氧化物基稀磁半导体材料的理论设计与实验制备
文献类型：学位论文
| 作者 | 裴广庆 |
| 学位类别 | 博士 |
| 答辩日期 | 2008 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐军 |
| 关键词 | 第一性原理计算 稀磁半导体 ZnO β-Ga2O3 |
| 其他题名 | First-principles materials design and experimental preparations of Oxide-based diluted magnetic |
| 中文摘要 | 集电荷特性和自旋特性于一身的稀磁半导体可以同时利用电子的电荷和自旋，兼备常规半导体电子学和磁电子学的优越性，被认为是21世纪最重要的电子学材料，它将被用于信息处理、通信、磁记录以及自旋极化电流的注入和量子计算的逻辑电路等。正因为此，它越来越受到人们的重视，成为一大研究热点。 目前，基于稀磁半导体材料的研究主要集中在ZnO和GaN基稀磁半导体的磁性机理和实验制备等领域。由于稀磁半导体材料铁磁性的产生过程十分复杂，且制备材料的方法多种多样，因此得到的结果出入很大，对于能否产生高居里温度也是众说纷纭。以上分析表明稀磁半导体的研究虽然在近年来十分的活跃，但是其中尚有许多问题需要解决。因此，本文从理论上采用第一性原理对ZnO和β-Ga2O3基稀磁半导体的磁性机理进行系统的研究和分析，并从实验上对部分理论预测结果尤其是ZnO基稀磁半导体的预测进行验证。 本论文第一部分介绍了氧化物基稀磁半导体的实验和理论研究进展以及第一性原理方法。首先介绍了ZnO、TiO2和其它一些氧化物基稀磁半导体的实验研究动态，接着介绍了稀磁半导体的一些理论研究进展，这些理论包括RKKY模型、平均场Zener模型和磁极化子模型等。对于第一性原理方法，首先介绍了两种近似，随后对第一性原理的核心构架，包括密度泛函理论和交换关联作用近似作了详细的说明。最后，我们还介绍了第一性原理所使用的VASP软件包的特点。 第二部分采用第一性原理计算了3d过渡金属元素（Fe、Co和Ni）掺质ZnO的能量和电子结构，研究了不同掺质构型的基态磁性以及稳定磁性的机理。在Fe和Co掺质的ZnO体系中，我们没有发现体系具有稳定的铁磁态。由于Fe2+和Co2+离子之间的超交换耦合作用很强，它在所有掺质构型中都起到主导作用，因此该两种离子掺质的ZnO稀磁半导体不具有稳定的铁磁性，而是反铁磁性的。由于大部分构型中Ni2+离子之间的双交换耦合作用很强，且该作用占据主导作用。同时这些构型的体系总能量（铁磁态和反铁磁态）都很低，因此Ni掺质的ZnO稀磁半导体可以实现稳定的铁磁基态。 第三部分我们研究了Ni掺质、(Ni,Al)共掺质和(Ni,Li)共掺质ZnO分别在GGA和GGA+U近似下以及(Co,Al) 和(Co,Li)共掺质ZnO在GGA近似下的磁性状态，比较了两种近似方法对计算结果的影响。对于Ni掺质ZnO，使用GGA方法时所有构型都具有铁磁基态，而采用GGA+U方法时得到的结果略有差别。但在后一种方法中，具有铁磁基态的构型的基态总能量比其它构型要低，说明无论采用哪种近似方法都证明Ni掺质ZnO具有稳定的铁磁基态。在(Ni,Al)掺质ZnO 体系中，我们分析它们的电子结构后得知，具有稳定铁磁基态的构型的Al-3p态和Ni-3d自旋向上态之间的杂化效应较大。自由载流子引起的双交换机理可以用来解释Al共掺后引起的铁磁性稳定机理。当Al-3p态与Ni-3d发生耦合时，引入的电子可以跳跃到Ni-3d态，这就为双交换机理提供了通道并降低了铁磁态的能量。对于(Ni,Li)共掺质的ZnO体系具有相似的道理，它的铁磁性稳定机理与(Ni, Al)共掺质ZnO的基本一致。然而，此时发生耦合的是Li-s态和Ni-3d态，并且是空穴跳跃到Ni-3d态达到稳定铁磁态的效果。对于(Co,Al) 或(Co,Li)掺质的ZnO，都没有发现具有铁磁基态的构型。 第四部分研究了不同过渡金属（Mn、Ni和Cu）掺质β-Ga2O3体系的电子结构及其磁性机理，通过比较铁磁态与反铁磁态之间的能量差来确定体系的基态磁性，通过电子结构来理解体系的磁性机理。研究表明：Mn掺质β-Ga2O3体系的基态为铁磁态，稳定铁磁态的作用是双交换作用（Double exchange），且这种作用与Mn离子之间的距离有很大的关系，由于Mn进入β-Ga2O3中的阳离子格位有两种方式，其中所有Mn取代八面体配位的Ga得到的构型是最稳定的，而且当Mn与Mn距离最近时最有可能出现铁磁基态；Ni和Cu掺质的β-Ga2O3体系与Mn掺质的β-Ga2O3体系相似，它们的基态仍然为铁磁态，同样所有Ni或者Cu取代八面体配位的Ga得到的构型是最稳定的，虽然并不是所有的构型都具有铁磁基态，但是对于能量最低或者最稳定的构型是具有铁磁基态的。 最后实验上合成了Ni掺质ZnO、Ni和Al共掺质ZnO、Co掺质ZnO以及Co和Al共掺质ZnO纳米陶瓷粉末，采用一系列实验表征手段对ZnO基的纳米粉末进行分析。结果表明：我们成功合成了具有室温铁磁性的Ni掺质ZnO和(Ni,Al)共掺质ZnO，这与我们的理论结果相符；对于Co掺质ZnO和(Co,Al)共掺质ZnO，并没有得到具有室温铁磁性，同样与我们的理论结果吻合；使用液相外延技术，在(0001) ZnO衬底上外延得到了高质量的ZnO薄膜，半峰宽为40弧秒，这一实验结果为我们制备氧化物基稀磁半导体薄膜打下了坚实的基础。 |
| 英文摘要 | Diluted magnetic semiconductors (DMSs) are formed by the partial substitution of the cations of the host semiconductors with a small amount of magnetic transition-metal (TM) ions. With charge and spin degrees of freedom in a single substance, DMSs have attracted a great deal of attention in recent years as potential materials for use in the field of information processing, communications, magnetic recording, as well as the injection of spin-polarized current and logic circuit of quantum computation. Adding the spin degrees of freedom to charge-based electronics will introduce more capabilities and better performance to devices. At present, the studies of DMSs are mainly concentrated on the areas of origin of ferromagnetism and experimental preparations of ZnO- and GaN-based