Titanium oxynitride thin films with tuneable double epsilon-near-zero behaviour for nanophotonic applications.

Titanium Oxynitride (TiOxNy) 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) behaviour with ENZ1 and ENZ2 wavelengths tunable in the 700-850 nm and in the 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual Oxygen in the background vacuum was above 2E-8Torr exhibit non- metallic behaviour, while the layers deposited when the level of residual Oxygen in the background vacuum was below 5E-9Torr, show metallic behaviour 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 TiOxNy and TiOx phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiOx and TiOxNy compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behaviour is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of TiO2 and TiN phases suggesting that double ENZ TiOxNy 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 non-reciprocal behaviour.