Annealing and passivation study of germanium on silicon (GOS) mid-infrared waveguide for sensing applications

Germanium on silicon (GOS) is an excellent platform for non-dispersive infrared sensing (NDIR) due to its broad mid-infrared (mid-IR) transparency. However, its optical waveguide propagation loss as a photonic sensing medium and its susceptibility to oxidation are concerns to be addressed. Herein, we study the effect of annealing GOS waveguide devices under forming gas and passivation using atomic layer deposition (ALD) of aluminum oxide (Al2O3) and aluminum nitride (AlN) on waveguide loss. Our findings showed that annealing helped reduce propagation loss as high as 17x at wavelength of ~ 5.85 μm and passivation with AlN was effective in minimising oxidation of germanium (Ge) in ambient, albeit at the expense of higher waveguide loss originating from the MIR absorption in the AlN film itself. Nevertheless, these results provide insights towards improving the performance and robustness of a GOS waveguide-based sensor.