Theoretical and material studies on thin-film electroluminescent devices

A theoretical study of resonant tunneling in multilayered heterostructures is presented based on an exact solution of the Schroedinger equation under the application of a constant electric field. By use of the transfer matrix approach, the transmissivity of the structure is determined as a function of the incident electron energy. The approach presented is easily extended to many layer structures where it is more accurate than other existing transfer matrix or WKB models. The transmission resonances are compared to the bound state energies calculated for a finite square well under bias using either an asymmetric square well model or the exact solution of an infinite square well under the application of an electric field. The results show good agreement with other existing models as well as with the bound state energies. The calculations were then applied to a new superlattice structure, the variablly spaced superlattice energy filter, (VSSEP) which is designed such that under bias the spatial quantization levels fully align. Based on these calculations, a new class of resonant tunneling superlattice devices can be designed

A layered structure surface acoustic wave for oxygen sensing

A novel layered structure surface acoustic wave (layered SAW) transducer has been employed for an oxygen sensing application. It is a SiO2(0.36 µm)/ST-cut quartz crystal transducer. The dominant mode propagating in the transducer is a combination of Rayleigh and Love modes. Such a structure has the advantage of confining the acoustic wave energy to the top selective layer, which increases the sensitivity of the device. A TiO2 thin film deposited by the sol-gel process has been used as the oxygen sensitive layer

An acoustic charge transport imager for high definition television applications

The primary goal of this research is to develop a solid-state television (HDTV) imager chip operating at a frame rate of about 170 frames/sec at 2 Megapixels/frame. This imager will offer an order of magnitude improvements in speed over CCD designs and will allow for monolithic imagers operating from the IR to UV. The technical approach of the project focuses on the development of the three basic components of the imager and their subsequent integration. The camera chip can be divided into three distinct functions: (1) image capture via an array of avalanche photodiodes (APD's); (2) charge collection, storage, and overflow control via a charge transfer transistor device (CTD); and (3) charge readout via an array of acoustic charge transport (ACT) channels. The use of APD's allows for front end gain at low noise and low operating voltages while the ACT readout enables concomitant high speed and high charge transfer efficiency. Currently work is progressing towards the optimization of each of these component devices. In addition to the development of each of the three distinct components, work towards their integration and manufacturability is also progressing. The component designs are considered not only to meet individual specifications but to provide overall system level performance suitable for HDTV operation upon integration. The ultimate manufacturability and reliability of the chip constrains the design as well. The progress made during this period is described in detail

Metal nanofilm in strong ultrafast optical fields

We predict that a metal nanofilm subjected to an ultrashort (single oscillation) optical pulse of a high field amplitude $\sim 3 \mathrm{V/\AA}$ at normal incidence undergoes an ultrafast (at subcycle times $\lesssim 1 \mathrm{fs}$) transition to a state resembling semimetal. Its reflectivity is greatly reduced, while the transmissivity and the optical field inside the metal are greatly increased. The temporal profiles of the optical fields are predicted to exhibit pronounced subcycle oscillations, which are attributed to the Bloch oscillations and formation of the Wannier-Stark ladder of electronic states. The reflected, transmitted, and inside-the-metal pulses have non-zero areas approaching half-cycle pulses. The effects predicted are promising for applications to nanoplasmonic modulators and field-effect transistors with petahertz bandwidth

An Acoustic Charge Transport Imager for High Definition Television Applications: Reliability Modeling and Parametric Yield Prediction of GaAs Multiple Quantum Well Avalanche Photodiodes

Reliability modeling and parametric yield prediction of GaAs/AlGaAs multiple quantum well (MQW) avalanche photodiodes (APDs), which are of interest as an ultra-low noise image capture mechanism for high definition systems, have been investigated. First, the effect of various doping methods on the reliability of GaAs/AlGaAs multiple quantum well (MQW) avalanche photodiode (APD) structures fabricated by molecular beam epitaxy is investigated. Reliability is examined by accelerated life tests by monitoring dark current and breakdown voltage. Median device lifetime and the activation energy of the degradation mechanism are computed for undoped, doped-barrier, and doped-well APD structures. Lifetimes for each device structure are examined via a statistically designed experiment. Analysis of variance shows that dark-current is affected primarily by device diameter, temperature and stressing time, and breakdown voltage depends on the diameter, stressing time and APD type. It is concluded that the undoped APD has the highest reliability, followed by the doped well and doped barrier devices, respectively. To determine the source of the degradation mechanism for each device structure, failure analysis using the electron-beam induced current method is performed. This analysis reveals some degree of device degradation caused by ionic impurities in the passivation layer, and energy-dispersive spectrometry subsequently verified the presence of ionic sodium as the primary contaminant. However, since all device structures are similarly passivated, sodium contamination alone does not account for the observed variation between the differently doped APDs. This effect is explained by the dopant migration during stressing, which is verified by free carrier concentration measurements using the capacitance-voltage technique

Has the phasing out of stavudine in accordance with changes in WHO guidelines led to a decrease in single-drug substitutions in first-line antiretroviral therapy for HIV in sub-Saharan Africa?

