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1
Kim, Kilyoung. „Super Collision Energy Transfer Studies in Single Collisions Between Vibrationally Hot Benzene Like Molecules and Ground State Bath Molecules: The Effect of Physical Properties of Donor and Bath Molecules on Super Collision Energy Transfer“. BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/2497.
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This research is focused on single-collision energy transfer events between highly vibrationally excited benzene-like donor molecules and small bath molecules, CO2 and N2O in the vibrational ground level. Measuring how much energy is transferred from donors to bath molecules was accomplished by probing bath molecules scattered into specific-rotational states using a tunable Δv=0.0003 cm-1 solid state diode laser. The normalized energy transfer probability distribution function, P(E,E'), determined from energy gain information, is very useful in comparing collisional energy transfer efficiency between various collision systems. P(E,E') is also used to investigate the effects of donor and bath physical properties on collisional energy transfer. The first chapter details the C6H5F–CO2 system, which is the basis of a study on the effect of donor fluorination on strong collision energy transfer. The second chapter is about all fluorobenzene–CO2 systems, which investigates the effect of excess vibrational excitation energy of donors on supercollision energy transfer efficiency as well as donor fluorination effect. The third chapter focuses on how the physical properties of bath molecules affect supercollision energy transfer by measuring state-specific energy gain of N2O scattered into 0000, J=59−75. Instead of CO2, N2O was used as a bath molecule with a pyrazine donor to compare energy gain results of bath molecules with somewhat different physical properties. N2O and CO2 are isoelectronic and have similar mass, but N2O has a small dipole moment. Comparison of P(E,E') obtained from pyrazine–CO2, –N2O, –DCl, and –H2O systems helps to elucidate the effect of the bath physical properties on supercollision energy transfer efficiency. The last chapter is dedicated to the extension of the measurement range of N2O energy gain to the mid J states (J=37–75). In this chapter I discuss reliability of P(E,E') obtained from only high J tail as well as the correction of overall energy transfer rate constant.
2
Park, Jeongho. „Molecular Beam Epitaxy (MBE) Growth of Rare Earth Doped Gallium Nitride for Laser Diode Application“. University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1148273402.
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3
Azarm, Ali, Paul Corkum und Pavel Polynkin. „Optical gain in rotationally excited nitrogen molecular ions“. AMER PHYSICAL SOC, 2017. http://hdl.handle.net/10150/626190.
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We pump low-pressure nitrogen gas with ionizing femtosecond laser pulses at 1.5 mu m wavelength. The resulting rotationally excited N-2(+) 2 molecular ions generate directional, forward-propagating stimulated and isotropic spontaneous emissions at 428 nmwavelength. Through high-resolution spectroscopy of these emissions, we quantify rotational population distributions in the upper and lower emission levels. We show that these distributions are shifted with respect to each other, which has a strong influence on the transient optical gain in this system. Although we find that electronic population inversion exists in our particular experiment, we show that sufficient dissimilarity of rotational distributions in the upper and lower emission levels could, in principle, lead to gain without net electronic population inversion.
4
Britton, Mathew. „Isolating the gain in the nitrogen molecular cation“. Thesis, Université d'Ottawa / University of Ottawa, 2020. http://hdl.handle.net/10393/41238.
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The nitrogen molecular cation is a promising gain medium for the air laser, which could be a powerful tool for remote sensing applications. After nearly a decade of international scientific effort, there are still questions about the gain mechanisms. This thesis explores gain in the nitrogen molecular ion using an experimental configuration that isolates the generation of gain from uncontrolled effects. We use a narrow nitrogen gas jet in vacuum to minimize propagation distance and a probe pulse to measure gain.
We first test the role of inelastic scattering during electron recollision as a mechanism to populate the excited state, and we find that it has a small contribution. Then, we measure gain dynamics by varying the probe delay. We measure short- and long-term gain in different conditions. The long-term gain is consistent with the decay of population inversion due to collisions between electrons and ions in the plasma. The decay is modulated due to rotational wave packets in the states of the ion. Rotational coherence decays on the same timescale as the gain due to the incoherent mixing of states.
We then introduce an additional non-ionizing pulse that interacts with the ion after ionization. The additional pulse changes the experiment to pump-probe spectroscopy on the ion itself. It can manipulate the rotational wave packets. It can also halt or redirect the emission following the probe pulse by coupling the ground lasing state with a middle state. In both cases, it controls the characteristics of gain and emission at a distance. These interactions highlight the role of the middle state, which is an essential ingredient for gain.
Short-term gain appears at low pump intensity when the pump or control pulses overlap the probe pulse. It is consistent with wave mixing due to Raman gain in a V-system.
5
Stephens, James M. „Gain characterization and donor molecule production for a proposed chemical laser system“. Diss., Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/31025.
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6
Panneerselvam, Nishanthi. „Exploring the sequence landscape of the model protein Rop to gain insights into sequence-stability relationship in proteins“. The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492735031524266.
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7
Wang, Yifan. „A Gain of Function Senescence Bypass Screen Identifies the Homeobox Transcription Factor DLX2 as a Regulator of ATM-P53 Signaling“. Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:26718730.
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Senescence stimuli activate multiple tumor suppressor pathways to initiate cycle arrest and a differentiation program characteristic of senescent cells. We performed a two-stage, gain-of function screen to select for the genes whose enhanced expression can bypass replicative senescence. We uncovered multiple genes known to be involved in p53 and Rb regulation, ATM regulation and two components of the CST complex involved in preventing telomere erosion and additional genes such as REST and FOXO4 that have been implicated in aging. Among the new genes now implicated in senescence we identified DLX2, a Homeobox transcription factor that has been shown to be required for tumor growth, metastasis and associates with poor cancer prognosis. Growth analysis showed that DLX2 expression led to increased cellular replicative lifespan. We found that DLX2 expression inhibited p53 activation, and DLX2 reduced the protein level of upstream activator kinases ATM and DNA-PK. Our data suggest that DLX2 expression reduces the protein components of the TTI1/TTI2/TEL2 complex, a key complex required for the proper folding and stabilization of ATM, DNA-PK and other members of the PIKK family. Over-expression of DLX2 exhibit mutual exclusivity with p53 alteration in cancer patients, suggesting DLX2 may attenuate the p53 pathway during tumor formation. Our functional screen identified novel players that may promote tumorigenesis by regulating the ATM-p53 pathway and senescence.Medical Sciences
8
Assefa, Kebebew. „Phenotypic and molecular diversity in the Ethiopian cereal, tef [Eragrostis tef (Zucc.) Trotter] : implications on conservation and breeding /“. Alnarp : Dept. of Crop Science, Swedish Univ. of Agricultural Sciences, 2003. http://epsilon.slu.se/a426-ab.html.
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9
Shen, Yu. „Neural Mechanisms of Gait Regulation and Olfactory Plasticity in Caenorhabditis elegans“. Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:14226051.
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One of the fundamental questions in biological science is to understand how the nervous system functions to generate behavior. The past decades have witnessed much progress in behavioral neuroscience, but it is often challenging to gain mechanistic insights at the molecular and cellular level. The small nervous system and experimental accessibility of the nematode Caenorhabditis elegans offer an opportunity to study neural mechanisms underlying behavior in greater detail. Because many of the genes and proteins are conserved across species, studies in C. elegans provide useful information to the broad research community. In this dissertation, I use the locomotory gait regulation and olfactory aversive learning as two examples to demonstrate that C. elegans neurobiology can offer unique insights into the organization of behavior in more complex organisms.
Chapter 2 of this dissertation characterizes a small neuronal circuit that modulates the amplitude of head deflection in C. elegans. C. elegans moves its head rhythmically along the dorsal-ventral axis during forward movement. By quantifying local head curvature, I found the cholinergic SMD neurons facilitate head deflection, whereas the GABAergic RME neurons restrain head deflection. I then examined the calcium dynamics in RME and found the activity is correlated with, but not dependent on, dorsal-ventral head movement. Using a combination of neurophysiological, behavioral and optogenetic approaches, I found that the SMD neurons drive the calcium oscillation in RME via cholinergic neurotransmission. In return, the activated RME releases GABA, tuning down SMD activity via the B-type GABA receptor, and negatively regulates the head bending amplitude. The interaction between SMD and RME contributes to an excitation-inhibition balance in the motor system, which fine-tunes the bending angle and thus optimizes the phase velocity during forward movement. This oscillatory circuit suggests a parsimonious model for a small neural network to regulate the locomotory gait.
The SMD motor neurons are also implicated in a sensori-motor circuit underlying olfactory learning. In Chapter 3, I investigate the plasticity of the circuit in pathogen-induced learning behavior. C. elegans learns to avoid the smell of pathogenic bacteria after being infected by the pathogen. I characterize a mutant that displays enhanced olfactory learning, eol-1, isolated from a forward genetic screen. eol-1 acts in the URX sensory neurons to inhibit learning. The protein product of eol-1 has many homologs in eukaryotes, including the mammalian protein Dom3Z implicated in pre-mRNA quality control. Expressing the mouse Dom3z in eol-1-expressing cells fully rescues the learning phenotype in eol-1 mutants, indicating that EOL-1 shares functional similarities with Dom3Z in regulating learning. Mutating the residues that are critical for the enzymatic activity of Dom3Z, and the equivalent residues in EOL-1, abolishes the function of these molecules in learning. These results provide insights into the function of a conserved protein in regulating experience-dependent behavioral plasticity.
In summary, this dissertation aims to understand how a small nervous system regulates complex behavior in C. elegans. I show that the neural circuits underlying rhythmic locomotion share common properties, and evolutionarily conserved molecules have similar functions in regulating neural plasticity. Some of the principles uncovered in C. elegans may be generalizable and informative to our understanding of the human brain.
10
Rosvall, Ola. „Enhancing gain from long-term forest tree breeding while conserving genetic diversity /“. Umeå : Swedish Univ. of Agricultural Sciences (Sveriges lantbruksuniv.), 1999. http://epsilon.slu.se/avh/1999/91-576-5643-6.pdf.
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11
Godavarty, Anuradha. „Fluorescence enhanced optical tomography on breast phantoms with measurements using a gain modulated intensified CCD imaging system“. Texas A&M University, 2003. http://hdl.handle.net/1969.1/2184.
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Fluorescence-enhanced optical imaging using near-infrared (NIR) light developed for in-vivo molecular targeting and reporting of cancer provides promising opportunities for diagnostic imaging. However, prior to the administration of unproven contrast agents, the benefits of fluorescence-enhanced optical imaging must be assessed in feasibility phantom studies.
A novel intensified charge-coupled device (ICCD) imaging system has been developed to perform 3-D fluorescence tomographic imaging in the frequency-domain using near-infrared contrast agents. This study is unique since it (i) employs a large tissue-mimicking phantom (~1087 cc), which is shaped and sized to resemble a female breast and part of the extended chest wall region, and (ii) enables rapid data acquisition in the frequency-domain by using a gain-modulated ICCD camera. Diagnostic 3-D fluorescence-enhanced optical tomography is demonstrated using 0.5-1 cc single and multiple targets contrasted from their surrounding by ??M concentrations of Indocyanine green (ICG) in the breast-shaped phantom (10 cm diameter), under varying conditions of target-to-background absorption contrast ratios (1:0 and 100:1) and target depths (up to 3 cm deep). Boundary surface fluorescence measurements of referenced amplitude and phase shift were used along with the coupled diffusion equation of light propagation in order to perform 3-D image reconstructions using the approximate extended Kalman filter (AEKF) algorithm, and hence differentiate the target from the background based on fluorescent optical contrast.
Detection of single and multiple targets is demonstrated under various conditions of target depths (up to 2 cm deep), absorption optical contrast ratio (1:0 and 100:1), target volumes (0.5-1 cc), and multiple targets (up to three 0.5 cc targets). The feasibility of 3-D image reconstructions from simultaneous multiple point excitation sources are presented. Preliminary lifetime imaging studies with 1:2 and 2:1 optical contrast in fluorescence lifetime of the contrast agents is also demonstrated. The specificity of the optical imager is further assessed from homogeneous phantom studies containing no fluorescently contrasted targets.
While nuclear imaging currently provides clinical diagnostic opportunities using radioactive tracers, molecular targeting of tumors using non-ionizing NIR contrast agents tomographically imaged using the frequency-domain ICCD imaging system could possibly become a new method of diagnostic imaging.
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Howard, C. Bradley Howard. „Development of gain-of-function reporters to probe trans-editing of misacylated tRNA in vivo“. The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1469118488.
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13
Kapasi, Purvi. „An Insight into GAIT Complex Mediated Translational Silencing“. Cleveland State University / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=csu1232567504.
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14
Miguez, Andrés. „Positional control of oligodendrocyte development : Role of hox homeoproteins and Tag-1 cell adhesion molecule“. Paris 6, 2010. http://www.theses.fr/2010PA066309.
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Les oligodendrocytes sont les cellules myélinisantes du système nerveux central. La gaine de myéline ainsi que l’oligodendrocyte qui la synthétise, sont les cibles du processus pathologique dans la Sclérose En Plaques (SEP), une maladie neurologique autoimmune et démyélinisante du système nerveux central. Les progrès thérapeutiques des 10-15 dernières années ont permis une meilleure prise en charge de la composante autoimmune de la maladie. En revanche nous sommes toujours aussi démuni pour corriger la composante démyélinisante, qui pourtant est la cause du handicap sensori-moteur permanent. Dans le but de mettre en place des thérapies de réparation myélinique chez les patients atteints de SEP, des efforts sont développés pour identifier des cibles thérapeutiques qui permettraient de contrôler la production et la dispersion des cellules précurseurs d’oligodendrocytes (OPCs) dans la moelle épinière et le cerveau. Le développement embryonnaire et néonatal constitue une période privilégiée pour examiner ces phénomènes. Dans ce contexte, mon travail de thèse a porté sur l’identification de nouvelles molécules contrôlant la spécification régionale et la migration des OPCs ainsi que leur interaction avec les axones dans le système nerveux central en développement chez la souris. Une première contribution, portant sur l’étude de la molécule d’adhésion TAG-1, m’a permis de montrer que TAG-1 régule la structure de la myéline et des axones dans le nerf optique de souris. Dans un deuxième temps, j’ai exploré l’origine des oligodendrocytes dans le tronc cérébral de souris, et mon travail m’a permis de montrer que le facteur de transcription Hoxa2 réprime l’oligodendrogenèse.
15
Peng, Jinrong. „Genetic and molecular analysis of the gai, ga4 and fhy2 mutations of Arabidopsis“. Thesis, University of East Anglia, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.359324.
