【作者】 周昌杰；

【导师】 康俊勇；

【作者基本信息】 厦门大学 ， 凝聚态物理， 2009， 博士

【摘要】 在当今追求科技产品多功能的信息时代,人们对微纳尺度光电器件和电子器件的要求日益提高。为获得新功能器件,制备稳定的n型和p型掺杂的新型化合物半导体材料凸显重要;同时,器件的比表面积随器件的尺度减小迅速增大,Si表面上金属的结构和特性对器件的性能影响极大。然而,在纤锌矿结构ZnO新型光电半导体中,存在非对称掺杂和获得的p型掺杂不稳定和极性表面非零偶极矩的静电不稳定的难题;在Si表面上的金属原子所形成的纳米团簇也存在结构稳定性和电子结构不确定等问题。极大地制约了这些半导体结构材料的应用。为此,本论文着重就纤锌矿结构ZnO中Ga-N等电子对共掺杂p型材料、Zn和O极性表面结构、Si（111）-（7×7）表面Zn纳米团簇结构的稳定性及其电子结构性质开展研究。首先,从纤锌矿结构ZnO的p型掺杂稳定性和能带结构调制的角度出发,计算了两种Ga-N等电子对共掺杂构型的总能、态密度和能带结构。总能计算表明,共掺有利于N_O受主杂质的稳定,并提高N在ZnO晶体中的掺杂浓度。态密度和能带结构结果显示,共掺构型Ⅰ较构型Ⅱ更有利于N_O引入的空穴和Ga_Zn引入的电子的完全互补偿,从而不改变完整晶格ZnO的基本电子结构性质,而仅仅抬高其价带顶,在价带顶处产生一个完全占据的非有效杂质态,有效降低了N_O的受主电离能,并提高了N_O的p型掺杂效率。其次,干净无再构Zn和O极性表面的几何结构和电子结构性质的计算表明,形成Zn极性表面后,最外Zn原子层稍微向外移动,并在完整晶格ZnO的导带底处产生了新的陡峭表面态,使得费米能级抬高进入导带,形成n型导电。形成O极性表面后,最外O原子层向内大幅度位移,并在完整晶格ZnO的价带顶处产生了新的平坦表面态,费米能级下移进入价带,使O面具有p型导电性。结合Zn和O极性表面的STM观测认为,Zn表面的稳定通过形成以O原子为台阶边沿的能量最低的{10（?）0}为台阶纳米小面的三角岛锯齿台面,改变表面Zn/O比,以补偿极性表面电荷。而O表面的稳定则主要通过表面电荷的转移和p型表面态的生成来补偿极性表表面电荷。最后,在Si（111）-（7×7）表面生长出了全同的Zn纳米团簇,结合扫描隧道显微镜和第一性原理总能计算及理论STM模拟研究结果显示,Zn纳米团簇中心倾向于被一个Zn原子所占据,使Zn纳米团簇不同于其它金属纳米团簇（N=6）,形成最稳定的Zn₇Si₃原子构型。STS测量结果表明,在扫描偏压为±0.5V时Zn₇Si₃纳米团簇几乎消失不见的现象是Si（111）-（7×7）表面位于-0.3和+0.5V处的Si顶戴原子表面悬挂键态被饱和,形成具有半导体态密度分布的结果;Zn₇Si₃纳米团簇占据层错半单胞,清空了与之近邻的无层错半单胞中最近邻中心Si顶戴原子在-0.3V处的占据电子,澄清了近邻无层错半单胞中最近邻3个中心Si顶戴原子在占据态STM形貌像中变暗的物理起源。不同Zn覆盖度下Zn/Si（111）-（7×7）表面的STM形貌研究表明,Zn薄层的生长模式为经典的层状-岛状生长模式。所生长的Zn薄层不是普通的六角密堆积金属Zn薄层,而是多层蜂窝状Zn纳米团簇阵列层。对不同衬底温度生长的Zn薄层STM形貌研究发现,随着Zn原子单层与衬底Si（111）-（7×7）表面间距的增大,Si（111）-（7×7）表面对Zn原子单层的作用逐渐减弱,并导致Zn原子层的生长从单层生长模式转变为岛状生长模式,并对不同Zn原子单层的电子态产生显著的影响。原位RHEED衍射图样和STM形貌像分析显示,第一Zn原子单层为Zn₇Si₃纳米团簇组成的纳米团簇阵列层。从第二Zn原子单层开始,金属Zn纳米团簇通过直接在Zn₇Si₃纳米团簇上柱状堆叠而成。金属Zn纳米团簇中的Zn原子以六角密堆积结构方式排布,与Si（111）-（7×7）表面的外延关系为Zn（0001）//Si（111）和Zn[11（?）0]//Si[11（?）]。更多还原

