Electrical transport in epitaxially grown undoped and Si-doped degenerate GaN films

Undoped and Si-doped GaN films were grown epitaxially on sapphire by reactive rf sputtering of GaAs (and Si) in Ar-N2 mixture. The resistivity of undoped GaN film grown at 100% N2 was ~2 x 105 {\Omega} cm, which reduced to ~1 {\Omega} cm in Si-doped film, revealing the effect of Si doping. With decrease of N2 from 100% to 75%, the carrier concentration of Si-doped films increased from ~7 x 1018 cm-3 to ~2 x 1019 cm-3, but remained practically unchanged as N2 was decreased to 20%, which is explained by effects due to saturation of Si doping and increase of Ga interstitials as well as compensation by N interstitials and Ga vacancies. Undoped and Si-doped films grown below 20% N2 displayed similar carrier concentrations (~1020 cm-3), due to dominance of Ga interstitials. Both undoped and Si-doped films were degenerate and displayed increase of mobility with carrier concentration and temperature, which was analyzed by the combined effect of ionized impurity and dislocation scattering, using compensation ratio as fitting parameter. At carrier concentrations below 1019 cm-3, the mobility was governed by both ionized impurity and dislocation scattering, while at higher carrier concentrations, ionized impurity scattering was found to dominate, limited by compensation due to acceptors. In spite of the degenerate character, the films displayed a small decrease of carrier concentration with temperature, along with a nearly linear decrease of mobility, which are explained by a marginal increase of compensation ratio with decrease of temperature, taking into account the band edge fluctuation effects

Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries

Abstract Background Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres. Methods This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries. Results In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia. Conclusion This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries

Microstructure dependent residual stress in reactively sputtered epitaxial Si-doped GaN films

Epitaxial Si-doped GaN films were grown on $\textit{c}$-sapphire by rf magnetron reactive co-sputtering of GaAs and Si at different N$_2$ percentages in Ar-N$_2$ atmosphere. High-resolution x-ray diffraction and $\phi$-scans reveal the mosaic growth of $\textit{c}$-axis oriented GaN films. Energy dispersive x-ray spectroscopy reveal $\thicksim$2 at.% Si in all the films, while the N/Ga ratio decreased substantially with N$_2$ percentage in sputtering atmosphere. The micro-strain, screw and edge dislocation densities were respectively obtained from $\omega$-2$\theta$, $\omega$, and in-plane $\phi$-rocking scans. The films grown at 30%-100% N$_2$ reveal dominance of edge over screw dislocations, with both approaching similar densities at lower N$_2$ percentages. The $\textit{c}$ and $\textit{a}$ parameters were independently determined to obtain the out-of-plane and in-plane strain components. The strain data was analyzed to separate hydrostatic and biaxial contributions and their dependences on N$_2$ percentage. The film grown at 100% N$_2$ displays large hydrostatic strain and micro-strain, attributed to excess/interstitial nitrogen. Both hydrostatic strain and micro-strain decrease substantially with initial decrease of N$_2$ percentage, but increase slightly in the films grown below 30% N$_2$, due to the incorporation of Ar. The films grown at $\gtrsim$75% N$_2$ reveal growth related, intrinsic biaxial stress, which is compressive and is attributed to the incorporation of excess nitrogen into grain boundaries and tensile side of edge dislocations. The films grown below 75% N$_2$ display stress reversal owing to the prevalence of coalescence related intrinsic tensile stress, which decreases in the films grown below 30% N$_2$, due to the incorporation of Ar and their voided structure.Comment: 23 pages and 7 figures along with 5 figures in supplementary informatio

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 degrees C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 x 1012 cm-2 to 8 x 1010 cm- 2. The films grown near room temperature showed large hydrostatic strain (-6 x 10-3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 degrees C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality

Effects of pre-operative isolation on postoperative pulmonary complications after elective surgery: an international prospective cohort study an international prospective cohort study

We aimed to determine the impact of pre-operative isolation on postoperative pulmonary complications after elective surgery during the global SARS-CoV-2 pandemic. We performed an international prospective cohort study including patients undergoing elective surgery in October 2020. Isolation was defined as the period before surgery during which patients did not leave their house or receive visitors from outside their household. The primary outcome was postoperative pulmonary complications, adjusted in multivariable models for measured confounders. Pre-defined sub-group analyses were performed for the primary outcome. A total of 96,454 patients from 114 countries were included and overall, 26,948 (27.9%) patients isolated before surgery. Postoperative pulmonary complications were recorded in 1947 (2.0%) patients of which 227 (11.7%) were associated with SARS-CoV-2 infection. Patients who isolated pre-operatively were older, had more respiratory comorbidities and were more commonly from areas of high SARS-CoV-2 incidence and high-income countries. Although the overall rates of postoperative pulmonary complications were similar in those that isolated and those that did not (2.1% vs 2.0%, respectively), isolation was associated with higher rates of postoperative pulmonary complications after adjustment (adjusted OR 1.20, 95%CI 1.05–1.36, p = 0.005). Sensitivity analyses revealed no further differences when patients were categorised by: pre-operative testing; use of COVID-19-free pathways; or community SARS-CoV-2 prevalence. The rate of postoperative pulmonary complications increased with periods of isolation longer than 3 days, with an OR (95%CI) at 4–7 days or ≥ 8 days of 1.25 (1.04–1.48), p = 0.015 and 1.31 (1.11–1.55), p = 0.001, respectively. Isolation before elective surgery might be associated with a small but clinically important increased risk of postoperative pulmonary complications. Longer periods of isolation showed no reduction in the risk of postoperative pulmonary complications. These findings have significant implications for global provision of elective surgical care. We aimed to determine the impact of pre-operative isolation on postoperative pulmonary complications after elective surgery during the global SARS-CoV-2 pandemic. We performed an international prospective cohort study including patients undergoing elective surgery in October 2020. Isolation was defined as the period before surgery during which patients did not leave their house or receive visitors from outside their household. The primary outcome was postoperative pulmonary complications, adjusted in multivariable models for measured confounders. Pre-defined sub-group analyses were performed for the primary outcome. A total of 96,454 patients from 114 countries were included and overall, 26,948 (27.9%) patients isolated before surgery. Postoperative pulmonary complications were recorded in 1947 (2.0%) patients of which 227 (11.7%) were associated with SARS-CoV-2 infection. Patients who isolated pre-operatively were older, had more respiratory comorbidities and were more commonly from areas of high SARS-CoV-2 incidence and high-income countries. Although the overall rates of postoperative pulmonary complications were similar in those that isolated and those that did not (2.1% vs 2.0%, respectively), isolation was associated with higher rates of postoperative pulmonary complications after adjustment (adjusted OR 1.20, 95%CI 1.05–1.36, p = 0.005). Sensitivity analyses revealed no further differences when patients were categorised by: pre-operative testing; use of COVID-19-free pathways; or community SARS-CoV-2 prevalence. The rate of postoperative pulmonary complications increased with periods of isolation longer than 3 days, with an OR (95%CI) at 4–7 days or ≥ 8 days of 1.25 (1.04–1.48), p = 0.015 and 1.31 (1.11–1.55), p = 0.001, respectively. Isolation before elective surgery might be associated with a small but clinically important increased risk of postoperative pulmonary complications. Longer periods of isolation showed no reduction in the risk of postoperative pulmonary complications. These findings have significant implications for global provision of elective surgical care