XRD and EXAFS studies on the structure of Er~(3+)-doped SiO_2–HfO_2 glass-ceramic waveguides: Er~(3+)-activated HfO_2 nanocrystals

摘要

This paper describes the structure of Er~(3+)-doped SiO_2—HfO_2 waveguides containing nanocrystals of HfO_2. Pure and 1 mol% Era+-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.