Matching Items (6)

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Synthesis, Characterization, and Optimization of Superconductor-Dielectric Interfaces

Description

The chemical, structural, and electrical properties of niobium-silicon, niobium-germanium, and YBCO-dielectric interfaces are characterized. Reduction in the concentration of interfacial defects in these structures can improve the performance of (i)

The chemical, structural, and electrical properties of niobium-silicon, niobium-germanium, and YBCO-dielectric interfaces are characterized. Reduction in the concentration of interfacial defects in these structures can improve the performance of (i) many devices including low-loss coplanar, microstrip, and stripline microwave resonators used in next-generation cryogenic communication, sensor, and quantum information technologies and (ii) layers used in device isolation, inter-wiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication.

Methods were developed to synthesize amorphous-Ge (a-Ge) and homoepitaxial-Si dielectric thin-films with loss tangents of 1–2×10 -6 and 0.6–2×10 -5 at near single-photon powers and sub-Kelvin temperatures (≈40 mK), making them potentially a better choice over undoped silicon and sapphire substrates used in quantum devices. The Nb/Ge interface has 20 nm of chemical intermixing, which is reduced by a factor of four using 10 nm Ta diffusion layers. Niobium coplanar resonators using this structure exhibit reduced microwave losses.

The nature and concentration of defects near Nb-Si interfaces prepared with commonly-used Si surface treatments were characterized. All samples have H, C, O, F, and Cl in the Si within 50 nm of the interface, and electrically active defects with activation energies of 0.147, 0.194, 0.247, 0.339, and 0.556 eV above the valence band maximum (E vbm ), with concentrations dominated by a hole trap at E vbm +0.556 eV (presumably Nb Si ). The optimum surface treatment is an HF etch followed by an in-situ 100 eV Ar ion mill. RCA etches, and higher energy ion milling processes increase the concentration of electrically active defects.

A thin SrTiO 3 buffer layer used in YBa 2 Cu 3 O 7-δ superconductor/high-performance Ba(Zn 1/3 Ta 2/3 )O 3 and Ba(Cd 1/3 Ta 2/3 )O 3 microwave dielectric trilayers improves the structural quality of the layers and results in 90 K superconductor critical temperatures. This advance enables the production of more compact high-temperature superconductor capacitors, inductors, and microwave microstrip and stripline devices.

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Created

Date Created
  • 2020

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Defects and statistical degradation analysis of photovoltaic power plants

Description

As the photovoltaic (PV) power plants age in the field, the PV modules degrade and generate visible and invisible defects. A defect and statistical degradation rate analysis of photovoltaic (PV)

As the photovoltaic (PV) power plants age in the field, the PV modules degrade and generate visible and invisible defects. A defect and statistical degradation rate analysis of photovoltaic (PV) power plants is presented in two-part thesis. The first part of the thesis deals with the defect analysis and the second part of the thesis deals with the statistical degradation rate analysis. In the first part, a detailed analysis on the performance or financial risk related to each defect found in multiple PV power plants across various climatic regions of the USA is presented by assigning a risk priority number (RPN). The RPN for all the defects in each PV plant is determined based on two databases: degradation rate database; defect rate database. In this analysis it is determined that the RPN for each plant is dictated by the technology type (crystalline silicon or thin-film), climate and age. The PV modules aging between 3 and 19 years in four different climates of hot-dry, hot-humid, cold-dry and temperate are investigated in this study.

In the second part, a statistical degradation analysis is performed to determine if the degradation rates are linear or not in the power plants exposed in a hot-dry climate for the crystalline silicon technologies. This linearity degradation analysis is performed using the data obtained through two methods: current-voltage method; metered kWh method. For the current-voltage method, the annual power degradation data of hundreds of individual modules in six crystalline silicon power plants of different ages is used. For the metered kWh method, a residual plot analysis using Winters’ statistical method is performed for two crystalline silicon plants of different ages. The metered kWh data typically consists of the signal and noise components. Smoothers remove the noise component from the data by taking the average of the current and the previous observations. Once this is done, a residual plot analysis of the error component is performed to determine the noise was successfully separated from the data by proving the noise is random.

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Created

Date Created
  • 2016

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First-principles study of thermodynamic properties in thin-film photovoltaics

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

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.

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Created

Date Created
  • 2011

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Efficiency-limiting recombination mechanisms in high-quality crystalline silicon for solar cells

Description

Recent technology advancements in photovoltaics have enabled crystalline silicon (c-Si) solar cells to establish outstanding photoconversion efficiency records. Remarkable progresses in research and development have been made both on the

Recent technology advancements in photovoltaics have enabled crystalline silicon (c-Si) solar cells to establish outstanding photoconversion efficiency records. Remarkable progresses in research and development have been made both on the silicon feedstock quality as well as the technology required for surface passivation, the two dominant sources of performance loss via recombination of photo-generated charge carriers within advanced solar cell architectures.

As these two aspects of the solar cell framework improve, the need for a thorough analysis of their respective contribution under varying operation conditions has emerged along with challenges related to the lack of sensitivity of available characterization techniques. The main objective of my thesis work has been to establish a deep understanding of both “intrinsic” and “extrinsic” recombination processes that govern performance in high-quality silicon absorbers. By studying each recombination mechanism as a function of illumination and temperature, I strive to identify the lifetime limiting defects and propose a path to engineer the ultimate silicon solar cell.

