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Impact of Ionizing Radiation and Electron Injection on Carrier Transport Properties in Narrow and Wide Bandgap Semiconductors
Title
Impact of Ionizing Radiation and Electron Injection on Carrier Transport Properties in Narrow and Wide Bandgap Semiconductors.
Creator
Lee, Jonathan, Flitsiyan, Elena, Chernyak, Leonid, Peale, Robert, Orlovskaya, Nina, University of Central Florida
Abstract / Description
This study investigated the minority carrier properties of wide and narrow bandgap semiconductors. Included specifically are wide bandgap materials GaN and ?-Ga2O3, and narrow bandgap InAs/GaSb type-II strain-layer superlattice. The importance of minority carrier behavior in bipolar device performance is utmost because it is the limiting component in current conduction. The techniques used to determine minority carrier properties include electron beam induced current (EBIC) and...
Show moreThis study investigated the minority carrier properties of wide and narrow bandgap semiconductors. Included specifically are wide bandgap materials GaN and ?-Ga2O3, and narrow bandgap InAs/GaSb type-II strain-layer superlattice. The importance of minority carrier behavior in bipolar device performance is utmost because it is the limiting component in current conduction. The techniques used to determine minority carrier properties include electron beam induced current (EBIC) and cathodoluminescence (CL) spectroscopy. The CL spectroscopy is complemented with time-resolved CL (TRCL) for direct measurement of carrier radiative recombination lifetime. The minority carrier properties and effect of high energy radiation is explored. The GaN TRCL results suggested an activation energy effecting carrier lifetime of about 90 meV which is related to nitrogen vacancies. The effects of 60Co gamma radiation are demonstrated and related to the effects of electron injection in GaN-based devices. The effects of various high energy radiations upon Si-doped ?-Ga2O3 minority carrier diffusion length and radiative lifetime are measured. The non-irradiated sample thermal activation energies found for minority carrier diffusion length were 40.9 meV, related to shallow Si-donors in the material. The CL results demonstrate that the bandgap of 4.9 eV is slightly indirect. The thermal activation energy decreased on 1.5 MeV electron irradiation but increased for 10 MeV proton irradiation. The increase in energy was related to higher order defects and their complexes, and influenced recombination lifetime significantly. Finally, the diffusion length is reported for narrow bandgap InAs/GaSb superlattice structure and the effect of 60Co gamma radiation is demonstrated.In general, the defects introduced by high energy radiations decreased minority carrier diffusion length, except for 60Co gamma on AlGaN/GaN HEMT devices and high-temperature proton irradiated ?-Ga2O3.
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Date Issued
2018
Identifier
CFE0007217, ucf:52239
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007217
Radiation Effects on Wide Band Gap Semiconductor Transport Properties
Title
Radiation Effects on Wide Band Gap Semiconductor Transport Properties.
Creator
Schwarz, Casey, Flitsiyan, Elena, Chernyak, Leonid, Peale, Robert, Schoenfeld, Winston, University of Central Florida
Abstract / Description
In this research, the transport properties of ZnO were studied through the use of electron and neutron beam irradiation. Acceptor states are known to form deep in the bandgap of doped ZnO material. By subjecting doped ZnO materials to electron and neutron beams we are able to probe, identify and modify transport characteristics relating to these deep accepter states. The impact of irradiation and temperature on minority carrier diffusion length and lifetime were monitored through the use of...
