Characterization of lattice defects by x-ray absorption spectroscopy at the Zn K-edge in ferromagnetic, pure ZnO films

Ferromagnetic, pure ZnO films were grown on Al2O3 substrates at various nitrogen pressures (0.01–1.0 mbar) and investigated with x-ray diffraction (XRD) and x-ray absorption spectroscopy. According to XRD data, the crystalline films were composed of crystallites of approximately 50 nm in size, oriented with respect to the substrates, and the lattice spacings show slight deviations with respect to single-crystalline ZnO of wurtzite structure. The parameters determined by XRD agree with those determined by extended x-ray absorption fine structure, except for the sample grown at the lowest N2 pressure of 0.01 mbar, which was attributed to deviations from the ZnO single crystals. The results for the ZnO films grown at 0.1 to 1.0 mbar partial N2 pressure indicate wurtzite unit cells compressed along the c axis. The x-ray absorption near-edge structure (XANES) spectra exhibited a strong dependence on the x-ray polarization and on nitrogen partial pressure, which was explained by the increase in the concentration of defects with nitrogen partial pressure and by interface or grain boundary effects. First-principles calculations using multiple-scattering formalism suggested that the XANES spectra changes were because of increasing Zn vacancy concentration with increasing nitrogen pressure. The results indicated that Zn vacancy defects play a significant role in the ferromagnetism of these films.