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Controlling the growth of zinc oxide nanowires is necessary to optimize the performance of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. With this in mind, we investigate the nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam electron diffraction. For growth on O-polar ZnO substrate, the nanowires are found to sit on O-polar pyramids. As growth proceeds, the inversion domain boundary moves up in order to remain at the top of the O-polar pyramids. For growth on sapphire substrates, the nanowires may also originate from the sapphire/ZnO interface. The presence of atomic steps and the non-polar character of sapphire could be the cause of the Zn-polar crystal nucleation on sapphire, whereas it is proposed that the segregation of aluminum impurities could account for the nucleation of inverted domains for growth on O-polar ZnO. [ABSTRACT FROM AUTHOR]

Nanowire-based light-emitting devices require multi-quantum well heterostructures with high room temperature optical efficiencies. We demonstrate that such efficiencies can be attained through the use of ZnO/Zn(1-x)MgxO core-shell quantum well heterostructures grown by metal organic vapor phase epitaxy. Varying the barrier Mg concentration from x = 0.15 to 0.3 leads to the formation of misfit induced dislocations in the multi-quantum wells. Correlatively, temperature dependent photoluminescence reveals that the radial well luminescence intensity decreases much less rapidly with increasing temperature for the lower Mg concentration. Indeed, about 54% of the 10 K intensity is retained at room temperature with x = 0.15, against 1% with x = 0.30. These results open the way to the realization of high optical efficiency nanowire-based light-emitting diodes. [ABSTRACT FROM AUTHOR]

Abstract: We report on ZnO nanorods grown by catalyst-free metal-organic chemical vapour deposition (MOCVD) in a commercial Epigress reactor using diethylzinc and N2O as precursors. Well-aligned ZnO nanorods with uniform diameter, length and density have been grown perpendicularly to the sapphire (0001) surface. Scanning electron microscopy (SEM) has been used to observe the morphology of the ZnO nanorods and X-ray diffraction and transmission electron microscopy (TEM) to investigate the crystalline structure of the nanorods. TEM observation as well as photoluminescence measurements confirm the very good crystalline quality of the nanorods. SEM observation on samples that have been prepared with various deposition times has been used in order to investigate the growth mechanism. Three types of ZnO morphologies have been identified: a thin two-dimensional ZnO layer formed at the sapphire surface, covered by three-dimensional hexagonal-shaped islands and hexagonal nanorods on top of them. [Copyright &y& Elsevier]