Show moreIn this research, the transport properties of ZnO were studied through the use of electron and neutron beam irradiation. Acceptor states are known to form deep in the bandgap of doped ZnO material. By subjecting doped ZnO materials to electron and neutron beams we are able to probe, identify and modify transport characteristics relating to these deep accepter states. The impact of irradiation and temperature on minority carrier diffusion length and lifetime were monitored through the use of the Electron Beam Induced Current (EBIC) method and Cathodoluminescence (CL) spectroscopy. The minority carrier diffusion length, L, was shown to increase as it was subjected to increasing temperature as well as continuous electron irradiation. The near-band-edge (NBE) intensity in CL measurements was found to decay as a function of temperature and electron irradiation due to an increase in carrier lifetime. Electron injection through application of a forward bias also resulted in a similar increase of minority carrier diffusion length.Thermal and electron irradiation dependences were used to determine activation energies for the irradiation induced effects. This helps to further our understanding of the electron injection mechanism as well as to identify possible defects responsible for the observed effects. Thermal activation energies likely represent carrier delocalization energy and are related to the increase of diffusion length due to the reduction in recombination efficiency. The effect of electron irradiation on the minority carrier diffusion length and lifetime can be attributed to the trapping of non-equilibrium electrons on neutral acceptor levels. The effect of neutron irradiation on CL intensity can be attributed to an increase in shallow donor concentration. Thermal activation energies resulting from an increase in L or decay of CL intensity monitored through EBIC and CL measurements for p-type Sb doped ZnO were found to be the range of Ea = 112 to 145 meV. P-type Sb doped ZnO nanowires under the influence of temperature and electron injection either through continuous beam impacting or through forward bias, displayed an increase in L and corresponding decay of CL intensity when observed by EBIC or CL measurements. These measurements led to activation energies for the effect ranging from Ea = 217 to 233 meV. These values indicate the possible involvement of a SbZn-2VZn acceptor complex. For N-type unintentionally doped ZnO, CL measurements under the influence of temperature and electron irradiation by continuous beam impacting led to a decrease in CL intensity which resulted in an electron irradiation activation energy of approximately Ea = 259 meV. This value came close to the defect energy level of the zinc interstitial. CL measurements of neutron irradiated ZnO nanostructures revealed that intensity is redistributed in favor of the NBE transition indicating an increase of shallow donor concentration. With annealing contributing to the improvement of crystallinity, a decrease can be seen in the CL intensity due to the increase in majority carrier lifetime. Low energy emission seen from CL spectra can be due to oxygen vacancies and as an indicator of radiation defects.
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Date Issued
2012
Identifier
CFE0004234, ucf:49018
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004234
IMPACT OF GAMMA-IRRADIATION ON THE CHARACTERISTICS OF III-N/GaN BASED HIGH ELECTRON MOBILITY TRANSISTORS
Title
IMPACT OF GAMMA-IRRADIATION ON THE CHARACTERISTICS OF III-N/GaN BASED HIGH ELECTRON MOBILITY TRANSISTORS.
Creator
Yadav, Anupama, Flitsiyan, Elena, Chernyak, Leonid, Peale, Robert, Richie, Samuel, University of Central Florida
Abstract / Description
In this study, the fundamental properties of AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have been investigated in order to optimize their performance in radiation harsh environment. AlGaN/GaN HEMTs were irradiated with 60Co gamma-rays to doses up to 1000 Gy, and the effects of irradiation on the devices' transport and optical properties were analyzed. Understanding the radiation affects in HEMTs devices, on carrier transport, recombination rates and traps creation play a...
Show moreIn this study, the fundamental properties of AlGaN/GaN based High Electron Mobility Transistors (HEMTs) have been investigated in order to optimize their performance in radiation harsh environment. AlGaN/GaN HEMTs were irradiated with 60Co gamma-rays to doses up to 1000 Gy, and the effects of irradiation on the devices' transport and optical properties were analyzed. Understanding the radiation affects in HEMTs devices, on carrier transport, recombination rates and traps creation play a significant role in development and design of radiation resistant semiconductor components for different applications. Electrical testing combined with temperature dependent Electron Beam Induced Current (EBIC) that we used in our investigations, provided critical information on defects induced in the material because of gamma-irradiation. It was shown that low dose (below ~250 Gy) and high doses (above ~250 Gy) of gamma-irradiation affects the AlGaN/GaN HEMTs due to different mechanisms. For low doses of gamma-irradiation, the improvement in minority carrier diffusion length is likely associated with the irradiation-induced growing lifetime of the non-equilibrium carriers. However, with the increased dose of irradiation (above ~ 250 Gy), the concentration of point defects, such as nitrogen vacancies, as well as the