Optical investigation of Zn1−xFexO films grown on Al2O3(0001) by radio-frequency sputtering

Abstract

Zn 1 − x Fe x O thin films were grown on Al2O3(0001) substrates by radio-frequency magnetron sputtering. The alloys show wurtzite crystal structure up to x=0.24 with reduced c-axis lattice constant compared to that of pure ZnO. Fe 2p core-level photoemission measurements reveal the evidence for the coexistence of the Fe3+ and Fe2+ ions substituting the tetrahedral the Zn2+ sites. The optical properties of the samples were measured by the spectroscopic ellipsometry at room temperature in the 1.5–5eV photon energy region. With increasing x, the optical band-gap (E0) absorption edge is found to shift slightly to lower energies (70meV for x=0.24) than that of the pure ZnO. Below the E0 edge, optical absorption structures are observed at about 1.7, 2.4, and 2.8eV. These structures are interpreted as due to the d-d transitions from the A16 ground state to the excited states, T14 (1.7eV), T24 (2.4eV), and E4 and A14 (2.8eV), of the crystal-field-split 3d5 multiplets of the tetrahedral Fe3+ ion.