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

Abstract

The interface formation of Nb-doped SrTiO 3 single crystals and ͑Ba, Sr͒TiO 3 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 Ն1.2 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 Ti 3+ related signal, the defects are identified as oxygen vacancies. The same effects are observed at interfaces between Pt and ͑Ba, Sr͒TiO 3 thin films with a smaller absolute value of the barrier height in the oxidized state of ϳ1 eV. Deposition of ͑Ba, Sr͒TiO 3 onto a metallic Pt substrate also results in a barrier height of 1.0 eV.

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

“…The strong dependence of the barrier height on the chemical state of the interface has also been reported by other groups [22,23]. The effect is not restricted to PZT/Pt but also observed at other oxide/metal interfaces [24,25].…”

“…As the defects are related to a chemical reduction of the interface, it is likely that these are related to oxygen deficiency. Hence, the lowest defect concentrations are expected for most oxidizing conditions, which consistently leads to a downward shift of the Fermi energy at the interface [16,24,25], corresponding to a lowering of the hole Schottky barrier. The lowest Fermi level position observed at the PZT/Pt interface is E F − E VB = 1.1 eV [16], which is significantly lower than the charge neutrality level calculated by Robertson and Chen [13].…”

“…Extrinsic defects can considerably affect the barrier heights [16,24,25,52,53]. It is therefore only reasonable to compare charge neutrality levels with experimental Schottky barrier heights for interfaces with low concentrations of extrinsic defects.…”

“…The Zr/Ti ratio is maintained during deposition as verified by chemical composition analysis using XPS. In order to obtain contamination-free surfaces prior to metal deposition, the PZT thin films were heated to 400 • C in 0.5 Pa oxygen for 1 h, which has effectively been used for cleaning other oxide surfaces [24,26]. Platinum and copper were deposited onto clean PZT thin films using radio-frequency magnetron sputtering at room temperature from 2" targets with a power of 5 W, an Ar gas pressure of 0.5 Pa and a substrate to target distance of 10 cm.…”

Low temperature dielectric relaxation and charged defects in ferroelectric thin films

“…Deep investigation of charged defects is underway including XPS, RBS/NRA measurements. [2][3][4] While the physics of defects in ferroelectric films has some specific features, it shares a lot of trends with related investigations in the parent bulk ferroelectric perovskites. 5 In single crystals and ceramics, from EPR spectroscopy or combining EPR and IR absorption, [6][7][8][9][10] clear evidence of polaronic states was observed, which is a highly probable outcome of charged defects and, in particular, oxygen-vacancies-related ones.…”