complexes involving native defects increases which results in the non-equilibrium carrier scattering. The impact of defect scattering is more pronounced at higher radiation, which leads to the degradation in the mobility and therefore the diffusion length. In addition for each device under investigation, the temperature dependent minority carrier diffusion length measurements were carried out. These measurements allowed the extraction of the activation energy for the temperature-induced enhancement of the minority carrier transport, which (activation energy) bears a signature of defect levels involved the carrier recombination process. Comparing the activation energy before and after gamma-irradiation identified the radiation-induced defect levels and their dependences. To complement EBIC measurements, spatially resolved Cathodoluminescence (CL) measurements were carried out at variable temperatures. Similar to the EBIC measurements, CL probing before and after the gamma-irradiation allowed the identification of possible defect levels generated as a result of gamma-bombardment. The observed decrease in the CL peak intensity after gamma-irradiation provides the direct evidence of the decrease in the number of recombination events. Based on the findings, the decay in the near-band-edge intensity after low-dose of gamma-irradiation (below ~250 Gy) was explained as a consequence of increased non-equilibrium carrier lifetime. For high doses (above ~250 Gy), decay in the CL intensity was observed to be related to the reduction in the mobility of charge carriers. The results of EBIC are correlated with the CL measurements in order to demonstrate that same underlying process is responsible for the changes induced by the gamma-irradiation. DC current-voltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation on transfer, gate and drain characteristics. Exposure of AlGaN/GaN HEMTs to high dose of 60Co gamma-irradiation (above ~ 250 Gy) resulted in significant device degradation. Gamma-rays doses up to 1000 Gy are shown to result in positive shift in threshold voltage, a reduction in the drain current and transconductance due to increased trapping of carriers and dispersion of charge. In addition, a significant increase in the gate leakage current was observed in both forward and reverse directions after irradiation. Post-irradiation annealing at relatively low temperature was shown to restore the minority carrier transport as well as the electrical characteristics of the devices. The level of recovery of gamma-irradiated devices after annealing treatment depends on the dose of the irradiation. The devices that show most recovery for a particular annealing temperature are those exposed to the low doses of gamma-irradiation, while those exposed to the highest doses results in no recovery of performance. The latter fact indicates that a higher device annealing temperature is needed for larger doses of gamma-irradiation.
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Date Issued
2016
Identifier
CFE0006424, ucf:51458
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006424
Development of an Efficient Molecular Single-Electron Transport Spectroscopy
Title
Development of an Efficient Molecular Single-Electron Transport Spectroscopy.
Creator
Rodriguez Garrigues, Alvar, Gonzalez Garcia, Enrique, Flitsiyan, Elena, Ishigami, Marsahir, University of Central Florida
Abstract / Description
In this thesis I present a complete and detailed guide for the development process and fabrication of efficient single-electron transistors (SETs) and a better single-molecule magnets (SMMs) deposition yield. Starting from a commercial Si/SiO2 wafer I show the steps for the deposition of different layers to fabricate a SET as well as the improvements achieved in those for a completely functional SET device. The development process is based on a combination of optical lithography and e-beam...
Show moreIn this thesis I present a complete and detailed guide for the development process and fabrication of efficient single-electron transistors (SETs) and a better single-molecule magnets (SMMs) deposition yield. Starting from a commercial Si/SiO2 wafer I show the steps for the deposition of different layers to fabricate a SET as well as the improvements achieved in those for a completely functional SET device. The development process is based on a combination of optical lithography and e-beam lithography with metal deposition in ultra-high vacuum. The improvements involve a better conductance in the Al gate component, with a controlled formation of the superficial oxide layer and a faster feedback electromigration-induced breaking of Au nanowires for the creation of nanogaps at room temperature. The gate component is improved by increasing its thickness and exposing it to plasma oxidation for the complete oxidation of its surface. The nanowire breaking is realized at room temperature to make use of the surface tension of Au, which, after a previous feedback procedure, eventually opens the final gap in the nanowire. Finally, I demonstrate a new technique that allows increasing the yield of having a SMM connected in the nanowire gap. This new technique is based on monitoring the resistance of the broken nanowires during the SMM deposition from a controlled liquid solution at room temperature. When the resistance ((>)G? for open gaps) drops to values below Mega-ohms (characteristic resistance of a molecule bridging the gap) for a number of nanowires in the chip, the device is then ready for low temperature measurements.
