Fabrication of Ordered Au‑Capped GaAs Nanopillar Arrays via Metal‑Assisted Chemical Etching

Background

III–V compound semiconductors such as GaAs are increasingly considered as next‑generation alternatives to silicon due to their superior carrier mobility and direct band‑gap. Ordered nanostructures—particularly those with high aspect ratios—are essential for advanced optical and optoelectronic devices, offering lower cost and higher conversion efficiencies compared to conventional thin films [1,2,3,4]. Traditional fabrication of low‑dimensional III–V structures relies on high‑precision, costly dry‑processes (e.g., MBE, VLS, MOVPE) that are limited in pattern size and throughput. Metal‑assisted chemical etching, first introduced by Li and Bohn in 2000 [8], provides a simpler, cheaper alternative, enabling the production of a variety of complex nanostructures in silicon [4,9,10,11,12]. However, its application to III–V semiconductors remains underexplored, especially for sub‑micron periodicities on GaAs [13,14]. Understanding the etching mechanism at the nanoscale is therefore critical for expanding MACE to III–V materials.

Previous work from our group fabricated microbump arrays on InP [15] and larger‑scale pillar arrays on GaAs [16] using MACE, but periodicities were in the micrometer range. Achieving nanometer‑scale control poses two challenges: (i) precise sizing of noble‑metal catalysts and (ii) the less‑understood etching behavior of GaAs compared to silicon. In this study, we address both by creating Au nanodot arrays with 70‑nm diameters on a 100‑nm hexagonal lattice, enabling the formation of ordered GaAs nanopillars via MACE. We also systematically investigate how etchant composition and temperature influence the resulting morphology.

Methods

The fabrication workflow is illustrated in Figure 1. A two‑layer anodized alumina mask with a hexagonal array of 300‑nm‑thick pores (top diameter ≈ 80 nm, bottom ≈ 70 nm) was prepared on an n‑type (100) GaAs wafer (Si‑doped, 2.35–2.67 × 10⁻³ Ω cm). Vacuum deposition of a 30‑nm Au film through the mask yielded nanodots that mirrored the mask’s pore pattern (see Figure 2). The mask was removed in 5 wt% phosphoric acid, leaving Au dots on the substrate.

Au‑coated wafers were then immersed in an etching solution comprising HF (0.5–20 mol dm⁻³) and KMnO₄ (0.001–0.01 mol dm⁻³) at 20–45 °C. The high HF content suppresses lateral etching while the low oxidant level limits GaAs oxidation, favoring vertical dissolution under the Au catalysts. The etch duration ranged from 5 s to 1 min. Morphology was examined by field‑emission scanning electron microscopy (FE‑SEM, JEOL JSM‑6701F) and chemical composition was verified by Auger electron spectroscopy (AES, JEOL JAMP‑9500F).

Schematic of the MACE process: a Au deposition through an alumina mask; b mask removal; c, d selective GaAs etching under Au nanodots.

Results and Discussion

The precise size of the Au nanodots directly dictates the resulting nanopillar dimensions. The 70‑nm diameter dots, deposited via the porous alumina mask, produced hexagonally ordered arrays with a 100‑nm lattice period (see Figure 2). Each dot’s height (~30 nm) was set by the deposition time.

a Surface and b cross‑sectional SEM images of the Au nanodot array.

Etching in 0.001 mol dm⁻³ KMnO₄ and 20 mol dm⁻³ HF at 45 °C produced well‑defined pores directly beneath the Au dots (Figure 3). The pores matched the dot diameters, confirming that etching occurs exclusively at the Au/GaAs interface and proceeds anisotropically along the <100> direction.

Top view SEM of GaAs after 600 s of MACE in 0.001 mol dm⁻³ KMnO₄ / 20 mol dm⁻³ HF at 45 °C.

Varying KMnO₄ concentration altered the pillar shape. With 0.01 mol dm⁻³ KMnO₄ and 5–20 mol dm⁻³ HF, hexagonal arrays of pillars emerged. Higher HF concentrations yielded taller pillars (up to 50 nm) while maintaining the 100‑nm period. At 20 °C, Au remained on the pillar tips (Figure 4a), indicating a site‑selective etching mechanism where the Au dots protect surrounding GaAs (inverse MACE). At 45 °C, Au detachment occurred due to enhanced lateral etching (Figure 4b).

Cross‑sectional SEM of GaAs nanopillars etched in 0.01 mol dm⁻³ KMnO₄ and 5, 10, or 20 mol dm⁻³ HF: a 20 °C, b 45 °C.

Prolonged etching (10 s–60 s) in 20 mol dm⁻³ HF / 0.01 mol dm⁻³ KMnO₄ at 20 °C produced pillars ~50 nm tall, each capped with Au (Figures 5 and 6). AES mapping confirmed Au localization at pillar tips, even after extended etching. Excessive etch time (1 min) led to pillar height reduction due to lateral etching and Au detachment, highlighting the need for optimized etch windows.

Cross‑sectional SEM of GaAs nanopillars after 10 s (a) and 60 s (b) of MACE in 20 mol dm⁻³ HF / 0.01 mol dm⁻³ KMnO₄ at 20 °C. Inset shows a surface image of the Au‑capped array.

AES analysis of the same sample: a SEM image; b Ga map; c Au map.

The successful fabrication of Au‑capped GaAs nanopillars demonstrates that MACE can precisely control III–V nanostructures without resorting to expensive dry‑etching techniques. The resulting arrays are promising for optoelectronic applications such as plasmonic solar cells, where metallic caps can enhance light trapping [23,24].

Conclusions

We have shown that a 100‑nm hexagonal lattice of 70‑nm Au nanodots, deposited through a porous alumina mask, serves simultaneously as a catalyst and a protective mask during MACE. By fine‑tuning etchant composition and temperature, Au‑capped GaAs nanopillar arrays with 50‑nm heights were reliably produced. This approach offers a scalable, low‑cost route to engineer ordered III–V nanostructures, overcoming the limitations of conventional dry processes and opening new possibilities for advanced optoelectronic devices.

Abbreviations

AES:

Auger electron spectroscopy

FE‑SEM:

Field‑emission scanning electron microscopy