Development of Ni/Al and Ni/Ti/Al ohmic contact materials for p-type 4H-SiC

New Ni/Al and Ni/Ti/Al ohmic contact materials (a slash ‘/’ indicates the deposition sequence) were developed by depositing on p-type 4H-SiC substrates with doping concentrations (NA) of 3.0–9.0×1018 cm−3 and subsequently annealing at 800 °C in an ultra high vacuum chamber. In contrast with conventional Ti/Al ohmic contacts, which must be annealed at temperatures above 1000 °C, both the Ni/Al contacts and Ni/Ti/Al contacts showed ohmic behavior after annealing at 800 °C. The lowest specific contact resistances of the Ni/Al contacts and the Ni/Ti/Al contacts (prepared by annealing at 800 °C) were 5×10−3 and 6.6×10−5 Ω cm2, respectively. The later value is the lowest contact resistance ever obtained by annealing at such a low temperature. A theory indicates that a contact resistance less than 1.0×10−5 Ω cm2 would be obtained for the Ni/Ti/Al contacts when the hole concentration in the SiC is greater than 1.0×1019 cm−3. In addition, the Ni/Ti/Al contacts showed excellent thermal stability during isothermal annealing at 400 °C for 10 h in Ar atmosphere. The microstructural analysis at the metal/SiC interfaces showed that the Ni/Al contacts and the Ni/Ti/Al contacts reacted with the SiC substrates after annealing at 800 °C; the Ni/Al contacts formed Ni2Si and NiAl3 compounds, and the Ni/Ti/Al contacts formed ternary Ti3SiC2 compounds in addition to these binary compounds. A correlation between the electrical properties and the microstructure is discussed.