Characterization of strain relaxation of (001) oriented SrTiO3 thin films grown on LaAlO3 (110) by means of reciprocal space mapping using x-ray diffraction

The strain relaxation of SrTiO3 r.f. magnetron sputter-deposited thin films on LaAlO3 substrates have been studied by x-ray diffraction mapping. An investigation of different x-ray optics shows that a, so called, hybrid mirror monochromator in combination with a triple-bounce analyser crystal provides very good conditions for characterization of thin distorted films grown epitaxially onto substrates with high structural order. The in-plane and out-of-plane lattice parameters of the SrTiO3 films could accurately be determined since the x-ray diffraction optics enabled the splitting of substrate peaks, caused by the twinning in the rhombohedral LaAlO3 to be resolved and, provided film peak intensities are high enough, to precisely establish their positions. Films in the thickness range 9.3-144.0 nm were found to be partially relaxed, having a tetragonal distortion due to in-plane strain that was found to decrease with increasing film thickness, approaching an undistorted SrTiO3 lattice parameter of 0.3927 nm. This value is 0.6% larger than the bulk indicating that the compositions of the films were slightly non-stoichiometric. The strain relaxation of the grown films was found to follow the general trend of a predicted strain-thickness relation based on energy density balance considerations regarding misfit dislocations and lattice strain.