Computer simulations invoking the Metropolis Algorithm and Monte Carlo techniques were constructed to model the behavior ofvarious systems of magnetic semiconductors. Among the systems modeled were bulk type II FCC antiferromagnets and simple cubic thin films and superlattices. Studies of bulk antiferromagnets revealed dependence of Neel Temperature on nearest neighbor interactions as well as next nearest neighbor contributions in contradiction to mean field theory. Thin film studies revealed the effects of film thickness on critical temperature as well as differing magnetization curves for different portions of the fIlm. Superlattice simulations showed agreement with experimental data regarding anisotropy studies and also investigated the correlation beween interlayer and intralayer ordering in a superlattice. These simulations predict that interlayer coupling begins either at a temperature equal to that of the onset of intralayer ordering or at some higher temperature. / Graduation date: 2004

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaf 137). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaf 137). Also available on the Internet.

All-optical time-resolved measurement techniques provide a powerful tool for investigating critical parameters that determine the performance of infrared photodetector and emitter semiconductor materials. Narrow-bandgap InAs/GaSb type-II superlattices (T2SLs) have shown great promise as a next generation source of these materials, due to superior intrinsic properties and versatility. Unfortunately, InAs/GaSb T2SLs are plagued by parasitic Shockley-Read-Hall recombination centers that shorten the carrier lifetime and limit device performance. Ultrafast pump-probe techniques and time-resolved differential transmission measurements are used here to demonstrate that Ga-free InAs/InAsSb T2SLs and InAsSb alloys do not have this same limitation and thus have significantly longer carrier lifetimes. Measurements at 77 K provided minority carrier lifetimes of 9 μs and 3 μs for an unintentionally doped mid-wave infrared (MWIR) InAs/InAsSb T2SL and InAsSb alloy, respectively; a two order of magnitude increase compared to the 90 ns minority carrier lifetime measured in a comparable MWIR InAs/GaSb T2SL. Through temperature-dependent lifetime measurements, the various carrier recombination processes are differentiated and the dominant mechanisms identified for each material. These results demonstrate that these Ga-free materials are viable options over InAs/GaSb T2SLs for potentially improved infrared photodetectors. In addition to carrier lifetimes, the drift and diffusion of excited charge carriers through the superlattice growth layers (i.e. vertical transport) directly affects the performance of photodetectors and emitters. Unfortunately, there is a lack of information pertaining to vertical transport, primarily due to difficulties in making measurements on thin growth layers and the need for non-standard measurement techniques. However, all-optical ultrafast techniques are successfully used here to directly measure vertical diffusion in MWIR InAs/GaSb T2SLs. By optically generating excess carriers near one end of a MWIR T2SL and measuring the transit time to a thin, 2 lower-bandgap superlattice placed at the other end, the time-of-flight of vertically diffusing carriers is determined. Through investigation of both unintentionally doped and p-type superlattices at 77 K, the vertical hole and electron diffusion coefficients are determined to be 0.04±0.03 cm2/s and 4.7±0.5 cm2/s, corresponding to vertical mobilities of 6±5 cm2/Vs and 700±80 cm2/Vs, respectively. These measurements are, to my knowledge, the first direct measurements of vertical transport properties in narrow-bandgap superlattices. Lastly, the widely tunable two-color ultrafast laser system used in this research allowed for the investigation of nonlinear optical properties in narrow-bandgap semiconductors. Time-resolved measurements taken at 77 K of the nondegenerate two-photon absorption spectrum of bulk n-type GaSb have provided new information about the nonresonant change in absorption and two-photon absorption coefficients in this material. Furthermore, as the nondegenerate spectrum was measured over a wide range of optical frequencies, a Kramers-Kronig transformation allowed the dispersion of the nondegenerate nonlinear refractive index to be calculated.

