Optical properties of single semiconductor nanowires and nanowire ensembles

This thesis presents a detailed investigation of the optical properties of semiconductor nanowires (NWs) in general and single GaN NWs and GaN NW ensembles in particular by photoluminescence (PL) spectroscopy. NWs are often considered as potential building blocks for future nanometer-scaled devices. This vision is based on several attractive features that are generally ascribed to NWs. For instance, they are expected to grow virtually free of strain and defects even on substrates with a large structural mismatch. In the first part of the thesis, some of these expectations are examined using semiconductor NWs of different materials. On the basis of the temperature-dependent PL of Auand selfassisted GaAs/(Al,Ga)As core-shell NWs, the influence of foreign catalyst particles on the optical properties of NWs is investigated. For the Au-assisted NWs, we find a thermally activated, nonradiative recombination channel, possibly related to Auatoms incorporated from the catalyst. These results indicate the limited suitability of catalyst-assisted NWs for optoelectronic applications. The effect of the substrate choice is studied by comparing the PL of ZnO NWs grown on Si, Al2O3, and ZnO substrates. Their virtually identical optical characteristics indicate that the synthesis of NWs may indeed overcome the constraints that limit the heteroepitaxial deposition of thin films. The major part of this thesis discusses the optical properties of GaN NWs grown on Si substrates. The investigation of the PL of single GaN NWs and GaN NW ensembles reveals the significance of their large surface-to-volume ratio. Differences in the recombination behavior of GaN NW ensembles and GaN layers are observed. First, the large surface-to-volume ratio is discussed to be responsible for the different recombination mechanisms apparent in NWs. Second, certain optical features are only found in the PL of GaN NWs, but not in that of GaN layers. An unexpected broadening of the donor-bound exciton transition is explained by the abundant presence of surface donors in NWs. The existence and statistical relevance of these surface donors is confirmed by PL experiments of single GaN NWs which are either dispersed or free-standing. In this context, the distinct optical characteristics of each individual NW is investigated in greater detail. Third, the increase of the PL intensity of GaN NW ensembles upon exposure to ultraviolet light is ascribed to a reduction of the electric field strengths. These fields are shown to be present in the entire NW volume due to a pinning of the Fermi level at the NW sidewalls. Finally, the quantum efficiency of GaN NWs is studied. The coupling of light with GaN NWs is assessed by comparing the reflectance and the Raman intensities of a GaN NW ensemble with that of a thick GaN layer. A considerably increased extraction efficiency is found for the NWs. In addition, the internal quantum efficiency of GaN NWs is studied by time-resolved PL measurements. The typically observed nonexponential recombination behavior of NW ensembles is explained with a model based on a distribution of recombination rates. This rate distribution is motivated by the above results of this thesis: each NW exhibits its own individual recombination behavior, which is influenced by nonradiative surface recombination, the presence and location of donors within the NW, and the strength of the electric fields. Preliminary simulations using this model describe the nonexponential decay of GaN NW ensembles satisfactorily. The results suggest that nonradiative recombination processes prevail in GaN NWs.