Microstructure and dielectric properties of compositionally-graded (Ba1−xSrx)TiO3 thin films

Compositionally-graded (Ba1−xSrx)TiO3 thin films (x varying from 0 to 0.25) were epitaxially grown by pulsed-laser deposition on (1 0 0)MgO single-crystal substrates with a conductive La0.5Sr0.5CoO3 layer as bottom electrode. The crystallinity and epitaxial growth behavior of the graded BST films were investigated by X-ray diffraction and transmission electron microscopy. The results show that the graded films are (1 0 0)-oriented with an orientation relationship of (1 0 0)[0 1 0]BST//(1 0 0)[0 1 0]MgO. Cross-sectional TEM images show that graded BST films grow with a columnar structure and a surface roughness of 10 nm. From planar TEM images granular and/or polyhedral shaped grains are observed with an average grain size of 65 nm. At room temperature, the dielectric constant and dielectric loss at 10 kHz for the graded film were 953 and 0.0103, respectively. A broad and flat profile of the dielectric constant versus temperature was demonstrated in the graded film. This was attributed to the presence of the compositional and/or residual strain gradients in the epitaxial compositionally-graded films. With such a graded structure, it is possible to build dielectric thin film capacitors with almost negligible temperature dependence of the capacitance, thereby operating without the need for controlling the environmental temperatures.