-amine solvent mixtures were first reported. Cu2-xSeyS1-y and Ag-doped Cu2-xSeyS1-y thin films were synthesized, and the interrelationship between structure, composition, and room temperature thermoelectric properties was studied. The precursor annealing temperature affects the metal:chalcogen ratio, and leads to charge carrier concentration changes that affect Seebeck coefficient and electrical conductivity. Incorporating Ag into the Cu2-xSeyS1-y film leads to appreciable improvements in thermoelectric performance. Overall, the room temperature thermoelectric properties of these solution-processed materials are comparable to measurements on Cu2-xSe alloys made via conventional thermoelectric material processing methods.
Finally, a new route to make soluble metal chalcogenide precursors by reacting organic dichalcogenides with metal in different solvents was reported. By this method, SnSe, PbSe, SnTe and PbSexTe1-x precursors were successfully synthesized, and phase-pure and impurity-free metal chalcogenides were recovered after precursor decomposition. Compared to the hydrazine and diamine-dithiol route, the new approach uses safe solvent, and avoids introducing unwanted sulfur into the precursor. SnSe and PbSexTe1-x thin films, both of which are interesting thermoelectric materials, were also successfully made by solution deposition. The thermoelectric property measurements on those thin films show a great potential for future improvements.
Next, thermoelectric properties of metal chalcogenide thin films deposited from precursors using thiol-amine solvent mixtures were first reported. Cu2-xSeyS1-y and Ag-doped Cu2-xSeyS1-y thin films were synthesized, and the interrelationship between structure, composition, and room temperature thermoelectric properties was studied. The precursor annealing temperature affects the metal:chalcogen ratio, and leads to charge carrier concentration changes that affect Seebeck coefficient and electrical conductivity. Incorporating Ag into the Cu2-xSeyS1-y film leads to appreciable improvements in thermoelectric performance. Overall, the room temperature thermoelectric properties of these solution-processed materials are comparable to measurements on Cu2-xSe alloys made via conventional thermoelectric material processing methods.
Finally, a new route to make soluble metal chalcogenide precursors by reacting organic dichalcogenides with metal in different solvents was reported. By this method, SnSe, PbSe, SnTe and PbSexTe1-x precursors were successfully synthesized, and phase-pure and impurity-free metal chalcogenides were recovered after precursor decomposition. Compared to the hydrazine and diamine-dithiol route, the new approach uses safe solvent, and avoids introducing unwanted sulfur into the precursor. SnSe and PbSexTe1-x thin films, both of which are interesting thermoelectric materials, were also successfully made by solution deposition. The thermoelectric property measurements on those thin films show a great potential for future improvements.
ContributorsMa, Yuanyu (Author) / Wang, Robert (Thesis advisor) / Newman, Nathan (Committee member) / Wang, Liping (Committee member) / Hildreth, Owen (Committee member) / Arizona State University (Publisher)
Created2016
Description
Counterfeiting of goods is a widespread epidemic that is affecting the world economy. The conventional labeling techniques are proving inadequate to thwart determined counterfeiters equipped with sophisticated technologies. There is a growing need of a secure labeling that is easy to manufacture and analyze but extremely difficult to copy. Programmable metallization cell technology operates on a principle of controllable reduction of a metal ions to an electrodeposit in a solid electrolyte by application of bias. The nature of metallic electrodeposit is unique for each instance of growth, moreover it has a treelike, bifurcating fractal structure with high information capacity. These qualities of the electrodeposit can be exploited to use it as a physical unclonable function. The secure labels made from the electrodeposits grown in radial structure can provide enhanced authentication and protection from counterfeiting and tampering.
