Adhesion of the electrodes on diamond device surfaces
Appropriate candidates of the metallic sheet used for the electrodes of diamond semiconductor are investigated using computational approaches based on density functional theory (DFT). For twenty kinds of metallic elements $x$, we modeled a diamond-metal interface and evaluated its work of separation, $W_\mathrm{sep}(x)$, as a possible measure of anti-peeling strength. The appropriateness of the Ohmic contact was inferred from DOS (density of states) analysis of diamond-metal interface by looking at whether an in-gap (isolated/localized) peak disappears as well as a sufficient amount of DOS value exists around the Fermi level. Our DFT simulation confirmed that a typical electrode, Au, is not adhesive enough for power devices [$W_\mathrm{sep}(\mathrm{Au}) = 0.80$ J/m$^2$], though showing the Ohmic contact. In contrast, some transition metals were found to possess Ohmic features with much stronger adhesion than Au [e.g., $W_\mathrm{sep}(\mathrm{Cr}/\mathrm{Ti}) = 6.02/4.03$ J/m$^2$].