Why do nanowires grow with their c-axis vertically-aligned in the absence of epitaxy?

Almog R Azulay,R S Goldman,Davide Del Gaudio,Yury Turkulets,Ilan Shalish

doi:10.1038/s41598-020-63500-y

Almog R Azulay, R S Goldman + Show 3 more

Open Access

https://doi.org/10.1038/s41598-020-63500-y

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Abstract

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template. Here, we reveal the physics underlying one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate electric fields need always be considered. Our results suggest a new concept, according to which the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self-assembly, wherein the internal built-in field in the polar material tends to align in parallel to an external field exerted by the substrate to minimize the interfacial energy of the system.

Highlights

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template
One peculiar observation that has become more apparent with the growth of ZnO nanowires is that on substrates such as glass and silicon, that do not provide an epitaxial template, ZnO grows in a direction along the c axis, i.e. it is preferentially c-oriented[13]
These were synthesized at temperatures ranging from 400 to 600 °C using metal organic vapor phase epitaxy[14], chemical vapor deposition (CVD)[15], atomic layer deposition[16], chemical vapor transport[17], or a hydrothermal approach[18,19]

Summary

Introduction

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template. C-oriented ZnO nanowire arrays have been grown on silicon or glass substrates without the use of a preexisting textured thin film These were synthesized at temperatures ranging from 400 to 600 °C using metal organic vapor phase epitaxy[14], chemical vapor deposition (CVD)[15], atomic layer deposition[16], chemical vapor transport[17], or a hydrothermal approach[18,19]. Claeyssens et al suggested a graphitic structure reconstruction of the ZnO of films thinner than 18 monolayers, over which c-oriented growth should be favored[21] This reconstruction was required in order to cancel the polarization charge present on the polar faces to allow the ZnO to grow on a substrate that was not charged. We propose a simple mechanism that resolves the instability problem of the polar surfaces and renders the reconstruction redundant, at least when the substrate is Si, though the same effect may not be limited to ZnO on Si

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