Material Adhesion Properties of Ruthenium Phosphides from First Principles

First-principles density functional theory (DFT) calculations are performed to investigate adhesion during a full contact cycle of conductive ruthenium phosphide surfaces (RuP and Ru2P). A range of work of adhesion is obtained, considering various lateral alignments of crystal grains constructed through translations and rotations. Surface reconstruction and oxidation are considered to simulate the micro- and nanoelectromechanical systems (MEMS/NEMS) operating environment. Our results indicate that ruthenium phosphides have work of adhesion lower than those of pure Ru self-mated metal contacts. The onset of chemical bond formation during the switch-closing process leads to anharmonic interaction energy curves deviating from the universal binding-energy relation. Step-by-step switch opening calculations provide additional insight into surface stability against chemical bond formation; in particular, the RuP surface shows no strong bonding across the interface. Surface reconstruction and oxidation lower the work of adhesion for RuP and Ru2P from as high as 3.85 J/m2 and 2.03 J/m2 to as low as 0.08 J/m2 and 0.03 J/m2, respectively, lowering the probability of material transfer, contact damage, or device failure due to surfaces remaining stuck closed. This work provides a systematic investigation of the adhesion properties of Ru, RuO2, RuP, and Ru2P, indicating the potential of ruthenium phosphides as stable conductive contacts for novel MEMS/NEMS switches.