X-Ray photoelectron spectroscopy of SiO2-HfO2 amorphous and glass-ceramic waveguides: a comparative study

X-ray photoelectron spectroscopy (XPS) was used to characterize xHfO(2) - (100-x) SiO2 (x = 10, 20, 30, 40 mol%) amorphous waveguides and xHfO(2) -(100-x) SiO2 (x = 10, 20, 30 mol%) glass-ceramic waveguides, doped with 0.3 mol% Er3+ ions, and prepared by the sol-gel route. The XPS analysis provided a detailed description of the mechanisms underlying the structural properties of the two different silica-hafnia systems. In particular the effect of the Hf concentration and of the annealing protocol on the material structure were worked out by studying the oxygen, silicon and hafnium core lines. In the case of SiO2-HfO2 amorphous waveguides with hafnia concentration <= 30 mol%, core lines show the formation of hafnium silicate homogeneously dispersed in the silica matrix. For higher hafnia concentrations the material undergoes a spinodal decomposition with generation of HfO2 rich domains. In the glass-ceramics the thermal treatments lead to the growth of crystallites also at Hf concentration as low as 10%. Optical, structural and spectroscopic properties of the amorphous and glass-ceramic waveguides are compared and discussed on the basis of the XPS results, allowing us to understand the chemical-physics of the fabricated waveguides, which are characterized by low-losses and by efficient luminescence @ 1.5 mu m.