<p>Trapping of 30 keV ion-implanted helium by the radiation-produced damage in hot isostatic pressed beryllium foils of 99.5 wt% purity has been investigated by a series of ion implantation/thermal desorption experiments. Different experimental regimes were designed to obtain some fundamental insight into the behaviour of helium in beryllium. The helium release was related to the surface morphology changes observed on the desorbed surfaces by scanning eIectron microscopy.</p> <p>The nature of helium trapping in beryllium has been found to strongly depend on the implantation parameters as well as on the thermal treatment of the implanted samples. Desorption peaks have been analyzed in terms of the dissociation of simple helium-vacancy trapping centres and/or helium release from microbubbles that nucleate and grow during the annealing of the implanted samples.</p> <p>Linear-ramp annealing following room-temperature 30 keV He⁺ implantation in beryllium to a total fluence in the range of 10²⁰ to 10²¹ /m² has produced two desorption stages above 890 K and below 830 K, respectively. The high temperature desorption peak was analyzed in terms of a first-order dissociation mechanism with an activation energy that depends on the relative occupation of the trapping site. The low temperature peak corresponds to a higher-order helium-vacancy cluster that begins to fill once the deeper trapping site approaches saturation. A third trapping site, with higher dissociation energy, has been inferred from the fact that a sizeable fraction of the implanted helium has not been released after heating up to 75% of the melting temperature.</p> <p>Some samples have been heated by a stepped anneal regime, for two hours at 573 or 773 K, that allows reconfiguration of the trapped helium before being desorbed. The release spectra and the desorbed surface morphology show that the nature of helium trapping has changed to a more stable form of helium bubbles. A model is proposed to account for the helium bubble nucleation and growth by a migration and coalescence mechanism. Linearly ramped thermal desorption after high temperature implantations at 600 and 773 K reveals the formation of more stable trapping sites. The helium release has been related to the formation of holes on the surface and the desorption was inferred to result from microbubble growth by thermal vacancy assisted mechanism.</p> <p>Some of the samples have been deliberately corroded to investigate the effect of the surface contamination on the helium release. The desorption curves show the formation of an additional broad desorption peak at temperatures higher than 950 K. This is associated with a relative drop in the population of the primary and secondary desorption peaks and implies that the corroded surface contains additional trapping sites that delay the release of the helium detrapped from the bulk.</p> / Doctor of Philosophy (PhD)

<p>This thesis reports on study of the effects of various plasmas on the growth of InP films by GSMBE. The samples were grown at temperatures ranging from 232 to 500°C. The plasmas were generated by electron cyclotron resonance, giving a broad distribution of ion energies in the 10-50 cV range. H. D, He and Ar plasmas were studied. Films were also grown at the same temperatures without plasma, as references.</p> <p>The samples were characterized by: Nomarski phase contrast microscopy, Xray double crystal diffraction, Hall effect measurements, photoluminescence (PL), variable-energy positron annihilation, thermal desorption, nuclear reaction analysis and capacitance/voltage profiling. The Xray, Hall and PL measurements were repeated after the samples were annealed at 730°C for 10 seconds. In no case was the material produced of comparable quality to that grown under standard conditions (465°C, no plasma).</p> <p>The films grown without plasma were n-type, reaching a carrier concentration of approximately 2x10¹⁸/cm³ at 300°C. This is in agreement with published results, which showed that the donor defect responsible is a P atom on an In site. This defect occurred in sufficiently high concentrations to mask the effects of the plasmas below 400°C.</p> <p>H plasma increased the carrier concentration by approximately 7x10¹⁶/cm³. Some H atoms appear to bond to P atoms in the crystal, resulting in an excess In electron. The carrier concentration was not affected by annealing. Additional H atoms may be present as interstitials, causing a reduction in mobility and a broad PL peak near 1.05 eV. These effects were removed by annealing.</p> <p>D plasma produced similar effects to H plasma, but the carrier concentration and mobility were lower. P interstitials, which act as deep acceptors, may be produced by recoiling surface P atoms into the bulk. Beryllium dopant, at a concentration of 2x10¹⁸/cm³), was passivated by this plasma. Silicon dopant was not strongly affected.</p> <p>He plasma produced P interstitials and P vacancies, by recoil displacement from the surface and in the bulk. More P interstitials were produced, compensating up to 10¹⁷/cm³ carriers. The interstitials were more mobile, diffusing out during the 500°C growth and during annealing.</p> <p>The effects of the Ar plasma were small enough to be masked by the weak H plasma that results from H backstreaming.</p> <p>Films grown at 400°C or less without plasma had textured surfaces. The plasmas usuaIly smoothed the surfaces at these temperatures, resulting in defect densities lower even than occured at 465°C without plasma. Above 400°C. none of the plasmas significantly affected the surface defect densities.</p> / Doctor of Philosophy (PhD)

