Inductively Coupled Plasma Etching of ITO Nanophotonic Structures for Visible Wavelengths

Indium tin oxide (ITO) is ubiquitously used as a transparent electrode for current injection and spreading in optoelectronic devices, including photonic crystal surface-emitting lasers and edge-emitting laser diodes. With a high refractive index and low absorption in the visible spectrum, ITO can play an additional and important role in optical modulation when directly integrated with laser diodes in the form of nanostructures, such as metasurfaces, photonic crystals, and circular gratings. To realize these functional nanostructures for integrated visible-wavelength photonic devices, a precise and controllable etching process is essential. Although a transparent conductive photonic structure is highly desirable, the literature on key parameters and fundamental mechanisms of dry etching of nanostructured ITO remain scarce. This work first systematically shows the inductively coupled plasma (ICP) etching of patterned ITO nanostructures with a feature size of around 100 nm. Using a mixed gas of methane and hydrogen, the influence of the gas ratio on the etch rate and sidewall cross-sectional morphology is thoroughly investigated, and key parameters, ICP power, radio frequency (RF) power, and chamber pressure, are optimized to achieve circular grating structures with steep sidewall profiles. Additionally, the effects of adding trace amount of Cl2 on the etching process are explored. Based on the etching processes, an ITO concentric circular grating with a period of Hankel waves is fabricated on a GaN epi-structure with multi-quantum wells. Angle-resolved spectroscopy confirms the formation of photonic band structures around 440 nm, showing multiple high-quality factor resonance peaks. The main peak aligns precisely with the target wavelength, verifying both the optical functionality and the fabrication precision of the ITO circular grating. This work thereby establishes a reliable ICP etching process for ITO nanophotonic structures, providing a critical fabrication solution applicable across the visible spectrum. The photonic bandgap in the blue-light regime highlights the potential of ITO in realizing functional devices such as photonic crystal surface-emitting lasers.