GaAs_1-xBi_x is an exciting new semiconductor material, which has been proposed as a new material for infrared light emitting devices. Recent advancements in the growth of GaAs_1-xBi_x films have made it possible to produce GaAs_1-xBi_x light emitting diodes for the first time. Throughout this research we have grown, fabricated and characterized GaAs_1-xBi_x light emitting diodes. Similarly structured In_xGa_1-xAs light emitting diodes were also produced and characterized for comparison to the GaAs_1-xBi_x devices. Strong electroluminescence was obtained from GaAs_1-xBi_x devices, showing two emission peaks, one corresponding to the GaAs_1-xBi_x layer and the other to the GaAs cladding. Emission from In_xGa_1-xAs devices was about 100 times brighter than from GaAs_1-xBi_x devices. Temperature dependent electroluminescence and photoluminescence measurements of a GaAa_1-xBi_x light emitting diode were made and showed some unusual results. The wavelength of the peak in the electroluminescence from the GaAs_1-xBi_x was independent of temperature in the range 100 K to 300 K while the GaAs peak shifted with temperature as expected. Photoluminescence measurements on the same structure show temperature dependence of the peak wavelength similar to the temperature dependence of GaAs.

I am reporting on lensless imaging of human red blood cell using coherent x-ray scattering (CXS)technique. The successful microfabrication of a sample-mask structure using focused ion beam (FIB) milling was the key element in this imaging technique. The sample-mask structure is 600 - 800 nm gold films deposited using sputtering or electron beam evaporation on Si₃N₄ membrane windows. We used commercially available 100 nm thick Si₃N₄ membranes held by 3 mm diameter silicon frames that are designed for use in transmission electron microscopy. The red blood cell (RBC) sample was mounted in front of a 3 μm hole milled through both gold and Si₃N₄ layers on the opposite side. Three smaller reference apertures with diameters 300, 250 and 200 nm on the gold side were milled all the way through both layers at a distance of 9 μm center-to center from the sample aperture. These holes are used for holographic lensless x-ray imaging. It was found that a gold surface roughens during ion milling due to a sputter instability and which produces cup-like features with a characteristic length up to few hundred nm. We found apertures milled through gold films deposited by sputtering show good circularity and sidewall roughness of 20 nm. We present result on CXS measurements in transmission geometry near Fe L₃- and C K-absorption edges on a single RBC. We captured high resolution images of the sample by simple Fourier inversion of the recorded far-field scattered intensity. We found 8.5 % reduction in the transmission intensity near Fe L₃-edge due to presence of Fe in the form of hemoglobin molecules inside RBC. This absorption agrees with estimated agrees well with the estimated value of 9 % within experimental uncertainty. From limited data measured below C K-edge we measured a presence of at least 300 nm thick carbon inside RBC which lies in the range of the estimated value of 1.8 μm. The resolution of our lensless imaging technique is about 55 nm near Fe L₃-edge and 78 nm near C K-edge.

Semiconductor alloys that are lattice matched to GaAs but have a smaller energy band gap are of interest for numerous applications, including infrared lasers for telecommunications, high efficiency solar cells, and high electron mobility transistors. For high optoelectronic efficiency, these materials must be highly perfect single crystals with low defect densities. In this thesis, two substitutional GaAs-based alloy families, nitrides and bismides, are investigated experimentally. In the first alloy, GaNAs, the addition of N results in a large band gap reduction, though the small size of the N relative to As introduces tensile strain into the lattice, and the high electronegativity of N attracts electrons. The second alloy, GaAsBi, also has a smaller band gap and is formed by the addition of the very large Bi atom to GaAs, which introduces compressive strain and tends to attract holes. The experimental investigations of these alloys focused on elucidating the relationships between the growth process, atomic structure, and electronic properties. Films were grown by molecular beam epitaxy (MBE) with in-situ process monitoring and subject to post-growth structural and electronic characterization. For GaNAs and a related alloy, InGaNAs, degradation in luminescence efficiency, mobility and structural integrity were observed as the nitrogen content of the alloy was increased. A comprehensive study of strain relaxation in compressively strained InGaNAs and InGaAs quantum wells revealed that the nitrogen alloying did not have an effect on the critical thickness for dislocation formation, or the dislocation density in relaxed films. At large lattice mismatch, InGaNAs quantum wells were observed to relax by means of unusually oriented pure edge-type misfit dislocations aligned with h100i directions, likely due to the high stress associated with the large misfit. Use of bismuth as a non-incorporating surfactant during growth was successfully applied to improve the material quality of the nitrides. The Bi surface layer during growth was investigated using in-situ electron diffraction intensity measurements, and was found to improve both the smoothness of nitride films, by promoting a layer-bylayer growth mode, as well as increasing the photoluminescence (PL) intensity by a factor of 2.4. The improvement in PL is attributed to a reduction in nitrogen Abstract iii clusters with the surfactant. In addition, an increase in nitrogen content of up to 50% was observed in films grown with Bi over films grown without the surfactant. The increase in the nitrogen incorporation scales with the Bi flux, and saturates at full Bi coverage. A modified Langmuir model was applied to describe the behaviour of Bi on the surface, as well as the increase in nitrogen incorporation. The bismide alloy family requires atypical MBE growth conditions such as low substrate temperature and low As overpressure in order to achieve Bi incorporation. The conditions are close to those where Ga and Bi droplets form. However, in- situ light scattering was used to identify and avoid growth with droplets, resulting in films with high structural quality as determined by x-ray diffraction, and strong photoluminescence. 1% Bi alloying in GaAs was also found to result in a minimal 15% decrease in electron mobility, as compared with a 94% reduction for 1% N alloying. Finally, a preliminary investigation was made into the concept of co-doping GaAs with both N and Bi. GaNAsBi films showed room temperature PL at long wavelengths commensurate with contributions to band gap reduction from both N and Bi. Since lattice matching to GaAs can be achieved with a Bi/N ratio of 1.7, GaNAsBi and GaAsBi are promising new alloys for the applications described.

