Electronic and Optical Properties of Substitutional and Interstitial Si-Doped ZnO

This study investigates the formation energies, electronic structures, and optical properties of pure and Si-doped ZnO using density functional theory and the Hubbard U (DFT + U-d + U-p) method. The difference in lattice constants between calculated results and experimental measurements is within 1%, and the calculated band gap of pure ZnO is in excellent agreement with experimental values. This study considers three possible Si-doped ZnO structures including the substitution of Si for Zn (Si-s(Zn)), interstitial Si in an octahedron (Si-i(oct)), and interstitial Si in a tetrahedron (Si-i(tet)). Results show that the formation energy of Si-s(Zn) defects is the lowest, indicating that Si-s(Zn) defects are formed more easily than Si-i(oct) and Si-i(tet). All three of the Si defect models exhibited n-type conductive characteristics, and except for the Si-i(oct) mode the optical band gap expanded beyond that of pure ZnO. In both the Si-i(oct) and Si-i(tet) models, a heavier effective mass decreased carrier mobility, and deeper donor states significantly decreased transmittance. Therefore, the existence of interestitial Si atoms was bad for the electric and optical properties of ZnO.