In this paper, high-k hafnium–aluminum oxide (HAO) films were synthesized by the sol–gel technique. The effects of the ratio of Hf and Al on the properties of the HAO films were investigated thoroughly. The average optical transmittance of the HAO films was above 88% within the visible light range and Al incorporation in HfO2 can enlarge the band gap of HAO films. X-ray diffraction (XRD) results showed that Al additive can suppress the crystallization of HfO2 and the HAO films were amorphous in structure. The refractive index of HAO films can be modulated with the ratio of Hf and Al in the HAO films. The HAO films with the ratio of Hf and Al = 2:1 obtained excellent performance including the root mean square (RMS) roughness of 0.26 nm, the relative permittivity of 12.1, the leakage current density of 1.69 × 10−7 A/cm2 at 2 MV/cm, and the etching rate in dilute HF solution less than 1 nm/s.