The popularity of III-nitride materials has taken up the semiconductor industry to newer applications because of their remarkable properties. In addition to having a direct and wide band gap of 3.4 eV, a very fascinating property of GaN is the band gap tuning from 0.7 to 6.2 eV by alloying with Al or In. The most common orientation to grow optoelectronic devices out of these materials are the polar c-plane which are strongly affected by the intrinsic spontaneous and piezoelectric polarization fields. Devices grown in no polar orientation such as (1 0 –1 0) m-plane or (1 1 –2 0) a-plane have no polarization in the growth direction and are receiving a lot of focus due to enhanced behaviour. The first part of this thesis deals with the development of non-polar epimGaN films of usable quality, on an m-plane sapphire by plasma assisted molecular beam epitaxy. Growth conditions such as growth temperature and Ga/N flux ratio were tuned to obtain a reasonably good crystalline quality film. MSM photodetectors were fabricated from (1 0 -1 0) m-GaN, (1 1 -2 0) a-GaN and semipolar (1 1 -2 2) GaN films and were compared with the polar (0 0 0 2) c-GaN epilayer. Later part of the thesis investigated (1 0 -1 0) InN/ (1 0 -1 0) GaN heterostructures. Further, we could successfully grow single composition nonpolar a-plane InxGa1-xN epilayers on (1 1 -2 0) GaN / (1 -1 0 2) sapphire substrate. This thesis focuses on the growth and characterisation of nonpolar GaN, InxGa1-xN and InN by plasma assisted molecular beam epitaxy and on their photodetection potential. Chapter 1 explains the motivation of this thesis work with an introduction to the III-nitride material and the choice of the substrate made. Polarization effect in the polar, nonpolar and semipolar oriented growth is discussed. Fabrication of semiconductor photodetectors and its principle is explained in details. Chapter 2 discusses the various experimental tools used for the growth and characterisation of the film. Molecular beam epitaxy technique is elaborately explained along with details of the calibration for the BEP of various effusion cells along with growth temperature at the substrate. Chapter 3 discusses the consequence of nitridation on bare m-sapphire substrate. Impact of nitridation step prior to the growth of GaN film over (1 0 -1 0) m-sapphire substrate was also studied. The films grown on the nitridated surface resulted in a nonpolar (1 0 -1 0) orientation while without nitridation caused a semipolar (1 1 -2 2) orientation. Room temperature photoluminescence study showed that nonpolar GaN films have higher value of compressive strain as compared to semipolar GaN films, which was further confirmed by room temperature Raman spectroscopy. The room temperature UV photodetection of both films was investigated by measuring the I-V characteristics under UV light illumination. UV photodetectors fabricated on nonpolar GaN showed better characteristics, including higher external quantum efficiency, compared to photodetectors fabricated on semipolar GaN. Chapter 4 focuses on the optimization and characterisation of nonpolar (1 0 -1 0) m-GaN on m-sapphire by molecular beam epitaxy. A brief introduction to the challenges in growing a pure single phase nonpolar (1 0 -1 0) GaN on (1 0 -1 0) sapphire without any other semipolar GaN growth is followed by our results achieving the same. Effect of the growth temperature and Ga/N ratio on the structural and optical properties of m-GaN epilayers was studied and the best condition was obtained for the growth temperature of 7600C and nitrogen flow of 1 sccm. Strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by RSM. Au/ nonpolar GaN schottky diode was fabricated and temperature dependent I-V characteristics showed rectifying nature. Chapter 5 demonstrates the growth of (1 0 -1 0) m-InN / (1 0 -1 0) m-GaN / (1 0 -1 0) m-sapphire substrate. Nonpolar InN layer was grown at growth temperature ranging from 3900C to 440C to obtain a good quality film at 4000C. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. RSM images on the asymmetric plane revealed highly strained layers. InN band gap was found to be around 0.8 eV from absorption spectra. The valance band offset value is calculated to be 0.93 eV for nonpolar m-plane InN/GaN heterojunctions. The heterojunctions form in the type-I straddling configuration with a conduction band offsets of 1.82 eV. Chapter 6 focuses on the optimization of nonpolar (1 1 -2 0) a-GaN on (1 -1 0 2) r-sapphire by molecular beam epitaxy. Effect of the growth temperature and Ga/N ratio on the structural and optical properties of a-GaN epilayers was studied and the best condition was obtained for the growth temperature of 7600C and nitrogen flow of 1 sccm. An in-plane orientation relationship is found to be [0 0 0 1] GaN || [-1 1 0 1] sapphire and [-1 1 0 0] GaN || [1 1 -2 0] sapphire for nonpolar GaN on r-sapphire substrate. Strain in the film was quantitatively measured using Raman spectroscopy and qualitatively analyzed by RSM. UV photo response of a-GaN film was measured after fabricating an MSM structure over the film with Au. EQE of the photodetectors fabricated in the (0 0 0 2) polar and (1 1 -2 0) nonpolar growth directions were compared in terms of responsively, nonpolar a-GaN showed the best sensitivity at the cost of comparatively slow response time. Chapter 7 demonstrates the growth of non-polar (1 1 -2 0) a-plane InGaN epilayers on a-plane (1 1 -2 0) GaN/ (1 -1 0 2) r-plane sapphire substrate using PAMBE. The high resolution X-ray diffraction (HRXRD) studies confirmed the orientation of the films and the compositions to be In0.19Ga0.81N, In0.21Ga0.79N and In0.23Ga0.77N. The compositions of the films were controlled by the growth parameters such as growth temperature and indium flux. Effect of variation of Indium composition on the strain of the epilayers was analyzed from the asymmetric RSM images. Further, we report the growth of self-assembled non-polar high indium clusters of In0.55Ga0.45N over non-polar (1 1 -2 0) a-plane In0.17Ga0.83N epilayer grown on a-plane (1 1 -2 0) GaN / (1 -1 0 2) r-plane sapphire substrate. The structure hence grown when investigated for photo-detecting properties, showed sensitivity to both infrared and ultraviolet radiations due to the different composition of InGaN region. Chapter 8 concludes with the summary of present investigations and the scope for future work.

