【作者】 梁培；

【导师】 江建军；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2009， 博士

【摘要】 自旋电子学是利用电子的电荷和自旋两个自由度作为信息载体，进而实现信息传输、处理和存储，目前已成为电子学、物理以及材料等多学科交叉研究中的热点之一，作为关键材料的稀磁半导体的研究备受关注。鉴于氧化锌半导体作为母体材料的稀磁半导体有可能实现较高的掺杂浓度，且掺杂离子的3d能带可以通过本征缺陷态而产生强铁磁耦合作用。因此，ZnO稀磁性半导体成为首选研究体系。结合寻求具有高居里温度的稀磁性半导体和分析稀磁性半导体内部磁性产生的机理这两个关键科学问题，本论文主要采用基于密度泛涵的第一性原理计算ZnO稀磁半导体的电子结构，并且分析和解释其磁性起源机理。在此基础上，研究第一性原理和蒙特卡洛方法耦合集成来计算ZnO稀磁半导体的居里温度，探讨居里温度的调控方法，重点讨论不同单掺杂和共掺杂体系对于ZnO稀磁半导体磁性的影响和作用机理。首先，阐明利用第一性原理赝势平面波方法和全势-缀加平面波方法计算ZnO稀磁性半导体电子结构理论的基础和计算方案，从能带角度研究氧化物磁性半导体中过渡金属的耦合作用，为解释磁性起源机理奠定了基础。进一步利用第一性原理和蒙特卡洛耦合集成算法，预测不同计算体系的居里温度，研究其调控方法。其次，利用全势-线性缀加平面波方法在广义梯度近似下计算了碳单掺杂的ZnO电子结构。结果表明，碳掺杂在氧位置或者是在间隙位置都会导致磁性，并且无论是间隙位置掺杂还是替代掺杂体系都具有半金属性，理论上具有较高的自旋极化率。利用第一性原理和蒙特卡洛耦合计算方法获得了碳掺杂ZnO的居里温度，在不同的掺杂浓度下，居里温度分布251～439 K之间。由此表明碳单掺杂的ZnO稀磁半导体在合适的制备条件下，可以得到室温铁磁性。利用全势-线性缀加平面波方法在库仑势修正下，计算了钕单掺杂的ZnO以及其分别含有一个V_Zn和一个V_o的电子结构和磁性。表明含有V_Zn缺陷的钕掺杂的ZnO可能具有较高的居里温度，而没有任何本征缺陷时则表现出顺磁性，含有V_o的体系具有弱反铁磁耦合。对于含有V_Zn缺陷的体系其磁性的起源，利用束缚极化子机理进行了相应地解释。最后，研究了过渡金属-金属共掺杂体系对于ZnO稀磁半导体的影响，分析交换作用产生磁性的载流子调控机理。研究了Co-Al掺杂ZnO体系，通过Al原子的引入，实现了共掺杂体系的反铁磁到铁磁性的转变。铁磁性的起源主要由于多余的电子调控下的Al-2p电子和近邻的Co-3d电子的相互作用，导致了体系的磁性，其相互作用符合载流子调控的p-d交换模型。Fe-Al掺杂ZnO体系的计算表明，Fe掺杂的ZnO体系在基态下表现出反铁磁性，当引入Al原子后，Al处于最近邻掺杂是Fe掺杂ZnO体系实现了反铁磁到铁磁态的转变，此时Al-2p电子和Fe-3d电子没有发生作用，铁磁性的产生是RKKY远程交换作用。研究了Cu-N过渡金属-非金属共掺杂ZnO稀磁半导体体系的载流子调控机理，通过N原子的引入，N-2p和Cu-3d发生了交换作用，这个交换作用使得掺杂系统更加稳定。同时在掺杂前后，体系的价带顶仍然都是有O-2p电子占据的，在导带顶中Cu-3d电子和4s电子占据了大部分的态。载流子调控模型被用来解释磁性的起源，由于N的掺入使得在掺杂体系中的载流子数量增加，磁性离子之间的交换作用通过自由载流子进行传递，导致了体系的磁有序。进一步研究了过渡金属-非金属共掺杂体系的居里温度调控方法和作用机理。表明Mn-N共掺杂的体系成功实现了基态下的铁磁性转变。从海森堡模型和平均场理论出发利用蒙特卡洛和第一性原理的方法预测得到共掺杂体系可以具有室温铁磁性。总之，通过第一性原理与蒙特卡洛方法耦合集成，计算了单掺杂和共掺杂的几种典型体系，表明其作用机理不尽相同，由此解释了稀磁半导体中磁性的起源和居里温度（T_c）提高的途径，为ZnO稀磁半导体掺杂工艺提供了实践设计方向。更多还原

