Vertical conduction properties of few-layer epitaxial graphene / n-type 4H-SiC heterojunctions at cryogenic temperatures

Vertical diodes of epitaxial graphene on n 4H-SiC were investigated. The graphene Raman spectraexhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film.Rectifying properties improved at low temperatures as the reverse leakage decreased over six ordersof magnitude without freeze-out in either material. Carrier concentration of 10 16 cm 3in the SiCremained stable down to 15 K, while accumulation charge decreased and depletion width increasedin forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emissionindicated majority carrier field emission as the dominant conduction mechanism.

Analysis of the modified optical properties and band structure of GaAs12xSbx-capped InAs/GaAs quantum dots

The origin of the modified optical properties of InAs/GaAs quantum dots (QD) capped with a thin GaAs1−xSbx layer is analyzed in terms of the band structure. To do so, the size, shape, and composition of the QDs and capping layer are determined through cross-sectional scanning tunnelling microscopy and used as input parameters in an 8 × 8 k·p model. As the Sb content is increased, there are two competing effects determining carrier confinement and the oscillator strength: the increased QD height and reduced strain on one side and the reduced QD-capping layer valence band offset on the other. Nevertheless, the observed evolution of the photoluminescence (PL) intensity with Sb cannot be explained in terms of the oscillator strength between ground states, which decreases dramatically for Sb > 16%, where the band alignment becomes type II with the hole wavefunction localized outside the QD in the capping layer. Contrary to this behaviour, the PL intensity in the type II QDs is similar (at 15 K) or even larger (at room temperature) than in the type I Sb-free reference QDs. This indicates that the PL efficiency is dominated by carrier dynamics, which is altered by the presence of the GaAsSb capping layer. In particular, the presence of Sb leads to an enhanced PL thermal stability. From the comparison between the activation energies for thermal quenching of the PL and the modelled band structure, the main carrier escape mechanisms are suggested. In standard GaAs-capped QDs, escape of both electrons and holes to the GaAs barrier is the main PL quenching mechanism. For small-moderate Sb (<16%) for which the type I band alignment is kept, electrons escape to the GaAs barrier and holes escape to the GaAsSb capping layer, where redistribution and retraping processes can take place. For Sb contents above 16% (type-II region), holes remain in the GaAsSb layer and the escape of electrons from the QD to the GaAs barrier is most likely the dominant PL quenching mechanism. This means that electrons and holes behave dynamically as uncorrelated pairs in both the type-I and type-II structures.

Properties and applications of high electron density structures based on InN and related compounds