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Date Issued
2013
Identifier
CFE0004742, ucf:49775
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0004742
Microwave Radiometer (MWR) Evaluation of Multi-Beam Satellite Antenna Boresight Pointing Using Land-Water Crossings, for the Aquarius/SAC-D Mission
Title
Microwave Radiometer (MWR) Evaluation of Multi-Beam Satellite Antenna Boresight Pointing Using Land-Water Crossings, for the Aquarius/SAC-D Mission.
Creator
Clymer, Bradley, Jones, W Linwood, Mikhael, Wasfy, Flitsiyan, Elena, University of Central Florida
Abstract / Description
This research concerns the CONAE Microwave Radiometer (MWR), on board the Aquarius/SAC-D platform. MWR's main purpose is to provide measurements that are simultaneous and spatially collocated with those of NASA's Aquarius radiometer/scatterometer. For this reason, knowledge of the MWR antenna beam footprint geolocation is crucial to mission success.In particular, this thesis addresses an on-orbit validation of the MWR antenna beam pointing, using calculated MWR instantaneous field of view ...
Show moreThis research concerns the CONAE Microwave Radiometer (MWR), on board the Aquarius/SAC-D platform. MWR's main purpose is to provide measurements that are simultaneous and spatially collocated with those of NASA's Aquarius radiometer/scatterometer. For this reason, knowledge of the MWR antenna beam footprint geolocation is crucial to mission success.In particular, this thesis addresses an on-orbit validation of the MWR antenna beam pointing, using calculated MWR instantaneous field of view (IFOV) centers, provided in the CONAE L-1B science data product. This procedure compares L-1B MWR IFOV centers at land/water crossings against high-resolution coastline maps. MWR IFOV locations versus time are computed from knowledge of the satellite's instantaneous location relative to an earth-centric coordinate system (provided by on-board GPS receivers), and a priori measurements of antenna gain patterns and mounting geometry.Previous conical scanning microwave radiometer missions (e.g., SSM/I) have utilized observation of rapid change in brightness temperatures (T_B) to estimate the location of land/water boundaries, and subsequently to determine the antenna beam-pointing accuracy. In this thesis, results of an algorithm to quantify the geolocation error of MWR beam center are presented, based upon two-dimensional convolution between each beam's gain pattern and land-water transition. The analysis procedures have been applied to on-orbit datasets that represent land-water boundaries bearing specific desirable criteria, which are also detailed herein. The goal of this research is to gain a better understanding of satellite radiometer beam-pointing error and thereby to improve the geolocation accuracy for MWR science data products.
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Date Issued
2015
Identifier
CFE0005591, ucf:50269
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005591
Theoretical Study of Laser Beam Quality and Pulse Shaping by Volume Bragg Gratings
Title
Theoretical Study of Laser Beam Quality and Pulse Shaping by Volume Bragg Gratings.
Creator
Kaim, Sergiy, Zeldovich, Boris, Flitsiyan, Elena, Leuenberger, Michael, Likamwa, Patrick, University of Central Florida
Abstract / Description
The theory of stretching and compressing of short light pulses by the chirped volume Bragg gratings (CBG) is reviewed based on spectral decomposition of short pulses and on the wavelength-dependent coupled wave equations. The analytic theory of diffraction efficiency of a CBG with constant chirp and approximate theory of time delay dispersion are presented. Based on those, we performed comparison of the approximate analytic results with the exact numeric coupled-wave modeling. We also study...
