Application of atomic layer deposited Al₂O₃ as charge injection layer for high-permittivity dielectrics

The current transport through (Ba,Sr)TiO₃ (BST)/Al₂O₃ bilayer structures with Pt electrodes is studied using current–voltage measurements. Due to its low permittivity compared to BST the Al₂O₃ layer, which is deposited by atomic layer deposition (ALD), enables electron injection by tunneling at low thickness, such that transport becomes limited by the bulk conductivity of BST. A current rectification of up to 10⁶ is observed at an Al₂O₃ thickness of 1.8 nm, indicating that hole transport does not contribute to the current. The measurements are complemented by the determination of the energy band alignment at the interface using photoelectron spectroscopy. A Fermi level pinning in the Al₂O₃ layer, which seems to be characteristic for ALD films, leads to a significant modification of the energy band alignment. This pinning does not prohibit electron injection, which relies on the potential drop across the Al₂O₃ layer. (invited article)