Effects of Al-Impurity Type on Formation Energy, Crystal Structure, Electronic Structure, and Optical Properties of ZnO by Using Density Functional Theory and the Hubbard-U Method

We systematically investigated the effects of Al-impurity type on the formation energy, crystal structure, charge density, electronic structure, and optical properties of ZnO by using density functional theory and the Hubbard-U method. Al-related defects, such as those caused by the substitution of Zn and O atoms by Al atoms (Al(s(Zn)) and Al(s(O)), respectively) and the presence of an interstitial Al atom at the center of a tetrahedron (Al(i(tet))) or an octahedron (Al(i(oct))), and various Al concentrations were evaluated. The calculated formation energy follows the order E(f)(Al(s(Zn))) < E(f)(Al(i(tet))) < E(f)(Al(i(oct))) < E(f)(Al(s(O))). Electronic structure analysis showed that the Al(s(Zn)), Al(s(O)), Al(i(tet)), and Al(i(oct)) models follow n-type conduction, and the optical band gaps are higher than that of pure ZnO. The calculated carrier concentrations of the Al(s(O)) and Al(i(tet))/Al(i(oct)) models are higher than that of the Al(s(Zn)) model. However, according to the curvature of the band structure, the occurrence of interstitial Al atoms or the substitution of O atoms by Al atoms results in a high effective mass, possibly reducing the carrier mobility. The average transmittance levels in the visible light and ultraviolet (UV) regions of the Al(s(Zn)) model are higher than those of pure ZnO. However, the presence of an interstitial Al atom within the ZnO crystal reduces transmittance in the visible light region; Al(s(O)) substantially reduces the transmittance in the visible light and UV regions. In addition, the properties of ZnO doped with various Al(s(Zn)) concentrations were analyzed.