Show moreThe theory of stretching and compressing of short light pulses by the chirped volume Bragg gratings (CBG) is reviewed based on spectral decomposition of short pulses and on the wavelength-dependent coupled wave equations. The analytic theory of diffraction efficiency of a CBG with constant chirp and approximate theory of time delay dispersion are presented. Based on those, we performed comparison of the approximate analytic results with the exact numeric coupled-wave modeling. We also study theoretically various definitions of laser beam width in a given cross-section. Quality of the beam is characterized by the dimensionless beam propagation products (?x???_x)?? , which are different for each of the 21 definitions. We study six particular beams and introduce an axially-symmetric self-MFT (mathematical Fourier transform) function, which may be useful for the description of diffraction-quality beams. Furthermore, we discuss various saturation curves and their influence on the amplitudes of recorded gratings. Special attention is given to multiplexed volume Bragg gratings (VBG) aimed at recording of several gratings in the same volume. The best shape of a saturation curve for production of the strongest gratings is found to be the threshold-type curve. Both one-photon and two-photon absorption mechanism of recording are investigated. Finally, by means of the simulation software we investigate forced airflow cooling of a VBG heated by a laser beam. Two combinations of a setup are considered, and a number of temperature distributions and thermal deformations are obtained for different rates of airflows. Simulation results are compared to the experimental data, and show good mutual agreement.
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Date Issued
2015
Identifier
CFE0005638, ucf:50210
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005638
Multi-Objective Optimization for Construction Equipment Fleet Selection and Management In Highway Construction Projects Based on Time, Cost, and Quality Objectives
Title
Multi-Objective Optimization for Construction Equipment Fleet Selection and Management In Highway Construction Projects Based on Time, Cost, and Quality Objectives.
Creator
Shehadeh, Ali, Tatari, Omer, Al-Deek, Haitham, Abou-Senna, Hatem, Flitsiyan, Elena, University of Central Florida
Abstract / Description
The sector of highway construction shares approximately 11% of the total construction industry in the US. Construction equipment can be considered as one of the primary reasons this industry has reached such a significant level, as it is considered an essential part of the highway construction process during highway project construction. This research addresses a multi-objective optimization mathematical model that quantifies and optimize the key parameters for excavator, truck, and motor...
Show moreThe sector of highway construction shares approximately 11% of the total construction industry in the US. Construction equipment can be considered as one of the primary reasons this industry has reached such a significant level, as it is considered an essential part of the highway construction process during highway project construction. This research addresses a multi-objective optimization mathematical model that quantifies and optimize the key parameters for excavator, truck, and motor-grader equipment to minimize time and cost objective functions. The model is also aimed to maintain the required level of quality for the targeted construction activity. The mathematical functions for the primary objectives were formulated and then a genetic algorithm-based multi-objective was performed to generate the time-cost Pareto trade-offs for all possible equipment combinations using MATLAB software to facilitate the implementation. The model's capabilities in generating optimal time and cost trade-offs based on optimized equipment number, capacity, and speed to adapt with the complex and dynamic nature of highway construction projects are demonstrated using a highway construction case study. The developed model is a decision support tool during the construction process to adapt with any necessary changes into time or cost requirements taking into consideration environmental, safety and quality aspects. The flexibility and comprehensiveness of the proposed model, along with its programmable nature, make it a powerful tool for managing construction equipment, which will help saving time and money within the optimal quality margins. Also, this environmentally friendly decision-support tool model provided optimal solutions that help to reduce the CO2 emissions reducing the ripple effects of targeted highway construction activities on the global warming phenomenon. The generated optimal solutions offered considerable time and cost savings.
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Date Issued
2019
Identifier
CFE0007863, ucf:52800
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007863
Simulation, Analysis, and Optimization of Heterogeneous CPU-GPU Systems
Title
Simulation, Analysis, and Optimization of Heterogeneous CPU-GPU Systems.
Creator
Giles, Christopher, Heinrich, Mark, Ewetz, Rickard, Lin, Mingjie, Pattanaik, Sumanta, Flitsiyan, Elena, University of Central Florida
Abstract / Description
With the computing industry's recent adoption of the Heterogeneous System Architecture (HSA) standard, we have seen a rapid change in heterogeneous CPU-GPU processor designs. State-of-the-art heterogeneous CPU-GPU processors tightly integrate multicore CPUs and multi-compute unit GPUs together on a single die. This brings the MIMD processing capabilities of the CPU and the SIMD processing capabilities of the GPU together into a single cohesive package with new HSA features comprising better...
