Characterizing the Ultrafast Charge Carrier Trapping Dynamics in Single ZnO Rods Using Two-Photon Emission Microscopy

Zinc oxide has emerged as an attractive candidate for a variety of optoelectronic and photonic applications, due in part to a large second-order nonlinear susceptibility, its wide band gap, and large exciton binding energy. We have used time-resolved nonlinear two-photon emission microscopy to characterize the excited-state dynamics of individual ZnO rods. Photoluminescence images reveal a rich structure in the spatial distribution of both the band-edge and trap emission, and spectra recorded following excitation at a specific point in the structure show the characteristic band-edge and defect emission. Time-resolved emission spectra reveal a dynamic red shift of the trap emission band in the as-grown structures. Our results suggest that the trap emission is composed of at least two overlapping emission bands. The higher-energy band is assigned to e h recombination between a conduction band electron and photogenerated hole bound to an acceptor defect lying within the band gap. The lower-energy band is attributed to a donor acceptor pair (DAP) transition in which an electron localized on a donor defect recombines with a nearby hole-bound acceptor. The DAP transition energy and recombination rate depend upon the spatial proximity of the two traps, with higher-energy transitions corresponding to closely spaced pairs and occurring more rapidly, resulting in the dynamic red shift. Reduction in the trap density following annealing suppresses the DAP emission and its signature time-dependent red shift. The blue shift of the static photoluminescence spectrum is attributed to the donor defects being preferentially annealed out of the structure, resulting in a larger contribution of the higher-energy band.