Stimulated emission, lasing, and related properties of III-V nitride heterostructures are studied. A strain-dependent semi-empirical tight-binding model is developed, using the valence force-field model of Keating, to predict the atomic positions in the strained wurtzite crystal lattice. Predicted deformation potentials and strain-induced exciton splitting are shown to closely match data in the literature. The spectral properties of the edge luminescence from GaN-AlGaN heterostructures is investigated. The existence of stimulated emission is demonstrated and a measurement of the optical gain spectra is reported. In addition, the light emission properties of GaN-AlGaN separate confinement heterostructures is studied. The measured luminescence properties are improved for active region designs with fewer, thicker wells. An analysis of the trend is presented demonstrating recombination at the well-barrier interface as a significant factor. The results also indicate that the quantum wells experience compressive strain from the lattice mismatch with the AlGaN cladding layers. Further experimental results demonstrate that the commonly observed surface stimulated emission is related to in-plane optical gain, and is observed most commonly in samples with rough surface morphology. Photopumping results from GaN-AlGaN laser platelets are presented and discussed. Laser oscillation in GaN-AlGaN separate confinement heterostructures is demonstrated in which the optical cavity is formed unintentionally by parallel cracks in the epilayer. The observed laser modes are broad and shift to shorter wavelengths with increasing pump intensity. An analysis is presented revealing that mode shifting resulting from carrier-induced refractive index changes restricts the observation of laser modes to short optical cavities. GaN-AlGaN Bragg reflectors are investigated through reflectivity modeling and characterization. A transfer-matrix model is developed with an empirical relation for the refractive indices and predictions of the model are compared with data in the literature. Experimental results are then presented and compared with the predictions of the model. The design and characterization of GaN-AlGaN vertical cavity surface emitting lasers is studied. Luminescence spectra are presented from two devices which demonstrate sharp, highly-polarized, regularly spaced modes for pump intensities above a threshold. The spacing of the laser modes is shown to match the mode spacing predicted by the reflectivity model.

This dissertation presents a theoretical study of a variety of charged polymer systems: Critical conditions are determined for adsorption of a charged polymer chain in an electrolyte solution by a curved, charged surface; the scaling behavior and density profiles of a polyelectrolyte brush is examined; the stretch-colapse transition of a charged, grafted polymer layers in a poor solvent is analyzed; and, the sequence dependence of heteropolymer configurations is calculated. These problems are studied theoretically using standard techniques of statistical mechanics.

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A study of InGaAs channel heterostructure field effect transistors (HFETs) on GaAs substrates was undertaken utilizing the low pressure organometallic chemical vapor phase epitaxial (OMVPE) growth technique. Excellent quality HFET material properties were obtained for a split level donor structure, in which the Schottky gate was placed on an undoped AlGaAs layer grown on top of the doped AlGaAs donor layer. A one micron gate length fabrication process was developed to examine the device properties of these materials. A very strong correlation between material characterization results and device performance was observed in all cases. After demonstrating the consistency of the growth and fabrication processes using an $\rm In\sb{0.15}Ga\sb{0.85}As$ channel as a baseline, improvements to the device were undertaken. A delta doping technique was successfully developed and optimized using SIMS and Hall measurements to study the diffusion of the dopant spike. The sheet charge density and device transconductance increased for delta doped material. Increasing the channel indium content reduced the 2DEG mobility, but the expected improvements in transconductance and RF performance were observed. Critical layer thickness (CLT) issues were examined using $\rm In\sb{0.33}Ga\sb{0.67}As$ channel HFETs. Lightly dislocated material still exhibited superior device performance. An asymmetry in dislocation formation was observed, with dislocations forming preferentially in the (011) direction. Devices with a 50 A well width displayed a sharp drop in current in the (0-11) direction. The transconductance and RF properties were not as strongly affected. As the CLT was further exceeded the dislocation network became more symmetric and dense and device performance was severely degraded. A linear channel indium grading methodology was developed to delay the onset of misfit dislocations. Grading from 25-33% produced device properties commensurate with the ungraded 33% indium channel structure, without the asymmetry effects due to dislocation formation. Efforts at developing lattice constant engineered substrates were undertaken. Linear grading to 53% indium at a low growth temperature of 575$\sp\circ$C reduced the amount of three dimensional growth compared to other techniques.