【Abstract】 Spintronics seek to exploit both spin and charge attributes of information carriers totransfer,process and store data,which has become a focused interdisciplinary fieldinvolving electronics,physics,materials science and other disciplines.As a crucial materialin this field,ZnO based diluted magnetic semiconductor has received a great deal ofattention.Zinc Oxide semiconductors base materials with high doping concentration and the3-d band of doped ions could generate strong ferromagnetic coupling interaction throughthe intrinsic defect states of the base material,which led to Zinc Oxide become the mostemerging hotspot in this field.Two key scientific problems in spintronics to both look formagnetic semiconductors with high Curie temperature and analyse the origin andmechanism of magnetism in these semiconductors could be sought for solution.Therefore,the electronic structure of diluted magnetic semiconductors through first-principlecalculation was firstly carried out,and the results were utilized to analyze and explain theorigin of ferromagnetism.Base on the result achieved in first-principles,a new couplingintegration algorithm to calculate the Curie temperature of ZnO DMS and discusse thecontrolling method of it was put forward.Then the effects of interaction mechanism ofdifferent single-doping and co-doping diluted magnetic semiconductor systems wereextensively discussed in detail.Firstly,the electronic structure of ZnO-based diluted magnetic semiconductor wascalculated by first-principles according to the Pseudopotential Plane Wave method （PP-PW）and Full Potential Augmented Plane Wave method （FP-LAPW）.The coupling interactionbetween transition metals in oxide magnetic semiconductors were analysed fromperspectives of energy bands theory,which described the origin of magnetism about dilutedmagnetic semiconductor.Furthermore,the coupling computations of first-principles andMonte Carlo method were developed by calculation code in our study,which was used toestimate Curie temperature of several diluted magnetic semiconductor systems and studiedthe further controlling method of it.Secondly,the electronic structure of carbon doped ZnO was calculated by the full potential linearized augmented plane wave method （FP-LAPW） with general gradualapproximation （GGA）.The results showed that the single carbon doping led toferromagnetism both in Oxygen position and in interstitial position,and they both result inhalf-metallic property,which theoretically had a high rate of spin-polarization.The Curietemperature of carbon doped ZnO was also predicted through a combination of first-principles and Monte Carlo coupling integrated calculation method.The results showedthat the Curie temperature of carbon doped ZnO system changed ranges from 251 to 439 Kas the concentration variation.As a result,room-temperature ferromagnetism of carbondoped ZnO could be derived in proper preparing conditions.With full potential linearized augmented plane wave method （FP-LAPW） and Coulombpotential amendment,electronic structure and magnetism of neodymium doped ZnO,theneodymium doped ZnO with an oxygen vacancy （ Vo ） and neodymium doped ZnO a Zincvacancy ( V_Zn) were calculated,respectively.The results indicated that neodymium dopedZnO with V_Zn defects might have high Curie temperature,while those without intrinsicdefects are paramagnetic,and those with Vo defects get weak anti-ferromagnetism.Furthermore,bound magnetic polaron theory can be employed to explain the origin ofmagnetism in systems with Zn vacancy.Finally,after the single doped ZnO was presented,the effect of metal-transition metalco-doped ZnO systems were extensively examined to described the mechanism of the inter-exchange and the origin of magnetism.Hence,Al-Co co-doped ZnO systems were alsoinvestigated;the calculated results showed that by co-doping with Al,the systems weretransferred from anti-ferromagnetism to ferromagnetism.The origin of ferromagnetismmainly attributes to the interaction between Al-2p electron and nearest Co-3d electronmodulated by extra electrons,and the inter-exchange effect can be explained by p-dexchange model.Al-Fe co-doped ZnO system is also investigated,and the results indicatethat by co-doping with Al the system is transferred from anti-ferromagnetism toferromagnetism.But the Al-2p and Fe-3d electrons do not have interaction with each other,and the origin of ferromagnetism can be explained by long range Ruderman-Kittel- Kasuya-Yoshida （RKKY） exchange interaction.In order to describe the different interaction mechanism of DMS,the influence of metal-non metal co-doping system were analyzed.Hence,the Cu-N co-doped ZnO systemswere also investigated.By introducing nitrogen,the interaction between N-2p and Cu-3delectrons stabilize the magnetism of the system.In the meanwhile,both before and after co-doping,the VBM is always occupied by O-2p electrons,while in conduction band states aremostly occupied by Cu-3d and 4s electrons.Carrier mediated model was employed toexplain the origin of magnetism.As nitrogen co-doping improved the density of chargecarriers in system,interaction mediated by free carriers between magnetic ions led toferromagnetic order as a whole.To obtain the Curie temperature controling method and the interaction mechanism,metal-nonmetal co-doped systems were also studied by co-doping nitrogen the Manganesedoped system showed stable ferromagnetism.Based on Heisenberg model and mean fieldtheory,with Monte Carlo and first-principles method,we predict that room-temperatureferromagnetism can achieve in ideal conditions theoretically.As a result,through the combination of first-principles and Monte Carlo method,single-doped and co-doped systems were extensively discussed.The results showed thatdifferent doped system has different inter-exchange mechanism.Our calculation can explainthe origin of magnetism in some doped systems and the way to increase the Curietemperature.All these can give a guide for the preparation techniques of ZnO dilutedmagnetic semiconductors.更多还原