Titanium Oxynitride Thin Films with Tunable Double Epsilon-Near-Zero Behavior for Nanophotonic Applications

Titanium oxynitride (TiO_xN_y) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained, and their optical properties are presented. The films grown when the level of residual oxygen in the background vacuum was between 5 nTorr to 20 nTorr exhibit double epsilon-near-Zero (2-ENZ) behavior with ENZ1 and ENZ2 wavelengths tunable in the 700-850 and 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual oxygen in the background vacuum was above 2 × 10^-8 Torr exhibit nonmetallic behavior, while the layers deposited when the level of residual oxygen in the background vacuum was below 5 × 10^-9 Torr show metallic behavior with a single ENZ value. The double ENZ phenomenon is related to the level of residual oxygen in the background vacuum and is attributed to the mixture of TiN and TiO_xN_y and TiO_x phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiO_x, and TiO_xN_y compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behavior is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of TiO₂ and TiN phases suggesting that double ENZ TiO_xN_y films are formed by inclusions of TiN within a TiO₂ matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges. Materials with ENZ properties are advantageous for designing the enhanced nonlinear optical response, metasurfaces, and nonreciprocal behavior.