Hybrid ScAlN-SiN Platform for Enhanced Nonlinear Photonics

Aluminum nitride (AlN) has wide applications in the field of photonic integrated circuits (PIC) due to the wide bandgap that enables a wide transparency window from ultraviolet (UV) to the infrared range. Additionally, it offers several exceptional properties such as piezoelectricity and second-order nonlinearity originate from the uncentrosymmetric wurtzite crystal structure, which enables its active functionalities in optomechanics, frequency conversions as well as electro-optic modulators. To further improve its properties, researchers have found that the addition of the rare earth element scandium (Sc) to AlN shows a five-time enhancement of piezoelectricity thus have great potential in the micro-electro-mechanical-system (MEMS) applications. Recently, ScAlN has also shown a great boost of second-order nonlinearity (<i>χ</i>⁽²⁾) compared to AlN, the <i>d</i>₃₃ is more than 60 pm/V when the Sc concentration reaches 36%, which is exceeding LiNbO₃. However, more investigations on the growth dynamics of high quality ScAlN are required for such new materials and a major challenge for its direct implementation in high performance integrated photonics is the increasing loss associated with the Sc doping. In the meantime, silicon nitride (Si₃N₄) has been extensively studied to reduce the propagation loss in the photonic devices, yet it lacks second-order nonlinearity, greatly limits its applications in quantum photonics. In this work, we present a hybrid ScAlN-Si₃N₄ microring resonator structure that shows a high <i>Q</i>-factor of 1.4 × 10⁵. We first deposit single-crystalline ScAlN on sapphire (Al₂O₃) substrate in a plasma assisted molecular beam epitaxy (PA-MBE) system, which provides high-quality materials with lower defect densities to suppress losses in the photonic components. Subsequently, Si₃N₄ is grown by low-pressure chemical vapor deposition (LPCVD) and fabricated to high <i>Q-</i>factor microring resonators by E-beam lithography and dry etching. The etch will stop at the ScAlN layer and optical guiding will be provided by the Si₃N₄. Such structure will benefit the device fabrication in terms of reducing optimization on the ScAlN etching condition and can potentially reduce the scattering loss from the ScAlN sidewall roughness. This device enhances the light-matter interactions in a platform combines both the high <i>χ</i>⁽²⁾ of ScAlN and the low-loss of Si₃N₄, which can be utilized as a fundamental component in quantum photonics.