This thesis focuses on a number of aspects governing the transformation of gibbsite, via intermediate phases, to a-alumina. These aspects include the size and morphology of the gibbsite grains, the influence of additions of foreign elements, the effect of a mechanical treatment of the gibbsite prior to calcination, and combinations of these factors. The materials were characterisedb y scanninge lectron microscopy, X-ray diffraction and surfacea rea measurementsF. or someo f the calcined materials an attempt was made to sinter the powders to a dense body to investigate if any of the treatments during calcination had an effect on this process. The literature review covers the current state of understanding of the production of bulk alumina powder by the Bayer process and the phase changes seen on calcination of precursors to the stable a-alumina phase. A detailed description of the phase changes is given and the various routes and conditions necessary for the transformations to occur are considered. The transformations are examined in relation to the morphology of the crystals and the variables controlling the phase transformation route are discussed. Calcination in air showed that the size of the gibbsite grain governs the calcination route taken to reach a-alumina. The standard gibbsites used in this work show a mixed calcination sequence transforming both via the boehmite phase, followed by the y, 8 and 0 phases, and via the x and K phases. The formation of boehmite is attributed to retention of water vapour within the grainDifferences in morphology of the starting materials showed that for the range of materials seen, the morphology of the grain is less important than its size. The super fine material confirmed that a small grain size transforms via the non-boehmite route only, with the other gibbsites taking intermediate routes as for the standard gibbsites. Of the additions made prior to calcination, aluminium fluoride was found to reduce the transformation temperature to a-alumina by approximately 300°C. Other additions had little effect on the transformation temperature although a reduction in grain size was seen with aluminium chloride. It was found that good mixing of the alumina fluoride was essential to obtain reliable and reproducible results. This is due to the small amounts of additive that are needed and the sensitivity of the process to concentration variations. Mineralisation of a range of gibbsites showed that the presence of sodium in the starting material was crucial in reducing the calcination temperature. This led to the conclusion that the sodium and fluoride react to form a liquid phase. The presence of a liquid phase increases the mobility of the aluminium and oxygen atoms resulting in a reduction of the transformation temperature. Fluoride additions to the gibbsites with different morphologies showed that the presence of sodium was the governing factor in reduction of the transformation temperature. Milling of the starting materials showed that there was a small reduction in the transformation temperature between some of the phases. The energy involved in milling leads to activation of the gibbsite. This activation takes the form of a reduction in the grain size and in a reduction of the crystallinity seen in the XRD patternFluoride additions during the calcination of sapphire with a standard gibbsite powder showed preferential grain growth. It was possible to initiate growth of small plate-like crystals on the polished surface of a piece of sapphire parallel to the basal plane. Crystal growth was also seen in scratches on a polished surface perpendicular to the basal plane