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16
He, Lei. „III-nitride Semiconductors Grown By Plasma Assisted Molecular Beam Epitaxy“. VCU Scholars Compass, 2004. http://scholarscompass.vcu.edu/etd/1019.
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III-nitride semiconductors are of great interest owing to their commercial and military applications due to their optoelectronic and mechanical properties. They have been synthesized successfully by many growth methods. Among them, molecular beam epitaxy (MBE) is a promising epitaxial growth method owing to precise control of growth parameters, which significantly affect the film properties, composition, and thickness. However, the understanding of growth mechanism of III-nitride materials grown in this growth regime is far from being complete.In this dissertation, GaN and AIGaN growth mechanism under metal-rich conditions were investigated. The Ga surface desorption behavior during the growth was investigated systematically using reflection high-energy electron diffraction (RHEED). It was found that desorption of Ga atoms from the (0001) GaN surfaces under different III-V ratios deviates from the zeroth-order kinetics in that the desorption rate is independent of the coverage of adsorbed atoms. The desorption energies of Ga are determined to be 2.76 eV with the Ga coverage closing to 100%, 1.89 eV for a ~45% coverage, and 0.82 eV for a 10% coverage, as monitored by the change of the RHEED specular beam intensity during growth. In addition, the GaN surface morphology under different III-V ratios on porous templates matches the dependence of Ga desorption energy on the metal coverage, and III/V ratio dominates the growth mode. In a related AIGaN growth mechanism study, a competition between A1 and Ga atoms to incorporate into the film was found under metal-rich conditions. Employing this mechanism, A1xGa1-xN layers with precisely controlled A1 mole fraction, x in the range 0xxGa1-xN films was determined to be about 1 eV. The A1xGa1-xN layers grown under metal-rich conditions, as compared to that under N-rich conditions, have a better structural and optical quality. Employing A1xGa1-xN layers grown under metal-rich conditions, a lateral geometry GaN/A1GaN MQW-based photodetector was fabricated. It exhibited a flat and narrow spectral response in the range of 297~352 nm in the backillumination configuration.
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Bougacha, Nadia. „Molecular characterization and functional analysis of poor-prognosis B-cell leukemias“. Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS146.
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L’objectif général de cette thèse a été d’approfondir la compréhension actuelle des bases génétiques et de la pathophysiologie de leucémies B agressives, à savoir deux sous-types de leucémie lymphoïde chronique (LLC) et la leucémie prolymphocytaire B (LPLB). La LLC, qui est la forme de leucémie adulte la plus fréquente en Occident, est caractérisée par une accumulation de lymphocytes B monoclonaux (CD20+, CD5+ and CD23+) dans le sang périphérique, la moelle osseuse ainsi que les organes lymphoïdes secondaires. La LLC est une maladie très hétérogène, où un large panel d’altérations génétiques mènent à des résultats cliniques variables. Le gain du bras court du chromosome 2 (gain 2p) est une anomalie chromosomique récurrente dans la LLC et d’autres cancers. Notre groupe avait décrit l’association du gain 2p avec la résistance aux traitements et des facteurs de mauvais pronostic comme l’IGHV non-muté et la délétion 11q. Des analyses cytogénétiques et moléculaires nous ont notamment permis d’identifier une région minimale de gain incluant entre autres les gènes XPO1 et REL. Dans la partie principale de ma thèse, l’analyse fonctionnelle du rôle de REL, par trois stratégies complémentaires d’inhibition pharmacologique, d’inactivation du gène et d’activation transcriptionelle, a permis l’identification de REL comme un acteur central de la survie cellulaire dans la LLC. De plus, j’ai développé plusieurs modèles cellulaires de LLC permettant la surexpression de n’importe quel gène, seul ou en combinaison, pour approfondir les études sur REL et XPO1, et identifier de potentielles coopérations oncogéniques menant au phénotype des LLC avec gain 2p. Enfin, nous avons analysé la hiérarchie et l’évolution clonale des anomalies chromosomiques dans les LLC avec gain 2p. La LLC avec délétion 17p est associée avec une absence de réponse aux traitements standards et donc avec la pire issue clinique possible. Nous avons montré que la délétion 17p et le gain 8q24 ont un impact synergique sur le résultat clinique, et que les patients ayant cette LLC « double-hit » ont un pronostic particulièrement défavorable. La LPLB est un lymphome agressif, habituellement résistant aux chimio-immunothérapies standard, et défini par la présence de prolymphocytes dans le sang périphérique excédant 55% des cellules lymphoïdes. Nous décrivons les aspects cytogénétiques et moléculaires d’une large cohorte de 34 cas de LPLB, ainsi que leurs réponses aux nouveaux inhibiteurs spécifiques in vitro. In fine, notre travail a permis une meilleure compréhension de la LLC et de la LPLB, et d’ouvrir la voie au développement de nouvelles stratégies thérapeutiquesThe overall aim of this thesis consisted of expanding the current understanding of the genetic basis and physiopathology of aggressive B-cell leukemias, namely in chronic lymphocytic leukemia (CLL) subtypes and in B-cell prolymphocytic leukemia (B-PLL). CLL, the most common form of adult leukemia in the West, is characterized by an accumulation of monoclonal B cells (CD20+, CD5+ and CD23+) in the peripheral blood, bone marrow, and secondary lymphoid organs. CLL is a highly heterogeneous disease, with a large panel of genetic alterations leading to variable clinical outcomes. Gain of the short arm of chromosome 2 (2p gain) is a frequent chromosomal abnormality in CLL and in other malignancies. Our group has reported that 2p gain was associated with drug refractoriness and poor prognosis factors such as unmutated IGHV and 11q deletion. Using cytogenetic and molecular analyses, we have notably identified a minimal region of gain which encompasses among others XPO1 and REL. In my main thesis project, functional analysis of the role of REL, using three complementary strategies of pharmacological inhibition, gene knockout and transcriptional activation, led to its identification as a key player driving cell survival in CLL. Moreover, I developed several CLL cellular models that allow the overexpression of any gene, alone or in combination, in order to further investigate the roles of REL and XPO1 in CLL and identify potential oncogenic cooperation driving phenotypic features of 2p gain CLL. Finally, we have analyzed the hierarchy and the clonal evolution of the chromosomal abnormalities in 2p gain CLL. CLL with 17p deletion, del(17p), is associated with a lack of response to standard treatment and thus the worst clinical outcome. Our findings showed that del(17p) and 8q24 gain have a synergistic impact on outcome, therefore patients with this “double-hit” CLL have a particularly poor prognosis. B-PLL is an aggressive leukemia, usually resistant to standard chemo-immuno therapy, defined by the presence of prolymphocytes in peripheral blood exceeding 55% of lymphoid cells. We described the cytogenetic and molecular features of a large cohort of 34 B-PLL cases, as well as their in vitro response to novel targeted drugs. Altogether, this work enabled a better understanding of CLL and B-PLL, as well as paving the way for the development of novel therapeutic strategies
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Michaud, Amadeo. „III-V / Silicon tandem solar cell grown with molecular beam epitaxy“. Electronic Thesis or Diss., Sorbonne université, 2019. http://www.theses.fr/2019SORUS247.
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Le photovoltaïque terrestre est actuellement largement dominé par des dispositifs à base de Silicium. La limite théorique d’efficacité de photoconversion pour les cellules solaires en silicium est de l’ordre de 29%. Avec des modules photovoltaïques ayant une efficacité de 26.3% sur le marché, la filière Si est à un niveau de maturité avancée et exploite déjà la quasi-totalité du potentiel de ce genre de cellule solaires. Le travail exposé ici traite d’une autre voie d’amélioration de l’efficacité de conversion des dispositifs photovoltaïques. En effet, les cellules solaires tandem, assemblées en empilant plusieurs cellules permettent de dépasser les limites associées aux cellules Si. La complémentarité importante des cellules solaire III-V avec les cellules Si permettrai en théorie d’atteindre plus de 40% d’efficacité. Cette thèse vise à l’élaboration de cellule III-V performante et compatible avec un usage en tandem. Dans un premier temps, l’épitaxie d’alliages phosphures a été étudiée et en particulier l’influence des conditions de croissance sur le GaInP. Une réduction de la pression en phosphore durant la croissance provoque des modulations de composition au sein de l’alliage. La température a un impact significatif sur la valeur de bande interdite qui diminue en augmentant la température. Des caractérisations de photoluminescence ont permis de définir les conditions optimales de croissance en maximisant le signal de luminescence de l’alliage. L’étude a notamment révélé que cru dans les conditions choisies, le GaInP présente moins de défaut et d’états profonds qu’à plus faibles températures de croissance. Enfin la capacité à atteindre des niveaux de dopages élevés dans l’alliage AlGaInP et l’impact de sa composition sur le dopage ont étés étudié. Dans un second temps, la structure des cellules solaires simple jonction GaInP a été optimisée. Nous illustrons l’impact de la passivation de la surface des cellules par AlInP et AlGaInP, ainsi que l’amélioration du photo-courant par l’amincissement de l’émetteur dopé n. L’introduction de couche non-dopée dans la structure ne permit pas de remédier au problème de collection des porteurs constaté dans les cellules. La couche limitant l’efficacité des cellules est composée de p-GaInP. Des caractérisations par Cathodoluminescence et Fluorescence résolue en temps d’échantillons identiques à cette couche ont été menées. Elles ont mis en avant une faible longueur de diffusion des porteurs générés dans le matériau. La comparaison de ces propriétés avec la littérature et celle mesurées pour GaInP épitaxié par MOCVD, indique que l’amélioration de l’efficacité des cellules passe par une augmentation de la mobilité des porteurs au sein du GaInP. Une solution pratique, combinant GaInP et AlGaAs dans une cellule à hétérojonction a été mise en œuvre. Ce type de structure est une autre perspective intéressante à l’avenir puisque des efficacités à l’état de l’art ont été mesurées. Enfin nous avons développé un procédé permettant d’adapter les cellules pour un usage tandem. Les structures sont crues en inversé puis transférées sur verre ou wafer de silicium sans endommager leur performance. Toutefois, des améliorations sont toujours nécessaires pour permettre l’assemblage d’une cellule tandem fonctionnelle. En effet, la non-planéité introduite par les contacts arrières de la cellule III-V cause actuellement des problèmes de collageTerrestrial photovoltaic is dominated by Silicon based devices. For this type of solar cells, the theory predicts an efficiency limit of 29%. With photovoltaic modules showing 26.6% efficiency already, Silicon-based modules is a mature technology and harvest almost their full potential. In this work, we intend to explore another path toward the enhancement of photovoltaic conversion efficiency. Tandem solar cells that consist in stacking sub-cells, allow to overcome the Si efficiency limit. Since solar cells made of III-V semiconductors are complementary to Silicon solar cells, theory predicts that efficiency above 40% is attainable when combining those types of cells. Here we focus on the elaboration of a performant III-V solar cell, compatible for a tandem use. The first stage of the PhD was to build know-how on phosphide alloys epitaxy with MBE. The influence of the growth conditions on GaInP properties was studied. We noted that composition modulations appear in the alloy when grown with low phosphorus pressure. The growth temperature also impacts the material bandgap, which reduces while increasing the temperature. Photoluminescence characterization served to select the best growth conditions by maximizing the photoluminescence efficiency. We could also highlight that in the conditions chosen, the GaInP exhibits less defect states. AlGaInP alloys are used for passivation purposes in the cells, the influence of the composition of the alloy on the Beryllium doping efficiency was studied. Then GaInP single junction solar cells were fabricated. The different layers composing the cells were optimized. The impact of the front surface passivation with AlGaInP and AlInP was emphasized; improvement of the cell photocurrent by the thinning of the n-doped GaInP layer was also demonstrated. The introduction of a non-intentionally-doped layer in the structure was tested in order to remedy the limits encountered with photocurrent collection. The p-GaInP composing the cells was eventually identified as the limiting factor. In depth characterization of samples mimicking the limiting layer was performed with cathodoluminescence and time-resolved fluorescence. A small diffusion length of the generated carriers was evidenced. Comparison with MOVPE and with literature values suggests that improving the carrier mobility in this layer is the main route to follow for improving of the GaInP cell efficiency. A practical solution was proposed and implemented: we designed a cell combining GaInP and AlGaAs in a heterojunction cell. This structure proves to be very relevant for the project since state of the art photoconversion efficiency of 18.7% was obtained. Finally a process was developed to adapt the III-V solar cells to the tandem configuration. Inverted PV cells structures were grown and transferred on glass or Silicon hosts without degradation of their efficiency. Further improvement of the process is needed to build a full tandem device, in particular the back metallization of the III-V cells must be compatible with the bonding of the cells on the host substrate
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Schiaber, Ziani de Souza. „Nanoestruturas de GaN crescidas pelas técnicas de epitaxia por magnetron sputtering e epitaxia por feixe molecular“. Universidade Estadual Paulista (UNESP), 2016. http://hdl.handle.net/11449/138237.
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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Nanosestruturas de GaN destacam-se devido à baixa densidade de defeitos e consequentemente alta qualidade estrutural e óptica quando comparadas ao material em forma de filme. O entendimento dos mecanismos de formação de nanofios e nanocolunas de GaN por diferentes técnicas é fundamental do ponto de vista da ciência básica e também para o aprimoramento da fabricação de dispositivos eletrônicos e optoeletrônicos baseados nesse material. Neste trabalho discorre-se sobre a preparação e caracterização de nanofios e nanoestruturas de GaN pelas técnicas de epitaxia por magnetron sputtering e epitaxia por feixe molecular em diferentes tipos de substratos. Pela técnica de epitaxia por magnetron sputtering foram obtidos nanocristais e nanocolunas de GaN, além de uma região com camada compacta. Visando criar uma atmosfera propícia para o crescimento de nanoestruturas de GaN não coalescida, atmosfera de N2 puro e um anteparo, situado entre o alvo e o porta-substratos, foram utilizados. O anteparo causou diferença no fluxo incidente de gálio no substrato, ocasionando a formação de diferentes tipos de estruturas. A caracterização das amostras se deu principalmente através de medidas de microscopia eletrônica de varredura, difração de raios X e espectroscopia de fotoluminescência. As nanocolunas, de 220 nm de altura, foram formadas na região distante 2 mm do centro da sombra geométrica do orifício do anteparo e apresentaram orientação [001] perpendicular ao substrato, comumente encontrada em nanofios de GaN depositados por MBE. Em relação aos nanofios obtidos pela técnica de MBE, investigou-se a possibilidade de controlar a densidade de nanofios através de uma camada de Si sobre o GaN–Ga polar visando inibir a coalescência. Diferentes quantidades de Si foram depositadas e a densidade dos nanofios foi diferenciada significativamente. Os nanofios apresentaram densidade média de 108 nanofios/cm2 com 0,60 nm de espessura da camada de Si. Espessuras menores não resultaram no crescimento de nanofios, porém espessuras superiores causaram uma alta densidade de nanofios de 1010 nanofios/cm2 que permaneceu constante, independentemente do tempo de deposição. Medidas de polo por difração de raios X evidenciaram que os nanofios nuclearam-se orientados e em uma camada cristalina de Si ou SixNy. Experimentos de ataque químico com KOH indicaram a polaridade N para o nanofio e as medidas de difração por feixe convergente confirmaram a polaridade de N para o nanofio e Ga para a buffer layer. Os resultados obtidos neste trabalho permitiram um melhor entendimento da nucleação e dos mecanismos de formação de nanoestruturas de GaN, viabilizando maior controle das características dessas nanoestruturas produzidas.
