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°C. 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°C, and this ternary contacts yielded a specific contact resistance of approximately 1 × 10^-4 ω cm² after annealing at a temperature as low as 600°C. 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°C) were observed to grow epitaxially on the SiC surface after annealing at temperatures as low as 600°C. 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 Ti₃SiC ₂ layers on the SiC surface.