Modeling and Characterization of Metal/SiC Interface for Power Device Application

Abstract

Silicon carbide is a wide band-gap semiconductor widely considered to be an excellent material for the fabrication of power devices able to operate in extreme environmental conditions. Its superior properties such as wide energy bandgap, high hardness, chemical inertness, high electrical field breakdown strength and high thermal conductivity enable electronic devices, based on it, to operate at high temperatures, high voltages and high frequencies and make it an attractive semiconducting material for the power electronics industry. Since 1999 a number of electronic devices based on silicon carbide are commercially available such as Schottky barrier diodes with voltage rating of 300 - 1700 V (as of 2011) which often show non-ideal electrical behavior. Non-ideal electrical behavior is manifested in the abnormal current-voltage characteristics and greater than unity ideality factors. Various theories exist as to the origin of these non-idealities some attribute them to different conduction mechanisms such as generation-recombination and edge-related currents and others to the inhomogeneous Schottky barrier. We have considered the approach, taken by Tung, which can explain all the non-ideal behaviors with thermionic emission theory alone by assuming the Schottky barrier height to be inhomogeneous. Inhomogeneous Schottky barrier implies spatially varying isolated low barrier height regions existing alongside a homogeneous high Schottky barrier. These regions are supposed to interact, in case of being situated together, resulting in the region with low barrier height to be pinched-off. If the pinch-off occurs the low barrier height region (or patch depending on the shape) has a Schottky barrier height equal to the 'saddle point potential' in front of that patch or low barrier region. Whole Schottky barrier is assumed to be composed of numerous such low barrier height patches. These patches are considered to be embedded into the high background Schottky barrier and define the overall current transport through the Schottky barrier diode. A similar model is the parallel conduction model presented by D. Defives et al. which instead of considering the Schottky barrier to be composed of various small patches, divides the Schottky barrier into two major parts each with different Schottky barrier height and both existing simultaneously within one Schottky barrier diode. Though accurate to some extent, this model considers the two Schottky barrier heights to be electrically independent of each other; which is not true in all situations. After applying Tung's theoretical model it was possible to extract nearly correct value of Richardson constant for the Schottky diodes with titanium and molybdenum Schottky contacts on 4H silicon carbide. It was also possible to fit the experimental data correctly with Tung's theoretical model. Note: The diodes used in this research work were fabricated during a research project involving Vishay and Politecnico di Torino