Preparation, property, and mechanism studies of amorphous ferroelectric (Ba, Sr)TiO₃ thin films for novel metal-ferroelectric-metal type hydrogen gas sensors

Ferroelectric (Ba_0.67Sr_0.33)Ti_xO₃ (BST) thin films with x = 0.98, 1.00, 1.02, and 1.04 were prepared by the sol-gel technology, and their thermal, structural, dielectric, and gas sensing properties were systematically characterized. The amorphous (Ba_0.67Sr_0.33)Ti_xO₃ thin film capacitive devices were made on Si substrate to detect hydrogen gas and to study hydrogen-induced interracial polarization potential. Experimental results showed that the Schottky I-V behavior appears in these Pd/amorphous BST thin film/metal capacitive devices and that enhanced interfacial dipole potentials as large as 4.5 V at 1000 ppm hydrogen gas in air were newly observed, which is about 7 times larger than the best value reported under similar testing conditions. It was 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 simple hydrogen interface-blocking model is also presented to explain this interesting phenomenon.