Matching Items (5)
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- All Subjects: Thin films
- All Subjects: Solar energy
- Creators: Bertoni, Mariana
Description
Carrier lifetime is one of the few parameters which can give information about the low defect densities in today's semiconductors. In principle there is no lower limit to the defect density determined by lifetime measurements. No other technique can easily detect defect densities as low as 10-9 - 10-10 cm-3 in a simple, contactless room temperature measurement. However in practice, recombination lifetime τr measurements such as photoconductance decay (PCD) and surface photovoltage (SPV) that are widely used for characterization of bulk wafers face serious limitations when applied to thin epitaxial layers, where the layer thickness is smaller than the minority carrier diffusion length Ln. Other methods such as microwave photoconductance decay (µ-PCD), photoluminescence (PL), and frequency-dependent SPV, where the generated excess carriers are confined to the epitaxial layer width by using short excitation wavelengths, require complicated configuration and extensive surface passivation processes that make them time-consuming and not suitable for process screening purposes. Generation lifetime τg, typically measured with pulsed MOS capacitors (MOS-C) as test structures, has been shown to be an eminently suitable technique for characterization of thin epitaxial layers. It is for these reasons that the IC community, largely concerned with unipolar MOS devices, uses lifetime measurements as a "process cleanliness monitor." However when dealing with ultraclean epitaxial wafers, the classic MOS-C technique measures an effective generation lifetime τg eff which is dominated by the surface generation and hence cannot be used for screening impurity densities. I have developed a modified pulsed MOS technique for measuring generation lifetime in ultraclean thin p/p+ epitaxial layers which can be used to detect metallic impurities with densities as low as 10-10 cm-3. The widely used classic version has been shown to be unable to effectively detect such low impurity densities due to the domination of surface generation; whereas, the modified version can be used suitably as a metallic impurity density monitoring tool for such cases.
ContributorsElhami Khorasani, Arash (Author) / Alford, Terry (Thesis advisor) / Goryll, Michael (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2013
Description
A series of pyrite thin films were synthesized using a novel sequential evaporation
technique to study the effects of substrate temperature on deposition rate and micro-structure of
the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using
sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures
(typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a
one-step process in which a constant growth temperature is maintained throughout growth, and a
three-step process in which an initial low temperature seed layer is deposited, followed by a high
temperature layer, and then finished with a low temperature capping layer. Analysis methods to
analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD),
Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM),
Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect
measurements. Our results show that crystallinity of the pyrite thin film improves and grain size
increases with increasing substrate temperature. The sticking coefficient of Fe was found to
increase with increasing growth temperature, indicating that the Fe incorporation into the growing
film is a thermally activated process.
technique to study the effects of substrate temperature on deposition rate and micro-structure of
the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using
sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures
(typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a
one-step process in which a constant growth temperature is maintained throughout growth, and a
three-step process in which an initial low temperature seed layer is deposited, followed by a high
temperature layer, and then finished with a low temperature capping layer. Analysis methods to
analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD),
Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM),
Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect
measurements. Our results show that crystallinity of the pyrite thin film improves and grain size
increases with increasing substrate temperature. The sticking coefficient of Fe was found to
increase with increasing growth temperature, indicating that the Fe incorporation into the growing
film is a thermally activated process.
ContributorsWertheim, Alex (Author) / Newman, Nathan (Thesis advisor) / Singh, Rakesh (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2014
Description
The Solar Powered Amphibious Transport (SPAT) is an amphibious hovercraft that uses solar energy as a power source and is fully controlled via iOS application on a phone or tablet. The hovercraft field is relatively unexplored with a solar power source, and one of the goals of the SPAT was to spark interest in sustainable hovercraft design. By challenging the potential of solar power, the SPAT proves that solar energy can be used in high power transportation applications. The second motive behind the creation a hovercraft was for it to serve as a disaster relief vehicle. A hovercraft can traverse both ground and water, which makes it ideal in flooded areas. With the SPAT being remote controlled it can allow the operator to stay at a safe distance while sending supplies or rescuing a person. The SPAT design covered multiple size options, however a small prototype version was built to serve as a proof of concept that a larger solar hovercraft is possible. Our analysis suggests that a larger craft will be able to carry more weight, and be more power efficient. A larger SPAT could help deliver supplies or rescue stranded people after a flood or hurricane. One issue faced however, was that many hovercrafts are highly expensive. The SPAT prototype was designed on a tight budget that did not exceed $800. The possibility of achieving this cost levels allows hovercraft to be a reasonable option for disaster relief agencies. After many long hours spent the SPAT became a fully operational remote control solar powered hovercraft.