semiconductors. Due to complexity of the origin of ferromagnetism in DMSs and the variety of preparation methods, the magnetic results and the possibility of obtaining high TC are under debate. The above analysis shows that the research of diluted magnetic semiconductors is very active in recent years, but there are still many issues which need to be resolved. Therefore, the first-principles calculations are used here to explore the magnetic mechanisms of ZnO- and β-Ga2O3-diluted magnetic semiconductors, and the auto-combustion method is used here to synthesize ZnO-based diluted magnetic semiconductor in order to testify the reliability of the theoretical results from experimental view. In the first part of the paper, we introduce the experimental and theoretical research progress and first-principles calculations of diluted magnetic semiconductor. We firstly introduce the experimental research progress of ZnO and TiO2 and other oxides based diluted magnetic semiconductors. Then the review of theoretical models of DMSs is presented, these model including RKKY interactions，mean-field Zener model and first-principles calculations. The main frame of first-principles calculations and the two approximations methods for exchange and correlations (GGA and GGA+U) are explained in detail. We also introduce the main characteristics of the Vienna ab initio Simulation Package (VASP). In the second part of the paper, we calculate the energy and electronic structures of 3d TM (Fe, Co, and Ni) doped ZnO, and investigate the ground state of magnetic properties and the mechanism for stabilizing magnetic ground state in different configurations. In the Fe- and Co-doped ZnO system, no ferromagnetic ground state is obtained. Due to the strong super exchange coupling between Fe2+ and Fe2+ ions or Co2+ and Co2+ ions, this kind of interaction plays a leading role in all doping configurations. Therefore, Fe- and Co-doped ZnO DMSs do not possess the stable ferromagnetic ground state, while the antiferromagnetic ground state is observed in the two doping systems. Due to the strong double exchange interaction between Ni2+ and Ni2+ ions, this kind of interaction plays a leading role in all doping configurations. At the same time, the total energies (ferromagnetic and antiferromagnetic state states) of these configurations are very low, thus ferromagnetic ground state can be achieved in the Ni-doped ZnO dilute magnetic semiconductors. In the third part of the paper, we investigate the magnetic properties of Ni-, (Ni,Al)- and (Ni,Li)-doped ZnO using GGA and GGA+U method, as well as (Co,Al)- and (Co,Li)-doped ZnO using GGA method. For Ni-doped ZnO, all configurations have ferromagnetic ground states in the GGA calculations frame, while the results vary a little using GGA+U method. But in the latter method, the ground-state energies of the configurations showing ferromagnetic ground state are lower than other configurations. We conclude that Ni-doped ZnO possesses the stable ferromagnetic ground state. For (Ni,Al)-doped ZnO system, the hybridization effects stabilizing the ferromagnetic ground states between