Electrical properties and microstructure of ternary Ge/Ti/Al ohmic contacts to p-type 4H-SiC

The high-power SiC devices require ohmic contact materials, which are prepared by annealing at temperatures lower than 800 degreesC. Recently, we demonstrated in our previous paper [J. Appl. Phys. 95, 2187 (2004)] that an addition of a small amount of Ge to the conventional binary Ti/Al contacts reduced the ohmic contact formation temperature by about 500 degreesC, and this ternary contacts yielded a specific contact resistance of approximately 1x10(-4) Omega cm(2) after annealing at a temperature as low as 600 degreesC. In this paper, the electrical properties and the microstructures of the Ge/Ti/Al contacts (where a slash / indicates the deposition sequence) were investigated by current-voltage measurements and transmission electron microscopy observations, respectively, in order to understand the ohmic contact formation mechanism. Ti3SiC2 compound layers (which were previously observed at the metal/SiC interface in the Ti/Al ohmic contacts after annealing at temperatures higher than 1000 degreesC) were observed to grow epitaxially on the SiC surface after annealing at temperatures as low as 600 degreesC. The Ti3SiC2 layers were believed to act as a p-type intermediate semiconductor layer, which played a key role to reduce the Schottky barrier height at the contacting metal/SiC interface. Further reduction of the contact resistances of the Ge/Ti/Al contacts would be achieved by increasing the coverage of the Ti3SiC2 layers on the SiC surface. (C) 2004 American Institute of Physics.