Shunyi Li

Energy band alignment at ferroelectric/electrode interface determined by photoelectron spectroscopy

The most important interface-related quantities determined by band alignment are the barrier heights for charge transport, given by the Fermi level position at the interface. Taking Pb(Zr,Ti)O3 (PZT) as a typical ferroelectric material and applying X-ray photoelectron spectroscopy (XPS), we briefly review the interface formation and barrier heights at the interfaces between PZT and electrodes made of various metals or conductive oxides. Polarization dependence of the Schottky barrier height at a ferroelectric/electrode interface is also directly observed using XPS.

Energy band alignment between Pb(Zr,Ti)O3 and high and low work function conducting oxides—from hole to electron injection

The interface formation between Pb(Zr,Ti)O3 (PZT) and RuO2 and between PZT and In2O3 : Sn (ITO), respectively, was characterized using in situ x-ray photoelectron spectroscopy (XPS). No interface reaction was observed for the interfaces studied. The Fermi level position at the interface (Schottky barrier height) is strongly different for the two electrode materials. A Fermi level position of 1.0 ± 0.1 eV above the valence band maximum (VBM) is observed for the contact between PZT and the high work function oxide RuO2. For the contact between PZT and the low work function oxide ITO a Fermi level position of 2.1 ± 0.2 eV above the VBM is found.

Electrical properties of (Ba, Sr)TiO3 thin films with Pt and ITO electrodes: dielectric and rectifying behaviour

The electrical properties of (Ba, Sr)TiO(3) (BST) thin films are studied using different combinations of Pt and tin-doped indium oxide (In(2)O(3):Sn, ITO) as electrode material. With Pt as bottom and top electrode the films show insulating behaviour with a low leakage current. A rectifying current-voltage characteristic is obtained by replacing the top electrode with ITO. As shown by photoemission as well as by electrical measurements, the property of the BST/ITO interface depends strongly on the deposition sequence, and can be related to the level of oxidation of the ITO film. Highly doped ITO as top electrode forms an Ohmic contact with BST. This enables the preparation of highly rectifying diodes that exhibit a space-charge-limited current behaviour. Larger barriers are obtained when ITO is used as bottom electrode. This is related to the oxidation of the ITO layer during BST deposition and results in a low interface-limited current. Due to the large energy gaps of both BST and ITO, the combination of these materials provides an additional route to transparent electronics.

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.

Polarisation dependence of Schottky barrier heights at ferroelectric BaTiO3 / RuO2 interfaces: influence of substrate orientation and quality

In situ x-ray photoelectron spectroscopy was employed to examine the change in Schottky barrier height at BaTiO3/RuO2 interfaces upon polarisation reversal for [1 0 0], [1 1 0] and [1 1 1] oriented BaTiO3 single crystals. Compared to previous measurements on BaTiO3/RuO2 interfaces (Chen and Klein 2012 Phys. Rev. B 86 094105), the crystals exhibit a significantly reduced dependence of barrier height on polarisation direction. This is connected to a much higher polarisation of the present cystals, which is comparable to the accepted bulk polarisation of BaTiO3 of and which exhibit the expected dependence on crystal orientation. This indicates a much higher crystal quality in the present experiments, which is also confirmed by a Kolmogorov–Avrami–Ishibashi like polarisation switching dynamics. It is observed that is reduced for the [1 1 0] and [1 1 1] orientation and scales with polarisation as long as crystals from the same batch are used. The fact, that a poor polarisation hysteresis behaviour relates to a high polarisation dependence of Schottky barrier height, indicates that the electrodes ability to screen ferroelectric polarisation charges depends sensitively on crystal and/or interface quality.

Modification of energy band alignment and electric properties of Pt/Ba0.6Sr0.4TiO3/Pt thin-film ferroelectric varactors by Ag impurities at interfaces

We report on the effects of Ag impurities at interfaces of parallel-plate Pt/Ba0.6Sr0.4TiO3/Pt thin film ferroelectric varactors. Ag impurities occur at the interfaces due to diffusion of Ag from colloidal silver paint used to attach the varactor samples with their back side to the plate heated at 600–750 °C during deposition of Ba0.6Sr0.4TiO3. X-ray photoelectron spectroscopy and secondary ion mass spectrometry suggest that amount and distribution of Ag adsorbed at the interfaces depend strongly on the adsorbent surface layer. In particular, Ag preferentially accumulates on top of the Pt bottom electrode. The presence of Ag significantly reduces the barrier height between Pt and Ba0.6Sr0.4TiO3 leading to an increased leakage current density and, thus, to a severe degradation of the varactor performance.

Application of atomic layer deposited Al2O3as 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.