Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy

A series of BaTiO3 thin films with various thicknesses from 10 to 400 nm we re epitaxially grown under various oxygen pressures from 2 x 10(-4) to 12 P a on SrTiO3 (001) substrates using laser molecular beam epitaxy. Being conf irmed by reflection high energy electron diffraction, atomic force microsco py, x-ray diffraction, and high resolution transmission electron microscopy the epitaxial single crystal BaTiO3 thin films are highly c-axis or a-axis oriented with a root-mean-square surface roughness of 0.14 nm. The observe d thickness and oxygen pressure dependent structural characteristics of the BaTiO3 thin films are discussed by taking into account both the misfits in thermal expansion and lattice constants between BaTiO3 films and SrTiO3 su bstrates, and the effect of the energy of the sputtered particles, which is consistent with the established strain relaxation mechanism. An abnormal e xpansion of lattice volume of a BaTiO3 unit cell is found and attributed to the effect of oxygen vacancies in the BaTiO3 films. (C) 2000 American Inst itute of Physics. [S0021-8979(00)03310-7].