Matching Items (78)
Filtering by
- All Subjects: Electrical Engineering
- Creators: Vasileska, Dragica
Description
The advent of silicon, germanium, narrow-gap III-V materials, and later the wide bandgap (WBG) semiconductors, and their subsequent revolution and enrichment of daily life begs the question: what is the next generation of semiconductor electronics poised to look like? Ultrawide bandgap (UWBG) semiconductors are the class of semiconducting materials that possess an electronic bandgap (EG) greater than that of gallium nitride (GaN), which is 3.4 eV. They currently consist of beta-phase gallium oxide (β-Ga2O3 ; EG = 4.6–4.9 eV), diamond (EG = 5.5 eV), aluminum nitride (AlN; EG =6.2 eV), cubic boron nitride (BN; EG = 6.4 eV), and other materials hitherto undiscovered. Such a strong emphasis is placed on the semiconductor bandgap because so many relevant electronic performance properties scale positively with the bandgap. Where power electronics is concerned, the Baliga's Figure of Merit (BFOM) quantifies how much voltage a device can block in the off state and how high its conductivity is in the on state. The BFOM has a sixth-order dependence on the bandgap. The UWBG class of semiconductors also possess the potential for higher switching efficiencies and power densities and better suitability for deep-UV and RF optoelectronics. Many UWBG materials have very tight atomic lattices and high displacement energies, which makes them suitable for extreme applications such as radiation-harsh environments commonly found in military, industrial, and outer space applications. In addition, the UWBG materials also show promise for applications in quantum information sciences. For all the inherent promise and burgeoning research efforts, key breakthroughs in UWBG research have only occurred as recently as within the last two to three decades, making them extremely immature in comparison with the well-known WBG materials and others before them. In particular, AlN suffers from a lack of wide availability of low-cost, highquality substrates, a stark contrast to β-Ga2O3, which is now readily commercially available. In order to realize more efficient and varied devices on the relatively nascent UWBG materials platform, a deeper understanding of the various devices and physics is necessary. The following thesis focuses on the UWBG materials AlN and β-Ga2O3, overlooking radiation studies, a novel device heterojunction, and electronic defect study.
ContributorsMontes, Jossue (Author) / Zhao, Yuji (Thesis advisor) / Vasileska, Dragica (Committee member) / Goodnick, Stephen (Committee member) / Sanchez Esqueda, Ivan (Committee member) / Arizona State University (Publisher)
Created2021
Description
As the single-junction silicon solar cell is approaching its theoretical efficiency limits, the loss from shading and resistance is gaining increasing attention. The metal grid pattern may cause an efficiency loss up to 1–3%abs (absolute percentage) depending on the grid’s materials and structure.Many attempts have been proposed to reduce the loss caused by the contacts and module. Among them, the monolithic solar cell, which is a solar cell with multiple string cells on the same wafer and connected in a series, presents advantages of low output current, busbar-free contact, minimized interconnection space, and ohmic loss reduction. However, this structure also introduces a lateral forward bias current through the base region, which severely degrades the cell’s performance. In addition, this interconnection in the base region has partially shunted certain solar cells in the monolithic cell, which created a mismatch between string cells.
For the last few decades, researchers have used different methods such as etching trenches or enlarging the distance between the neighboring string cells to solve this problem. However, these methods were both ineffective and defective.
In this work, a novel method of suppressing the lateral forward bias current is proposed. By adding a very high surface recombination to the mid-region between the string cells, the carrier density in the mid-region can be decreased close to the doping density. Thus, the resistivity in the mid-region can be increased tenfold or more. As a result, the lateral forward bias current is greatly reduced.
Other methods to reduce lateral forward bias current include optimizing the width of the mid-region, shading the mid-region, reducing the base doping and base thickness which can be used to reduce the mismatch as well.
Another method has been proposed to calculate the minimum efficiency loss of a monolithic cell compared to the baseline solar cell. As a result, the monolithic cell could potentially gain more advantages over the baseline solar cells with a thinner and low-doped wafer.
A monolithic solar cell with innovative designs is presented in this work which shows an efficiency that is potentially higher than that of normal solar cells.
