Amorphous ferroelectric (Ba_0.67Sr_0.33)Ti_1.02O₃ thin films with enhanced H₂ induced interfacial polarization potential

Ferroelectric (Ba_0.67Sr_0.33)Ti_1.02O₃ thin films have been prepared by the sol-gel technology and characterized using thermogravimetric analysis, differential thermal analysis, x-ray diffraction, dielectric characterizations, and gas sensing measurements. The (Ba_0.67Sr_0.33)Ti_1.02O₃ thin film capacitive devices are made on Si substrate to detect hydrogen gas and to study hydrogen induced interfacial polarization potential. Experimental results show that the Schottky I-V behavior appears in these Pd/amorphous (Ba,Sr)TiO₃ thin film/metal capacitive devices, both in air and in diluted hydrogen gas environment, and that the enhanced interfacial dipole potential as large as 4.5 V at 1042 ppm hydrogen gas in air has been observed. Compared with the available data in the literature, the obtained value of hydrogen induced interfacial polarization potential in our experiment is about seven times larger than the best one reported under similar testing conditions. It has been clearly shown that the hydrogen induced interfacial polarization potential is closely correlated with the microstructure of ferroelectric thin films and the enhancement of this interfacial polarization potential is mainly attributed to the high dielectric constant of amorphous ferroelectric thin films. A model is also proposed to explain this interesting phenomenon. In this model, hydrogen H₂ molecules are dissociated at the top surface of the catalytic Pd layer and ionized under the positive bias. The H⁺ ions then diffuse through this Pd layer, accumulate at the interface between the Pd layer and the amorphous ferroelectric passivation film. Dipoles are thus formed so that the polarization potential is built up at the interface. Moreover, the high dielectric constant of ferroelectric films enhances dipole polarization, thus greatly improves the H₂ gas induced polarization potential. Though in a preliminary stage, our experimental results offer great promise in fabricating large-scale, Si based ferroelectric thin film gas sensors and related electronic devices.