Nanostructuring of Si substrates by a metal-assisted chemical etching and dewetting process

Uniform large-area surface patterning achieved by metal dewetting for the top-down fabrication of GaN nanowire ensembles

The dewetting of thin Pt films on different surfaces is investigated as a means to provide the patterning for the top-down fabrication of GaN nanowire ensembles. The transformation from a thin film to an ensemble of nanoislands upon annealing proceeds in good agreement with the void growth model. With increasing annealing duration, the size and shape uniformity of the nanoislands improves. This improvement speeds up for higher annealing temperature. After an optimum annealing duration, the size uniformity deteriorates due to the coalescence of neighboring islands. By changing the Pt film thickness, the nanoisland diameter and density can be quantitatively controlled in a way predicted by a simple thermodynamic model. We demonstrate the uniformity of the nanoisland ensembles for an area larger than 1 cm2. GaN nanowires are fabricated by a sequence of dry and wet etching steps, and these nanowires inherit the diameters and density of the Pt nanoisland ensemble used as a mask. Our study achieves advancements in size uniformity and range of obtainable diameters compared to previous works. This simple, economical, and scalable approach to the top-down fabrication of nanowires is useful for applications requiring large and uniform nanowire ensembles with controllable dimensions.

Dewetting-Assisted Patterning: A Lithography-Free Route to Synthesize Black and Colored Silicon

We report the use of thermal dewetting to structure gold-based catalytic etching masks for metal-assisted chemical etching (MACE). The approach involves low-temperature dewetting of metal films to generate metal holey meshes with tunable morphologies. Combined with MACE, dewetting-assisted patterning is a simple, benchtop route to synthesize Si nanotubes, Si nanowalls, and Si nanowires with defined dimensions and optical properties. The approach is compatible with the synthesis of both black and colored nanostructured silicon substrates. In particular, we report the lithography-free fabrication of silicon nanowires with diameters down to 40 nm that support leaky wave-guiding modes, giving rise to vibrant colors. Additionally, micrometer-sized areas with tunable film composition and thickness were patterned via shadow masking. After dewetting and MACE, such patterned metal films produced regions with distinct nanostructured silicon morphologies and colors. To-date, the fabrication of colored silicon has relied on complicated nanoscale patterning processes. Dewetting-assisted patterning provides a simpler alternative that eliminates this requirement. Finally, the simple transfer of resonant SiNWs into ethanolic solutions with well-defined light absorption properties is reported. Such solution-dispersible SiNWs could open new avenues for the fabrication of ultrathin optoelectronic devices with enhanced and tunable light absorption.

A review of cost-effective black silicon fabrication techniques and applications

Ever since the discovery of black silicon, scientists around the world have been trying to come up with novel, cost-effective methods of utilizing this super material in a variety of different industries due to its remarkably low reflectivity and excellent electronic and optoelectronic properties. In this review, many of the most common methods of black silicon fabrication are exhibited, including metal-assisted chemical etching, reactive ion etching, and femto-second laser irradiation. Different nanostructured silicon surfaces are assessed based on their reflectivity and applicable properties in both the visible wavelength range and the infrared range. The most cost efficient technique for the mass production of black silicon is discussed, as well as some promising contender materials ready to replace silicon. Also, solar cell, IR photo-detector, and antibacterial applications are looked into, along with their respective challenges to date.

Fabrication of Black Silicon via Metal-Assisted Chemical Etching—A Review

The metal-assisted chemical etching (MACE) technique is commonly employed for texturing the wafer surfaces when fabricating black silicon (BSi) solar cells and is considered to be a potential technique to improve the efficiency of traditional Si-based solar cells. This article aims to review the MACE technique along with its mechanism for Ag-, Cu- and Ni-assisted etching. Primarily, several essential aspects of the fabrication of BSi are discussed, including chemical reaction, etching direction, mass transfer, and the overall etching process of the MACE method. Thereafter, three metal catalysts (Ag, Cu, and Ni) are critically analyzed to identify their roles in producing cost-effective and sustainable BSi solar cells with higher quality and efficiency. The conducted study revealed that Ag-etched BSi wafers are more suitable for the growth of higher quality and efficiency Si solar cells compared to Cu- and Ni-etched BSi wafers. However, both Cu and Ni seem to be more cost-effective and more appropriate for the mass production of BSi solar cells than Ag-etched wafers. Meanwhile, the Ni-assisted chemical etching process takes a longer time than Cu but the Ni-etched BSi solar cells possess enhanced light absorption capacity and lower activity in terms of the dissolution and oxidation process than Cu-etched BSi solar cells.

