Hf1-xAlxNy: A versatile material system for visible-range anti-reflection and high-reflection optical coatings

Transition-metal aluminum nitride thin films are widely known for their use as hard protective coatings due to their high hardness, stiffness, thermal stability, and oxidation resistance. In addition, their optical properties can be tailored through compositional control. In this work, the optical properties of Hf 1-x Al x N y and HfN y thin films deposited on Si substrates by ion-assisted reactive magnetron sputtering are investigated. Spectroscopic ellipsometry was used to determine the refractive index and extinction coefficient of Hf 1-x Al x N y films ranging from metallic ( 0 ≤ x < 0.3 ) to dielectric ( 0.5 ≤ x ≤ 1 ) compositions, as well as overstoichiometric metallic HfN y ( 1.12 ≤ y ≤ 1.46 ). Based on the measured optical constants, several coating designs for visible range anti-reflection and high reflection applications were explored through optical calculations and experimental validation. Anti reflection behaviour was achieved using a single quarter wavelength dielectric layer on Si, yielding reflectance below 1% at the designed wavelength. A thinner dielectric layer deposited on opaque metallic HfN y films resulted in reflectance below 10% over the 470 −660 nm range. High-reflection performance was demonstrated through calculated quarter wavelength multilayers of dielectric HfAlN. Furthermore, a novel HR multilayer design combining ultrathin metallic HfN y layers and thicker dielectric Hf 1-x Al x N y layers achieved a maximum reflectance of approximately 60% at tunable wavelengths, with potential for further enhancement. These results showcase the versatility of the Hf-Al-N material system for advanced optical coating applications. • Optical constants of Hf₁₋ₓAlₓNᵧ tuned from metallic to dielectric by composition. • Structural phase transition controls optical absorption and free-carrier density. • Wide refractive-index range enables flexible optical coating design strategies. • Broadband anti-reflection achieved on Si and metallic HfN surfaces. • Hybrid metallic/dielectric multilayers enable tunable visible-range high reflection.