【作者】 何燕；

【导师】 蒋建中；

【作者基本信息】 浙江大学 ， 材料学， 2010， 博士

【摘要】 稀磁半导体(DMS)是将磁性元素掺入半导体材料中制备而成,由于在自旋电子器件领域的潜在应用前景而受到广泛的关注。因理论预测ZnO基稀磁半导体在室温时仍具有铁磁性,引起大家对过渡金属掺杂甚至非金属掺杂ZnO基稀磁半导体研究的极大兴趣。然而,到目前为止该体系的磁性机理仍处于争议之中。本论文主要研究内容之一是探讨Co、Ga单掺及共掺ZnO的铁磁性来源。样品制备主要采用三种方法：热分解法,溶胶-凝胶法及脉冲激光沉积法,结合实验测试及第一性原理计算研究ZnO基稀磁半导体的室温铁磁性机理。采用热分解法制备了具有室温铁磁性的纳米尺寸Zn0.95Co0.05O样品,ZnO中Co2+取代了Zn2+位置。Co的K边XANES图谱及吸收光谱均表明在Co2+周围有一定量氧空位的存在。第一性原理计算结果表明处于某一特定位置的氧空位的存在导致最近邻Co-Co离子之间的铁磁耦合。这些氧空位极易捕获电子而形成F-中心,被捕获的电子局域化后占据的轨道与相邻Co2+的3d轨道重迭。Co2+的3d轨道中只有未占满的自旋向下轨道,被捕获的电子会与每个单个Co2+的自旋方向相反,导致两个相邻的Co2+之间产生铁磁耦合。采用溶胶-凝胶法制备了具有室温铁磁性的Co、Ga共掺ZnO样品,其中Co2+取代了Zn2+位置,Ga3+则占据了八面体间隙位置,在其周围伴随着一个氧空位的存在。Ga-4p与Co-3d壳层电子之间发生强的杂化作用,并且Co2+周围的氧离子被部分极化。相邻两个Co2+的3d电子之间以Ga-4p电子以及被极化的氧离子为媒介产生间接交换交互作用,进而使Co、Ga共掺ZnO具有铁磁性。采用PLD法生成纯ZnO和Ga掺杂ZnO薄膜。Ga3+占据ZnO内的间隙位置,并且电阻测试表明载流子引起的铁磁交换交互作用不是薄膜铁磁性的真正来源。结合第一性原理计算,我们发现在(0001)氧终止面上存在的Zn空位使得O价带中存在大量的2p空穴,最终导致纯ZnO薄膜的铁磁性。而Ga掺杂ZnO薄膜中,在(0001)氧终止面上存在的O空位均导致Ga-4s电子发生自旋极化,而Ga原子的自旋极化进一步导致其周围O和Zn原子的极化,从而使得Ga掺杂ZnO薄膜显示铁磁性。综合同步辐射高压实验及第一性原理计算对纤锌矿结构ZnO纳米棒的相变路径进行研究,发现12GPa发生的由纤锌矿结构转变为氯化钠结构的相变首先沿着[0001]方向急剧变形,而其垂直方向变化不大,随后两个方向同时发生变形。通过同步辐射原位高压X射线衍射实验,研究了大块及纳米SnO2的高压相变行为,SnO2相变压力随着晶粒尺寸的减小而增大。研究发现相变压力的提高主要来源于两个方面：SnO2金红石与萤石立方相之间的表面能差异,及随着SnO2晶粒尺寸减小能带宽度的增大。更多还原

【Abstract】 Diluted magnetic semiconductors (DMSs), which are fabricated by doping semiconductors with magnetic elements, are attracting a great deal of interests because of their potential applications in spintronic devices. ZnO-based DMSs were theoretically predicted to be good candidates for room temperature ferromagnetism, and these predictions also initiated intensive research on ZnO DMSs with or even without transition metal (TM) doping. However, the origin of intrinsic ferromagnetism in these systems is still under controversy. This thesis presents magnetism investigation on ZnO DMSs doped or co-doped with dopants Co and Ga. The ZnO DMSs samples are prepared by different methods, such as the thermal decomposition, the sol-gel route and pulsed laser deposition (PLD). Experimental measurements together with first-principles calculations are used to investigate the room temperature ferromagnetic mechanism of ZnO DMSs samples.Nanocrystalline Co-doped ZnO sample was synthesized by the thermal decomposition that exhibits ferromagnetism above the room temperature. It was found that Co ions with a valence 2+ in the sample indeed substituted Zn ions in the wurtzite ZnO lattice. The Co K-edge XANES together with the absorption spectra indicated the presence of oxygen vacancies surrounded the Co center. An electron trapped in the oxygen vacancy constituted an F-center, and the electron occupied an orbital which overlaps the d shells of both Co neighbors, leading to a ferromagnetic coupling between the two Co ions. Finally, the Co-doped ZnO sample showed the intrinsic ferromagnetism above the room temperature.The origin of high temperature ferromagnetism in (Co, Ga)-codoped ZnO, prepared by the sol-gel method, has been investigated. It was found that Co ions substitute Zn sites, while Ga ions were located at octahedral interstitial sites together with one O vacancy. The strong hybridization between the charge carriers in the Co-3d and Ga-4p states and electronic polarization for surrounding O ions at Co ions were detected. Finally, the Ga-4p electrons merged with conduction band and polarized O ions acted as medium for an indirect exchange between the Co ions, which could be the origin of ferromagnetism in the (Co, Ga)-codoped ZnO.Thin films of pure ZnO and Ga-doped ZnO were prepared by pulsed laser deposition. It is found that Ga ions were located at interstitial sites with three plus valences in Ga-doped ZnO film, and the large bulk resistance of films rules out the existence of carrier-mediated ferromagnetism in the present films. On the basis of first principles calculations, it was found that the Zn vacancies at O-terminated (0001) surface led to a high number of 2p holes in the O valance band, which might be the FM source of pure ZnO film. With the Ga doping, the O vacancies at O-terminated (0001) surface resulted in the spin polarization of Ga-4s electrons. Thus, the Ga exchange splitting further induced the polarity of O and Zn atoms surrounding the Ga atom.The phase transformation path for wurtzite ZnO nanorods were studied using the combined techniques of high pressure synchrotron X-ray diffraction measurements and first-principles calculations. It was found that the transformation path for the WZ-to-RS transition occurred at about 12GPa for ZnO nanorods was governed by first a dramatic deformation along the [0001] direction with little change along the vertical direction, and then the deformation was observed along the two directions together.High-pressure behaviors of bulk and nanocrystalline SnO2 samples have been studied by in situ high-pressure synchrotron radiation X-ray diffraction measurements at ambient temperature. It was found that the smaller the nanocrystal SnO2, the higher the onset transition pressure for the rutile-to-cubic phase transformation in SnO2. The enhancement of the transition pressure was mainly due to the surface energy differences between the rutile and cubic phases and/or the enhancement of energy band gap when the SnO2 crystallite size gets smaller.更多还原