First-principle Study On The Electronic Structure And Optical Properties Of SnO₂ Based Semiconductors

Tin oxide is a new wide gap IV-VI semiconductor with a direct band gap of 3.6eV and a large exciton binding energy of 130meV. SnO2-based semiconductors have become the promising photonic material because of this large exciton binding energy, and therefore they are widely used in many fields of opto-electronic devices, flat-panel displays, and in solar cells or organic semiconductors as effective electrodes. In this paper, by using the full-potential linearized augmented plane-wave method (FP-LAPW) based on the first-principles of density functional theory (DFT), we have performed the whole work of calculating SnO2 based semiconductor, within the generalized gradient approximation (GGA) for the exchange and correlation effects in the framework.First of all, we present the electronic structure and optical properties of SnO2 eigenstate. From the calculated density of the states (DOS), bandstructure and mult-optical properties, we know that the total DOS consists of the partial states of Sn 5s, Sn 5p, O 2s and O 2p states. Besides the reflectivity, extinction and other optical properties are corresponding to the imaginary part of the dielectric function related to the transition of electrons. It is pointed that there are relationships between electronic structure and optical properties in theory, and we compared them with some references, so it is hoped that our present work may inspire experimental research and theory calculation in future.Secondly, we have investigated the electronic structures and the optical properties including dielectric function and absorption spectra of Sn1-xAlxO2 (x= 0, 0.0625, 0.125, 0.1875, 0.25) via substituting for Sn with Al in a 2×2×2 SnO2 supercell. Calculated results show that the introduced Al incorporation could induce the band gap widening, and this can be attributed to the increased folded states, which lead to the contraction of the valence band and the less Sn 5s states in the bottom of the conduction band. With the increase of Al concentrations, both the imaginary part of dielectric function and the absorption spectrum have taken blue shift corresponding to the change of the band gaps. The III metal elements Ga and In doped in SnO2 supercell also have been calculated, and a comparison has been made to the properties of Al doped to get a meaningful and comparable results. Thirdly, the isosurface of spin density,total density of states and optical properties of substitutional N to O and substitutional N to Sn in SnO2 have been given, and also there it has been compared to the situation of undoped SnO2. The results show that the band gaps in two kinds of N-doped SnO2 are wider than that of SnO2 eigenstate, and the imaginary part of the dielectric function makes a blue shift corresponding to the increscent band gaps. The refraction spectrum is related to the imaginary of the dielectric function, and the absorption edge is decided by the band gap indicating the transition capability of electrons on the maximum of the valence band to the bottom of the conduction band.Finally, we select In and N as the co-doped ions respectively substituting Sn and O atoms, and change the concentration of N from one atom to two to calculate the two co-doped crystals. While the third and fourth chapter about the In-doped and the N-doped cases have been compared, it is found that there has been a strong part of the very thin local band in -11 ~ -12 eV energy range for both of the co-doped structures. And in the band gaps, the spin-down direction also forms local levels, indicating that now the two formed co-doped compounds also have half-metallic nature. Through the incorporation of activated In donor element, it can effectively improve the acceptor doping concentration of doping elements N and improve the stability of the doped system. Imaginary part of dielectric spectra, mainly in the vicinity of 8.58 eV there is a main peak size of 7.64, and this main peak intensity significantly increased and has made a blue shift. So it is indicating that the doped N 2p states and In 5s states have taken a strong interaction between states, which reduced the acceptor energy levels and increased the donor levels.