Coalescence, crystallographic orientation and luminescence of ZnO nanowires grown on Si(001) by chemical vapour transport

TL;DR

This study investigates the growth, structure, and luminescence of ZnO nanowires on silicon, revealing coalescence effects, crystallographic orientation, and inhomogeneous luminescence linked to core-shell defect structures.

Contribution

It provides a detailed analysis of nanowire coalescence, crystallography, and luminescence distribution, introducing a correlation between structural features and optical properties.

Findings

Coalescence degree can be quantified via cross-sectional shape analysis.

Nanowires crystallize in the wurtzite phase, elongate along c-axis, and are randomly oriented.

Luminescence is inhomogeneous, with core emission and defect-related green luminescence near sidewalls.

Abstract

We analyse the morphological, structural and luminescence properties of self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The examination of nanowire ensembles by scanning electron microscopy reveals that a non-negligible fraction of nanowires merge together forming coalesced aggregates during growth. We show that the coalescence degree can be unambiguously quantified by a statistical analysis of the cross-sectional shape of the nanowires. The examination of the structural properties by X-ray diffraction evidences that the nanowires crystallize in the wurtzite phase, elongate along the c-axis, and are randomly oriented in plane. The luminescence of the ZnO nanowires, investigated by photoluminescence and cathodoluminescence spectroscopies, is characterized by two bands, the nearband-edge emission and the characteristic defect-related green luminescence of ZnO. The cross-correlation of scanning electron micrographs and monochromatic cathodoluminescence intensity maps reveals that: (i) coalescence joints act as a source of non-radiative recombination, and (ii) the luminescence of ZnO nanowires is inhomogeneously distributed at the single nanowire level. Specifically, the near-band-edge emission arises from the nanowire cores, while the defect-related green luminescence originates from the volume close to the nanowire sidewalls. Two-dimensional simulations of the optical guided modes supported by ZnO nanowires allow us to exclude waveguiding effects as the underlying reason for the luminescence inhomogeneities. We thus attribute this observation to the formation of a core-shell structure in which the shell is characterized by a high concentration of green-emitting radiative point defects with respect to the core.