GaN nanowires and nanocolumns stand out due to the low defect density and high structural and optical quality compared to the corresponding thin films. The understanding of the formation mechanism of the different GaN structures using different techniques is critical to improving the manufacture of the electronic and optoelectronic devices based on this material. This thesis focuses on the preparation and characterization of GaN nanowires and nanostructures. The molecular bem epitaxy (MBE) and magnetron sputtering epitaxy (MSE) were used and different substrates were tested. Concerning GaN nanocrystals and nanocolumns obtained by MSE, optimization of the deposition conditions was necessary in order to produce non-coalesced GaN nanostructures. The best conditions were: pure N2 atmosphere, silicon substrate, and a perforated screen placed between the target and the substrate holder. The later produced differences on the Ga flow to the substrate, inducing the formation of different structures, depending on the position of growth spot. Samples were characterized using scanning electron microscopy, X-ray diffraction and photoluminescence spectroscopy. Nanocolumns were observed, mainly in sites corresponding to a disc of radius 2 mm from the geometric centre of the hole. The columns were oriented with the GaN [001] axis perpendicular to the Si (111) substrate surface, situation which is commonly found in GaN nanowires deposited by MBE. Regarding the nanowires prepared by MBE technique, in order to inhibit coalescence and to investigate the possibility of controlling the numerical density of nanowires, we have used Si cap layers on top of the Ga-polar GaN buffer layer. Different amounts of Si have been deposited, and the density of the nanowires was significantly modified. With Si layer thickness of 0.60 nm, the nanowires had an average density of 108 nanowires/cm2 . Lower thickness did not result in the growth of nanowires, but higher thickness caused a high density of nanowires of 1010 nanowires/cm2 which remained constant regardless of the deposition time. X-ray diffraction pole figures showed that the different nanowires grown up in oriented fashion in a crystalline layer of Si or SixNy. Etching with KOH indicated N polarity for the grown nanowires, in spite of the fact that they were grown using Ga polar GaN buffer layers. Measurements by convergent beam electron diffraction confirmed the N polarity to the nanowire and Ga polarity for the buffer layer. Aspects obtained in this study allowed a better understanding of nucleation and nanostructures formation mechanisms of GaN, enabling greater control of the characteristics of these nanostructures produced.
FAPESP: 2011/22664-2
FAPESP: 2013/25625-3
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Yu, Kuan-Hung. „Optical Spectroscopy of GaN/Al(Ga)N Quantum Dots Grown by Molecular Beam Epitaxy“. Thesis, Department of Physics, Chemistry and Biology, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-19821.
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GaN quantum dots grown by molecular beam epitaxy are examined by micro-photoluminescence. The exciton and biexciton emission are identified successfully by power-dependence measurement. With two different samples, it can be deduced that the linewidth of the peaks is narrower in the thicker deposited layer of GaN. The size of the GaN quantum dots is responsible for the binding energy of biexciton (EbXX); EbXX decreases with increasing size of GaN quantum dots. Under polarization studies, polar plot shows that emission is strongly linear polarized. In particular, the orientation of polarization vector is not related to any specific crystallography orientation. The polarization splitting of fine-structure is not able to resolve due to limited resolution of the system. The emission peaks can be detected up to 80 K. The curves of transition energy with respect to temperature are S-shaped. Strain effect and screening of electric field account for blueshift of transition energy, whereas Varshni equation stands for redshifting. Both blueshifting and redshifting are compensated at temperature ranging from 4 K to 40 K.
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Poddar, Darshana Ph D. „Study of Role of Ribosomal Protein L13a in Resolving Inflammation“. Cleveland State University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=csu1400587453.
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Zettler, Johannes Kristian. „Growth of GaN nanowire ensembles in molecular beam epitaxy: Overcoming the limitations of their spontaneous formation“. Doctoral thesis, Humboldt-Universität zu Berlin, 2018. http://dx.doi.org/10.18452/18926.
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Dichte Ensembles aus GaN-Nanodrähten können in der Molekularstrahlepitaxie mithilfe eines selbstinduzierten Prozesses sowohl auf kristallinen als auch amorphen Substraten gezüchtet werden. Aufgrund der Natur selbstgesteuerter Prozesse ist dabei die Kontrolle über viele wichtige Ensembleparameter jedoch eingeschränkt.
Die Arbeit adressiert genau diese Einschränkungen bei der Kristallzucht selbstinduzierter GaN-Nanodrähte. Konkret sind das Limitierungen bezüglich der Nanodraht-Durchmesser, die Nanodraht-Anzahl-/Flächendichte, der Koaleszenzgrad sowie die maximal realisierbare Wachstumstemperatur. Für jede dieser Einschränkungen werden Lösungen präsentiert, um die jeweilige Limitierung zu umgehen oder zu verschieben. Als Resultat wurde eine neue Klasse von GaN Nanodrähten mit bisher unerreichten strukturellen und optischen Eigenschaften geschaffen.
Mithilfe eines Zwei-Schritt-Ansatzes, bei dem die Wachstumstemperatur während der Nukleationsphase erhöht wurde, konnte eine verbesserte Kontrolle über die Flächendichte, den Durchmesser und den Koaleszenzgrad der GaN-Nanodraht-Ensembles erreicht werden.
Darüber hinaus werden Ansätze präsentiert, um die außerordentlich lange Inkubationszeit bei hohen Wachstumstemperaturen zu minimieren und damit wesentlich höhere Wachstumstemperaturen zu ermöglichen (bis zu 905°C). Die resulierenden GaN-Nanodraht-Ensembles weisen schmale exzitonische Übergänge mit sub-meV Linienbreiten auf, vergleichbar zu denen freistehender GaN-Schichten.
Abschließend wurden Nanodrähte mit Durchmessern deutlich unterhalb von 10 nm fabriziert. Mithilfe eines Zersetzungsschrittes im Ultrahochvakuum direkt im Anschluss an die Wachstumsphase wurden reguläre Nanodraht-Ensembles verdünnt. Die resultierenden ultradünnen Nanodrähte weisen dielektrisches Confinement auf. Wir zeigen eine ausgeprägte exzitonische Emission von puren GaN-Nanodrähten mit Durchmessern bis hinab zu 6 nm.In molecular beam epitaxy, dense arrays of GaN nanowires form spontaneously on crystalline as well as amorphous substrates. Due to the nature of spontaneous formation, the control over important parameters is limited. This thesis addresses the major limitations of spontaneous nanowire formation, namely the nanowire diameter, number density, and coalescence degree but also the maximum achievable growth temperature, and presents approaches to overcome the same. Thereby, we have fabricated a new class of nanowires with unprecedented structural and optical properties. We find that a two-step growth approach, where the substrate temperature is increased during the nucleation stage, is an efficient method to gain control over the area coverage, average diameter, and coalescence degree of GaN nanowire ensembles. Furthermore, we present growth approaches to minimize the long incubation time that precedes nanowire nucleation at elevated temperatures and to thus facilitate significantly higher growth temperatures (up to 905°C). The GaN nanowire ensembles grown at so far unexplored substrate temperatures exhibit excitonic transitions with sub-meV linewidths comparable to those of state-of-the-art free-standing GaN layers grown by hydride vapor phase epitaxy. Finally, we fabricate nanowires with diameters well below 10 nm, the lower boundary given by the nucleation mechanism of spontaneously formed nanowires. Here, regular nanowire arrays are thinned in a post-growth decomposition step in ultra-high vacuum. In situ monitoring the progress of decomposition using quadrupole mass spectrometry enables a precise control over the diameter of the thinned nanowires. These ultrathin nanowires show dielectric confinement, which is potentially much stronger than quantum confinement. We demonstrate intense excitonic emission from bare GaN nanowires with diameters down to 6 nm.
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Kour, Ravinder. „Insights into the ribosomal, extra-ribosomal and developmental role of RP L13a in mammalian model“. Cleveland State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=csu1572548728931568.
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Clark, Kendal. „Ultra High Vacuum Low Temperature Scanning Tunneling Microscope for Single Atom Manipulation on Molecular Beam Epitaxy Grown Samples“. Ohio University / OhioLINK, 2005. http://www.ohiolink.edu/etd/view.cgi?ohiou1125611713.
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Müllhäuser, Jochen R. „Properties of Zincblende GaN and (In,Ga,Al)N Heterostructures grown by Molecular Beam Epitaxy“. Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 1999. http://dx.doi.org/10.18452/14382.
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Während über hexagonales (alpha) GaN zum ersten Mal 1932 berichtet wurde, gelang erst 1989 die Synthese einer mit Molekularstrahlepitaxie (MBE) auf 3C-SiC epitaktisch gewachsenen, metastabilen kubischen (eta) GaN Schicht. Die vorliegende Arbeit befaßt sich mit der Herstellung der Verbindungen eta-(In,Ga,Al)N mittels RF-Plasma unterstützter MBE auf GaAs(001) und den mikrostrukturellen sowie optischen Eigenschaften dieses neuartigen Materialsystems. Im Vergleich zur hexagonalen bietet die kubische Kristallstruktur auf Grund ihrer höheren Symmetrie potentielle Vorteile für die Anwendung in optischen und elektronischen Bauelementen. Viele wichtige Materialgrößen der kubischen Nitride sind jedoch noch gänzlich unbekannt, da sich die Synthese einkristalliner Schichten als sehr schwierig erweist. Das Ziel dieser Arbeit ist es daher erstens, die technologischen Grenzen der Herstellung von bauelementrelevanten kubischen (In,Ga,Al)N Heterostrukturen auszuweiten und zweitens, einen Beitrag zur Aufklärung der bis dato wenig bekannten optischen und elektronischen Eigenschaften des GaN und der Mischkristalle In GaN zu leisten. Zunächst wird ein optimierter MBE Prozess unter Einsatz einer Plasmaquelle hohen Stickstofflusses vorgestellt, welcher nicht nur die reproduzierbare Epitaxie glatter, einphasiger GaN Nukleationsschichten auf GaAs ermöglicht. Vielmehr können damit auch dicke GaN. Schichten mit glatter Oberflächenmorphologie hergestellt werden, welche die Grundlage komplizierterer eta-(In,Ga,Al)N Strukturen bilden. An einer solchen GaN Schicht mit einer mittleren Rauhigkeit von nur 1.5 nm werden dann temperaturabhängige Reflexions- und Transmissionsmessungen durchgeführt. Zur Auswertung der Daten wird ein numerisches Verfahren entwickelt, welches die Berechnung des kompletten Satzes von optischen Konstanten im Spektralgebiet 2.0 = 0.4 wären grün-gelbe Laserdioden. Zusammenfassung in PostScriptWhile the earliest report on wurtzite (alpha) GaN dates back to 1932, it was not until 1989 that the first epitaxial layer of metastable zincblende (eta) GaN has been synthesized by molecular beam epitaxy (MBE) on a 3C-SiC substrate. The present work focuses on radio frequency (RF) plasma-assisted MBE growth, microstructure, and optical properties of the eta-(In,Ga,Al)N material system on GaAs(001). Due to their higher crystal symmetry, these cubic nitrides are expected to be intrinsically superior for (opto-) electronic applications than the widely employed wurtzite counterparts. Owing to the difficulties of obtaining single-phase crystals, many important material constants are essentially unknown for the cubic nitrides. The aim of this work is therefore, first, to push the technological limits of synthesizing device-relevant zincblende (In,Ga,Al)N heterostructures and, second, to determine the basic optical and electronic properties of GaN as well as to investigate the hardly explored alloy InGaN. An optimized MBE growth process is presented which allows not only the reproducible nucleation of smooth, monocrystalline GaN layers on GaAs using a high-nitrogen-flow RF plasma source. In particular, thick single-phase GaN layers with smooth surface morphology are obtained being a prerequisite for the synthesis of ternary eta-(Ga,In,Al)N structures. Temperature dependent reflectance and transmittance measurements are carried out on such a GaN film having a RMS surface roughness as little as 1.5 nm. A numerical method is developed which allows to extract from these data the complete set of optical constants for photon energies covering the transparent as well as the strongly absorbing spectral range (2.0 -- 3.8 eV). Inhomogeneities in the refractive index leading to finite coherence effects are quantitatively analyzed by means of Monte Carlo simulations. The fundamental band gap EG(T) of GaN is determined for 5 < T < 300 K and the room temperature density of states is investigated. Systematic studies of the band edge photoluminescence (PL) in terms of transition energies, lineshapes, linewidths, and intensities are carried out for both alpha- and GaN as a function of temperature. Average phonon energies and coupling constants, activation energies for thermal broadening and quenching are determined. Excitation density dependent PL measurements are carried out for both phases in order to study the impact of nonradiative recombination processes at 300 K. A recombination model is applied to estimate the internal quantum efficiency, the (non)radiative lifetimes, as well as the ratio of the electron to hole capture coefficients for both polytypes. It is seen that the dominant nonradiative centers in the n-type material investigated act as hole traps which, however, can be saturated at already modest carrier injection rates. In summary, despite large defect densities in GaN due to highly mismatched heteroepitaxy on GaAs, band edge luminescence is observed up to 500 K with intensities comparable to those of state-of-the-art alpha-GaN. For the first time, thick InGaN films are fabricated on which blue and green luminescence can be observed up to 400 K for x=0.17 and x=0.4, respectively. Apart from bulk-like InGaN films, the first coherently strained InGaN/GaN (multi) quantum wells with In contents as high as 50 % and abrupt interfaces are grown. This achievement shows that a ternary alloy can be synthesized in a metastable crystal structure far beyond the miscibility limit of its binary constituents despite the handicap of highly lattice mismatched heteroepitaxy. The well widths of these structures range between 4 and 7 nm and are thus beyond the theoretically expected critical thickness for the strain values observed. It is to be expected that even higher In contents can be reached for film thicknesses below 5 nm. The potential application of such InGaN/GaN multi quantum wells with x >= 0.4 would thus be diode lasers operating in the green-yellow range. abstract in PostScript
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Ye, Wei. „Nano-epitaxy modeling and design: from atomistic simulations to continuum methods“. Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/50304.
