Mechanism Behind The Large Dielectric Constant Of The CaCu₃Ti₄O₁₂ Ceramics

With the development of the microelectronic technology and industry, many microelectronic devices are becoming more and more miniaturized and integrated. High-dielectric-constant oxides are very desirable for the application in microelectronics, especially for the application in 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 origin of high dielectric constant of this material has not yet been clarified. In order to apply CCTO to the microelectronic devices, the establishment of the mechanism behind high dielectric constant of this novel material is becoming very crucial. So, it is the issuefocused on this thesis.In this thesis, the contact effects at the electrode/sample interfaces on the dielectric properties of the CaCu₃Ti₄O₁₂ (CCTO) ceramic were studied firstly through post-annealing the sample and employing the different metal electrodes. It is found that the contact has the effect on the dielectric properties of CCTO depending on the surface resistivity of the sample. When the surface resistivity of the ceramic is as high as 1.2×10⁸Ω·cm, no obvious mobile space charges can be observed, and the dielectric properties of the sample is inert to the different metal electrodes and various sample thicknesses, indicating the colossal dielectric constant is due to the true properties of the ceramic itself (IBLC, grain boundary effect) . However, after the surface resistivity is lowered to 3.1×10⁷Ω·cm through post-annealing the sample in N₂ atmosphere at 750℃, obvious mobile space charges can be observed, and the dielectric properties of the sample become sensitive to the different types of contacts. The dielectric constant of the sample with Pt electrode shows a significant enhancement, comparing with that of the sample with Ag electrode. Obviously, in this case, the extrinsic contact contributes partly to the colossal dielectric constant of CCTO ceramic. Thus, the experimental results provide clear evidence that the colossal dielectric constant in CCTO is related to the ceramic itself, however, may also partly originated from the electrode/sample contact effects.