Study of Amorphous Barium Titanate Thin Films

Although the conduction mechanisms in dielectrics have been studied for many years, several issues are still not well understood from a fundamental point of view. For examples: in differentiating the Schottky emission and Frenkel-Poole transport mechanisms the factor of two difference in the slopes of ln(J) versus V^{1/2 } plots has never been unambiguously observed; also how fast a carrier moves in the dielectric is another controversial issue. It is fundamentally important to understand the basis of various conduction phenomena. This thesis presents an experimental study on the conduction mechanisms in amorphous BaTiO_3 thin films. The amorphous BaTiO_3 samples are prepared by a reactive thin film growth technique which involves a physical vapor deposition onto a substrate in an oxygen ambient with a concurrent low energy ion bombardment of the substrate during deposition. This growth technique is referred to as the reactive partially ionized beam (RPIB) deposition technique. Two types of amorphous BaTiO _3 thin films are deliberately created using different deposition conditions. These two types of thin films show very different charged defect density, characterized by capacitance-voltage (C-V) measurements. Also different transport phenomena are observed in current voltage (I -V) characterization. The different transport phenomena are explained by the different conduction mechanisms, the Schottky emission and Frenkel-Poole transport mechanisms. Several important conclusions are obtained: (1) the factor of two difference in the slopes of ln(J) versus V^ {1/2} plots, which is essential in differentiating the Schottky emission and Frenkel-Poole conduction mechanisms, is observed for the first time; and (2) the static dielectric constant should be used in interpreting the conduction mechanisms, which implies that the motion of carriers in the dielectric is slow and the polaron model proposed by Mott is valid. Also, a correlation of conduction mechanisms with the charged defect density in the film is obtained. We also study the microcrystalline structure of amorphous BaTiO_3 thin films using structural characterization. The formation of microcrystalline structure is achieved by annealing the amorphous BaTiO _3 thin films deposited on Si (100) using the RPIB deposition technique. The existence of BaTiO_3 microcrystallites in the film is indicated by X-ray diffraction measurement. The first direct observation of BaTiO_3 microcrystallites is realized using transmission electron microscope (TEM). The typical size of microcrystallites is about 70A x 400A. Diffraction fringes are observed in some BaTiO_3 microcrystallites, which indirectly indicate that these microcrystallites may have a ferroelectric domain structure. The annealing induced dielectric properties change of the amorphous BaTiO _3 thin films is discussed. The study finds that the densification of the film during the annealing is primarily responsible for the change of the dielectric properties.