Effect of annealing temperature on the structural, morphological and optical properties of ThO₂ thin films grown by photochemical metal–organic deposition

In this work, we report the influence of annealing temperature on the structural, morphological and optical properties of ThO₂ thin films deposited on fused quartz and (100) silicon substrates by photochemical metal–organic deposition (PMOD) using hinokitiolate thorium (IV) complex as the precursor. X-ray photoelectron spectroscopy (XPS) confirmed the deposition of the ThO₂ films. The effect of thermal annealing (from 300 °C to 1100 °C) on the structural properties of the ThO₂ films was evaluated with X-ray diffraction (XRD), UV–Vis transmittance spectroscopy, spectroscopic ellipsometry and atomic force microscopy (AFM). XRD patterns of the films annealed above 300 °C revealed that all diffraction peaks belong to a cubic ThO₂ structure without preferential orientation. The average crystallite size increased from 2.3 nm to 3.7 nm as the annealing temperature increased from 300 °C to 750 °C. Annealing at 1100 °C promoted the formation of huttonite (β-ThSiO₄) in the ThO₂ layer. Films deposited on fused quartz showed 80% transmittance in the visible range, and the annealing temperature did not impact the band gap energy. The AFM studies revealed that as the annealing temperature increased, the film surface roughness decreased. Between 750 °C and 950 °C, the film surface was relatively smooth with an of RMS surface roughness in the range 2.3–5.0 nm. The values of the refractive index of the films were in the range of n = 1.6 for the as-deposited film to n = 2.1 for the films annealed at 750 °C. This result suggests the potential use of ThO₂ films as a transparent reflector material for short wavelengths.