So far only microscale radial structures and electrodeposits have been fabricated which limits their use to labeling only high value items due to high cost associated with their fabrication and analysis. Therefore, there is a need for a simple recipe for fabrication of macroscale structure that does not need sophisticated lithography tools and cleanroom environment. Moreover, the growth kinetics and material characteristics of such macroscale electrodeposits need to be investigated. In this thesis, a recipe for fabrication of centimeter scale radial structure for growing Ag electrodeposits using simple fabrication techniques was proposed. Fractal analysis of an electrodeposit suggested information capacity of 1.27 x 1019. The kinetics of growth were investigated by electrical characterization of the full cell and only solid electrolyte at different temperatures. It was found that mass transport of ions is the rate limiting process in the growth. Materials and optical characterization techniques revealed that the subtle relief like structure and consequently distinct optical response of the electrodeposit provides an added layer of security. Thus, the enormous information capacity, ease of fabrication and simplicity of analysis make macroscale fractal electrodeposits grown in radial programmable metallization cells excellent candidates for application as physical unclonable functions.
So far only microscale radial structures and electrodeposits have been fabricated which limits their use to labeling only high value items due to high cost associated with their fabrication and analysis. Therefore, there is a need for a simple recipe for fabrication of macroscale structure that does not need sophisticated lithography tools and cleanroom environment. Moreover, the growth kinetics and material characteristics of such macroscale electrodeposits need to be investigated. In this thesis, a recipe for fabrication of centimeter scale radial structure for growing Ag electrodeposits using simple fabrication techniques was proposed. Fractal analysis of an electrodeposit suggested information capacity of 1.27 x 1019. The kinetics of growth were investigated by electrical characterization of the full cell and only solid electrolyte at different temperatures. It was found that mass transport of ions is the rate limiting process in the growth. Materials and optical characterization techniques revealed that the subtle relief like structure and consequently distinct optical response of the electrodeposit provides an added layer of security. Thus, the enormous information capacity, ease of fabrication and simplicity of analysis make macroscale fractal electrodeposits grown in radial programmable metallization cells excellent candidates for application as physical unclonable functions.
ContributorsChamele, Ninad (Author) / Kozicki, Michael (Thesis advisor) / Barnaby, Hugh (Thesis advisor) / Newman, Nathan (Committee member) / Arizona State University (Publisher)
Created2017
Description
This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors.
The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.
The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.
The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
ContributorsLiu, Hanxiao (Author) / Ponce, Fernando A. (Thesis advisor) / Zhao, Yuji (Committee member) / Newman, Nathan (Committee member) / Fischer, Alec M (Committee member) / Arizona State University (Publisher)
Created2020
Description
The availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able to reach their true potential due to material growth related issues. Power devices typically have patterned p-n, and p-i junctions in lateral, and vertical direction relative to the substrate. Identifying the variations from the intended layer design is crucial for failure analysis of the devices. A most commonly used dopant profiling technique, secondary ion mass spectroscopy (SIMS), does not have the spatial resolution to identify the dopant distribution in patterned devices. The possibility of quantitative dopant profiling at a sub-micron scale for GaN in a scanning electron microscope (SEM) is discussed. The total electron yield in an SEM is shown to be a function of dopant concentration which can potentially be used for quantitative dopant profiling.
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.
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.
ContributorsAlugubelli, Shanthan Reddy (Author) / Ponce, Fernando A. (Thesis advisor) / McCartney, Martha (Committee member) / Newman, Nathan (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2019
Description
Realization of efficient, high-bandgap photovoltaic cells produced using economically viable methods is a technological advance that could change the way we generate and use energy, and thereby accelerate the development of human civilization. There is a need to engineer a semiconductor material for solar cells, particularly multijunction cells, that has high (1.6-2.0 eV) bandgap, has relatively inactive defects, is thermodynamically stable under normal operating conditions with the potential for cost-effective thin-film growth in mass production.This work focuses on a material system made of gallium, indium, and phosphorus – the ternary semiconductor GaInP. GaInP based photovoltaic cells in single-crystal form have demonstrated excellent power conversion efficiency, however, growth of single-crystal GaInP is prohibitively expensive. While growth of polycrystalline GaInP is expected to lower production costs, polycrystalline GaInP is also expected to have a high density of electronically active defects, about which little is reported in scientific literature. This work presents the first study of synthesis, and structural and optoelectronic characterization of polycrystalline GaInP thin films.
