Microstructural properties and sensitive performance of TbTa_xO_y sensing membranes for electrolyte-insulator-semiconductor pH sensors

The sensitivity and stability of sensing film material are mainly factors to impede the practical applications of ion-selective field effect transistor (ISFET) or electrolyte-insulator-semiconductor (EIS) pH sensors. Metal oxide films have demonstrated great advances on achieving high-performance pH sensors. However, some metal oxide films with low sensitivity and poor reliability still hinder the biochemical, environmental and medical applications. Therefore, choosing a proper sensing film material plays a very important role for the fabrication of highly sensitive and reliable sensors. In this article, the novel TbTa_xO_y sensing membrane demonstrating a super-Nernstian pH response and excellent stabililty was developed for a solid-state pH sensor. We investigated the relationship between sensing performance and microstructural property of the TbTa_xO_y sensing membranes deposited onto Si substrates through a reactive co-sputtering system and subsequently post-deposition annealing (PDA) at four temperatures (from 600°C to 900°C). To check the influence of PDA treatment on the microstructural properties of the TbTa_xO_y films, atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were employed to manifest the morphological, microstructural, and chemical features, respectively, of these films. Among these PDA temperatures, the TbTa_xO_y EIS device after annealing at 900 °C exhibited the best sensing performances including, the super-Nernstian sensitivity (67.04 mV/pH), the lowest drift rate (0.12 mV/h), and the smallest hysteresis voltage (1 mV). Probably, this PDA temperature can enhance the surface roughness, form the stoichiometry of TbTaO₄ film, and reduce the crystal defects. The super-Nernstian response may be contributed to a variation in the oxidation state of Tb ion changing from Tb³⁺ to Tb²⁺, thereby generating two H⁺ ions and one electron transferred in the chemical reaction. Moreover, this TbTa_xO_y sensing film demonstrates better sensing performance compared to previously fabricated sensing materials available in the literature.