【作者】 杨春；

【导师】 李言荣；

【作者基本信息】 电子科技大学 ， 材料物理与化学， 2004， 博士

【摘要】 氧化锌（ZnO）是一种应用广泛的直接带隙、宽禁带半导体材料,以蓝宝石（α-Al₂O₃）为基片制得的ZnO薄膜在短波光电器件如发光二极管、激光器件等领域有重要的应用前景。虽然实验上采用激光分子束外延技术（PLD-MBE）,在α-Al₂O₃（0001）上已制得了高质量的ZnO薄膜,然而对其生长机理缺乏理论研究。本文在分析和总结薄膜生长模拟方法与模型基础上,选择了基于第一原理的计算方法,运用CASTEP总能量计算软件包,在周期边界条件下的κ空间中,针对复杂氧化物薄膜生长过程模拟异常困难的问题,建立了ZnO在α-Al₂O₃（0001）表面吸附生长的系列模型。采用基于密度泛函理论的平面波超软赝势,应用了局域密度近似（LDA）和广义梯度近似（GGA）方法,对α-Al₂O₃（0001）表面及其对ZnO的吸附生长进行了计算机模拟研究。通过研究α-Al₂O₃（0001）表面原子与电子结构、ZnO表面吸附位置与成键特性、不同温度下ZnO生长过程、Al₂O₃/ZnO界面结构以及薄膜生长中的缺陷等问题,详细论述了ZnO在α-Al₂O₃（0001）表面吸附机理、表面O、Al空位缺陷对吸附生长的影响、表面界面结构与ZnO薄膜生长取向关系、温度对表面界面原子行为的影响特征、扩散对ZnO薄膜生长模式的影响规律等。所得到的主要结论同相关文献报导的实验现象一致。 通过理论计算,证实了α-Al₂O₃（0001）最表层终止原子为单层Al的表面结构最稳定,表面弛豫主要发生在最外表面的Al-O层,Al向内弛豫距离为0.079～0.082nm。弛豫后电子将有更大的几率定域在最表层O原子的周围,从而出现了新的分裂能级,所产生的大部分电子态为O的电子态,主要来自于O的2p轨道。弛豫后表面局部区域电子密度增大,能增强对阳离子以及带正电荷粒子的吸附作用。 ZnO在α-Al₂O₃（0001）表面发生了强烈的化学吸附,Zn在表面较稳定的化学吸附位置正好偏离表面O六角对称约30°。ZnO的O^2-与表面上的Al³⁺所形成的化学键具有强离子键特征;而其Zn²⁺同基片表面O^2-形成的化学键表现为明显的共价键特征,主要来自于Zn 4s与O 2p的杂化带,以及部分Zn 3d与O2p的杂化带。从吸附前后态密度变化、吸附能量（424.5±38.6 kJ/mol）和吸附位置来看,在这些偏离基片表面O位置30°处,有利于Zn 4s轨道与表面相邻的3个O原子2p轨道产生sp³杂化,形成四面体配位,有利于ZnO薄膜的铅锌矿结构形成。这些吸附位置是ZnO薄膜最优生长点。 通过ZnO在表面吸附与生长的动力学模拟与表面界面吸附能的计算,表明更多还原

【Abstract】 The high-quality ZnO thin films growth on the sapphire(0001) substrate, using pulse laser deposition (PLD) or molecule beam epitaxy (MBE) technology, has a wide application prospect in short wavelength optical devices including light emitting diodes or laser diodes. But there is no theoretical calculation on the initial growing mechanism of the ZnO thin films deposited on the α-Al2O3(0001) surface. The first principle is considered for ZnO films heterogeneous growth simulation through an analysis of the difficulties of complex oxide films growth and a summary of the modeling and computer simulation methods of films growth. A series of models of ZnO films growth on the α-Al2O3(0001) surface and their theoretical calculation are carried out by using the CASTEP code based on the density-functional theory(DFT). Electron wave functions are expanded in terms of a plane wave basis set in the periodic k-space and the ultrasoft pseudo-potentials (USPs) are employed. The electron-electron interaction is treated within the local density approximation (LDA) and the generalized gradient approximation(GGA). In this paper, the author attempts to have a detail discussion of the following points such as the geometric and electronic structure of the α-Al2O3(0001) surface, the adsorption geometries sites on the substrate surface, the ZnO films growing process in different temperature conditions, the surface and interface structure of the Al2O3/ZnO, and the defect in films growth. The mechanism of ZnO adsorption on the α-Al2O3(0001) surface, the surface and interface structure and the growing orientation, the temperature dependent influence of the atoms behavior character of the surface/interface, and the point defect of the surface AI and the O effect on ZnO adsorption growth etc. are calculated the first time by the author. The results are well in accordance with experimental reports.The theoretical calculation gives a further evidence that the single Al-terminated structure of the a-Al2O3(0001) surface is much more stable, showing that the surface relaxation mainly takes place in the top Al -O layer with an inward distance of 0.079~0.082nm. The relaxation has caused the re-distribution of large quantity of surface O2- charge which is mainly from the O-2p states, thus the locations which the electronic density increase on the surface areas can enhance the adsorption of cations and the particles with positive charge.ZnO has experienced strong chemisorbed on the α-Al2O3(0001) surface where the preferential adsorption sites has a 30° away from the axis of hexagonal symmetry of oxygen cells on the surface. The chemical bonding of the (Zn)O-Al(substrate) is shown as having the character of ionic bonding, while chemical bonding of Zn-O(substrate) has a clearly covalent feature, manly from the hybridization between Zn4s and O2p, as well as part of the hybridization between Zn3d and O2p. In view of the adsorption energy and the adsorptive position, it is favorable for forming the tetrahedral coordination by sp3 hybridization between Zn 4s and the 2p orbits of the surface O atoms, which is favorable for forming the tetrahedral coordination in ZnO wurtzite structure. These adsorption sites are the most preferred growth sites of ZnO films.The ab initio dynamic simulation of the ZnO adsorption on the α-Al2O3(0001) surface and the calculation on the surface and interface energy indicate that the growth structure at c-axis is of substrate-O-layer-Al layer-Zn layer-O-layer structure, and that the films with (0001) -O surface is the most stabilized structure. The temperature has an evident impact on the reaction diffusivity of the O atoms so as to significantly act on the regular films growth, which has a decisive effect on the ZnO films’ growth model. ZnO films with a spiral-in plane growth at 400℃, the temperature around 400 ℃ is favorable for forming the Zn-terminated surface, while an aligned in-plane growth at 600℃ is observed. There are a surface phase transition during ZnO films growth at 600 ℃, one with the surface structure characteristic of the Zn-terminated surface and the ZnO films [1010] // substrate[1010], and the other with the O-terminated surface and the films [1010]// substrate [1120]. The barrier energy in the two changes of the surface structure is about 1.6 eV.The simulation results reveal that there is more disordered layer structure of ZnO films at 400℃. With the temperature up to 800℃, the dissociation of the Al-O atoms on the Al2O3 surface results in forming intermixed amorphous interface. The temperature of about 600 ℃ is the ideal temperature condition of ZnO heterogeneous growth on sapphire (0001) surface. The vacancies of the Zn on the interface where close to the α-Al2O3(0001) surface are more than that of the O. Therefore, we suggest that a proper increase of the deposition rate and a treatment of the α-Al2O3(0001) surface with quantitative Zn atoms as a surfactant should serve as the nucleation sites for the further formation of high quality films.更多还原