A route for the top-down fabrication of ordered ultrathin GaN nanowires

TL;DR

This paper presents a straightforward top-down method to fabricate highly ordered, ultrathin GaN nanowires with high aspect ratios and diameters below 20 nm, using lithography, etching, and digital etching techniques.

Contribution

The authors introduce a novel, efficient fabrication route for GaN nanowires that achieves diameters down to 5 nm and high uniformity, with potential for regrowth and application to other nanostructures.

Findings

Achieved GaN nanowires with diameters below 20 nm and aspect ratios over 10.

Demonstrated a digital etching process to reduce NW diameter to 5 nm.

Identified mechanical instability and buckling behavior at small diameters, mitigated by critical point drying.

Abstract

Ultrathin GaN nanowires (NWs) are attractive to maximize surface effects and as building block in high-frequency transistors. Here, we introduce a facile route for the top-down fabrication of ordered arrays of GaN NWs with aspect ratios exceeding $10$ and diameters below $20\,$nm. Highly uniform thin GaN NWs are first obtained by using electron beam lithography to pattern a Ni/SiN$_x$ hard mask, followed by dry etching and wet etching in hot KOH. The SiN$_x$ is found to work as an etch stop during wet etching in hot KOH. Arrays with NW diameters down to $(33 \pm5)\,$nm can be achieved with a yield exceeding $99.9\,\%$. Further reduction of the NW diameter down to $5\,$nm is obtained by applying digital etching which consists in plasma oxidation followed by wet etching in hot KOH. The NW radial etching depth is tuned by varying the RF power during plasma oxidation. NW breaking or bundling is observed for diameters below $\approx 20\,$nm, an effect that is associated to capillary forces acting on the NWs during sample drying in air. This effect can be principally mitigated using critical point dryers. Interestingly, this mechanical instability of the NWs is found to occur at much smaller aspect ratios than what is predicted for models dealing with macroscopic elastic rods. Explicit calculations of buckling states show an improved agreement when considering an inclined water surface, as can be expected if water assembles into droplets. The proposed fabrication route can be principally applied to any GaN/SiN$_{x}$ nanostructures and allows regrowth after removal of the SiN$_{x}$ mask.