Computational observation of the strengthening of Cu/TiN metal/ceramic interfaces by sub-nanometer interlayers and dopants

First principles density functional theory calculations were used to explore the enhancement of the structural integrity of Cu/TiN metal/ceramic interfaces by substitutional doping of the interface with Ni, Zn and Sn. To evaluate the interfacial strength, energy barriers to shear displacement and maximum tensile stress before fracture were calculated. Enthalpy of mixing dictated that Sn was not energetically suitable for doping at the interface, whereas both Ni and Zn were. Ni segregated at the interface forming sub-nanometer interlayers between Cu and TiN, whereas Zn formed a solid solution with Cu. While Zn-doping increased the resistance to shear, it led to a weakening of tensile strength. Ni interlayers increased both the shear and tensile strength to a significant degree coinciding with an increase in electron density between the layers. Using analysis form their partial density of states, Ni interlayers were found to accept more electrons from interfacial Ti into their more compact 3d-orbitals than Cu, which accepted more into available 4s-orbitals. Zn doping increased resistance to shear due to its lower electronegativity than Cu, causing interfacial Zn to have a more positive charge, which also raised the barrier to shear when in close contact with positively charged interfacial Ti atoms.