Formation of low-resistivity Au/Mo/Ti ohmic contacts to $p$-diamond epitaxial layers

The formation of Au/Mo/Ti ohmic contacts to $p$-diamond epitaxial films has been studied. Specifically, the influence of annealing on the electrical properties and structure of contacts has been investigated. It has been shown that the upper gold layer protects the contact system against oxidation up to 850$^\circ$C during RTA unlike the case of a “simplified” Au-free Mo/Ti system frequently used in today’s solutions. In Mo-free Au/Ti systems, high-temperature annealing causes effective diffusion of titanium into the gold layer, which deteriorates the protective properties of the latter and enhances oxygen diffusion toward the interface with diamond. Oxidation of the Ti/C contact area prevents the formation of a titanium carbide conducting layer, which has high adhesion to diamond. The role of various factors, namely, annealing to form titanium carbide, heavy doping of diamond with boron, and crystal perfection of diamond films, in lowering the contact resistance, has been estimated. For doped epitaxial films grown on single-sector substrates, unalloyed ohmic contacts with a record low contact resistance of 4 $\times$ 10$^{-7}$ $\Omega$/cm$^{2}$ have been obtained.