High-Quality Ta₃N₅ Photoelectrodes for Photoelectrochemical Energy Conversion

For photoelectrochemical energy conversion, metal nitride semiconductors have the potential to overcome several limitations associated with the more intensively investigated class of metal oxides. Among these materials, Ta₃N₅ is especially promising. However, it is commonly synthesized by nitridation of Ta₂O₅ films in ammonia atmosphere at high temperatures, which results in high concentrations of residual oxygen, nitrogen vacancies, and low-valent Ta cations within the Ta₃N₅ lattice. These defects often dominate the (opto)electronic properties of Ta₃N₅ photoelectrodes, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport mechanisms. Here, we deposit tantalum nitride thin films by reactive magnetron sputtering and explore the role of subsequent NH₃ annealing.[1] This synthesis process leads to thin films with near-ideal stoichiometry, as well as significantly reduced native defect and oxygen impurity concentrations compared to the commonly used nitridation of Ta₂O₅. By analyzing structural, optical, and photoelectrochemical properties as a function of NH₃ annealing temperature, we provide new insights into the basic semiconductor properties of Ta₃N₅, as well as the role of defects on its optoelectronic characteristics. For example, the high material quality enables us to unambiguously identify the nature of the Ta₃N₅ bandgap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. Improved understanding of not only the basic properties of this material, but also of how defect concentrations can be optimized, provides a path to high efficiency photoelectrodes.<br/>[1] Eichhorn et al., J. Mater. Chem. A, 9, 20653 (2021)