【作者】 张旺；

【导师】 徐法强；

【作者基本信息】 中国科学技术大学 ， 核科学与技术， 2010， 博士

【摘要】 随着近年来高密度存储设备的发展需求,Fe、Co等磁性超薄膜与各种氧化物衬底的界而研究引起了广泛的关注,包括薄膜的磁性,界而的电性能,化学稳定性在内的一系列界面的物理化学性能都成了研究的重点。随着磁性薄膜在氧化物表面的生长,界面作用诸如界面的化学态,界面之间的互扩散,以及不同的界面结构都会随着薄膜厚度的增加而相应的变化,而这些界面作用的不同必然导致整个界面性能的改变,通过研究界面作用随厚度的改变可以分析出相应的界面性能的变化,这将对界面体系的应用极具实际意义。同时,通过界面的退火可以有效促进磁性介质向氧化物中的扩散,这种方法正作为稀磁半导体的一种有效制备手段而得到重视。本文主要通过光电子能谱技术研究了磁性Fe、Co金属超薄膜在极性ZnO(000+1)表而的生长和退火时电子结构的变化,并辅以原子力显微镜(AFM)和场发射扫描电子显微镜(FESEM),以及超导量子干涉仪(SQUID)等对样品的表面形貌,磁性等作相应的考察。主要的研究结果如下：1.利用同步辐射角分辨光电子能谱(SRARPES)技术和光电子衍射(PD)技术研究了获得的清洁ZnO(000-1)表面,并初步获得了ZnO在ΓA、ΓM方向上的能带色散及ZnO在[-1100]平面内的光电子衍射结果。2.室温下,对Fe与不同极性ZnO界面的XPS和SRPES研究结果显示,Fe的生长模式依赖于ZnO衬底的极性,Fe在ZnO(0001)表面呈三维岛状生长,而在ZnO(000-1)表面呈从层状到岛状生长转变的SK模式。对Fe/ZnO(000-1)界面的研究结果显示,初始沉积的Fe明显被表面氧氧化为Fe2+离子,在Fe覆盖度为0-3nm的范围内,分别观察到与界面电荷传输、化学作用以及磁性相关的0.5ML,1ML,3ML等三个有意义的临界厚度。同时室温下对Fe/ZnO(000-1)在沉积时的共振光电发射研究显示,Fe初始时与ZnO的作用并没有引起ZnO导带的显著变化,Fe与ZnO呈现弱相互作用。3.对3nm Fe/ZnO(000-1)退火过程的同步辐射光电子能谱(SRPES),原子力显微镜(AFM)研究显示,在300℃附近的低温退火过程中,室温下Fe在ZnO表面形成的3D岛状结构出现解聚过程；在300℃到600℃之间,Fe的解聚薄膜再次团聚形成较大的颗粒薄膜,并导致衬底在600℃时重新暴露；Fe与ZnO的界面化学作用发生在600℃左右,Fe随着温度的上升逐渐被氧化成Fe2+,温度超过800℃,Fe可以被进一步氧化为Fe3+。在600℃以上的退火温度下,随着ZnO衬底在高温下的分解,所释放出来的活性氧成为界面氧化—还原反应的一个主要原因。同时,对不同退火温度下的3nmFe/ZnO (000-1)样品的磁性SQUID研究结果显示,样品的矫顽力随着退火温度增加而增大。900℃高温退火样品中的铁磁性则说明界面反应中铁磁相的形成。4.对Co/ZnO界面的生长和退火过程的同步辐射光电子能谱(SRPES)研究显示。室温下Co在不同极性ZnO面上的生长都显示较强的反应活性,在Co膜厚度达到0.45nm时,依然能够显示很强的Co2+氧化峰。1.1nm-Co/ZnO界面的退火结果显示,200℃的退火导致了ZnO衬底峰的加强,并且在室温到300℃之间的退火并没有导致明显的Co-ZnO之间的氧化-还原反应,Co在300℃之后逐渐由金属态氧化为Co2+。一系列的RPES结果显示,Co与ZnO的界面作用主要形成了Eb=2.0eV,11.0eV和3.4eV,6.8eV两组峰结构,这两组峰可能分别起源于CoO的3d6和3d7L电子态与磁性掺杂相Znl-xCoxO的3d6和3d7L电子态。对不同Co覆盖度下的Zn3d部分电子产额谱(PEYS)研究显示, Co的出现也导致了ZnO导带结构发生了改变,形成了可能起源于Zn1-xCoxO磁性掺杂相的边前峰结构。对退火过程中Co/ZnO的价带研究发现,600℃的退火温度可以大大促进Eb=3.4eV峰的增强,反映了此温度下可能更适合Co掺杂相的形成。更多还原

