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

## TL;DR

This paper studies how annealing and passivation can improve the performance of germanium-on-silicon waveguides for mid-infrared sensing.

## Contribution

The study demonstrates that annealing and ALD passivation can significantly reduce waveguide loss and oxidation in GOS.

## Key findings

- Annealing reduced propagation loss by up to 17 times at ~5.85 μm wavelength.
- AlN passivation minimized Ge oxidation but increased waveguide loss due to MIR absorption in AlN.
- The results offer insights for improving the performance and durability of GOS waveguide-based sensors.

## Abstract

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.

The online version contains supplementary material available at 10.1038/s41598-026-35766-1.