Hybrid Scandium Aluminum Nitride/Silicon Nitride Integrated Photonic Circuits

The article discusses the development of a hybrid Si₃N₄–ScAlN integrated photonic circuit platform, aimed at overcoming the challenge of optical loss in ScAlN-based photonics. Scandium-doped aluminum nitride (ScAlN) combines strong second-order nonlinearity, ferroelectricity, and piezoelectricity, making it a promising material for quantum photonics, optical modulation, and sensing. However, the significant optical loss in pure ScAlN (>2.4 dB/cm) has hindered its broader application.

The key breakthrough in this paper is the integration of ScAlN with Si₃N₄ waveguides, which preserves the beneficial properties of ScAlN while mitigating optical loss. The hybrid Si₃N₄–ScAlN waveguides exhibit an intrinsic quality factor (Q) of 3.35 × 10⁵, corresponding to a propagation loss of 1.03 dB/cm, comparable to that of commercial silicon-on-insulator (SOI) waveguides. This innovation enables a low-loss, scalable photonic platform that can leverage the advantages of both materials, supporting advanced photonic functionalities.

The hybrid platform's design focuses on minimizing loss from material absorption and scattering due to rough sidewalls. The waveguide confines light within the Si₃N₄ layer while keeping the ScAlN layer unetched, preserving its unique properties. This design balances optical confinement and bending loss, which was further optimized through numerical simulations.

The paper also presents the fabrication process, which includes the growth of ScAlN on sapphire substrates, followed by the deposition of Si₃N₄ and a-Si layers, and the use of a hardmask for precise waveguide etching. The resulting waveguides exhibit smooth sidewalls and low optical loss.

Experimental results, including microring resonator measurements, show that the hybrid waveguide platform achieves high-Q factors, with the best resonators showing Qi values up to 3.35 × 10⁵ and low propagation loss (1.03 dB/cm for TE modes). The integration of Euler racetrack resonators further reduces optical loss, demonstrating the platform’s potential for applications in high-speed optical communication, modulation, and quantum optics.

Overall, the paper highlights the potential of Si₃N₄–ScAlN hybrid integrated photonic circuits for scalable, low-loss photonic systems, compatible with CMOS fabrication, and capable of supporting advanced functionalities like nonlinear optics and electro-optic modulation.