We report a real-time observation of strain relaxation during in situ growth of SrTiO 3 thin films by measuring the in-plane lattice constant at the film surface using reflection high-energy electron diffraction. The initial misfit strain in the SrTiO 3 film is tensile on MgO and compressive on LaAlO 3 as expected from the lattice mismatches between the film and the substrates. Strain relaxation begins immediately after the deposition starts, but is not complete until the film thickness reaches

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... 500-2500 Å depending on the substrate and the deposition temperature. The strain relaxation at the growth temperature influences the film strain at room temperature, which is compressive for both substrates for thin SrTiO 3 films.