The problem of a complete theory describing the far-from-equilibrium statistical mechanics of epitaxial crystal growth remains unsolved. Besides its academic importance, this problem is also interesting from the point of view of device manufacturing. In order to improve on the quality and performance of lateral nanostructures at the lengthscales required by today's technology, a better understanding of the physical mechanisms at play during epitaxial growth and their influence on the evolution of the large-scale morphology is required. In this thesis, we present a study of the morphological evolution of GaAs (001) during molecular beam epitaxy by experimental investigation, theoretical considerations and computational modeling. Experimental observations show that initially rough substrates smooth during growth and annealing towards a steady-state interface roughness, as dictated by kinetic roughening theory. This smoothing indicates that there is no need for a destabilizing step-edge barrier in this material system. In fact, generic surface growth models display a much better agreement with experiments when a weak, negative barrier is used. We also observe that surface features grow laterally, as well as vertically during epitaxy. A growth equation that models smoothing combined with lateral growth is the nonlinear, stochastic Kardar-Parisi-Zhang (KPZ) equation. Simulation fits match the experimentally observed surface morphologies quite well, but we argue that this agreement is coincidental and possibly a result of limited dynamic range in our experimental measurements. In light of these findings, we proceed by developing a coupled growth equations (CGE) model that describes the full morphological evolution of both flat and patterned starting surfaces. The resulting fundamental model consists of two coupled, spatially dependent rate equations that describe the interaction between diffusing adatoms and the surface through physical processes such as adatom diffusion, deposition, and incorporation and detachment at step edges. In the low slope, small amplitude limit, the CGE model reduces to a nonlinear growth equation similar to the KPZ equation. From this, the apparent applicability of the KPZ equation to surface shape evolution is explained. The CGE model is based on fundamental physical processes, and can therefore explain the underlying physics, as well as describe macroscopic pattern evolution during growth.

Optical coherence tomography (OCT) is an emerging medical imaging technology based on the coherent interference of light. Current use of OCT in clinical settings is limited by the lack of a suitable light source. This thesis describes the design of a new type of source for OCT, based on the GaInNAs semiconductor materials system.

A semiconductor heterostructure consisting of several different quantum wells is discussed as a device for generating broadband (>100 nm) light in the near infrared (900-1500 nm). The use of temperature to control spectral shape and intensity is examined. Other aspects of device design are investigated, including models for quantum well emission and for the band gaps of dilute nitride semiconductors.

Photoluminescence measurements are presented, providing a proof of principle demonstration of the source design. Emission centred at 1225 nm with a 195 nm bandwidth is achieved. The use of temperature to control inter-well carrier transfer is demonstrated and successfully modelled. Localization effects of nitrogen cluster states are shown to be greatly reduced in quantum well structures, as compared to bulk samples.

