ZnO-Nanodrähte: Optische Eigenschaften und Ladungsträgerdynamik

Abstract

Nanostructures of ZnO have attracted much attention in recent years since they can be easily fabricated reproducibly and may be used as building blocks for nanometer-scale electronic and photonic devices operating in the blue to UV spectral region. Especially, self-assembled nanowires offer a large variety of peculiar properties. In this work the optical properties and the carrier dynamics of individual ZnO nanowires and of nanowire ensembles are systematically studied in time-integrated and time-resolved micro-photoluminescence experiments.Based on optical measurements performed on the as-grown nanowire ensembles the observed emission lines and bands are clearly assigned to excitonic recombination processes. Due to the wire diameter which is much larger than the exciton bohr-radius no confinement occurs and their optical properties are similar to those of the bulk material. At high excitation intensities the p-band was absent mainly due to resonator effects in the wires.To address individual ZnO nanowires with diameters in the range of 90nm-620nm single wires are prepared from the as-grown ensemble. It is shown that the majority of nanowires in the ensemble are of good crystalline quality and thus easily performed ensemble measurements reflect the properties of typical single nanowires. For the first time, a broad and asymmetric excitonic surface-related emission is observed at 3.367eV (SX) in ZnO nanowires. This clear assignment to the near surface region is based on the observed saturation behaviour at high excitation intensity and the increasing contribution in the optical spectrum of individual wires with decreasing diameter. A first rough estimation yields to a surface-layer thickness of a few nanometers.For nanowires with diameters in the range of 40nm-130nm the SX emission strongly dominates the near band-edge emission. To study the dynamics of the SX emission band in detail transients are measured as a function of spectral position, temperature, and excitation intensity. The measured transients point to a biexponential decay. A henomenological rate-equation model is developed and discussed. The observed dependence of the transients on the spectral position is explained by the relaxation and decay of excitons separated into two fractions of weakly and strongly localized excitons. Furthermore, the weakly trapped excitons are activated with increasing temperature into less localized near surface centers. With increasing excitation intensity a distinct saturation behaviour of the SX band is observed in time-integrated and time-resolved measurements which is clearly related to the limited number of localization centers in the near surface region.In addition, a further narrow surface-related emission line is observed on the low-energy side of the SX emission showing a different temperature dependence with respect to the SX band and a monoexponential decay. It is assumed that this line is caused by recombination of strongly localized excitons. The dynamics of this line point to a connection with the SX recombination and relaxation processes.Moreover, the influence of ion implantation on the optical properties of ZnO bulk crystals and nanowires is studied. After implantation for bulk crystals a donor-acceptor-pair transition is found and clearly assigned. Opposite to this, for nanowires only additional sharp emission lines in the near band-edge region are observed caused by implantation induced temperature stable defects.