ContributorsXue, Shujian (Author) / Bowden, Studart (Thesis advisor) / Goodnick, Stephen (Committee member) / Vasileska, Dragica (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2022
Description
The metallization and interconnection of Si photovoltaic (PV) devices are among some of the most critically important aspects to ensure the PV cells and modules are cost-effective, highly-efficient, and robust through environmental stresses. The aim of this work is to contribute to the development of these innovations to move them closer to commercialization.Shingled PV modules and laser-welded foil-interconnected modules present an alternative to traditional soldered ribbons that can improve module power densities in a cost-effective manner. These two interconnection methods present new technical challenges for the PV industry. This work presents x-ray imaging methods to aid in the process-optimization of the application and curing of the adhesive material used in shingled modules. Further, detailed characterization of laser welds, their adhesion, and their effect on module performances is conducted. A strong correlation is found between the laser-weld adhesion and the modules’ durability through thermocycling. A minimum laser weld adhesion of 0.8 mJ is recommended to ensure a robust interconnection is formed.
Detailed characterization and modelling are demonstrated on a 21% efficient double-sided tunnel-oxide passivating contact (DS-TOPCon) cell. This technology uses a novel approach that uses the front-metal grid to etch-away the parasitically-absorbing poly-Si material everywhere except for underneath the grid fingers. The modelling yielded a match to the experimental device within 0.06% absolute of its efficiency. This DS-TOPCon device could be improved to a 23.45%-efficient device by improving the optical performance, n-type contact resistivity, and grid finger aspect ratio.
Finally, a modelling approach is explored for simulating Si thermophotovoltaic (TPV) devices. Experimentally fabricated diffused-junction devices are used to validate the optical and electrical aspects of the model. A peak TPV efficiency of 6.8% is predicted for the fabricated devices, but a pathway to 32.5% is explained by reducing the parasitic absorption of the contacts and reducing the wafer thickness. Additionally, the DS-TOPCon technology shows the potential for a 33.7% efficient TPV device.
ContributorsHartweg, Barry (Author) / Holman, Zachary (Thesis advisor) / Chan, Candace (Committee member) / Bertoni, Mariana (Committee member) / Yu, Zhengshan (Committee member) / Arizona State University (Publisher)
Created2023
Fill Factor Loss Mechanisms: Analysis and Basic Understanding in Silicon Hetero-junction Solar Cells
Description
The objective of this thesis is to achieve a detailed understanding of the loss mechanisms in SHJ solar cells. The working principles of these cells and what affects the cell operation, e.g. the IV characteristics at the maximum power point (MPP) and the correspondingly ll factor (FF) are investigated. Dierent loss sources are analyzed separately, and the weight of each in the total loss at the MPP are evaluated. The total series resistance is measured and then compared with the value obtained through summation over each of its components. In other words, series resistance losses due to recombination, vertical and lateral carrier transport, metalization, etc, are individually evaluated, and then by adding all these components together, the total loss is calculated. The concept of ll factor and its direct dependence on the loss mechanisms at the MPP of the device is explained, and its sensitivity to nearly every processing step of the cell fabrication is investigated. This analysis provides a focus lens to identify the main source of losses in SHJ solar cells and pave the path for further improvements in cell efficiency.
In this thesis, we provide a detailed understanding of the FF concept; we explain how it can be directly measured; how it can be calculated and what expressions can better approximate its value and under what operating conditions. The relation between FF and cell operating condition at the MPP is investigated. We separately analyzed the main FF sources of losses including recombination, sheet resistance, contact resistance and metalization. We study FF loss due to recombination and its separate components which include the Augur, radiative and SRH recombination is investigated. We study FF loss due to contact resistance and its separate components which include the contact resistance of dierent interfaces, e.g. between the intrinsic and doped a-Si layers, TCO and a-Si layers. We also study FF loss due to lateral transport and its components that including the TCO sheet resistance, the nger and the busbars resistances.
In this thesis, we provide a detailed understanding of the FF concept; we explain how it can be directly measured; how it can be calculated and what expressions can better approximate its value and under what operating conditions. The relation between FF and cell operating condition at the MPP is investigated. We separately analyzed the main FF sources of losses including recombination, sheet resistance, contact resistance and metalization. We study FF loss due to recombination and its separate components which include the Augur, radiative and SRH recombination is investigated. We study FF loss due to contact resistance and its separate components which include the contact resistance of dierent interfaces, e.g. between the intrinsic and doped a-Si layers, TCO and a-Si layers. We also study FF loss due to lateral transport and its components that including the TCO sheet resistance, the nger and the busbars resistances.
