Electron scattering at Co(0001) surfaces: Effects of Ti and TiN capping layers

In situ transport measurements on epitaxial 7.6-nm-thick Co(0001)/Al2O3(0001) films with and without Ti and TiN capping layers during O2 exposure are used to investigate the effects of surface chemistry on electron scattering at Co(0001) surfaces. The Co sheet resistance Rs increases with increasing thickness dTi and dTiN of the Ti and TiN capping layers, saturating at 8% and 31% above the uncoated Co(0001) for dTi > 0.2 nm and dTiN > 0.1 nm, respectively. This increase is attributed to electron scattering into local surface states, which is less pronounced for Ti than TiN. In situ resistance measurements taken during a continuously increasing O2 partial pressure from 0 Pa to 40 Pa indicate a relatively steep 24% increase in Rs at an exposure of ∼50 Pa s, which can be attributed to Co surface oxidation that leads to atomic level roughness and a decrease in the electron scattering specularity p. Ti and TiN cap layers with dTi ≥ 0.5 nm and dTiN ≥ 0.13 nm exhibit no resistance change upon air exposure, indicating suppression of Co oxidation. These results indicate a promising Co–Ti interface with an electron scattering specularity of p = 0.4–0.5, which is retained during oxygen exposure, while, in contrast, electron scattering at the Co–TiN interface is completely diffuse (p = 0), suggesting that Ti barrier layers facilitate higher-conductivity Co interconnects than TiN barriers, as long as the Ti layer is sufficiently thick (dTi ≥ 0.5 nm) to suppress Co oxidation.