El objetivo principal del presente trabajo es estudiar y explotar estructuras que presentan un gas bidimensional de electrones (2DEG) basadas en compuestos nitruros con alto contenido de indio. Existen muchas preguntas abiertas, relacionadas con el nitruro de indio y sus aleaciones, algunas de las cuales se han abordado en este estudio. En particular, se han investigado temas relacionados con el análisis y la tecnología del material, tanto para el InN y heteroestructuras de InAl(Ga)N/GaN como para sus aplicaciones a dispositivos avanzados. Después de un análisis de la dependencia de las propiedades del InN con respecto a tratamientos de procesado de dispositivos (plasma y térmicos), el problema relacionado con la formación de un contacto rectificador es considerado. Concretamente, su dificultad es debida a la presencia de acumulación de electrones superficiales en la forma de un gas bidimensional de electrones, debido al pinning del nivel de Fermi. El uso de métodos electroquímicos, comparados con técnicas propias de la microelectrónica, ha ayudado para la realización de esta tarea. En particular, se ha conseguido lamodulación de la acumulación de electrones con éxito. En heteroestructuras como InAl(Ga)N/GaN, el gas bidimensional está presente en la intercara entre GaN y InAl(Ga)N, aunque no haya polarización externa (estructuras modo on). La tecnología relacionada con la fabricación de transistores de alta movilidad en modo off (E-mode) es investigada. Se utiliza un método de ataque húmedo mediante una solución de contenido alcalino, estudiando las modificaciones estructurales que sufre la barrera. En este sentido, la necesidad de un control preciso sobre el material atacado es fundamental para obtener una estructura recessed para aplicaciones a transistores, con densidad de defectos e inhomogeneidad mínimos. La dependencia de la velocidad de ataque de las propiedades de las muestras antes del tratamiento es observada y comentada. Se presentan también investigaciones relacionadas con las propiedades básicas del InN. Gracias al uso de una puerta a través de un electrolito, el desplazamiento de los picos obtenidos por espectroscopia Raman es correlacionado con una variación de la densidad de electrones superficiales. En lo que concierne la aplicación a dispositivos, debido al estado de la tecnología actual y a la calidad del material InN, todavía no apto para dispositivos, la tesis se enfoca a la aplicación de heteroestructuras de InAl(Ga)N/GaN. Gracias a las ventajas de una barrera muy fina, comparada con la tecnología de AlGaN/GaN, el uso de esta estructura es adecuado para aplicaciones que requieren una elevada sensibilidad, estando el canal 2DEG más cerca de la superficie. De hecho, la sensibilidad obtenida en sensores de pH es comparable al estado del arte en términos de variaciones de potencial superficial, y, debido al poco espesor de la barrera, la variación de la corriente con el pH puede ser medida sin necesidad de un electrodo de referencia externo. Además, estructuras fotoconductivas basadas en un gas bidimensional presentan alta ganancia debida al elevado campo eléctrico en la intercara, que induce una elevada fuerza de separación entre hueco y electrón generados por absorción de luz. El uso de metalizaciones de tipo Schottky (fotodiodos Schottky y metal-semiconductormetal) reduce la corriente de oscuridad, en comparación con los fotoconductores. Además, la barrera delgada aumenta la eficiencia de extracción de los portadores. En consecuencia, se obtiene ganancia en todos los dispositivos analizados basados en heteroestructuras de InAl(Ga)N/GaN. Aunque presentando fotoconductividad persistente (PPC), los dispositivos resultan más rápidos con respeto a los valores que se dan en la literatura acerca de PPC en sistemas fotoconductivos. ABSTRACT The main objective of the present work is to study and exploit the two-dimensionalelectron- gas (2DEG) structures based on In-related nitride compounds. Many open questions are analyzed. In particular, technology and material-related topics are the focus of interest regarding both InNmaterial and InAl(Ga)N/GaNheterostructures (HSs) as well as their application to advanced devices. After the analysis of the dependence of InN properties on processing treatments (plasma-based and thermal), the problemof electrical blocking behaviour is taken into consideration. In particular its difficulty is due to the presence of a surface electron accumulation (SEA) in the form of a 2DEG, due to Fermi level pinning. The use of electrochemical methods, compared to standard microelectronic techniques, helped in the successful realization of this task. In particular, reversible modulation of SEA is accomplished. In heterostructures such as InAl(Ga)N/GaN, the 2DEGis present at the interface between GaN and InAl(Ga)N even without an external bias (normally-on structures). The technology related to the fabrication of normally off (E-mode) high-electron-mobility transistors (HEMTs) is investigated in heterostructures. An alkali-based wet-etching method is analysed, standing out the structural modifications the barrier underwent. The need of a precise control of the etched material is crucial, in this sense, to obtain a recessed structure for HEMT application with the lowest defect density and inhomogeneity. The dependence of the etch rate on the as-grown properties is observed and commented. Fundamental investigation related to InNis presented, related to the physics of this degeneratematerial. With the help of electrolyte gating (EG), the shift in Raman peaks is correlated to a variation in surface eletron density. As far as the application to device is concerned, due to the actual state of the technology and material quality of InN, not suitable for working devices yet, the focus is directed to the applications of InAl(Ga)N/GaN HSs. Due to the advantages of a very thin barrier layer, compared to standard AlGaN/GaN technology, the use of this structure is suitable for high sensitivity applications being the 2DEG channel closer to the surface. In fact, pH sensitivity obtained is comparable to the state-of-the-art in terms of surface potential variations, and, due to the ultrathin barrier, the current variation with pH can be recorded with no need of the external reference electrode. Moreover, 2DEG photoconductive structures present a high photoconductive gain duemostly to the high electric field at the interface,and hence a high separation strength of photogenerated electron and hole. The use of Schottky metallizations (Schottky photodiode and metal-semiconductor-metal) reduce the dark current, compared to photoconduction, and the thin barrier helps to increase the extraction efficiency. Gain is obtained in all the device structures investigated. The devices, even if they present persistent photoconductivity (PPC), resulted faster than the standard PPC related decay values.

