Optical description of HfO2/Al/HfO2 multilayer thin film devices

A three-layer system of dielectric/metal/dielectric (D/M/D) has been prepared on Marienfeld commercial glass substrates with Metal = Al, and Dielectric = HfO2 for energy efficient windows applications. Subsequently, HfO2/Al/HfO2 multilayers have been deposited with 10 nm each HfO2 layer and 5 nm thick Al layer using electron beam evaporation. The microstructural characteristics of D/M/D thin films have been investigated using X-ray diffraction (XRD) and atomic force microscopy (AFM). Present results indicate the formation of HfO2 weak polycrystals embedded in the disordered lattice. AFM data reveals quite a smooth surface involving a structure of slightly elongated grains with almost Gaussian size distribution with mean grain size in the range from 7 to 23 nm. Regarding optical properties, maximum transmittance of the D/M/D structure is noticed to occur in the UV-region, whereas reflectance rises to ∼60% in the visible to near infrared (NIR)-regions. To optimize the performance of these D/M/D devices, computer calculations have been performed by varying either the thickness of both HfO2 layers and/or thickness of metallic Al layer. A satisfactory agreement between theoretical and experimental spectra is noticed. Such D/M/D structures can be useful in heat mirror applications involving energy efficient windows etc.