Phase mapping of aging process in InN nanostructures: oxygen incorporation and the role of the zincblende phase

TL;DR

This study maps the aging process of InN nanostructures, revealing oxygen incorporation and phase transformations from wurtzite to zincblende and cubic phases, which impact their fundamental properties.

Contribution

It introduces a novel TEM-based phase mapping method using geometrical phase algorithms to distinguish phases and grains in aging InN nanostructures.

Findings

Disappearance of InN-w phase from surface and formation of textured cubic layer.

Presence of metastable InN:O zincblende phase as intermediate during aging.

High twinning ratio in cubic phases affecting phase identification.

Abstract

Uncapped InN nanostructures undergo a deleterious natural aging process at ambient conditions by oxygen incorporation. The phases involved in this process and their localization is mapped by Transmission Electron Microscopy (TEM) related techniques. The parent wurtzite InN (InN-w) phase disappears from the surface and gradually forms a highly textured cubic layer that completely wraps up a InN-w nucleus which still remains from original single-crystalline quantum dots. The good reticular relationships between the different crystals generate low misfit strains and explain the apparent easiness for phase transformations at room temperature and pressure conditions, but also disable the classical methods to identify phases and grains from TEM images. The application of the geometrical phase algorithm in order to form numerical moire mappings, and RGB multilayered image reconstructions allows to discern among the different phases and grains formed inside these nanostructures. Samples aged for shorter times reveal the presence of metastable InN:O zincblende (zb) volumes, which acts as the intermediate phase between the initial InN-w and the most stable cubic In2O3 end phase. These cubic phases are highly twinned with a proportion of 50:50 between both orientations. We suggest that the existence of the intermediate InN:O-zb phase should be seriously considered to understand the reason of the widely scattered reported fundamental properties of thought to be InN-w, as its bandgap or superconductivity.