Valence band and bandgap states of ferroelectric SrBi2Ta2O9 thin films

Ultraviolet photoelectron spectroscopy (UPS), X-tay photoelectron spectrosc opy (XPS) and surface photovoltage spectroscopy (SPV) were used to determin e the electronic structure near the bandgap of strontium bismuth tantalate (SBT) thin films. The UPS results for neatly stoichiometric SET have been c ompared with tight-binding calculations. The spectra for bismuth-excess SET indicate additional density of states (DOS) in the wide bandgap of the mat erial. SPV studies indicate that the surface bandgap of bismuth-excess SET is approximately 2 eV, which also confirms that there are additional surfac e states in the bandgap. These electronic structural data are used to expla in the observed dependency of the electrical properties of the SBT/electrod e junction on the bismuth concentration.