The properties of sputter-deposited multilayered superconductors have been studied, including Mo/Ta metallic superlattices and thin films of YBa₂Cu₃O₇₋ₓ. The former have been prepared with the same integer number of atomic planes of Mo and Ta modulating the layered composition. In contrast to behavior observed in other metal-metal superlattices, Mo/Ta exhibits long range structural coherence and metallic resistivity behavior over the entire range of wavelengths down to the monolayer limit. The structural properties of these superlattices are used to explain an anomalous decrease in the c₄₄ elastic stiffness constant previously observed in Mo/Ta for 20 Å ≤ Λ ≤ 50 Å. Superconductivity measurements indicate "universal" T(c) versus ρ behavior in Mo/Ta, and tunneling results show that these superlattices are weakly-coupled BCS superconductors. The second part of this dissertation examines the properties of superconducting thin films of YBa₂Cu₃O₇₋ₓ prepared by dc triode sputtering from metallic targets of Y and Ba₂Cu₃. Post-depression annealing in O₂ is necessary to form the superconducting oxide. Various substrates were used, including sapphire and MgO, both with and without buffer layers of Ag, and SrTiO₃. The buffer layers are used to decrease the interaction of the substrate with the film. The best results occur with films deposited on MgO with a Ag buffer layer, exhibiting T(c) onsets as high as 90 K and zero resistance by 60 K. I find that the crystalline orientation of films deposited on (100) SrTiO₃ are influenced by the substrate, and re-annealing a sample can sometimes improve its superconducting properties. Overall, reproducibility is the biggest problem with this technique, as Ba metal is highly reactive with the environment.

No description available.

by Tsoi Hing Lun. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 98-102). / Abstracts in English and Chinese. / Acknowledgement --- p.i / 摘要 --- p.ii / Abstract --- p.iii / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- General review --- p.1 / Chapter 1.2 --- Our work --- p.4 / Chapter Chapter 2 --- Experimental setup and procedures / Chapter 2.1 --- Sample preparation --- p.6 / Chapter 2.1.1 --- ZnSe heterostructure --- p.6 / Chapter 2.1.2 --- Quantum dot --- p.7 / Chapter 2.2 --- Common aspects of Raman scattering --- p.8 / Chapter 2.3 --- General studies --- p.11 / Chapter 2.3.1 --- Excitation power density dependent studies --- p.11 / Chapter 2.3.2 --- Angular dependent studies --- p.12 / Chapter 2.3.3 --- Excitation energy dependent studies --- p.14 / Chapter 2.4 --- Peripheral measurements --- p.16 / Chapter 2.4.1 --- Spectral calibration --- p.16 / Chapter 2.4.2 --- Alignment and system stability checks --- p.18 / Chapter 2.5 --- The method of measuring the Raman scattering efficiency --- p.19 / Chapter Chapter 3 --- Band Bending at the interface of ZnSe/GaAs / Chapter 3.1 --- Results and discussions --- p.27 / Chapter Chapter 4 --- Characteristics of QD phonons / Chapter 4.1 --- Angular dependent studies --- p.38 / Chapter 4.2 --- Excitation energy dependent studies --- p.60 / Chapter 4.3 --- The nature of the QD phonons --- p.64 / Chapter 4.4 --- The measurement of the Raman scattering efficiency of QDs --- p.67 / Conclusions --- p.75 / Future work --- p.73 / Appendix1 --- p.79 / Appendix2 --- p.82 / Appendix3 --- p.86 / Appendix4 --- p.88 / Appendix5 --- p.95 / References --- p.93

