摘要
自旋电子学器件由于具备运算速度高、能耗和发热率小等特点将对未来信息技术带来变革。稀磁半导体(DMSs)作为自旋电子学器件的核心材料融合了半导体和铁磁特性,研究其磁性起源及微观磁耦合机制成为材料科学和信息技术领域前沿热点。本论文依托实验室正电子装置平台,利用正电子湮没技术能够灵敏观察材料微观结构和点缺陷的独特优势,深入分析某些DMSs中的磁性和微结构之间关系、d0磁性起源以及缺陷对磁性离子相互作用影响等问题。理论上采用基于密度泛函的第一性原理计算研究半导体在不同离子掺杂或缺陷条件下体系的电子自旋态密度和磁矩,结合实验测量结果分析DMSs在不同微结构条件下的磁耦合机制,着力解决国际上对磁性起源问题存在的争议,并对半导体缺陷不同微观条件下的磁效应给出深入的探索。论文的主要研究内容如下：
1.针对非过渡金属氧化物中d0磁性起源的争议问题,实验选择绝缘性的MgO和Al203单晶作为典型的d0磁性载体材料,利用分布式离子注入方法进行非磁性阴离子掺杂和引入晶格缺陷,研究在缺少载流子调控的条件下样品的磁性和长程磁有序的宏观表现,分析束缚磁极子模型(BMP)在d0磁性起源和磁耦合相互作用中的适用性。实验以正电子湮没技术为材料微观结构和缺陷的主要分析手段,结合X射线衍射(XRD)、次级离子质谱(SIMS)等其它结构表征方法,研究不同掺杂体系样品的宏观磁性表现与微观结构之间的关系。实验发现样品中阳离子空位引入局域磁矩并产生铁磁耦合相互作用,证实了束缚磁极子模型可用于描述局域磁矩之间的耦合相互作用。提出了MgO中的Mg空位在不同结构条件下具有双重磁耦合效应的观点,并得到磁性理论计算的证实。
2.宽禁带半导体ZnO具有较高的过渡金属离子固溶度和自旋注入效率使其成为自旋电子学领域的热门材料。最具潜在应用价值的Co或Mn掺杂ZnO体系由于磁性测量结果的多样性导致机理阐述不清,阻碍了其在自旋电子学器件中的应用和发展,因此有必要从微观结构角度进一步分析其磁性起源机制。实验采用脉冲激光沉积法(PLD)在蓝宝石衬底上制备Co或Mn掺杂的ZnO薄膜,并在全同生长条件下制备纯ZnO薄膜用于对比分析和N离子注入实验。实验以正电子湮没技术结合其它结构分析手段来研究样品磁性表现与掺杂离子的关系,并且分析样品缺陷变化对离子之间磁相互作用的影响。实验在Co掺杂ZnO的样品中观察到室温铁磁性和超顺磁性共存的状态,认为室温铁磁性来源于Vo调节的Co-Co离子之间磁相互作用,超顺磁性则与铁磁性分布不均匀有关。Mn掺杂的ZnO样品中仅有居里温度小于50K的低温铁磁性存在,这一结果与理论计算Mn-O-Mn自旋间接交换作用相符合。N离子注入纯ZnO薄膜的实验表明VZn可以在样品中引入缺陷磁性,而中性的Vo却与d0磁性起源无关。
3.实验采用离子注入方法在6H-SiC单晶中引入Zn离子并产生了一定的晶格缺陷。对比C离子注入6H-SiC样品的磁性测量结果,实验发现缺陷本身可以产生局域磁矩并具有铁磁相互作用。对Zn离子注入的6H-SiC样品进行阶梯性退火发现随退火温度升高样品的磁性先减小趋于消失,后重现并逐步增大至一定水平。样品中缺陷与Zn离子共存条件下的磁性表现无法用同一种起源机制进行阐述。通过对样品不同退火温度条件下的微观结构分析,实验认为两个退火阶段的磁性具有不同的来源,前者占据Si位置的Zn离子具有重要贡献,后者则主要来源于O侵入与Si结合导致的大量Si空位。
4.为了进一步探索DMSs磁性起源机制并对实验结果进行深入分析,我们采用基于密度泛函理论的第一性原理计算方法分别对实验中涉及到的磁性结构体系进行理论计算。计算工作主要采用VASP软件完成,通过静态性质计算得到体系电子自旋态密度和基态能量,进而对体系进行磁性分析。结果表明MgO体系中的缺陷磁矩主要来源于Mg空位周围O原子的2p电子自旋极化,并且证实了Mg空位与N或C离子之间具有完全不同的磁相互作用表现。ZnO样品中的Zn空位可以产生局域磁矩并耦合形成长程磁有序。完整晶格中的Co-Co(或Mn-Mn)之间磁相互作用具有稳定的反铁磁态,O空位可以对磁性离子耦合起调节作用。此外,我们对Al203和Zn离子掺杂的SiC单晶体系也进行了磁性理论计算工作,结果与实验分析符合得很好。
Spintronic devices will bring great changes to the IT field in the future due to their outstanding properties such as high-speed operation, low power consumption, etc. Dilute magnetic semiconductors (DMSs) have attracted increasing attention in the past decade because of their promising technological applications in the field of spintronics. A lot of groups focus their efforts on the topic of magnetic origin, while no agreed mechanism of observed ferromagnetism (FM) was established clearly. This topic rigorously analyzed the relationship between the magnetic properties and microstructures in some of DMSs by using positron annihilation spectroscopy, which was proved to be a sensitive tool for study of microstructures and defects in materials. Based on the positron facilities in the State Key Laboratory of Particle Detection and Electronics of China, we studied the magnetic mechanisms in DMSs, including origin of DO magnetism, defect-induced FM, and the effect of microstructures on the coupling between magnetic ions. Moreover, the first-principle calculation (based on density functional theory) also was developed to study the spin-density states and magnetic interaction in different magnetic microstructures. We try to give original insight in magnetic origin and coupling mechanism in different magnetic systems, and resolve some controversial issues in the field of DMSs. Research on this topic will provide theoretical basis for spintronics application and benefit the development of information technology in the future. The main contents are as follows.