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The dissertation starts from the understanding of dislocation dissipation mechanism due to the image force acting on the dislocation. This work implements a screw dislocation in solids with free surfaces by a novel finite element model, and then image forces of dislocations embedded in various shaped GaN nanorods are calculated. As surface stress could dramatically influence the behavior of nanostructures, this work has developed a novel analytical framework to solve the stress field of solids with dislocations and surface stress. It is successfully implemented in this framework for the case of isotropic circular nanowires (2D) and the analytical result of the image force has been derived afterwards. Based on the finite element analysis and the analytical framework, this work has a semi-analytical solution to the image force of isotropic nanorods (3D) with surface stress. The influences of the geometrical parameter and surface stress are illustrated and compared with the original finite element result. In continuation, this work has extended the semi-analytical approach to the case of anisotropic GaN nanorods. It is used to analyze image forces on different dislocations in GaN nanorods oriented along polar (c-axis) and non-polar (a, m-axis) directions. This work could contribute to a wide range of nanostructure design and fabrication for dislocation-free devices.
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Ajay, Akhil. „Nanofils de GaN/AlGaN pour les composants quantiques“. Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY030/document.
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Ce travail se concentre sur l'ingénierie Intersubband (ISB) des nanofils où nous avons conçu des hétérostructures de GaN / (Al, Ga) N intégrées dans un nanofil GaN pour le rendre optiquement actif dans la région spectrale infrarouge (IR), en utilisant un faisceau moléculaire assisté par plasma épitaxie comme méthode de synthèse. Les transitions ISB se réfèrent aux transitions d'énergie entre les niveaux confinés quantiques dans la bande de conduction de la nanostructure.Un contrôle précis des niveaux élevés de dopage est crucial pour les dispositifs ISB. Par conséquent, nous explorons Ge comme un dopant alternatif pour GaN et AlGaN, pour remplacer le Si couramment utilisé. Nous avons cultivé des couches minces de GaN dopé Ge avec des concentrations de porteurs atteignant 6,7 × 1020 cm-3 à 300 K, bien au-delà de la densité de Mott, et nous avons obtenu des couches minces conductrices AlxGa1-xN dopées Ge avec une fraction molaire Al jusqu'à x = 0,64. Dans le cas de GaN, la présence de Ge n'affecte pas la cinétique de croissance ou les propriétés structurales des échantillons. Cependant, dans des échantillons AlxGa1-xN dopés par Ge avec x> 0,4, la formation de grappes riches en Ge a été observée, avec une baisse de la concentration du porteur.Ensuite, nous avons réalisé une étude comparative du dopage Si vs Ge dans des hétérostructures GaN / AlN pour des dispositifs ISB dans la gamme IR à courte longueur d'onde. Nous considérons les architectures planaire et nanofils avec des niveaux de dopage et des dimensions de puits identiques. Sur la base de cette étude, nous pouvons conclure que les deux Si et Ge sont des dopants appropriés pour la fabrication d'hétérostructures GaN / AlN pour l'étude des phénomènes optoélectroniques ISB, à la fois dans les hétérostructures planaires et nanofils. Dans cette étude, nous rapportons la première observation de l'absorption d'ISB dans des puits quantiques GaN / AlN dopés au Ge et dans des hétérostructures de nanofils GaN / AlN dopés au Si. Dans le cas des nanofils, nous avons obtenu une largeur de ligne d'absorption ISB record de l'ordre de 200 meV. Cependant, cette valeur est encore plus grande que celle observée dans les structures planaires, en raison des inhomogénéités associées au processus de croissance auto-assemblé.En essayant de réduire les inhomogénéités tout en gardant les avantages de la géométrie des nanofils, nous présentons également une analyse systématique de l'absorption de l'ISB dans les micro et nanopillars résultant d'un traitement top-down des hétérostructures planaires GaN / AlN. Nous montrons que lorsque l'espacement du réseau de piliers est comparable aux longueurs d'onde sondées, les résonances des cristaux photoniques dominent les spectres d'absorption. Cependant, lorsque ces résonances sont à des longueurs d'onde beaucoup plus courtes que l'absorption ISB, l'absorption est clairement observée, sans aucune dégradation de son amplitude ou de sa largeur de raie.Nous explorons également la possibilité d'étendre cette technologie de nanofils à des longueurs d'onde plus longues, pour les absorber dans la région IR à mi-longueur d'onde. En utilisant des hétérostructures de nanofils GaN / AlN, nous avons fait varier la largeur du puits GaN de 1,5 à 5,7 nm, ce qui a conduit à un décalage rouge de l'absorption ISB de 1,4 à 3,4 μm. Remplaçant les barrières AlN par Al0.4Ga0.6N, le composé ternaire représente une réduction de la polarisation, ce qui conduit à un nouveau décalage rouge des transitions ISB à 4,5-6,4 um.L'observation de l'absorption de l'ISB dans des ensembles de nanofils nous a motivés pour le développement d'un photodétecteur infrarouge à puits quantiques à base de nanofils. La première démonstration d'un tel dispositif, incorporant une hétérostructure de nanofils GaN / AlN qui absorbe à 1,55 μm, est présentée dans ce manuscritDue to its novel properties nanowires have emerged as promising building blocks for various advanced device applications. This work focuses on Intersubband (ISB) engineering of nanowires where we custom design GaN/(Al,Ga)N heterostructures to be inserted in a GaN nanowire to render it optically active in the infrared (IR) spectral region. ISB transitions refer to energy transitions between quantum confined levels in the conduction band of the nanostructure. All the structures analised in this thesis were synthesized by plasma-assisted molecular beam epitaxy.Precise control of high doping levels is crucial for ISB devices. Therefore, we explored Ge as an alternative dopant for GaN and AlGaN, to replace commonly-used Si. We grew Ge-doped GaN thin films with carrier concentrations of up to 6.7 × 1020 cm−3 at 300 K, well beyond the Mott density, and we obtained conductive Ge-doped AlxGa1-xN thin films with an Al mole fraction up to x = 0.66. In the case of GaN, the presence of Ge does not affect the growth kinetics or structural properties of the samples. However, in Ge doped AlxGa1-xN samples with x > 0.4 the formation of Ge rich clusters was observed, together with a drop in the carrier concentration.Then, we performed a comparative study of Si vs. Ge doping in GaN/AlN heterostructures for ISB devices in the short-wavelength IR range. We considered both planar and nanowire architectures with identical doping levels and well dimensions. Based on this study, we concluded that both Si and Ge are suitable dopants for the fabrication of GaN/AlN heterostructures for the study of ISB optoelectronic phenomena, both in planar and nanowire heterostructures. Within this study, we reported the first observation of ISB absorption in Ge-doped GaN/AlN quantum wells and in Si-doped GaN/AlN nanowire heterostructures. In the case of nanowires, we obtained a record ISB absorption linewidth in the order of 200 meV. However, this value is still larger than that observed in planar structures, due to the inhomogeneities associated to the self-assembled growth process.Trying to reduce the inhomogeneities while keeping the advantages of the nanowire geometry, we also presented a systematic analysis of ISB absorption in micro- and nanopillars resulting from top-down processing GaN/AlN planar heterostructures. We showed that, when the spacing of the pillar array is comparable to the probed wavelengths, photonic crystal resonances dominate the absorption spectra. However, when these resonances are at much shorter wavelengths than the ISB absorption, the absorption is clearly observed, without any degradation of its magnitude or linewidth.We also explore the possibility to extend this nanowire technology towards longer wavelengths, to absorb in the mid-wavelength IR region. Using GaN/AlN nanowire heterostructures, we varied the GaN well width from 1.5 to 5.7 nm, which led to a red shift of the ISB absorption from 1.4 to 3.4 µm. Replacing the AlN barriers by Al0.4Ga0.6N, the reduction of polarization led to a further red shift of the ISB transitions to 4.5-6.4 µm.The observation of ISB absorption in nanowire ensembles motivated us for the development of a nanowire-based quantum well infrared photodetector (NW-QWIP). The first demonstration of such a device, incorporating a GaN/AlN nanowire heterostructure that absorbs at 1.55 µm, is presented in this manuscript
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Comyn, Rémi. „Développement de briques technologiques pour la co-intégration par l'épitaxie de transistors HEMTs AlGaN/GaN sur MOS silicium“. Thesis, Université Côte d'Azur (ComUE), 2016. http://www.theses.fr/2016AZUR4098.
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L’intégration monolithique hétérogène de composants III-N sur silicium (Si) offre de nombreuses possibilités en termes d’applications. Cependant, gérer l’hétéroépitaxie de matériaux à paramètres de maille et coefficients de dilatation très différents, tout en évitant les contaminations, et concilier des températures optimales de procédé parfois très éloignées requière inévitablement certains compromis. Dans ce contexte, nous avons cherché à intégrer des transistors à haute mobilité électronique (HEMT) à base de nitrure de Gallium (GaN) sur substrat Si par épitaxie sous jets moléculaires (EJM) en vue de réaliser des circuits monolithiques GaN sur CMOS SiThe monolithic integration of heterogeneous devices and materials such as III-N compounds with silicon (Si) CMOS technology paves the way for new circuits applications and capabilities for both technologies. However, the heteroepitaxy of such materials on Si can be challenging due to very different lattice parameters and thermal expansion coefficients. In addition, contamination issues and thermal budget constraints on CMOS technology may prevent the use of standard process parameters and require various manufacturing trade-offs. In this context, we have investigated the integration of GaN-based high electron mobility transistors (HEMTs) on Si substrates in view of the monolithic integration of GaN on CMOS circuits
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Fang, Zhihua. „N and p-type doping of GaN nanowires : from growth to electrical properties“. Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY007/document.
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Les nanostructures à base de nitrures d’éléments III suscitent un intérêt croissant, en raison de leurs propriétés singulières et de leurs applications technologiques potentielles, dans les diodes électroluminescentes (LED) notamment. La maîtrise et le contrôle du dopage de ces nanostructures est un enjeu crucial, mais difficile. A ce sujet, cette thèse apporte une contribution nouvelle, en explorant le processus de dopage de type n et p des nanofils (NFs) de GaN crus par épitaxie par jets moléculaires (EJM). En particulier, les propriétés électriques de ces structures ont été caractérisées par une approche multi-technique, à l’échelle du NF unique.Tout d'abord, les propriétés structurales et électriques d'une série de NFs de GaN dopés au Si (type n) ont été étudiées. Des mesures de spectroscopie de rayons X à haute résolution sur des NFs individuels ont mis en évidence une incorporation de Si plus élevée dans les NFs que dans les couches minces épitaxiées, ainsi qu’une migration du Si à la surface du NF pour le fil ayant le niveau de dopage le plus élevé. Des mesures de transport sur des NFs uniques (quatre contacts avec une température allant de 300 K jusqu’à 5 K) ont démontré un contrôle du dopage, avec une résistivité allant de 10^2 à 10^-3 Ω.cm et une concentration de porteurs comprise entre 10^17 et 10^20 cm-3. Des mesures réalisées sur des transistors à effet de champ à NFs uniques non intentionnellement dopés ont démontré qu’ils sont de type n avec une mobilité de porteurs élevée.Parallèlement à cela, les conditions de croissance de NFs de GaN dopés au Mg (p-type) et de jonctions p-n ont été déterminées afin d’obtenir une incorporation significative en Mg. Les propriétés électriques de jonctions p-n axiale à base de NFs de GaN posées sur un substrat de SiO2 et contactés avec de l’oxyde d’indium-étain (ITO) ont été étudiées en utilisant la technique du courant induit par faisceau électronique (EBIC). L’analyse EBIC a permis de localiser la jonction p-n le long du fil et de clairement montrer son bon fonctionnement en polarisation directe ou inverse. L'analyse EBIC a démontré que le GaN de type p est hautement résistif, confirmant ainsi les difficultés à réaliser des mesures de transport sur ce matériau.Cette étude originale a permis de décrire les propriétés électriques et de dopage de ces NFs de GaN à une échelle nanoscopique, facilitant ainsi la fabrication des futurs dispositifs incorporant des nanostructures à base de GaNIII-nitride nanostructures have been attracting increasing attention due to their peculiar properties and potential device applications as lighting LEDs. The control and evaluation of the doping in the nanostructures is a crucial, yet a challenging issue. This thesis advances the field by exploring the n and p type doping process of GaN nanowires (NWs) grown by molecular beam epitaxy (MBE). In particular, their electrical properties have been revealed through a multi-technique approach at the single NW level.Firstly, the structural and electrical properties of a series of Si-doped (n-type) GaN NWs have been studied. High resolution energy dispersive X-ray spectroscopy measurements on single NWs have illustrated the achievement of a higher Si incorporation in NWs than in epilayers, and Si segregation at the edge of the NW with the highest doping. Furthermore, direct transport measurements (four probes measurements from 300 K down to 5 K) on single NWs have shown a controlled doping with resistivity from 10^2 to 10^-3 Ω.cm, and a carrier concentration from 10^17 to 10^20 cm-3. Field effect transistor measurements have evidenced the n-type nature and a high electron mobility of the non-intentionally doped NWs.Secondly, the growth conditions of Mg-doped (p-type) and axial GaN p-n junction NWs have been determined to achieve significant Mg incorporation. Furthermore, the electrical properties of the axial GaN p-n junction NWs, dispersed on SiO2 and contacted by ITO, have been studied using electron beam induced current (EBIC) technique. EBIC technique revealed the location of the p-n junction and clearly demonstrated its operation under reverse and forward polarization. Moreover, EBIC showed highly resistive p-GaN in accordance with the difficulties to perform direct transport measurements on p-GaN NWs.This original study provides a nanoscale description of the electrical and doping properties of the GaN NWs, facilitating the fabrication of the future GaN nanostructures based devices
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Wölz, Martin. „Control of the emission wavelength of gallium nitride-based nanowire light-emitting diodes“. Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2013. http://dx.doi.org/10.18452/16753.