The neuroprotective agent SR 57746A abrogates experimental autoimmune encephalomyelitis and impairs associated blood–brain barrier disruption: Implications for multiple sclerosis treatment

Experimental autoimmune encephalomyelitis (EAE) is a T cell autoimmune disorder that is a widely used animal model for multiple sclerosis (MS) and, as in MS, clinical signs of EAE are associated with blood–brain barrier (BBB) disruption. SR 57746A, a nonpeptide drug without classical immunosuppressive properties, efficiently protected the BBB and impaired intrathecal IgG synthesis (two conventional markers of MS exacerbation) and consequently suppressed EAE clinical signs. This compound inhibited EAE-induced spinal cord mononuclear cell invasion and normalized tumor necrosis factor α and IFN-γ mRNA expression within the spinal cord. These data suggested that pharmacological intervention aimed at inhibiting proinflammatory cytokine expression within the central nervous system provided protection against BBB disruption, the first clinical sign of EAE and probably the key point of acute MS attacks. This finding could lead to the development of a new class of compounds for oral therapy of MS, as a supplement to immunosuppressive agents.

An apical permeability barrier to NH3/NH4+ in isolated, perfused colonic crypts.

Fermentation of nonabsorbed nutrients in the colon generates high concentrations of NH3/NH4+ in the colonic lumen. NH3 is a small, lipophilic neutral weak base that readily permeates almost all cell membranes, whereas its conjugate weak acid NH4+ generally crosses membranes much more slowly. It is not known how colonocytes maintain intracellular pH in the unusual acid-base environment of the colon, where permeant acid-base products of fermentation exist in high concentration. To address this issue, we hand dissected and perfused single, isolated crypts from rabbit proximal colon, adapting techniques from renal-tubule microperfusion. Crypt perfusion permits control of solutions at the apical (luminal) and basolateral (serosal) surfaces of crypt cells. We assessed apical- vs. basolateral-membrane transport of NH3/NH4+ by using fluorescent dyes and digital imaging to monitor intracellular pH of microvacuolated crypt cells as well as luminal pH. We found that, although the basolateral membranes have normal NH3/NH4+ permeability properties, there is no evidence for transport of either NH3 or NH4+ across the apical borders of these crypt cells. Disaggregating luminal mucus did not increase the transport of NH3/NH4+ across the apical border. We conclude that, compared to the basolateral membrane, the apical border of crypt colonocytes has a very low permeability-area product for NH3/NH4+. This barrier may represent an important adaptation for the survival of crypt cells in the environment of the colon.

