Ultrathin SrTiO3 films: epitaxy and optical properties

Ultrathin (12–15 nm) SrTiO3 films are grown by pulsed laser deposition on various single-crystal substrates. The crystal structure, orientation, and strain state of the films are studied by x-ray diffraction. The room-temperature optical properties of the films are experimentally determined using ellipsometric spectroscopy in the 1–6 eV spectral range. Epitaxial films with biaxial in-plane strain are obtained on LaAlO3, DyScO3, and KTaO3 substrates, with the critical thickness for pseudomorphic growth being less than 10 nm. Abrupt strain relaxation has been detected. The optical properties of the films with different microstructure are compared with each other and with those of single-crystal SrTiO3. Based on the comparison, surface effects are suggested to be dominant in the visible range, while the interband transitions are smeared and suppressed due to small film thickness and the presence of biaxial strain. The energies of gaps can increase due to strain-induced polarization. The absorption edge is affected by all the factors mentioned (surface, thickness, strain, and polarization).