79 research outputs found
Growth and characterization of gallium arsenide phosphide and gallium phosphide on silicon for III-V/Si multi-junction solar cells
No full textSubmission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2024-08-01The student, Ryan Hool, accepted the attached license on 2022-07-12 at 16:55.The student, Ryan Hool, submitted this Dissertation for approval on 2022-07-12 at 17:39.This Dissertation was approved for publication on 2022-07-14 at 15:06.DSpace SAF Submission Ingestion Package generated from Vireo submission #18267 on 2022-11-16 at 10:55:57Solar photovoltaic (PV) technologies offer the ability to provide a significant percent of the world’s increasing energy demand while reducing overall greenhouse gas emissions. At the same time, PV technology is critical for the future of power generation in space. Silicon single-junction (1J) solar cells dominate the terrestrial solar market due to high production efficiencies and low material and processing costs from decades of investment. However efficiency is a key driver to further reduce the cost of PV technology and little room remains to improve the efficiency of Si toward its theoretical limit. Multi-junction (MJ) solar cells using a combination of III-V compound semiconductors achieve much higher efficiencies through tailoring bandgaps of sub-cells, leading to their utilization in space technology. Nevertheless, III-V solar cells are hindered by small-diameter, high-cost substrates, excluding their use in terrestrial power generation and placing a barrier to pursuing a greater variety of space missions. Through epitaxial integration of III-V semiconductors on Si, there is great potential to dramatically reduce the cost of III-V MJ cells and to realize competitive, high-efficiency solar cells for both space and terrestrial power.
While epitaxial III-V/Si MJ solar cells offer the possibility of a much-desired combination of high efficiency and low cost, the difficulty of growing high-quality III-V materials on Si substrates has left the potential of this PV technology unfilled. The combination of a ~1.7 eV III-V top cell and a ~1.1 eV Si bottom cell is close to the ideal double-junction (2J) bandgap pairing and has a > 25% (relative) higher theoretical maximum efficiency than for Si by itself. However, epitaxial III-V/Si MJ solar cells have yet to demonstrate efficiency higher than Si 1J solar cells. Extended defects resulting from crystal mismatches between III-V materials and Si hamper III-V sub-cell performance, and reducing the densities of these defects is critical to advancing epitaxial III-V/Si technologies. The larger lattice constants for III-V materials than for Si inevitably cause threading dislocations to form that extend through III-V sub-cell(s). Gallium Arsenide Phosphide (GaAsyP1-y) is an ideal partner to Si due to possessing a tunable direct bandgap from 1.42 – 2.01 eV and the smallest lattice mismatch to Si for ~1.7 eV III-V materials (~3.1%). Gallium Phosphide (GaP), with the closest lattice constant to Si for III-V materials, offers a convenient starting point for III-V epitaxial growth on Si before further expanding the lattice constant with compositional grading of GaAsyP1-y.
Obtaining low defect densities in ~1.7 eV GaAs0.77P0.23 (GaAsP) has remained challenging with threading dislocation densities (TDD) typically around an order of magnitude higher than desired for high performance (targeting ≤ 1 – 2 × 10^6 cm-2). More troubling is that relaxed GaP on Si can result in TDD > 10^7 cm-2, even though the lattice mismatch is less than an eighth of that for ~1.7 eV GaAsP. In this dissertation, I focus on defect management for relaxed GaP and GaAsP to improve material quality and advance GaAsP/Si 2J solar cell development. To this end, I investigated defects in GaP and determined growth strategies to reduce TDD to the lowest reported values I am aware of for both p- and n-type relaxed GaP on Si. I observed a strong dependence of TDD with doping and demonstrated methods to reach parity for relaxed p- and n-GaP on Si. For the GaAsP top cell I identified additional performance-damaging defects and strategies to mitigate them to realize high-current top cells. As a result, my collaborators and I realized the highest reported efficiencies for both GaAsP/Si 2J and ~1.7 eV GaAsP 1J solar cells on Si. Lastly, I studied radiation hardness and temperature-dependent device performance of ~1.7 eV GaAsP solar cells to broaden the space testing literature for this material
A framework for the analysis of mineral tax policy in sub-Saharan Africa
No full textGiven the dual role played by the Government as resource owner and tax collector in many sub - Saharan economies, it is important to separate"resource factor payments"from taxes through the use of different instruments. The instruments to be considered are: (1) a factor payment system that includes"ad rem"or"ad valorem"royalties. Production sharing, resource rent schemes, and fixed fees could also be used, but some form of unit payment is necessary and justified, because natural resources in the ground are inputs into the production process; (2) a cash flow and withholding tax system initially for the mineral sectors and eventually for other sectors of the economy. The cash flow tax would capture a share of the"economic rent"from each sector and be neutral across sectors; and (3) a depletion account to preserve the nations capital stock. Natural resources are part of an economy's capital stock, which will fall unless"replacement investment"is made as the resource is depleted.Economic Theory&Research,Environmental Economics&Policies,Banks&Banking Reform,Public Sector Economics&Finance,Health Economics&Finance
Effects of Graded Buffer Design and Active Region Structure on GaAsP Single-Junction Solar Cells Grown on GaP/Si Templates
No full text
The impact of CFO gender on bank loan contracting
No full textMotivated by recent studies that show female CFOs are more risk averse than male CFOs when making various corporate decisions, we examine whether banks take into consideration the gender of CFOs when pricing bank loans. We find that in our sample, firms under the control of female CFOs on average enjoy about 11% lower bank loan price than firms under the control of male CFOs. In addition, loans given to female CFO-led companies have longer maturities and are less likely to be required to provide collateral than loans given to male CFO led companies. Our results are robust to a series of robustness tests, such as a firm and year-fixed effect regression, a Heckman two-stage self selection model, a propensity score match method and a differences-in-differences approach. Overall, our results suggest that banks tend to recognize the role of female CFOs in providing more reliable accounting information ex ante and reducing default risk ex post, and grant firms with female CFOs lower loan price and more favourable contract terms.CFOs; gender; accounting information; bank loans
Supplementary document for Low-threshold InP quantum dot and InGaP quantum well visible lasers on silicon (001) - 5506150.pdf
No full textHigh-temperature L-I, Comparison with literature, I-V characteristics, Cavity length effect
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