Crystal Facet Engineering in Ga-Doped ZnO Nanowires for Mid-Infrared Plasmonics

The metal–organic chemical vapor deposition growth of various Ga-doped ZnO nanostructures for plasmonics is investigated, with a particular focus on the nanowire facet transformations induced by the addition of trimethylgallium in the gas phase. For nonintentionally doped spontaneous ZnO nanowires, the aspect ratio is strongly decreased due to residual Ga in the reactor, and the shape evolves rapidly toward Christmas-tree-like and hierarchical structures upon intentional Ga doping. Regarding ZnO/ZnO:Ga core–shell structures, a change of the smooth initial M-oriented facets occurs, with the development of {202̅1} surfaces, and further {101̅1} and {0001} surfaces. Interestingly, a similar evolution of the lateral roughness is observed in Au-catalyzed doped nanowires. High concentrations of Ga in the grown nanostructures are revealed by photoluminescence and confirmed by Rutherford backscattering spectrometry. First photoacoustic measurements show an optical absorption at 6 μm, evidencing that the degenerated material is suitable for plasmonics applications in the infrared range. The influence of Ga doping on the facet transformations and the occurrence of unexpected {0001} polar surfaces are discussed. The results can be mainly understood by a Ga surfactant effect (at least partial) responsible for the modification of the surface energies and kinetics. Density functional calculations support the floating behavior of the negatively charged Ga^– ion on the growing surface