ContributorsDavis, Parker William (Co-author) / Clenney, Jacob (Co-author) / Nachman, Michael (Co-author) / Melillo, Nick (Co-author) / Bertoni, Mariana (Thesis director) / Kozicki, Michael (Committee member) / Electrical Engineering Program (Contributor, Contributor) / Barrett, The Honors College (Contributor)
Created2017-05
Description
Polycrystalline CdS/CdTe solar cells continue to dominate the thin-film photovoltaics industry with an achieved record efficiency of over 22% demonstrated by First Solar, yet monocrystalline CdTe devices have received considerably less attention over the years. Monocrystalline CdTe double-heterostructure solar cells show great promise with respect to addressing the problem of low Voc with the passing of the 1 V benchmark. Rapid progress has been made in driving the efficiency in these devices ever closer to the record presently held by polycrystalline thin-films. This achievement is primarily due to the utilization of a remote p-n heterojunction in which the heavily doped contact materials, which are so problematic in terms of increasing non-radiative recombination inside the absorber, are moved outside of the CdTe double heterostructure with two MgyCd1-yTe barrier layers to provide confinement and passivation at the CdTe surfaces. Using this design, the pursuit and demonstration of efficiencies beyond 20% in CdTe solar cells is reported through the study and optimization of the structure barriers, contacts layers, and optical design. Further development of a wider bandgap MgxCd1-xTe solar cell based on the same design is included with the intention of applying this knowledge to the development of a tandem solar cell constructed on a silicon subcell. The exploration of different hole-contact materials—ZnTe, CuZnS, and a-Si:H—and their optimization is presented throughout the work. Devices utilizing a-Si:H hole contacts exhibit open-circuit voltages of up to 1.11 V, a maximum total-area efficiency of 18.5% measured under AM1.5G, and an active-area efficiency of 20.3% for CdTe absorber based devices. The achievement of voltages beyond 1.1V while still maintaining relatively high fill factors with no rollover, either before or after open-circuit, is a promising indicator that this approach can result in devices surpassing the 22% record set by polycrystalline designs. MgxCd1-xTe absorber based devices have been demonstrated with open-circuit voltages of up to 1.176 V and a maximum active-area efficiency of 11.2%. A discussion of the various loss mechanisms present within these devices, both optical and electrical, concludes with the presentation of a series of potential design changes meant to address these issues.
ContributorsBecker, Jacob J (Author) / Zhang, Yong-Hang (Thesis advisor) / Bertoni, Mariana (Committee member) / Vasileska, Dragica (Committee member) / Johnson, Shane (Committee member) / Arizona State University (Publisher)
Created2017
Description
With an abundance of sunshine, the state of Arizona has the potential for producing large amounts of solar energy. However, in recent years Arizona has also become the focal point in a political battle to determine the value and future of residential solar energy fees, which has critical implications for distributed generation. As the debate grows, it is clear that solar policies developed in Arizona will influence other state regulators regarding their solar rate structures and Net Energy Metering; however, there is a hindrance in the progress of this discussion due to the varying frameworks of the stakeholders involved. For this project, I set out to understand and analyze why the different stakeholders have such conflicting viewpoints. Some groups interpret energy as a financial and technological object while others view it is an inherently social and political issue. I conducted research in three manners: 1) I attended public meetings, 2) hosted interviews, and 3) analyzed reports and studies on the value of solar. By using the SRP 2015 Rate Case as my central study, I will discuss how these opposing viewpoints do or do not incorporate various forms of justice such as distributive, participatory, and recognition justice. In regards to the SRP Rate Case, I will look at both the utility- consumer relationship and the public meeting processes in which they interact, in addition to the pricing plans. This work reveals that antiquated utility structures and a lack of participation and recognition justice are hindering the creation of policy changes that satisfy both the needs of the utilities and the community at large.
ContributorsGidney, Jacob Robert (Author) / Richter, Jennifer (Thesis director) / Jurik, Nancy (Committee member) / School of Mathematical and Statistical Sciences (Contributor) / Department of Economics (Contributor) / Barrett, The Honors College (Contributor)
Created2015-12