MOCVD growth mechanisms of ZnO nanorods

TL;DR

This paper investigates the MOCVD growth mechanisms of ZnO nanorods on sapphire, revealing the role of crystal polarity and nucleation processes in achieving well-aligned nanorod arrays.

Contribution

It provides a detailed analysis of the growth process, including the formation of wetting layers, pyramids, and nanorods, and highlights the importance of crystal polarity in growth mechanisms.

Findings

Nanorods are Zn-polar with O-polar pyramids.

Growth involves nucleation at the interface or on pyramids.

Threading dislocations are mainly in the wetting layer.

Abstract

ZnO is a promising material for the fabrication of light emitting devices. One approach to achieve this goal is to use ZnO nanorods because of their expected high crystalline and optical quality. Catalyst free growth of nanorods by metalorganic chemical vapour deposition (MOCVD) was carried out on (0001) sapphire substrates. Arrays of well-aligned, vertical nanorods were obtained with uniform lengths and diameters. A thin wetting layer in epitaxy with the sapphire substrate is formed first, followed by pyramids and nanorods. The nucleation of nanorods occurs either directly at the interface, or later on top of some of the pyramids, suggesting various nucleation mechanisms. It is shown that crystal polarity plays a critical role in the growth mechanism with nanorods of Zn polarity and their surrounding pyramids with O polarity. A growth mechanism is proposed to explain that most threading dislocations lie in the wetting layer, with only a few in the pyramids and none in the nanorods.