ContributorsLeilaeioun, Mohammadmehdi (Ashling) (Author) / Goodnick, Stephen (Thesis advisor) / Goryll, Michael (Thesis advisor) / Bertoni, Mariana (Committee member) / Bowden, Stuart (Committee member) / Stuckelberger, Michael (Committee member) / Arizona State University (Publisher)
Created2018
Description
Achieving high efficiency in solar cells requires optimal photovoltaics materials for light absorption and as with any electrical device—high-quality contacts. Essentially, the contacts separate the charge carriers—holes at one terminal and electrons at the other—extracting them to an external circuit. For this purpose, the development of passivating and carrier-selective contacts that enable low interface defect density and efficient carrier transport is critical for making high-efficiency solar cells. The recent record-efficiency n-type silicon cells with hydrogenated amorphous silicon (a-Si:H) contacts have demonstrated the usefulness of passivating and carrier-selective contacts. However, the use of a-Si:H contacts should not be limited in just n-type silicon cells.
In the present work, a-Si:H contacts for crystalline silicon and cadmium telluride (CdTe) solar cells are developed. First, hydrogen-plasma-processsed a-Si:H contacts are used in n-type Czochralski silicon cell fabrication. Hydrogen plasma treatment is used to increase the Si-H bond density of a-Si:H films and decrease the dangling bond density at the interface, which leads to better interface passivation and device performance, and wider temperature-processing window of n-type silicon cells under full spectrum (300–1200 nm) illumination. In addition, thickness-varied a-Si:H contacts are studied for n-type silicon cells under the infrared spectrum (700–1200 nm) illumination, which are prepared for silicon-based tandem applications.
Second, the a-Si:H contacts are applied to commercial-grade p-type silicon cells, which have much lower bulk carrier lifetimes than the n-type silicon cells. The approach is using gettering and bulk hydrogenation to improve the p-type silicon bulk quality, and then applying a-Si:H contacts to enable excellent surface passivation and carrier transport. This leads to an open-circuit voltage of 707 mV in p-type Czochralski silicon cells, and of 702 mV, the world-record open-circuit voltage in p-type multi-crystalline silicon cells.
Finally, CdTe cells with p-type a-Si:H hole-selective contacts are studied. As a proof of concept, p-type a-Si:H contacts enable achieving the highest reported open-circuit voltages (1.1 V) in mono-crystalline CdTe devices. A comparative study of applying p-type a-Si:H contacts in poly-crystalline CdTe solar cells is performed, resulting in absolute voltage gain of 53 mV over using the standard tellurium contacts.
In the present work, a-Si:H contacts for crystalline silicon and cadmium telluride (CdTe) solar cells are developed. First, hydrogen-plasma-processsed a-Si:H contacts are used in n-type Czochralski silicon cell fabrication. Hydrogen plasma treatment is used to increase the Si-H bond density of a-Si:H films and decrease the dangling bond density at the interface, which leads to better interface passivation and device performance, and wider temperature-processing window of n-type silicon cells under full spectrum (300–1200 nm) illumination. In addition, thickness-varied a-Si:H contacts are studied for n-type silicon cells under the infrared spectrum (700–1200 nm) illumination, which are prepared for silicon-based tandem applications.
Second, the a-Si:H contacts are applied to commercial-grade p-type silicon cells, which have much lower bulk carrier lifetimes than the n-type silicon cells. The approach is using gettering and bulk hydrogenation to improve the p-type silicon bulk quality, and then applying a-Si:H contacts to enable excellent surface passivation and carrier transport. This leads to an open-circuit voltage of 707 mV in p-type Czochralski silicon cells, and of 702 mV, the world-record open-circuit voltage in p-type multi-crystalline silicon cells.
Finally, CdTe cells with p-type a-Si:H hole-selective contacts are studied. As a proof of concept, p-type a-Si:H contacts enable achieving the highest reported open-circuit voltages (1.1 V) in mono-crystalline CdTe devices. A comparative study of applying p-type a-Si:H contacts in poly-crystalline CdTe solar cells is performed, resulting in absolute voltage gain of 53 mV over using the standard tellurium contacts.
