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A Study of GaSb Quantum Ring Vertical-Cavity Surface-Emitting Devices at Telecommunications Wavelengths
Get PDFThis work presents the results of studies that were conducted in the context of gallium antimonide (GaSb) quantum ring (QR) vertical-cavity surface-emitting lasers (VCSELs). The aim of this project was to produce GaSb QR VCSELs operating at telecomswavelengths for applications in data transfer and in mobile device sensing. Simulations of the optical and electrical properties of distributed Bragg reflectors (DBRs) and VCSELs were performed in order to inform and optimise the design of several generations of QR VCSELs. Numerous experimental techniques were used to characterise and provide insight into the optical, electrical and material properties of samples produced throughout the duration of this project, and to enable comparison of designs, models and experiments. Characterisation techniques included optical transmittance and reflectance measurements, electrical characterisation, photoluminescence and time-resolved photoluminescence (PL and TRPL), electroluminescence (EL), scanning electron and transmission electron microscopy (SEM and TEM), energy-dispersive Xray spectroscopy (EDS), beam-exit cross-sectional polishing in conjunction with atomic force microscopy (BEXP+AFM) and X-ray photoelectron spectroscopy (XPS). Epitaxial growth was performed in-house using molecular beam epitaxy (MBE), and externally using an industrial metal organic chemical vapour deposition (MOCVD) reactor. Device fabrication and processing studies were conducted in the Lancaster Quantum Technology Centre (QTC). A study into the annealing of Au/Ni/Au contacts to p-type GaAs yielded a new ohmic contact scheme with specific contact resistance as low as (1.6 ± 0.2) × 10−5 Ω cm2. An in-house reflectance monitoring setup was shown to provide accurate measurements of layer thickness for GaAs and AlAs layers grown by MBE and demonstrated that the approach is suitable for the calibration of group-III growth rates for future GaSb QR VCSEL growth runs. DBRs with a step-graded interface between GaAs/Al0.9Ga0.1As layers and grown by MBE show improved electrical resistance when compared to ungraded structures. The step-graded structures are p-doped with beryllium as highly as 5 × 1018 cm−3 without detriment due to the low optical absorption at 1300 nm. A set of continuously-graded DBRs were grown by IQE using MOCVD and demonstrated comparable electrical performance to conventional AlxGa1−xAs-based DBRs. The peak reflectance of such structures approached that of the conventional DBRs; however, the conventional structures exhibited a broader stopband. TRPL measurements provided new insight into the carrier recombination lifetimes of QR structures in the presence of an optical cavity and stimulate further interest in the area. QR VCSEL devices are shown to operate within the telecoms ‘O’-band but are plagued by background emission. Low temperature (77 K) operation of the devices reveals interesting device physics associated with the quenching of the output emission when driven at high biases. Improvements to the QR VCSELs are presented as well as avenues in need of further investigation regarding the fundamental device physics and the measurement of device performance
Anthropogenic Water Withdrawals Modify Freshwater Inorganic Carbon Fluxes across the United States
No full textQuantifying inorganic carbon fluxes to and from freshwater environments is essential for the accurate determination of the total amount of carbon exported to both the atmosphere and oceans. However, understanding of how anthropogenic freshwater withdrawals perturb land-freshwater-ocean and freshwater-atmosphere inorganic carbon fluxes is limited. Using the United States (US) as an exemplar, we estimate that fresh surface water withdrawals across the country during the year 2015 resulted in a median gross dissolved inorganic carbon (DIC) retention flux of 8.2 (uncertainty range: 6.7–9.9) Tg C yr–1, equivalent to 28.3% of the total export of DIC to the oceans from US rivers. The median gross retention flux due to fresh groundwater withdrawals was 6.9 (uncertainty range: 5.3–8.8) Tg C yr–1, over eight times the magnitude of the DIC flux to the oceans by US subterranean groundwater discharge. The degassing of CO2 supersaturated groundwater following withdrawal emitted 3.6 (uncertainty range: 2.2–5.5) Tg of CO2 yr–1, 112% larger than previous estimates. On a county level, these CO2 emissions exceeded CO2 emissions from major emitting facilities across 45% of US counties. Reported results and a data analysis framework have important implications for the accurate development of carbon budgets across the US and around the world
Supervised Dimensionality Reduction for the Algorithm Selection Problem
Get PDFInstance space analysis extends the algorithm selection framework by enabling the visualisation of problem instances via dimensionality reduction (DR). The lower dimensional projection can also be used as input to predict algorithm performance, or to perform algorithm selection. In this paper we consider two supervised DR methods - partial least squares (PLS) and linear discriminant analysis (LDA) - both as visualisation tools and for the purpose of constructing classification models for algorithm selection. Multinomial logistic regression models are used for the classification problem. We compare PLS and LDA to DR methods previously used in this context on three combinatorial optimisation problems, and show that these methods are as competitive
Do Audit Firms’ Financial Statements Provide Information about Audit Quality?
