Robert Schafranek

Surface versus bulk electronic/defect structures of transparent conducting oxides: I. Indium oxide and ITO

Carefully prepared bulk ceramic specimens of In2O3 and Sn-doped In2O3 (ITO) were analysed with x-ray and UV photoelectron spectroscopy before and after heat treatment in vacuum and oxygen atmosphere. The results on ex situ prepared ceramic specimens were shown to be comparable to those of in situ deposited-measured thin films in terms of core levels, Fermi levels and ionization potentials. This suggests a viable path for rapid synthesis and screening of surface electronic-defect properties for other transparent conducting oxides (TCO) materials. A strong correlation exists between the surface electronic-defect structure of In2O3-based TCOs and their underlying electronic-defect structure, owing to the unique crystal-defect properties of the bixbyite structure. This leads to formation of a chemical depletion at the surface and the formation of a peroxide surface species for higher preparation temperatures. The results are discussed with respect to the use of ITO as hole injection electrode in organic light emitting devices.

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.

Reduction-induced Fermi level pinning at the interfaces between Pb(Zr,Ti)O3 and Pt, Cu and Ag metal electrodes

The interface formation between Pb(Zr,Ti)O3 and Pt, Cu and Ag was studied using in situ photoelectron spectroscopy. A strong interface reaction and a reduction of the substrate surface is observed for all three interfaces as evidenced by the appearance of metallic Pb species. Despite the different work function of the metals, nearly identical barrier heights are found with EF − EVB = 1.6 ± 0.1 eV, 1.8 ± 0.1 eV and 1.7 ± 0.1 eV of the as-prepared interfaces with Pt, Cu and Ag, respectively. The barrier heights are characterized by a strong Fermi level pinning, which is attributed to an oxygen deficient interface induced by the chemical reduction of Pb(Zr,Ti)O3 during metal deposition.

Barrier heights, polarization switching, and electrical fatigue in Pb(Zr,Ti)O3 ceramics with different electrodes

The influence of Pt, tin-doped In2O3, and RuO2 electrodes on the electrical fatigue of bulk ceramic Pb(Zr,Ti)O3 (PZT) has been studied. Schottky barrier heights at the ferroelectric/electrode interfaces vary by more than one electronvolt for different electrode materials and do not depend on crystallographic orientation of the interface. Despite different barrier heights, hysteresis loops of polarization, strain, permittivity, and piezoelectric constant and the switching kinetics are identical for all electrodes. A 20% reduction in polarization after 106 bipolar cycles is observed for all the samples. In contrast to PZT thin films, the loss of remanent polarization with bipolar switching cycles does not significantly depend on the electrode material.

Formation and modification of Schottky barriers at the PZT/Pt interface

A determination of the Schottky barrier height at the interface between ferroelectric Pb(Zr,Ti)O3 thin films and Pt by photoelectron spectroscopy is presented. Stepwise Pt deposition was performed in situ onto a contamination-free Pb(Zr,Ti)O3 thin film surface. The substrate surface is reduced in the course of Pt deposition as evident from the observation of metallic Pb. The Fermi level is found at EF − EVB = 1.6 ± 0.1 eV above the valence band maximum of the as-prepared interface. Annealing of the sample in an oxygen pressure of 0.1 and 1 Pa strongly reduces the amount of metallic Pb and leads to a reduction in the Fermi level position at the interface to EF − EVB = 1.1 ± 0.1 eV. Storage in vacuum at room temperature strongly reduces the interface leading to a significantly higher Fermi level position (EF − EVB = 2.2 ± 0.1 eV). The reduction is attributed to the presence of hydrogen in the residual gas. The change in barrier height might be a severe issue for stable device operation with Pt contacts even at ambient temperatures.

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.

Characterization of Acoustic Effects in Ferroelectric Thin-Films for Microwave Components

In this paper we present the impact of acoustic resonances excited by electrostriction in BST thin-films on the Q-factor of a varactor in the GHz range. By changing the process-parameter substrate to target distance while sputtering the thin-films, we observe a change in the acoustic resonance frequencies. To characterize the material properties which determine the acoustic resonance frequencies a model based on a 1D acoustic transmission line is used. The extracted material properties results in a proportional dependency to the changed process parameter. For verification of the acoustic transmission line model 3D simulations have been performed which show good agreement to the line model for lower resonance frequencies, where the difference at higher frequencies becomes significant due to additional higher order modes.