The bandgap of SrBi2Ta2O9

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Abstract
The electronic structure of SrBi2Ta2O9 thin films, which are of interest for ferroelectric random access memories, has been investigated. The valence band has been analysed using surface sensitive X-ray photoemission spectroscopy (XPS), while the partial density of unoccupied electronic states associated with the conduction band has been investigated using near edge X-ray absorption spectroscopy (NEXAS). The shapes of the NEXAFS spectra are roughly in agreement with calculations presented previously, however, the metal states (especially bismuth) appear to be broader. The NEXAFS measurements were performed for two different information depths (∼100Å and ∼2000Å) which indicated no significant electronic structural differences between the bulk and the near-surface region. The bandgap has been measured using bulk sensitive UV-visible absorption and surface sensitive electron energy loss spectroscopy. The data indicated that the bandgap of the thin film material is ca. 4 eV and lower than suggested previously.