Papers by Author: Sergio Ferrero

Abstract: Various attempts have been made to evaluate the correct value (A*=146 A/cm2.K2) ofRichardson's constant. In 2005 S. Ferrero et al. published their research in which they performedan analysis of electrical characterizations of twenty Ti/4H-SiC(titanium on silicon carbide) Schottkydiodes with the help of thermionic emission theory and evaluated the value of Richardson's constantto be 17±8 A/cm2.K2; which is very low as compared to the theoretical value of 146 A/cm2.K2.Wehave tried in this paper to evaluate the Richardson's constant's value by nearly same experimental tech-niques followed by S. Ferrero et al. and additionally, have applied Tung's theoretical approach whichdeals with the incorrect value of A* in the perspective of Schottky barrier inhomogeneities caused bythe presence of nanometer size low barrier patches present in the uniform high barrier of the Schottkydiode.We have fabricated two Ti/4H-SiC (titanium on silicon carbide) Schottky diodes with differentareas and oneMo/4H-SiC (molybdenumon silicon carbide) Schottky diode. In this paper we have pre-sented a comparative analysis of forward current-voltage characteristics of all three Schottky diodes.In all three cases we were successful in the evaluation of nearly correct value of Richardson's constant.This work emphasizes the effects of differentmetal-SiC combinations and laboratory environments onthe evaluation of Richardson's constant and the effective area involved in the current transport. As pre-dicted by Tung's model the effective area is seen to be substantially different from the geometric areaof the Schottky diode. Evaluated values of A*, with an error of ±2, come out to be 145.39, 148.33and 148.33 A/cm2.K2for Ti/4H-SiC(large area), Mo/4H-SiC and Ti/4H-SiC(small area) Schottkydiodes, respectively.

Abstract: In this work we studied different Schottky contacts to 4H-SiC with the aim to obtain Schottky Barrier diodes (SBDs) and Junction Barrier Schottky diodes (JBS) able to operate at high temperatures, frequencies and power densities with low power losses. Schottky contacts were fabricated using Mo and Mo/Al layers annealed up to 600 °C using a Rapid Thermal Process (RTP). A comparison with previous results obtained with Ni, Ti and Ti/Al layers annealed up to 400 °C is also proposed. The Schottky contacts were characterized by means of standard Current-Voltage (I-V) and Capacitance-Voltage (C-V) techniques. X-ray Photoelectron Spectroscopy (XPS) analyses were performed in depth profile mode in order to study the structural evolution of the interface Mo/SiC and Al/Mo during annealing treatments. Mo/Al contacts show a lower barrier height and better overall performances in forward polarization when compared to the Ti- and Ni-based contacts, and they are very promising for Schottky contact fabrication on SBD and JBS.

Abstract: In this work we present the results of electrical characterization of 4H-SiC power Schottky diodes with a Mo metal barrier for high power applications. A comparison between different Schottky Barrier Height (SBH) evaluation methods (capacitance-voltage and current-voltage measurements), together with the comparison with other authors’ works, indicates that thermionic current theory is the dominant transport mechanism across the barrier from room temperature (RT) to 450K, while at T < 300K some anomalies in J-V curves appear and SBH and ideality factor significantly change their values. These deviations from ideality are attributed to Schottky barrier inhomogeneities. In particular, a model based on two SBHs seems appropriate to properly describe the electrical behavior of our devices.

Abstract: We have studied different Schottky and ohmic contacts on 4H-SiC with the aim to obtain Schottky barrier diodes (SBDs) capable to operate at high temperatures, frequencies and power densities for long periods of time, and showing low power losses. The control of the Schottky barrier plays an important role in minimizing the power loss of a SBD, and the metal-semiconductor interface properties strongly affect the overall performances of such a device. Schottky contacts were deposited using Ni, Ti, Ti/Al, Mo and Mo/Al layers, and the annealing treatments have been performed up to 600 °C using a rapid thermal annealing process (RTA). Ohmic contacts have been deposited on the wafer backside using Ti/Al or Ti/Ni/Ag layers. The Schottky diodes have been characterized by means of standard current-voltage (I-V) and capacitance-voltage (C-V) techniques. Schottky diodes with Mo and Mo/Al barriers show a lower barrier height and better overall performances in forward polarization when compared to the Ti- and Ni-based contacts.

Abstract: Vertically aligned multiwall carbon nanotubes were directly grown by means of thermal Chemical Vapor Deposition onto epitaxial and bulk double side polished 4H-SiC substrates. Their structure and morphology have been examined through Field Emission Scanning Electron Microscopy and Raman spectroscopy. The results have been compared with CNTs carpets grown in the same conditions on Si substrates. Preliminary analysis of their electrical properties has been performed using the four-point probe technique in order to evaluate their resistivity.

Abstract: The formation of metal/diamond Ohmic contacts is essential to most electronic devices. In order to form a good Ohmic contact to diamond a carbide-forming metal such as Ti or Cr is necessary. In this study, Cr/Au contacts to heavily boron-doped single crystal CVD diamond were fabricated by subsequent deposition of Cr and Au. The surface morphology and specific contact resistance of diamond/Cr/Au contacts has been investigated. The reaction between the Cr metal and the diamond during annealing gives an improved specific contact resistance. However, this reaction also causes a significant change in the surface morphology. The surface morphology of singlecrystal diamond is shown to greatly influence the properties of metal contacts to diamond. Shearforce mode atomic force microscopy (AFM) investigations have been used to examine the diamond surface before metallization, and after removing the metal contact. The initial diamond surface was predominantly smooth, apart from some scratches from the polishing process. Surface RMS roughness values of around 0.4nm were found. Correlation between surface morphology and contact resistance has been found, with rougher surfaces exhibiting a barrier to conduction. An understanding of the contact formation process is an essential step in achieving high quality Ohmic contacts which are vital in the fabrication of high quality diamond devices.

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.