Show moreWith the computing industry's recent adoption of the Heterogeneous System Architecture (HSA) standard, we have seen a rapid change in heterogeneous CPU-GPU processor designs. State-of-the-art heterogeneous CPU-GPU processors tightly integrate multicore CPUs and multi-compute unit GPUs together on a single die. This brings the MIMD processing capabilities of the CPU and the SIMD processing capabilities of the GPU together into a single cohesive package with new HSA features comprising better programmability, coherency between the CPU and GPU, shared Last Level Cache (LLC), and shared virtual memory address spaces. These advancements can potentially bring marked gains in heterogeneous processor performance and have piqued the interest of researchers who wish to unlock these potential performance gains. Therefore, in this dissertation I explore the heterogeneous CPU-GPU processor and application design space with the goal of answering interesting research questions, such as, (1) what are the architectural design trade-offs in heterogeneous CPU-GPU processors and (2) how do we best maximize heterogeneous CPU-GPU application performance on a given system. To enable my exploration of the heterogeneous CPU-GPU design space, I introduce a novel discrete event-driven simulation library called KnightSim and a novel computer architectural simulator called M2S-CGM. M2S-CGM includes all of the simulation elements necessary to simulate coherent execution between a CPU and GPU with shared LLC and shared virtual memory address spaces. I then utilize M2S-CGM for the conduct of three architectural studies. First, I study the architectural effects of shared LLC and CPU-GPU coherence on the overall performance of non-collaborative GPU-only applications. Second, I profile and analyze a set of collaborative CPU-GPU applications to determine how to best optimize them for maximum collaborative performance. Third, I study the impact of varying four key architectural parameters on collaborative CPU-GPU performance by varying GPU compute unit coalesce size, GPU to memory controller bandwidth, GPU frequency, and system wide switching fabric latency.
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Date Issued
2019
Identifier
CFE0007807, ucf:52346
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007807
Microstrip Patch Electrically Steerable Parasitic Array Radiators
Title
Microstrip Patch Electrically Steerable Parasitic Array Radiators.
Creator
Luther, Justin, Gong, Xun, Wahid, Parveen, Jones, W, Wu, Xinzhang, Flitsiyan, Elena, University of Central Florida
Abstract / Description
This dissertation explores the expansion of the Electrically Steerable Parasitic Array Radiator (ESPAR) technology to arrays using microstrip patch elements. Scanning arrays of two and three closely-coupled rectangular patch elements are presented, which incorporate no phase shifters. These arrays achieve directive radiation patterns and scanning of up to 26(&)deg; with maintained impedance match. The scanning is effected by tunable reactive loads which are used to control the mutual coupling...
Show moreThis dissertation explores the expansion of the Electrically Steerable Parasitic Array Radiator (ESPAR) technology to arrays using microstrip patch elements. Scanning arrays of two and three closely-coupled rectangular patch elements are presented, which incorporate no phase shifters. These arrays achieve directive radiation patterns and scanning of up to 26(&)deg; with maintained impedance match. The scanning is effected by tunable reactive loads which are used to control the mutual coupling between the elements, as well as additional loads which compensate to maintain the appropriate resonant frequency. The design incorporates theoretical analysis of the system of coupled antennas with full-wave simulation. A prototype of the three-element array at 1 GHz is fabricated and measured to exhibit a maximum gain of 7.4 dBi with an efficiency of 79.1%. Further, the microstrip ESPAR is thoroughly compared to uniformly-illuminated arrays of similar size. To satisfy the need for higher directivity antennas with inexpensive electronic scanning, the microstrip ESPAR is then integrated as a subarray. The three-element subcell fabrication is simplified to a single layer with an inverted-Y groove in the ground plane, allowing for DC biasing without the need for the radial biasing stubs or tuning stubs found in the two-layer design. The 1 GHz ESPAR array employs a corporate feed network consisting of a Wilkinson power divider with switchable delay line phase shifts, ring hybrid couplers, and achieves a gain of 12.1 dBi at boresight with (&)#177;20(&)deg; scanning and low side lobes. This array successfully illustrates the cost savings associated with ESPAR subarray scanning and the associated reduction in required number of phase shifters in the RF front end.
