Amorphous-Silicon Carbide Integrated Photonics

Wide bandgap semiconductors, including silicon carbide (SiC), gallium nitride,lithium niobate, and aluminum nitride, have attracted growing interest in integrated photonics due to their broad transparent window and negligible nonlinear propagation losses, attributed to the absence of two-photon absorption at telecom wavelengths. SiC is particularly notable for its widespread adoption in power electronics while also exhibiting excellent optical properties. SiC has strong second- and third-order optical nonlinearities, as well as potential applications in quantum optics, given its ability to host color centers. While various polytypes of SiC, such as 3C-, 4H-, and 6H-SiC, have been studied, these crystalline forms face challenges in achieving a commercially viable realization of the SiC-on-insulator platform. Amorphous-SiC (a-SiC) offers a promising alternative, as it can be easily deposited on substrates, exhibiting significant nonlinear characteristics and a high thermo-optic coefficient. These attributes, combined with the ability to tune elemental ratios and adjust the optical properties of a-SiC, make it suitable for a broader range of applications compared to other photonic materials. This thesis investigates the potential of a-SiC-on-insulator (a-SiCOI) in integrated photonics and demonstrates several devices to explore its capabilities. The thesis begins with the characterization of a-SiC thin film properties across different refractive indices, utilizing techniques such as ellipsometry, prism coupler method, and X-ray photoelectron spectroscopy. It then details the development of a complementary metal-oxide semiconductor (CMOS)-compatible fabrication process for devices on the a-SiCOI platform, with optimizations aimed at reducing propagation losses. This is followed by the realization of low-loss couplers for the a-SiCOI platform, including grating couplers and edge couplers that utilize lensed fibers and single-mode fibers to couple, respectively. The thesis further explores the third-order nonlinearity of a-SiC. While a-SiC does not exhibit second-order nonlinearity due to the absence of non-centrosymmetry, its high third-order nonlinearity still presents promising applications in nonlinear optics. Additionally, two widely studied devices—photonic crystal waveguides and thermo-optic modulators—are demonstrated on the a-SiCOI platform. One key finding of this thesis is that the propagation loss of a-SiCOI can be reduced to as low as 5.66 dB/cm after optimizing the fabrication process. Secondly, various high-efficiency couplers have been developed. The grating couplers achieve coupling loss of 6.9 dB per side, enabling the characterization of devices with hundreds of distributions on a single chip, such as photonic crystal waveguides and thermo-optic modulators. The edge couplers for lensed fibers achieve 3.2 dB/facet, while the edge couplers for single-mode fibers achieve 3.0 dB/facet. Thirdly, this project extracts the nonlinear refractive index in different a-SiC waveguides with varying refractive indices through four-wave mixing experiments. We find that the nonlinear refractive index increases with the refractive index. For the sample with the highest refractive index, which is 2.80, a value of 6.70 ± 0.11 × 10−18 m2/W is achieved, an order of magnitude higher than that of crystalline SiC. Fourthly, photonic crystal waveguides are successfully demonstrated, with the bandgap tuned by adjusting the hole radius, lattice size, and their ratios. Lastly, the thermo-optic coefficient of the a-SiC is characterized as 7.11 × 10−5/ ◦C for a sample with a refractive index of 2.52, which is also higher than that of crystalline SiC. Based on this high thermo-optic coefficient, an integrated thermo-optic modulator is demonstrated. I hope this thesis will contribute to the advancement of a-SiCOI integrated photonics and provide inspiration for future research in this promising field.