MgO- doped Ba0.5Sr0.5TiO3 BST thin films were synthesized by radio-frequency (rf) magnetron sputtering at substrate temperature 750 S C using single- phase targets with different MgO contents ranging from 0 to 5 mol %. Microstructure, surface morphology, dielectric constant, and leakage current of the MgO- doped BST films were characterized to understand the influence of the MgO dopant on film properties. Polycrystalline and single- phase solid solution films with a dense microstructure were obtained in all deposition conditions. The electrical and dielectric properties of the BST- containing capacitors are both found to be improved significantly by doping MgO in the BST films. The leakage current density of the 5 mol % MgO- doped BST capacitors is nearly two orders of magnitude lower and the dielectric constant is about 45% times higher than that of the undoped BST capacitors. It is speculated that the increase in the dielectric constant is due to the formation of electric dipole M-Ti(2-)- V-0(2+) complex resulting in an increase in the residual stress, whereas the improvement in the leakage properties is attributed to the oxygen vacancies by MgO doping, resulting in a reduced electron concentration in the materials. (c) 2005 The Electrochemical Society.