Spatially Resolved Dynamics of Cobalt Color Centers in ZnO Nanowires by Nanoscale X-Ray Analysis

Color centers in semiconductors provide properties of great importance for quantum technologies. Their most prominent characteristics are the capability to act as qubits and to offer high quality single photon sources. A promising wide-bandgap semiconductor material for defect-related emission in the visible range is zinc oxide. Creation of color centers in zinc oxide can be achieved by doping it with rare earth elements or transition metals, such as coper, nickel, iron or cobalt. The transition metals provide particularly fast decay times and therefore fast responses. The potential of a coupling platform between single photon emitters and photonic circuits can additionally be gained by embedding such color centers into nanowires, because they provide waveguiding and a cavity for the emitted photons. Investigating the photocarrier dynamics, with respect to the local environment, in such a system is thus of great importance. We apply high spatial resolution synchrotron-based methods to evaluate how the carrier dynamics and the luminescence is influenced by the elemental composition and the local environment of cobalt centers in zinc oxide nanowires. Such systems were previously investigated by different techniques such as photo- or cathodoluminescence measurements. However, these techniques lack crucial complementary information, e.g. spatial or compositional variations of the system, respectively. Simultaneous spatially resolved measurements of the X-ray fluorescence and the X-ray excited optical luminescence provide both information at once. Our findings show an anti-correlation between the band edge emission of the zinc oxide host and the intra-3d cobalt luminescence indicating two competing recombination paths. The luminescence dynamics are temporally resolved with a streak camera providing unique insights into the different decay paths of the intra-shell luminescence of the cobalt atoms incorporated into the lattice of zinc oxide nanowires. Two different exponential decays of the cobalt-related emission are revealed. A fast and newly observed decay is attributed to a recombination cascade within the cobalt atom, resulting from direct excitation.