Development of thick epsilon-near-zero ITO metafilms for use in metastructure and metasurface devices

We report our development of Indium tin oxide (ITO) films with thicknesses greater than the typical optical telecommunication wavelength bands (~1550 nm) having epsilon-near-zero (ENZ) property at 1550 nm wavelength for the purpose of providing a new ENZ material platform for building high-contrast metastructure and metasurface devices. The films were grown using a high-power impulse magnetron sputtering (HiPIMS) tool, which allows for more control over film growth. A post-growth thermal annealing allowed the ITO film to reach the ENZ condition at the desirable wavelength. Our goal is to understand how deposition parameters and post deposition annealing conditions affect the film’s optical properties, therefore obtaining a controllable fabrication process for a desired optical property. Using spectroscopic ellipsometry to characterize the films, we show that the thick ITO films grown with HiPIMS exhibit ENZ behavior after post deposition annealing. The regime in which the material exhibits ENZ behavior is shown to be tunable within the wavelength range of 1500-1650 nm by varying the anneal temperature, anneal time, and oxygen exposure during anneal. In comparison with other thick ITO films grown with conventional pulsed DC magnetron sputtering, the optical constants of HiPIMS ITO films are shown to be much more constant with less variation throughout the bulk of the film. This result shows that these ITO films can be used to design a new family of opto-electronic devices that use ENZ ITO as the low-index base for high-contrast metasurface devices and as cladding for waveguides or optical cavities.