Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire

TL;DR

This study investigates the nucleation and growth mechanisms of ZnO nanowires on O-polar ZnO and sapphire, revealing substrate-dependent growth behaviors and the role of impurities and atomic steps in nucleation.

Contribution

It provides new insights into how substrate type influences ZnO nanowire growth mechanisms and the role of impurities and surface steps in nucleation processes.

Findings

ZnO nanowires are Zn-polar regardless of substrate.

Growth on O-polar ZnO involves pyramids and inversion domain boundaries.

Growth on sapphire can originate from the interface, influenced by atomic steps and impurities.

Abstract

Controlling the growth of zinc oxide nanowires is necessary to optimize the performances of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. In this view, 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.