<p>A detailed characterization of the In1- x Gax Asy P 1-y quaternary material system lattice matched to GaAs, grown by gas source Molecular Beam Epitixy (MBE) has been performed. Photoluminescence, X-ray diffraction and Transmission Electron Microscopy (TEM) were used to study the lateral composition modulation (LCM) which was observed in this material system. Optimization of the growth process and the substrate orientation resulted in a significant reduction of the LCM. Additionally, a comprehensive analysis of the optical constants was performed which resulted in the first publication of wavelength and composition dependent index of refraction data for this material system. The combination of growth optimization and index of refraction data lead to the demonstration of efficient, low threshold operation of InGaAsP/GaAs based multiple quantum well lasers. In order to efficiently couple the above laser diodes to an external cavity to facilitate the generation of ultrashort pulses, antireflection facet coatings were required. As such, optical interference filters have been fabricated using a plasma enhanced chemical vapor deposition system, based on the SiO x Ny material system. High quality antireflection facet coatings, suitable for application to the InGaAsP/GaAs diode lasers have been designed and fabricated, resulting in modal reflectivities of 1-2 × 10-4 . Finally, an ultrashort-pulse external-cavity diode laser system was designed and manufactured which allowed the laser diode to be wavelength tuned and emit mode-locked ultrashort optical pulses. Pulses with sub 2 ps duration and greater than 1 mW average output power have been achieved. A study of the novel application of an asymmetric quantum well structure to the generation of ultrashort optical pulses has been proposed and initiated.</p> / Doctor of Philosophy (PhD)

<p>This thesis details the development of enabling technologies required for inexpensive, monolithic integration of Si-based wavelength division multiplexing (WDM) components and photodetectors. The work involves the design and fabrication of arrayed waveguide grating demultiplexers in silicon-on-insulator (SOI), the development of advanced SiGe photodetectors capable of photodetection at 1.55 μm wavelengths, and the development of a low cost fabrication technique that enables the high volume production of Si-based photonic components. Arrayed waveguide grating (AWG) demultiplexers were designed and fabricated in SOI. The fabrication of AWGs in SOI has been reported in the literature, however there are a number of design issues specific to the SOI material system that can have a large effect on device performance and design, and have not been theoretically examined in earlier work. The SOI AWGs presented in this thesis are the smallest devices of this type reported, and they exhibit performance acceptable for commercial applications. The SiGe photodetectors reported in the literature exhibit extremely low responsivities at wavelengths near 1.55 μm. We present the first use of three dimensional growth modes to enhance the photoresponse of SiGe at 1.55 μm wavelengths. Metal semiconductor-metal (MSM) photodetectors were fabricated using this undulating quantum well structure, and demonstrate the highest responsivities yet reported for a SiGe-based photodetector at 1.55 μm. These detectors were monolithically integrated with low-loss SOI waveguides, enabling integration with nearly any Si-based passive WDM component. The pursuit of inexpensive Si-based photonic components also requires the development of new manufacturing techniques that are more suitable for high volume production. This thesis presents the development of a low cost fabrication technique based on the local oxidation of silicon (LOCOS), a standard processing technique used for Si integrated circuits. This process is developed for both SiGe and SOI waveguides, but is shown to be commercially suitable only for SOI waveguide devices. The technique allows nearly any Si microelectronics fabrication facility to begin manufacturing optical components with minimal change in processing equipment or techniques. These enabling technologies provide the critical elements for inexpensive, monolithic integration in a Si-based system.</p> / Doctor of Philosophy (PhD)