1. We implanted virgin MgO and Al2O3single crystals with non-magnetic anions at room temperature to investigate the origin of DO magnetism in these systems. The annealing effects on the microstructures and induced defects of these samples were determined by positron annihilation spectroscopy, X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), etc. All samples with high-dose implantation show room temperature ferromagnetism, and the relevance of the ferromagnetism on microstructures was studied. Experimental results indicate that ferromagnetism can be introduced to metal oxides by cation vacancies. The implanted C/N ions played more effective role in ferromagnetic performance than VMg in MgO systems. However, the VMg coexistence with C or N ions may play coupling and inhibition role in magnetic performance in these MgO samples, respectively. The magnetic moment possibly occurred from the localized wave function of unpaired electrons and the exchange interaction formed a long-range magnetic order. The phenomenological bound magnetic polaron model (BMP) was employed to understand magnetic coupling of localized moments in these insulated metal oxides.
2. ZnO-based DMSs were extensively investigated as a promising candidate for spintronics applications due to their outstanding characteristics such as wide-band gap, large solubility of transition metal ions and spin injection efficiency, etc. Co-and Mn-doped ZnO have aroused widespread interest as the most attractive DMS candidates, while the origin and mechanism of the observed ferromagnetism is far from being clearly understood, since experimental and theoretical studies on the magnetic properties of Co/Mn-doped ZnO show a number of contradictions. Further studies are required to understand the magnetic mechanism of this system in detail. We investigated the structural and magnetic properties of Co-and Mn-doped ZnO films deposited on sapphire substrates by pulsed-laser deposition. Also the pure ZnO film was prepared under identical growth conditions for comparison and further N-implantation. Positron annihilation spectroscopy with other characterized methods was used to investigate the dependence of Magnetic properties on doped-ions and crystal defects. Experimental results reveal the coexistence of ferromagnetism with super-paramagnetic behavior in Co-doped ZnO films. We confirm the ferromagnetism is intrinsic property of samples and oxygen vacancies play an important mediation role in Co-Co coupling. Super-paramagnetism probably arises from the nanosize effect or nano-scale aggregation of ferromagnetism. Only low temperature ferromagnetism (Curie temperature lower than50K) was observed in Mn-doped samples, which might be interpreted as p-d hybridization from indirect coupling of Mn ions (Mn-O-Mn). N-implantation implies that DO ferromagnetism in ZnO films may arise from VZn instead of neutral oxygen vacancies.
3. We made use of ion implantation method to induce Zn ions in6H-SiC single crystal, certain defects were also produced in materials after implantation. Firstly we found that lattice defects can induce ferromagnetism in C-implanted6H-SiC sample. Following the increase of annealing temperature, magnetization in Zn-implanted6H-SiC sample reduced gradually until disappear at first stage and then remerged, and increased to a certain level at second stage. Experimental results indicate that ferromagnetic performance cannot be subscribed by only one coupling mechanism in Zn-doped6H-SiC samples. Based on the microstructural analysis, we consider that substitutional Zn ions contribute to ferromagnetism in the first stage of annealing sample, and the O-Si combined bond induced Si vacancies play an important role in magnetization increase in the second annealing stage.
4. For further analysis the experimental results and study the magnetic mechanism of DMSs, we employed first-principle calculation (based on density functional theory) to calculate the magnetic structures of semiconductor systems mentioned in above experimental works. Density functional theory calculations were performed using the plane-wave pseudopotential method in the Vienna AB initio simulation package (VASP). Calculation results show that room-temperature ferromagnetism in MgO supercell originated in the electronic status around Mg vacancies due to the spin polarization of2p electrons of the oxygen atoms surrounding the Mg vacancies. Also it's proved the VMg involve in complex interaction with N or C ions and play different role in magnetic performance in these samples. Zn vacancies can introduce localized magnetic moments in ZnO lattices and coupling interaction form long-range magnetic order. The magnetic interaction between Co-Co (Mn-Mn) in defect-free supercell of ZnO may tend to form antiferromagnetic ground state. Oxygen vacancies play an important mediation role in coupling of magnetic ions. Besides, we also calculated the defect induced magnetism in Al2O3systems and the Zn-doped SiC systems, the results are in good agreement with the experimental discussions.
引文
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