| Due to the superior electronic and optical performances, transparent conducting oxide (TCO) film material is one of highlights of recent scientific research and industrial development. In this thesis, we systemically studied the affects of species, content and distribution of dopants on the properties of SnO2 type TCO films using the first principles calculations. The emphases concerned were the changes and mechanisms of the electronic and optical properties of these materials. In addition, considering the universality of strain in thin film and its strong impact on the properties, ZnO film was selected as a sample to investigate the influence of in-plane strain on the electronic, optical and related properties. The effects of lattice and atomic relaxation was clarified through comparisons with the classic linear elastic biaxial strain model.The traditional concept claimed that the limitation of Sb concentration in SnO2 was 20 at.%. However, the value in recent experiment has reached a maximum of 85 at.%, which can't only been reasonably explained with the traditional concept, but may also indicated the probability of obtaining TCO materials in a larger range of Sb concentration. In our calculations using the CASTEP module based on the density function theory, the influences of Sb content on the structural, electronic and optical properties of SnO2:Sb were studied in details. It's found that with increasing Sb content, the lattice expands with different deviations in the lattice constant. The calculations on thermal stability suggest the probability of a continuous solid solution of the SnO2:Sb system in the content range of 0~100% for Sb element. With elevated Sb concentrations the semiconductor-metal- semimetal transition occurs, along with the changes in the electronic structure and optical properties. The many body effect and the atomic symmetry play the essential role in these transitions.Due to the different radii and electronic configurations of Sn and In atoms, the lattice of SnO2:In expands and the structure distorts with increasing In content. According to the thermal stability calculation, the preferred In3+ distribution is to occupy the Sn sites in different (110) slabs. In dopant brings about an acceptor band located slightly above the Fermi level, resulting in the small p-type conductivity taking into account of the large effective mass of electron holes in the valence band maximum. The stable conduction band and the bandgap lead to the unaltered optical spectra of SnO2:In in the ultraviolet-visible region, while the transition from the occupied levels to the empty band near Ef is considered to interpret the dramatic rise in the dielectric function, reflectivity and absorption in the infrared region. In our calculations the reflectivity and absorption will be doubled if the content of In increases 12.5 at.% in the range of 0~25 at.%.The calculations on SnO2:F with different F contents and distributions show that, with increasing F content in the range of 0~12.5 at.%, the lattice expands and the thermal stability decreases, but the segregation of F atom will increase the stability energetically. The F leads to the apparance of a new narrow band composed by F 2s orbitals in the original bandgap, causing the increase of electric conductivity. The SnO2:F thin film with the minimum F interspace possesses the best Low-E performance. Due to the great electronegativity of F atom, the content of polar chemical bond increases, leading to improvment of combination of thin film with the glass substrate.The calculations on the deformation of wurtzite ZnO under in-plane stress indicate that, free lattice and atom relaxation leads to the plastic relaxation. Comparing with the classic linear elastic deformation behavior, this peculiar plastic relaxation results in a metastable state with larger thermal stability, smaller varieties in lattice volume and Poisson ratio and no stress in any direction. The plastic relaxation reduces the piezoelectric effect and ultimately decreases the energy conversion efficiency. The Eg atΓpoint decreases with the absolute value of strains, while the inhomogeneity in charge distribution between the two kinds of Zn-O bonds is reduced under tensile strain but enlarged under compressive strain. The variation is smaller in plastic relaxation than the elastic one under the same in-plane strain. |