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Halbleiter-Nanosäulen (auch -Nanodrähte) werden als Baustein für Leuchtdioden (LEDs) untersucht. Herkömmliche LEDs aus Galliumnitrid (GaN) bestehen aus mehreren Kristallschichten auf einkristallinen Substraten. Ihr Leistungsvermögen wird durch Gitterfehlpassung und dadurch hervorgerufene Verspannung, piezoelektrische Felder und Kristallfehler beschränkt. GaN-Nanosäulen können ohne Kristallfehler auf Fremdsubstraten gezüchtet werden. Verspannung wird in Nanosäulen elastisch an der Oberfläche abgebaut, dadurch werden Kristallfehler und piezoelektrische Felder reduziert. In dieser Arbeit wurden GaN-Nanosäulen durch Molukularstrahlepitaxie katalysatorfrei gezüchtet. Eine Machbarkeitsstudie über das Kristallwachstum von Halbleiter-Nanosäulen auf Metall zeigt, dass GaN-Nanosäulen in hoher Kristallqualität ohne einkristallines Substrat epitaktisch auf Titanschichten gezüchtet werden können. Für das Wachstum axialer (In,Ga)N/GaN Heterostrukturen in Nanosäulen wurden quantitative Modelle entwickelt. Die erfolgreiche Herstellung von Nanosäulen-LEDs auf Silizium-Wafern zeigt, dass dadurch eine Kontrolle der Emissionswellenlänge erreicht wird. Die Gitterverspannung der Heterostrukturen in Nanosäulen ist ungleichmäßig aufgrund des Spannungsabbaus an den Seitenwänden. Das katalysatorfreie Zuchtverfahren führt zu weiteren statistischen Schwankungen der Nanosäulendurchmesser und der Abschnittlängen. Die entstandene Zusammensetzung und Verspannung des (In,Ga)N-Mischkristalls wird durch Röntgenbeugung und resonant angeregte Ramanspektroskopie ermittelt. Infolge der Ungleichmäßigkeiten erfordert die Auswertung genaue Simulationsrechnungen. Eine einfache Näherung der mittleren Verspannung einzelner Abschnitte kann aus den genauen Rechnungen abgeleitet werden. Gezielte Verspannungseinstellung erfolgt durch die Wahl der Abschnittlängen. Die Wirksamkeit dieses allgemeingültigen Verfahrens wird durch die Bestimmung der Verspannung von (In,Ga)N-Abschnitten in GaN-Nanosäulen gezeigt.Semiconductor nanowires are investigated as a building block for light-emitting diodes (LEDs). Conventional gallium nitride (GaN) LEDs contain several crystal films grown on single crystal substrates, and their performance is limited by strain-induced piezoelectric fields and defects arising from lattice mismatch. GaN nanowires can be obtained free of defects on foreign substrates. In nanowire heterostructures, the strain arising from lattice mismatch can relax elastically at the free surface. Crystal defects and piezoelectric fields can thus be reduced. In this thesis, GaN nanowires are synthesized in the self-induced way by molecular beam epitaxy. A proof-of-concept study for the growth of semiconductor nanowires on metal shows that GaN nanowires grow epitaxially on titanium films. GaN of high crystal quality is obtained without a single crystal substrate. Quantitative models for the growth of axial (In,Ga)N/GaN nanowire heterostructures are developed. The successful fabrication of nanowire LED devices on silicon wafers proves that these models provide control over the emission wavelength. In the (In,Ga)N/GaN nanowire heterostructures, strain is non-uniform due to elastic relaxation at the sidewalls. Additionally, the self-induced growth leads to statistical fluctuations in the diameter and length of the GaN nanowires, and in the thickness of the axial (In,Ga)N segments. The (In,Ga)N crystal composition and lattice strain are analyzed by x-ray diffraction and resonant Raman spectroscopy. Due to the non-uniformity in strain, detailed numerical simulations are required to interpret these measurements. A simple approximation for the average strain in the nanowire segments is derived from the detailed numerical calculation. Strain engineering is possible by defining the nanowire segment lengths. Simulations of resonant Raman spectra deliver the experimental strain of (In,Ga)N segments in GaN nanowires, and give a proof of this universal concept.
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Chmielewski, Daniel Joseph. „III-V Metamorphic Materials and Devices for Multijunction Solar Cells Grown via MBE and MOCVD“. The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534707692114982.
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Zhang, Xin. „Growth and characterization of GaN/lnGaN nanowire heterostructures“. Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY107.
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Les nanostructures de nitrures d’éléments III sont considérées comme des candidats prometteurs visant à la réalisation de divers dispositifs innovants. Depuis quelques années, l'intérêt croissant des nano-LEDs basés sur l’InGaN a été relevé dans le domaine de l'éclairage et de l'affichage. Ce travail de thèse porte sur la croissance par épitaxie par jets moléculaires assistée plasma (PA-MBE) et sur la caractérisation d'hétérostructures InGaN/GaN à base de nanofils.Tout d'abord, un modèle de croissance cinétique de nanofils de nitrures d’éléments III a été établi, en vue d'une analyse en profondeur et d'un meilleur contrôle des processus cinétiques atomiques impliqués dans la croissance MBE. Ce travail de modélisation construit la base théorique et guide l'interprétation expérimentale dans cette thèse.Ensuite, les propriétés morphologiques, structurelles, compositionnelles et optiques des hétérostructures de nanofils GaN/InGaN/GaN axiaux ont été étudiées à nano-échelle par une combinaison de microscopie électronique (SEM / STEM / TEM), photoluminescence (PL), nano-cathodoluminescence (nano-CL), spectroscopie de rayons-X à énergie dispersive (EDX). Sur la base des résultats expérimentaux, nous avons obtenu une description statistique du paysage morphologique pour tous les NWs InGaN/GaN sous différentes conditions thermodynamiques et de flux atomiques. De plus, la corrélation entre les caractéristiques morphologiques & compositionnelles et les propriétés électroniques & optiques des NWs InGaN/GaN a été établie.En outre, divers types de superstructures InGaN à base de nanofils ont été étudiés. On constate que tant le taux de croissance axiale que la composition réelle d’Indium vont diminuer dans le cas de l'excès d'Indium, en raison de l'effet surfactant de l’Indium et d'un flux effectif d’azote réduit. En même temps, un élargissement spectaculaire des sections de l'InGaN a été observé sous des conditions riches en azote, suggérant que la condition riche en métal n'est pas nécessaire pour l'élargissement de l'InGaN. En conséquence, nous proposons le mécanisme de la croissance d’InGaN, pour lequel la croissance axiale est un processus cinétique déterminé par le flux et l'élargissement latéral est principalement induit par la contrainte.Enfin, nous avons étudié l'influence du processus de recuit pour l'efficacité de la luminescence et proposé nos recettes de croissance de plaques LEDs, visant à aborder la fabrication de plaques LED développées par MBEGroup-III-nitride nanostructures are considered as promising candidates aiming at the realization of various innovative devices, including fields from electronics, photonics, to biochemistry and energy. Since recent years, a growing interest of InGaN-based nano-LEDs has been raised in the field of lighting and display. This PhD work focuses on the growth by plasma-assisted molecular beam epitaxy and on the characterization of nanowire-based InGaN/GaN heterostructures.Firstly, a kinetic growth model of III-nitride nanowires has been established, aiming at an in-depth analysis and a better control of atomically kinetic processes involved in MBE growth. This modeling work constructs the theoretical basis and guides the experimental interpretation in this thesis.Then, the morphological, inner-structural, compositional and optical properties of axial GaN/InGaN/GaN nanowire heterostructures have been investigated at nano-scale by a combination of electron microscopy (SEM/STEM/TEM), photoluminescence (PL), nano-cathodoluminescence (nano-CL), energy-dispersive X-ray spectroscopy (EDX). On the basis of experimental results, we have achieved a statistical description of morphological landscape for all InGaN/GaN NWs under different thermodynamic and atomic fluxes conditions. Meanwhile, the correlation between the morphological & compositional features and the electronic & optical properties of InGaN/GaN NWs has been established.Furthermore, various types of NW-based InGaN superstructures have been grown and investigated. It is found that both axial growth rate and real In composition will decrease in the case of Indium excess, due to In surfactant effect and a reduced effective nitrogen flux. Meanwhile, a dramatic widening of InGaN sections has been observed under N-rich condition, suggesting the metal-rich condition is not necessary for the InGaN enlargement. Accordingly, we propose the driving mechanism of InGaN growth, for which the axial growth is a flux-determined kinetic process and the lateral widening is mainly strain-induced.Finally, we have investigated the influence of post-growth annealing process for the luminescence efficiency and proposed our growth recipes of LED plates, aiming at approaching the fabrication of MBE-grown LED plates
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Ive, Tommy. „Growth and investigation of AlN/GaN and (Al,In)N/GaN based Bragg reflectors“. Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2006. http://dx.doi.org/10.18452/15395.
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Die Synthese von AlN/GaN- und (Al,In)N/GaN-Braggreflektoren wird untersucht. Die Strukturen wurden mittels plasmaunterstützter Molekularstrahlepitaxie auf 6H-SiC(0001)-Substraten abgeschieden. Ferner wurde der Einfluß der Si-Dotierung auf die Oberflächenmorphologie sowie die strukturellen und elektrischen Eigenschaften der AlN/GaN-Braggreflektoren untersucht. Es wurden rißfreie Braggreflektoren mit einer hohen Reflektivität (R>99%) und einem bei 450 nm zentrierten Stopband erhalten. Die Si-dotierten Strukturen weisen eine ohmsche I-V-Charakteristik im gesamten Meßbereich sowie einen spezifischen Widerstand von 2-4 mOhmcm2 auf. Die Ergebnisse der (Al,In)N-Wachstumsversuche wurden in einem Phasendiagramm zusammengefaßt, welches den optimalen Parameterraum für (Al,In)N klar aufzeigt.We study the synthesis of AlN/GaN and (Al,In)N/GaN Bragg reflectors. The structures were grown by plasma-assisted molecular beam epitaxy (MBE) on 6H-SiC(0001) substrates. In addition, we study the impact of Si-doping on the surface morphology and the structural and electrical properties of the AlN/GaN Bragg reflectors. Crack-free and high-reflectance (R>99%) Bragg reflectors were achieved with a stopband centered at 450 nm. The Si-doped structures exhibit ohmic I-V behavior in the entire measurement range. The specific series resistance is 2-4 mOhmcm2. The results of the (Al,In)N growth experiments are summarized in a phase diagram which clearly shows the optimum growth window for (Al,In)N.
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Monavarian, Morteza. „Beyond conventional c-plane GaN-based light emitting diodes: A systematic exploration of LEDs on semi-polar orientations“. VCU Scholars Compass, 2016. http://scholarscompass.vcu.edu/etd/4198.
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Despite enormous efforts and investments, the efficiency of InGaN-based green and yellow-green light emitters remains relatively low, and that limits progress in developing full color display, laser diodes, and bright light sources for general lighting. The low efficiency of light emitting devices in the green-to-yellow spectral range, also known as the “Green Gap”, is considered a global concern in the LED industry. The polar c-plane orientation of GaN, which is the mainstay in the LED industry, suffers from polarization-induced separation of electrons and hole wavefunctions (also known as the “quantum confined Stark effect”) and low indium incorporation efficiency that are the two main factors that contribute to the Green Gap phenomenon. One possible approach that holds promise for a new generation of green and yellow light emitting devices with higher efficiency is the deployment of nonpolar and semi-polar crystallographic orientations of GaN to eliminate or mitigate polarization fields. In theory, the use of other GaN planes for light emitters could also enhance the efficiency of indium incorporation compared to c-plane.
In this thesis, I present a systematic exploration of the suitable GaN orientation for future lighting technologies. First, in order to lay the groundwork for further studies, it is important to discuss the analysis of processes limiting LED efficiency and some novel designs of active regions to overcome these limitations. Afterwards, the choice of nonpolar orientations as an alternative is discussed. For nonpolar orientation, the (1-100)-oriented (m-plane) structures on patterned Si (112) and freestanding m-GaN are studied. The semi-polar orientations having substantially reduced polarization field are found to be more promising for light-emitting diodes (LEDs) owing to high indium incorporation efficiency predicted by theoretical studies. Thus, the semi-polar orientations are given close attention as alternatives for future LED technology.
One of the obstacles impeding the development of this technology is the lack of suitable substrates for high quality materials having semi-polar and nonpolar orientations. Even though the growth of free-standing GaN substrates (homoepitaxy) could produce material of reasonable quality, the native nonpolar and semi-polar substrates are very expensive and small in size. On the other hand, GaN growth of semi-polar and nonpolar orientations on inexpensive, large-size foreign substrates (heteroepitaxy), including silicon (Si) and sapphire (Al2O3), usually leads to high density of extended defects (dislocations and stacking faults). Therefore, it is imperative to explore approaches that allow the reduction of defect density in the semi-polar GaN layers grown on foreign substrates.
In the presented work, I develop a cost-effective preparation technique of high performance light emitting structures (GaN-on-Si, and GaN-on-Sapphire technologies). Based on theoretical calculations predicting the maximum indium incorporation efficiency at θ ~ 62º (θ being the tilt angle of the orientation with respect to c-plane), I investigate (11-22) and (1-101) semi-polar orientations featured by θ = 58º and θ = 62º, respectively, as promising candidates for green emitters. The (11-22)-oriented GaN layers are grown on planar m-plane sapphire, while the semi-polar (1-101) GaN are grown on patterned Si (001).
The in-situ epitaxial lateral overgrowth techniques using SiNx nanoporous interlayers are utilized to improve the crystal quality of the layers. The data indicates the improvement of photoluminescence intensity by a factor of 5, as well as the improvement carrier lifetime by up to 85% by employing the in-situ ELO technique. The electronic and optoelectronic properties of these nonpolar and semi-polar planes include excitonic recombination dynamics, optical anisotropy, exciton localization, indium incorporation efficiency, defect-related optical activities, and some challenges associated with these new technologies are discussed. A polarized emission from GaN quantum wells (with a degree of polarization close to 58%) with low non-radiative components is demonstrated for semi-polar (1-101) structure grown on patterned Si (001). We also demonstrated that indium incorporation efficiency is around 20% higher for the semi-polar (11-22) InGaN quantum wells compared to its c-plane counterpart. The spatially resolved cathodoluminescence spectroscopy demonstrates the uniform distribution of indium in the growth plane. The uniformity of indium is also supported by the relatively low exciton localization energy of Eloc = 7meV at 15 K for these semi-polar (11-22) InGaN quantum wells compared to several other literature reports on c-plane. The excitons are observed to undergo radiative recombination in the quantum wells in basal-plane stacking faults at room temperature. The wurtzite/zincblende electronic band-alignment of BSFs is proven to be of type II using the time-resolved differential transmission (TRDT) method. The knowledge of band alignment and degree of carrier localization in BSFs are extremely important for evaluating their effects on device properties. Future research for better understanding and potential developments of the semi-polar LEDs is pointed out at the end.
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Rossner, Ulrike. „Epitaxie des nitrures de gallium et d'aluminium sur silicium par jets moléculaires : caractérisation structurale et optique“. Université Joseph Fourier (Grenoble), 1995. http://www.theses.fr/1995GRE10181.
