Annealing Environment Controlled Structural, Optical, and Electrical Properties of Heteroepitaxial ITO/YSZ Thin Films

The development of low-loss, tunable, and Si-compatible photonic and epsilon-near-zero (ENZ) materials is essential for advancing high-density optical integration and metamaterials research, driving efficient nonlinear optical devices, energy harvesting, and thermal imaging. Significant work has been devoted so far to understanding the optical and electrical properties of indium-tin-oxide (ITO) materials due to its refractory nature and strong nonlinear optical responses. However, the fundamental connections between material structure, electrical transport, and optical dispersion of sputtered ITO thin films remain elusive, motivating a comprehensive investigation. We demonstrate tunability in heteroepitaxial ITO films grown on (100) yttria-stabilized zirconia (YSZ), where post-deposition annealing in vacuum, argon, and oxygen atmospheres induces distinct modifications in morphology, crystallinity, and oxygen vacancies, revealed through XRD, SEM/AFM, XPS, and Raman analyses. The fabricated materials yield carrier densities of 10²⁰-10²¹ cm⁻³, mobilities up to 65 cm²/Vs, resistivities as low as 2.2×10⁻⁴ Ωcm, and ENZ wavelength tunability from 1.2–2.1 μm. An application-oriented figure of merit (σ/α) is presented that highlights oxygen control as key, balancing high conductivity, optical transparency, and low-loss ENZ behavior for NIR photonic integration of nonlinear ENZ materials on the Si platform.