Indirect bandgap, optoelectronic properties, and photoelectrochemical characteristics of high-purity Ta₃N₅ photoelectrodes

The (opto)electronic properties of Ta3N5 photoelectrodes are often dominated by defects, such as oxygen impurities, nitrogen vacancies, and reduced tantalum centers, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport. Here, we synthesize high quality Ta3N5 thin films by reactive magnetron sputtering and subsequent NH3 annealing at varying temperatures. The resulting films are characterized by nearly-ideal N/Ta stoichiometry, low O content, and small Urbach energies. Both the crystallinity and material quality improve with increasing annealing temperatures up to 940 °C, while higher annealing temperatures introduce additional disorder within the Ta3N5 lattice, leading to reduced photoelectrochemical performance. These changes are also reflected in the surface and bulk composition, showing the elimination of oxygen impurities at moderate annealing temperatures and the loss of nitrogen at high annealing temperatures. As a consequence, defect-related sub-gap optical absorption initially decreases due to reduced oxygen impurity concentration, and subsequently increases due to increased formation of nitrogen vacancies. The high material quality enables us to unambiguously identify the nature of the Ta3N5 band gap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. The assignment of Ta3N5 as indirect semiconductor is further supported by the suppression of disorder-induced band-edge photoluminescence with improved structural order within the Ta3N5 films.