【Abstract】 In the information age,multi-functional products have attracted much attention as the rapid development of science and technology.In order to obtain the products, people make great efforts to new feature optoelectronic semiconductors and Si electronic devices in nano-scale.However,a crucial problem of structure stability should be overcome before new optoelectronic devices and the nano-scale Si devices could potentially make inroads into the world.For a new optoelectronic semiconductor,wurtzite ZnO faces the difficulty and instability of p-type conductivity and the electrostatic instability of Zn and O polar surfaces.For the nano-scale Si devices,the geometrical and electronic structures of metal-clusters adsorbed on Si surfaces are subject to instability and unknown problems.Therefore,in the thesis we mainly studied the structural and electronic properties of isoelectronic Ga-N codoping of wurtzite ZnO,Zn and O polar surfaces of wurtzite ZnO,and self-assembled Zn nanoclusters on the Si（111）-（7×7）surfaces.Firstly,we demonstrated that the isoelectronic Ga-N complex in wurtzite ZnO can enhance the stability of the N_O acceptor and the N concentration by using the first-principles total energy calculations.As indicated by the calculated electronic structures,one of the isoelectronic Ga-N complexes can form a totally passive donor-acceptor complex by almost keeping on the basic electronic structure of undoped wurtzite ZnO,but only generates an additional fully occupied band above the top of the valence band.Then the ionization energy of the excess N_O acceptors will be reduced largely and the p-type conductivity will be enhanced significantly.Secondly,we calculated the geometrical and electronic structures of the Zn and O polar surfaces.For the Zn polar surface,steep surface states appear in the band gap of bulk ZnO and follow the bottom of bulk conduction band.Moreover,Fermi level shifts up into the conduction band,which leads to the n-type conduction behaviour. For the O polar surface,flat surface states emerge above the top of the valence band of bulk ZnO as Fermi level shifts down a little into the valence band.Thus,the O polar surface can be predicted to have the p-type conduction behaviour.STM measurements showed that the Zn polar surface can be stabilized by reducing the surface Zn/O stoichiometry with 0 atoms occupied at the edge of the triangular terraces.Different from the stabilization mechanism of the Zn polar surface,the O polar surface can be stabilized by transforming the surface charges and forming the p-type surface states.Finally,we successfully fabricated the identical-size Zn nanoclusters grown on Si（111）-（7×7）at room-temperature and demonstrated the atomic structure of the clusters by combining in situ STM and theoretical simulation.Due to the varying valence,Zn nanoclusters are favor in forming the Zn₇Si₃ geometrical structures with one characteristic Zn atom occupying the center and therefore distinguish this system from other nanoclusters（N=6）.STS measurements indicate that the drastic depressions of the Zn₇Si₃ nanoclusters with respective to the corner Si adatoms at low bias voltages of±0.5V are attributed to the saturation of the metallic Si adatom dangling bond states at about-0.3 and +0.5V,which further reveal the semiconducting characteristics of the Zn₇Si+3 nanoclusters.Due to vanishing the Si adatom surface dangling bond state at about-0.3V,the closest edge Si adatoms in the nearest neighboring uncovered UHUCs are strongly influenced and thus almost darkened in the filled-state STM images,indicating the charge transfer from the closest edge Si adatoms to the Zn₇Si₃ nanocluster.We also demonstrated the Stranski-Krastanov（SK） growth mode of the multilayered Zn nanocluster arrays on the Si（111）-（7×7）surfaces. STM measurements showed that the interaction between the Zn atoms and the Si（111）-（7×7）surface become weak as the distance between the Zn atomlayer and the Si（111）-（7×7）surface increases,which will induce the transformation from the Frank-VanderMerwe（FV）mode to the Volmer-Weber（VW）mode,and further influence significantly the electronic properties of different Zn atomlayers.In-situ RHEED measurements suggest that the 1^stZn atomlayer is composed of the Zn₇Si₃ nanoclusters.The 2^ndand 3^rdZn atomlayers are made up of the Zn nanoclusters, which directly stack on the top of the Zn₇Si₃ nanoclusters with the hexagonal-closed-pack atomic geometry.And due to the specific atomic structure of Zn₇Si₃ nanoclusters,Zn（0001）and Si（111）are parallel with each other with an in-plane epitaxial relationship as Zn[112^-0]//Si[112^-].更多还原