Thickness-Dependent Strain Evolution of Epitaxial SrTiO3 Thin Films Grown by Ion Beam Sputter Deposition

Epitaxial SrTiO3 (STO) thin films are grown on vicinal (001)-oriented LaAlO3 (LAO) substrates with various thicknesses (3-108nm) by an ion beam sputter deposition (IBSD). Thickness-dependent strain in the STO films is investigated comprehensively through the X-ray analysis. The STO films grow on the LAO substrates under a compressive strain along in-plane direction (a- and b-axes) and a tensile strain along the growth direction (c-axis). With increasing film thickness, the STO unit cells evolve from more to less tetragonal structure, indicating the occurrence of strain relaxation. Quantitative strain analysis is also carried out for all STO films in terms of biaxial and hydrostatic components. It is found that the major cause of strain in STO films is caused by biaxial component over hydrostatic component. The surface morphology of STO films exhibit a step-and-terrace structure with roughness close to the LAO substrate (approx. 0.1nm), indicating a 2D growth mode. However, as the film thickness increases, the surface terrace become rougher, which is caused by the effect of strain relaxation.