Modified Confinement Model for Size Dependent Raman Shift and Linewidth of Silicon Nanocrystals

A modified phonon confinement model considering the size distribution, an improved phonon dispersion curve and a confinement function is developed for the calculation of size dependent Raman spectra of the silicon (Si) nanocrystals. The model is capable in simultaneous calculation of the Raman shift, intensity and linewidth. The calculated size dependent redshift and linewidth of Raman spectra are in good agreement with the available experimental data in literature and better than previously reported theoretical results. The rapid rise in the redshift and linewidth for relatively smaller Si nanocrystals are well reproduced. The asymmetric behavior of Raman spectra is also obtained from the present model.Comment: 15 pages, 4 figure

A convenient band-gap interpolation technique and an improved band line-up model for InGaAlAs on InP

The band-gap energy and the band line-up of InGaAlAs quaternary compound material on InP are essential information for the theoretical study of physical properties and the design of optoelectronics devices operating in the long-wavelength communication window. The band-gap interpolation of In1-x-y Ga (x) Al (y) As on InP is known to be a challenging task due to the observed discrepancy of experimental results arising from the bowing effect. Besides, the band line-up results of In1-x-y Ga (x) Al (y) As on InP based on previously reported models have limited success by far. In this work, we propose an interpolation solution using the single-variable surface bowing estimation interpolation method for the fitting of experimentally measured In1-x-y Ga (x) Al (y) As band-gap data with various degree of bowing using the same set of input parameters. The suggested solution provides an easier and more physically interpretable way to determine not only lattice matched, but also strained band-gap energy of In1-x-y Ga (x) Al (y) As on InP based on the experimental results. Interpolated results from this convenient method show a more favourable match to multiple independent experiment data sets measured under different temperature conditions as compared to those obtained from the commonly used weighted-sum approach. On top of that, extended framework of the model-solid theory for the band line-up of In1-x-y Ga (x) Al (y) As/InP heterostructure is proposed. Our model-solid theory band line-up result using the proposed extended framework has shown an improved accuracy over those without the extension. In contrast to some previously reported works, it is worth noting that the band line-up result based on our proposed extended model-solid theory has also shown to be more accurate than those given by Harrison's mode

Evolutionary algorithms for compiler-enabled program autotuning

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 79-82).Millions of patients worldwide are hospitalized each year due to an acute coronary syndrome (ACS). Patients who have had an acute coronary syndrome are at higher risk for developing future adverse cardiovascular events such as cardiovascular death, congestive heart failure, or a repeat ACS. Currently, there have been several electrocardiographic metrics used to assess the risk of ACS patients for a future cardiovascular death including heart rate variability, heart rate turbulence, deceleration capacity, T-wave alternans, and morphologic variability. This thesis introduces new ECG-based metrics that can be used to risk-stratify post-ACS patients for future cardiovascular death and evaluates the clinical utility of the existing electrocardiogram based metric known as morphologic variability (MV). We first analyze a metric called weighted morphologic variability (WMV) which is based on assessment of beat-to-beat morphology changes in the ECG. In addition, we introduce machine learning methods with morphology based features to separate post-ACS patients into high risk or low risk for cardiovascular death. Finally, we aim to increase the clinical utility of MV by creating a metric that can achieve good risk stratification when applied to a small amount of data. The body of this work suggests that morphologic variability is an effective metric in prognosticating post- ACS patients into high risk and low risk for cardiovascular dearth.by Priya Parayanthal.M.Eng