Papers by Author: Bernard Enrico Watts

Abstract: We have investigated 3C-SiC layers grown on silicon and on poli-Si in order to realize test MEMS structures. The strain of the films were investigated by the fabrication of cantilevers, beams, springs and we successfully fabricated a Double-Ended-Tuning-Fork double clamped SiC resonator on the film, with perfectly aligned actuation electrode.

Abstract: β-SiC/SiO₂ core-shell NWs have been synthesized on patterned silicon wafers in a CVD system, using carbon oxide as single precursor and nickel nitrate as catalytic element, in nitrogen or argon atmosphere at 1100°C. The coaxial structure and the crystallinity of the NW core are examined by (scanning) transmission electron microscopy. The patterning of the substrate allows to grow NWs in selected areas only, as imaged by SEM. Cathodoluminescence (CL) panchromatic images of the same areas point out that the light emitted under electron excitation is localized only in the area covered with NWs. The room-temperature CL spectrum has three different components peaked at 2.45 eV, related to the 3C-SiC near-band-edge emission, and at 2.75 and 3.75 eV, that are induced by the triplet and singlet states of oxygen-deficiency centers ODC(II) in silicon dioxide shell.

Abstract: In this work we report on the growth of cubic silicon carbide using CBr₄ and silane as precursors at different C₃H₈/CBr₄ flow ratios. The layers were deposited on 2’’ (001) Si wafers by means of the VPE technique in a horizontal cold-wall reactor with induction heating. The growths were performed at atmospheric pressure, in H₂ atmosphere and involved several steps: Si thermal etching; carburisation; epitaxial growth. Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were performed to observe the film morphology and defects and correlate them with the gas phase composition. Results show that the addition of CBr₄ to the standard SiH₄ and C₃H₈ precursor can change the crystalline nature and the morphology of the grown SiC.

Abstract: Cubic silicon carbide nanowires (-SiC or 3C-SiC NW) have been grown by Vapour Phase Epitaxy on (001) silicon substrates patterned by conventional photolithography and on Micro Electro Mechanical Systems (MEMS, e.g. cantilevers, springs, bridges) fabricated on (001) Silicon On Insulator (SOI) wafers. The NW morphology was investigated by scanning electron microscopy, showing that the nanowires grew selectively where a nickel thin layer was previously deposited, thanks to its catalytic action. High resolution transmission electron microscopy studies showed that the NWs are predominantly 3C polytype with <111> growth axis and stacking defects on (111) planes.

Abstract: To assess deformation issues in SiC/Si, different pre-growth procedures were investigated, involving the addition of SiH4 to C3H8 during the temperature ramps used for the carbonization. 3C-SiC layers were deposited on (001) and (111) Si substrates by VPE. The mechanical deformation of the wafer was measured by makyoh, obtaining 2D maps of the entire wafers. For the same pre-growth procedures, the substrate curvature depends strongly on the orientation of the substrate, (001) or (111), being generally lower for (111) substrates. The deformation results were compared with XRD and Raman spectroscopy. Plastic deformation of the substrate was evidenced by XRD, while the presence of tensile stress is suggested by Raman analysis.

Abstract: -SiC and -SiC/SiO2 core-shell nanowires (NWs) grown on silicon substrates by three different processes, based on the use of i) carbon monoxide, ii) silane with propane and iii) carbon tetrachloride precursors, are analysed by structural and optical techniques. Spectroscopic cathodoluminescence studies show a luminescence enhancement in core-shell structures, ascribed to an effective role of the shell as both carrier injecting barrier and passivation layer. In NWs grown using CCl4 precursor, a peculiar luminescence with dominant red component at about 2 eV has been detected and ascribed to point defects related to an unintentional oxygen incorporation.

Abstract: 3C-SiC films were grown on Si by VPE using CBr4 as the carbon source, at temperatures ranging between 1100 to 1250°C. XRD, TEM, AFM, and SEM results indicate that the epitaxy proceeds as a 3D growth of uncoalesced islands at low temperature, whereas a continuous layer with hillocks on top is obtained above 1200°C. The shape and faceting of the islands are analyzed by AFM, showing (311) preferred facets.

Abstract: A comparative study of the morphology of 3C-SiC films prepared with different C:Si ratios is presented. The silane precursor controls the growth rate at all values of C:Si ratio but combined of observations using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) indicates that the C:Si ratio is critical in determining the grain size and at values of C:Si close to 1 texturing and faceting become evident. Makyoh Topography reveals various surface defects, a slight mesoscale roughness and bending of the epiwafers.

Abstract: M-type lead hexaferrites (PbFe12O19) have been prepared using sol-gel methods. Lead and iron precursors were dissolved in butoxyethanol and stabilised with acetylacetone to produce a sol with good wetting and surface coating characteristics. Powder samples were investigated to optimise conditions for the growth of thin films. The «spin on» technique was used to provide homogeneous films. X-ray diffraction and thermomagnetic analysis vibrating sample magnetometry have been performed to assess phase distribution and transformation. The results show that the hexaferrite phase can be crystallised above 700 °C and that for the first indication that this phase is metastable in air below 650 °C.

Abstract: In recent years several ferroelectric thin films have been studied at microwave frequencies; lead zirconate titanate (PZT) and barium-strontium titanate (BST) has been widely investigated. However, the microwave dielectric properties of strontium-bismuth tantalate (SBT) have not yet been investigated so widely [1]. The purpose of this work is the microwave characterization of the dielectric properties of an SBT thin film biased at different DC voltages. The dielectric properties of SBT make it a good material for the production of FERAM memories. Microwave characterizations may show other properties that could promote the SBT as good candidate for capacitors to be employed also in microwave circuits (e.g. resonators and filters). In this work a study of high frequency dielectric properties has been performed and equivalent circuit model has been used to correct the measurements.