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Les nitrures de gallium et d'aluminium ont ete elabores par epitaxie par jets moleculaires (mbe) sur des substrats de silicium. La phase cubique de ces nitrures a ete obtenue sur le si(001). Elle est initiee par une carburation de la surface du silicium precedante a la croissance des nitrures: cette carburation permet de recouvrir completement la surface des substrats par des cristallites de sic cubique. La phase hexagonale du gan et de l'ain a ete observee sur le si(111). Pour la croissance du gan hexagonal, des couches tampons d'ain ont ete utilisees. Le nitrure d'aluminium a ete caracterise structuralement (par rheed, diffraction x et microscopie electronique). Pour le nitrure de gallium, semiconducteur a grand gap (3. 4 ev) qui constituait le but principal de notre travail, une etude structurale detaillee ainsi qu'une etude optique (par photoluminescence et cathodoluminescence) ont ete realisees. L'azote reactif pour la croissance des nitrures a ete obtenu par deux methodes differentes: par la creation d'un plasma d'azote par resonance cyclotron-electronique (ecr-mbe) ou par la decomposition de l'ammoniac sur la surface de l'echantillon (gas-source-mbe). Pour travailler en ecr-mbe, une source de plasma a ete developpee et caracterisee: elle est concue telle que le rapport ions/atomes neutres d'azote soit faible pour minimiser l'endommagement eventuel des nitrures par des ions energetiques
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Arlery, Magali. „Etude par microscopie électronique à transmission de couches et structures semi-conductrices GaN/AlxGa(1-x)N“. Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10047.
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Nous presentons une etude structurale de depots epais de gan sur alumine et d'heterostructures gan / al#xga#1#-#xn de basse dimensionnalite. Les systemes etudies ont ete elabores par epitaxie par jet moleculaire (mbe) ou par depot en phase vapeur d'organo-metalliques (mocvd). La technique d'investigation est la microscopie electronique a transmission, utilisee en modes conventionnel, haute resolution et faisceau convergent. La premiere partie de l'etude traite de la croissance optimisee de couches de gan sur substrat d'alumine. La polarite des depots est le parametre clef de cette croissance. En effet, la presence de domaines d'inversion de polarite ga dans une matrice a polarite n provoque une croissance sous forme d'ilots hexagonaux, tandis qu'une matrice a polarite ga assure une surface de croissance lisse. Les depots realises contiennent tous des dislocations en grande densite (10#+#1#0/cm#2), dont les vecteurs de burgers sont egaux a 1/3 <2,1,1,0>, 1/3 <2,1,1,3> et 0001. Des nanotubes sont formes par l'ouverture locale du coeur de dislocations 0001. Des defauts prismatiques 2,1,1,0 et 0,1,1,0 ont aussi ete observes. Les derniers forment les parois des domaines d'inversion de polarite et introduisent une translation de 1/20001 entre les cristaux de polarites opposees. Dans une seconde partie, differentes heterostructures gan / al#xga#1#-#xn (puits, super-reseaux et boites quantiques) sont etudiees a l'echelle atomique par analyse quantitative des images haute resolution. L'epitaxie de gan sur al#. #1#5ga#. #8#5n reste coherente pour 16 monocouches deposees. Par contre, une relaxation partielle apparait dans un super-reseau gan / aln dont les couches ont une epaisseur superieure a 11 monocouches. Les heterostructures aln / gan sont caracterisees par une faible miscibilite et des interfaces relativement abruptes. Selon la temperature, les couches de gan deposees sur aln adoptent un mode de croissance bidimensionnel ou de type stranski-krastanov. Ce dernier mode de croissance permet le confinement d'ilots de gan dans aln et ouvre la voie a la realisation de boites quantiques.
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Widmann, Frédéric. „Epitaxie par jets moléculaires de GaN, AlN, InN et leurs alliages : physique de la croissance et réalisation de nanostructures“. Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10234.
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Ce travail a porte sur la croissance epitaxiale des nitrures d'elements iii gan, aln, et inn, en utilisant l'epitaxie par jets moleculaires assistee par plasma d'azote. Nous avons optimise les premiers stades de la croissance de gan ou aln sur substrat al#2o#3 (0001). Le processus utilise consiste a nitrurer la surface du substrat a l'aide du plasma d'azote, afin de la transformer en aln, puis a faire croitre une couche tampon d'aln ou de gan a basse temperature, avant de reprendre la croissance de gan ou aln a haute temperature (680 a 750c). Nous avons en particulier etudie les proprietes d'une couche de gan en fonction de la temperature a laquelle est realisee l'etape de nitruration. Lorsque les conditions de demarrage de la croissance sont optimisees, nous avons pu observer des oscillations de rheed pendant la croissance de la couche de gan. Nous avons etudie l'effet du rapport v/iii sur la morphologie de surface et les proprietes optiques et structurales de cette couche. Nous avons propose l'utilisation de l'indium en tant que surfactant pour ameliorer ces proprietes. Nous avons ensuite aborde la realisation de superreseaux gan/aln dont nous avons optimise les interfaces. Les mecanismes de relaxation des contraintes de aln sur gan et gan sur aln ont ete etudies. Nous avons egalement elabore les alliages algan et ingan, comme barrieres quantiques dans les heterostructures. Nous avons montre que la relaxation elastique des contraintes de gan en epitaxie sur aln donne lieu a la formation d'ilots de tailles nanometriques, qui se comportent comme des boites quantiques. Leur densite et leur taille dependent de la temperature de croissance, et des conditions de murissement apres croissance. Les proprietes optiques de ces ilots sont gouvernees a la fois par les effets de confinement quantique et par le fort champ piezo-electrique induit par la contrainte dans les ilots.
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Fiorini, Claudia. „“Molecular mechanisms induced by p53 reactivating molecules in p53 mutant pancreatic adenocarcinoma cell lines”“. Doctoral thesis, 2014. http://hdl.handle.net/11562/706761.
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Le mutazioni del gene TP53, che compromettono la funzione trascrizionale di p53, si verificano in più del 50% dei tumori umani, tra cui l’adenocarcinoma pancreatico, e rendono le cellule tumorali più resistenti alle terapie convenzionali. Negli ultimi anni, molti sforzi sono stati indirizzati per identificare molecole-riattivanti la proteina p53 mutata ri-conferendole una conformazione wild-type e attiva trascrizionalmente.
Nella presente tesi di Dottorato, dimostriamo che due di queste molecole, CP-31398 e RITA, possono indurre inibizione della crescita di cellule di adenocarcinoma pancreatico umano, apoptosi e autofagia attivando il legame p53/DNA e la fosforilazione di p53 (ser15), senza modificare la quantità della proteina totale. Questi effetti si verificano sia nelle linee cellulari di adenocarcinoma pancreatico p53 wild-type che nelle p53 mutate (mutp53), mentre sono molto meno pronunciati nelle normali linee cellulari primarie di fibroblasti umani.
Inoltre, CP-31398 e RITA regolano l'asse SESN1- 2/AMPK/mTOR inducendo la fosforilazione di AMPK in Thr172, che ha un ruolo cruciale nella risposta autofagica. Il ruolo protettivo dell'autofagia con inibizione della crescita delle cellule da CP-31398 e RITA è supportato dall’ osservazione che l'inibitore dell’AMPK compound C o gli inibitori dell’autofagia clorochina e 3-metiladenina sensibilizzano le linee cellulari di adenocarcinoma pancreatico alla risposta apoptotica indotta dalle molecole riattivanti p53. I nostri risultati dimostrano per la prima volta il ruolo di sopravvivenza dell'autofagia indotta dalle molecole riattivanti p53 in cellule tumorali p53 mutate.
Le proteine p53 mutate perdono non solo la loro funzione di soppressore tumorale, ma anche acquistano nuove proprietà oncogene note come (GOF- “gain of function”) "guadagno di funzione".
Generalmente, le proteine p53 mutate sono sovra-espresse nelle cellule tumorali e promuovono le attività GOF aumentando la proliferazione delle cellule tumorali e la loro resistenza ad una varietà di farmaci chemioterapici comunemente utilizzati in pratica clinica. Con il guadagno di funzione la proteina mutp53 si comporta come fattore di trascrizione oncogenico interagendo con altri regolatori trascrizionali, come E2F1, NF-Y e VDR. Inoltre, le proteine p53 mutate possono anche interagire con proteine oncosoppressorie inibendo le loro funzioni.
Con questo lavoro dimostriamo che la gemcitabina (GEM), chemioterapico standard, attiva fortemente mutp53 stimolando la sua fosforilazione (Ser15) e la traslocazione nucleare. Questi eventi provocano l’attivazione della GOF in cellule tumorali p53 mutate, come evidenziato dalla stimolazione del ciclo cellulare promossa dai geni, come Cdk1 e CCNB1, dopo il trattamento con GEM. Inoltre, abbiamo dimostrato che il silenziamento di mutp53 aumenta la sensibilità delle cellule tumorali a GEM e che l'aggiunta di CP-31398 o RITA al trattamento con GEM può sinergicamente indurre apoptosi in linee cellulari di adenocarcinoma pancreatico sia p53 wt che p53 mutate, e che questi effetti non sono presenti in cellule tumorali che non esprimono il gene per p53. Questa combinazione di composti induce fortemente la fosforilazione di p53 in Ser15 (senza modificare la concentrazione totale di p53), l'apoptosi, e la formazione dell'autofagosoma. Inoltre, abbiamo dimostrato che l'autofagia provocata da GEM/CP-31398 ha un ruolo protettivo per le cellule tumorali. Infatti, l'aggiunta del inibitori dell’autofagia, clorochina o 3-metiladenina, aumenta l'apoptosi indotta dal trattamento GEM/CP-31398. I nostri risultati supportano lo sviluppo di una strategia antitumorale basata sull'inibizione autofagia associata al trattamento combinato di molecole riattivanti p53 con la chemioterapia tradizionale, nell’adenocarcinoma pancreatico sia con p53wt che con p53 mutato.TP53 gene mutations compromising p53 transcriptional function occur in more than 50% of human cancers, including pancreatic adenocarcinoma, and render cancer cells more resistant to conventional therapy. In the last few years, many efforts have been addressed to identify p53-reactivating molecules able to restore the wild-type transcriptionally competent conformation of the mutated proteins. In the present thesis, we show that two of these compounds, CP-31398 and RITA, can induce cell growth inhibition, apoptosis, and autophagy by activating p53/DNA binding and p53 phosphorylation (Ser15), without affecting the total amount of p53. These effects occur in both wild-type and mutant p53 (mutp53) pancreatic adenocarcinoma cell lines, whereas they are much less pronounced in normal human primary fibroblasts. Furthermore, CP-31398 and RITA regulate the axis SESN1- 2/AMPK/mTOR by inducing AMPK phosphorylation in Thr172, which has a crucial role in the autophagic response. The protective role of autophagy in cell growth inhibition by CP-31398 and RITA is supported by the finding that the AMPK inhibitor compound C or the autophagy inhibitors chloroquine or 3-methyladenine sensitize pancreatic adenocarcinoma cell lines to the apoptotic response induced by p53-reactivating molecules. Our results demonstrate for the first time a survival role for autophagy induced by p53 reactivating molecules in p53 mutant cancer cells. Mutp53 proteins not only lose their tumor suppressive function but also gain new oncogenic properties known as “gain-of-function” (GOF). Generally, mutp53 proteins are over-expressed in cancer cells and promote GOF activities enhancing the proliferation of cells and their resistance to a variety of chemotherapeutic drugs commonly used in the clinical practice. GOF activities are carried out because mutp53 proteins behave as oncogenic transcription factors by interacting with other transcriptional regulators, such as E2F1, NF-Y and VDR. Moreover, these p53 mutant proteins can also interact with oncosuppressor proteins inhibiting their function. Here, we show that the standard drug gemcitabine (GEM) strongly activates mutp53 by stimulating both its phosphorylation (Ser15) and nuclear translocation. These events result in the stimulation of mutp53 GOF in cancer cells bearing mutant p53, as revealed by the stimulation of cell cycle promoting genes, as Cdk1 and CCNB1, after GEM treatment. Furthermore, we demonstrate that silencing of mutp53 strongly increases sensitivity of cancer cells to GEM and that the addition of CP-31398 or RITA to GEM treatment can synergistically induce apoptotic cell death in both wt and mutant p53 pancreatic adenocarcinoma cell lines, whereas these effects are missing in p53-null cancer cells. This drug combination strongly induces p53 phosphorylation in Ser15 (without affecting the total amount of p53), apoptosis, and autophagosome formation. Furthermore, we demonstrate that autophagy stimulation by GEM/CP-31398 has a protective role for cancer cells. In fact, the addition of the autophagy inhibitors, chloroquine or 3-methyladenine, increases apoptosis induced by GEM/CP-31398 treatment. Our results support the development of an anti-tumoral strategy based on autophagy inhibition associated to the combined treatment of p53-reactivating molecules with standard chemotherapy, for both wild-type and mutant p53 pancreatic adenocarcinoma cell types.
39
Chandrasekaran, Vasudevan. „Structure and ligand-based applications of molecular modeling to gain insights into the structural features of proteins and small molecules of biological interest“. 2006. http://purl.galileo.usg.edu/uga%5Fetd/chandrasekaran%5Fvasudevan%5F200608%5Fphd.
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Sills, Gavin Rene'. „Molecular markers associated with gain from selection, and maternal phenotypic effects in Arabidopsis“. 1994. http://catalog.hathitrust.org/api/volumes/oclc/31615068.html.
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41
Cardnell, Robert John Gunn. „In vivo analysis of gain-of-function mutations in the Drosophila eag-encoded potassium ion channel“. Thesis, 2006. http://hdl.handle.net/1911/18879.
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Neuronal Na+ and K+ channels elicit currents in opposing directions and thus have opposing effects on neuronal excitability. Mutations in genes encoding Na+ or K+ channels often interact genetically, leading either to phenotypic suppression or enhancement for genes with opposing or similar effects on excitability respectively. For example, the effects of mutations in Shaker (Sh), which encodes a K+ channel subunit, are suppressed by loss of function mutations in the Na+ channel structural gene para, but enhanced by loss of function mutations in a second K + channel encoded by eag.
