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Published online by Cambridge University Press: 01 February 2011
Nickel (Ni) contacts to n-type silicon carbide (n-SiC) are annealed to ∼950°C in order to achieve their good ohmic properties through the reaction of the Ni with Si from the SiC to form nickel silicides. Unfortunately, the physical contact, and therefore the reliability, is poor. A possible cause is that the silicidation reaction liberates C from the SiC, which then diffuses throughout the contact. The reaction also produces a poor morphology and voids form at the metal-SiC interface. To try to understand the processes that produce the good electrical properties and at the same time improve the physical properties, we studied the reactions of Si/Ni contacts with 1:1 and 1:2 stoichiometric ratios as well as Ni-only contacts on n- and p-type SiC, both visually and electrically in situ using a hot stage and microprobe-equipped scanning electron microscope (SEM). The visual observations of the Ni-only film show that it does not react with the SiC until the temperature reaches 500–550°C. For the n-type SiC, the electrical measurements show a decrease in contact resistivity as the anneal temperature is increased from 500°C to 650°C. Increasing the anneal temperature further increases the resistivity until it begins to drop precipitously as the temperature approaches 950°C and higher. The visual observations of the Si/Ni contacts show that the Si and Ni are reacting at ∼600°C, with phases nucleating and then growing laterally. The electrical measurements for the n-type samples show that the contact resistance initially drops at 100–300°C indicating that there may be reactions, unseen by the SEM, at lower temperatures. The resistance continues to rise and fall over the intervening temperatures but begins to consistently and significantly fall at temperatures above 850°C, and then reaches ohmic values at 900–950°C. Because the silicidation reactions are seen to occur at temperatures far below those required to achieve ohmic properties, it is clear that silicide formation, while it may be necessary, is not sufficient for the formation of Ni-ohmic contacts to n-SiC. In this work, it has been observed that reaction of the Ni with the SiC appears to be necessary for achieving ohmic properties. While this may form a more intimate contact, it is proposed that damaging the SiC surface with this reaction is an important part of ohmic contact formation, possibly through increased current tunneling through interface defect states.
