Papers by Author: Samuele Porro

Abstract: Since their discovery, carbon nanotubes received a great deal of attention because of unique physical and chemical properties. However, in order to become of interest in the field of super resistant fibers for nanocomposite materials or in the production of textile material, very long carbon nanotubes are needed. Massive samples of well packed, vertically aligned and very long selfstanding multi wall carbon nanotubes (MWNT) were synthesized on uncoated silicon by a very efficient thermal CVD process, which involved the co-evaporation of camphor and ferrocene in a nitrogen atmosphere. We obtained structures with diameter between 20 and 80 nm with an average growth rate of about 400 nm/s, organized in thick carpets of entangled nanotubes. By the weight of the deposited carpet of MWNTs (density circa 0.8 g/cm3) the conversion of about 30% of the total hydrocarbon feedstock was calculated. Morphology and physical properties were characterized by electron microscopy techniques, Micro- Raman spectroscopy and thermogravimetric analysis. The analyses performed showed the absence of secondary carbonaceous products, whereas only 6% in weight of ferromagnetic iron clusters are present. BET analysis was used to calculate the porosity and the specific surface area density of the as grown samples, which resulted approximately 70 m2/g. Hydrophobicity of the CNT carpet was also investigated.

Abstract: We present a theoretical and experimental study on the design, fabrication and characterization of Schottky Barrier Diodes (SBD) on commercial 4H-SiC epitaxial layers. Numerical simulations were performed with a commercial tool on different edge termination structures, with the aim of optimizing the device behavior. For each termination design, SBD were fabricated and characterized by means of electrical measurements vs. temperature. Simulations provided also useful data for the assessment of the device process technology.

Abstract: In this work we report an analysis on Ni/4H-SiC interfaces aimed at optimizing the ohmic contacts. Several thermal cycles have been performed by rapid thermal annealing checking the possible chemical reactions at the metal semiconductor interfaces. Micro x-ray diffraction and micro Raman techniques have been performed in order to study the interface micro structural evolution. Inter diffusion of each element at the Ni - SiC interface was examined using Auger spectroscopy. Electrical measurements have been performed in order to check the ohmic behavior of the contacts. Finally, a correlation between microstructures evolution and electrical behaviors is reported.

Abstract: Specific on-resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick layers exhibits variations from tens of mW.cm2 to tens of W.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with a literature reported values. Decrease in mobility with the temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples respectively were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy, an influence of shallow boron related levels and D-center on dependence of on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism thus resulting in high Ron.