Electronic structures and ferroelectric properties of Ba-doped ZnO

Abstract

Wurtzite ZnO has long been considered to be a promising candidate material for photovoltaic application due to its high power conversion efficiency. More interestingly and very recently, some research results suggested that the ferroelectric property of the photovoltaic material introduced by chemical elements doping can promote its power conversion efficiency significantly. Therefore, in order to understand the effect of Ba doping on the electronic structure and the ferroelectric properties of ZnO and to reveal the potentially optoelectronic properties of Zn1-xBaxO, the energy band structure, the density of states, and the polarizability and the relative dielectric constant of the bulk Ba-doped ZnO supercell system, in which the Zn atoms are partly and uniformly substituted by the Ba atoms, are investigated by using the first-principles method based on the density functional theory and other physical theory. The norm-conserving pseudopotentials and the plane-wave basis set with a cut-off energy of 600 eV are used in the calculation. The generalized gradient approximation refined by Perdew and Zunger (GGA-PBE), the local density approximation (LDA) and the local density approximation added Hubbard energy (LDA+U) are employed for determining the exchange-correlation energy respectively. Brillouin zone is set to be within 4×4×5K point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 2.0×10-6 eV/atom. Additionally, in order to obtain a stable and accurate calculation result, the cell structure is optimized prior to calculation. The calculated results suggest that the bulk Ba-doped ZnO semiconductor system is still a semiconductor with a direct wide band gap. The band gap of Zn1-xBaxO increases gradually with Ba atom doping percentage increasing from 12.5% to 87.5%. Consequently, the ferroelectric polarization properties and the dielectric properties of the bulk Ba-doped wurtzite ZnO materials are tailored by doping Ba atoms. It indicates that the polarizability of Zn1-xBaxO system increases with Ba doping atomic percentage increasing, especially, the polarizability reaches to a maximum when the atomic percentage of doping is 75%. Meanwhile, the relative dielectric constant inversely decreases with Ba atomic percentage increasing. This is attributed to the effective contribution of Ba atoms to the density of state at the bottom of the valence band. The diagonalized components of polarizability imply that there are possible micro-domains in the supercell while applying externally electric field to it. And the supercell presents a nearly isotropic polarizability macroscopically due to the strong interaction among the electric dipole moments existing in the different domains.