Formation of N₂O gas bubbles in anodic films on NbN_x alloys

Sputter-deposited niobium, and Nb₉₀N₁₀, Nb₈₃N₁₇ and Nb₆₀N₄₀ alloys, have been anodized at 50 A m^-2 in 0.1 mol dm^-3 ammonium pentaborate electrolyte at 293 K and resultant amorphous anodic films have been characterized by transmission electron microscopy, glow discharge optical emission spectroscopy, Rutherford backscattering spectrometry and Fourier-transform, infra-red spectroscopy. Except for the Nb₆₀N₄₀ alloy, which shows a breakdown at approximately 60 V, the deposited layers reveal a linear voltage increase to more than 150 V, with the rate decreasing with the niobium content. Additionally, anodic films, with flat and parallel metal/film and film/electrolyte interfaces, are formed on the specimens with the formation ratio decreasing with increasing nitrogen content in the deposited films. Nitrogen is incorporated into the inner ∼70-75% of the film thickness as N₂O molecules, forming fine bubbles, with typical sizes up to 10 nm, in a Nb₂O₅-based matrix material. The remaining 25-30% of the film comprises a layer of Nb₂O₅ with the outer regions containing boron species derived from the electrolyte. Release of high pressure N₂O gas is associated with the relatively low breakdown voltage for the Nb₆₀N₄₀ alloy of highest nitrogen content. The two-layered nature of the anodic films arises from the film growth mechanism that involves outward migration of niobium species and inward migration of oxygen species; in contrast nitrogen species are immobile due either to the relatively high energy of the N-O bond or to their presence in bubbles.