Computational Design Of Si-based Optoelectronic Materials And Diluted Magnetic Materials

As well known, silicon is the dominant material for microelectronics and the fabrication technology is quite mature. However, due to its indirect band-gap, the light emission can occur only when accompanied with an emission of phonons and it has been considered unsuitable as bulk material for optoelectronic applications. Using Si-based nanostructures, this problem can be solved partly as has been recently reported. In conventional electronic structure theory of solids, about the band-gap type, direct gap or indirect-gap under a given crystal structure, the answer is always provided after the electronic structure calculations. As a result, a great challenge to experimenters and theorists is that how to make or design an efficient Si-based light emission material using an advanced technology and physics principles.As one known, the band structure of solid depends on the crystal symmetry, this is a fact known for a long time. Based on a statistical analysis for the band gap type and crystal symmetry, we find that all of the semiconductors with Oh point group symmetry have an indirect gap property, whereas those of C6v symmetry have a direct gap, and the semiconductors with Td symmetry are laid between them. The results indicate clearly that the symmetry reduction (i.e., decreasing a group order) will be advantageous to develop a direct band-gap semiconductor. In order to reduce symmetry, there are two conventional methods can be provided to selection: one is the atomic substitute method, i.e., substitute other atoms for silicons in Si lattice. Another method is so-called the intercalation growth, in which the intercalation atoms are located on a same crystal plane in silicon lattice. With the consideration of the size of core states and the electro-negativity difference between the component atoms in solids, we design several kinds of Si-based superlattices (SLs). The results show that some of them are direct band-gap semiconductors.On the other hand, diluted magnetic semiconductors (DMSs) have stimulated a great deal interest because of their potential application in the spintronics, in which the electron spin becomes another degree of freedom in addition to the usual charge degree of freedom. Ferromagnetism (FM) in the MnxGe1-x compound has been reported and the Curie temperature is up to 285K. Moreover, a recent calculation where both MnxGe1-x and MnxSi1-x were studied declared that it were possible to make room-temperature FM MnxSi1-x. However, the origin of the FM in IV semiconductors is still a matter of debate, and several different mechanisms, such as the competition between a long-range FM interaction and a short-range AFM interaction, mean field theory, Ruderman-Kittel-Kasuya-Yoshida (RKKY), have been proposed. In the dissertation, a similar study is extended to the different transition metals (TM). Comparison between these DMSs allows us to investigate variations of magnetic properties with a change of dopants and their site locations. The Curie temperature is also predicted by RKKY theory.This dissertation consists of two parts. The first part presents the calculation methods and its theoretical basis. We firstly introduce the density functional theory (DFT), including Hobenberg-Kohn theorem, Kohn-Sham equations, the approximations for exchange and correlations and GW approximation. And then, we describe the details of computational methods used in our work, i.e., the ab initio pseudopotential methods with the plane wave basis expansion of the wavefunction. We also present the major characters of the Vienna ab initio Simulation Packages (VASP). The second part which divided into three chapters presents the main results of the present dissertation.In the third chapter, ab initio pseudoptential method has been employed to investigate the electronic properties of Si-based SLs designed by the intercalation growth method, which consisted of one monolayer inserted atoms, such as group-VI, group-V or group III and V atoms, along the Si(001) direction periodically. The different surface reconstruction models, (i.e., (2x1) and (2x2)), were also discussed. The results show that the SLs Se/Si5/VI/Si5/Se and Se/Si6/VI/Si6/Se (VI=O,S,Se) have a direct band-gap atГpoint. Furthermore, the band structure of Se/Si5/VI/Si5/Se with a (2x2) surface structure is better than that of Se/Si6/VI/Si6/Se with a (2x1) structure. In the fourth chapter, we presented computational design of SLs Si1-yXy/Si (X=C,Ge,Sn,Ti,Zr; y=0.125,0.25,0.5) using the atomic substitute method. Our calculations reveal that the SLs Si1-ySny/Si (y=0.125) and Si1-yGey/Si (y=0.125,0.25) are theГ-point direct band-gap semiconductors. Based on the analysis of those SLs mentioned above, we draw some conclusions that the symmetry reduction, accompanied with a bigger core states of inserted atoms and less electronegativity difference between the component atoms, will be advantageous to develop a direct band-gap semiconductor.In the fifth chapter, the magnetic properties of TM-doped Si or Ge (TM=V,Cr,Mn,Fe,Co, Ni) has been studied. It is believed that the magnetic properties on Cr-, Mn- and Fe-doped DMSs is better, especially for Mn. Furthermore, we investigated the variations of magnetic properties on Cr-, Mn- and Fe-doped DMSs respectively, with a change of their site locations. The main results can be described as below. For the Cr-doped DMSs, the AFM order is energetically more favored than the FM order. For the Mn-doped Si DMSs can be explained by the competition between a long-range FM interaction and a short-range AFM interaction, and for the Mn-doped Ge DMSs are more like a RKKY FM semiconductors. For the Fe-doped DMSs, we found the higher Fe concentrations and larger lattice constants will enhance the ferromagnetism.