Interfacial charge transfer between metal oxides and organic semiconductors has been found to limit the efficiency of organic optoelectronic devices. Although a number of investigations of inorganic/organic systems exist, very few generally applicable rules for oxide/organic interfaces have been developed and many questions about these systems remain unanswered. Thus the studies presented in this dissertation were designed to improve the understanding of the fundamental interface physics of metal oxide/organic systems. Single molecule fluorescence microscopy was employed to determine the charge transfer mechanism while photoelectron spectroscopy was used to determine the energy level alignment of model systems. Additional computational studies allowed the examination of the properties of the charged organic molecules involved in charge transfer and modeling of the molecule-surface interaction. Calculations of the ground state properties and excited state transitions of the neutral and singly charged states of a modified perylene molecule were performed to provide insight into the orbitals of the initial and final states involved in the interfacial charge transfer process. The design and implementation of a novel UHV single molecule microscope is described. This microscope was used to observe the excited state charge transfer between a modified perylene molecule and Al₂O₃ (0001). The charge transfer mechanism was identified as involving activated trapping and detrapping of the defect derived states within the Al₂O₃ band gap, which resulted in the observation of strongly distributed kinetics for this system. The influence of defects and adsorbates on the electronic structure of ZnO and its interface with organic semiconductors was determined from photoelectron spectroscopy. Modified perylene molecules were found to have strong chemisorptive interactions with the ZnO surface involving charge transfer from defect derived ZnO states to the LUMO, while magnesium phthalocyanine molecules appear to have only weak physisorptive interactions with the ZnO surface. The interfacial investigations of the organic/oxide systems demonstrate the rich defect structure present in metal oxides. In both cases, defects were found to control the interfacial interactions between the metal oxide surface and the modified perylene molecules. Thus the manipulation of these defects states is of fundamental importance for optoelectronic device design.

No description available.

No description available.

The Micro-Raman scattering technique has been used for the study of GaN and ZnO. Capabilities of the Raman technique and existing literature on Raman spectroscopy in GaN and ZnO are reviewed. About 50 GaN and ZnO samples with a wide range of properties are studied. From the analysis of positions of the E2H and A1(LO) phonon modes, biaxial stress and plasmon coupling of the Al(LO) mode are observed and compared to a bulk GaN sample. The stress-related shift rate for the AI(LO) mode in hexagonal GaN is established to be 2.7 ± 0.4 cm-1/GPa through series of GaN with low free carrier concentration. Bulk ZnO and ZnO layers grown on sapphire have been studied, and no biaxial stress is found in ZnO layers. Doping and impurity modes resulted in disorder-activated scattering in ZnO. The choice of the laser for study of GaN and ZnO layers on sapphire substrate is discussed.

Master of Science / Department of Electrical and Computer Engineering / William B. Kuhn / A technique for measuring and characterizing transistor noise is presented. The primary goal of the measurements is to locate the 1/f noise corner for select transistors in Silicon-on-Sapphire processes. Additionally, the magnitude of the background channel noise of each transistor is measured. With this data, integrated circuit (IC) engineers will have a qualitative and quantitative resource for selecting transistors in designs with low noise requirements. During tests, transistor noise behavioral change is investigated over varying channel lengths, device type (N-type and P-type), threshold voltage, and bias voltage levels. Noise improvements for increased channel lengths from minimal, 1.0μm, and 4.0μm are measured. Transistors with medium and high threshold voltages are tested for comparison of their noise performance. The bias voltages are chosen to represent typical design values used in practice, with approximately 400 mV overdrive and a drain-to-source voltage range of 0.5 to 3.0V. The transistors subjected to tests are custom designed in Peregrine’s 0.5μm (FC) and 0.25μm (GC) Silicon-on-Sapphire (SOS) processes. In order to allow channel current noise to dominate over other circuit noise, the transistors have extraordinarily large aspect ratios (~2500 - 5000). The transistor noise produced is amplified and measured over a frequency range of 1kHz - 100MHz. This range allows the measurement of each device’s low and high frequency noise spectrum and resulting noise corner.

