Tantalum Nitride Films Integrated With Transparent Conductive Oxide Substrates via Atomic Layer Deposition (ALD)

Hamed Hajibabaei, Daniel J. Little, Thomas W. Hamann*

Abstract. Recently, tantalum nitride (Ta₃N₅) has emerged as a promising photoanode for PEC water oxidation. Ta₃N₅ films (electrodes) are commonly synthesized by oxidation of Ta(0) to Ta(IV), followed by ammonolysis at elevated temperatures (> 850 °C). Although this method is simple and resulted in the best PEC water oxidation performance of T_a3N₅, from synthetic point of view however, multiple draw backs makes this method unsuitable to realize efficient photoelectrodes as this method: (1) is highly energy intensive, (2) produces a sizable quantity of chemical wastes, (3) inefficient on chemical utilization, (4) provides highly reducing conditions which limits Ta₃N₅ to only be compatible to Ta-substrate or Nobel metals, e.g. Pt, (5) precludes application of Ta₃N₅ in the tandem cell – the most efficient cell configuration for overall solar water splitting- as Ta or other metal substrates are not transparent to the sub-bandgap photons, (6) results in the formation of electronic resistive phases at the Ta₃N₅|Ta junction which further limits the electron collection efficiency. To overcome these challenges we used ALD to deposit tantalum nitride at lower temperatures.

While the low-temperature ALD deposition of tantalum nitride resulted in the amorphous films of TaO_xN_y, they are nitridized to crystalline and pure phase of Ta₃N₅ at considerably milder ammonolysis conditions (750 °C, 30 min). Furthermore, we investigated Ta-doped TiO₂ (TTO) as a promising TCO, stable (unlike FTO or ITO) under the reducing conditions. Subsequently, we were able to integrate these two layers to fabricate the 1^st example of Ta₃N₅ on TCO. The resultant Ta₃N₅ films on TTO produced a promising solar water oxidation performance. We found that the performance of the photoelectrodes correlated to the conductivity of the TCO. Thus, it would beneficial to directly deposit crystalline Ta₃N₅ films on the state-of-art TCO such as FTO, which do not require a subsequent ammonolysis step. Thus, we designed and built a fully automated high-temperature ALD system which enables us to directly deposit metal nitrides on commercially