【作者】 张敏；

【导师】 赵勇； 羊新胜；

【作者基本信息】 西南交通大学 ， 材料学， 2015， 博士

【摘要】 拓扑绝缘体(TI)作为非传统意义的绝缘体,其特点是材料的内部有能隙,而表面态却无能隙无散耗。由于强自旋耦合作用,这种奇特的表面态受时间反演对称保护,因此能抵抗体系中晶体的缺陷、非磁性杂质等外界环境的影响。理论上预言拓扑绝缘体和其他体系相(磁性材料或超导材料)结合的复合体系的界面还可能发现新的物质相和新奇的物理性质,如量子反常霍尔效应、Majorana费米子和磁单极子。利用这些物理特性,人们可以构造新型量子器件,并最终应用到自旋电子学和量子计算机等领域。本论文选取典型的3D TI Bi2Se3作为研究对象。TI在低温强磁场下的作用下出现拓扑量子态,由此能观察到量子霍尔效应,然而这还不算是真正的拓扑绝缘体。科学家们期望有真正的TI,它们不依赖外加的强磁场也能够观察到量子霍尔效应,即反常量子霍尔效应,这是利用样品本身所具有的的铁磁性导致霍尔平台。而想要得到这种效应可以通过材料设计在TI中引入铁磁性。实现的手段是在TI中进行过渡族掺杂从而引入铁磁性,而理论计算出非磁性的2p轻元素X(X-B,C和N)掺杂TI,可以诱导出铁磁性,调控拓扑性质。故本论文对Bi2Se3进行了磁性元素Co、Ni和非磁性元素C的掺杂研究。另一个手段是把TI和铁磁材料结合在一起,利用异质结的邻近效应诱导出铁磁性。本论文选择在钙钛矿锰氧化物(La0.7Sr0.3MnO3简称LSMO)薄膜上沉积Bi2Se3薄膜,形成异质结构。探索铁磁性的强关联体系对Bi2Se3磁性、电输运性能和表面态调控方面的影响。全文的内容为以下几部分：对Bi2Se3单晶体进行磁性元素的掺杂,掺杂元素有Co和Ni。Bi2Se3本身样是抗磁性的,而Co掺杂后的样品磁化率温度曲线为顺磁(PM)行为,12K低温下的磁化率磁场曲线存在明显磁滞现象,证实铁磁相的存在。样品的饱和磁化率、矫顽力和剩余磁化率,随着Co浓度的增加而逐渐增大,说明Co浓度与铁磁性密切相关。FM的来源有两个可能的解释。第一,生成了很少量的铁磁体Co-Se化合物。第二,磁性杂质之间的RKKY作用。样品的电阻率的趋势在30K以上和30K两个温区不同,T>30K的温区,主要是电子-声子散射作用主要是电子-电子之间的散射作用。所有样品都表现出正磁电阻行为。Ni掺杂样品NixBi2-xSe3同样具有层状结构。Ni掺杂对样品磁性影响很大,x=0即Bi2Se3为抗磁材料,x=0.03样品是典型的PM材料,x≥0.05的样品中观察到了复杂的铁磁性,有多个磁转变点,这可能与Ni-Se化合物有关。所有样品都表现出弱金属导电行为,样品的电阻率随Ni含量的增加而增加。不同含量的样品表现出不同的磁电阻行为,可能与样品散射机制和磁有序有关。C掺杂的Bi2Se3晶体的生长是沿c轴取向,结晶性良好且有良好的周期性。晶格参数先减小后增加。样品仍然是n型半导体,说明C掺杂补偿Se缺失是有限的。C掺杂样品的电阻率变化趋势基本相同,随温度降低而降低,30K附近出现金属绝缘转变,电磁阻也在该温度点翻转,说明C掺杂增强了Bi2Se3的表面态对整个电导的贡献,使得电输运中表面电导的作用凸显出来。磁性质在C掺杂之后出现了抗磁到铁磁的变化。探索真空蒸发镀膜的方法制备Bi2Se3拓扑绝缘体薄膜的工艺条件。制备蒸镀Bi2Se3薄膜的最佳后退火处理参数为退火温度300℃,保温时间5h。适当增加退火温度Ta和保温时间可以提高Bi2Se3薄膜的结晶性,而较高的温度和较长的保温时间又会造成Se缺失。300℃退火的薄膜的电阻率在30K以下有一个很小的上升,显示出金属-绝缘转变,而转变温度点几乎随外加磁场增加而线性增加。300℃退火样品的磁电阻在高场下线性增加,这种现象被认为是一种量子线性霍尔效应；低场下由于拓扑绝缘体表面态中的自旋-动量锁定和自旋轨道的相互作用出现了弱反局域效应。Si(111)单晶,LaAl03单晶、石英和玻璃做基底蒸镀Bi2Se3薄膜,都能成的Bi2Se3相。所有薄膜的Se/Bi相对原子的比C,都比1.5小,虽然是在富Se环境下后退火处理,但过量的Se分子并没有进入薄膜内部。探索了磁控溅射方法制备Bi2Se3拓扑绝缘体薄膜的工艺条件。溅射沉积的Bi2Se3薄膜的进行后退火处理的最佳参数为：退火温度300℃,保温时间2h。未退火的Bi2Se3薄膜为非晶薄膜,随着Ta的增加,晶粒逐渐从圆形晶粒长成三角形和六边形结构。Ta对样品的电阻率也有影响,对于未退火的薄膜,电阻率随测量温度降低而增加(dρ)/dT<0)。当Ta≥200≥时,表现出金属行为(dρ/dT<0)。Si(100)基底上溅射的不同厚度的Bi2Se3薄膜都具有很好的c轴取向。300nm薄膜样品表现出弱金属性,在零场下,电阻率随着测试温度的降低而呈现线性降低。然而其他厚度的样品却表现出了不同的特征,这是由于电导在高温和低温区域有不同的输运机制。霍尔效应测试显示载流子与厚度相关,随着厚度增加而增加。MR曲线反应出高场线性磁阻和低场弱反局域效应。采用蒸发和溅射的方法制备拓扑绝缘体/钙钛矿锰氧化物(Bi2Se3/LSMO)异质结。蒸发制备的异质结呈现出绝缘体特性,电阻率随温度降低而逐渐增加,在30K附近饱和,这个温度与Bi2Se3体电导自由电子的冻结温度一致,说明在LSMO薄膜与Bi2Se3之间存在着某些相互作用。扣除LSMO的铁磁信号,还存在明显的磁滞现象,由于Bi2Se3本身是抗磁性的,说明LSMO在Bi2Se3薄膜中诱导出了铁磁性。在通过磁控溅射得到的Bi2Se3/LSMO异质结中,异质结的连续性很好,没有裂缝和孔洞,有层状结构和六方结构。不同厚度的薄膜电阻行为不同,但都能在薄膜中观察到金属-绝缘体相变,Bi2Se3层存在明显的自旋极化电流注入效应。这可能是由于LSMO中长程载流子进入到Bi2Se3当中,不同类型的载流子之间的强耦合及竞争作用造成的。磁性测试证实薄膜中存在铁磁性,说明Bi2Se3薄膜中可能由于基底的磁近邻效应作用而引入了铁磁序。更多还原

【Abstract】 Topological insulator (TI) as a non-traditional insulator has a new quantum state. Topological insulators also has dual nature of conductor and insulator, namely in the bulk interior is insulating state energy gap, and the surface has a metallic state gapless. The surface states (SSs) are protected by time-reversal symmetry owing to strong spin-orbit coupling. Therefore, TI is not easily affected by the defects, non-magnetic impurities and other external environment in the system. TI has important potential application value in low energy consumption of electronic devices and fault tolerant quantum computation. Many exotic physical