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Date Issued
2013
Identifier
CFE0005107, ucf:50754
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005107
Light Trapping in Thin Film Crystalline Silicon Solar Cells
Title
Light Trapping in Thin Film Crystalline Silicon Solar Cells.
Creator
Boroumand Azad, Javaneh, Chanda, Debashis, Peale, Robert, Del Barco, Enrique, Flitsiyan, Elena, Schoenfeld, Winston, University of Central Florida
Abstract / Description
This dissertation presents numerical and experimental studies of a unified light trapping approach that is extremely important for all practical solar cells. A 2D hexagonal Bravais lattice diffractive pattern is studied in conjunction with the verification of the reflection mechanisms of single and double layer anti-reflective coatings in the broad range of wavelength 400 nm - 1100 nm. By varying thickness and conformity, we obtained the optimal parameters which minimize the broadband...
Show moreThis dissertation presents numerical and experimental studies of a unified light trapping approach that is extremely important for all practical solar cells. A 2D hexagonal Bravais lattice diffractive pattern is studied in conjunction with the verification of the reflection mechanisms of single and double layer anti-reflective coatings in the broad range of wavelength 400 nm - 1100 nm. By varying thickness and conformity, we obtained the optimal parameters which minimize the broadband reflection from the nanostructured crystalline silicon surface over a wide range of angle 0(&)deg;-65(&)deg;. While the analytical design of broadband, angle independent anti-reflection coatings on nanostructured surfaces remains a scientific challenge, numerical optimization proves a viable alternative, paving the path towards practical implementation of the light trapping solar cells. A 3 (&)#181;m thick light trapping solar cell is modeled in order to predict and maximize combined electron-photon harvesting in ultrathin crystalline silicon solar cells. It is shown that the higher charge carrier generation and collection in this design compensates the absorption and recombination losses and ultimately results in an increase in energy conversion efficiency. Further, 20 (&)#181;m and 100 (&)#181;m thick functional solar cells with the light trapping scheme are studied. The efficiency improvement is observed numerically and experimentally due to photon absorption enhancement in the light trapping cells with respect to a bare cell of same thickness.
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Date Issued
2017
Identifier
CFE0006936, ucf:51654
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006936
Electrostatic control over temperature-dependent tunneling across single-molecule junctions
Title
Electrostatic control over temperature-dependent tunneling across single-molecule junctions.
Creator
Rodriguez Garrigues, Alvar, Del Barco, Enrique, Flitsiyan, Elena, Ishigami, Masa, Hernandez, Eloy, University of Central Florida
Abstract / Description
The aim of the present dissertation is to improve the understanding and methodology of temperature-dependent tunnel conduction through individual molecules by single-electron transport spectroscopy. New advances in electrochemistry present individual molecular diodes as a realistic option for the implementation on molecular circuits thanks to their high current rectification ratios. Therefore, a major requisite in this field is to understand and control the conduction behaviors for a large...
Show moreThe aim of the present dissertation is to improve the understanding and methodology of temperature-dependent tunnel conduction through individual molecules by single-electron transport spectroscopy. New advances in electrochemistry present individual molecular diodes as a realistic option for the implementation on molecular circuits thanks to their high current rectification ratios. Therefore, a major requisite in this field is to understand and control the conduction behaviors for a large variety of conditions. This work focuses on the electric conduction through ferrocene-based molecules as a function of temperatures within a wide range of bias and gate voltages by means of three-terminal electromigrated-broken single-electron transistors (SETs).The results show that the temperature dependence of the current (from 80 to 260 K) depends strongly on the bias and gate voltages, with areas in where the current increases exponentially with temperature (at the Coulomb blockade regimes), and others where the increase of the temperature makes the current only to vary slightly (at resonance) or to decrease monotonically (at the charge degeneracy points). These different observed behaviors of the tunneling current with increasing temperatures can be well explained by a formal single-level coherent tunneling model where the temperature dependence relies on the thermal broadening of the Fermi distributions of the electrons in the leads. The model portraits the molecule as a localized electrostatic level capacitively coupled to the transistor leads, and the electrical conduction through the junction as coherent sequential tunneling.
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Date Issued
2016
Identifier
CFE0006171, ucf:51132
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006171