In addition, this work models the best performance of polycrystalline solar cells achievable with a given grain size with grain-boundary/surface recombination velocity as a variable parameter. The effects of defect characteristics at the surface and layer properties such as doping and thickness on interface recombination velocity are also modeled.
Recombination velocities at the free surface of single-crystal GaInP and after deposition of various dielectric layers on GaInP are determined experimentally using time-resolved photoluminescence decay measurements. In addition, experimental values of bulk lifetime and surface recombination velocity in well-passivated single crystal AlInP-GaInP based double heterostructures are also measured for comparison to polycrystalline material systems.
A novel passivation method – aluminum-assisted post-deposition treatment or Al-PDT – was developed which shows promise as a general passivation and material improvement technique for polycrystalline thin films. In the GaInP system, this aluminum post-deposition treatment has demonstrated improvement in the minority carrier lifetime to 44 ns at 80 K. During development of the passivation process, aluminum diffusivity in GaInP was measured using TEM-EDS line scans. Introduction, development, and refinement of this novel passivation mechanism in polycrystalline GaInP could initiate the development of a new family of passivation treatments, potentially improving the optoelectronic response of other polycrystalline compound semiconductors as well.
ContributorsChikhalkar, Abhinav (Author) / King, Richard R (Thesis advisor) / Honsberg, Christiana (Committee member) / Newman, Nathan (Committee member) / Tongay, Sefaattin (Committee member) / Arizona State University (Publisher)
Created2021
Description
Lateral programmable metallization cells (PMC) utilize the properties of electrodeposits grown over a solid electrolyte channel. Such devices have an active anode and an inert cathode separated by a long electrodeposit channel in a coplanar arrangement. The ability to transport large amount of metallic mass across the channel makes these devices attractive for various More-Than-Moore applications. Existing literature lacks a comprehensive study of electrodeposit growth kinetics in lateral PMCs. Moreover, the morphology of electrodeposit growth in larger, planar devices is also not understood. Despite the variety of applications, lateral PMCs are not embraced by the semiconductor industry due to incompatible materials and high operating voltages needed for such devices. In this work, a numerical model based on the basic processes in PMCs – cation drift and redox reactions – is proposed, and the effect of various materials parameters on the electrodeposit growth kinetics is reported. The morphology of the electrodeposit growth and kinetics of the electrodeposition process are also studied in devices based on Ag-Ge30Se70 materials system. It was observed that the electrodeposition process mainly consists of two regimes of growth – cation drift limited regime and mixed regime. The electrodeposition starts in cation drift limited regime at low electric fields and transitions into mixed regime as the field increases. The onset of mixed regime can be controlled by applied voltage which also affects the morphology of electrodeposit growth. The numerical model was then used to successfully predict the device kinetics and onset of mixed regime. The problem of materials incompatibility with semiconductor manufacturing was solved by proposing a novel device structure. A bilayer structure using semiconductor foundry friendly materials was suggested as a candidate for solid electrolyte. The bilayer structure consists of a low resistivity oxide shunt layer on top of a high resistivity ion carrying oxide layer. Devices using Cu2O as the low resistivity shunt on top of Cu doped WO3 oxide were fabricated. The bilayer devices provided orders of magnitude improvement in device performance in the context of operating voltage and switching time. Electrical and materials characterization revealed the structure of bilayers and the mechanism of electrodeposition in these devices.
ContributorsChamele, Ninad (Author) / Kozicki, Michael (Thesis advisor) / Barnaby, Hugh (Committee member) / Newman, Nathan (Committee member) / Gonzalez-Velo, Yago (Committee member) / Arizona State University (Publisher)
Created2020