Strain evolution in GaN Nanowires: from free-surface objects to coalesced templates

TL;DR

This study investigates how strain evolves in GaN nanowires during growth and coalescence, combining experimental measurements with numerical modeling to understand strain relaxation and its impact on the final material properties.

Contribution

It provides a detailed experimental and theoretical analysis of strain relaxation in GaN nanowires, identifying the height at which they become strain-free and how coalescence affects the final strain state.

Findings

Strain profiles match numerical models based on continuous media approach.

Nanowires above a certain height are effectively strain-free.

The final GaN layer remains strained despite initial strain-free nanowire templates.

Abstract

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top facet for both initial compressive and tensile strains. The relaxation behaviors derived from optical and structural characterizations perfectly match the numerical results of calculations based on a continuous media approach. By monitoring the elastic relaxation enabled by the lateral free-surfaces, the height from which the nanowires can be considered strain-free has been estimated. Based on this result, NWs sufficiently high to be strain-free have been coalesced to form a continuous GaN layer. X-ray diffraction, photoluminescence and cathodoluminescence clearly show that despite the initial strain-free nanowires template, the final GaN layer is strained.