Synthesis, Characterization and Optical Constants of Silicon Oxycarbide

High refractive index glasses are preferred in integrated optics applications to realize higher integration scale of passive devices. With a refractive index that can be tuned between silica (1.46) and amorphous silicon... [ view full abstract ]

High refractive index glasses are preferred in integrated optics applications to realize higher integration scale of passive devices. With a refractive index that can be tuned between silica (1.46) and amorphous silicon carbide (3.2), silicon oxycarbide SiOC offers this flexibility. The typical methods to deposit SiOC such as CVD and sol-gel-pyrolysis introduce hydrogen and large absorption. In this work, SiOC thin films have been deposited using reactive RF magnetron sputtering a pure SiC target in a controlled environment of argon and oxygen gases at room temperature. Quantitative characterization of the SiOC thin films was performed extensively by profilometry, ellipsometry, prism-coupling, XPS, SEM, and AFM. The optical constants, index-of-refraction n and extinction coefficient k, of the prepared silicon oxycarbide films were derived from the ellipsometry data acquired with JA-Woollam-VASE-ellipsometer in the UV-VIS-NIR wavelength regions (300-1600nm) and at multiple angles of incidence. A four-layer optical model including: surface roughness/SiOC/native-SiO2/c-Si was used in WVASE32 software to perform numerical fitting to the experimental data. A Kramers-Kroning consistent Tauc-Lorentz oscillator was used to describe SiOC films, hence the obtained optical functions were Kramers-Kroning consistent. Controlling the deposition parameters, the refractive index of the SiOC films was tuned from 1.39 to 1.86 as measured by ellipsometer and prism-coupler at 1550 nm which showed almost a linear dependence on RF power of sputtering. The material absorption coefficient k has been estimated to be less than 10-4 for all the deposited silicon oxycarbide films in the visible and near infrared regions. The SiOC films thickness was determined from 80 nm to 2200 nm using P-15 profilometer. The deposition rate followed an increasing trend with RF power and large rate of 150 nm/min was achieved. XPS analysis confirmed the presence of silicon, oxygen and carbon in SiOC films that exist in mixed bonding structure. The rms roughness of SiOC films as low as 0.5 nm was obtained from AFM 2D images. In this work, we demonstrate synthesis of silicon oxycarbide with good transparency in Visible-NIR regions, high refractive index, and low roughness which suggest silicon oxycarbide is a promising candidate for integrated photonics applications.