Distinct roles of Mg and Li in affecting the electro-optic properties of wurtzite ZnO thin films

Abstract

ZnO-based thin films unfold unique potentials for compact and ultrafast electro-optic (EO) modulation with low power consumption, because they show a strong Pockels effect, low loss of light propagation, and moderate permittivity. This work clarifies the distinct roles of Mg and Li incorporation in affecting the effective Pockels EO coefficients (rc) of wurtzite ZnO thin films by deeply analyzing the lattice distortion and charge density distributions. Mg incorporation significantly enhances internal parameter u and reduces the anion–cation bond lengths along the c-axis direction. This facilitates Zn displacement along the c-axis direction and increases electric susceptibility, enhancing the ionic contribution to rc. Li incorporation produces slight lattice expansion and increases anion–cation bond lengths along the c-axis direction. First-principle calculations show that Li interstitials in oxygen octahedron markedly change the charge density around the nearest-neighbor O. With the evidence of enhancing the second-order nonlinear susceptibility under the optical frequency electrical field, Li incorporation improves rc by enhancing electronic contribution. The Li incorporated Zn0.7Mg0.3O thin film, with a thickness of 400 nm and an optical bandgap of 3.99 eV, exhibits a rc of 2.85 ± 0.01 pm/V. These findings uncover the great potential of widegap ZnMgO:Li thin films for electro-optic modulation in ultraviolet band.