| Summary: | 博士 === 國立中興大學 === 材料工程學研究所 === 93 === Thin films of rf-sputtered Ba1-xSrxTiO3 (BST) with appropriate MgO doping contents were deposited on Pt/Ti/SiO2/Si substrate. BST thin film doped with MgO is expected to improve the dielectric and leakage properties. Microstructure, chemistry, and dielectric properties of BST thin films under different substrate temperatures, various MgO doping contents, and post-annealing were carried out by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), analytical electron microscopy (AEM) combined with energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), differential scanning calorimetry (DSC), Rutherford backscattering spectrometer (RBS), X-ray photoelectron spectroscopy/Auger electron spectroscope (XPS/AES), and capacitance-voltage (C-V) and current-voltage (I-V) analyzer.
The analytical results of XPS show that Ba/(Ba+Sr) and Sr/(Ba+Sr) molar ratio are invariable irrespectively of substrate temperatures. The crystallinity of the BST films depends on the substrate temperature as well as the measured (Ba+Sr)/Ti molar ratio. The analytical results of XRD and plan-view/cross-sectional TEM show that the perovskite phase in both films began to appear when the substrate temperature was increased to 450 oC. For both BST films deposited between 450 and 650 oC, it shows a coexistence of crystalline and amorphous phases. An almost completely crystallized BST film with columnar grain structure was obtained for the film deposited at substrate temperature 750 oC. Electrical properties such as dielectric constant and leakage current are closely related to the microstructural changes of the BST films.
In addition, the influence of the BST films deposited at substrate temperature 750 oC with various MgO doping contents on the microstructure and dielectric properties is investigated. The analytical results of XRD and cross-sectional TEM showed that MgO doping can suppress grains growth in the BST-based films and the grain size decreases with increasing MgO doping contents. The AES and RBS analysis revealed a uniform distribution from the BST film surface to the interface between BST and Pt bottom layer and a sharp interface with no significant interdiffusion of the constituent elements between BST and Pt bottom layer. The DSC revealed that MgO addition causes the Curie temperature (Tc) of the BST films to shift toward higher temperature. Higher dielectric constant was obtained for the poor crystalline MgO-doped BST film, which, apparently, contradicts the fact that a better crystallinity results in a higher dielectric constant. A reason proposed here based on the analytical results, which is speculated that it may be due to an increase in the residual stress. On the other hand, the leakage current of the BST film with MgO addition is improved by reduction of grain size, reducing the electron concentration, increasing the barrier height, and expansion of depletion layer width.
The influence of post-annealing on the microstructure, dielectric, and leakage properties is further investigated. The film deposited at substrate temperature 350 oC followed by post-annealing at 750 oC in ambient air for 2 hrs displays a granular structure, whereas that deposited at substrate temperature 750 oC followed by post-annealing at 750 oC in ambient air for 1 hr still displays a columnar structure. The results indicate that MgO-doped BST films followed by post-annealing in ambient air can further improve the dielectric and leakage properties. The pores are mostly pronounced in the undoped BST film and then decrease with increasing MgO doping contents. In addition, the dielectric and leakage properties with columnar grains structure are higher than that with granular grains structure, which may be relative to the grains structure, porosity, and oxygen vacancies.
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