Oxynitrogenography: Controlled Synthesis of Single-Phase Tantalum Oxynitride Photoabsorbers

We propose an in situ UV-vis monitoring technique called "oxynitrogenography" as an approach toward the controlled and reproducible synthesis of thin films of different Ta-O-N phases, including the elusive β-TaON phase. The optical absorption changes are measured during annealing of the film at increasing NH₃/H₂O ratios, and can be directly correlated to the presence of different phases (Ta₂O₅, β-TaON, mixed TaON-Ta₃N₅, Ta₃N₅) due to the abrupt change in the absorption edge. After additional XRD analysis, the thermodynamic equilibrium conditions to obtain these various phases are determined, and a phase diagram is constructed. We observe that there is a very narrow range of parameters for the thermodynamic stability of the β-TaON phase. We observe that the carrier mobility increases with the nitrogen content in the sample, from 1 × 10^-5 cm²/(V s) in Ta₂O₅, to 1 × 10^-2 cm²/(V s) in β-TaON and the mixed TaON-Ta₃N₅, until 1 × 10^-1 cm²/(V s) in Ta₃N₅. Although the carrier mobility of β-TaON and Ta₃N₅ is comparable to that of BiVO₄, the lifetime in the order of milliseconds is comparable to that of crystalline silicon. This is much higher than previously reported and compares favorably with the currently most promising metal oxide-based semiconductors (BiVO₄, Fe₂O₃, WO₃, Cu₂O) for photoelectrochemical (PEC) water splitting. Although these long lifetimes may be partly caused by (de)trapping from shallow trap states, these results clearly demonstrate that a high phase purity is an essential prerequisite for efficient (oxy)nitride-based absorber materials. © 2015 American Chemical Society.