Many fabrication processes for semiconductor nanostructures rely on the understanding of surface pattern evolution during crystal growth and etching. In this thesis, the morphological evolution of GaAs surfaces during thermal chlorine etching and molecular beam epitaxial growth is investigated by atomic force microscopy and light scattering. The experimental results are compared to numerical simulations based on continuum models. For both etching and growth, the evolution of flat surfaces and small amplitude (<30 nm) random surface patterns can be modeled with excellent accuracy with stochastic differential equations for the surface height as predicted by kinetic roughening theory. For MBE growth this equation is the Kardar-Parisi-Zhang (KPZ) equation while etching requires the extension of the KPZ model with a fourth-order linear term. Anisotropic etch rates with respect to crystal orientation are found to be a major consideration for surface pattern transfer by thermal chlorine etching. It is shown how pattern transfer of one- and two-dimensional gratings can be predicted and optimized by varying the orientation of the pattern and by the use of a directional molecular beam to supply the chlorine. To describe the complex shapes evolving from etching and growth on microfabricated gratings, models based on two coupled differential equations for the surface concentration of etchant or adatoms and the surface height are developed. Excellent fits to the experimental shapes observed over a wide range of etching and growth conditions can be obtained with these models and they emerge as a powerful tool to understand the pattern evolution in terms of the underlying microscopic physics such as surface diffusion, spatial flux inhomogeneity, sticking coefficients, step edge incorporation and diffusion bias.

The morphology of GaAs (001) single crystals during thermal chlorine etching was studied in this work. Models that can predict the evolution of 3 μm pitch, 100 nm amplitude wet etched gratings were developed. A model was developed in which the etch rate and the angle of the exposed crystal plane with respect to t he (001) plane were associated. Additional effects such as the formation of chlor ine adatoms and surface diffusion were included to provide better agreement with the experimental data. The possibility of chlorine sliding along the surface d ue to the initial momentum from the incident beam was also considered. Experime ntal data measured using atomic force microscopy compared with the simulated dat a found that the average diffusion length was determined to be approximately 50  nm. By including the sliding effect, better agreement with the experimental dat a was observed when using an average sliding distance of approximately 11 nm.

The design of a closed cycle optical cryostat for semiconductor crystal characterization is discussed. The system designed and developed is capable of performing photoluminescence, resistivity, and Hall measurements as a function of temperature from 10 K to higher than 300 K. Preliminary photoluminescence experiments are carried out as a test of the system. Results from the photoluminescence measurements show evidence for the existence of nitrogen clusters in GaN_xAs_1-x. The clusters are shown to produce states with energies inside the band gap. It has also been found that the introduction of bismuth, as a surfactant, during the growth process tends to reduce the density of the nitrogen clusters in the material.

The photochemical reaction of Cl₂ gas with (100) GaAs was studied in this work. Thermal effects due to light induced heating of the GaAs substrate were isolated from photochemical effects. Light induced heating was calculated and the corresponding photothermal Cl₂ etching of GaAs was accounted for. The temperature dependence of the photochemical etch rate was examined. The light-induced etch rate potentially follows an Arrhenius temperature dependence with an activation energy of 0.161 eV. Near 200^oC the light-induced etch rate does not increase with increasing temperature. The photochemical etch rate was found to depend linearly on intensity. A possible explanation for the temperature dependence of the photochemical etch rates is presented. It is proposed that the desorption of GaCl₃ is the important mechanism in the photochemical Cl₂-GaAs etch. GaCl₃ is photodesorbed with illumination, which increases the rate of etching.

In_yGa_1-yAs_1-xN_x containing a small amount of nitrogen (x<0.05) is a new narrow bandgap semiconductor alloy that has advantageous properties for the fabrication of optoelectronic devices. In this thesis, we seek to improve the material quality of InGaAsN and GaAsN by studying how the epitaxial growth conditions affect both the structural and elec tronic properties of the alloy. We describe a novel RF plasma source based on a helical resonator design that was developed for the incorporation of nitrogen into GaAsN and InGaA sN thin films grown by molecular beam epitaxy. The plasma source is equipped with a baffle apparatus that decreases the ion content of the flux. We show how the structural and electronic properties of InGaAsN epilayers depend on the growth conditions. In sit u light scattering measurements and atomic force microscopy show that a faceted surface morphology occurs when growth conditions increase adatom surface diffusion: slow growth rate, high substrate temperature and high V/III ratio. Large nitrogen concentrations also favour the faceted growth mode. The residual strain in relaxed InGaAsN films is found to be higher than in InGaA s epilayers having the same lattice mismatch. In situ substrate curvature measurements were used to monitor the strain state of the sample in real time during the growth. Ex situ transmission electron microscopy and x-ray d iffraction measurements agree with the residual strain determined with the in situ monitor. These characterization results also indicate that threading dislocation glide is slower in InGaAsN than in InGaAs. Finally, we find that the electronic properties of InGaAsN are generally degrade d with increasing nitrogen content. However, by choosing appropriate growth conditions, we demonstrate InGaAsN quantum wells with room temperature photoluminescence efficiencies that are comparable to InGaAs structures. These photoluminescence r esults may be related to the faceting transition that was observed during GaAsN growth. In contrast with the findings of other groups, rapid thermal anneals only moderately improve the photoluminescence intensity and line shape of InGaAsN single quantum wells. We observe peak intensity gains on the order of 2 after one minute anneals at 785^oC. Hall measurements ind icate that the electron mobility of Si-doped GaAsN is inversely proportional to the nitrogen content. We conclude that nitrogen-related neutral impurity scattering is the limiting fa ctor in the electron mobility of GaAsN. The use of Bi as a surfactant during growth is shown to improve the surface morphology of GaAsN epilayers and the photoluminescence properties of InGaAsN single quantum wells. This work provides insight into some of the key issues that must be taken into account in the growth of dilute nitrides.