properties of 3D TIs confirmed by experimental and theoretical prediction, such as Quantum anomalous Hall effect, Majorana fermion, and magnetic monopole, leading to great potential for future application.In this work, we selected the typical 3D TI Bi2Se3 in the series of V2VI3 compound as the research object. Topological quantum states appear under low temperature and strong magnetic field in TIs. The TIs are not real TIs at all.In the real TI, strong magnetic field which is not dependent on external can also the quantum Hall effect can be observed independent of low temperature and strong magnetic field. Namely the anomalous quantum Holzer effect could stems from ferromagnetic of sample itself. Materials design is effective in getting this kind TIs. To introduce ferromagnetism in TI, two methods are proposed. Doping with transition metal elements can make the 3D TIs to become magnetically ordered, result in gap opening at the Dirac point, and lead to other exotic phenomena. There is a theoretical prediction that non-magnetic 2p light elements X (X=B, C and N) can induce ferromagnetism and manipulate SSs in TIs by substitutional doping at the host anions. In this thesis, we mainly studied the structure and properties of magnetic element (Co and Ni) and non-magnetic element C doping Bi2Se3 bulk crystal. Some groups’works demonstrate that the ferromagnetism at ambient temperature can be induced in heterojunction interface by the magnetic proximity effec. This result opens a new path to interface-controlled. Based on above considerations we studied the properties and modification effect of double membrane structure of Bi2Se3 and perovskite manganite oxide. At last, we explore two techniques for preparing Bi2Se3 thin film, vacuum evaporation and magnetron sputtering. The primary research work and achievements are as follows:The Bi2Se3 matrix is diamagnetic and doped samples CoxBi2-xSe3 is a superposition of ferromagnetism (FM) and paramagnetism (PM) behavior at low temperature. The values of Msmol, Hc, Mr increase as the Co concentration increasing. There are two possible explanations to origin of FM:one is the bulk ferromagnetic behaviors of Co-doped Bi2Se3 are due to nanoclusters of Co-Se compound in the crystals, and the other is localized Co ions diluted into host matrix. All samples exhibited weak metallic resistivity. The resistivity data could be fitted by different formulas below and above 30 K in all samples result from different mechanism corresponding to different temperature regions. The magnetoresistance (MR) of Bi2Se3 topological insulator showed a change dependence of concentration. The Hall mobility data indicated that incorporating Co in Bi2Se3 single crystal can reduce native defect of Bi2Se3.Transition metal doped topological insulators NixBi2-xSe3 were prepared by the self-flux method. The lattice constant c decreased with the increasing Ni concentration. All samples are highly the c-axis oriented and exhibit weak metallic resistivity. The resistivity increased with increasing applied magnetic field and Ni concentration. The magnetic changes monotonously as the Ni dopant concentrations increased, which implies the nickel