Photoelectric properties of Ti doped ZnO: First principles calculation

Abstract

Nowadays, the studies on absorption spectra and conductivities of Ti doped ZnO systems have presented distinctly different experimental results when the atom fraction of impurity increases in a range from 1.04 at% to 1.39 at% To solve this contradiction, all calculations in this paper are carried out by the CASTEP tool in the Materials Studio software based on the first-principals generalized gradient approximation (GGA) plane wave ultra-soft pseudopotential method of the density functional theory. The supercell geometric structures of ZnO, Zn0.9792Ti0.208O and Zn0.9722Ti0.278O systems are used as the calculation models. For all the geometry optimization models, the band structures, densities of states, electron density differences, population and absorption spectra are calculated by the method of GGA+U. The results show that with the Ti doping amount increasing from 1.04 at% to 1.39 at%, the lattice parameters and also the volume of the doping system increase. The higher the total energy of the doping system, the higher the formation energy of the doping system is, thereby making doping difficult and lower stability of the doping system. The increase of Ti-doping concentration weakens the covalent bond, but strengthens the ionic bond. As the Ti substitutional doping concentration increases, the Mulliken bond populations decrease, but bond lengths of Ti-O increase for the doping system Meanwhile, the higher the Ti doping content, with all the doping systems converted into n-type degenerate semiconductor the wider the band gap of the doping system will be and the more significant the blue shift of absorption spectra of Ti-doped ZnO systems. In this paper the mechanism of band gap widening is reasonably explained. In addition, the higher the Ti doping content, the higher the electronic effective mass of doping systems is The higher the electronic concentration of doping systems, the lower the electronic mobility of doping systems is. The lower the electronic conductivity of doping systems, the worse the doping systems conductivity is. The calculation results of absorption spectrum and conductivity of Ti-doped ZnO system are consistent with the experimental data. And the contradiction between absorption spectrum and conductivity of Ti-doped ZnO system in experiment is explained reasonably by temperature effect. In this paper, the comprehensive optical and electrical properties of Ti-doped ZnO systems are calculated by first-principals GGA+U method. And these results may improve the design and the preparation of photoelectric functional materials for Ti-doped ZnO at quite a low temperature.