Local structure of Co-implanted ZnO nanowires

Co-doped ZnO nanowires offer unique advantages for spintronics applications owing to their large geometrical aspect ratio and predicted room-temperature ferromagnetism [1]. However, controlled doping of semiconductor nanowires with transition metals during growth remains a challenge. Ion implantation is an alternative to doping that allows a better control of both concentration and distribution of transition metal ions. It is important to know the doping homogeneity and local order of individual nanostructures in order to understand their behaviour in nanodevices. The average local atomic structure and secondary phases in ensembles of nanowires have already been studied by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) [2]. However, the examination of single nanowires remains an experimental challenge mainly due to the beam instability during the energy scan and chromaticity of the nanofocussing lens. In this work, we have used a 100 x 100 nm² monochromatic X-ray beam at station ID22NI to explore the short-range order in single Co implanted ZnO nanowires. In addition, the linear polarisation of the synchrotron nano-beam enabled us to detect preferentially oriented defects induced by the ion implantation process. Also noteworthy, the spectroscopic techniques used in this study avoided averaging effects observed in the local order characterisation of ensembles of nanostructures.

Figure 1a shows the scanning electron microscopy (SEM) image of a single nanowire. The positions indicated by 1, 2, and 3 correspond to the areas where XANES and EXAFS spectra were recorded. The elemental map of Co with the respective concentration estimated from the X-ray fluorescence (XRF) quantification is displayed in Figure 1b, showing a homogeneous distribution of Co along the nanowire without any signature of clusters or nano-aggregates. XANES data from the nanowire with its c-axis oriented perpendicular and parallel to the polarisation vector of the X-ray beam are shown in Figures 1c and 1d, respectively. The spectra exhibit the peaks associated to the hexagonal structure, without any evidence of lattice damage in the nanowire.

Figure 1. (a) SEM image of a single Co-implanted nanowire. (b) Elemental map collected at 12 keV for Co with the respective atomic fraction estimated from the XRF quantification. Zn K edge XANES spectra recorded at various points along the nanowire with the c-axis oriented perpendicular (c), and parallel (d) to the electric field vector of the X-ray nanobeam.

The chemical state of the implanted Co ions was investigated by nano-XANES. Figure 2a shows the XANES spectra around the Zn K edge (solid circles) and Co K edge (open circles) taken at points 1 and 2. Despite the low Co content, the quality of the XANES data around the Co K edge is good and reproduces well the oscillations of the Zn K edge spectra at both points, suggesting Co ions incorporated into the wurtzite host lattice on the Zn sites. Furthermore, the good match between the XANES spectra of the nanowire and that of a high quality wurtzite Zn_0.9Co_0.1O epitaxial film [3] (not shown), suggests oxidation state 2+ for implanted Co ions. Finally, nano-EXAFS measurements around the Zn K-edge allow us to study the local order of the host lattice. Figure 2b shows the magnitude of the Fourier transforms of the EXAFS functions. The spectra show two dominant peaks related to the first O- and second Zn-shells. In general, there is no evidence of amorphisation, and the Zn-O and Zn-Zn spacings remain equal to those of pure ZnO (1.98 and 3.25 Å) in the nanowire. This confirms perfect recovery of the radiation damaged ZnO lattice through the thermal annealing (within the sensitivity of our experimental techniques).

Figure 2. (a) XANES spectra around the Zn K edge (solid circles) and Co K edge (open circles) taken at points 1 and 2. For comparison between Zn and Co XANES spectra, the energy has been rescaled to the respective absorption K edges calculated from the first derivative of the XANES signal. (b) Magnitude of the Fourier transforms of the EXAFS functions (open symbols) and their best fits (solid lines).

In summary, we have carried out nano-X-ray absorption spectroscopy in different regions of single nanowires. Our findings revealed implanted ions incorporated homogeneously in the nanowire as Co²⁺ and occupied Zn sites into the host lattice. The radiation damage in the ZnO host lattice was completely recovered through thermal annealing.

Principal publication and authors
Nano-X-ray absorption spectroscopy of single Co-implanted ZnO nanowires, J. Segura-Ruiz (a), G. Martinez-Criado (a), M.H. Chu (a), S. Geburt (b), and C. Ronning (b), Nano Lett. 11, 5322–5326 (2011).

References
[1] Y.W. Heo, D. Norton, L. Tien, Y. Kwon, B. Kang, F. Ren, S. Pearton, J. LaRoche, Mater. Sci. Eng. R 47, 1–47 (2004).
[2] B.D. Yuhas, S. Fakra, M.A. Marcus, P. Yang, Nano Lett. 7, 905–909 (2007).
[3] A. Ney, K. Ollefs, S. Ye, T. Kammermeier, V. Ney, T.C. Kaspar, S.A. Chambers, F. Wilhelm, A. Rogalev, Phys. Rev. Lett. 100, 157201 (2008).

Top image: Location of Co atoms within a Co-implanted ZnO nanowire: elemental map for Co overlaid on SEM image (atomic fraction estimated from the XRF quantification).