Microstructures and electrical properties of CaCu 3 Ti 4 O 12 thin films on Pt/TiO 2 /SiO 2 /Si substrates by pulsed las
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Ji Hye Lee and William Job) Department of Physics, Ewha Womans University, Seoul 120-750, Korea
Young-Hwan Kim and Kyoung Jin Choi Nano-Materials Center, Korea Institute of Science and Technology, Seoul 136-791, Korea (Received 14 April 2011; accepted 6 July 2011)
We investigated microstructures, compositional distributions, and electrical properties of dielectric CaCu3Ti4O12 (CCTO) thin films deposited on Pt/TiO2/SiO2/Si substrates from 700 to 800 °C by pulsed laser deposition. With increasing the deposition temperature from 700 to 750 °C, the dielectric constants (er) of CCTO films were greatly enhanced from ;300 to ;2000 at 10 kHz, respectively. However, the er values of CCTO films were gradually decreased above 750 °C, which was surely attributable to the formation of a TiO2-rich dead layer at the interface between CCTO and Pt electrode. Compositional analyses by Auger electron spectroscopy, energy dispersive spectroscopy, and electron energy loss spectroscopy revealed that the TiO2-rich dead layer became thicker because of severe Cu diffusion from CCTO films to Pt electrode. The leakage current behaviors of CCTO films are in good agreement with Poole–Frenkel conduction mechanism, where both the TiO2-rich dead layer and rutile TiO2 nanocrystalline particles are considered to play a role of charge trapping centers.
I. INTRODUCTION
The unprecedentedly high dielectric constant (er;10 ) of the cubic perovskite CaCu3Ti4O12 (CCTO) ceramics without any phase transition over a wide temperature region of 100–600 K has drawn considerable attention for both its physical origin and practical applications.1 First of all, related to the physical origin of the colossal dielectric response (CDR) of CCTO ceramics2–6 and films,7,8 many research groups seem to accept the grain boundary internal barrier layer capacitor (IBLC) model as the mechanism for CDR. The CDR is, however, observed also in CCTO single crystals9,10 without grain boundaries, indicating that it is unexplainable only by the grain boundary IBLC model. Thus, for explaining the CDR in both CCTO ceramics and single crystals, it has been suggested as an alternative mechanism that CDR is attributable to the presence of insulating barriers such as domain boundaries11–13 and twin boundaries.10,14 Recently, Ferrarelli et al.15 have proposed that a nonohmic contact between the electrodes (Au, InGa) and CCTO single crystal is 4
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2011.226 J. Mater. Res., Vol. 26, No. 19, Oct 14, 2011
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responsible for CDR. Consequently, it can be said that the mechanism of CDR in CCTO is still controversial. On the other hand, for practical applications, CCTO should be made in the form of film devices. Thus many research groups fabricated CCTO films and reported their properties.7,8,16–37 Up to date, various deposition techniques, including pulsed laser deposition (PLD),7,16–28 sol– gel,8,29–33 sputtering,34–36 and metalorganic v
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