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An apparatus to experimentally verify the accuracy of filmwise condensation heat transfer theories for small diameter horizontal tubes was developed. The apparatus was designed so that the condensation pattern was visible at all locations, all of the parameters necessary to calculate the average heat transfer coefficient were accurately determinable, and the removal of noncondensible gases was effected before the start of the condensation process. A method was devised to insulate the test specimen and cooling water from the rest of the apparatus to obtain an accurate measure of the heat transfer rate. The surface temperature of the tubes was directly measured by copper-nickel thermocouples which were electroplated onto the tube. Initial testing of the apparatus was performed using steam as the condensing vapor. Difficulty in obtaining and maintaining filmwise condensation was caused by small amounts of impurities which were introduced during the assembly of the apparatus. The surface thermocoup]es produced output of approximately l mv per 75 F and were responsive to transient temperature fluctuations. / Master of Science

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Thèse numérisée par la Direction des bibliothèques de l’Université de Montréal / While content selection has been intensively explored in the sentence extraction approach to automatic swnmarization, there is generally little work on the other process of content condensation. To understand this process of condensation, we propose a partial typology based on whether a linguistic unit is replaced, deleted, compressed into fewer essential units, or combined with another unit. Four important categories of condensation processes: generalization, deletion, compression, and aggregation, including their inverse processes, e.g. insertion, and expansion, which were occasionally observed, are proposed. To guide the usage of the same tenu for similar operations, we borrow definitions from linguistics. The type and function of the linguistic units involved are also discussed. We carried out an empirical analysis of 57 author-written abstracts of on-line journal articles in entomology, tracing each abstract sentence back to the plausible source sentences in the corresponding full text. Unlike other studies which focus on the resultant abstract, our study focuses on the processes leading to the production of abstract sentences from corresponding full-text sentences. We do not, however, propose an algorithm for abstracting, or account for all the conditions under which individual condensation operations may apply. While a range of substitutes were used in abstracting, about half of the stems of lexical units in our abstracts share the same stem as their source words, or are their derived forms. Only a small proportion of substitutes were synonyms, and the rest were (quasi-)synonyms, or imprecise equivalents. Authors tend to use less technical forms in abstracts possibly in anticipation of non-specialist abstract readers. Numerical expressions are rendered less precise although no less accurate: absolute numbers and decimals are rounded off, and percentages replaced by ratios or fractions. These observations are consistent with the "new" context of an abstract where only the gist of a document s content need be re-conveyed. Among the linguistic units commonly deleted are metadiscourse phrases, and segments of text (e.g. parenthetical texts, and apposed texts), which provide details and precision in the full text, but are out of place in an abstract. Redundancies inserted for various reasons, or units deemed to be implicit to the comprehension of targeted readers are also often removed. While deletion is an important sub-process of condensation, we observed some instances of adding experimental and other details to compact more information into abstract. The expansion or "unpacking" of compact linguistic units was also observed. The secondary role of inverse processes observed calls for a review of the meaning of condensation from "not giving as much detail or using fewer words" to include the adding of information in order to make a unit of text informatively compact. Among the linguistic units compressed are verbal complexes containing a support verb, or a catenative. Like semantically empty support verbs (e.g. X caused decreases in Y = X reduced Y), some catenatives too may be deleted without significant changes in meaning to the verbal complex (e.g. X was allowed to hatch E-e X hatched). Redundancy in meaning between an adjective and a noun in a noun phrase, e.g. functional role, may be removed, and the phrase compressed to just the stem of the adjective, i.e. function. While not frequently occurring in the corpus studied, the compression of such units may be described by rules, and hence, might be operationalized for automatic abstracting. Aggregation, the combining of units of text within or between sentences, is an important sub-process of condensation. Two-thirds of sentences in abstracts studied were written using multiple sentences, and more sentences were combined without than with the use of an explicit sign, such as a connective, a colon or a semi-colon. If research in summarization is to progress beyond sentence selection, then we must work towards: (a) a clear distinction between operations that are condensation processes, and those that are not; (b) bringing operationally similar processes together under the same designation, and (c) a greater understanding of sub-processes constitutiiig condensation. To this end, our provisional typology for condensation, the range of type of linguistic units involved and their functions sets the first step to advance research into content condensation. We have only just begun to identify the condensation sub-processes in operation during abstracting. The factors that are critical on the interplay of these processes still need to be investigated.

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