In this work, we report on the tunability of carrier concentration and epsilon-near-zero (ENZ) wavelength (i.e. the region where the real dielectric permittivity of a material approaches zero) in ultrathin (<100 nm) Al-doped ZnO (AZO) nano-layers fabricated through the atomic layer deposition (ALD) technique. ALD is a variation of chemical vapor deposition in which a substrate is exposed to only one self-limiting reactant at a time, allowing for ultra-smooth, conformal deposition and precise control over film-thickness at the nanometer scale. To create the AZO meta-films, fused silica substrates are exposed to alternating cycles of Diethylzinc (DEZ) and water vapor, with periodic dopant cycles of Trimethylaluminium (TMA).
Optical and material properties of the meta-films are determined using spectroscopic ellipsometry. Using the Drude model and regression analysis with measured values, properties such as film thickness, ENZ wavelength, and complex refractive index are then determined. Furthermore, excitation of ENZ modes in the fabricated films has been demonstrated experimentally using the Kretschmann-Raether configuration. It was found that by varying the deposition temperature, Al:ZnO doping ratio, and film thickness, the ENZ wavelength of AZO thin films could be precisely tuned in the near infrared region from 1520 to 1700 nm. The results of this work allow for the precise engineering of optical properties of AZO films for zero-index photonic applications.