This dissertation presents a detailed description of the experimental procedure required to deconvolute surface recombination contributions from bulk recombination contributions when performing lifetime spectroscopy analysis. This work proves that temperature- and injection-dependent lifetime spectroscopy (TIDLS) sensitivity can be extended to impurities concentrations down to 109 cm-3, orders of magnitude below any other characterization technique available today. A new method for the analysis of TIDLS data denominated Defect Parameters Contour Mapping (DPCM) is presented with the aim of providing a visual and intuitive tool to identify the lifetime limiting impurities in silicon material. Surface recombination velocity results are modelled by applying appropriate approaches from literature to our experimentally evaluated data, demonstrating for the first time their capability to interpret temperature-dependent data. In this way, several new results are obtained which solve long disputed aspects of surface passivation mechanisms. Finally, we experimentally evaluate the temperature-dependence of Auger lifetime and its impact on a theoretical intrinsically limited solar cell. These results decisively point to the need for a new Auger lifetime parameterization accounting for its temperature-dependence, which would in turn help understand the ultimate theoretical efficiency limit for a solar cell under real operation conditions.

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Created

Date Created
  • 2018

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Characterization of perovskite oxide/semiconductor heterostructures

Description

Integrated oxide/semiconductor heterostructures have attracted intense interest for device applications which require sharp interfaces and controlled defects. The research of this dissertation has focused on the characterization of perovskite oxide/oxide

Integrated oxide/semiconductor heterostructures have attracted intense interest for device applications which require sharp interfaces and controlled defects. The research of this dissertation has focused on the characterization of perovskite oxide/oxide and oxide/semiconductor heterostructures, and the analysis of interfaces and defect structures, using scanning transmission electrom microscopy (STEM) and related techniques.

The SrTiO3/Si system was initially studied to develop a basic understanding of the integration of perovskite oxides with semiconductors, and successful integration with abrupt interfaces was demonstrated. Defect analysis showed no misfit dislocations but only anti-phase boundaries (APBs) in the SrTiO3 (STO) films. Similar defects were later observed in other perovskite oxide heterostructures.

Ferroelectric BaTiO3 (BTO) thin films deposited directly onto STO substrates, or STO buffer layers with Ge substrates, were grown by molecular beam epitaxy (MBE) in order to control the polarization orientation for field-effect transistors (FETs). STEM imaging and elemental mapping by electron energy-loss spectroscopy (EELS) showed structurally and chemically abrupt interfaces, and the BTO films retained the c-axis-oriented tetragonal structure for both BTO/STO and BTO/STO/Ge heterostructures. The polarization displacement in the BTO films of TiN/BTO/STO heterostructures was investigated. The Ti4+ atomic column displacements and lattice parameters were measured directly using HAADF images. A polarization gradient, which switched from upwards to downwards, was observed in the BTO thin film, and evidence was found for positively-charged oxygen vacancies.

Heterostructures grown on Ge substrates by atomic layer deposition (ALD) were characterized and compared with MBE-grown samples. A two-step process was needed to overcome interlayer reaction at the beginning of ALD growth. A-site-rich oxide films with thicknesses of at least 2-nm had to be deposited and then crystallized before initiating deposition of the following perovskite oxide layer in order to suppress the formation of amorphous oxide layers on the Ge surface. BTO/STO/Ge, BTO/Ge, SrHfTiO3/Ge and SrZrO3/Ge thin films with excellent crystallinity were grown using this process.

Metal-insulator-metal (MIM) heterostructures were fabricated as ferroelectric capacitors and then electrically stressed to the point of breakdown to correlate structural changes with electrical and physical properties. BaTiO3 on Nb:STO was patterned with different top metal electrodes by focused-ion-beam milling, Au/Ni liftoff, and an isolation-defined approach.

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Created

Date Created
  • 2018

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Mechanisms of microwave loss tangent in high performance dielectric materials

Description

The mechanism of loss in high performance microwave dielectrics with complex perovskite structure, including Ba(Zn1/3Ta2/3)O3, Ba(Cd1/3Ta2/3)O3, ZrTiO4-ZnNb2O6, Ba(Zn1/3Nb2/3)O3, and BaTi4O9-BaZn2Ti4O11, has been investigated. We studied materials synthesized in our own

The mechanism of loss in high performance microwave dielectrics with complex perovskite structure, including Ba(Zn1/3Ta2/3)O3, Ba(Cd1/3Ta2/3)O3, ZrTiO4-ZnNb2O6, Ba(Zn1/3Nb2/3)O3, and BaTi4O9-BaZn2Ti4O11, has been investigated. We studied materials synthesized in our own lab and from commercial vendors. Then the measured loss tangent was correlated to the optical, structural, and electrical properties of the material. To accurately and quantitatively determine the microwave loss and Electron Paramagnetic Resonance (EPR) spectra as a function of temperature and magnetic field, we developed parallel plate resonator (PPR) and dielectric resonator (DR) techniques. Our studies found a marked increase in the loss at low temperatures is found in materials containing transition metal with unpaired d-electrons as a result of resonant spin excitations in isolated atoms (light doping) or exchange coupled clusters (moderate to high doping) ; a mechanism that differs from the usual suspects. The loss tangent can be drastically reduced by applying static magnetic fields. Our measurements also show that this mechanism significantly contributes to room temperature loss, but does not dominate. In order to study the electronic structure of these materials, we grew single crystal thin film dielectrics for spectroscopic studies, including angular resolved photoemission spectroscopy (ARPES) experiment. We have synthesized stoichiometric Ba(Cd1/3Ta2/3)O3 [BCT] (100) dielectric thin films on MgO (100) substrates using Pulsed Laser Deposition. Over 99% of the BCT film was found to be epitaxial when grown with an elevated substrate temperature of 635 C, an enhanced oxygen pressures of 53 Pa and a Cd-enriched BCT target with a 1 mol BCT: 1.5 mol CdO composition. Analysis of ultra violet optical absorption results indicate that BCT has a bandgap of 4.9 eV.

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Created

Date Created
  • 2013