Formation and role of graphite and nickel silicide in nickel based ohmic contacts to n-type silicon carbide

Abstract

Nickel-based contacts, deposited on 4H-SiC C-face substrates, were annealed at temperatures ranging from 800to1040°C and the phase composition of the contact layers analyzed by x-ray diffraction techniques. Ni2Si was identified as the dominant phase for annealing temperatures exceeding 925°C, with further increases in concentration with increasing temperature. At the highest annealing temperature of 1040°C, a 40nm thick nanocrystalline graphite film at the Ni2Si–SiC interface was discovered and its presence confirmed by Raman spectroscopy. The roles of the Ni2Si and graphite films in the formation of ohmic contacts were determined by their subsequent exclusion from the contact composition. Following deposition and annealing, the Ni2Si and graphite layers were etched away selectively and replaced with new metal films deposited at room temperature and without any annealing. Measurement of the current-voltage characteristics revealed that the ohmic nature of the contacts was preserved after removal of the Ni2Si and the graphite layers. It is concluded that the main reason for the conversion of Schottky to ohmic contacts during high-temperature annealing is a change of the electrical properties of the underlying SiC rather than being attributable to the presence of nickel silicides or graphite. It is proposed that a solid-state reaction between nickel and silicon carbide, similar to catalytic graphitization of carbon, may take place during Ni–SiC contact annealing at the temperature of 1040°C. This process may result in the creation of sufficient carbon vacancies in the near-interface region of the SiC to allow increased electron transport through the Schottky barrier.

Contact Formation on Silicon Carbide by Use of Nickel and Tantalum in a Materials Science Point of View

“…The total C content was higher than that of Si in the reaction product when the temperature is at or above 650 o C, indicating the formation of C deficiency region in the near interface side of the SiC. It has been proposed that the transition of Schottky to ohmic contacts during high temperature annealing is due to the creation of sufficient C vacancies in the near interface region of the SiC Nikitina et al, 2005). The C vacancies then act as donors to increase the net concentration of electrons and thus change the electrical properties of the SiC in the near surface region, resulting in the formation of ohmic contact.…”

Initial formation of contact layers on Ni/SiC samples studied by XPS

“…[3,7,9] Since we are dealing with thin film coatings with maximum thickness of around some hundred nanometres, one important means of detailed characterisation is by surface chemical analysis using XPS, AES or SIMS. Among these methods, XPS has the advantage of being the best tool for quantitative surface analysis.…”

Improved Thermal Stability Observed in Ni-Based Ohmic Contacts to n-Type SiC for High-Temperature Applications

“…The 'free carbon' is a product of the reaction between Ni and SiC to form Ni 2 Si and C during the ohmic contact annealing step, [7,8] while the oxygen is a result of exposure to air between the ohmic contact anneal and the subsequent TaSi x deposition. The AES depth profiles obtained from set A contacts shortly before electrical failure (24 h) and after electrical failure (36 h) show oxidation throughout the TaSi x layer.…”

“…In this study, we attempted to control the stoichiometry of the TaSi 2 layer and increased the thicknesses of the Pt and TaSi 2 layers to equal those in Pt/TaSi 2 /Ti/SiC contacts that showed electrical stability at 600 °C. 7 Contact structures with thin and thick Pt and TaSi x layers were compared via electrical measurements and a combination of Auger depth profiles, transmission electron microscope (TEM) images, electron energy loss spectroscopy (EELS), and energy dispersive x-ray spectroscopy (EDS) to identify morphological and chemical changes at key points during the heat treatment tests. The associated phases formed are discussed in terms of contact degradation.…”