In situ scanning tunneling microscopy studies of the evolution of surface morphology and microstructure in epitaxial TiN(001) grown by ultra-high-vacuum reactive magnetron sputtering

The group IV-B transition metal nitride TiN is widely employed as a wear-resistant coating on mechanical components and as a diffusion barrier in microelectronic devices. We use the epitaxial growth of TiN as a model system for insight on the evolution of surface morphology and microstructure in more complex polycrystalline films. Atomically-flat MgO(001) substrates, prepared by air annealing at 950°C for 12 h, are verified by atomic force microscopy (AFM). Epitaxial TiN layers are grown by reactive magnetron sputter deposition in pure N₂ at 650 ≤ T_s ≤ 750°C. Scanning tunneling microscopy (STM) results show that the development of surface morphology is dominated by growth mounds with an aspect-ratio of ≈ 0.006; both the roughness amplitude and average separation between mounds follow an approximate power law dependence on film thickness, t^γ, with γ = 0.25 ± 0.07. The films grow in a two-dimensional multilayer mode in which island edges exhibit dendritic geometries characteristic of limited step-edge mobility. Transmission electron microscopy (TEM) shows that the films are epitaxial with dislocation loops on {111} planes and 〈001〉 misfit dislocations at the interface. Low-energy N⁺₂ ion irradiation during film growth leads to surface smoothing with the smoothest layers, having a surface width of ≅ 0.22 nm, obtained with V_s = 43 V. Increasing V_s ≥ 43 V leads to surface roughening with decreased in-plane length scales.