The Fe/ZnO Interface Study By Synchrotron Radiation

Metal-oxide interfaces play a key role for some special materials such asfunctional ceramics with metals, oxide dispersion-strengthened alloys, oxide coatingson metals, catalysts etc. It is well known that the macroscopic properties of materialsare decided by their microstructures. In order to improve the properties of materials,one needs to understand the microstructure of interface of the materials, such asinterface atomic structure, space charge distribution, contact potential, band bending,work function, interface chemical reaction as well as the evolution of morphology,structure and property of the interface during annealing process. Although there aretremendous researches on the metal-semiconductor interfaces last century, the studiesof metal-oxide interface is relatively less because the properties of metals and oxidesusually differ extremely from each other. Contrary to metals, the oxides are usuallyvery brittle, elastically stiff, insulating and exhibit less thermal expansion and theircrystal lattice constants are different from metals. Moreover, the preparation of cleanmetal-oxide interface is relatively difficult. In this paper, the MBE was used tofabricate the Fe-ZnO interface. Synchrotron radiation photoelectron spectroscopy,combining with the Low Energy Electron Diffraction (LEED), Atom ForceMicroscopy (AFM), X-ray Diffraction (XRD), Scanning Electronic Microscopy(SEM) and Superconductor Quanta Interference Device (SQUID) techniques wereemployed to systematically investigate the structures and properties of the interface.1. The ARPES have performed to investigate the valence band states and the band dispersion of ZnO(000-1). The band structure along the FA direction has been obtained and agrees well with the theory calculation. The normal emission and off-normal emission spectra were recorded by using ARPES method. From the spectra we obtained the band structure along the FA direction and observed two surface states. From the off-normal emission spectra we got the 2D band structure of the surface states along the FM direction. According to the comparison between experimental results and theory calculation, we conclude that these two surface states are originated from the Zn4p-O2p and Zn4s-O2p mixing states 2. The growth, electronic properties and interface reaction of the Fe atoms depositing on ZnO two different polar planes have been performed using SRPES in UHV. The results showed that Fe grew in a Stranski-Krastanov(SK) mode on the ZnO(OOO-l) and a Volmer-Weber (VW) mode on the ZnO(0001)surface at RT. The oxidized thickness is about 1ML Fe for Fe/ZnO(000-1) system and 0.25ML Fe atom for Fe/ZnO(0001) system. With increasing Fe film thickness, the Fermi edge emerged at 3ML Fe film for Fe/ZnO(000-1) and 1ML Fe film for Fe/ZnO(0001). For the Fe/ZnO(000-l) system, the shifts of Zn3d and work function are ascribed to the electron transfer from the adsorbed metal to the substrate which induced the dipole layer on the surface and lowered the surface potential. But for Fe/ZnO(0001) system, the shifts of Zn3d and work function may come from the charging effect.3. The resonant photoelectron spectroscopy (RPES) is used for farther study of the interface electronic structure. In Fe oxides, RPES is explained due to the hybridization between the O2p and Fe3d orbits and therefore RPES can be used to interpret the contribution of Fe3d-derived states to the valence band. For Fe/ZnO(000-1) system, the valence band changed evidently when photon energy reached the Fe 3p-3d excitation threshold. Constant-initial-state (CIS) curves showed that the state at 0.9eV was due to Fe3p-3d resonant emission, the state at 5.5eV mostly belonged to metallic Fe MVV Auger transition, the state at 10.7eV might come from the degeneracy of the two Zn3d related bands.4. The evolution of valence band, Fe2p and Fe3p have been studied by SRPES during annealing process of Fe(3nm)/ZnO(000-1) system. The metallic Fe atoms began to diffuse into the bulk and react with the substrate at 600℃. A great deal of oxygen diffused out the surface at 900℃to make the Fe²⁺ changing into Fe³⁺. So the annealing leads to a stepwise oxidation of the Fe to FeO and Fe₂O₃. The images of AFM and FE-SEM showed that the sample morphology changed with temperature. The Fe film was oxidized and emerged phase transition at 500℃～600℃. The steady Fe₂O₃ phase appeared at 900℃. The magnetism of the Fe/ZnO(000-1) system changed at two stepwise oxidized processes. The coercive force and remanence ratio at two stepwise oxidized process were studied using the SQUID technique, but the origin of the magnetism at different stages may be different and needs further investigation.