Effect of structures and substrate temperatures on BaZn0.06Bi0.94O3-δ perovskite-based NTC thermistor thin films

Abstract

Negative temperature coefficient (NTC) thermistor thin films based on perovskite-type BaBiO3 (BB) and BaZn0.06Bi0.94O3-δ (BZB) were successfully prepared by radio frequency (RF) magnetron sputtering method on Pt substrate. The crystal structure and grain morphology of the BZB and BB films deposited at different substrate temperatures (25–200 °C and 200 °C, respectively) were examined by X-ray diffraction (XRD) and atomic force microscope (AFM). The substrate temperature had a significant influence on the crystallinity and phase structure, in which the main crystalline phases with the cubic and tetragonal BaBiO3-based perovskite structures could be obtained on the substrate temperatures of 100 °C and 200 °C, respectively. On the other hand, the electrical properties were analyzed by measuring the resistance temperature (ρ-T) characteristics, and all the thin films exhibited good NTC thermistor characteristics. In addition, the grain (Rg) and grain boundary (Rgb) contribution to the total resistance were estimated by three parallel R-CPE equivalent circuits in series, in which the main contribution of resistance for BZB thin film deposited at 200 °C was derived from Rg response rather than that in Rgb at the higher temperature range. This was one of the most important reasons why this sample had a good NTC behavior, and this result was confirmed by the current-voltage (I-V) characteristics analysis. For the BB film, the room-temperature resistivity (ρ25) was 1540 Ω cm (B25/85 –3240 K), while the ρ25 value decreased to approximately 1156 Ω cm (B25/85 –3183 K) for BZB film deposited at a same substrate temperature (200 °C). This is mainly due to the change in crystal structural characteristics. It is believed that the substitution of Zn2+ for BB thin films deposited at an appropriate temperature will be useful for low-resistance applications as novel thin film NTC thermistors.