Energy level alignment and electrical properties of (Ba,Sr)TiO₃/Al₂O₃ interfaces for tunable capacitors

The interface formation between Ba₀.₆Sr₀.₄TiO₃ and Al₂O₃ has been studied using photoelectron spectroscopy with in situ sample preparation. A negligible valence band discontinuity, corresponding to a ∼5.6 eV barrier for electron transport at the BST/Al₂O₃ interface is determined. Current-voltage measurements show that the leakage current can be significantly reduced by inserting the Al₂O₃ barrier layer between barium strontium titanate (BST) and Pt electrode. Different charge injection behavior depending on Al₂O₃ thickness is observed, which correspond well with the experimentally determined energy band diagrams. Direct tunneling from the metal electrode into the BST conduction band through the Al₂O₃ barrier layer is observed.