Deposition And Characterization Of High Dielectric Constant CaCu₃Ti₄O₁₂ Thin Films

With the trend of size reduction of many microelectronic devices, high-dielectric-constant oxides have become increasingly important in microelectronics, especially in the application of dynamic random access memories (DRAM) devices. Recently, much attention has been paid to an unusual cubic perovskite material CaCu3Ti4O12 (CCTO) because of its extraordinarily high dielectric constant. However, the study of this material has mainly focused on the bulk material and the study of the CCTO films is confined to the ones prepared on the single crystal oxide substrates. There has been no report on the CCTO thin films deposited on Si substrates, which are more compatible with very large-scale integrated circuits than oxide substrates.In this thesis, we study the deposition conditions of the CCTO thin films on Pt/Ti/SiO2/Si (100) substrates by pulsed-laser deposition (PLD). With the help of X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM), thin films with pure perovskite structure were obtained at the substrate temperature above 700癈 and oxygen pressure above 13.3 Pa. The values of the dielectric constant and loss and their temperature dependence under different frequency are comparable with those obtained in the epitaxial CCTO films grown on oxides substrates. In addition, we also study the effect of the film thickness on the dielectric constant. The results of the experiments are tentatively discussed.In order to get the higher value of the dielectric constant of the CCTO films, LaNiO3 (LNO) have been deposited by PLD as the bottom electrode. The microstructures of the CCTO thin films were obviously affected by the bottom electrode. The room-temperature dielectric constant of the 350-nm-thick CCTO films on LaNiO3/Pt/Ti/SiO2/Si substrates at 100 kHz was found to be 2300, which was increased significantly compared with that obtained in the films on Pt/Ti/SiO2/Si substrates.In this thesis, we also study the effect of post-annealing conditions on the CCTOthin films and found that the dielectric properties are very sensitive to the post-annealing atmosphere and temperature. Post-annealing in nitrogen atmosphere produces larger low-frequency dielectric relaxation as the annealing temperature increases, while annealing in oxygen atmosphere at high temperature suppresses the relaxation and decreases the dielectric constant of the thin films. The mechanism of this phenomenon was explained.The origin of the high dielectric constant of CCTO is still to be established. With the help of impedance spectroscopy, the ac impedance measurements were performed on the CCTO films deposited on different electrodes and on different post-annealing conditions, respectively. Together with the results of XRD and SEM, we proposed that the polycrystalline CCTO thin films are composed of semiconducting grains separated by insulating grain boundaries. High-dielectric-constant phenomenon is thus attributed to the grain-boundary mechanism.