Optische Eigenschaften von ZnO-Nanodrähten:Einfluss von Oberflächenbehandlungen und hohen Anregungsdichten

Abstract

ZnO nanowires are promising building blocks for optoelectronic applications. Lasers andlight-emitting diodes fabricated from ZnO nanowires will emit in the blue and near-UVspectral region due to the wide band gap of ZnO. Their large refractive index makes themuseful as active waveguides in optoelectronic devices. In addition to the near UV emission,ZnO can show several defect related emission bands in the visible spectral region. By carefullydoping the material, these defect bands can be tailored to generate a white-light spectrumwhich will be beneficial for applications as light emitting diodes. Much research interestfocused on the various defect bands in the past years. Previous work of the author on ZnOnanowires embedded in polymer matrices shows a strong decrease of the defect luminescencefor the embedded samples, combined with a stronger surface exciton emission band.In this work, the dependence of the defect and near-band-edge luminescence of ZnO nanowireson surrounding materials necessary to fabricate nanowire-LED devices is investigated. First,coating of the nanowires with a dielectric shell of amorphous Al2O3 is found to stronglysuppress the defect luminescence while enhancing the surface excitonic emission. This effectis similar to the previously investigated polymer samples and is explained by a model basedon the dielectric properties of the material. A dielectric coating shields the surface states andreduces the band bending typically observed for the ZnO nanowire surfaces. The band bendinginfluences the activation mechanism of deep centers which cause the defect luminescence andredistributes the excitons near the surface in favour of the surface excitonic recombinationmechanism. As a next step, the influence of metallic coatings on the optical properties ofZnO nanowires is investigated and found to result in an opposite behaviour compared to thedielectric coating. An increased defect luminescence and a reduced surface emission bandare observed, completely independent of the work function of the metal. These findings areexplained by the formation of metal-induced gap states at the nanowire surface, which trapexcitons close to the surface.The interaction between plasmonic resonances of metal nanoparticles and excited states ofsemiconductors has attracted much attention in the research community. The use of a plasmasputter-coater is one method to deposit such metal nanoparticles. In this work, experimentalevidence is presented which shows that the plasma of such a system itself changes theoptical properties of ZnO nanowires by implantation of hydrogen into the crystal. The defectluminescence is weakened while the near-UV emission is enhanced, but does not show astronger surface excitonic emission.The lifetime of the charge carriers excited by a fs laser is investigated in pump-and-probeexperiments on ZnO bulk surfaces. Even at excitation densities below the lasing threshold, anelectron-hole-plasma forms with lifetimes of several tens of picoseconds. In order to investigatethe lasing properties of single ZnO nanowires, a setup based on the variable-stripe-lengthmethod is developed and used to determine the modal gain of single nanowires. When excitingwith a fs laser, modal gain values of up to 4900 cm−1 are found which are in good agreementwith theoretical predictions. The gain is found to depend on the nanowire diameter.