【作者】 黄俊；

【导师】 刘志国；

【作者基本信息】 哈尔滨工业大学 ， 凝聚态物理， 2008， 硕士

【摘要】 稀磁半导体材料,由于其在自旋电子器件上的潜在应用价值,在最近几年里引起了人们广泛的兴趣。为了能实际应用,必须要获得居里温度在室温以上的铁磁性稀磁半导体。因为ZnO是宽禁带半导体(带宽为3.3ev),有着很高的激子束缚能(60mev),很有希望作为下一代的用光学性质或者电学性质来控制磁学性质的自旋电子器件材料。然而,关于过渡金属(Fe、Co、Ni)掺杂ZnO的磁半导体,其磁性的机理仍存在着大量有争议。有人认为ZnO基稀磁半导体是反铁磁性的,也有人认为它是顺磁性的,还有人认为它是铁磁性。本论文针对ZnO基稀磁半导体磁性上的争议,先对Zn1-xCoxO进行了制备上的探索,然后利用多种表征手段对样品的物理性质进行了研究,最后在实验现象的基础上提出了一个关于Zn1-xCoxO磁性机理的物理模型。由于目前已报道的大多数ZnO掺磁性过渡金属的样品都是薄膜,而薄膜的质量很小,磁性很弱,很难探测,再加薄膜一般是长在反铁磁衬底上的,分析磁性时需要将衬底的背底扣除,而衬底的信号一般比薄膜大很多,所以最后得到的结果误差会比较大。为了解决这个问题,我们采用水热法合成样品Zn1-xCoxO。这是因为水热法能够直接合成样品而不需要用衬底,从而解决了扣除背底引发的问题。在制备过程中我们尝试了各种制备条件,最终成功的合成了出了单相的Zn1-xCoxO纳米晶体。为了对样品的结构及微观性质进行了研究,我们采用了多种表征手段。XRD测试结果表明,样品Zn1-xCoxO(x≤0.1)是典型的ZnO结构;XPS分析表明了样品中的Co离子是正二价的;傅立叶红外光谱证实了Co离子进入了ZnO的晶格并影响了分子的振动;拉曼光谱检测到了Zn1-xCoxO的Co离子周围的施主缺陷的局域振动模;扫描电镜显示Zn1-xCoxO是一种50nm左右的块状纳米颗粒,扫描电镜附带的X射线能谱分析表明Zn1-xCoxO纳米晶体中只含有Zn,Co,O三种元素;高分辨透射电镜对Zn1-xCoxO纳米晶体的表面晶格结构进行了成像,未观察到不同结构的团簇出现。对Zn1-xCoxO纳米晶体进行了室温的M-H曲线的测试,结果显示在300k下有磁滞现象,这表明样品中存在室温下具有铁磁性。为了说明Zn1-xCoxO稀磁半导体的磁性来源,本论文提出了一个Co离子通过杂质能级提供的电子进行相互作用的理论模型,并将此模型用于分析目前存在的各种有争议报道,从而合理的解释了这些看似互相矛盾的实验现象。更多还原

【Abstract】 Diluted magnetic semiconductors (DMSs) materials have been attracting intense interests in recent years for their potential applications in spintronic devices. For practical applications, ferromagnetic DMSs with Curie temperatures TC greatly exceeding room temperature is required. ZnO-based DMSs are paid much attention. For its wide band gap (3.3 eV) semiconductor with a high exciton binding energy (60 meV), ZnO is a promising material for the next generation of spintronic devices utilizing electronically or optically controlled magnetism. However , numerous contradictory results on the magnetic properties of transition metal (TM) doped ZnO DMSs have been reported, ranging from antiferromagnetism and paramagnetism to ferromagnetism.This paper is for the magnetic controversies of ZnO-based dilute magnetic semiconductors. We first has carried on the preparation exploration to Zn1-xCoxO, then use many kinds of measurements to research the physical properties of samples, finally proposed a physical model about the Zn1-xCoxO magnetic mechanism by combining the theory with experiments.As most of magnetic transition metal-doped ZnO samples have been reported are films and because the quality of film is very small, the magnetic signal is very weak, it is difficult to detect, and what’s more, a film usually grows on the anti-ferromagnetic substrate, one need to deducted the background of the substrate when analyses the magnetism, but the substrate signal is generally larger than the thin film in several magnitudes, therefore the errors of the final result will be quite big. To solve this problem, we have adopted hydrothermal synthesis Zn1-xCoxO samples. This is because the hydrothermal method can prepare sample directly need no to use any substrate, thus solving the problem cause by deducting background. In the process of preparation conditions, we tried a variety of conditions, finally we have successfully synthesized single phase Zn1-xCoxO nano-crystals.In order to study the structure and microscopic nature of the samples, we characterized them by varieties of method. XRD test results showed that the samples Zn1-xCoxO (x≤0.1) is a typical ZnO structure; X-ray photoelectron spectrum analysis showed that the Co ions in the samples were divalent; Fourier transform infrared spectroscopy confirmed the Co ions into the ZnO crystal lattice and impact the vibration of molecular; Raman spectrum showed confined vibration mold of donor defects around the Co ions of Zn1-xCoxO; Scanning electron microscope showed that Zn1-xCoxO are block-liked nanoparticles around 50 nm , analysis through energy dispersive spectrometer indicated that Zn1-xCoxO nano-crystals contained only Zn, Co, O three elements; High-resolution transmission electron microscopy has carried on the analysis to the surface lattice structure of Zn1-xCoxO nano-crystals, no any redundant cluster has been observed.Results of M-H curve test at room temperature on Zn1-xCoxO nano-crystals showed a magnetic hysteresis under 300k which indicated that the sample has room temperature ferromagnetism. In order to explain magnetic source of Zn1-xCoxO dilute magnetic semiconductors, this paper proposed a model of Co ions interact with each other through electrons come from impurities level. We then used the model to analyze current controversy, and make a reasonable explanation for these seemingly contradictory phenomenon of experiments.更多还原