New waveguide material Si₃N₄ for photonic integration

Photonic integration is one of the most important issues in scientific research. Silicon-on-insulator (SOI) and other materials play an important role in photonic integration, but their losses are large and affect the integration, so it is very important to find new waveguide materials. As a consequence, Si₃N₄ came into being and is now a hot area of research. Its crystal structure is stable, with a wide energy band of E_g~5.1 eV, so it is transparent in the whole optical range from ultraviolet to infrared. Also, its optical loss in this range is very low, α~0.045±0.04 dB/m, which is lower than that of SOI waveguides by 3—4 orders of magnitude. Its refractive index at 1550 nm is ~2, so combined with Si and SiO₂, high-performance dielectric waveguide structures can be designed. Its thermal expansion coefficient is ~2.35×10^-6/℃, smaller than that of Si, so its growth on Si will introduce a larger tensile stress and may produce cracks. Thus, growing thick films of large area is very difficult. By using low-pressure or plasma-enhanced chemical vapor deposition, Si₃N₄ films can be deposited on low refractive index SiO₂ to form Si₃N₄-SiO₂ waveguides, which are smaller in size and allow better integration. Current research progress in this field is reviewed, and future application prospects are reviewed.