Here I characterize three novel mutations that suppress the effects of a Sh mutation on behavior and neuronal excitability. Recombination mapping localized the mutations to the eag locus, and I used sequence analysis to determine that two of the mutations are caused by a single amino acid substitution (G297E) in the S2-S3 linker of Eag. Because these novel eag mutations confer opposite phenotypes to eag loss of function mutations, I suggest that eag G297E causes an eag gain of function phenotype. I hypothesize that the G297E substitution may cause premature, prolonged or constitutive opening of the Eag channels by favoring the "unlocked" state of the channel. The third mutation has two amino acid substitutions in Eag (A259V and E762V) and may also be a gain of function allele of eag . Interestingly, these mutations appear to manifest their most obvious phenotypes under conditions that prolong the action potential.
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Madhangi, M. „Functional characterization of WD40-repeat protein, WDR8, in Zebrafish to gain insight into its role in Isolated Microspherophakia“. Thesis, 2017. https://etd.iisc.ac.in/handle/2005/5700.
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WD40-repeat (WDR) proteins are a family of proteins that are characterized by
widespread occurrence, low level of sequence conservation, common structural
conformation (β propeller structure) and functional diversity. They act as
scaffolds for multi-protein complex assembly during cellular processes like
DNA repair, cell division, apoptosis, etc. Chapter 1 introduces WDR proteins
and reviews the various features that characterize this family of proteins. The
functions and the significance of WDR proteins have been described and the
importance of characterizing the WDR proteins of unknown function, which
have been implicated in human disorders, is discussed. Specifically, the link
between a putative missense mutation in a relatively unstudied WDR protein,
WDR8 and isolated Microspherophakia is elaborated on. Microspherophakia is a
congenital, autosomal recessive disorder in humans characterized by the
presence of a smaller, more spherical lens. The clinical, phenotypic and
genotypic characteristics of this developmental disorder are explained.
Chapter 2 lists the various protocols and the experimental techniques and
methods used in this study.
Chapter 3 details the results regarding the function of WDR8 in zebrafish eye
development. Morpholino-mediated knockdown of WDR8 during development
caused a decrease in cell numbers in the lens and retinal layers, ultimately
resulting in a reduction in eye size and lens size, without affecting the gross
morphology of the eye. When embryos were supplied with exogenous WDR8
lacking the morpholino-binding site, this reduction in eye size was rescued
proving that the phenotype was due to the knock down of WDR8. Further, this
phenotype was specific to WDR8, since knock down of another centrosomal
WDR protein, WDR62 (mutations in which cause Microcephaly), did not affect
the eye size in the embryos. Cell cycle analysis of whole embryos at 24 hpf
(hours-post-fertilization) and the retinal cells at 48 hpf revealed cell cycle arrest
selectively in the retina of the WDR8 morphants.
The results discussed in this chapter also reveal an abnormal persistence of
Phospho-Histone H3 (PH3+, a marker for mitosis) positive cells in the eyes of
morphants, suggesting mitotic arrest in the retinal cells. Moreover, WDR8
morphants showed an increase in the PH3+ retinal cells undergoing
programmed cell death, indicating the removal of the cells arrested during
mitosis by apoptosis. Results from flow-cytometric analysis of co-stained retinal
cells showed that the cells undergoing cell cycle arrest in the WDR8 morphants
were predominantly PAX6+. Paired Box 6 (PAX6) is a major transcriptional
regulator of early eye development and differentiation. Interestingly, it is also
shown that unlike the knock down, the over-expression of WDR8 affected cell
division ubiquitously, resulting in extensive apoptosis and decreased survival of
the embryos. Thus, the results from the knock down experiments showed that
WDR8 is involved in the regulation of cell division in the eye during zebrafish
development.
Chapter 3 also contains the results of the experiments aimed at understanding
the mechanism by which a putative mutation (p.Pro383Leu) in WDR8 could
affect the function of the protein and contribute to Microspherophakia. The
results presented reveal that, when the WDR8 morphants were injected with the
plasmid expressing either the human wild type or mutant WDR8 in order to
compensate for the deficiency in zebrafish WDR8, the wild type human WDR8
could suppress the morphant phenotype and rescue the cell cycle defect and the
increased apoptosis. However, the mutant human WDR8 failed to suppress the
reduction in eye size and restore the level of cell division in the retina,
suggesting that the mutation indeed abrogated the protein function.
Further, Chapter 3 also presents the findings from the expression of either the
human wild type or mutant WDR8 in HeLa cells that was carried out in order to
identify differences between the wild type and mutant proteins in terms of the
localization and the interaction with binding partners. The centrosomal
localization of the mutant protein was found to be unaltered in the presence of
the endogenous wild type protein. Also, the over-expression of either the wild
type or the mutant WDR8 resulted in cell cycle arrest in HeLa cells. Importantly,
co-immunoprecipitation experiments showed that the mutation interferes with
the interaction of the WDR8 protein with its binding partners, such as OFD1
(oral-facial-digital syndrome 1), a centrosomal protein.
Chapter 4 discusses the findings and the conclusions of the present study.
Based on the results explained in Chapter 3, the function of WDR8 during eye
development and its role in the causation of microspherophakia are explained.
The present study offers the following insights:
1. WDR8 plays an important role in the cell-cycle progression in the
precursor cells of the developing optic vesicle. Thus, WDR8 is required in
the developing eye for attaining the optimal cell numbers in the lens and
retina of zebrafish.
2. The missense mutation (p.Pro383Leu) in WDR8 diminishes its interaction
ability and affects its function. Thus, homozygous missense mutation in
WDR8 can abrogate its function, leading to the disease phenotype
suggesting that WDR8 is a causative gene for Microspherophakia.
43
Bhat, Thirumaleshwara N. „Group III Nitride/p-Silicon Heterojunctions By Plasma Assisted Molecular Beam Epitaxy“. Thesis, 2012. https://etd.iisc.ac.in/handle/2005/2454.
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The present work focuses on the growth and characterizations of GaN and InN layers and nanostructures on p-Si(100) and p-Si(111) substrates by plasma-assisted molecular beam epitaxy and the studies of GaN/p-Si and InN/p-Si heterojunctions properties. The thesis is divided in to seven different chapters.
Chapter 1 gives a brief introduction on III-nitride materials, growth systems, substrates, possible device applications and technical background.
Chapter 2 deals with experimental techniques including the details of PAMBE system used in the present work and characterization tools for III-nitride epitaxial layers as well as nanostructures.
Chapter 3 involves the growth of GaN films on p-Si(100) and p-Si(111) substrates. Phase pure wurtzite GaN films are grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) are found to be phase mixtured, containing both cubic and hexagonal modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural and optical properties of GaN films grown with silicon nitride buffer layer grown at 800 oC, when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 oC. Core-level photoelectron spectroscopy of SixNy layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism is observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibites superior rectifying nature with reduced trap concentrations. Lowest ideality factors (~1.5) are observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. A brief comparison of the structural, optical and heterojunction properties of GaN grown on Si(100) and Si(111) has been carried out.
Chapter 4 involves the growth and characterizations of InN nanostructures and thinfilms on p-Si(100) and p-Si(111) substrates. InN QDs are grown on Si(100) at different densities. The PL characteristics of InN QDs are studied. A deterioration process of InN QDs, caused by the oxygen incorporation into the InN lattice and formation of In2O3/InN composite structures was established from the results of TEM, XPS and PL studies. The results confirm the partial oxidation of the outer shell of the InN QDs, while the inner core of the QDs remains unoxidized. InN nanorods are grown on p-Si(100), structural characterizations are carried out by SEM, and TEM. InN nanodots are grown on p-Si(100), structural characterizations are performed. InN films were grown on Si(100) and Si(111) substrates and structural characterizations are carried out.
Chapter 5 deals with the the heterojunction properties of InN/p-Si(100) and InN/p-Si(111).The transport behavior of the InN NDs/p-Si(100) diodes is studied at various bias voltages and temperatures. The temperature dependent ZB BH and ideality factors of the forward I-V data are observed, while it is governed through the modified Richardson’s plot. The difference in FB BH and C-V BH and the deviation of ideality factor from unity indicate the presence of inhomogeneities at the interface. The band offsets derived from C-V measurements are found to be Δ EC=1.8 eV and Δ EV =1.3 eV, which are in close agreement with Anderson’s model. The band offsets of InN/p-Si heterojunctions are estimated using XPS data. A type-III band alignment with a valence band offset of Δ EV =1.39 eV and conduction band offset of ΔEC=1.81 eV is identified. The charge neutrality level model provides a reasonable description of the band alignment of the InN/p-Si interface. The interface dipole deduced by comparison with the electron affinity model is 0.06 eV. The transport studies of InN NR/p-Si(100) heterojunctions have been carried out by conductive atomic force microscopy (CAFM) as well as conventional large area contacts. Discussion of the electrical properties has been carried out based on local current-voltage (I-V) curves, as well as on the 2D conductance maps. The comparative studies on transport properties of diodes fabricated with InN NRs and NDs grown on p-Si(100) substrates and InN thin films grown on p-Si(111) substrates have also been carried out.
Chapter 6 deals with the growth and characterizations of InN/GaN heterostructures on p-Si(100) and p-Si(111) substarets and also on the InN/GaN/p-Si heterojunction properties. The X-ray diffraction (XRD), scanning electron microscopy (SEM) studies reveal a considerable variation in crystalline quality of InN with grown parameters. Deterioration in the rectifying nature is observed in the case of InN/GaN/p-Si(100) heterojunction substrate when compared to InN/GaN/p-Si (111) due to the defect mediated tunneling effect, caused by the high defect concentration in the GaN and InN films grown on Si(100) and also due to the trap centers exist in the interfaces. Reduction in ideality factor is also observed in the case of n-InN/n-GaN/p–Si(111) when compared to n-InN/n-GaN/p–Si(100) heterojunction. The sum of the ideality factors of individual diodes is consistent with experimentally observed high ideality factors of n-InN/n-GaN/p–Si double heterojunctions due to double rectifying heterojunctions and metal semiconductor junctions. Variation of effective barrier heights and ideality factors with temperature are confirmed, which indicate the inhomogeneity in barrier height, might be due to various types of defects present at the GaN/Si and InN/GaN interfaces. The dependence of forward currents on both the voltage and temperatures are explained by multi step tunneling model and the activation energis were estimated to be 25meV and 100meV for n-InN/n-GaN/p–Si(100) and n-InN/n-GaN/p–Si(111) heterojunctions, respectively.
Chapter 7 gives the summary of the present study and also discusses about future research directions in this area.
44
Bhat, Thirumaleshwara N. „Group III Nitride/p-Silicon Heterojunctions By Plasma Assisted Molecular Beam Epitaxy“. Thesis, 2012. http://etd.iisc.ernet.in/handle/2005/2454.
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The present work focuses on the growth and characterizations of GaN and InN layers and nanostructures on p-Si(100) and p-Si(111) substrates by plasma-assisted molecular beam epitaxy and the studies of GaN/p-Si and InN/p-Si heterojunctions properties. The thesis is divided in to seven different chapters.
Chapter 1 gives a brief introduction on III-nitride materials, growth systems, substrates, possible device applications and technical background.
Chapter 2 deals with experimental techniques including the details of PAMBE system used in the present work and characterization tools for III-nitride epitaxial layers as well as nanostructures.
Chapter 3 involves the growth of GaN films on p-Si(100) and p-Si(111) substrates. Phase pure wurtzite GaN films are grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) are found to be phase mixtured, containing both cubic and hexagonal modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural and optical properties of GaN films grown with silicon nitride buffer layer grown at 800 oC, when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 oC. Core-level photoelectron spectroscopy of SixNy layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism is observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibites superior rectifying nature with reduced trap concentrations. Lowest ideality factors (~1.5) are observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. A brief comparison of the structural, optical and heterojunction properties of GaN grown on Si(100) and Si(111) has been carried out.
Chapter 4 involves the growth and characterizations of InN nanostructures and thinfilms on p-Si(100) and p-Si(111) substrates. InN QDs are grown on Si(100) at different densities. The PL characteristics of InN QDs are studied. A deterioration process of InN QDs, caused by the oxygen incorporation into the InN lattice and formation of In2O3/InN composite structures was established from the results of TEM, XPS and PL studies. The results confirm the partial oxidation of the outer shell of the InN QDs, while the inner core of the QDs remains unoxidized. InN nanorods are grown on p-Si(100), structural characterizations are carried out by SEM, and TEM. InN nanodots are grown on p-Si(100), structural characterizations are performed. InN films were grown on Si(100) and Si(111) substrates and structural characterizations are carried out.
Chapter 5 deals with the the heterojunction properties of InN/p-Si(100) and InN/p-Si(111).The transport behavior of the InN NDs/p-Si(100) diodes is studied at various bias voltages and temperatures. The temperature dependent ZB BH and ideality factors of the forward I-V data are observed, while it is governed through the modified Richardson’s plot. The difference in FB BH and C-V BH and the deviation of ideality factor from unity indicate the presence of inhomogeneities at the interface. The band offsets derived from C-V measurements are found to be Δ EC=1.8 eV and Δ EV =1.3 eV, which are in close agreement with Anderson’s model. The band offsets of InN/p-Si heterojunctions are estimated using XPS data. A type-III band alignment with a valence band offset of Δ EV =1.39 eV and conduction band offset of ΔEC=1.81 eV is identified. The charge neutrality level model provides a reasonable description of the band alignment of the InN/p-Si interface. The interface dipole deduced by comparison with the electron affinity model is 0.06 eV. The transport studies of InN NR/p-Si(100) heterojunctions have been carried out by conductive atomic force microscopy (CAFM) as well as conventional large area contacts. Discussion of the electrical properties has been carried out based on local current-voltage (I-V) curves, as well as on the 2D conductance maps. The comparative studies on transport properties of diodes fabricated with InN NRs and NDs grown on p-Si(100) substrates and InN thin films grown on p-Si(111) substrates have also been carried out.
Chapter 6 deals with the growth and characterizations of InN/GaN heterostructures on p-Si(100) and p-Si(111) substarets and also on the InN/GaN/p-Si heterojunction properties. The X-ray diffraction (XRD), scanning electron microscopy (SEM) studies reveal a considerable variation in crystalline quality of InN with grown parameters. Deterioration in the rectifying nature is observed in the case of InN/GaN/p-Si(100) heterojunction substrate when compared to InN/GaN/p-Si (111) due to the defect mediated tunneling effect, caused by the high defect concentration in the GaN and InN films grown on Si(100) and also due to the trap centers exist in the interfaces. Reduction in ideality factor is also observed in the case of n-InN/n-GaN/p–Si(111) when compared to n-InN/n-GaN/p–Si(100) heterojunction. The sum of the ideality factors of individual diodes is consistent with experimentally observed high ideality factors of n-InN/n-GaN/p–Si double heterojunctions due to double rectifying heterojunctions and metal semiconductor junctions. Variation of effective barrier heights and ideality factors with temperature are confirmed, which indicate the inhomogeneity in barrier height, might be due to various types of defects present at the GaN/Si and InN/GaN interfaces. The dependence of forward currents on both the voltage and temperatures are explained by multi step tunneling model and the activation energis were estimated to be 25meV and 100meV for n-InN/n-GaN/p–Si(100) and n-InN/n-GaN/p–Si(111) heterojunctions, respectively.
