XRD and EXAFS studies on the structure of Er³⁺-doped SiO₂–HfO₂glass-ceramic waveguides: Er³⁺-activated HfO₂nanocrystals

This paper describes the structure of Er 3+ -doped SiO 2 –HfO 2 waveguides containing nanocrystals of HfO 2 . Pure and 1 mol% Er 3+ -doped 70SiO 2 –30HfO 2 films were deposited by the sol–gel method on amorphous SiO 2 substrates using the dip-coating technique. Each waveguide has experienced a single thermal treatment at temperatures ranging from 900 to 1200 °C, for either short (30 min) or long (24 h) durations. Crystallization and microstructure were studied by x-ray diffraction (XRD). The local environments of hafnium and erbium ions were determined, respectively, from Hf and Er L 3 -edges extended x-ray absorption fine structure (EXAFS) experiments. Both XRD and EXAFS results demonstrate the substitution of Hf 4+ by Er 3+ ions in the crystalline structure. XRD shows the nucleation of tetragonal HfO 2 nanocrystals after heat treatment at 1000 °C for 30 min in the pure waveguide, and at 900 °C for 24 h in the waveguide doped with Er 3+ . In both series, partial transformation from tetragonal to monoclinic HfO 2 nanocrystals starts after heat treatment at 1100 °C for 24 h. The average crystallite size and size distribution can be controlled by thermal annealing temperature and duration, respectively, with brief treatment yielding a more homogeneous nanocrystal size.