Phase change observed in ultrathin Ba0.5Sr0.5TiO3 films by in situ resonant photoemission spectroscopy

Epitaxial Ba0.5Sr0.5TiO3 thin films were prepared on Nb-doped SrTiO3 (100) substrates by the pulsed laser deposition technique and were studied by measuring the Ti 2p→3d resonant photoemission spectra in the valence-band region as a function of film thickness, both at room temperature and low temperature. The results demonstrated an abrupt variation in the spectral structures between 2.8nm (∼7 ML) and 2.0nm (∼5 ML) Ba0.5Sr0.5TiO3 films, suggesting that there exists a critical thickness for phase change in the range of 2.0–2.8nm. This may be ascribed mainly to the intrinsic size effects.