Thermal annealing study of the mid-infrared aluminum nitride on insulator (AlNOI) photonics platform

Aluminum nitride on insulator (AlNOI) photonics platform has great potential for mid-infrared applications thanks to the large transparency window, piezoelectric property, and second-order nonlinearity of AlN. However, the deployment of AlNOI platform might be hindered by the high propagation loss. We perform thermal annealing study and demonstrate significant loss improvement in the mid-infrared AlNOI photonics platform. After thermal annealing at 400°C for 2 hours in ambient gas environment, the propagation loss is reduced by half. Bend loss and taper coupling loss are also investigated. The performance of multimode interferometer, directional coupler, and add/drop filter are improved in terms of insertion loss, quality factor, and extinction ratio. Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction spectroscopy suggest the loss improvement is mainly attributed to the reduction of extinction coefficient in the silicon dioxide cladding. Apart from loss improvement, appropriate thermal annealing also helps in reducing thin film stress.