Surface stability of WN ultrathin films under O2 and H2O exposure: A first-principles study

Recently developed tungsten nitride (WN) ultrathin films are reported to be potentially good diffusion barriers in electronic and protective coatings and have exceptional mechanical strength. However, the surface interactions with the impurities in ambient conditions may play a crucial role in their stability, modifying the electronic and mechanical properties. In this paper, we investigate the molecular and dissociative adsorptions of O-2 and H2O on both polar and nonpolar WN surfaces using density functional theory. The results show that the nonpolar and the W-terminated polar surfaces are highly reactive to O-2. On the other hand, the N-terminated surfaces are stable on which the O-2 dissociation occurs with finite energy barriers. Moreover, H2O dissociation on WN surfaces is not spontaneous irrespective of the surface direction or termination. The results, therefore, suggest that experiments growing the energetically stable W-terminated ultrathin films should employ the controlled conditions to obtain defect-free surfaces that offer excellent mechanical strength.

Automated summary

Recently developed tungsten nitride (WN) ultrathin films are potentially good diffusion barriers in electronic and protective coatings. However, the stability and properties of these films are influenced by the surface interactions with impurities. Molecular and dissociative adsorptions of O-2 and H2O on both polar and nonpolar WN surfaces were investigated. The results show that nonpolar and W-terminated polar surfaces are highly reactive to O-2, while N-terminated surfaces have finite energy barriers for O-2 dissociation. Moreover, dissociation of H2O on WN surfaces is not spontaneous. Controlled conditions are recommended to obtain defect-free surfaces for growing energetically stable W-terminated ultrathin films with excellent mechanical strength.