Formation of polyhedral N2 bubbles during reactive sputter deposition of epitaxial TiN(100) films

Abstract

Cross-sectional transmission electron microscopy has been used to investigate the formation of polyhedral N2 bubbles in single-crystal TiN(100). Thin TiN films were deposited on MgO(100) substrates by reactive magnetron sputtering in pure N2 atmospheres at growth temperatures Ts between 550 and 800 °C and negative substrate biases Vs between 0 and 500 V resulting in a deposition rate between 0.35 and 0.33 nm s−1. The energy per incident nitrogen was ∼eVs/2 and the irradiation fluxes were ≤2.5×1015 N+2 cm−2 s−1. Nitrogen bubbles were observed in films grown at 550≤Ts<800 °C with Vs>300 V. In as-deposited films grown at Ts=650 °C with Vs=500 V, the bubbles were distributed in two shapes and size distributions. The largest bubbles were orthorhombohedra, i.e., cubes elongated along [100] directions normal to the growth direction, with 5–20 nm faces on {100} planes most of which exhibited {110} and {111} faceting. Small (≤5 nm) octahedral bubbles with {111} faces were also observed. In situ annealing experiments in the transmission electron microscope showed that the bubbles transformed from orthorhombohedra to octahedra at Ts=350 °C and then returned to orthorhombohedral shape with either continued heating to Ts=600 °C or upon cooling to room temperature. Assuming quasiequilibrium conditions, the surface energy γ of N2 bubbles in TiN was found to be anisotropic and to vary as γ100<γ110<γ111. Some of the smaller orthorhombohedral bubbles exhibited Moiré fringes with a spacing indicative of solid-phase precipitation of cubic α-N2.