Ultra-thin Si₃N₄ films were grown on Si(111) surface by radio frequency (RF)-N₂ plasma exposure at 900 °C with 1–1.2 sccm of a flux of atomic nitrogen. We discuss the effect of various conditions such as N₂ flow rate, nitriding time and RF power on the optical, chemical, and structural properties of a nitrided Si₃N₄ layer. The optical properties, surface morphology and chemical composition are investigated by using ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Cross-sectional TEM images show that an RF power of 350 W induced some damage to the Si(111) surface. The thickness of nitrided Si₃N₄ was measured to be about 5–7 nm. XPS results shown that the binding energy of Si 2p_3/2 located at 101.9 ± 0.1 eV is attributed to the Si–N bonds in the Si₃N₄ compound. Smooth Si₃N₄ ultra-thin films were obtained at a nitridation time close to 1 h with an RF power of 300 W, with a measured refractive index (n) nearly to 1.88 at 632 nm. The increase in refractive index with decreased RF-plasma power and nitrogen flow rate is probably attributed to the change in the stoichiometry of the film and less surface damage.

Terahertz (THz) plays a pivotal part in numerous technology fields in modern times, including the system of the 6^th generation wireless communication, imaging and elemental analysis. In addition to light sources and detectors, THz applications require quasi optics as lenses, waveguides, and reflectors for the design of a THz system. Three-dimensional (3D) printing has many advantages. However, 3D printing materials always have higher THz-wave absorption rates. In this study, we attempted to discover the optimum process parameters of the combustion temperature of rice husk ash (RHA) and mix RHA with acrylonitrile butadiene styrene (ABS). According to the THz spectrum, the mixture powder would be very useful in applications of 3D printing to increase the refractive index and decrease the absorption coefficient with RHA added into ABS. This improvement will benefit 3D printing technique applications in the far-infrared and THz range.