【作者】 王婷霞；

【导师】 杨春；

【作者基本信息】 四川师范大学 ， 凝聚态物理， 2010， 硕士

【摘要】 在激光分子束外延(LMBE)生长BaTiO3(BTO)薄膜过程中,初期多粒子碰撞反应是薄膜形成的关键过程。本文利用密度泛函理论中的广义梯度近似(DFT/GGA),在PW91/DNP水平上分别对BTO原胞的形成机理,以及BTO薄膜形成初期的粒子(以BaO、TiO2和BTO分子为主)在Si基表面的吸附生长过程作了理论研究。所有计算均采用Materials Studio 4.0中的DMol3和CASTEP两个软件包来完成。BTO薄膜生长初期存在一个Ba、O、Ti粒子共存的高真空沉积气氛,计算这些粒子碰撞反应机理及其中间体的形成机理后,获得了相应中间体的几何结构、过渡态及反应活化能,并运用前线轨道理论分析了BTO分子形成的机理。研究表明在BTO薄膜生长初期会以TiO2分子为中心,结合BaO分子成核生长,并经由类似于BTO单原胞中的钛酸钡分子结构形成大量的具有钙钛矿结构雏形的BTO原胞。对BTO初期粒子(以BaO和TiO2分子为例)在Si基表面吸附生长的研究采用从头计算分子动力学方法。模拟计算得到吸附过程的运动轨迹及每个时刻的能量,并找出每个阶段的吸附能。通过吸附能大小的比较,发现TiO2优先于BaO分子在驰豫的Si基表面发生吸附,说明生长将以TiO2为中心。另外,利用Mulliken电荷布局和前线轨道理论分别对BaO分子和TiO2分子在Si基表面的吸附过程作了具体的成键分析,发现BaO分子的吸附过程中,O原子与基底表面的Si原子结合后,Ba—O键发生断裂;而TiO2分子的吸附过程中,Si原子先与其中一个O原子结合,再与Ti原子结合,形成以Ti为中心的稳定四面体结构。这些分析结果都与动力学过程相一致。最后,对于BTO分子在Si基表面的吸附情况,我们结合前面研究的BTO原胞形成机理,并对比分析BaO和TiO2分子吸附过程,得出这样的猜测:基底表面Si原子先与BTO分子中的两个O原子吸附,再与Ti原子发生结合,而BTO分子中剩下的一个O原子将在孤电子对的作用下靠近Ba原子,形成类似于金刚体的稳定结构。更多还原

【Abstract】 Finding out the processes of the microcosmic reactions and particle states in the early growth of BaTiO3 (BTO) thin films is very important for the preparation and control of good structure in the growth of BTO thin films. The precedence reacton processed of Ba, Ti, and O atoms, the formation of BaO and TiO2 molecules, and the microcosmic reaction mechanism underlying the formation of BTO, the BaO and TiO2 molecules adsorption on Si(001) surface have been investigated using a generalized gradient approximation of density functional theory (DFT) at the PE91/DNP level. And the DMol3 and CASTEP package of Materials Studio 4.0 are used for the calculations.It was found that BaO, TiO2, and BTO molecules are the primary particle states in the early growth of BTO thin films. As it acts as a combining center, the TiO2 molecule is preferentially formed. Subsequently, BaO molecules combin with this center and then BTO molecule, which is stable and somewhat similar to the BTO unit cell, is constructed. Due to the mechanisms, BTO thin films have been formed.The theoretical study of BaO and TiO2 molecules adsorption on Si(001) surface has been investigated using ab initio molecular dynamics method. We also have found the trajectory and energy of every moment in the process. At the time, the adsorption energys of corresponding stage has been calculated.By comparison the adsorption energy, we concluded that TiO2 molecule have been adsorbed growth priority than SrO molecule on the Si(001) surface.while TiO2 molecule have been adsorbed growth priority than SrO molecule on the SrO-SrTiO3(100) surface. In this instance, SrO and TiO2 layer are growth alternating layered cycle. The results are well in accordance with experimental.At the same time, the bonding process of SrO molecule on the TiO2-SrTiO3(100) surface have been investigated by using Mulliken, highest occupied and lowest unoccupied molecular orbital theory and density of states. Consistent with the results of kinetic studies, it indicated that Sr atoms in SrO molecule combine with O atoms in the surface priority. And then O atoms in SrO molecule combine with Ti in the surface, Sr-O bond was broken in SrO molecule.The bonding process of TiO2 molecule on the SrO-SrTiO3(100) surface has been investigated by the same method. Ti atoms in TiO2 molecule combine with O atoms in the surface combine first, and then O atoms in TiO2 molecule combine with Sr atoms in the surface.We can see that the final configurations of SrO molecule on the TiO2-SrTiO3(100) and TiO2 molecule on the SrO-SrTiO3(100) surface are similar with structure of SrTiO3 unit cell. It confirms that the model of SrTiO3 early growth is migration of unit cell and layer growth. In addition, the chemical bond of TiO2 and BaO molecules in the adsorption process of Si-based was analyzed respectively by using the Mulliken charge distribution and frontier orbital theory. It was found that in the adsorption process of BaO molecules, O atoms was connected with the Si atoms located on the surface of the substrate. Then Ba-O bond was broken; at the same time, the Si atom first was combined with one O atom, and then combined with the Ti atom, to form a stable Ti-centered tetrahedral structure, which are consistent with the dynamics.Finally, for BTO molecules in the adsorption process based on Si-based molecular, by using the BTO formation mechanism, and comparative analysis of BaO and TiO2 molecular adsorption process, it can be concluded that the Si atoms was first connected with the two O atomics which comes from the BTO, and then combined with the Ti atoms, while the remaining elements of a O atom will be influenced by the lone electron pairs near the Ba atom ro form a stable structure which is similar to the diamond.更多还原