entering the matrix structure. For the sample with small amount of Ni (x=0.03), a behavior in the curves of temperature dependent of magnetism (M (T)) closely resembling a paramagnet. The samples with (x≥0.05) showed clear hysteresis loops, which suggest the existence of ferromagnetism ordering. It was considered that there are two possible explanations to the origin of ferromagnetism:Ni-Se compound and magnetic contamination.C doped samples had the same lattice structure as pure Bi2Se3, the lattice parameter c firstly increased and then decreased. The surface of samples showed the typical terrace-and-step morphology. From the p-T curves, obvious M-I transformation in all samples were observed near 30K. p-T and MR-T characteristics around 30 K was observed which indicated that contribution of surface state had been significantly enhanced by C doping. After C doping, the magnetic properties change from diamagnetic to ferromagnetic.Bi2Se3/Si (100) thin films with good crystalline quality and highly c-axis orientation were synthesized by thermal evaporation deposition with post annealing treatment. The sintering parameters of holding time and post annealing temperature obviously affected the phase structure and electrical properties. The best sintering parameter was holding for 5 h and post annealing temperature 300 ℃. The annealing temperature affects the phase structures and electrical properties. The relative atomic ratio of Se/Bi decreases with increasing annealing temperature. WAL and resistivity upturn are both observed only in the film annealed at 300℃.Bi2Se3 thin films with good crystalline quality and highly c-axis orientation were synthesized by magnetron sputtering with post annealing treatment. The films have good crystalline quality, and their surfaces exhibit terracelike quintuple layers. Most good thin nanoplatelets exhibit hexagonal morphologies with planar dimensions about 400 nm, while some of them show truncated triangle morphology. The resistivity of films with different thicknesses exhibit different characteristics at different temperature regions. The weak antilocalization (WAL) cusp was observed in the magneto-transport measurements of topological insulators at low temperatures and low magnetic fields. We also find the linear-like magnetoresistance (MR) at high-field, which is associated with the gapless topological surface states and of quantum origin.LSMO thin films have been grown on LaAl03 (100) single crystal substrates by the polymer-assisted chemical solution deposition method. Then Bi2Se3 films were deposited on LSMO films. Bi2Se3/LSMO films have good crystalline quality with hexagonal structure. The p-T curves still showed different behavior in different thickness films, while the metal-insulator (M-I) transition in low temperature region was obvious in all samples. This may be due to long distance carriers in LSMO access to the Bi2Se3, resulting in strong coupling and competition between various types of carriers. Magnetic measurement has been confirmed the presence of ferromagnetic in Bi2Se3 thin films which may attribute to the magnetic proximity effect of the substrate introduce a ferromagnetic order.更多还原