Electronic characteristics of Metal/Insulator Super-lattice, based on the first principle electronic structure calculation

基于密度泛函理论和FLAPW方法，对金属Ti/绝缘体MgO超晶格进行了第一原理电子结构计算，首先通过计算总能量的比较确定了超晶格界面原子的组态。计算结果表明，Ti原子与O原子在界面处接触。基于这种界面原子组态，计算了超晶格的电子能带结构。结果表明，存在两种电子态：一种是主要存在于Ti或MgO层中的受量子限制效应影响的准二维电子态;另一种是同时存在于Ti和MgO层中的具有有限能量色散的态。在绝缘体MgO层的带隙内，Ti层中也存在金属态，这些金属态构成了金属Ti层中的准二维电子态.根据MgO的电子结构和实验带隙，得到了Ti/MgO超晶格的能带图，它显示了Ti层和MgO层中导电子态和价电子态的相对位置.此外，计算的电子能带结构用于计算隧穿电流，即，在金属Ti/绝缘体MgO超晶格中测量了X射线光电子能谱（XPS），并讨论了XPS与价带态密度的关系。刻蚀前和刻蚀5分钟后的两个谱图显示出差异，这反映了MgO层和Ti层中的电子态，与理论态密度很好地吻合。这个XPS实验被认为是显示我们的理论电子结构计算的可靠性。

Based on the density functional theory and the FLAPW method, the first principle electronic structure calculation has been performed for the metal Ti/Insulator MgO Superlattice.At first, atomic configuration in the superlattice interface is determined from the comparison of calculated total energies. The calculated result shows that Ti atom is in contact with the 0 atom at the interface. Based on this atomic configuration at the interface, electronic band structure of the superlattice is calculated. It is found that there are two kinds of electronic states.One is the states, which exist mainly in only the Ti or MgO layer and receive the quantum confinement effects (quasi-two dimensional electronic states). Another is the states which exist in the both Ti and MgO layers and have the finite dispersion for energy. Also there are metallic states in a Ti layer within the band gap in an insulator MgO layer and then the metallic states constitute the quasi two-dimensional electronic states in a metallic Ti layer.Based on the electronic structure and the experimental band gap in MgO, band diagram of Ti/MgO superlattices, which shows the relative position of the conduction and valence electronic states in Ti and MgO layers is obtained.Also, calculated electronic band structure is used to calculate a tunneling current, i.e., a tunneling of an electron between two metallic Ti layers through the thin MgO layer under an electric field.Xray photoelectron spectra (XPS) has been measured in the metal Ti/insulator MgO superlattice and has been discussed in relation to the valence band density of states. Two spectra before etching and after 5-minutes etching show the difference, which reflect the electronic states in MgO layer and Ti layer, being in good agreement of the theoretical density of the state. This XPS experiment is considered to show the reliability of our theoretical electronic structure calculation.