【作者】 熊志华；

【导师】 江风益；

【作者基本信息】 南昌大学 ， 材料物理与化学， 2008， 博士

【摘要】 与计算机技术相结合的计算材料和材料设计是现代材料科学研究的重要方面,本论文应用基于密度泛函理论的第一性原理方法和分子动力学等计算材料科学方法对纤锌矿ZnO中的几个基本问题进行了理论研究,包括以下五个方面:ZnO及其在压力作用下的电子结构和光学性质、ZnO的n型和p型掺杂机理、M_xZn_1-xO（M=Cd,Ca）合金机理、非磁性金属掺杂ZnO基DMSs的磁性来源和机理、H吸附ZnO（10-10）表面的金属化转变等。1、研究了本征ZnO的电子结构和光学性质。（1）利用GGA近似计算了ZnO的能带,ZnO为直接禁带半导体材料,其带隙为1.02eV。（2）计算了ZnO的介电函数、吸收系数等光学常数,并结合ZnO电子结构信息和半导体带间跃迁理论对上述光谱进行了指认和判别。（3）进一步研究了压力作用下ZnO的电子结构和光学性质,分析了压力作用下蓝移现象的机理。所得理论结果和实验相符。2、研究了ZnO的n型、p型掺杂改性机理。计算了ZnO中单掺Ag、N等受主元素,单掺Al、Ga等施主元素以及N_o-H共掺的电子结构,并结合分子动力学方法研究了H在ZnO中的行为。结果表明:（1）Ga掺杂较Al掺杂具有更好的电导性,适合制备n型低阻ZnO。（2）单掺Ag形成深受主能级,单掺N会产生窄的深受主能级,局域化严重,导致掺入的N原子活性差,单掺Ag和单掺N都不利于制备高质量p型ZnO薄膜。（3）H是浅施主而且解溶度非常高,这是ZnO的n型导电性的起因。（4）N_o-H共掺改性计算表明,N_o-H形成能要低于N的形成能,说明适量H有利于提高了N原子的固溶度,同时发现N被H钝化,不利于p型掺杂。但是,H在ZnO中呈现热不稳定性,高温退火处理N_o-H共掺ZnO薄膜可以获得较高质量的p型ZnO薄膜。以上计算结果很好地解释了本组ZnO掺杂实验,并为获得高质量的n或p型ZnO提供了设计方案和理论基础。3、研究了M_xZn_1-xO（M=Cd,Ca）合金的电子结构,从原子层次阐述了M掺杂ZnO的能带调整机理。研究表明:（1）掺杂Cd导致ZnO禁带宽度减小,其机理是Cd_xZn_1-xO价带顶始终由O 2p占据,而导带底部则由O 2p、Cd 5s与Zn 4s的杂化轨道控制,随着掺杂量x的增加,导带底逐步下降,出现红移效应。（2）掺杂Ca导致ZnO禁带宽度增加,其机理是Ca_xZn_1-xO价带顶仍由O 2p占据,而导带底部则Zn 4s控制,随着掺杂量x的增加,导带底逐步升高,出现蓝移效应。以上计算结果为ZnO合金的设计提供了有效的理论依据。4、研究了非磁性金属（Cu和Ti）掺杂ZnO基DMSs的电子结构、磁性来源和磁性机理,寻找居里温度高于室温的新型ZnO基DMSs。研究表明:（1）非磁性金属（Cu和Ti）掺杂ZnO具有室温下铁磁性基态,且Cu和Ti分别作为受主和施主出现。（2）我们采用磁性离子d电子间的交换作用模型即d-d双交换模型解释了非磁性金属（Cu和Ti）掺杂ZnO基DMSs的磁性机理。（3）载流子浓度影响体系磁性,如O缺陷的生成会破坏ZnO基DMSs磁性。以上研究结果丰富了ZnO基DMSs的理论内涵,提供了制备新型ZnO基DMSs的思路,并被最近实验所证实。5、研究了ZnO（10-10）表面及其吸附H原子的几何、电子结构和电荷转移现象。研究表明:（1）ZnO（10-10）表面弛豫后,Zn原子向体内移动幅度明显,表面Zn-O二聚体发上扭曲。（2）ZnO（10-10）表面吸附H后空间电荷重新分布,导致ZnO（10-10）表面发生金属化转变。本论文得到下列课题的支持:国家863纳米专项课题（No.2003AA302160）和信息产业部电子发展基金（2004-125号）。更多还原

