Impact of annealing on low-loss sputtered aluminum nitride: insights from optics to advanced material characterization

Aluminium nitride (AlN) exhibits Pockels electro-optic, non-linear, and piezo-electric coefficients, which, together with a large bandgap (i.e., 6.2 eV), make it a very interesting material for photonic integrated circuits (PICs) with operation down to the ultraviolet wavelength range. In this work, AlN films were deposited by reactive RF sputter deposition. The influence of thermal post-deposition treatment at temperatures ranging from 400 °C to 1150 °C in a nitrogen atmosphere on the optical, morphological, chemical, and crystallographic properties of AlN films is studied. Preliminary slab propagation losses as low as 4.03 dB/cm at 633 nm of wavelength have been measured for as-deposited layers. The losses reduce to ∼1.60 dB/cm at 633 nm after annealing at 600 °C. Atomic force microscopy (AFM) measurements show that the surface roughness experiences a drastic reduction after annealing at 600 °C, which greatly reduces the propagation losses. X-ray photoelectron spectroscopy (XPS) reveals that annealing at 600 °C decreases the oxygen content, leading to a more stoichiometric AlN. High-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) show that the as-deposited material exhibits a hexagonal wurtzite polycrystalline structure, the degree of crystallinity increasing after annealing at increasing temperatures with preferential orientation along the C axis.