Combining metal dewetting and lateral etching for the scalable top-down fabrication of GaN nanowire arrays with independently tunable diameter and spacing

TL;DR

This work identifies an intrinsic coupling between nanowire diameter and spacing in metal-dewetting top-down fabrication and proposes two strategies to decouple them: modifying the dewetting system through substrate and metal choice, and adding a lateral etching step to shrink diameters while increasing spacing. Using GaN nanowires as a model, the authors demonstrate that alloying Pt with Au tunes the diameter–spacing ratio within a limited range, whereas lateral etching substantially expands independent control over diameter and pitch and extends the accessible design space. The lateral-etching protocol combines O2 plasma oxidation to form GaOx with a subsequent KOH reduction step, and is described by a robust oxidation model with high fit quality. The approach is compatible with wafer-scale fabrication, transferable to other semiconductors, and holds promise for low-cost, scalable nanowire-based photonic and optoelectronic devices.

Abstract

The top-down fabrication of nanowires based on patterning via metal dewetting is a cost-effective and scalable approach that is particularly suited for applications requiring large arrays of nanowires. Advantageously, the nanowire diameter can be tailored by the initial metal film thickness. However, we show here that metal dewetting inherently leads to a coupling between the nanowire diameter and spacing. To overcome this limitation, we introduce two strategies that are exemplified for GaN nanowires: (i) modification of the surface and interface energies within the dewetting system, and (ii) thinning of the nanowires by lateral etching. In the first strategy, GaN(0001), SiOx, and SiNx substrate surfaces are combined with Au, Pt, and Pt-Au alloy dewetting metals to tune the dewetting behavior. The differences in interface energies affect the relation between nanowire diameter and spacing, albeit within a limited range. The second strategy adds a lateral etching step to the conventional top-down nanowire fabrication process. This step at the same time reduces the nanowire diameter and increases the spacing, thus enabling combinations beyond the constraints of metal dewetting alone. When in addition different initial nanowire diameters are employed, it is possible to independently control diameter and spacing over a substantially extended range. Therefore, the inherent limitation of conventional dewetting-based patterning approaches for the top-down fabrication of nanowires is overcome.

Figures (5)

• Figure 1: (a) and (b) show the dependence of nanoisland diameter and density on Pt film thickness for different dewetting substrates, as indicated by the text labels. (c) and (d) show correlations of the dewetting-formed nanoislands' mean diameter and the average spacing between nanoislands for different substrates and dewetting metals, respectively. Data points correspond to experiment results, and the solid lines are the result of fits as described in the main text.
• Figure 2: Step flow diagram and schematics for the lateral etching of top-down GaN nanowires.
• Figure 3: (a) Reduction in the mean radius of GaN nanowires as a function of RF power for a fixed exposure time of 2 min. (b) Reduction in the mean radius of GaN nanowires via lateral etching as a function of oxidation time (2--15 min) at a fixed RF power of 100 W. The dashed lines represent fits to equation \ref{['eq: CP4_LE: oxidation thickness']}. Light blue points denote the individual measured values from nanowire samples, while red points indicate the corresponding mean values with error bars.
• Figure 4: SE micrographs of top-down GaN nanowires prepared by metal dewetting and subsequent lateral etching. (a) Top-view of nanowires from a 15 nm Pt film (mean diameter $\approx$190 nm, spacing $\approx$170 nm). (b) Top-view and (c) side-view of the same nanowire ensemble after lateral etching (mean diameter $\approx$70 nm, spacing $\approx$380 nm). (d) Top-view of nanowires from a 12 nm Au film (mean diameter $\approx$310 nm, spacing $\approx$430 nm). (e) Top-view and (f) side-view of the same nanowire ensemble after lateral etching (mean diameter $\approx$170 nm, spacing $\approx$750 nm). (g) Top-view and (h) side-view of a reference nanowire ensemble from a 10 nm Pt film without lateral etching that exhibits similar diameter but very different spacing compared to the sample shown in (e) and (f) (mean diameter $\approx$140 nm, spacing $\approx$190 nm).
• Figure 5: Combinations of GaN nanowire diameter and spacing obtained by top-down fabrication based on metal dewetting on a SiO$_\mathrm{x}$ buffer layer. The symbols indicate experimental values for the dewetting of Pt (diamonds) and Au (spheres). The solid lines correspond to linear fits. The square-shaped symbols depict experimental values resulting from lateral etching for 15 min with an RF power of 100 W, as marked by the arrows. The yellow-shaded area is accessible by changing the Au-Pt alloy composition. The blue-shaded region is unlocked by lateral etching, with the deep blue-shaded region requiring larger nanowire diameter than in the samples fabricated for the present study.