  Aluminum nitride (AlN) is an emerging material for integrated quantum
photonics with its excellent linear and nonlinear optical properties. In
particular, its second-order nonlinear susceptibility $\chi^{(2)}$ allows
single-photon generation. We have grown AlN thin films on silicon nitride via
reactive DC magnetron sputtering. The thin films have been characterized using
X-ray diffraction, optical reflectometry, atomic force microscopy, and scanning
electron microscopy. The crystalline properties of the thin films have been
improved by optimizing the nitrogen to argon ratio and the magnetron DC power
of the deposition process. X-ray diffraction measurements confirm the
fabrication of high-quality c-axis oriented thin films with a full width at
half maximum of the rocking curves of 3.9 deg. for 300-nm-thick films. Atomic
force microscopy measurements reveal a root mean square surface roughness below
1 nm. The AlN deposition on SiN allows us to fabricate hybrid photonic circuits
with a new approach that avoids the challenging patterning of AlN.