Photoconductivity and Electro-Optical Properties of Wide Band Gap Metal Oxides

The PhD activity described in this Thesis was focused on the study of metal-oxide wide-bandgap materials, aiming at fabricating new optoelectronic devices such as solar-blind UV photodetectors, high power electronics, and gas sensors. Photocurrent spectroscopy and DC photocurrent time evolution were used to investigate the performance of prototypes under different atmospheres, temperatures and excitation wavelengths (or dark conditions). Cathodoluminescence, absorption spectroscopy, XRD and SEM were used to assess structural, morphologic, electrical and optical properties of materials. This thesis is divided into two main sections, each describing the work done on a different metal-oxide semiconductor. 1) MOVPE-grown Ga2O3 thin films for UV solar-blind photodetectors and high power devices The semiconducting oxides, among them Ga2O3, have been employed for several decades as transparent conducting oxide (TCO) electrodes for fabrication of solar cells, displays, electronic, and opto-electronic devices. The interest was mainly confined to such applications, as these materials tend to grow intrinsically n-type, and attempts to get an effective p-type doping has consistently failed. The key requirements of TCO electrodes are indeed high electrical conductivity and good transparency, while crystallographic perfection is a minor issue. Furthermore, for a long period no high-quality substrates and epi-layers were available, which in turn impeded the development of a truly full-oxide electronics. Recently, Ga2O3 has attracted renewed interest, as large single crystals and high-quality homo- and hetero-epitaxial layers became available, which paved the way to novel application areas. Our research group spent the last two years in developing a low temperature (500-700°C) MOVPE growth procedure to obtain thin films of Ga2O3 on different substrates (Dept. of Physics and IMEM-CNR at UNIPR). We obtained a significant result growing on oriented sapphire epitaxial films of high crystalline, undoped, pure phase -Ga2O3 (hexagonal). The crystallographic properties of this phase were investigated by XRD, in order to clarify the lattice parameters of the hexagonal cell. First design and development of solar blind UV photodetectors based on -phase was carried out and the optoelectronic performance is evaluated by means of photocurrent spectroscopy. The UV-response is adequately fast and reliable to render this unusual phase a subject of great interest for future applications. The availability of a hexagonal phase of Ga2O3 stable up to 700°C, belonging to the same space group of gallium nitride, with high crystallinity and tunable electrical properties, is intriguing in view of the development of nitride-based devices, by taking advantage of the more favorable symmetry and epitaxial relationships with respect to the monoclinic β-phase. In addition, annealing at temperatures higher than 700°C demonstrate that the hexagonal phase converts totally in the monoclinic one. 2) ZnO nano-tetrapods: charge transport mechanisms and time-response in optoelectronic devices and sensors Size and morphology of ZnO at the nanometer scale play a key role in tailoring its physical and chemical properties. Thanks to the possibility of growing zinc oxide in a variety of different nanostructures, there is a great variety of applications, among which gas sensors, light emitting diodes, transparent conducting oxides, solar cells. Even if the operation of ZnO nanostructure-based devices has been recently demonstrated, the mechanisms of charge transport in these assembly is still under debate. The candidate performed an accurate investigation by photocurrent spectroscopy and DC-photocurrent time evolution of electrical response of both single-tetrapod and tetrapod-assembly devices. During the research done for this thesis, a thermal activation energy enables the performance of samples at high temperatures (above about 300°C). The energy barrier is related to the leg-to-leg interconnection in the assembly of nanotetrapods. Percolation mechanisms are responsible for both the very slow photo-response (minutes to hours or days) and the significant persistent photocurrent. Below the bandgap energy, electronic states were investigated but their contribution to the photocurrent are two-three order of magnitude lower than the band edge. Such devices are suitable for employ in photodetectors as well as in gas sensors, provided that the mechanism by which the photo-current is generated and gas adsorption on the surface modify the conductivity of the material are known.

Surface, Thermal and Antimicrobial Release Properties of Plasma-Treated Zein Films

The effects of dielectric barrier discharge plasma treatment on zein film containing thymol as an active ingredient were evaluated. The plasma discharge was optically characterized to identify the reactive species. A significant increase in the film roughness (p < 0.05) was observed due to the etching effect of DBD plasma, which was correlated with the increase in the diffusion rate of thymol in the food simulant. The diffusion of thymol from the zein film was measured in aqueous solution. The kinetics of thymol release followed the Fick’s law of diffusion as shown by the high correlation coefficients between experimental and theoretical data. No significant change (p > 0.05) was observed for the thermal properties of the antimicrobial films after DBD plasma treatment.

Functional properties of sodium and calcium caseinate antimicrobial active films containing carvacrol

Active edible films were prepared by adding carvacrol into sodium caseinate (SC) and calcium caseinate (CC) matrices plasticized with two different glycerol concentrations (25 and 35 wt%) prepared by solvent casting. Functional characterisation of these bio-films was carried out by determination of some of their physico-chemical properties, such as colour, transparency, oxygen barrier, wettability, dye permeation properties and antibacterial effectiveness against Gram negative and Gram positive bacteria. All films exhibited good performance in terms of optical properties in the CIELab space showing high transparency. Carvacrol was able to reduce CC oxygen permeability and slightly increased the surface hydrophobicity. Dye diffusion experiments were performed to evaluate permeation properties. The diffusion of dye through films revealed that SC was more permeable than CC. The agar diffusion method was used for the evaluation of the films antimicrobial effectiveness against Escherichia coli and Staphylococcus aureus. Both SC and CC edible films with carvacrol showed inhibitory effects on both bacteria.