<p>The operation and safety of both fossil-fuel and nuclear power stations depend on adequate cooling of the thermal source involved. This is usually accomplished using liquid coolants that are forced through the high temperature regions by a pumping system; this fluid then transports the thermal energy to another section of the power station. However, fluids that undergo boiling during this process create vapor that can be detrimental, and influence safe operation of other system components. The behavior of this vapor, or void, as it is generated and transported through the system is critical in predicting the operational and safety performance. This study uses two advanced penetrating radiation techniques, Real Time Neutron Radiography (RTNR), and High Speed X-Ray Tomography (HS-XCT), to examine void generation and transport behavior in a flow boiling system. The geometries studied were tube side flow boiling in a cylindrical configuration, and a similar flow channel with an internal twisted tape swirl flow generator. The heat transfer performance and pressure drop characteristics were monitored in addition to void distribution measurements, so that the impact of void distribution could be determined. The RTNR and heat transfer pipe flow studies were conducted using boiling Refrigerant 134a at pressures from 500 to 700 kPa, inlet subcooling from 3 to 12°C and mass fluxes from 55 to 170kg/m 2 -s with heat fluxes up to 40 kW/m2 . RTNR and HS-XCT were used to measure the distribution and size of the vapor phases in the channel for cylindrical tube-side flow boiling and swirl-flow boiling geometries. The results clearly show that the averaged void is similar for both geometries, but that there is a significant difference in the void distribution, velocity and transport behavior from one configuration to the next. Specifically, the void distribution during flow boiling in a cylindrical-tube test section showed that the void fraction was largest near the tube center and decreased with increasing radial distance. For swirling flow, the void concentration was highest in the center of each subchannel formed by the twisted tape insert, producing two local void maxima at each axial position. Furthermore, the instantaneous RTNR results show that the effects of bubble agglomeration change from one geometry to the next. To further examine the application of RTNR for void distribution measurement, both vertical and horizontal orientations were examined. These experimental results show similar cross sectional averaged axial distributions of the void fraction but significant differences in the local void behavior. The HS-XCT experiments were conducted on swirl-flow boiling of Refrigerant 123 at similar conditions as the RTNR experiments. These tests were conducted to qualitatively compare and verify the void distribution and behavior obtained using RTNR techniques. The HS-XCT results verify that during smooth flow boiling in a vertical tube the void tends to concentrate in the center of the channel and decrease outward to the channel walls. For swirl flow, the void tends to concentrate near the center of each subchannel formed by the twisted tape. Furthermore, wall region void fraction for smooth-flow boiling was significantly higher than swirling flow conditions due to the significant centrifugal forces present in swirl-flow. These centrifugal forces may improve the heat transfer and dryout behavior during swirl-flow conditions. This work contributes to the development of two-phase flow diagnostics based on penetrating radiative techniques, i.e., RTNR and HS-XCT for void distribution measurement, and enhances the knowledge of flow boiling systems. The application of HS-XCT and RTNR for the study of flow boiling phenomena using smooth and swirl-flow geometries has clearly demonstrated that differences in local void distribution result in differences in heat transfer behavior.</p> / Doctor of Philosophy (PhD)

<p>This thesis presents dielectric-enhanced quantum-well intermixing (QWI) studies of InGaAsP/InP-based λ = 1.55 μm laser structures for photonic integration. QWI is studied as a function of MBE growth, dielectric film (SiO x Ny ) composition and thickness, annealing temperature and time. The results suggest there are at least two processes that cause intermixing in samples without dielectric coatings, related to grown-in defects. It is found that plasma-enhanced chemical-vapor-deposited SiOx Ny films of refractive index 1.65 are best at enhancing the QWI process. Based on SIMS measurements of these films before and after anneal we observe the migration of Group III atoms into the dielectric film. Consequently we believe that Group V interstitials are injected into the laser structure and are responsible for the enhanced QWI. However, the amount of intermixing does not correlate with the amount of Group III that migrates into the dielectric film, suggesting that another factor, such as film stress, may affect the amount of injected interstitials. This postulated mechanism for InGaAsP/InP-based structures differs from the accepted belief that in the GaAs materials system the dielectric film injects Group III vacancies into material which promote the intermixing process, and that the number of injected vacancies is related to the film porosity. The thesis concludes with a presentation of materials properties and processing issues which are important for device fabrication. It is shown that although the electrical and optical properties of the intermixed material remain desirable, the process leads to difficulties in wet chemical etching and regrowth over material annealed with dielectric films.</p> / Doctor of Philosophy (PhD)

<p>Asymmetric Multiple Quantum Well (AMQW) lasers have wells of varying thickness and/or composition within a single active region. The different wells emit at different wavelengths. The thesis begins with a description of the basic properties of AMQW lasers which have been discovered through experimentation. Next, an experimental technique is introduced which employs AMQW lasers to study experimentally the non-uniform carrier distribution in MQW lasers. Results of three studies carried out using this new technique are presented. A theoretical model is developed to explain some of our experimental results. The theoretical treatment of carrier capture and carrier escape is discussed in detail and different treatments are compared to experimental data taken from AMQW lasers. Finally, the application of AMQW lasers as broadly tunable lasers is discussed and the key design issues for designing broadly tunable AMQW lasers are presented.</p> / Doctor of Philosophy (PhD)

