Bixbyite-Type Zirconium Tantalum Oxynitride Thin Films as Composition-Tunable High Refractive Index Semiconductors

Multinary nitrides and oxynitrides offer a range of tunable structural and optoelectronic properties. However, much of this vast compositional space remains to be explored due to the challenges associated with their synthesis. Here, reactive sputter deposition is used to synthesize isostructural polycrystalline zirconium tantalum oxynitride thin films with varying cation ratios and systematically explore their structural and optical properties. All films possess the cubic bixbyite-type structure and n-type semiconducting character, as well as composition-tunable optical bandgaps in the visible range. Furthermore, these compounds exhibit remarkably high refractive indices that exceed a value 2.8 in the non-absorbing sub-bandgap region and reach 3.2 at 589 nm for Ta-rich compositions. Photoemission spectroscopy reveals non-uniform shifts in electron binding energies that indicate a complex interplay of structural and compositional effects on interatomic bonding. In addition to being high-index materials, the measured band edge positions of the films align favorably with the water oxidation and reduction potentials. Thus, this tunable materials family offers prospects for diverse optoelectronics application, including for production of photonic metamaterials and for solar water splitting.