Chapter 7 gives the summary of the present study and also discusses about future research directions in this area.
45
Lin, Yeh-Fon, und 林業峰. „Molecular Dynamics Simulations to Gain Insights into the Structural Stability and Aggregation Behavior of the VEALYL and LYQLEN Peptides Derived from Human Insulin“. Thesis, 2009. http://ndltd.ncl.edu.tw/handle/jmn84r.
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碩士國立臺北科技大學
生物科技研究所
97
The LYQLEN and VEALYL peptides from the A chain (residues 13-18) and B chain (residues 12-17) of insulin has been shown to form amyloid-like fibrils. Recently, the atomic structures of the LYQLEN and VEALYL oligomers have been determined by x-ray microcrystallography and reveal a dry, tightly self-complementing structure between the neighboringβ-sheet layers, termed as “steric zipper”. In this study, several molecular dynamics simulations with all-atom explicit water were conducted to investigate the structural stability and aggregation behavior of the LYQLEN and VEALYL peptides with various sizes and their single glycine replacement mutations. The results of our single-layer models showed that the structural stability of the LYQLEN and VEALYL oligomers increases significantly with increasing the number ofβ-strands. We further suggested that the minimal nucleus seed for LYQLEN and VEALYL fibril formation could be as small as trimer or tetramer. Our results also revealed that the hydrophobic interaction between glutamate and tyrosine plays an important role in stabilizing the adjacentβ-strands within the same layer for LYQLEN and VEALYL oligomers. For the case VEALYL oligomers, the hydrophobic steric zipper formed via the side chains of V1, A3, L4, Y5, and L6 plays a critical role in holding the two opposingβ-sheets together. Mutation simulations showed that single glycine substitution at V1, A3, L4, Y5 and L6 directly destroyed the steric zipper, leading to the destabilization of the VEALYL oligomers. For the case of LYQLEN oligomers, the steric zipper via the side chains of L1, Q3, L4, and N6 associates two neighbouringβ-sheet layers together. Mutation simulations showed that the replacement of Y2 or E5 by a single glycine residue exhibits strong destabilizing effects on the adjacentβ-strands within the same layer; whereas single glycine substitution at L1, Q3, L4, and N6 directly disrupts the steric zipper between the two neighbouringβ-sheet layers, resulting in the destabilization of the entire LYQLEN oligomers. The results of this study provide detailed atomistic insights into the factors stabilizing the LYQLEN and VEALYL oligomers and the aggregation behaviour of these two peptides. It may also provide helpful information for designing new or modified inhibitor able to prevent the fibrillization of the insulin protein.
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(9183557), Amanpreet Kaur. „A Novel Maize Dwarf Resulting From a Gain-of-Function Mutation In a Glutamate Receptor Gene“. Thesis, 2020.
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Plant height is an important agronomic trait and a major target for crop improvement. Owing to the ease of detection and measurement of plant stature, as well as its high heritability, several height-related mutants have been reported in maize. The genes underlying a few of those mutants have also been identified, with a majority of them related to the biosynthesis or signaling of two key phytohormones - gibberellins (GAs) and brassinosteroids (BRs). However, most other maize dwarfing mutants, and especially those that result from gain-of-function mutations, remain uncharacterized. The present study was undertaken to characterize a novel dominant dwarfing mutant, named D13. This mutant appeared in the M1 population of the inbred B73 that was generated by mutagenesis with ethyl methanesulfonate (EMS). Like most other maize dwarfing mutants, the reduction in D13 height was largely due to the compression of the internodes. However, unlike the GA or BR mutants, D13 had no defects in the female or male inflorescences. Further, in contrast to the GA and BR mutants, the mesocotyl elongation during etiolation was not impacted in D13. D13 seedlings developed red coloration in two to three lowermost leaves. In addition, D13 also showed enhanced tillering when the phenotype was very severe. The size of the shoot apical meristem of D13 was reduced slightly, and significant aberrations in the structure of vascular bundles in the mutant were observed. All anatomical and phenotypic features of D13 were highly exaggerated in homozygous state, indicating the partially dominant nature of the D13 mutation. Interestingly, the heterozygous mutants showed remarkable variation in their phenotype, which was maintained across generations. Moreover, the D13 phenotype was found to be sensitive to the genetic background, being completely suppressed in Mo17, Oh7B, enhanced in CML322, P39 and changed to different degrees in others. To identify the genetic defect responsible for the D13 mutant phenotype, a map-based cloning approach was used, which identified a single base-pair change from G to A (G2976A) in the coding region of a glutamate receptor gene (Zm00001d015007). The G2976A missense mutation resulted in the replacement of alanine with threonine at the location 670. The replaced alanine is highly conserved in glutamate receptors across all domains of life from cyanobacteria to plants to mammals, suggesting a causal relationship between the G2976A substitution and the D13 phenotype. To validate this relationship, a targeted EMS-based mutagenesis approach was used to knock-out (inactivate) the D13 mutant allele. A suppressor mutant was found in which the D13 mutant phenotype reverted to the normal tall phenotype. The sequence of the revertant allele, designated D13*, revealed that the original D13 mutant allele underwent a second G to A mutation (G1520A) to change glycine into aspartic acid at position 473. This intragenic second-site mutation in the D13 allele suppressed the function of the D13 allele, thereby preventing it from interfering with the function of the wild type allele. To further unveil the genes and underlying mechanisms that enable the D13 mutant to confer a dwarf phenotype, transcriptomic and metabolomic analyses of D13 mutants were conducted and compared to the wild type sibs. While the omics analysis confirmed that stress responses were upregulated and genes related to shoot system development were downregulated in the mutant, the data did not allow us to pinpoint the underlying mechanisms that connect the D13 mutation with its dwarfing phenotype. Furthermore, it remains unclear whether these stress and shoot system-related changes result in the manifestation of D13 phenotype, or the dwarf phenotype due to D13 mutation activates the stress-related mechanisms. This is the first study that signifies the importance of a glutamate receptor gene in controlling plant height.
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Mohan, Lokesh. „III- Nitride Thin Films and Nanostructures on Si(111) by Plasma Assisted Molecular Beam Epitaxy“. Thesis, 2017. http://etd.iisc.ac.in/handle/2005/4297.
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This thesis focuses on studying heterostructures of GaN, Silicon and AlN. GaN nanostructures are grown on bare Si (111) with and without a GaN buffer layer and GaN film was grown on an AlN layer. Apart from the material characterization of the grown samples we have studied the carrier transport across GaN/Si and AlN/Si heterojunctions by means of the I-V-T curves from these junctions and we also studied the band alignment across GaN/AlN and AlN/Si heterojunctions by means of X-ray photoelectron spectroscopy.
The thesis is divided in 7 chapters. The first chapter deals with general introduction of the field, choice of the substrate, different growth techniques and an overview of nanostructures.
In the second chapter different experimental techniques used in the current study are briefly mentioned. These techniques include Growth by Plasma Assisted Molecular Beam Epitaxy (PAMBE), X-Ray Diffraction (XRD), Raman spectroscopy, Photoluminescence spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).
Then in the 3rd chapter the growth and characterization of GaN nanostructures on Silicon (111) is discussed. Attention has been paid to the effect of substrate temperature and V-III ratio on the morphology and optical quality of the grown structures when other growth parameters has been kept constant. However due to the complexity involved in forming proper electrical contacts from such rods and low yield of single nanowire-based devices, the need to grow compact nanorods was felt.
Hence in the fourth chapter compact GaN nanorods were grown on n-Si with a buffer layer for improved quality and elimination of any possibility of electrical short with the substrate during metallization. The main focus in this chapter is the electrical characterization of GaN nanorods/Si
(111) heterojunction. The temperature dependent current-voltage characteristics from GaN/n-Si (111) junctions are analyzed and explained as the result of a lateral inhomogeneity in barrier heights with Gaussian distribution and temperature dependent Gaussian parameters.
The importance of AlN as a buffer layer for many III-Nitride based devices and as an active layer in many electromechanical devices drew our attention towards band off-set studies of the GaN/AlN/Si heterojunction and electrical transport across the AlN/n-Si junction, in 5th and 6th chapter respectively. The 5th chapter starts with the structural and optical characterization of AlN/Si (111) templates and overgrown GaN thin film. The rest of the 5th chapter is dedicated to the band off sets studies on GaN/AlN and AlN/Si (111) heterojunctions with X-ray photoelectron spectroscopy (XPS). A band diagram of GaN/AlN and AlN/Si is suggested based on our studies.
In the 6th chapter, which happens to be the last work chapter, the temperature dependent electrical characterization of AlN/n-Si (111) heterojunction was carried out from 100K to 400K and the transport mechanism was explained with the help of the trap states at the interface. Finally, the thesis is concluded and insights for future work is presented in the seventh chapter.
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Mukundan, Shruti. „Epitaxial Nonpolar III-Nitrides by Plasma-Assisted Molecular Beam Epitaxy“. Thesis, 2015. http://etd.iisc.ac.in/handle/2005/3930.
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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.
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Mukundan, Shruti. „Epitaxial Nonpolar III-Nitrides by Plasma-Assisted Molecular Beam Epitaxy“. Thesis, 2015. http://etd.iisc.ernet.in/2005/3930.
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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.
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Roul, Basanta Kumar. „Group III-Nitride Epitaxial Heterostructures By Plasma-Assisted Molecular Beam Epitaxy“. Thesis, 2012. https://etd.iisc.ac.in/handle/2005/2514.
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Group III-nitride semiconductors have received much research attention and witnessed a significant development due to their ample applications in solid-state lighting and high-power/high-frequency electronics. Numerous growth methods were explored to achieve device quality epitaxial III-nitride semiconductors. Among the growth methods for III-nitride semiconductors, molecular beam epitaxy provides advantages such as formation of abrupt interfaces and in-situ monitoring of growth. The present research work focuses on the growth and characterizations of III-nitride based epitaxial films, nanostructures and heterostructures on c-sapphire substrate using plasma-assisted molecular beam epitaxy system. The correlation between structural, optical and electrical properties of III-nitride semiconductors would be extremely useful. The interfaces of the metal/semiconductor and semiconductor heterostructures are very important in the performance of semiconductor devices. In this regard, the electrical transport studies of metal/semiconductor and semiconductor heterostructures have been carried out. Besides, studies involved with the defect induced room temperature ferromagnetism of GaN films and InN nano-structures have also been carried out.
The thesis is organized in eight different chapters and a brief overview of each chapter is given below.
Chapter 1 provides a brief introduction on physical properties of group III-nitride semiconductors. It also describes the importance of III-nitride heterostructures in the operation of optoelectronic devices. In addition, it also includes the current strategy of the emergence of room temperature ferromagnetism in III-nitride semiconductors.
Chapter 2 deals with the basic working principles of molecular beam epitaxy system and different characterization tools employed in the present work.
Chapter 3 describes the growth of GaN films on c-sapphire by plasma-assisted molecular beam epitaxy. The effects of N/Ga flux ratio on structural, morphological and optical properties have been studied. The flux ratio plays a major role in controlling crystal quality, morphology and emission properties of GaN films. The dislocation density is found to increase with increase in N/Ga flux ratio. The surface morphologies of the films as seen by scanning electron microscopy show pits on the surface and found that the pit density on the surface increases with flux ratio. The room temperature photoluminescence study reveals the shift in band-edge emission towards the lower energy with increase in N/Ga flux ratio. This is believed to arise from the reduction in compressive stress in the GaN films as it is evidenced by room temperature Raman study. The transport studies on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and is found to be caused by the increase in dislocation density in the GaN films.
Chapter 4 deals with the fabrication and characterization of Au/GaN Schottky diodes. The temperature dependent current–voltage measurements have been used to determine the current transport mechanism in Schottky diodes. The barrier height (φb) and the ideality factor (η) are estimated from the thermionic emission model and are found to be temperature dependent in nature, indicating the existence of barrier height inhomogeneities at the Au/GaN interface. The conventional Richardson plot of ln(Is/T2) versus 1/kT gives Richardson constant value of 3.23×10-5 Acm-2 K-2, which is much lower than the known value of 26.4 Acm-2 K-2 for GaN. Such discrepancy of Richardson constant value was attributed to the existence of barrier height inhomogeneities at the Au/GaN interface. The modified Richardson plot of ln(Is/T2)-q2σs2/2k2T2 versus q/kT, by assuming a Gaussian distribution of barrier heights at the Au/GaN interface, provides the Schottky barrier height of 1.47 eV and Richardson constant value of 38.8 Acm-2 K-2 which is very close to the theatrical value of Richardson constant. The temperature dependence of barrier height is interpreted on the basis of existence of the Gaussian distribution of the barrier heights due to the barrier height inhomogeneities at the Au/GaN interface.
Chapter 5 addresses on the influence of GaN underlayer thickness on structural, electrical and optical properties of InN thin films grown using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals superior crystalline quality for the InN film grown on thicker GaN film. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Also, we present the studies involving the dependence of structural, electrical and optical properties of InN films, grown on thicker GaN films, on growth temperature. The optical absorption edge of InN film is found to be strongly dependent on carrier concentration. Kane’s k.p model is used to describe the dependence of optical absorption edge on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band.
Chapter 6 deals with the analysis of the temperature dependent current transport mechanisms in InN/GaN heterostructure based Schottky junctions. The barrier height (φb) and the ideality factor (η) of the InN/GaN Schottky junctions are found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height and the ideality factor obtained by TFE model are 1.43 eV and 1.21, respectively.
Chapter 7 focuses on the defect induced room temperature ferromagnetism in Ga deficient GaN epitaxial films and InN nano-structures grown on c-sapphire substrate by using plasma-assisted molecular beam epitaxy. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm-1 in Raman spectra confirms the existence of Ga vacancies in GaN films. The ferromagnetism in Ga deficient GaN films is believed to originate from the polarization of the unpaired 2p electrons of nitrogen surrounding the Ga vacancy. The InN nano-structures of different size are grown on sapphire substrate, the structural and magnetic properties are studied. The room temperature magnetization measurement of InN nano-structures exhibits the ferromagnetic behavior. The saturation magnetization is found to be strongly dependent on the size of the nano-structures.
Finally, Chapter 8 gives the summary of the present work and the scope for future work in this area of research.