ContributorsShi, Jianwei (Author) / Holman, Zachary (Thesis advisor) / Bowden, Stuart (Committee member) / Bertoni, Mariana (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2018
Description
Gallium Nitride (GaN) based Current Aperture Vertical Electron Transistors (CAVETs) present many appealing qualities for applications in high power, high frequency devices. The wide bandgap, high carrier velocity of GaN make it ideal for withstanding high electric fields and supporting large currents. The vertical topology of the CAVET allows for more efficient die area utilization, breakdown scaling with the height of the device, and burying high electric fields in the bulk where they will not charge interface states that can lead to current collapse at higher frequency.
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.
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.
ContributorsWarren, Andrew (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2019
Description
In this dissertation, I investigate the electronic properties of two important silicon(Si)-based heterojunctions 1) hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) which has already been commercialized in Heterojunction with Intrinsic Thin-layer (HIT) cells and 2) gallium phosphide/silicon (GaP/Si) which has been suggested to be a good candidate for replacing a-Si:H/c-Si in HIT cells in order to boost the HIT cell’s efficiency.
In the first part, the defect states of amorphous silicon (a-Si) and a-Si:H material are studied using density functional theory (DFT). I first employ simulated annealing using molecular dynamics (MD) to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. I find that H atoms decrease the density of mid-gap states and increase the band gap of a-Si by binding to Si atoms with strained bonds. My results also indicate that Si atoms with strained bonds creates high-localized orbitals in the mobility gap of a-Si, and the binding of H atoms to them can dramatically decrease their degree of localization.
In the second part, I explore the effect of the H binding configuration on the electronic properties of a-Si:H/c-Si heterostructure using density functional theory studies of models of the interface between a-Si:H and c-Si. The electronic properties from DFT show that depending on the energy difference between configurations, the electronic properties are sensitive to the H binding configurations.
In the last part, I examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen H passivation at the interface in comparison to interface mixing, through DFT calculations. My calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. I find that the addition of H leads to additional bonding at the interface which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction.
In the first part, the defect states of amorphous silicon (a-Si) and a-Si:H material are studied using density functional theory (DFT). I first employ simulated annealing using molecular dynamics (MD) to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. I find that H atoms decrease the density of mid-gap states and increase the band gap of a-Si by binding to Si atoms with strained bonds. My results also indicate that Si atoms with strained bonds creates high-localized orbitals in the mobility gap of a-Si, and the binding of H atoms to them can dramatically decrease their degree of localization.
In the second part, I explore the effect of the H binding configuration on the electronic properties of a-Si:H/c-Si heterostructure using density functional theory studies of models of the interface between a-Si:H and c-Si. The electronic properties from DFT show that depending on the energy difference between configurations, the electronic properties are sensitive to the H binding configurations.
In the last part, I examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen H passivation at the interface in comparison to interface mixing, through DFT calculations. My calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. I find that the addition of H leads to additional bonding at the interface which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction.
ContributorsVatan Meidanshahi, Reza (Author) / Goodnick, Stephen Marshall (Thesis advisor) / Vasileska, Dragica (Committee member) / Bowden, Stuart (Committee member) / Honsberg, Christiana (Committee member) / Arizona State University (Publisher)
Created2019
Description
Silicon photonic technology continues to dominate the solar industry driven by steady improvement in device and module efficiencies. Currently, the world record conversion efficiency (~26.6%) for single junction silicon solar cell technologies is held by silicon heterojunction (SHJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). These solar cells utilize the concept of carrier selective contacts to improve device efficiencies. A carrier selective contact is designed to optimize the collection of majority carriers while blocking the collection of minority carriers. In the case of SHJ cells, a thin intrinsic a-Si:H layer provides crucial passivation between doped a-Si:H and the c-Si absorber that is required to create a high efficiency cell. There has been much debate regarding the role of the intrinsic a-Si:H passivation layer on the transport of photogenerated carriers, and its role in optimizing device performance. In this work, a multiscale model is presented which utilizes different simulation methodologies to study interfacial transport across the intrinsic a-Si:H/c-Si heterointerface and through the a-Si:H passivation layer. In particular, an ensemble Monte Carlo simulator was developed to study high field behavior of photogenerated carriers at the intrinsic a-Si:H/c-Si heterointerface, a kinetic Monte Carlo program was used to study transport of photogenerated carriers across the intrinsic a-Si:H passivation layer, and a drift-diffusion model was developed to model the behavior in the quasi-neutral regions of the solar cell. This work reports de-coupled and self-consistent simulations to fully understand the role and effect of transport across the a-Si:H passivation layer in silicon heterojunction solar cells, and relates this to overall solar cell device performance.
