Theses and Dissertations
This collection includes both ASU Theses and Dissertations, submitted by graduate students, and the Barrett, Honors College theses submitted by undergraduate students.
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- All Subjects: Condensed Matter Physics
- All Subjects: Surface passivation
Description
As crystalline silicon solar cells continue to get thinner, the recombination of carriers at the surfaces of the cell plays an ever-important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. The focus of this work is to understand the properties of charges present in the SiNx films and then to develop a mechanism to manipulate the polarity of charges to either negative or positive based on the end-application. Specific silicon-nitrogen dangling bonds (·Si-N), known as K center defects, are the primary charge trapping defects present in the SiNx films. A custom built corona charging tool was used to externally inject positive or negative charges in the SiNx film. Detailed Capacitance-Voltage (C-V) measurements taken on corona charged SiNx samples confirmed the presence of a net positive or negative charge density, as high as +/- 8 x 1012 cm-2, present in the SiNx film. High-energy (~ 4.9 eV) UV radiation was used to control and neutralize the charges in the SiNx films. Electron-Spin-Resonance (ESR) technique was used to detect and quantify the density of neutral K0 defects that are paramagnetically active. The density of the neutral K0 defects increased after UV treatment and decreased after high temperature annealing and charging treatments. Etch-back C-V measurements on SiNx films showed that the K centers are spread throughout the bulk of the SiNx film and not just near the SiNx-Si interface. It was also shown that the negative injected charges in the SiNx film were stable and present even after 1 year under indoor room-temperature conditions. Lastly, a stack of SiO2/SiNx dielectric layers applicable to standard commercial solar cells was developed using a low temperature (< 400 °C) PECVD process. Excellent surface passivation on FZ and CZ Si substrates for both n- and p-type samples was achieved by manipulating and controlling the charge in SiNx films.
ContributorsSharma, Vivek (Author) / Bowden, Stuart (Thesis advisor) / Schroder, Dieter (Committee member) / Honsberg, Christiana (Committee member) / Roedel, Ronald (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2013
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 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.
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.
ContributorsBernardini, Simone (Author) / Bertoni, Mariana I (Thesis advisor) / Coletti, Gianluca (Committee member) / Bowden, Stuart (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2018
Description
Chemical Vapor Deposition (CVD) is the most widely used method to grow large-scale single layer graphene. However, a systematic experimental study of the relationship between growth parameters and graphene film morphology, especially in the industrially preferred cold wall CVD, has not been undertaken previously. This research endeavored to address this and provide comprehensive insight into the growth physics of graphene on supported solid and liquid Cu films using cold wall CVD.
A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures.
The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape.
The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.
A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures.
The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape.
The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.
ContributorsDas, Shantanu, Ph.D (Author) / Drucker, Jeff (Thesis advisor) / Alford, Terry (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2018
Description
Two dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) are a new class of atomically thin polar materials. In these materials, the top and the bottom atomic layer are made of different chalcogen atoms. To date, several theoretical studies have shown that a broken mirror symmetry induces a colossal electrical field in these materials, which leads to unusual quantum properties. Despite these new properties, the current knowledge in their synthesis is limited only through two independent studies; both works rely on high-temperature processing techniques and are specific to only one type of 2D Janus material - MoSSe. Therefore, there is an urgent need for the development of a new synthesis method to (1) Extend the library of Janus class materials. (2) Improve the quality of 2D crystals. (3) Enable the synthesis of Janus heterostructures. The central hypothesis in this work is that the processing temperature of 2D Janus synthesis can be significantly lowered down to room temperatures by using reactive hydrogen and sulfur radicals while stripping off selenium atoms from the 2D surface. To test this hypothesis, a series of controlled growth studies were performed, and several complementary characterization techniques were used to establish a process–structure-property relationship. The results show that the newly proposed approach, namely Selective Epitaxy and Atomic Replacement (SEAR), is effective in reducing the growth temperature down to ambient conditions. The proposed technique benefits in achieving highly crystalline 2D Janus layers with an excellent optical response. Further studies herein show that this technique can form highly sophisticated lateral and vertical heterostructures of 2D Janus layers. Overall results establish an entirely new growth technique for 2D Janus.layers, which pave ways for the realization of exciting quantum effects in these materials such as Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, Majorana fermions, and topological p-wave superconductors.
ContributorsSayyad, Mohammed Yasir (Author) / Tongay, Sefaattin (Thesis advisor) / Crozier, Peter (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2020