Silicon quantum dots (QDs) in SiO2 superlattices were fabricated by alternate deposition of silicon oxide (SiO2) and silicon-rich oxide (SRO), i.e. SiOx (x<2), and followed by high temperature annealing. A deposited SRO film is thermodynamically unstable below 1173oC and phase separation and diffusion of Si atoms in the amorphous SiO2 matrix creates nano-scaled Si quantum dots. The quantum-confined energy gap was measured by static photoluminescence (PL) using an Argon ion laser operating at 514.5 nm. The measured energy band gaps of crystalline Si QDs in SiO2 matrix at room temperature (300 K) show that the emission energies from 1.32 eV to 1.65 eV originating Si dot sizes from 6.0 nm to 3.4 nm, respectively. There is a strong blue-shift of the PL energy peak position with decreasing the quantum dot size and this shows the evidence of quantum confinement of our fabricated Si QDs in SiO2 matrix. The PL results indicate that the fabricated Si QDs in SiO2 matrix could be suitable for the device application such as top cell material for all-silicon tandem solar cells. Silicon QD superlattices in nitride matrix were fabricated by alternate deposition of silicon nitride (Si3N4) and silicon-rich nitride (SRN) by PECVD or co-sputtering of Si and Si3N4 targets. High temperature furnace annealing under a nitrogen atmosphere was required to form nano-scaled silicon quantum dots in the nitride matrix. The band gap of silicon QD superlattice in nitride matrix (3.6- 7.0 nm sized dots) is observed in the energy range of 1.35- 1.98 eV. It is about 0.3- 0.4 eV blue-shifted from the band gap of the same sized quantum dots in silicon oxide. It is believed that the increased band gap is caused by a silicon nitride passivation effect. Silicon-rich carbide (SRC, i.e. Si1-xCx) thin films with varying atomic ratio of the Si to C were fabricated by using magnetron co-sputtering from a combined Si and C or SiC targets. Off-stoichiometric Si1-xCx is of interest as a precursor to realize Si QDs in SiC matrix, because it is thermodynamically metastable when the composition fraction is in the range 0 < x < 0.5. Si nanocrystals are therefore able to precipitate during a post-annealing process. SiC quantum dot superlattices in SiC matrix were fabricated by alternate deposition of thin layers of carbon-rich silicon carbide (CRC) and SRC using a layer by layer deposition technique. CRC layers were deposited by reactive co-sputtering of Si and SiC targets with CH4. The PL energy band gap (2.0 eV at 620 nm) from 5.0 nm SRC layers could be from the nanocrystalline ??-SiC with Si-O bonds and the PL energy band gap (1.86 eV at 665 nm) from 6.0 nm SRC layers could be from the nanocrystalline ??-SiC with amorphous SiC clusters, respectively. The dielectric material for an all-silicon tandem cell is preferably silicon oxide, silicon nitride or silicon carbide. It is found that for carrier mobility, dot spacing for a given Bloch mobility is in the order: SiC > Si3N4 > SiO2. By ab-initio simulation and PL results, the band gap for a given dot size is in the order: SiC > Si3N4 > SiO2. However, the PL intensity for a given dot size is in the order: SiC < Si3N4 < SiO2.

Neutron diffraction experiments on europium telluride/lead telluride super-lattice systems reveal pronounced correlations between the antiferromagnetic EuTe layers. The results from an assortment of varying experimental conditions (applied magnetic field, temperature, etc.) and their relevance to the physical mechanism underlying this striking phenomena are discussed. Stochastic computer simulations are compared to this real data to examine the effects of varying the neutron coherence length and the amount of correlations in the model sample. There are rising theoretical efforts to determine the physical mechanism causing the observed correlations and a laconic overview of current theories in interlayer coupling is given herein. The investigated structures do not exhibit perfect correlations between the magnetic layers but most samples clearly show correlation effects. The description of these partial correlations, characterized by an order parameter p, is examined and a straightforward method for determining p is derived. / Graduation date: 1998

We present full-scale relativistic pseudopotential calculations of the first-order susceptibility in p-type SiGe/Si multiple quantum well structures with a view to exploring the suitability of such systems for infrared applications in the 3-5yrn and 8-15itm ranges. A derivation of an expression for the linear susceptibility, or absorption, is given and the frequency dependence of the linear response due to transitions between the valence minibands is determined. The microscopic origin of the absorption is demonstrated for both parallel and normal incident light. Comparisons between calculated and experimental results are presented and shown to be in good agreement. The effects of changing well width, temperature, doping concentration and germanium concentration in the well are considered. We also consider Auger recombination and discuss the possibility of engineering the miniband structure in order to prevent certain Auger processes occuring, Preliminary results from full scale Auger calculations are also presented.