ContributorsMuralidharan, Pradyumna (Author) / Goodnick, Stephen M (Thesis advisor) / Vasileska, Dragica (Thesis advisor) / Honsberg, Christiana (Committee member) / Ringhofer, Christian (Committee member) / Arizona State University (Publisher)
Created2019
Description
Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion.
In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.
In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.
In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.
In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.
ContributorsFu, Houqiang (Author) / Zhao, Yuji (Thesis advisor) / Vasileska, Dragica (Committee member) / Goodnick, Stephen (Committee member) / Yu, Hongbin (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2019
Description
Due to the ever increasing relevance of finer machining control as well as necessary reduction in material waste by large area semiconductor device manufacturers, a novel bulk laser machining method was investigated. Because the cost of silicon and sapphire substrates are limiting to the reduction in cost of devices in both the light emitting diode (LED) and solar industries, and the present substrate wafering process results in >50% waste, the need for an improved ingot wafering technique exists.
The focus of this work is the design and understanding of a novel semiconductor wafering technique that utilizes the nonlinear absorption properties of band-gapped materials to achieve bulk (subsurface) morphological changes in matter using highly focused laser light. A method and tool was designed and developed to form controlled damage regions in the bulk of a crystalline sapphire wafer leaving the surfaces unaltered. The controllability of the subsurface damage geometry was investigated, and the effect of numerical aperture of the focusing optic, energy per pulse, wavelength, and number of pulses was characterized for a nanosecond pulse length variable wavelength Nd:YAG OPO laser.
A novel model was developed to describe the geometry of laser induced morphological changes in the bulk of semiconducting materials for nanosecond pulse lengths. The beam propagation aspect of the model was based on ray-optics, and the full Keldysh multiphoton photoionization theory in conjuncture with Thornber's and Drude's models for impact ionization were used to describe high fluence laser light absorption and carrier generation ultimately resulting in permanent material modification though strong electron-plasma absorption and plasma melting. Although the electron-plasma description of laser damage formation is usually reserved for extremely short laser pulses (<20 ps), this work shows that it can be adapted for longer pulses of up to tens of nanoseconds.
In addition to a model describing damage formation of sub-band gap energy laser light in semiconducting and transparent crystalline dielectrics, a novel nanosecond laser process was successfully realized to generate a thin plane of damage in the bulk of sapphire wafers. This was accomplished using high numerical aperture optics, a variable wavelength nanosecond laser source, and three-dimensional motorized precision stage control.
The focus of this work is the design and understanding of a novel semiconductor wafering technique that utilizes the nonlinear absorption properties of band-gapped materials to achieve bulk (subsurface) morphological changes in matter using highly focused laser light. A method and tool was designed and developed to form controlled damage regions in the bulk of a crystalline sapphire wafer leaving the surfaces unaltered. The controllability of the subsurface damage geometry was investigated, and the effect of numerical aperture of the focusing optic, energy per pulse, wavelength, and number of pulses was characterized for a nanosecond pulse length variable wavelength Nd:YAG OPO laser.
A novel model was developed to describe the geometry of laser induced morphological changes in the bulk of semiconducting materials for nanosecond pulse lengths. The beam propagation aspect of the model was based on ray-optics, and the full Keldysh multiphoton photoionization theory in conjuncture with Thornber's and Drude's models for impact ionization were used to describe high fluence laser light absorption and carrier generation ultimately resulting in permanent material modification though strong electron-plasma absorption and plasma melting. Although the electron-plasma description of laser damage formation is usually reserved for extremely short laser pulses (<20 ps), this work shows that it can be adapted for longer pulses of up to tens of nanoseconds.
In addition to a model describing damage formation of sub-band gap energy laser light in semiconducting and transparent crystalline dielectrics, a novel nanosecond laser process was successfully realized to generate a thin plane of damage in the bulk of sapphire wafers. This was accomplished using high numerical aperture optics, a variable wavelength nanosecond laser source, and three-dimensional motorized precision stage control.
ContributorsLeBeau, James (Author) / Bowden, Stuart (Thesis advisor) / Honsberg, Christiana (Committee member) / Bertoni, Mariana (Committee member) / Cotter, Jeffrey (Committee member) / Arizona State University (Publisher)
Created2015