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Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 199-210). Available also in a digital version from Dissertation Abstracts.

Th. doct.-ing.--Besançon, 1981. N°: 120.

Th.--Sci.--Nancy--I.N.P.L., 1979.

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This thesis is concerned with a quantitative study of intermixing in GaAs/AlGaAs and ZnSe/ZnCdSe single quantum well semiconductor structures. In this study, a method of iterative isothermal anneals and photoluminescence used to characterize this phenomenon has enabled the evolution of the diffusion coefficients for the interdiffusion process with anneal time to be followed. The blue-shift emissions arising from this method are predicted by a model based on Fick's law of diffusion. This model is developed in an attempt to relate the energy shift that is observed experimentally to the diffusion length. The mixing is modelled using an error function expression to solve the diffusion equation so as to describe the variation in well shape which is attributed to compositional disordering induced during thermal processing. Using this approach, where evidence of intermixing was monitored, the emission would be expected to shift measurably. Data has been taken to cover a wide temperature range to establish values for the activation energy EA. From this data, it has been found that the diffusion coefficients at various temperatures are thermally activated with an energy of 3.6 +/- 0.2 eV in GaAs/AlGaAs. The data is compared with the available literature data taken under a wide range of experimental conditions. We show that despite the range of activation energies quoted in the literature all the data appears to be consistent with a single activation energy. Departures from the 'mean' value are ascribed to experimental uncertainties in determining the diffusion coefficients for example, to fluctuations in the composition of the material, to techniques used, or to a wide range of perturbations. Photoluminescence observations on ZnSe/ZnCdSe show that an improvement in the optical quality of these quantum well structures was found for anneals at temperatures (~500°C). A value of EA = 2.9 +/- 0.3 eV was derived from the experiments for the interdiffusion process over a 250 K temperature range and four decades of interdiffusion coefficient. The interdiffusion process of both these systems was inferred to be Fickian with no dependence on alloy composition or strain.

The purpose of this project has been twofold: firstly, to develop further FTIR reflectance spectroscopy as a technique for the quantitative characterisation of SOI multilayer structures and secondly, to use it in combination with other techniques, such as RBS/Channelling, XTEM, SIMS, AES and SRP, to investigate the evolution and structure of novel SOI materials. In order to evaluate the FTIR results, many SIMOX samples were fabricated with a wide range of Si and buried SiO2 layer thicknesses with one or two buried oxide layers. The FTIR results are compared to those obtained with the other experimental techniques and with theoretical calculations. We show that, given an "a priori" knowledge of the structure which is used to define the initial structural model, FTIR can be used as a non-destructive, fast, and inexpensive control method for characterising SOI structures. FTIR offers +/-2 nm accuracy in layer thicknesses and +/-5 nm in interfacial region thicknesses, over a wide (0.5 to >10mum) depth range, while for shallower depths an error of 15% on average is likely. FTIR gives Si thickness values which are within 5%, and buried layer or interfacial region thickness values which are within 10% to 20%, from the values obtained by other techniques. The sensitivity to oxygen content variations is down to 5 x 1020 O cm-3 for the top of a Gaussian profile, and 1 X 1022 O cm-3 over a 20 nm thick layer. FTIR gives a value for the retained dose within 5% of the nominal dose for both unannealed and annealed samples. Novel SOI materials, such as deep buried oxide layers and double SIMOX structures were fabricated and characterised, and the processing parameters optimised. We show that, for 2 MeV oxygen implantation into Si at 700°C to a dose of 2 x 1018 O+ cm-2 and after annealing at 1300°C for 6 hours, a buried layer is formed which is continuous but contains Si islands. It is found that by increasing the annealing time to 12 hours, the homogeneity of the buried layer is improved. We propose that an increase of both the oxygen dose and the annealing temperature would result in a higher quality buried oxide layer. The adverse effect of high beam current densities to the sample structure after annealing is demonstrated for the high energy SIMOX samples. We show that double SIMOX structures can be fabricated by a three step process, with one only annealing step at the end. Such structures can be used for Si waveguiding applications and we present preliminary results of waveguiding loss measurements, where a value of 18 dB/cm was obtained.

Recycled concrete aggregates (RCA) are an eco-sustainable alternative to traditional aggregates. The equivalent mortar volume method has been developed to design RCA concrete with comparable mechanical and strength properties to natural aggregate concrete by accounting for the adhered mortar that distinguishes RCA from natural aggregates. This method also presents an opportunity to reduce the cement content of RCA concrete, increasing its cement efficiency. Modifications to mix design procedures were introduced to improve the fresh state properties of RCA concrete with a 19% cement reduction compared to the control mix. Beams cast with the RCA concrete had similar flexural capacity to corresponding control beams, however the shear strength varied as a function of RCA content in the concrete. Polished core samples taken from RCA members showed that crack development and aggregate interlock in RCA members is influenced by the aggregate properties and the spacing of coarse aggregates, indicating that knowing the compressive strength of RCA concrete is not the only parameter required for member design.

Submitted by Marcele Costal de Castro (costalcastro@gmail.com) on 2017-09-06T13:30:34Z No. of bitstreams: 0 / Made available in DSpace on 2017-09-06T13:30:34Z (GMT). No. of bitstreams: 0 Previous issue date: 1984-05 / This work is concerned with the application of the finite element method to the study of creep brittle rupture of structural components. In the formulation material behavior is described by an elastocreep model in which the total strain rates are assumed to be the sum of elastic and creep components. The elastic strain rates are given by Hooke’s law while the creep strain rates and the damage rates are espressed by the multiaxial form of the Kachanov-Rabotnov equations proposed by Leckie and Hayhurst. The incremental equations of motion are derived from the principle of virtual work using an updated Lagrangian formulation which accounts for geometric effects due to large displacements, large rotations and deformation dependent loadings. The finite element incremental equations are developed according to a displacement-based formulation. Isoparametric elements with quadratic shape functions are employed for the domain discretization and simple numerical procedures are developed to deal with the presence of partially and/or fully ruptured elements in the mesh. For integration of the creep strain rate equations a family of implicit time marching schemes is developed which can be regarded as Runge-Kutta methods of second order. The integration of the coupled damage rate equations is performed using a first order predictor-corrector scheme with automatic time step length control. For material nonlinear problems only, a substructuring technique is employed in conjunction with the time integration algorithms. Selected numerical applications are presented and discussed in detail. Comparison with alternative numerical, analytical and/or experimental results is made whenever possible.