En dépit de leurs simplicités chimiques, de leurs fusion congruente, de leurs performances mécaniques et de leurs propriétés optiques, les monocristaux de saphir contiennent comme défauts des bulles aussi connus sous le nom de micro-vides. Quelle que soit la technologie de croissance, les cristaux obtenus sont caractérisés par la présence de micro-et macro-bulles qui affectent leurs qualités optiques et mécaniques limitant ainsi leurs applications. Ces bulles dégradent les propriétés optiques et l'efficacité des lasers produits par une réduction de la transparence des saphirs; elles induisent également des défauts de surface pendant le processus de polissage. Afin d'améliorer la qualité des cristaux, il est important d'éliminer les bulles, de connaître la raison de leurs formations, les causes de leurs propagations, de leurs constitutions, et de leurs diffusions dans le cristal. Nous avons étudié la distribution des bulles et leurs tailles dans les cristaux de saphir non dopés et dopés titane obtenus par la technique Czochralski (Cz). Les données expérimentales recueillies ont permis de connaître l'effet de différents paramètres de croissance sur la distribution, la densité et la taille des bulles. La propagation des bulles et leurs distributions dans les cristaux ne sont pas influencées par le type de germe. Si les vitesses de tirages augmentent, le diamètre des bulles diminuent et leurs densités augmentent. Les bulles formées dans le cristal de saphir sont influencées par la matière formant la charge de départ. L’utilisation de saphirs craquelés comme charge de départ pourrait être une bonne façon de minimiser la création de bulles et de limiter leurs propagations. Les résultats obtenus dans le cadre de cette thèse décrivent l'ensemble des phénomènes impliqués lors de l'incorporation de bulles dans les cristaux de saphir non dopés et dopés titane / In spite of the chemical simplicity, the congruent melt behaviour and it’s performed mechanical and optical properties sapphire single crystals contain bubbles defects also known as micro-voids. Whatever the growth technology, the grown crystals are characterized by the presence of micro and macro bubbles which affect the optical and mechanical quality of the crystal limiting their application. They degrade the optical properties and the laser efficiency by reduction of the transparency; they also induce surface defects during substrate polishing process. In order to improve the crystal quality, it is important to eliminate bubbles defects and know the reason of their formation, the causes of their propagation, their incorporation and their distribution in the crystal. We have studied bubbles distribution and their size in undoped and Ti-doped sapphire crystals grown by Czochralski (Cz) technique. The collected experimental data made it possible to know the effect of several growth parameters on the distribution, the density and the size of the bubbles. The bubbles propagation and distribution in the crystal are not influenced by the seed type. If the pulling rate increases, the diameter of bubbles decreases and their density increases. The bubbles formed in sapphire crystal are influenced by the starting charge material. Using sapphire crackle as starting charge could be a good way to minimise bubbles creation and limited their propagation. The obtained results in the frame of this thesis describe the whole phenomena involved during bubbles incorporation in undoped and Ti-doped sapphire crystals

<p>Flowcytometric technology has been widely used for measurement of the absolute numbers of T-lymphocytes subsets in Human Immunodeficiency virus (HIV), defining the disease state, monitoring antiviral treatment, and identifying any risk for opportunistic infections. A manual preparing of the samples is required. More recently an automated and enclosed blood cell counting, Celldyn Sapphire has been introduced. In this study the performance of the Flow cytometer EPICS XL as a reference method for analysis of CD3+, CD4+ and CD8 T-lymphocytes was evaluated with blood from 40 individual’s samples. EPICS XL was also compared with Celldyn Sapphire in the analysis of T-lymphocyte subsets in 39 blood samples from patients with low, high and normal lymphocyte counts. The result showed that the precision was high for both EPICS XL (2.5%) and Celldyn (10%). The method was linear over a wide range. Comparisons of CD3+, CD4+, and CD8+ T-lymphocytes analysis showed high coefficients of correlation (r0.9) and agreement (y>0.9x) between two instruments. A lower degree of agreement was observed at low concentration of CD3+ and CD4+ T-lymphocytes (0.757, 0.739). This means that cell counts obtained by Celldyn were 30% lower than those obtained with EPICS XL. This study shows that both EPICS XL and Celldyn Sapphire were suitable for CD4+ and CD8+ T cell counts. It is however preferable to use Flowcytometry for counting of low concentration of CD4+ T-lymphocytes (<200 cells/µL).</p>

Plastic relaxation was observed in InᵡGa₁₋ᵡN/GaN epilayers grown on c-plane sapphire substrates. The relaxation obeys the universal hyperbolic relation between the strain and the reciprocal of the layer thickness. Plastic relaxation in this material system reveals that there is no discontinuous relaxation at critical thickness and once a layer starts to relieve, it follows the same strain-thickness dependence, unconstrained by the original misfit until the material system work hardens. From x-ray diffraction calibration, the in-plane and normal relaxation constants KP0 and KN0 for the InᵡGa₁₋ᵡN/GaN grown on sapphire were found to be −0.98 ± 0.03 and +0.51 ± 0.03 nm, respectively. / Singapore-MIT Alliance (SMA)