Piezoelectricity in poled silica films achieved using super-lattice structure with tetravalent metal dopants

We previously reported piezoelectricity in thin films of poled silica glass doped with germanium (Ge:SiO₂). Similarly, tetravalent-metal-doped SiO₂ (M⁴⁺:SiO₂) films were prepared on Si substrates by RF magnetron sputtering for this experiment. We used germanium, titanium, and tin as the doping materials. We compared the piezoelectricity of the films with the piezoelectricity of quartz. Piezoelectricity with the same order of magnitude as that in quartz was observed in the M⁴⁺:SiO₂ films. However, less than a week later, the piezoelectricity disappeared almost completely in all the samples. To prevent this in the poled M⁴⁺:SiO₂ films, we have tried to pin the displacement of the doping ions with a poling treatment. We have developed a pinning technique based on the structure of a Ge:SiO₂-Ti:SiO₂-Sn:SiO₂ super-lattice. This super-lattice structure was very effective in preventing the piezoelectricity from disappearing. It is known that an anomalous photovoltaic effect only exists in materials with spontaneous polarization. In the M⁺⁴:SiO₂ super-lattice film, an anomalous photovoltaic phenomenon could clearly be observed. Therefore, the poled super-lattice silica film was piezoelectric.