【作者】 王国栋；

【导师】 徐法强；

【作者基本信息】 中国科学技术大学 ， 核技术及应用， 2008， 博士

【摘要】 金属-氧化物界面（Metal-oxide interface）在很多先进的应用材料中起着非常重要的作用,有时甚至起着决定性的作用,比如:功能金属陶瓷材料、氧化物弥散强化合金、金属的氧化物防护、催化剂等等。众所周知,材料的宏观性质是由其微观结构所决定的,因此,为了改善材料的宏观性能,有必要弄清楚材料的界面微观结构,如界面的原子结构、空间电荷分布、接触势垒、能带弯曲、功函数、界面化学反应,以及在退火过程中,整个界面体系的形貌、结构和性质的变化等等。上世纪,人们已对金属-半导体的界面结构进行了大量的研究,然而,对金属-氧化物界面结构的研究却相对少一些,这主要是由于金属与氧化物之间的性质相差非常大,与金属相反,氧化物通常很脆、绝热、热膨胀系数小,晶格常数也不同于金属,有的甚至相差很大,而且,制备金属-氧化物界面比较困难。本文利用分子束外延（MBE）方法在超高真空条件下在ZnO单晶上生长了金属Fe薄膜,利用同步辐射光电子能谱技术,并结合低能电子衍射（LEED）、原子力显微镜（AFM）、X射线衍射（XRD）、扫描电子显微镜（SEM）、超导量子干涉仪（SQUID）等表征技术对界面结构和性能的变化进行了系统的研究。1、利用表面物理实验站的角分辨光电子能谱研究了ZnO（000-1）表面的能带色散。通过分析垂直出射时的光电子能谱,得到了沿ΓA方向的体能带结构以及两个表面态;通过分析非垂直出射时的光电子能谱,得到了两个表面态在ΓM方向的二维能带色散,实验结果与理论计算进行了比较,并分析了其可能的来源。2、在超高真空中,利用同步辐射光电子能谱对金属Fe与ZnO两个不同极性面的生长、电子结构和界面作用等进行了系统的研究。结果表明金属Fe在ZnO（000-1）表面的生长模式更接近SK模式（先二维后三维的生长模式）,而在ZnO（0001）表面的生长模式是VW模式（三维岛状生长模式）。Fe/ZnO（000-1）界面相互作用的氧化层厚度约在1ML的Fe原子层,而Fe/ZnO（0001）界面相互作用的氧化层厚度更小,只有约0.25ML。随着Fe薄膜厚度的增加,分别在3ML和1ML厚度时,形成了清晰的金属Fermi边。对于Fe/ZnO（000-1）体系,Zn3d和功函数的移动是由于沉积初始阶段界面电子传输导致表面偶极层的形成,降低表面电子的电离电位所引起的。而对于Fe/ZnO（0001）体系中,Zn3d和功函数的移动更多的是受到荷电效应的影响。3、利用共振光电子能谱（RPES）技术研究了Fe/ZnO（000-1）体系不同轨道电子的跃迁机制、杂化效应以及Fe3d电子对价带电子结构的贡献。光子能量达到过渡金属Fe3p-3d激发阈值时观察到明显的光电发射增强效应。从恒初态模式（CIS）曲线中可知,E_b=0.9eV处信号的增强,来自于Fe3p-3d的共振发射,E_b=5.5eV处的信号增强,主要是由Fe的俄歇电子跃迁引起的;而E_b=10.7eV附近的信号增强,则是因ZnO能带色散引起的Zn3d的两个子带的简并态。4、利用同步辐射光电子能谱技术,测试了Fe（3nm）/ZnO（000-1）体系界面在退火过程中价带、Fe2p和Fe3p能谱的演变。当退火温度达到600℃,表面的金属Fe开始向体相扩散,并且和衬底ZnO发生化学反应,当温度达到900℃后,衬底释放出大量的氧气,使得二价态的Fe被进一步氧化成三价态Fe,从而确定了金属Fe在ZnO（000-1）界面的两步氧化过程,即Fe→FeO→Fe₂O₃。AFM和FE-SEM图片显示Fe在ZnO（000-1）表面形貌随退火温度的改变而发生明显的变化,Fe薄膜从瓦解并团簇化,到被进一步氧化,出现了表面相变,相变起始温度大约为500℃到600℃左右,并在900℃时达到了稳定的氧化铁相。两步氧化过程中Fe/ZnO（000-1）薄膜的磁性质也发生了变化,SQUID研究了两步氧化过程中矫顽力和剩磁比的变化,但两个阶段磁性的来源可能会有所不同。更多还原

【Abstract】 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.更多还原