Effect of phase structures and substrate temperatures on NTC characteristics of Cu-modified Ba–Bi–O-based perovskite-type thermistor thin films

Based on the formula of BaBi0·92Cu0·08O3, Cu-modified Ba–Bi–O-based perovskite-like negative temperature coefficient (NTC) thermistor thin films were successfully prepared by radio frequency (RF) magnetron sputtering method with the various deposition temperatures (25 °C, 100 °C, 200 °C and 250 °C). The effects of the substrate temperatures on the phase structures and electrical properties were respectively characterized and investigated by X-ray diffraction (XRD), resistance-temperature measurement, and complex impedance analysis for the Ba–Bi–O-based thin films. Additionally, the cross-sectional microstructures and grain morphologies of the BaBi0·92Cu0·08O3 thin films were analyzed by scanning electron microscope (SEM) and atomic force microscope (AFM). The XRD patterns results showed that the perovskite monoclinic structure with a small amount of BaBiO2.5 s phase could be obtained for the BaBiO3 thin film deposited at 200 °C, and also, the main crystalline phase was transferred from BaBiO2.82 to BaBiO2.5 when the BaBi0·92Cu0·08O3 thin films were deposited at 200 °C and 250 °C, respectively. Temperature dependent complex impedance spectroscopy was carried out to make clear the resistance contribution of grain (Rg), grain boundary (Rgb) and contact (Rs) response for the conduction mechanisms in the Ba–Bi–O-based thin film deposited at a given substrate temperature. Compared with the electrical properties of the BaBiO3 thin film deposited at 200 °C [such as the room-temperature resistivity (ρ25 ~ 1548 Ω cm) and the thermistor constant (B25/85–3245 K)], the better NTC characteristics of a lower ρ25 value of about 956 Ω cm and a higher B value of approximately 3480 K were found in the BaBi0·92Cu0·08O3 thin film deposited at a same temperature (200 °C). It is believed that the phase structures and electrical properties of the BaBiO3 perovskite-type NTC thermistor thin films could be adjusted and optimized by a trace amounts of Cu-ions substitutions and depositing at an appropriate substrate temperature, which also showed that such as these novel BaBiO3-based thin films are feasible with the extensive of actual applications for the NTC thermistors.