Examination of the high-temperature optical properties of HfO₂ thin films for metamaterial selective-emitter applications

Metamaterial selective-emitters can enhance the efficiency of thermophotovoltaic (TPV) energy conversion systems by selecting the wavelengths of light emitted towards the TPV photodiodes. As such, the optical properties of any material used as part of a metamaterial selective-emitter must be known and controlled. As one of the highest melting point dielectric materials, hafnium dioxide (HfO₂, melting point of ~ 2760°C) has attracted researchers’ attention for possible use in high-temperature applications; such as metamaterial selective-emitters. In this work, HfO₂ was deposited on a Si substrate by atomic layer deposition (ALD) and then diced into smaller pieces. The frequency-dependent complex permittivity of the HfO₂ films was measured via ellipsometry during annealing at temperatures from room temperature (RT) up to 600°C and then again at RT after annealing. The ellipsometric measurements were conducted over a wavelength range of 200 – 500 nm. The degree of crystallization and film density is correlated with the measured permittivity of HfO₂ by X-ray reflectivity (XRR) and X-ray diffraction (XRD).