【Abstract】 With the increasing demand of high-density storage devices, the behavior of ultrathin Fe,Co films on various oxide surfaces has been studied extensively in recent years. The physical and chemical properties of the interface including the film magnetic, electrical properties and chemical stability have become the main focus of the researches. As the growth of the magnetic thin film on oxide surface, the interfacial interaction in terms of chemical states, interdiffusion and geometric structure will show a corresponding change with the film thickness, which will lead to different interfacial properties. Such a research as the film thickness vs interfacial interaction will be very meaningful in the application of these materials. Meanwhile, the annealing process at the interface will effectively prompt the diffusion of the magnetic particles into the oxide substrate. This method attracted a growing attention in the recent DMS studies.In this dissertation, by using the photoemission technique mainly, the electronic structure evolution of Fe/ZnO(000±1) and Co/ZnO(000±1) interfaces have been investigated systematically during the growth and annealing processes. In addition, techniques such as Atomic Force Microscopy(AFM), Field-Emission Scanning electronic microscopy(FE-SEM) and Superconductor Quantum Interference Devices(SQUID) have also been employed to obtain the surface morphology and magnetic properties of the interfaces. The main results are described as follows:1. Synchrotron radiation angle-resolved photoemission spectroscopy (SRARPES) and photoelectron diffraction (PD) techniques were used to investigate the clean ZnO(000-1) surface. The band structures along the FA, FM directions and the photoelectron diffraction in the [-1100] plane have been given as primary results.2. At room temperature, the XPS and SRPES studies for the Fe deposition on different polar ZnO surfaces have shown that the growth mode of Fe film depends on the polarity of ZnO substrate, the Volmer-Weber (VW) mode on the ZnO(0001) surface and the Stranski-Krastanov (SK) mode on the ZnO(000-1) surface have been observed respectively. On the Fe/ZnO(000-1) interface, the results showed obvious Fe2+formation at the initial Fe deposition stage. During the deposition of Fe on ZnO up to 3nm, three meaningful and critical thicknesses of 0.5ML,1ML,3ML have been obtained which may be related to the charge transfer, chemical reaction, and magnetic property, respectively. Meanwhile, the resonant photoemission results at different Fe thicknesses have shown that the initially oxidized Fe atoms do not cause any significant changes in the conduction band of ZnO indicating the weak interaction with ZnO at RT.3. The annealing process of 3nm-Fe/ZnO(000-1) have been studied by SRPES and AFM. Evident proof shows that the ultrathin Fe film has a disagglomeration process at low temperature annealing around 300℃. And a subsequent aggregation process at 600℃leads to the size growth of iron clusters and the reexposure of the substrate. The interfacial reaction takes place at 600℃, metallic Fe is gradually oxidized into Fe2+with the increase of annealing temperature, and even Fe3+above 800℃. Active oxygen released from the ZnO substrate on annealing the samples above 600℃is responsible for the oxidation reaction at the interface. The magnetic coercive force of the sample has been observed to increase with the annealing temperature. The existing ferromagnetism of the 900℃annealing sample may indicate a ferromagnetic phase formation from the interfacial reaction.4. The electronic structures of Co/ZnO interface have been investigated by SRPES. At RT, with both of the polar ZnO surfaces the Co film showed strong reactivity and obvious Co2+oxidation state can be detected even when the film thickness reaches 0.45nm. The annealing process of 1.1nm-Co/ZnO(000-1) have shown that, the 200℃annealing led to the increase of the Zn3d peak from ZnO substrate but no significant oxidation-reduction reaction between Co and ZnO has been detected below 300℃. Metallic Co is gradually oxidized into Co2+with the increase of annealing temperature above 300℃. From a series of RPES results, Co-ZnO interfacial interaction leads to the appearance of four new peaks with the binding energies of 2.0eV, 11.0eV,3.4eV and 6.8eV, respectively, the 2.0eV and 11.0eV peaks should be derived from Co3d6 and Co3d7L of CoO, and the 3.4eV and 6.8eV peaks should be attributed to the magnetic doping structure Znl-xCoxO. The RPES results at different Co thicknesses have shown that the initially oxidized Co atoms lead to a pre-edge structure in Zn3d PEYS which will derived from Co doping into the ZnO lattice. From the valence band structures at different annealing temperatures, a peak with binding energy of 3.4 eV was greatly enhanced at 600℃, which may suggest the formation of Co-doped phase at this temperature.更多还原