A new defect model to explain the anomalous dielectric properties of A(copper-titanium oxide) phases (A: calcium, cadmium)

The anomalous high dielectric properties of CaCu₃Ti ₄O₁₂ (CaCTO) have been studied in detail for bulk material and thin film samples. The dielectric properties and dielectric relaxation of CaCTO and CdCu₃Ti₄O₁₂ (CdCTO) polycrystalline samples sintered at various temperatures were evaluated. It was found that the dielectric properties of both samples were very sensitive to sintering temperatures. A new Defect Model was suggested, and successfully explained the correlation between the dielectric constant and the sintering temperature of CaCTO and CdCTO samples. The Defect Model developed the concept of localized semiconducting domains produced by electronic defects quenched in the CaCTO make it as responsible for the anomalous behaviors. The origin of defects was described as two distinct possibilities—Cu vacancy or oxygen vacancy formation. The results and the Defect Model suggested that the very high dielectric constant and the dielectric relaxation for CaCTO and CdCTO are unique properties of the ACu₃Ti₄O₁₂ phase.; To fully advance the Defect Model, the dielectric properties of CaCTO were examined by an impedance spectroscopy. It was found that the semiconducting domain conductivities and dielectric relaxation frequencies were also sensitive to sintering temperatures, which strongly supported the Defect Model. A concentration difference in defects in CaCTO grains was considered as the origin of the varying semiconducting domain conductivities.; The D.C conductivities of epitaxial thin films and polycrystalline thin film were evaluated and related to the Defect Model. (001) oriented epitaxial CaCTO thin films and the polycrystalline thin films were fabricated on LaAlO ₃ (LAO) substrates by pulsed laser deposition (PLD). It was found that the epitaxial thin films were semiconducting, and conductivity was very sensitive to the annealing conditions. The polycrystalline thin film, however, showed insulating properties. Oxygen vacancy formation in CaCTO grains was studied by X-ray photoelectron spectroscopy (XPS) in support of the Defect Model. XPS results clearly showed that there was oxygen vacancy formation accompanied by Cu2+ reduction to Cu1+. Small amounts of Cu1+ ions along with oxygen vacancies in CaCTO grains seemed to be a possible origin of defects for the Defect Model.; The dielectric properties of epitaxial thin films and the polycrystalline thin films were also measured and compared. (001) oriented epitaxial CaCTO thin film and the polycrystalline thin film capacitors were fabricated on SrRuO₃/LaAlO₃ substrates by PLD. The dielectric properties of the thin film capacitors were evaluated by the impedance spectroscopy in the temperature range over 90 K to 273 K. The average dielectric constant of CaCTO epitaxial thin films was 800, which make CaCTO a promising candidate for capacitive elements for dynamic random access memory (DRAM). However, a large leakage current for CaCTO epitaxial thin films is a challenge to overcome for practical applications. The dielectric constant of the polycrystalline thin film was 30–25 times less than that of the epitaxial thin films. The Defect Model was successfully applied to explain the dielectric constant differences between the two thin film CaCTO samples.