Get PDFWhether audit firms should disclose financial statements is controversial among investors, practitioners, and regulators. The debate centers on whether audit firms’ financial statements provide information about audit quality. Using hand-collected data from U.K. audit firms’ financial statements, we construct four measures that capture audit firms’ resource investments (i.e., human capital, workplace environment, technologies) and risk exposures (i.e., litigation provisions). We find that increases in audit firms’ staff costs, investments in tangible assets and IT software, and reductions in litigation provisions are associated with improved audit quality. The information in audit firms’ financial statements is not contained in their transparency reports or regulatory inspection reports. Additional tests show that increases in audit firms’ staff costs and tangible assets, as well as reductions in litigation provisions, are associated with improved audit efficiency. JEL Classifications: M40; M42
Peer review and Palliative Medicine : Guiding reviewers’ contributions to ensuring high quality publications
Get PDFGRADE guidance 40 : The GRADE evidence-to-decision framework for environmental and occupational health
No full textObjective To provide guidance for the use of the Grading of Recommendations Assessment, Development and Evaluation (GRADE) Evidence-to-Decision (EtD) framework for environmental and occupational health (EOH). Study design and setting We conducted a systematic review and narrative synthesis of published and public EOH decision frameworks, followed by a modified Delphi process leading to development of a draft GRADE EtD framework for EOH. We pilot tested the provisional framework through a virtual workshop series, which further informed guidance for the framework’s application. We presented a summary of the results to all attendees of the GRADE Working Group meeting for feedback in July 2022 and November 2022, and for approval in May 2023. Results Consistent with existing GRADE EtD frameworks, the EtD framework for EOH includes a scoping and contextualization process and twelve assessment criteria. Modifications to the existing EtD frameworks include: consideration of the socio-political context when making judgments about the priority of the problem and feasibility of different alternatives; the addition of timing when making judgments about benefits and harms, the balance of effects, and feasibility; broadening of the equity criterion to include considerations beyond health equity; and more explicit accommodation of variable or conflicting stakeholder views when considering values and acceptability. The new EtD framework is also accompanied by a user guide intended to support its implementation in the EOH context. Conclusion Policymakers, regulators, and other stakeholders may use this GRADE EtD framework to approach decision-making about environmental and occupational exposures and interventions
ZTF SN Ia DR2 : Overview
No full textWe present the first homogeneous release of several thousand spectroscopically classified type Ia supernovae (SNe Ia) with spectroscopic redshifts. This release, named “DR2”, contains 3628 nearby (z < 0.3) SNe Ia discovered, followed, and classified by the Zwicky Transient Facility survey between March 2018 and December 2020. Of these, 3000 have good-to-excellent sampling and 2667 pass standard cosmology light curve quality cuts. This release is thus the largest SN Ia release to date, increasing by an order of magnitude the number of well-characterized low-redshift objects. With DR2, we also provide a volume-limited (z < 0.06) sample of nearly a thousand SNe Ia. With such a large, homogeneous, and well-controlled dataset, we are studying key current questions on SN cosmology, such as the linearity SNe Ia standardization, the SN and host dependencies, the diversity of the SN Ia population, and the accuracy of current light curve modeling. These, and more, are studied in detail in a series of articles associated with this release. Alongside the SN Ia parameters, we publish our forced-photometry gri-band light curves, 5138 spectra, local and global host properties, observing logs, and a Python tool to facilitate the use and access of these data. The photometric accuracy of DR2 is not yet suited for cosmological parameter inference, which will follow as the “DR2.5” release. We nonetheless demonstrate that our Hubble diagram of several thousands of SNe Ia has a typical 0.15 mag scatter
Use of Proceeds in Private Equity-Backed Initial Public Offerings
No full textThis paper examines differences in the disclosure and efficacy of intended use of proceeds between private equity (PE)-backed and non-PE-backed initial public offerings (IPOs). We find that PE-backed issuers have a significantly higher (lower) probability of disclosing “repayment of debt” and “repayment to selling shareholders” (“M&A”) than non-PE-backed issuers. Moreover, PE-backed issuers that disclose “repayment of debt” deleverage significantly more post-IPO to reduce their above-average debt-to-asset ratios to the level of non-PE-backed issuers. This is consistent with the idea that leveraged buyouts do not lead to a sustained change in optimal capital structure. While non-PE-backed issuers that disclose “R&D” (“M&A”) increase their R&D intensity (M&A deal volume) post-IPO, PE-backed issuers do not. Our results suggest that this is due to a trade-off with the need to repay claimholders in PE-backed IPOs. Finally, we show that PE backing reduces underpricing only if the use-of-proceeds disclosure is vague. Hence, we provide evidence that the well-documented PE “certification effect” depends on the information content of the prospectus
Assessing temporal shifts in trophic diversity in fish assemblages after the Fundão dam collapse
Get PDFThe rupture of the Fundão dam stands as one of the most significant environmental disasters of its kind on a global scale, profoundly affecting the aquatic ecosystem of Doce River Basin. By employing stable isotopes of carbon and nitrogen, we were able to trace matter and energy flow within ecosystems. In this study, we assessed the spatial and temporal variation, between 2020 and 2022, in species richness and trophic diversity in areas exposed (along a gradient in the main channel of the river) or unexposed (control sites in tributaries systems) to the mine ore tailings in the Doce River Basin. We tested the hypothesis that tailings reduce species richness, and that trophic stability is negatively affected by mining tailings. To estimate trophic stability for each sampling site, we calculated the standard ellipse area (SEA) and six community-wide metrics based on stable isotopes. The three regions studied presented distinct patterns on trophic diversity. Control sites exhibited stability in trophic metrics over time. Affected regions close to the rupture of the dam exhibited significant fluctuations on all six community-wide metrics analyzed than the affected regions farther from the rupture. Sites close to the rupture exhibited lower species richness, affecting mainly herbivores and piscivores. Our findings show the potential of using the isotopic approach in monitoring the ecological recovery of impacted ecosystems