The band gap and optical absorption edge is measured in semi-insulating and p-type GaNAs as a function of nitrogen content using a photoconductivity technique. The band gap is found to decrease with nitrogen, from 1.42 eV with 0% nitrogen to 1.20 eV with 0.9% nitrogen, and to 1.14 eV with 1.73% nitrogen content. The characteristic energy of the exponentional absorption edge (Urbach parameter) for p-type GaNAs is found to increase with nitrogen, from 6.7 meV with 0% nitrogen to 14 meV with 0.8% nitrogen content.

The mobility and carrier concentration is measured as a function of nitrogen con tent in p-type and n-type GaNAs using Hall measurements. The electron mobility decreases from 3000 cm²/Vs with 0% nitrogen, to 650 cm²/Vs with 0.1% nitrogen, and to 300 cm²/Vs with 1.0% nitrogen content. The hole mobility is relatively una ffected by nitrogen and stays constant around 300 cm²/Vs for up to 1% nitrogen con tent. The carrier concentration in p-type GaNAs is found to decrease for highly doped (2.5x10¹⁶ cm^-3) GaNAs and increase for low doped (4.5x10¹⁴ cm^{- 3}) GaNAs with increasing nitrogen content. The carrier concentration converges to 7x10¹⁵ cm^-3 for both low and high doping at greater than 0.8% nitrogen content, which sugge sts there is a trap that is pinning the Fermi level. This behaviour is modeled using conservation of charge in the band gap, and a trap level at 0.18 eV above the top of the valence band is found to explain the experimental data.

A photoconductivity technique for measuring the band gap and Urbach edge is pres ented. In this method the sample is illuminated with monochromatic light, and the photoconductivity is measured as a function of incident wavelength. Light with energy greater than th e band gap is absorbed by the sample and increases the photoconductivity signal. The absorption coefficient is determined from the photoconductivity signal. The electrical properties, such as the mobility and carrier concetration are obtaine d from Hall measurements.

We have explored the use of coherent resonant x-ray scattering as a powerful technique to study, characterize and reconstruct magnetic domains for antiferromagnetic (AFM) and ferromagnetic (FM) thin films. This method is capable of high-resolution imaging (as it is not limited by optical aberrations), is able to probe buried interfaces and is operational in the presence of other fields. Here we report the first experimental observation of x-ray speckle patterns generated by AFM domains. Resonant x-ray scattering was performed on LaFeO3 thin films possessing two types of domains with their AFM orientations perpendicular to each other. X-ray magnetic linear dichroism (XMLD) at the Fe L3 absorption edge has been exploited in order to give rise to modulations of the scattering amplitudes according to domain distributions, resulting in magnetic speckle.

We also report resonant x-ray scattering in the transmission geometry from FM domains of Co/Pt multilayers. Magnetic x-ray circular dichroism (MXCD) has been utilized with the contrast arising from the dependence of scattering amplitude on magnetization direction of FM domains, which are oriented normal to the surface (i.e. parallel or antiparallel to photon helicity) due to the perpendicular interfacial anisotropy provided by the broken symmetry at the Co-Pt interface. By tuning the energy to the Co L3 edge, magnetic speckle is very clearly demonstrated. We have analytically shown that upon reversal of magnetic contrast (tuning of the scattering energy to either of the two crystal field split peaks of the Fe L3 edge in the first experiment, and changing the photon helicity in the second experiment) changes in speckle patterns will be observed solely arising from the interference between roughness and/or pinhole scattering with magnetic scattering.

