Ti and TiAl-based ohmic contacts to 4H-SiC

This paper describes successfully formed ohmic contacts to 4H-SiC with a view to their use in the technology of bipolar devices. The individual activities included in this investigation were divided into two main parts concerning n-type and p-type silicon carbide processing and different issues have been discussed in relation to that division. In the first part of the experiment titanium-based ohmic contacts to n-type 4H-SiC have been fabricated. The influence of annealing temperature within the range of 850-1100°C and the composition of working gases in RTP reactor on I-V characteristics and contact resistance have been examined. Furthermore, the effect of surface preparation by thermal oxidation of SiC substrate and removal of the oxide immediately prior to contact metallization deposition was investigated. The results obtained for the Si-face (0001) and C-face (000-1) of n-type 4H-SiC were compared. Further research concerns ohmic contacts formation to p-type 4H-SiC based on titanium-aluminum alloys. Four metallization compositions differing in the aluminum layer thickness (25, 50, 75, 100 nm) at a constant thickness of the titanium layer (50 nm) were examined. The structures were annealed within temperature range of 800°C - 1100°C and then electrically characterized. The best electrical parameters and linear, ohmic character of contacts were obtained for structures with Al layer thickness equal or greater then Ti layer thickness and annealing at temperature of 1000°C or higher. Circular Transmission Line Measurement (c-TLM) technique was adopted in both described cases to define a variation in contact resistance (Rc), transfer length (LT), specific contact resistance (ρc) and sheet resistance (Rsh) between metal contacts and silicon carbide.