Fabrication of sol–gel 70GeO₂–30SiO₂thick films from TEOG and DEOS and investigation of the 5 eV band

Abstract

Crack-free germanosilicate films, 3.6 µm-thick and containing up to 70 mol% germanium dioxide, which are inaccessible through the conventional sol–gel process, were fabricated using tetraethyl orthogermanate (TEOG) and diethylorthosilicate (DEOS) as precursors for germania and silica, respectively. The studies using viscosity, TEM and SEM revealed that DEOS contributed largely to stabilizing the GeO2–SiO2 sol and suppressing crack formation in thick films. XRD study showed that the films remained amorphous after being sintered at 600 °C in air for 60 min and annealed at 550 °C under a flowing H2/N2 atmosphere for 120 min. An intense absorption band at around 241 nm was distinctly observed in the films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy were detected near 375 and 275 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy. A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 389 cm−1 for 70GeO2–30SiO2 films.

Preparation of thick GeO2 glass film with high UV photosensitivity

“…Glass is a suitable waveguide material because it has no scattering sources such as specific crystal surface roughness or grain boundary and is highly transparent in visible and IR wavelength regions [8,10]. To obtain a GeO 2 glass, the hexagonal GeO 2 needs to be melted at a temperature higher than its melting point (1115°C), followed by a fast cooling to its glass transition temperature (~580°C) [17,18].…”

“…The 5.06 eV absorption band is photobleachable, and the bleaching process involves conversion of NOMVs to Ge E' centers [8]. Truthfully, this photobleaching behavior plays a key role for second-harmonic generation and hill grating growth [1,[7][8][9][10][11].…”

“…Presently there are some difficulties such as low photosensitivity and lack of adequate fabrication procedure for GeO 2 based films. These difficulties have so far been preventing the GeO 2 based photosensitive optical devices, especially the complex-shaped ones, from their full commercialization [1,10,11].…”

“…Thus, a thick GeO 2 film has much potential for applications in second-harmonic generation and growth of hill gratings [8,10,13]. Various methods including melting, flame hydrolysis, vapor axial deposition (VAD), sol-gel, sputtering deposition and chemical vapor deposition (CVD) have been used to synthesize SiO 2 -GeO 2 and GeO 2 films [1,2,[5][6][7][8][9][10][11][12][13]. Among these, the sol-gel method is attractive since it has potential for producing film on plane or complex-shaped substrates [2,8,10].…”

“…Various methods including melting, flame hydrolysis, vapor axial deposition (VAD), sol-gel, sputtering deposition and chemical vapor deposition (CVD) have been used to synthesize SiO 2 -GeO 2 and GeO 2 films [1,2,[5][6][7][8][9][10][11][12][13]. Among these, the sol-gel method is attractive since it has potential for producing film on plane or complex-shaped substrates [2,8,10]. However, GeO 2 sol-gel precursors (i. e germanium alkoxides) are very expensive and moisture-sensitive, and the maximum film thickness achieved without cracking via a single-step sol-gel coating is usually quite small (below 0.1 μm) [2,8,14].…”