We have developed a new reconstruction technique, upon extension of Fourier transform iterative algorithms previously utilized in other reconstruction tasks, capable of reconstructing AFM and FM magnetic structure from resonant x-ray scattering intensity measurements. This technique is shown to be very successful upon application to noisy simulated data. Using this method, experimental speckle data from the FM domains of the Co/Pt multilayer have been inverted resulting in magnetic domains showing a remarkable similarity to the worm-domain structure of the actual domain distribution imaged using magnetic force microscopy (MFM). This, to our knowledge, has been the first reconstruction of magnetic domains from experimental data. Moreover, direct (non-iterative) reconstruction of FM domains has been shown to be possible upon using small pinholes and/or rough samples with roughness scale comparable to the size of domains.

The surface of a film grown epitaxially on a crystalline substrate is generally rough, even if the initial growth surface is smooth on the atomic scale. In the case of strained-layer epitaxy, in which the composition of the film is such that it does not share the same lattice constant as the substrate, the roughness often develops in response to strain-relief processes occurring in the film during growth. In this thesis we show that a careful analysis of the time evolution of the surface morphology during strained-layer growth, can reveal quantitative information about both the strain-relief mechanisms acting within the film and the diffusion processes occurring at the surface.

Two complementary measurement techniques, namely atomic force microscopy (AFM) and elastic light scattering, are used to acquire the surface morphology information necessary for the analysis. In this work we demonstrate quantitative agreement between roughness measurements obtained by both techniques. A major advantage of light scattering over AFM is its suitability to real-time monitoring of the surface during growth.

We consider the growth of In_0.18Ga_0.82As and InAs, on (001)-oriented GaAs substrates, by molecular beam epitaxy (MBE). In both cases the film is compressively strained owing to the 7% lattice mismatch between InAs and GaAs. In the case of In_0.18Ga_0.82As growth, the strain is relieved plastically as the film thickness increases, through the introduction of misfit dislocations at the film/substrate interface. A characteristic crosshatch pattern develops at the surface, consisting of ridges aligned along the <110> crystal directions. We present an analytical model to describe this roughening, in which the ridges arise from surface diffusion in response to the dislocation strain fields. Although it has only three fitting parameters, the model is able to reproduce both the time dependence and the length scale dependence of the surface morphology, as measured by light scattering and AFM.

For the case of InAs growth on GaAs, the strain is relieved elastically through a morphological transition in which nearly identically-sized three-dimensional islands form, known in the literature as quantum dots. For typical growth conditions the islands are too small and closely spaced to be detected using visible wavelengths. An ultraviolet light scattering apparatus is described, which we show can detect the onset of quantum dot formation. The UV scattering signal increases linearly with time after the dots have formed, which we interpret as evidence that the dots are diffusing on the surface. During prolonged annealing we observe the emergence and growth of larger islands that initially consume material from the quantum dots and then compete with each other for material. The in situ light scattering measurements reveal that these processes are sensitively dependent on annealing temperature and arsenic overpressure.

Control over the surface structure of semiconductor films during growth is critical for devices of recent technological importance. Typically the length scales of interest range from nanometers to micrometers. Examples include the size and spacing of quantum dots in quantum dot lasers, and the pitch and amplitude of grating structures for distributed Bragg reflectors.

Elastic light scattering has atomic height sensitivity to this surface structure, on lateral length scales as low as half the incident wavelength, and is easily implemented for in-situ monitoring during film growth [Pinnington et al., PRL 79, 1698-1701 (1997)]. For the smooth surfaces of interest here, the distribution of the scattered light intensity as a function of scattering angle directly maps out the power spectral density (PSD). The PSD gives the 'root mean square' roughness of the surface structure as a function of inverse length scale, or spatial frequency.

Here we present in-situ light scattering measurements performed during III-V semiconductor film growth by molecular beam epitaxy (MBE). We have used the technique to monitor the smoothing of one-dimensional grating structures during regrowth. For the regrowth experiments, the grating pitch was chosen such that the detection angle of the in-situ measurement coincided with the scattering peak associated with a harmonic of the grating periodicity. Because the initial shape of the patterned surface is known, it is possible to reconstruct the shape of the grating from the PSD as it evolves in time during growth. We find that for homoepitaxy of gallium arsenide (GaAs) on textured substrates, the time evolution follows the Kardar-Parisi-Zhang (KPZ) model [Kardar et al., PRL 56, 889-892 (1986)].