Surface states, surface potentials, and segregation at surfaces of tin-doped In₂O₃

Surfaces of In₂O₃ and tin-doped In₂O₃ (ITO) were investigated using photoelectron spectroscopy. Parts of the measurements were carried out directly after thin film preparation by magnetron sputtering without breaking vacuum. In addition samples were measured during exposure to oxidizing and reducing gases at pressures of up to 100Pa using synchrotron radiation from the BESSY II storage ring. Reproducible changes of binding energies with temperature and atmosphere are observed, which are attributed to changes of the surface Fermi level position. We present evidence that the Fermi edge emission observed at ITO surfaces is due to metallic surface states rather than to filled conduction band states. The observed variation of the Fermi level position at the ITO surface with experimental conditions is accompanied by a large apparent variation of the core level to valence band maximum binding energy difference as a result of core-hole screening by the free carriers in the surface states. In addition segregation of Sn to the surface is driven by the surface potential gradient. At elevated temperatures the surface Sn concentration reproducibly changes with exposure to different environments and shows a correlation with the Fermi level position

In situ Photoelectron study of the (Ba,Sr)TiO3/RuO2 contact formation

The interface formation between (Ba,Sr)TiO3 and RuO2 has been studied using photoelectron spectroscopy. A barrier height of 0.85 +/- 0.1 eV is determined. The result is discussed in relation to BST/metal interfaces, which can exhibit a variable Schottky barrier height

Barrier height at (Ba,Sr)TiO₃/Pt interfaces studied by photoemission

The interface formation of Nb-doped SrTiO₃ single crystals and (Ba,Sr)TiO₃ thin films with Pt has been studied by using photoelectron spectroscopy with in situ sample preparation. For the single crystal sample, a Schottky barrier height for electrons of 0.5–0.6 eV is determined after deposition of Pt in vacuum environment. After annealing in 0.05 Pa oxygen pressure, a strong increase in the barrier height to ≥1.2 eV is observed. X-ray induced photovoltages of up to 0.7 eV are observed in this case and have to be taken into account for a proper determination of the barrier height. A subsequent annealing in vacuum reduces the barrier again. Hence, the barrier height can be reversibly switched between an oxidized state with a large barrier height and a reduced state with a low barrier height. Quantitative analysis of the barrier heights indicates that the changes are related to the changes of interfacial defect concentration. Due to the occurrence of a Ti³⁺ related signal, the defects are identified as oxygen vacancies. The same effects are observed at interfaces between Pt and (Ba,Sr)TiO₃ thin films with a smaller absolute value of the barrier height in the oxidized state of ∼1 eV. Deposition of (Ba,Sr)TiO₃ onto a metallic Pt substrate also results in a barrier height of 1.0 eV

Barrier height at (Ba,Sr)TiO3/Pt interfaces studied by photoemission

The interface formation of Nb-doped SrTiO3 single crystals and (Ba,Sr)TiO3 thin films with Pt has been studied by using photoelectron spectroscopy with in situ sample preparation. For the single crystal sample, a Schottky barrier height for electrons of 0.5–0.6 eV is determined after deposition of Pt in vacuum environment. After annealing in 0.05 Pa oxygen pressure, a strong increase in the barrier height to >=1.2 eV is observed. X-ray induced photovoltages of up to 0.7 eV are observed in this case and have to be taken into account for a proper determination of the barrier height. A subsequent annealing in vacuum reduces the barrier again. Hence, the barrier height can be reversibly switched between an oxidized state with a large barrier height and a reduced state with a low barrier height. Quantitative analysis of the barrier heights indicates that the changes are related to the changes of interfacial defect concentration. Due to the occurrence of a Ti3+ related signal, the defects are identified as oxygen vacancies. The same effects are observed at interfaces between Pt and (Ba,Sr)TiO3 thin films with a smaller absolute value of the barrier height in the oxidized state of ~1 eV. Deposition of (Ba,Sr)TiO3 onto a metallic Pt substrate also results in a barrier height of 1.0 eV

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)O(3) (PZT) and RuO(2) and between PZT and In(2)O(3) : 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 RuO(2). 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