This thesis investigates the possibility of enhancing the efficiency of thin film solar cells by including periodic material nonhomogeneities in combination with periodically corrugated back reflectors. Two different types of solar cell are investigated; p-i-n junctions solar cells made from alloys of hydrogenated amorphous silicon (a-Si:H) (containing either carbon or germanium), and Schottky barrier junction solar cells made from alloys of indium gallium nitride (InξGa1-ξN). Material nonhomogeneities are produced by varying the fractions of the constituent elements of the alloys. For example, by varying the content of carbon or germanium in the a-Si:H alloys, semiconductors with bandgaps ranging from 1:3 eV to 1:95 eV can be produced. Changing the bandgap alters both the optical and electrical properties of the material so this necessitates the use of coupled optical and electrical models. To date, the majority of solar cell simulations either prioritise the electrical portion of the simulation or they prioritise the optical portion of the simulation. In this thesis, a coupled optoelectronic model, developed using COMSOL Multiphysics®, was used to simulate solar cells: a two-dimensional finite-element optical model, which solved Maxwell's equations throughout the solar cells, was used to calculate the absorption of incident sunlight; and a finite-element electrical drift-diffusion transport model, either one- or two-dimensional depending on the symmetries of the problem, was used to calculate the steady state current densities throughout the solar cells under external voltage biases. It is shown that a periodically corrugated back reflector made from silver can increase efficiency of an a-Si:H alloy single p-i-n junction solar cell by 9:9% compared to a baseline design, while for a triple junction the improvement is a relatively meagre 1:8%. It is subsequently shown that the efficiency of these single p-i-n junction solar cells with a back reflector can be further increased by the inclusion of material nonhomogeneities, and that increasing the nonhomogeneity progressively increases efficiency, especially in thicker solar cells. In the case of InξGa1-ξN Schottky barrier junction solar cells, the gains are shown to be even greater. An overall increase in efficiency of up to 26:8% over a baseline design is reported.

A novel solid state illumination source has been developed. A two terminal dual LED has been created with the ability to control the relative intensities of the two emission peaks by varying drive current. Doping profiles have been used to extend the dynamic range of the dual LED over other reported devices. Operation of the dual LEDs is explained as a function of drive current. In addition, novel use of phosphor mixtures allows the creation of a broadband spectral power distribution that can be varied using a dual LED as an excitation source. Combinations of phosphors that have varied excitation spectra provide the ability to selectively excite different phosphors with the different LED emission peaks. First and second generations of the two terminal dual LED and the phosphor combination are discussed. The final source has the ability to mimic the light of a blackbody radiator over a range of 3200 K - 5300 K. The development of a three terminal dual LED as a pump source was prohibited by the need for a III-nitride tunnel junction, that proved unattainable in the scope of this work. However, several novel doping schemes were investigated toward this end. Finally, a circadian light source has also been developed that can affect physiological changes in humans, and a light box for entrainment of circadian rhythms in rats has been built.

By an improved understanding of Mg-doped GaN through an exhaustive review of current limitations, increased control over the material was achieved by addressing several of these issues. To address the issues of the memory effect, low sticking coefficient and high vapor pressure of Mg, a new Mg dopant source was implemented, characterized and modeled for p-type doping of GaN. The device enhanced the sticking coefficient of Mg by energizing the outgoing Mg flux, and also allowed the first reported demonstration of an abrupt junction between two non-zero Mg concentrations and a graded Mg-doped GaN film. The significant compensation of Mg acceptors at high dopant concentrations was used advantageously to develop a new ex situ resistivity analysis technique using the energy distributions of SIMS to characterize doping of buried layers. The new technique was used to identify the barrier between conductive and resistive Mg doping for increased Mg concentration, which was then used to optimize Mg-doped GaN. Because Mg doping exhibits a dependence upon the growth regime, a new growth and regime characterization technique was developed using specific RHEED intensity responses to repeat growth conditions. During the development of this technique, a new surface kinetics growth model for III-nitrides was discovered based on DMS observations, which suggests preferential buildup of the metal bilayer before growth begins with an unfamiliar cation-anion exchange process initially upon metal shutter opening. Using the new RHEED growth and regime characterization technique, a new growth technique called metal modulated epitaxy (MME) was developed to increase repeatability, uniformity and smoothness. The MME technique was enhanced with a closed-loop control using real-time feedback from RHEED transients to control shutter transitions. This enhancement, called smart shuttering, led to improved growth rate and further improvement of surface roughness and grain size, which were repeatable within low percentages. Effects of smart-shuttering MME were observed with Si, Mg and In during GaN growths. Repeatable Mg-doped GaN was achieved with a variation of less than 8%, and a peak hole carrier concentration of 4.7E18 cm^-3.

