Development of Ni/Al and MUM 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 (N(A)) of 3.0-9.0 x 10(18) cm(-3) and subsequently annealing at 800degreesC in an ultra high vacuum chamber. In contrast with conventional Ti/Al ohmic contacts, which must be annealed at temperatures above 1000degreesC, both the Ni/Al contacts and Ni/Ti/Al contacts showed ohmic behavior after annealing at 800degreesC. The lowest specific contact resistances of the Ni/Al contacts and the Ni/Ti/Al contacts (prepared by annealing at 800degreesC) were 5 x 10(-3) and 6.6 x 10(-5) Omega 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 X 10(-5) Omega cm(2) would be obtained for the Ni/Ti/Al contacts when the hole concentration in the SiC is greater than 1.0 x 10(19) cm-3. In addition, the Ni/Ti/Al contacts showed excellent thermal stability during isothermal annealing at 400degreesC for 10 It 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 800degreesC; 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. (C) 2003 Elsevier Science B.V. All rights reserved.