A permanent increase in the refractive index of $GeO_2$ doped silica waveguides by UV illumination can be used in the fabrication of Bragg gratings. This photosensitivity is due to the generation of Ge E' centers by photobleached 5-eV absorption band of oxygen deficient centers. Since the $GeO_2$ dopant in the $GeO_2-SiO_2$ system was responsible for the photosensitivity of the glass waveguides, it was to be expected that pure $GeO_2$ glass thin films would be even more strongly photosensitive.
Transparent and homogeneous $GeO_2$ glass thin films were fabricated on Si(100) wafer and fused silica glass substrates by the sol-gel spin coating method. In order to obtain a homogeneous glass film without cracking and crystalline phase during drying and densification process, the $GeO_2$ films were heat treated at 600℃ for 10min under air atmosphere using controlled heating rate.
The molecular structures, hydroxyl contents, and crystallinity of the $GeO_2$ films in relation to the heat treatments were investigated by FT-IR spectroscopy. Structural hydroxyl groups were removed during sintering process and any interactions were not detected in the germanium atoms and hydrogen atoms during heat treatment under reduced ($H_2/N_2$ = 1 : 9 volume ratio) atmosphere.
Since the porosity and the refractive index of the porous film can be related, the consolidation of the $GeO_2$ films were monitored by measuring the change of refractive index using a ellipsometer, after heat treatment in air at various temperature for 10minutes, using heating rate of 4°/min. So we obtain a fully densified $GeO_2$ glass thin films with refractive index of 1.607 at 632.8 nm.
Formation and photobleaching of optical absorption bands in the 5-eV region were examined in $GeO_2$ and $50GeO_2-50SiO_2$ glass thin films using UV/VIS/NIR spectrophotometer. Heat treatment of a $GeO_2$ and $50GeO_2-50SiO_2$ glass thin films in a reduced ($H_2/N_2$ = 1/9 volume ratio) atmosphere increased intensities of an absorption band center at 5-eV. The intensities of the 5-eV band were increased exponentially with the annealing temperature and increased linearly with annealing time. But, heat treatment of a $GeO_2$ films in a reduced atmosphere above 500℃ induced the formation of Ge and $GeO_2$ crystalline phase. The value evaluated in $GeO_2$ films after heat treatment in a reduced atmosphere at 500℃ for 2h was $525 cm^{-1}$ and that in $50GeO_2-50SiO_2$ films after heat treatment in a reduced atmosphere at 600℃ for 1h was $845 cm^{-1}$.
UV irradiation of films was carried out at room temperature using a 300W Xe lamp with 248 nm bandpass filter. Intensities of the 5-eV band were decreased almost linearly with illumination time at the initial stage and approach a saturated level. The saturated absorption coefficient changes (-Δα) for $GeO_2$ and $50GeO_2-50SiO_2$ glass thin films were 175 and $85 cm^{-1}$, respectively.
Assuming that the peak intensities are the sum of intensities of the UV-bleachable band and the unbleachable band, ratio of the concentrations of the NOMV and NODV in the $GeO_2$ and $50GeO_2-50SiO_2$ films were 0.113 and 0.025, respectively, using the molar extinction coefficients ($4×10^4$ L/mol/cm for the NOMV and $9×10^3$ L/mol/cm for the NODV). Although the intensities of the 5-eV band in $50GeO_2-50SiO_2$ films were larger than that in pure $GeO_2$ because of higher annealing temperature, pure $GeO_2$ glass thin films were more strongly photosensitive.
Therefore, we expect that pure $GeO_2$ glass thin films deposited by the sol-gel spin coating method will be promising for application to optical devices, which utilize refractive index change, induced by UV irradiation.
