Optical analysis of hafnium oxide-aluminum multilayer structures for transparent heat mirrors

We report on HfO2/Al/HfO2 multilayer thin films for heat mirror applications prepared on corning glass substrates by electron beam evaporation. Films fabricated at a substrate temperature of 100 °C show nano-polycrystals of HfO2 embedded in a disordered lattice according to X-ray diffraction results. Atomic force microscopy revealed that HfO2/Al/HfO2 layers possess smooth surface that is appropriate for optical heat mirror applications. Study of optical properties by UV-Visible spectrophotometer demonstrated that transmittance of HfO2/Al/HfO2 device was decreasing from UV to VIS and then slightly increasing in the NIR regions, with an opposite trend followed by reflectance. Optical constants i.e. refractive index, extinction coefficient, band gap energy, Urbach energy has also been calculated. The optical band gap and Urbach energy are found to be 4.34 eV and 3.164 eV, respectively. The collective oscillation energy loss for heat mirrors applications are also observed.