Berreman-assisted optical characterization of sub-percolation threshold, ultrathin near-zero-index films

Abstract Due to its transparent and conductive nature, indium tin oxide (ITO) offers substantial benefits in several industries, such as thin film transistors, displays, and nanophotonics. Previous studies on ultrathin ITO have so far focused on its electrical properties but have neglected the technologically important epsilon-near-zero (ENZ) optical features due to the difficulty of extracting the refractive index and the thickness-dependent degradation of the optical properties. Here, we demonstrate a complementary metal-oxide-semiconductor (CMOS)-compatible deposition procedure for sub-percolation thickness (below 4 nm) ITO using a dry-etch assisted radiofrequency magnetron sputtering technique that yields continuous films in a precisely controlled manner. Through interface engineering and post-deposition annealing optimization, we show that these ITO films can retain high carrier mobility (43 cm 2 V −1 s −1 ) while achieving a tunable near-zero-index (NZI) regime throughout the telecommunications band using a Berreman-assisted optical characterization technique. Our result opens the possibility of efficiently designing ENZ/NZI materials at the nanoscale using a robust fabrication approach for applications in nanophotonics.