Impedance spectroscopy study on post-annealing-tuned polycrystalline   CaCu3Ti4O12 films: Evidence of Barrier Layer Capacitor Effects
  In this paper, impedance spectroscopy study was performed to establish the electrical property and microstructure relations of the as-deposited and post-annealed polycrystalline CCTO films prepared on Pt/Ti/SiO2/Si (100) substrates by pulsed-laser deposition (PLD). Our results demonstrated that the as-deposited polycrystalline CCTO film was made of insulating grain boundaries with semiconducting grains, indicating that the high-dielectric-constant is attributed to the barrier layer capacitor (BLC) effects. The simple resistor-capacitor (RC) equivalent circuit and the modified constant phase element (CPE) circuit were used to describe the impedance spectroscopy, and excellent agreement between the calculated and measured curves was obtained in the CPE circuit. The resistance and capacitance of the grains and grain boundaries can be tuned by changing the annealing atmosphere and temperature. Under oxygen-absent annealing atmosphere, the electric resistances of the grain boundaries changed greatly but the resistance of the grains has almost no change. While under oxygen annealing atmosphere, the reverse happened. On the basis of this result, it is demonstrated that the origin of the semiconductivity of the grains in CCTO polycrystalline films arises from their oxygen-loss, while the grain boundaries are close to oxygen- stoicheometry. 
Liang Fang, Mingrong Shen, Zhenya Li and Wenwu Cao