Al-3p states and Ni-3d states are strong, thus double exchange mediated by free Carriers can be used to explain the mechanism for stabilizing the ferromagnetic ground state after Al codoping. When the coupling between Al-3p and Ni-3d states occurred, the additional electrons can hop to the Ni-3d states, which provide channels for the double exchange interactions and reduce the energy of ferromagnetic state. For (Ni,Li)-doped ZnO system, similar phenomena can occur, and the mechanism for stabilizing ferromagnetic states could be basically the same as that of (Ni,Al)-doped ZnO. However, the coupling take place between Li-s states and Ni-3d states, and the hole hop to the Ni-3d states to stabilize the ferromagnetic state. For (Co,Al)- or (Co,Li)-doped ZnO, no ferromagnetic ground states have been observed in all doping configurations. In the fourth part of the paper, we calculate the total energy and electronic structure of different TM (Mn, Ni, and Cu) doped β-Ga2O3 system. The energy differences between ferromagnetic and antiferromagnetic state are calculated in order to determine the magnetic ground state, and the electronic structures are also calculated for the purpose of understanding the magnetic mechanism underlying. The results show: Mn-doped β-Ga2O3 system has the ground state of ferromagnetic state, and the exchange interaction for stabilizing ferromagnetic state is double exchange mechanism. And this effect has great relation with the distance between the Mn ions. There are two cases for Mn entering into the β-Ga2O3 cation sites, the most stable one occurs when all Mn ions replace octahedral sites. Ferromagnetic ground state will most likely occur when the distance between Mn and Mn ions is small. Similar to Mn-doped β-Ga2O3 system, Ni- and Cu-doped β-Ga2O3 system still possess ferromagnetic ground state, and the most stable case also occurs when all Ni or Cu ions substitute octahedral sites. Although not all of the configurations have ferromagnetic ground state, it shows ferromagnetic ground state for the most stable conformation. In the last part of this paper, Ni-, (Ni,Al)-, Co-, and (Co,Al)-doped ZnO nanoparticles were synthesized from an auto-combustion method, and a series of characterization methods are employed here to analysis the obtained ZnO samples. The results show that: We have successfully synthesized Ni- and (Ni,Al)-doped ZnO with ferromagnetism at room temperature, which is consistent with our theoretical results; The Co- and (Co,Al)-doped ZnO do not show ferromagnetic property at room temperature, which is the same as our theoretical results; using liquid phase epitaxy technology, high quality ZnO films are obtained on the (0001) ZnO single crystal substrates and it has a FWHM of 40 arcsec, which lays a solid foundation for the preparation of oxide based DMSs thin films. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15222]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 裴广庆. 氧化物基稀磁半导体材料的理论设计与实验制备[D]. 中国科学院上海光学精密机械研究所. 2008. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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