Wafer-scale selective area growth of GaN hexagonal prismatic nanostructures on c-sapphire substrate

TL;DR

This paper presents a novel process for wafer-scale selective growth of high-quality GaN nanostructures on c-sapphire, achieving improved homogeneity and structural quality through a two-step nucleation and annealing process.

Contribution

A new high-temperature growth process with initial nucleation and ammonia annealing enhances homogeneity and quality of GaN nanostructures on sapphire substrates.

Findings

GaN nanostructures grow epitaxially with lateral overgrowth on the mask.

Lateral overgrowth areas exhibit low defect density and stronger NBE emission.

Strain and dislocations are present at the GaN/sapphire interface due to lattice mismatch.

Abstract

Selective area growth of GaN nanostructures has been performed on full 2" c-sapphire substrates using Si3N4 mask patterned by nanoimprint lithography (array of 400 nm diameter circular holes). A new process has been developed to improve the homogeneity of the nucleation selectivity of c-oriented hexagonal prismatic nanostructures at high temperature (1040\circ C). It consists of an initial GaN nucleation step at 950 \circ C followed by ammonia annealing before high temperature growth. Structural analyses show that GaN nanostructures are grown in epitaxy with c-sapphire with lateral overgrowths on the mask. Strain and dislocations are observed at the interface due to the large GaN/sapphire lattice mismatch in contrast with the high quality of the relaxed crystals in the lateral overgrowth area. A cathodoluminescence study as a function of the GaN nanostructure size confirms these observations: the lateral overgrowth of GaN nanostructures has a low defect density and exhibits a stronger near band edge (NBE) emission than the crystal in direct epitaxy with sapphire. The shift of the NBE positions versus nanostructure size can be mainly attributed to a combination of compressive strain and silicon doping coming from surface mask diffusion.