Luminescence quenching due to ion cluster formation in erbium ion doped amorphous aluminium oxide, limits the maximum doping concentration that can be incorporated into the material and, consequently, the maximum achievable optical gain. By controlling the reactive sputtering deposition parameters, layers with different morphologies can be deposited. In this work, we investigate low propagation loss poly-crystalline aluminium oxide thin films and the effect of erbium doping on the crystallinity. We have developed a reactive sputter process to reproducibly obtain high refractive index (n~1.72 at 633 nm) poly-crystalline thin films with very low slab waveguide losses from the near-UV to the midinfrared wavelength range. Slab waveguide losses as low as 1.8 dB/cm at 407 nm and less than 0.1 dB/cm at 1550 nm of wavelength have been experimentally characterized. Both the undoped and erbium doped layers were deposited by reactive sputter coating with, a set substrate temperature of 700 °C. Preliminary TEM analyses show no discernible change in the crystallinity of the doped layers with respect to their undoped counterparts. The high optical quality of this material, in combination with a potentially increased rare-earth ion doping concentration, could pave the way towards high-gain on-chip amplifiers in different wavelength ranges and efficient on-chip lasers.