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

Abstract 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 ( N A ) of 3.0–9.0×10 18 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 cm 2 , 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 cm 2 would be obtained for the Ni/Ti/Al contacts when the hole concentration in the SiC is greater than 1.0×10 19 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 Ni 2 Si and NiAl 3 compounds, and the Ni/Ti/Al contacts formed ternary Ti 3 SiC 2 compounds in addition to these binary compounds. A correlation between the electrical properties and the microstructure is discussed.