Epitaxial strain and its relaxation at the LaAlO{sub 3}/SrTiO{sub 3} interface
A series of LaAlO{sub 3} thin films with different thicknesses were deposited by pulsed laser deposition at temperatures from 720 °C to 800 °C. The results from grazing incidence x-ray diffraction and reciprocal space mapping indicate that a thin layer of LaAlO{sub 3} adjacent to the SrTiO{sub 3} substrate remains almost coherently strained to the substrate, while the top layer starts to relax quickly above a certain critical thickness, followed by a gradual relaxation at larger film thickness when they are grown at lower temperatures. The atomic force microscopy results show that the fast relaxation is accompanied by the formation of cracks on the film surface. This can be ascribed to the larger energy release rate when compared with the resistance of LaAlO{sub 3} to cracking, according to calculations from the Griffith fracture theory. For films grown at 720 °C, a drop in sheet resistance by two orders of magnitude is observed when the top layer starts to relax, indicating a relationship between the strain and the conductivity of the two-dimensional electron gas at the LaAlO{sub 3}/SrTiO{sub 3} interface. The strain engineered by growth temperature provides a useful tool for the manipulation of the electronic properties of oxide heterointerfaces.