Modeling and fabrication of Ge-on-Si<inf>3</inf>N<inf>4</inf> for low bend-loss waveguides

Abstract

Silicon and germanium are waveguide core materials for Si-based Group-IV integrated photonic circuit applications in the mid-infrared (MIR) wavelength range defined here as 2 to 15 um. In particular, Si and Ge are transparent up to 8 and 15 μm respectively, thus offering applications in communications and the sensing of biochemical, medical, and environmental variables [1]. The major problem with the MIR transition is that the most dominant platform of SOI can be used only up to 4 μm due to the high absorption loss of silicon dioxide. Therefore alternative material platforms have to be utilized for longer wavelengths. Presently, the Ge-on-Si platform (known as GOS) has been demonstrated to be workable to 15 μm-thereby offering potential for MIR applications. However, the refractive index contrast between Ge (n=4.1) and Si (n=3.4) is considerably less than that of SOI. As a result, the bend radii in GOS must be considerably larger than those in SOI, causing the footprint of GOS on-chip sensor circuits to be generally larger than those of SOI, an unwanted increase in “real estate.” What is wanted is a better alternative Ge-waveguide platform that will provide a larger core-clad index contrast than GOS as well as useful transparency and smaller channel-bend radii. To achieve those goals, the new structure proposed and realized here is Germanium on Silicon Nitride (Ge-on-Si 3 N 4 ) which is referred-to here as GON. The refractive index of silicon nitride is 2.4 and it is transparent up to ∼7 μm. So the index contrast in GON is 1.7 compared to 0.7 in GOS. Our simulation results show that this structure can provide a more compact integrated network whose bend radii are only a few μm. Most importantly, we have developed a new and practical wafer bonding and layer transfer technique, reported here, to realize GON.