# High-\(Q\) TeO\({}_{2}\)-Si Hybrid Microring Resonators
## 2 Microring Resonator Fabrication and Design  
The microring resonator structure used in this work is displayed in Figure 1. It consists of an integrated silicon microring and bus waveguide coated with a thin film of tellurium oxide (TeO\({}_{2}\)). The silicon structure was fabricated in a silicon photonics foundry on a wafer-scale SOI platform with a 220 nm silicon layer thickness and consists of a 30 \(\upmu\)m-radius silicon microring constructed using a 0.5 \(\upmu\)m-wide waveguide. The bus waveguide is 0.4 \(\upmu\)m wide, and the point coupling gap is 1.0 \(\upmu\)m. The structure was cladded with SiO\({}_{2}\), and subsequently, a window on top of the microring resonator was etched to the silicon layer for use in the post-processing TeO\({}_{2}\) deposition or unclad device experiments. A set of devices with identical dimensions but without the SiO\({}_{2}\) cladding etched was also fabricated for comparison. Deep etching was used for end-facet preparation and wafer dicing. For the TeO\({}_{2}\)-clad devices, the structure was coated with a 270 nm-thick TeO\({}_{2}\) film deposited using a room-temperature reactive RF co-sputtering post-processing step with 145 W of tellurium target sputtering power, 2.8 mTorr chamber pressure, and 12 and 7.6 sccm of argon and oxygen flow, respectively. The substrate temperature was set at 20 \({}^{\circ}\)C. A top-view scanning electron microscope (SEM) image and the cross-section diagram of the TeO\({}_{2}\)-coated Si microring resonator are displayed in Figure 1a,b, respectively. Figure 1c,d show the image of the experimental setup and a microscopic image of the microring resonator with an open window structure. The electric field profiles of the transverse-electric (TE)-polarized fundamental modes calculated using a finite element method (FEM) modesolver for the (cladding-core) TeO\({}_{2}\)-Si, SiO\({}_{2}\)-Si, and air-Si waveguides at a 1550 nm wavelength are displayed in Figure 2a-c, respectively. We also summarize the calculated optical properties of the TeO\({}_{2}\)-Si, SiO\({}_{2}\)-Si, and air-Si microring resonator structures in Figure 2d. The microring resonators and the bus waveguides were designed to achieve single-mode waveguide conditions at 1.55 to 2 \(\upmu\)m wavelengths. The ring waveguide structure supports the TE-polarized and TM-polarized modes, and it has a low bending radiation loss at 1550 nm for TE only. The effective index of the TeO\({}_{2}\)-coated resonator is 2.8, which is almost 7.7 and 12% higher than that of the air- and SiO\({}_{2}\)-clad devices. For the TeO\({}_{2}\)-coated Si microring resonator, 21.7% of the optical power is confined in the TeO\({}_{2}\) coating layer, while 65.0% is confined in the silicon layer. The rest of the optical power is confined in the lower SiO\({}_{2}\) cladding. Slightly lower cladding confinement of 15.8 and 17.2%, respectively, is observed in the air- and SiO\({}_{2}\)-clad cases. The effective area is slightly larger in the TeO\({}_{2}\)-coated Si microring resonator than SiO\({}_{2}\)-coated and uncoated silicon microring resonators, although the mode is pulled upward more into the cladding, both of which can influence the ring-bus waveguide coupling. Figure 1: (a) Cross-section profile of the TeO\({}_{2}\)-coated Si microring showing the microring structure. (b) Top-view SEM image of a TeO\({}_{2}\)-coated Si microring resonator. (c) Photograph of the coupling setup during measurement showing the SOI chip with window opening for TeO\({}_{2}\) post-processing deposition. (d) Microscopic image of the open window microring resonator structure. The TeO\({}_{2}\)-coated resonator has a higher mode area of approximately 0.20 \(\upmu\)m\({}^{2}\) at 1.55 \(\upmu\)m. The effective index increases for the TeO\({}_{2}\) film cladding as the resonant mode becomes more confined in the TeO\({}_{2}\) layer. The expansion of the optical mode at longer wavelengths decreases the effective index.