Refractory metal boride ohmic contacts to P-Type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC (1.3 × 10¹⁹ cm^-3) using CrB₂, W₂B, and TiB₂. The boride layers (∼100-200 nm) were sputter-deposited in a system with a base pressure of 3×10^-7 Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry. All as-deposited contacts exhibited rectifying characteristics. Ohmic behavior was observed following short anneals (2-10 min) at 1100°C and 5 × 10^-7 Torr. Current-voltage characteristics were linear for CrB₂ and W₂B and quasi-linear for TiB₂. The lowest values of the specific contact resistance (r_c in Ω-cm²) measured at room temperature for CrB₂ and W₂B were 8.2 × 10^-5 and 5.8 × 10^-5 respectively. The specific contact resistance for TiB₂ was not determined accurately. Longer anneals (30 min for W₂B and 90 min for CrB₂) reduced the room temperature values of r_c to 6.1 × 10^-5 for W₂B and 1.9 × 10^-5 for CrB₂. Backscattering spectra revealed substantial concentrations of oxygen in all as-deposited boride films. The short anneal cycle removed the oxygen in the CrB₂ films and reduced the concentration substantially in the W₂B films; however, annealing had no affect on the oxygen concentration in the TiB₂ films. The CrB₂/SiC interface remained stable during annealing; i.e., Si and carbon were not observed in the boride layers after annealing. In contrast, W₂B and TiB₂ reacted with the SiC epilayers, and after annealing, Si and carbon were observed at the surface of each boride layer.