Nitride Based Metal Insulator Semiconductor Heterostructure Material and Device Design and Characterization

Al2O3/AlGaN/GaN metal-insulator-semiconductor-heterostructures (MISH) were designed, fabricated and characterized. The effects of different dielectric deposition techniques, surface treatments and post deposition treatments were investigated by comprehensive material and electrical characterization to understand the Al2O3 dielectric and Al2O3/AlGaN interfacial properties.Thermal ALD and PEALD Al2O3 thin films were successfully deposited on MBE grown AlGaN/GaN layers. An XPS study reveals the band offset of Al2O3/AlGaN interface. In addition, pre-deposition treatments show a reduction of Ga-O bonds at the interface after ALD growth. The fabrication of Al2O3/AlGaN/GaN MISH diodes were achieved with deposition of Ti/Al/Ni/Au ohmic contacts and Ni/Au gate contacts. C-V characterization of MISH diodes was applied to evaluate Al2O3/AlGaN interface states. Traps with different energy levels were differentiated by C-V hysteresis curves and multi-frequency C-V. C-V analysis suggests that PEALD provides better film quality with lower defect densities than thermal ALD. The implementation of NH3 and N2 pre-deposition surface plasma treatment and N2 post-deposition annealing can also improve interfacial properties. Al2O3 dielectric thin film leakage current and conduction mechanisms were also studied by I-V characterization. PEALD Al2O3 thin films exhibit better leakage current suppression compared to thermal ALD films. Temperature dependent I-V characterization shows that Poole-Frenkel emission dominates in dielectric current transport at medium electric fields, while at high electric fields, Fowler-Nordheim tunneling and trap-assisted tunneling dominate at low and high temperatures, respectively. Various dielectric reliability tests were employed on Al2O3 thin films. The results of time dependent dielectric breakdown (TDDB) test can be fit into 1/E field dependent model and a Weibull slope of 2.87 is extracted for PEALD Al2O3 thin films. The dielectric breakdown field distribution statistics show that PEALD Al2O3 films have a larger average dielectric breakdown field than thermal ALD films, and the plasma N2O post deposition annealing improves the average breakdown field. The improvements from integration of pre-deposition and post deposition treatments may offer a better device performance and reliability in MIS-HEMTs, and enable further progress and development of nitride based power electronics

Systems Perspectives on Modelling and Managing Future Anthropogenic Emissions in Urban Areas : Nitrogen, Phosphorus and Carbon Studies in Stockholm, Sweden

Managing anthropogenic emissions in urban areas is a major challenge in sustainable environmental development for cities, and future changes and increasing urbanisation may increase this challenge. Systems perspectives have become increasingly important in helping urban managers understand how different changes may alter future emissions and whether current management strategies can efficiently manage these emissions. This thesis provides some systems perspectives that have been lacking in previous studies on modelling and managing future anthropogenic emissions in urban areas. The city of Stockholm, Sweden, was selected as the study site and studies about nitrogen, phosphorus and carbon were chosen, given world-wide urban eutrophication and global concerns about climate change. A substance flow analysis (SFA) structured model, comprising a source model coupled with a watershed model in an SFA structure, was developed to investigate future nutrient loading scenarios under various urban changes in small urban lake catchments. The results demonstrated that climate change potentially posed a greater threat to future nutrient loads to a selected lake catchment in Stockholm than the other scenarios examined. Another SFA-based study on future phosphorus flows through the city of Stockholm indicated that the best management option may depend on the perspective applied when comparing future scenarios of phosphorus flows and that both upstream and downstream measures need to be considered in managing urban phosphorus flows. An evaluation approach for examining current management plans and low-carbon city initiatives using the Driving forces-Pressure-States-Impact-Response (DPSIR) framework, was formulated. With such an evaluation approach, investigation of how well selected plans cover different aspects of the DPSIR framework and whether root causes and systematic measures are highlighted is possible. The results revealed that the current low-carbon city initiative in Stockholm falls within pressure-based, driver-orientated plans and that technical, institutional and cognitional measures are generally well covered. QC 20160510</p

Nitride Based Metal Insulator Semiconductor Heterostructure Material and Device Design and Characterization

