Au-Capped GaAs Nanopillar Arrays Fabricated by Metal-Assisted Chemical Etching

Metal-assisted chemical etching beyond Si: applications to III-V compounds and wide-bandgap semiconductors

Metal-assisted chemical etching (MacEtch) has emerged as a versatile technique for fabricating a variety of semiconductor nanostructures. Since early investigations in 2000, research in this field has provided a deeper understanding of the underlying mechanisms of catalytic etching processes and enabled high control over etching conditions for diverse applications. In this Review, we present an overview of recent developments in the application of MacEtch to nanomanufacturing and processing of III-V based semiconductor materials and other materials beyond Si. We highlight the key findings and developments in MacEtch as applied to GaAs, GaN, InP, GaP, InGaAs, AlGaAs, InGaN, InGaP, SiC, β-Ga2O3, and Ge material systems. We further review a series of active and passive devices enabled by MacEtch, including light-emitting diodes (LEDs), field-effect transistors (FETs), optical gratings, sensors, capacitors, photodiodes, and solar cells. By reviewing demonstrated control of morphology, optimization of etch conditions, and catalyst-material combinations, we aim to distill the current understanding of beyond-Si MacEtch mechanisms and to provide a bank of reference recipes to stimulate progress in the field.

Facile synthesis of size- and shape-controlled freestanding Au nanohole arrays by sputter deposition using anodic porous alumina templates

To fabricate a freestanding through-hole Au membrane using an anodic porous alumina template, Au was deposited on the outermost surface of an anodic film followed by the removal of the template. Alumina templates with different dimensions (e.g. diameters and number of pores) were prepared by two-step anodization in the range of 40-80 V and pore-widening. The Au thin films were deposited onto alumina templates with well-controlled surface morphologies by sputter deposition using a commercially available ion sputter coater. After the removal of the alumina template, a variety of Au membranes with nanoholes, nanotubes, or branched pores were obtained, which reflect the morphology of the alumina template. When the sputtered Au penetrates the pores of the alumina film, Au nanotube arrays with an aspect ratio of ∼3 can be fabricated. The present method is much simpler than the traditional template process involving multi-step replication because there is no need to separate the alumina template from the aluminum substrate before Au deposition.

Black GaAs: Gold-Assisted Chemical Etching for Light Trapping and Photon Recycling

Thanks to its excellent semiconductor properties, like high charge carrier mobility and absorption coefficient in the near infrared spectral region, GaAs is the material of choice for thin film photovoltaic devices. Because of its high reflectivity, surface microstructuring is a viable approach to further enhance photon absorption of GaAs and improve photovoltaic performance. To this end, metal-assisted chemical etching represents a simple, low-cost, and easy to scale-up microstructuring method, particularly when compared to dry etching methods. In this work, we show that the etched GaAs (black GaAs) has exceptional light trapping properties inducing a 120 times lower surface reflectance than that of polished GaAs and that the structured surface favors photon recycling. As a proof of principle, we investigate photon reabsorption in hybrid GaAs:poly (3-hexylthiophene) heterointerfaces.

Unravelling the strain relaxation processes in silicon nanowire arrays by X-ray diffraction

Investigations performed on silicon nanowires of different lengths by scanning electron microscopy revealed coalescence processes in longer nanowires. Using X-ray diffraction (XRD), it was found that the shape of the pole figure in reciprocal space is ellipsoidal. This is the signature of lattice defects generated by the relaxation of the strain concentrated in the coalescence regions. This observation is strengthened by the deviation of the XRD peaks from Gaussianity and the appearance of the acoustic phonon mode in the Raman spectrum. It implies that bending, torsion and structural defects coexist in the longer nanowires. To separate these effects, a grazing-incidence XRD technique was conceived which allows the nanowire to be scanned along its entire length. Both ω and φ rocking curves were recorded, and their shapes were used to extract the bending and torsion profiles, respectively, along the nanowire length. Dips were found in both profiles of longer nanowires, while they are absent from shorter ones, and these dips correspond to the regions where both bending and torsion relax. The energy dissipated in the nanowires, which tracks the bending and torsion profiles, has been used to estimate the emergent dislocation density in nanowire arrays.

