Electronic structure of defects in III-VI and II-VI semiconductors and novel ytterbium-based intermetallics.

摘要

In recent years there has been a revival of interest in the III-VI family of semiconductors (GaS, GaSe, GaTe and InSe) due to their exciting nonlinear optical properties and their possible application in detector devices. These materials crystallize in layered crystal structure and their physical properties display a quasi two-dimensional character. An important characteristic of these systems is the existence of Ga-Ga (or In-In) dimers. It is well known that defects control the physical properties of semiconductors. In this thesis, we have carried out electronic structure calculations to study the nature of defect states in these materials. The defects we have studied include substitutional impurities at the cation and the anion sites as well as cationic and anionic vacancies. The failure of the hydrogenic effective mass approximation (EMA) to reproduce the experimental binding energies for the substitutional Cd and Sn defect states in GaSe, indicates the presence of large central cell corrections and the necessity of incorporating short range interactions in the calculation of defect binding energy. This has been done using a supercell model and self-consistent ab initio electronic structure method within density functional theory (DFT), which is known to be quite successful in tackling the problem of defects in semiconductors. Analyzing the defects from first-principles, we have been able to explain the detailed microscopic mechanism of the formation of Ga-site defects in GaSe and GaTe. When Ga is replaced by an impurity or when it is removed from the system to create a vacancy, the Ga dimer states can be strongly perturbed and this perturbation can give rise to defect states in the band gap.;Defect formation energy calculations, based on total energy differences between the pure and defect containing systems, can give valuable insight into the solubility of different impurities in a host compound. The formation energies of Ge and Sn impurities reveal that under Ga-rich growth conditions it is easier to incorporate Sn in GaTe, whereas in the Te-rich limit Ge becomes more soluble than Sn. This information can be used to reduce the large leakage current due to the presence of native acceptors (Ga vacancies) in GaSe and GaTe by Ge or Sn doping. Furthermore, the formation energy calculations provide information about the preferred location of an impurity inside the host lattice. Using this idea, we developed a model which explains the experimentally observed improvement in the mechanical properties of In doped GaSe. In p-type GaSe, In becomes positively charged and can occupy an interstitial site, improving drastically the shear rigidity of the layered material.;Using the same theoretical methods we have investigated the nature of H defects in CdTe. The formation energy calculations indicate that the ground state position of H inside the CdTe lattice depends on the charge state: the lowest energy positions for H0 and H+ is at the bond center site, while H- prefers the low electron density site surrounded by Cd cations. H in CdTe acts as an amphoteric impurity as expected. In the case of H on Cd site, the system undergoes Jahn-Teller distortion, due to the presence of a partially occupied degenerate t 2 state at the top of the VB. The symmetry of the system is lowered (the H atom moves closer to one of the four nearest neighbor Te atoms) and the t2 level is split by ∼74 meV at the Gamma-point.;In order to study the properties of strongly correlated systems, one has to go beyond the local density approximation (LDA)to the DFT and take into consideration the strong Coulomb interaction within the localized electronic shell. In this thesis we have used the LDA+U formalism to investigate the electronic, magnetic and structural properties of several Yb-base systems, which involve highly localized and strongly correlated f electrons. We find that the configuration of the f shell plays a crucial role in the physical properties of many Yb containing intermetallics.