Al2O3/AlGaN/GaN metal-insulator-semiconductor-heterostructures (MISH) were designed, fabricated and characterized. The effects of different dielectric deposition techniques, surface treatments and post deposition treatments were investigated by comprehensive material and electrical characterization to understand the Al2O3 dielectric and Al2O3/AlGaN interfacial properties.Thermal ALD and PEALD Al2O3 thin films were successfully deposited on MBE grown AlGaN/GaN layers. An XPS study reveals the band offset of Al2O3/AlGaN interface. In addition, pre-deposition treatments show a reduction of Ga-O bonds at the interface after ALD growth. The fabrication of Al2O3/AlGaN/GaN MISH diodes were achieved with deposition of Ti/Al/Ni/Au ohmic contacts and Ni/Au gate contacts. C-V characterization of MISH diodes was applied to evaluate Al2O3/AlGaN interface states. Traps with different energy levels were differentiated by C-V hysteresis curves and multi-frequency C-V. C-V analysis suggests that PEALD provides better film quality with lower defect densities than thermal ALD. The implementation of NH3 and N2 pre-deposition surface plasma treatment and N2 post-deposition annealing can also improve interfacial properties. Al2O3 dielectric thin film leakage current and conduction mechanisms were also studied by I-V characterization. PEALD Al2O3 thin films exhibit better leakage current suppression compared to thermal ALD films. Temperature dependent I-V characterization shows that Poole-Frenkel emission dominates in dielectric current transport at medium electric fields, while at high electric fields, Fowler-Nordheim tunneling and trap-assisted tunneling dominate at low and high temperatures, respectively. Various dielectric reliability tests were employed on Al2O3 thin films. The results of time dependent dielectric breakdown (TDDB) test can be fit into 1/E field dependent model and a Weibull slope of 2.87 is extracted for PEALD Al2O3 thin films. The dielectric breakdown field distribution statistics show that PEALD Al2O3 films have a larger average dielectric breakdown field than thermal ALD films, and the plasma N2O post deposition annealing improves the average breakdown field. The improvements from integration of pre-deposition and post deposition treatments may offer a better device performance and reliability in MIS-HEMTs, and enable further progress and development of nitride based power electronics

Scientific Journal of the Ternopil National Technical University, No 1 (81), 2016

Managing anthropogenic emissions in urban areas is a major challenge in sustainable environmental development for cities, and future changes and increasing urbanisation may increase this challenge. Systems perspectives have become increasingly important in helping urban managers understand how different changes may alter future emissions and whether current management strategies can efficiently manage these emissions. This thesis provides some systems perspectives that have been lacking in previous studies on modelling and managing future anthropogenic emissions in urban areas. The city of Stockholm, Sweden, was selected as the study site and studies about nitrogen, phosphorus and carbon were chosen, given world-wide urban eutrophication and global concerns about climate change. A substance flow analysis (SFA) structured model, comprising a source model coupled with a watershed model in an SFA structure, was developed to investigate future nutrient loading scenarios under various urban changes in small urban lake catchments. The results demonstrated that climate change potentially posed a greater threat to future nutrient loads to a selected lake catchment in Stockholm than the other scenarios examined. Another SFA-based study on future phosphorus flows through the city of Stockholm indicated that the best management option may depend on the perspective applied when comparing future scenarios of phosphorus flows and that both upstream and downstream measures need to be considered in managing urban phosphorus flows. An evaluation approach for examining current management plans and low-carbon city initiatives using the Driving forces-Pressure-States-Impact-Response (DPSIR) framework, was formulated. With such an evaluation approach, investigation of how well selected plans cover different aspects of the DPSIR framework and whether root causes and systematic measures are highlighted is possible. The results revealed that the current low-carbon city initiative in Stockholm falls within pressure-based, driver-orientated plans and that technical, institutional and cognitional measures are generally well covered. QC 20160510</p

Climate change effects on nitrogen loading to urban lakes : The case of Råcksta Träsk, Stockholm, Sweden

Nutrient loads to aquatic recipients can be expected to change due to climate change. In this work, we focus on nitrogen loads to the lake Råcksta Träsk in Stockholm, Sweden as an example of an urban ecosystem. A substance flow model is developed to describe the sources and pathways of nitrogen at present. A feed-back table approach is applied to indicate potential climate change effects on nitrogen source strengths and processes in pathways, using existing regional climate change scenarios. The tentative results indicate that biological, hydrological, meteorological and biogeochemical effects and change in human behavior as response to climate change may lead to altered nitrogen flows through an urban catchment.QC 20160509</p