Gallium nitride (GaN) has been targeted for use in high power (>30 W/mm) and high frequency (>160 GHz) application due to its wide band gap and its large break down field. One of the most significant advances in GaN devices has evolved from the AlGaN/GaN high electron mobility transistor (HEMT). As a result of the large power densities being applied to these devices there can develop intense hot spots near areas of highest electric field. The hot spot phenomenon has been linked to a decrease in device reliability through a range of degradation mechanisms. In order to minimize the effect that hot spot temperatures have on device reliability a detailed understanding of relevant transport mechanisms must be developed. This study focuses on two main aspects of phonon transport within GaN devices. The first area of focus was to establish an understanding of phonon relaxation times within bulk GaN. These relaxation times were calculated from an application of Fermi's Golden Rule and explicitly conserve energy and crystal momentum. This analysis gives insight into the details behind the macroscopic thermal conductivity parameter. Once relaxation times for GaN were established a multiscale phonon transport modeling methodology was developed that allowed the Boltzmann Transport Equation to be coupled to the energy equation. This coupling overcomes some computational limits and allows for nanoscale phenomena to be resolved within a macroscopic domain. Results of the transport modeling were focused on benchmarking the coupling method as well as calculating the temperature distribution within an operating 6 finger HEMT.

The objective of the research was to develop high quality GaN epitaxial growth on alternative substrates that could result in higher external quantum efficiency devices. Typical GaN growth on sapphire results in high defect materials, typically 10⁸⁻¹⁰cm⁻², due to a large difference in lattice mismatch and thermal expansion coefficient. Therefore, it is useful to study epitaxial growth on alternative substrates to sapphire such as ZnO which offers the possibility of lattice matched growth. High-quality metalorganic chemical vapor deposition (MOCVD) of GaN on ZnO substrate is hard to grow due to the thermal stability of ZnO, out-diffusion of Zn, and H₂back etching into the sample. Preliminary growths of GaN on bare ZnO substrates showed multiple cracks and peeling of the surface. A multi-buffer layer of LT-AlN/GaN was found to solve the cracking and peeling-off issues and demonstrated the first successful GaN growth on ZnO substrates. Good quality InGaN films were also grown showing indium compositions of 17-27% with no indium droplets or phase separation. ZnO was found to to sustain a higher strain state than sapphire, and thereby incorporating higher indium concentrations, as high as 43%, without phase separation, compared to the same growth on sapphire with only 32%. Si doping of InGaN layers, a known inducer for phase separation, did induce phase separation on sapphire growths, but not for growths on ZnO. This higher strain state for ZnO substrates was correlated to its perfect lattice match with InGaN at 18% indium concentration. Transmission electron microscopy results revealed reduction of threading dislocation and perfectly matched crystals at the GaN buffer/ZnO interface showing coherent growth of GaN on ZnO. However, Zn diffusion into the epilayer was an issue. Therefore, an atomic layer deposition of Al₂O₃was grown as a transition layer prior to GaN and InGaN growth by MOCVD. X-ray and PL showed distinct GaN peaks on Al₂O₃/ZnO layers demonstrating the first GaN films grown on Al₂O₃/ZnO. X-ray photoelectron spectroscopy showed a decrese in Zn diffusion into the epilayer, demonstrating that an ALD Al₂O₃layer was a promising transition layer for GaN growth on ZnO substrates by MOCVD.

No description available.

The material and device characteristics of InGaN/GaN heterojunction bipolar transistors (HBTs) grown by metalorganic chemical vapor deposition are examined. Two structures grown on sapphire with different p-InxGa1-xN base-region compositions, xIn = 0.03 and 0.05, are presented in a comparative study. In a second experiment, NpN-GaN/InGaN/GaN HBTs are grown and fabricated on free-standing GaN (FS-GaN) and sapphire substrates to investigate the effect of dislocations on III-nitride HBT epitaxial structures. The performance characteristics of HBTs on FS-GaN with a 20×20 m2 emitter area exhibit a maximum collector-current density of ~12.3 kA/cm2, a D.C. current gain of ~90, and a maximum differential gain of ~120 without surface passivation. For the development of deep-ultraviolet optoelectronics, several various structures of optically-pumped lasers at 257, 246, and 243 nm are demonstrated on (0001) AlN substrates. The threshold-power density at room temperature was reduced to as low as 297 kW/cm2. The dominating polarization was measured to be transverse electric in all cases. InAlN material was developed to provide lattice matched, high-bandgap energy cladding layers for a III-N UV laser structure. This would alleviate strain and dislocation formation in the structure, and also mitigate the polarization charge. However, a gallium auto-doping mechanism was encountered which prevents the growth of pure ternary InAlN, resulting instead in quaternary InAlGaN. This phenomenon is quantitatively examined and its source is explored.

