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This paper reports on the growth, structural and optical properties of GaN free-stranding nanowires synthesized in catalyst-free mode on Si(111) substrate by plasma-assisted molecular beam epitaxy. Cylindrical nanowires with a hexagonal cross-section defined by {1010} planes and diameters down to 20 nm were observed. The nanowire length increases as a function of their diameter, following the Gibbs-Thomson expression. The growth rate in the lateral direction was studied using thin AlN marker layers showing that the lateral over axial growth rate ratio can be tuned from ∼1% to ∼10% by changing the III/V flux ratio, with the lateral growth remaining homogeneous along the NW axis. Nanowire ensembles showed a strong near band edge photoluminescence up to room temperature. Low-temperature micro-photoluminescence from a single wire is peaked at 3.478 eV with broadening of 6-10 meV. This emission is similar to the luminescence of nanowire ensembles, which demonstrates strain homogeneity from wire to wire. The optical properties along the wire axis probed by micro-cathodoluminescence were found to be uniform, with no evidence of a higher defect density in the bottom part of the nanowires next to the Si substrate.

We bury vertical free-standing core-shell GaAs/AlGaAs nanowires by a planar GaAs overgrowth. As the nanowires get buried, their crystalline structure progressively transforms: whereas the upper emerging part retains its initial wurtzite structure, the buried part adopts the zinc blende structure of the burying layer. The burying process also suppresses all the stacking faults that existed in the wurtzite nanowires. We consider two possible mechanisms for the structural transition upon burying, examine how they can be discriminated from each other, and explain why the transition is favorable.

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Strongly compensated Ga2O3 is shown to be an intrinsic (or native) p-type conductor with the largest bandgap for any reported p-type transparent semiconductor oxide which may shift the frontiers in fields such as power electronics and photonics.

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ZnO one-dimensional nanostructures were synthesized using the vapour-liquid-solid (VLS) process. Highly C-axis oriented ZnO nanowires, exhibiting a gold droplet on their top, were observed by scanning electron microscopy. Growth mechanisms are discussed, and the issue of supersaturation and precipitation of zinc in the gold droplet is debated. Transmission electron microscopy was carried out to investigate the crystalline properties and the polarity of the synthesized structures. In particular, an unexpected result shows that VLS ZnO nanowires grow with O-polarity, whereas spontaneous (catalyst-free) ZnO nanowires grow with Zn-polarity. The process at the origin of this polarity inversion is discussed.

Whereas nanowire (NW)-based devices offer numerous advantages compared to bulk ones, their performances are frequently limited by an incomplete understanding of their properties where surface effect should be carefully considered. Here, we demonstrate the ability to spatially map the electric field and determine the exciton diffusion length in NW by using an electron beam as the single excitation source. This approach is performed on numerous single ZnO NW Schottky diodes whose NW radius vary from 42.5 to 175 nm. The dominant impact of the surface on the NW properties is revealed through the comparison of three different physical quantities recorded on the same NW: electron-beam induced current, cathodoluminescence, and secondary electron signal. Indeed, the space charge region near the Schottky contact exhibits an unusual linear variation with reverse bias whatever the NW radius. On the contrary, the exciton diffusion length is shown to be controlled by the NW radius through surface recombination. This systematic comparison performed on a single ZnO NW demonstrates the power of these complementary techniques in understanding NW properties.