Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability

Hydrogenated amorphous silicon (a-Si: H) wire waveguides were fabricated by plasma-enhanced chemical vapor deposition and anisotropic dry etching. With the optimized fabrication process, the propagation losses of as low as 3.2 +/- 0.2 dB/cm for the TE mode and 2.3 +/- 0.1 dB/cm for the TM mode were measured for the 200 nm (height) x 500 nm (width) wire waveguides at 1550 nm using the standard cutback method. The loss becomes larger at shorter wavelength (similar to 4.4 dB/cm for TE and similar to 5.0 dB/cm for TM at 1520 nm) and smaller at longer wavelength (similar to 1.9 dB/cm for TE and similar to 1.4 dB/cm for TM at 1620 nm). With the waveguide width shrinking from 500 nm to 300 nm, the TM mode loss keeps almost unchanged whereas the TE mode loss increases, indicating that the predominant loss contributor is the waveguide sidewall roughness, similar to the crystalline silicon waveguides. Although the a-Si: H and the upper cladding SiO(2) were both deposited at 400 degrees C, the propagation loss of the fabricated a-Si: H wire waveguides starts to increase upon furnace annealing under atmosphere at a temperature larger than 300 degrees C: similar to 13-15 dB/cm after 400 degrees C/30 min annealing and >70 dB/cm after 500 degrees C/30 min annealing, which can be attributed to hydrogen out-diffusion. Even higher temperature (i.e., >600 degrees C) annealing leads to the propagation loss approaching to the polycrystalline silicon counterparts (similar to 40-50 dB/cm) due to onset of a-Si: H solid-phase crystallization. (C) 2010 Optical Society of America