The present invention provides semiconductor structures and methods for forming semiconductor structures which include monocrystalline oxide films exhibiting both high dielectric constants and low leakage current densities. In accordance with various aspects of the invention, a semiconductor structure includes a monocrystalline semiconductor substrate and one or more stoichiometrically graduated monocrystalline oxide layers. The stoichiometrically graduated monocrystalline oxide layer may include a perovskite material, such as an alkaline-earth metal titanate. Semiconductor devices fabricated in accordance with aspects of the present invention exhibit a high dielectric constant as well as a reduced leakage current density.