Microstructure, dielectric properties and hydrogen gas sensitivity of sputtered amorphous Ba_0.67Sr_0.33TiO₃ thin films

A novel metal-ferroelectric hydrogen gas sensing device was fabricated on platinum-coated silicon wafer with an amorphous ferroelectric Ba_0.67Sr_0.33TiO₃ layer using the RF magnetron co-sputtering process and was characterized by X-ray diffraction, transmission electron microscopy, dielectric characterization and gas sensing measurement. Experimental results show that the microstructure and the dielectric properties are closely correlated with the deposition parameters. The studies on the dielectric properties indicate that the non-stoichiometric defects in the amorphous films are largely reduced by depositing in 50% oxygen content just below the crystallization temperature of the films. J-E performances exhibit the typical Schottky behavior, both in air and in hydrogen gas and a voltage shift 0.6 V has been observed in 1042 ppm hydrogen diluted in air. Compared to the sol-gel case, it is believed that the electronic defects, both in bulk and at interface, cause the degradation of the hydrogen gas sensitivity and weaken the induced H₂ potential built-up across the space charge layer at the interface. The gas sensing mechanism based on the proton induced Pd/BST interfacial polarization potential is also discussed. Related to the MIS hydrogen sensor device, it is believed that the high permittivity of the amorphous ferroelectric thin films enhances the proton polarization at the Pd/BST interface and, in turn, greatly improves the built-up interfacial potential induced by the hydrogen.