<p>The use of InGaAsP semiconductor lasers as radiation sources in gas and liquid detection is described in this thesis. Single mode operation and tunability were studied in several schemes including diode lasers with a short external cavity (SXC), diode lasers with multiple short external cavities (MSXC), and a grating external cavity (GEC) diode laser. Comparisons of SXC, MSXC and GEC lasers are given in terms of tunability, side mode suppression ratio (SMSR), stability, and ease of construction and operation. In highly sensitive gas detection, the harmonic content of residual amplitude modulation (RAM) for current modulation of the diode lasers was studied based on the concept that the light intensity rather than the electric field is directly modulated by the injection current. Formulae for RAM and the absorption signals are given for injection current modulation spectroscopy with diode lasers. Water vapour was detected by using InGaAsP SXC and DFB diode lasers, and an electronic subtracter was employed to reduce the detection noise. A sensitivity of $\approx$1.6 $\times$ 10$\sp{-6}$ in units of equivalent absorbance in an equivalent noise bandwidth of 1.25Hz was obtained. In liquid detection, InGaAsP laser diodes with multiple short external cavities (MSCX's) were developed to provide a wide spectral coverage, up to 72nm spectral coverage was achieved. Liquid detection by MSXC diode lasers was studied in conjunction with multivariate calibration methods, i.e., principal component regression (PCR) and partial least squares (PLS). A sensitivity of 0.1% H$\sb2$O in D$\sb2$O was achieved and the limiting noise source was assessed. Three component mixtures of H$\sb2$O, acetone and methanol were studied in terms of regression factors and outlier detection in the PCR and the PLS algorithms. To achieve even broader tunability by means of external cavity, work on making broad gain peak InGaAsP/InP lasers was initiated.</p> / Doctor of Philosophy (PhD)

<p>Intranuclear mean free paths of nucleons which are incident on heavy nuclei with a kinetic energy in the range 100 MeV to 800 MeV are investigated. The heavy nucleus is modelled as a Fermi gas of nucleons. Relativistic scattering theory is employed under the assumption that on average the nucleon which is struck will initially be at rest. Constraints imposed by the Pauli Exclusion Principle are incorporated. The results can be used to solve problems in accelerator shielding and accelerator breeder technology.</p> / Master of Engineering (ME)

<p>A novel optoelectronic switching device, the bistable field effect transistor (BISFET) has been developed. The BISFET is an inversion channel heterostructure device containing a positive feedback loop transverse to the channel, between the gate and collector terminals. This leads to the existence of two distinct operating states. In one state, the feedback loop is in a high-impedance OFF state, associated with which are a large carrier population in the conduction channel and a correspondingly high drain current. In the other state, the feedback loop is in a low-impedance ON state, characterized by a small carrier population and relatively low drain current. These transitions, referred to as switchup and switchdown, sweep out a large hysteresis loop in the drain characteristics of the BISFET, making the device strongly bistable. A mathematical model is developed to describe the operation of the device.</p> <p>Implementation of the BISFET in the GaAs/AIGaAs material system is reported. A study of the electrical characteristics of the device has been carried out, using two separate device configurations. In one configuration, a separate collector terminal is used to supply feedback current in the device. In the other, known as the integral collector configuration, the source contact simultaneously serves as the collector terminal. The electrical characteristics reveal a sharp drop in gate current corresponding with that in the drain current. This confirms the feedback loop as the origin of the switching. The gate characteristics are shown to exhibit S-type negative differential resistance (NDR) associated with the feedback loop. The transfer characteristics of the BISFET are found to contain hysteresis corresponding with that seen in the drain characteristics.</p> <p>A single set of transitions is observed in the BISFET when the integral collector configuration is used. The feedback loop in this case lies between the gate and source terminal. Two distinct sets of transitions have been observed in this mode. The previously-observed set resulting from the gate-source feedback loop (integral collector configuration) is still present. In addition to this, however, a second set of transitions is seen, which is associated with the feedback loop between the gate and the separate collector terminal.</p> <p>An enhancement in drain current is reported in the BISFET at low drain voltage. This increase, seen at high gate bias, is believed to result from a geometric and electrical symmetry between the source and drain terminals at low drain voltages. The turn-off of the enhancement with increasing drain bias varies from a gradual decline at lower gate voltages to an abrupt drop at higher values.</p> <p>Optical bistability in the GaAs/AIGaAs BISFET is reported. The device is found to emit light from the active region when it is in the ON state. The light output exhibits abrupt transitions and hysteresis corresponding directly with those seen in the electrical characteristics. Optical control of the bistability is also reported. The drain voltages at which the current transitions occur are found to change when optical excitation is applied to the device. As the optical intensity is increased, the transition voltages decrease, then saturate, then increase.</p> <p>The structure and fabrication process used for the BISFET are compatible with a range of other devices, including light sources and detectors. The feasibility of constructing optoelectronic integrated circuits using the BISFET is demonstrated using a circuit consisting of an LED and a BISFET current driver.</p> / Doctor of Philosophy (PhD)