Black GaAs by Metal-Assisted Chemical Etching

Large area surface microstructuring is commonly employed to suppress light reflection and enhance light absorption in silicon photovoltaic devices, photodetectors, and image sensors. To date, however, there are no simple means to control the surface roughness of III-V semiconductors by chemical processes similar to the metal-assisted chemical etching of black Si. Here, we demonstrate the anisotropic metal-assisted chemical etching of GaAs wafers exploiting the lower etching rate of the monoatomic Ga<111> and <311> planes. By studying the dependence of this process on different crystal orientations, we propose a qualitative reaction mechanism responsible for the self-limiting anisotropic etching and show that the reflectance of the roughened surface of black GaAs reduces up to ∼50 times compared to polished wafers, nearly doubling its absorption. This method provides a new, simple, and scalable way to enhance light absorption and power conversion efficiency of GaAs solar cells and photodetectors.

Ordered Al xGa1- xAs Nanopillar Arrays via Inverse Metal-Assisted Chemical Etching

The ternary III-V semiconductor compound, Al _xGa_{1 -x}As, is an important material that serves a central role within a variety of nanoelectronic, optoelectronic, and photovoltaic devices. With all of its uses, the material itself poses a host of fabrication difficulties stemming from conventional top-down processing, including standard wet-chemical etching and reactive-ion etching (RIE). Metal-assisted chemical etching (MacEtch) techniques provide low-cost and benchtop methods that combine many of the advantages of RIE and wet-chemical etching, without being hindered by many of their disadvantages. Here, inverse-progression MacEtch (I-MacEtch) of Au-patterned Al _xGa_{1 -x}As is demonstrated for the first time and is exploited for the generation of vertical and ordered nanopillar arrays. The etching solution employed here consists of citric acid (C₆H₈O₇) and hydrogen peroxide (H₂O₂). The I-MacEtch evolution is tracked in time for Al _xGa_{1 -x}As samples with compositions defined by x = 0.55, x = 0.60, and x = 0.70. The vertical and lateral etch rates (VER and LER, respectively) are shown to be tunable with Al fraction and temperature of the etching solution, based on modification of catalytically injected hole distributions. Control over the VER/LER ratio is demonstrated by tailoring etch conditions for single-step fabrication of ordered AlGaAs nanopillar arrays with predefined aspect ratios. Maximum VER and LER values of ∼40 nm/min and ∼105 nm/min, respectively, are measured for Al_0.55Ga_0.45As at a process temperature of 65 °C. The I-MacEtch nanofabrication methodology outlined in this study may be utilized for the processing of many devices, including high electron mobility transistors, distributed Bragg reflectors, lasers, light-emitting diodes, and multijunction solar cells containing AlGaAs components.

Fabrication of Suspended III-V Nanofoils by Inverse Metal-Assisted Chemical Etching of In0.49Ga0.51P/GaAs Heteroepitaxial Films

Metal-assisted chemical etching (MacEtch) has been established as a low-cost, benchtop, and versatile method for large-scale fabrication of semiconductor nanostructures and has been heralded as an alternative to conventional top-down approaches such as reactive-ion etching. However, extension of this technique to ternary III-V compound semiconductor alloys and heteroepitaxial systems has remained relatively unexplored. Here, Au-assisted and inverse-progression MacEtch (I-MacEtch) of the heteroepitaxial In_0.49Ga_0.51P/GaAs material system is demonstrated, along with a method for fabricating suspended InGaP nanofoils of tunable thickness in solutions of hydrofluoric acid (HF) and hydrogen peroxide (H₂O₂). A comparison between Au- and Cr-patterned samples is used to demonstrate the catalytic role of Au in the observed etching behavior. Vertical etch rates for nominally undoped, p-type, and n-type InGaP are determined to be ∼9.7, ∼8.7, and ∼8.8 nm/min, respectively. The evolution of I-MacEtch in the InGaP/GaAs system is tracked, leading to the formation of nanocavities located at the center of off-metal windows. Upon nanocavity formation, additional localized mass-transport pathways to the underlying GaAs substrate permit its rapid dissolution. Differential etch rates between the epilayer and substrate are exploited in the fabrication of InGaP nanofoils that are suspended over micro-trenches formed in the GaAs substrate. A model is provided for the observed I-MacEtch mechanism, based on an overlap of neighboring injected hole distribution profiles. The nanofabrication methodology shown here can be applied to various heteroepitaxial III-V systems and can directly impact the conventional processing of device applications in photonics, optoelectronics, photovoltaics, and nanoelectronics.