Toward sustainable management of phosphorus flows in a changing rural–urban environment: recent advances, challenges, and opportunities

While food is mainly produced in rural areas, the main drivers of consumption are urban environments. The increasing demand for food in cities leads to an accumulation of phosphorus (P) in urban environments, which affects P flows at multiple scales. This study reviewed recent advances, challenges, and opportunities for sustainable management of P flows in decoupled rural–urban environments. We discussed recent advances in Substance Flow Analysis (SFA) that enable the characterization of P flows in rural–urban environments and illustrate how urban consumption affects rural food production systems. Most challenges of SFA are associated with the (1) spatial decoupling of food production in rural areas and food consumption in urban systems at multiple scales, (2) temporal implications of substance flows on system analysis, and (3) increasing complexity of resource flows in rural–urban environments. We identified three main opportunities for future research on sustainable management of P flows. These opportunities lie in linking urban and rural SFA through multi-scale analysis, increasing both spatial and temporal resolution of P flows within various environments and identifying how linking urban to rural environments can reduce pressure on primary production systems. Also, we highlighted that methodological development on high-resolution SFA at multiple scales is needed to close P cycles between rural and urban systems and to allow the development of future, sustainable P management systems in the anthropogenic food production chain.</p

Anthropogenic phosphorus flows under different scenarios for the city of Stockholm, Sweden

Today, concerns prevail about the unsustainable use of phosphorus and worldwide eutrophication, thus requiring efficient management of phosphorus flows. With increasing population and associated urban growth, urban management of phosphorus flows in the perspectives of recycling, eutrophication and total budget becomes increasingly important. This study mapped phosphorus flows for a reference year (2013) and a future year (2030) using different scenarios for the city of Stockholm, Sweden. The results indicated that the Swedish goal of recycling phosphorus from wastewater would cover the majority of the total phosphorus budget for Stockholm. However, in 2013, only 10% of phosphorus was recycled for agricultural use, around half of which was from sewage sludge and the other half from food waste. Almost 50% of total phosphorus was sent to landfill/mining waste capping with sewage sludge, for economic reasons and lack of market. Among the scenarios of upstream and downstream urban management options studied in combination with population growth, recovery of phosphorus from sewage sludge had the greatest potential to increase the fraction recycled to agriculture. However, only upstream measures, e.g. changed diet, were able to reduce the total phosphorus budget. Urban management of phosphorus flows based on the different perspectives of recycling, eutrophication or total budget was shown to potentially result in different preferred management actions and both upstream and downstream measures need to be considered. Moreover, management needs to pay attention to small but environmentally sensitive flows, particularly when setting city goals on phosphorus recycling by percentage in a large budget.QC 20160105</p

Measuring parents’ acceptance of non-national immunization program vaccines for children and its influencing factors during the COVID-19 pandemic in Shanghai, China

This study aimed to understand the willingness of and affecting factors of non-national immunization program (non-NIP) vaccines among children’s parents during the COVID-19 era in Shanghai, China. A cross-sectional survey was conducted with parents who attended vaccination clinics in four out of 16 districts in Shanghai, China. Data was obtained using a self-administered structured questionnaire. A multivariate logistic regression model was used to analyze factors associated with vaccination acceptability. In total, 1691 valid questionnaires were obtained. Of the participants, 69.5% (1,176/1,691) reported being interested in non-NIP vaccines for their children. Further, respondents were more likely to be willing to get non-NIP vaccines for their children if they had an income of 10,000–20,000CNY or more, an educational level of college or above, and if getting the vaccination was moderately convenient or convenient. Respondents were less likely to be willing to get the vaccines if they were in the 30–39 age group and had moderate or low satisfaction with the vaccine. Many parents are willing to get non-NIP vaccines for their children. However, some demographic factors, perceived convenience and satisfaction of vaccination, perceived necessity, safety and price barrier of non- NIP influenced the acceptability of non-NIP vaccines in Shanghai. Our findings can help guide future efforts to increase non-NIP vaccines acceptability