Ce travail de thèse porte sur l’étude des propriétés optiques et électroniques des puits quantiques de GaN / AlGaN grâce à des techniques classiques de réflectivité résolue en angle et de photoluminescence, ainsi qu’avec la technique de photoluminescence résolue temporellement. Les expériences de photoluminescence en régime continu ont permis d’estimer les énergies des transitions excitoniques qui sont également accessibles en réflectivité. Ces techniques ont ainsi permis de mettre en évidence l’effet Stark dans les puits quantiques GaN / AlGaN. L’effet Stark sur les énergies de transition est cohérent avec la théorie des fonctions enveloppes. Les spectres de réflectivité permettent d’accéder à la force d’oscillateur des excitons grâce à leur modélisation par le formalisme des matrices de transfert, prenant en compte les phénomènes d’élargissement homogène et inhomogènes des transitions optiques. Enfin, les mesures de photoluminescence résolue en temps en fonction de la température, ont également permis d’extraire la force d’oscillateur qui est inversement proportionnelle au temps de recombinaison radiative. Cette étude a également permis de mettre en évidence l’effet Stark responsable de la diminution de la force d’oscillateur en fonction de l’épaisseur du puits quantique mais aussi en fonction de la composition d’aluminium. L’augmentation de l’épaisseur du puits entraîne une diminution du recouvrement des fonctions d’onde, et une augmentation de la composition d’aluminium intensifie le champ électrique et diminue également le recouvrement des fonctions d’onde. / This work deals with the study of optical and electronic properties of GaN / AlGaN quantum wells, by classical techniques of spectroscopy including angle resolved reflectivity or photoluminescence, but also by time resolved photoluminescence. The continuous wave photoluminescence experiments allowed estimating the energies of the excitonic transitions, which are also available through reflectivity. These techniques highlighted the Stark effect in GaN / AlGaN quantum wells. The influence of the Stark effect on the energies of the excitonic transitions is well reproduced by envelop functions theory. Reflectivity spectra give access to the oscillator strength via their fitting by transfer matrix formalism, taking in account both homogeneous and inhomogeneous broadenings of the optical transitions. Finally, time resolved photoluminescence measurements as a function of temperature were performed to extract the oscillator strength, which is proportional to the inverse of the radiative recombination time. This technique also highlighted the Stark effect which is responsible of the vanishing of the oscillator strength with the thickness of the well and the aluminium composition. Increasing of the quantum well’s thickness induces decreasing of wave functions overlap, as well as an increasing of the aluminium composition which intensifies the electric field and splits the wave functions.

Polystyrene block polyethyleneimine (PS-b-PEI) copolymer prepared by combining PS and poly(2-methyl-2-oxazoline) (PMeOx) segments together through two strategies. Furthermore, PMeOx block was hydrolysis to produce PEI block which linked with PS block. Macroinitiator route is one of these two ways to prepare PS-b-PEI copolymer. Polystyrene macroinitiator or poly(2-methyl-2-oxazoline) macroinitiator prepared through Nitroxide Mediate Radical Polymerization (NMRP) or Cationic Ring Opening Polymerization (CROP) respectively. Each macroinitiator has active initiated terminal group toward another block monomer. Second strategy based on coupling of PS segment with PMeOx block through “click” coupling chemistry. Polystyrene modified with terminal azide moiety are combined with PMeOx functionalized with alkyne group via 1,3 dipolar cycloaddition reaction “click reaction”. PS-b-PMeOx was hydrolysis in alkaline medium to produce amphiphilic PS-b-PEI copolymer. A set of block copolymer with different block ratios was prepared and investigated to select suitable block copolymer for further applications. Stichiometric PS-b-PEI copolymer selected to stabilize gold nanoparticle (Au NPs) in polymer matrix. PEI segment work as reducing and stabilizing agent of gold precursor in aqueous solution. Various concentrations of gold precursor were loaded and its effect on UVVIS absorbance, particle size and particle distribution studied. In addition, reduction efficiency of PEI block was determined from XPS measurements. The thickness of Au NPs/PS-b-PEI thin film was determined with a novel model for composite system. On the other hand, Gallium nitride quantum dots (GaN QDs) stabilized in PS-b-PEI copolymer after annealing. Our amphiphilic block copolymer exhibit nice thermal stability under annealing conditions. GaN QDs prepared in narrow nano-size with fine particle distribution. Blue ray was observed as an indication to emission activity of GaN crystal. Over all, PS-b-PEI copolymer synthesized through macroinitiator and click coupling methods. It was successfully stabilized Au NPs and GaN QDs in polymer matrix with controlled particle size which can be post applied in tremendous industrial and researcher fields.

Thesis (M.App.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 73-76). Also available in electronic format on the Internet.