Effect of oxygen on sputtered tantalum nitride thin films for photoelectrochemical water splitting

With the rising pressure of climate change pushing research into various areas of renewable energy sources, solar generated hydrogen represents a compelling research avenue. Using solar hydrogen farms, it would be possible to generate enough hydrogen from water and sunlight to support a theoretical hydrogen economy. Since the discovery of photocatalytic water splitting using TiO2[ four decades of research has focused on developing efficient solar to hydrogen water splitting technologies. However, despite large improvements in the capabilities of TiO2, efficient water splitting devices utilising it and other oxides have not eventuated. In recent years, various nitride materials have been highlighted as possessing great potential for efficient water splitting. Tantalum nitride, specifically Ta3N5, is one such nitride, possessing appropriate band edge positions for efficient, bias free overall water splitting and a band gap allowing absorption of visible light. The goal of this project is to deposit thin films of Ta3N5 for use in photo-electrochemical cells as novel photo-electrodes. In the current literature, nearly all reports include the use of oxygen in the synthesis of Ta3N5, whether thermal nitridation or sputtering routes. The formation of these films is catalyzed by oxygen due to the tendency of high oxidation state transition metals, in this case Ta5+, to draw stability from the inductive effect of a more electronegative element. As such, the role of oxygen in the synthesis of Ta3N5 is important and warrants investigation. A number of films were deposited via RF sputtering with the deposition parameters listed in Table 1, using an AJA Orion 5 magnetron sputtering system (AJA International, Scituate MA). A film of tantalum was deposited prior to oxygen and nitrogen being introduced into the atmosphere. Surface feature imaging and elemental quantification was performed with a JEOL JSM-7001F (JEOL, Tokyo, Japan) Scanning Electron Microscopy (SEM) equipped with a Bruker XFlash 6I10 (Bruker, MA) detector for Electron-Dispersive Spectroscopy (EDS). Oxygen presence in the sputtering atmosphere had an impact on surface structure, elemental composition and film thickness. X-Ray Diffraction (not displayed) indicated the presence of Ta3N5 in the two films with lowest oxygen partial pressures (p(O2)), with a phase transition to TaN taking place at the highest p(O2). Figure 1 displays surface structures (below) and film cross sections (above). Surface roughness gradually decreased with increasing p(O2), due to the increasingly amorphous nature of the films. The p(O2) correlated to the presence of oxygen within the films, however even with a p(O2) of 0 mTorr, a not insignificant amount of oxygen was still detected in the films; the source of this is still to be determined. Despite the p(O2) matching the nitrogen partial pressure (p(N2)) at 0.3 mTorr, a larger amount of nitrogen was still observed in the films, suggesting a decrease in oxygen gettering at higher p(O2) values. The observation of TaN at p(O2) = 0.3 mTorr further indicates this. This data provides a basic view of the relationship between oxygen, nitrogen and tantalum in our systems atmosphere while utilising RF magnetron sputtering. It is apparent that oxygen must be controlled to within a few % of the total sputtering atmosphere in order to deposit Ta3N5 films.