Effects of thermal anneal on the UV photosensitivity for writing of Bragg gratings on Ge-doped silica thin films by a plasma CVD method

We studied the effects of thermal anneal and UV photon irradiation on GeO₂-SiO₂ thin films by a plasma CVD method. It was confirmed that annealing in the forming gas containing 5% H₂ and 95% N₂ at 1000 C for 4 hours could increase the density of neutral oxygen monovacancy (NOMV) that is responsible for UV photosensitivity and also decrease concentration of OH group that is the most abundant impurity in CVD films. On the other hand, the UV photosensitivity at 5.0 eV due to the NOMV was decreased by annealing in the atmosphere of nitrogen or oxygen. A UV-induced absorption band due to Ge E' center was observed at 6.3 eV, whose precursor is the NOMV. Also, two absorption bands associated with two kinds of germanium electron center (GEC) were induced at 4.7 and 5.7 eV. These absorption bands are responsible for UV-induced refractive index change. In particular, the amount of the induced Ge E' center in samples annealed in the forming gas was 20-fold larger than that of samples annealed in nitrogen. By UV irradiation through a phase mask, formation of Bragg gratings with a period of 0.5 μm was observed by an atomic force microscope (AFM). It was confirmed that the induction of the gratings was due to compression in the films by means of observing on the interface between un-irradiated and irradiated area by an AFM.