In this work, a broadband ellipsometric investigation of aluminium zinc oxide (AZO) from the infrared to the ultraviolet spectral range is presented. Aluminium zinc oxide samples were fabricated using spatial atomic layer deposition (SALD) having a range of percent aluminium incorporation. The AZO was deposited using a coflow of diethyl zinc (DEZ) and dimethyl aluminium isopropoxide (DMAI), where the amount of aluminium incorporation was controlled by varying the DMAI partial pressure. The broadband permittivity is reported as a function of the aluminium content of these aluminium zinc oxide thin films. A model dielectric function, which includes the contribution of recently predicted broad shallow donor states, is presented and discussed. We find that an increase in the aluminium content of the aluminium zinc oxide thin films results in a monotonically decreasing scattering time while the carrier density increases with diminishing efficiency. To demonstrate the necessity of broadband ellipsometric measurements for the accurate determination of the dielectric function of aluminium zinc oxide, a comparison with the classical Drude model and a semi-empirical extended Drude dispersion model, which are frequently employed in the literature to describe the optical response of aluminium zinc oxide, is given.