【Abstract】 Computational materials and materials design combined with computer techniques are important contents in materials science.Here five main aspects of the dissertation have been achieved using first-principle theory based Density Functional Theory（DFT）and molecular dynamics methods:electronic structures,optical properties of perfect ZnO,ZnO with native defects and under strain ZnO,respectively; doping and alloy mechanisms in ZnO;the magnetism origin and mechanisms of ZnO-based DMSs doped with non magnetic metal;the metallic transition of ZnO（10-10）surface induced by hydrogen adsorption.1.The electronic structures of perfect ZnO and ZnO with native point defects were investigated with the methods of first principles.（1）Using GGA,the calculated results indicated that ZnO is a direct wide band gap semiconductor material with energy gap of 1.02eV.（2）The optical parameters including dielectric functions, absorption coefficient were calculated,which were furthermore identified with electronic structure information of ZnO and theory of electron inter-band transitions. （3）The electronic structure and optical properties of ZnO under strain were calculated and the bule-shift effects were also been studied in details.2.We studied the electronic structure of n-type,p-type and p-type codoping ZnO, such as:Ag,N,Al,Ga single doping and N-H codoping,respectively.Moreover,the action of H atom was studied using the methods of molecular dynamics.It was shown: （1）Ga was more suitable for fabricating n-type low resistance ZnO due to its better conductivity compared with Al.（2）Substitute Ag produced a deep acceptor level,and doping N also produced a deep acceptor level,which were hardly localized near the top of the valence band.Therefore,single doping of Ag and N made against to fabricate high quality p-type ZnO.（3）It was confirmed that H is the main factor to induce the native n-type coductivity in ZnO for its shallow acceptor level and large concentration in ZnO.（4）The formation energy of N-H complex was lower than that of N single doping,which indicated that some of H atoms-doped made for increasing the concentration of N.As well,it was found that N was passivated by H atom,which is obviously not benefit for p-type doping.Furthermore,using molecular dynamics simulation,we found that H was unstable thermodynamicly.It means we could obtain the high quality p-type ZnO by annealing for N-H codoped ZnO.3.The electronic structure of M_xZn_1-xO（M=Cd,Ca）were calculated to analyse the mechanism of ZnO band gap engineering.It was shown:（1）The band gap of ZnO narrows with increasing Ca-doping concentration.Our work shows that the top of valence band is determined by the O 2p electron,while the bottom of conduction band is occuupied by the hybrid of O 2p,Cd 5S and Zn 4s electrons which can lower the bottom of conduction band.（2）On the contrarly,the band gap of ZnO broadens with increasing Ca-doping concentration.Our work shows that the top of valence band is still determined by the O 2p electron,while the bottom of conduction band is only determined by the Zn 4s electron states which can shift to a higher energy due to Ca doping.4 We presented first-principles spin polarized calculations of the eleotronic structure and magnetic properties of Cu（Ti）doped in ZnO.We showed that:（1）It is likely for Cu（Ti）to order ferromagnetically in ZnO,forming a dilute magnetic semiconductor.The coupling between Cu（Ti）atoms is found to be ferromagnetic despite Cu（Ti）being nonmagnetic in its natural phase.Cu is acceptor and Ti is donor in ZnO,respectively.（2）The ferromagnetic ground state in Cu（Ti）doped ZnO can be explained in terms of Zener’s double exchange mechanism.（3）Cartier concentration can affect the magnetism of ZnO-based DMSs,for example,oxygen vacancies tend to destroy the ferromagnetism and therefore should be avoided during sample fabrication.These results are consistent well with the recent experimental discovery of ferromagnetism in Cu-doped ZnO.5.The clean ZnO（10-10）surface and hydrogen adsorbed ZnO（10-10）surface are studied by first principles.It was shown:（1）Zn atom move inside obviously after relaxation,which induced surface Zn-O dimer distorting.（2）The alteration of the charge density redistribution of the ZnO（10-10）surface shows the electron transfer from surface layer to the adsorbed H atoms,which leads to a metallization of the ZnO（10-10）surface.The mechanism of alteration for charge density redistribution is discussed,which provides a theoretical background of electrical properties of ZnO（10-10）surface.This work was supported by the 863 project（contract No.2003AA302160）and China foundation for development of electronic information technology.更多还原