Institute of Fundamental Technological Research

Virgin and Yb-implanted epitaxial ZnO films grown using atomic layer deposition (ALD) were investigated by X-ray absorption spectroscopy (XAS). XAS study revealed a strong polarization dependence of films determined by the orientation of the polarization vector of the synchrotron radiation to the sample surface. It also indicated that the implantation and subsequent annealing have an important influence on the native point defect complexes in the ZnO. Comparison of experimental spectra with the modelled ones, which are computed based on the linear combination of model spectra corresponding to the selected point defects and their complexes, confirmed the presence of donor-acceptor complexes (mVZn - nVO, m = 1,4; n = 1,2) in the samples under study. The mechanism of vacancy complexes formation is unclear as it takes place under non-equilibrium conditions, for which any theoretical method has not been well established. Exploring the 3 d → 4 f absorption, it was found that oxidation state of Yb in ZnO is 3+, which is consistent with the XPS findings and previously conducted Resonant Photoemission Spectroscopy (RPES) investigations. The inversion of the polarization dependence for samples with different Yb fluences visible in Yb M5 spectra can be associated with a tilt of the oxygen pseudo octahedra or/and with their distortion. The analysis of the presented data suggests that the donor-acceptor complexes are present both in as grown and implanted films and may influence their electrical properties. This suggestion was confirmed by previous Hall measurements showing that the resistivity of annealed ZnO:Yb film with a fluence of 5e15 ions/cm2 decreases by about one order compared to the one with a fluence of 5e14 ions/cm2.

Resonant photoemission spectroscopy (RPES), which is a useful tool for extracting photoemission response of the localized Rare Earth (RE) impurity levels from the host electronic band structure, was used to study ZnO:Yb films. The resonant enhancement of the photoemission signal at binding energy around 7.5 and 11.7 eV was observed when photon energy was tuned to the Yb 4d-4f absorption threshold (182 eV). It was found that the 4f and the valence band (VB) maximum binding energies do not depend on the Yb dose, suggesting that the measurement of only one concentration is sufficient to determine the binding energies of the Yb 4f in the examined system. Subsequent annealing did not change the arrangements of implanted ytterbium atoms in host matrix: the majority of them remain in 3 + state having pseudo-octahedral local arrangement similar to Yb2O3.

The electronic structure of Yb implanted ZnO has been studied by the resonant photoemission spectroscopy. The contribution of the Yb 4f partial density of states is predominant at binding energy about 7.5 and approximate to 11.7 eV below the VB maximum. At photon energy about 182 eV the multiplet structure around 11.7 eV shows the strongest resonance that corresponds to the I-1 multiplet which is almost exclusively responsible for this resonance, while H-3 and F-3 states are responsible for the resonance around 7.5 eV. It was also found that the Yb 4f partial density of states distribution shows some similarity to Yb2O3.