This version is the Accepted Manuscript and is published in final edited form as: AIDS. 2017 January 02; 31(1): 147–157. doi:10.1097/QAD.0000000000001307OBJECTIVE: We assessed the relationship between phasing out stavudine in first-line antiretroviral therapy (ART) in accordance with WHO 2010 policy and single-drug substitutions (SDS) (substituting the nucleoside reverse transcriptase inhibitor in first-line ART) in sub-Saharan Africa. DESIGN: Prospective cohort analysis (International epidemiological Databases to Evaluate AIDS-Multiregional) including ART-naive, HIV-infected patients aged at least 16 years, initiating ART between January 2005 and December 2012. Before April 2010 (July 2007 in Zambia) national guidelines called for patients to initiate stavudine-based or zidovudine-based regimen, whereas thereafter tenofovir or zidovudine replaced stavudine in first-line ART. METHODS: We evaluated the frequency of stavudine use and SDS by calendar year 2004-2014. Competing risk regression was used to assess the association between nucleoside reverse transcriptase inhibitor use and SDS in the first 24 months on ART. RESULTS: In all, 33 441 (8.9%; 95% confience interval 8.7-8.9%) SDS occurred among 377 656 patients in the first 24 months on ART, close to 40% of which were amongst patients on stavudine. The decrease in SDS corresponded with the phasing out of stavudine. Competing risks regression models showed that patients on tenofovir were 20-95% less likely to require a SDS than patients on stavudine, whereas patients on zidovudine had a 75-85% decrease in the hazards of SDS when compared to stavudine. CONCLUSION: The decline in SDS in the first 24 months on treatment appears to be associated with phasing out stavudine for zidovudine or tenofovir in first-line ART in our study. Further efforts to decrease the cost of tenofovir and zidovudine for use in this setting is warranted to substitute all patients still receiving stavudine

An Acoustic Charge Transport Imager for High Definition Television Applications: Low-Voltage SAW Amplifiers on Multilayer GaAs/ZnO Substrates

This thesis addresses the acoustoelectric issues concerning the amplification of surface acoustic waves (SAWs) and the reflection of SAWs from slanted reflector gratings on GaAs, with application to a novel acoustic charge transport (ACT) device architecture. First a simple model of the SAWAMP was developed, which was subsequently used to define the epitaxially grown material structure necessary to provide simultaneously high resistance and high electron mobility. In addition, a segmented SAWAMP structure was explored with line widths on the order of an acoustic wavelength. This resulted in the demonstration of SAWAMPS with an order of magnitude less voltage and power requirements than previously reported devices. A two-dimensional model was developed to explain the performance of devices with charge confinement layers less then 0.5 mm, which was experimentally verified. This model was extended to predict a greatly increased gain from the addition of a ZnO overlay. These overlays were experimentally attempted, but no working devices were reported due to process incompatibilities. In addition to the SAWAMP research, the reflection of SAWs from slanted gratings on GaAs was also studied and experimentally determined reflection coefficients for both 45 deg grooves and Al stripes on GaAs have been reported for the first time. The SAWAMp and reflector gratings were combined to investigate the integrated ring oscillator for application to the proposed ACT device and design parameters for this device have been provided

Assessing the impact of interfering organic matter on soil metaproteomic workflow

Funding: Matthias Waibel was funded by the University of Galway College of Science and the Irish Research Council under GOIPG/2016/1215. The James Hutton Institute receives funding support from the Rural and Environment Science and Analytical Services Division of the Scottish Government. Open access funding provided by IReL.Soil organic matter (SOM) is biologically, chemically, and physically complex. As a major store of nutrients within soil, it plays an important role in nutrient provision to plants. An enhanced understanding of SOM utilisation processes could underpin better fertiliser management for plant growth, with reduced environmental losses. Metaproteomics can allow the characterisation of protein profiles and could help gaining insights into SOM microbial decomposition mechanisms. Here, we applied three different extraction methods to two soil types to recover SOM with different characteristics. Specifically, water extractable organic matter, mineral associated organic matter and protein-bound organic matter were targeted with the aim to investigate the metaproteome enriched in those extractions. As a proof-of-concept replicated extracts from one soil were further analysed for peptide identification using liquid chromatography followed by tandem mass spectrometry. We employ a framework for mining mass spectra for both peptide assignment and fragmentation pattern characterisation. Different extracts were found to exhibit contrasting total protein and humic substance content for the two soils investigated. Overall, water extracts displayed the lowest humic substance content (in both soils) and the highest number of peptide identifications (in the soil investigated) with most frequent peptide hits associated with diverse substrate/ligand binding proteins of Proteobacteria and derived taxa. Our framework also highlighted a strong peptidic signal in unassigned and unmatched spectra, information that is currently not captured by the pipelines employed in this study. Taken together, this work points to specific areas for optimisation in chromatography and mass spectrometry to adequately characterise SOM associated metaproteomes.Publisher PDFPeer reviewe

Homocysteine, type 2 diabetes mellitus, and cognitive performance: The Maine-Syracuse Study

Type 2 diabetes mellitus and higher total plasma homocysteine concentrations are each associated with an increased incidence of cardiovascular disease and with diminished cognitive performance. Relations between homocysteine concentrations and cardiovascular disease incidence are stronger in the presence of type 2 diabetes mellitus. Therefore, we hypothesized that relations between homocysteine concentrations and cognitive performance would be stronger in the presence of type 2 diabetes. We related homocysteine concentrations and cognitive performance on the Mini-Mental State Examination in 817 dementia- and stroke-free participants of the Maine-Syracuse Study, 90 of whom were classified with type 2 diabetes mellitus. Regardless of statistical adjustment for age, sex, gender, vitamin co-factors (folate, vitamin B6, vitamin B12), cardiovascular disease risk factors, and duration and type of treatment for type 2 diabetes mellitus, statistically significant inverse associations between homocysteine concentrations and cognitive performance were observed for diabetic individuals. The weaker inverse associations between homocysteine concentrations and cognitive performance obtained for non-diabetic individuals were not robust to statistical adjustment for some covariates. Interactions between homocysteine concentrations and type 2 diabetes mellitus are observed such that associations between homocysteine and cognitive performance are stronger in the presence of diabetes