Annealing-free fabrication of high-quality indium tin oxide films for free-carrier-based hybrid metal-semiconductor nanophotonics

Abstract

Abstract Recent discoveries have revealed that indium tin oxide (ITO), due to the presence of an epsilon-near-zero (ENZ) point and suitable carrier concentration and mobility, can be used to modulate the refractive index, confine fields in the nanoscale, enhance nonlinear effects, achieve ultrafast light switching or to construct so-called time-varying media. While this potential positions ITO as a key material for future nanophotonic devices, producing ITO films with precisely engineered properties remains a significant challenge. Especially when the device's complex geometry or incorporated materials require the fabrication process to be conducted at substrate temperatures below 100°C and without any post-annealing treatment. Here we present a comprehensive study on the low-temperature deposition of 70 nm thick ITO films using an e-beam PVD system. The nanolayers evaporated under different conditions were characterized by SEM and AFM microscopy, Hall effect measurement system as well as spectroscopic ellipsometry. We discuss the factors influencing the optical, electrical, and morphological properties of ITO films. We show that smooth nanolayers of similar quality to annealed samples can be obtained at 80°C by controlling the oxygen plasma parameters, and the ENZ wavelength can be tuned throughout the NIR spectral range. Finally, we show that using the proposed methodology, we fabricated ITO films with resistivity as low as 5.2x10-4 Ωcm, smooth surface with RMS < 1 nm, high carrier concentration reaching 1.2x1021 cm-3 and high transmittance (85%) in the VIS/NIR spectrum.