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors has been elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitated the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of wafers and particles with arbitrary shape and size. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. Metal-assisted catalytic etching (MACE) can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances, and the properties of the etch product with special emphasis on the etching of silicon powders.

The Kinetics and Stoichiometry of Metal Cation Reduction on Multi-Crystalline Silicon in a Dilute Hydrofluoric Acid Matrix

In this study, the process of metal cation reduction on multi-crystalline silicon in a dilute hydrofluoric acid (HF) matrix is described using Ag(I), Cu(II), Au(III) and Pt(IV). The experimental basis utilized batch tests with various solutions of different metal cation and HF concentrations and multi-crystalline silicon wafers. The metal deposition kinetics and the stoichiometry of metal deposition and silicon dissolution were calculated by means of consecutive sampling and analysis of the solutions. Several reaction mechanisms and reaction steps of the process were discussed by overlaying the results with theoretical considerations. It was deduced that the metal deposition was fastest if the holes formed during metal ion reduction could be transferred to the valence bands of the bulk and surface silicon with hydrogen termination. By contrast, the kinetics were lowest when the redox levels of the metal ion/metal half-cells were weak and the equilibrium potential of the H3O+/H2 half-cells was high. Further minima were identified at the thresholds where H3O+ reduction was inhibited, the valence transfer via valence band mechanism was limited by a Schottky barrier and the dissolution of oxidized silicon was restricted by the activity of the HF species F-, HF2- and H2F3-. The findings of the stoichiometric conditions provided further indications of the involvement of H3O+ and H2O as oxidizing agents in addition to metal ions, and the hydrogen of the surface silicon termination as a reducing agent in addition to the silicon. The H3O+ reduction is the predominant process in dilute metal ion solutions unless it is disabled due to the metal-dependent equilibrium potential of the H3O+/H2 half-cell and the energetic level of the valence bands of the silicon. As silicon is not oxidized up to the oxidation state +IV by the reduction of the metal ions and H3O+, water is suspected of acting as a secondary oxidant. The stoichiometric ratios increased up to a maximum with higher molalities of the metal ions, in the manner of a sigmoidal function. If, owing to the redox level of the metal half-cells and the energetic level of the valence band at the metal-silicon contact, the surface silicon can be oxidized, the hydrogen of the termination is the further reducing agent.

Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders

The recently discovered low-load metal-assisted catalytic etching (LL-MACE) creates nanostructured Si with controllable and variable characteristics that distinguish this technique from the conventional high-load variant. LL-MACE employs 150 times less metal catalyst and produces porous Si instead of Si nanowires. In this work, we demonstrate that some of the features of LL-MACE cannot be explained by the present understanding of MACE. With mechanistic insight derived from extensive experimentation, it is demonstrated that (1) the method allows the use of not only Ag, Pd, Pt, and Au as metal catalysts but also Cu and (2) judicious combinations of process parameters such as the type of metal, Si doping levels, and etching temperatures facilitate control over yield (0.065-88%), pore size (3-100 nm), specific surface area (20-310 m²·g^-1), and specific pore volume (0.05-1.05 cm³·g^-1). The porous structure of the product depends on the space-charge layer, which is controlled by the Si doping and the chemical identity of the deposited metal. The porous structure was also dependent on the dynamic structure of the deposited metal. A distinctive comet-like structure of metal nanoparticles was observed after etching with Cu, Ag, Pd, and, in some cases, Pt; this structure consisted of 10-50 nm main particles surrounded by smaller (<5 nm) nanoparticles. With good scalability and precise control of structural properties, LL-MACE facilitates Si applications in photovoltaics, energy storage, biomedicine, and water purification.

Gold Nanoisland Agglomeration upon the Substrate Assisted Chemical Etching Based on Thermal Annealing Process

In this study, we proposed the self-organization process and its localized surface plasmon resonance property (LSPR) to study the effect of chemically treated quartz glass substrates for gold nanoisland array formation. Firstly, we etched a quartz glass substrate using a sputter etching machine. Secondly, n-butanol was treated on the surface of the substrate. Then, we deposited a gold thin film on the substrate with assisted chemical etching. Finally, the self-organization method examined the thermal annealing of gold nanoisland arrays on a substrate. The results showed that the gold nanoisland that was aggregated on an etched quartz glass substrate was large and sparse, while the gold nanoisland aggregated on a chemically treated substrate was small and dense. Further, it was revealed that a substrate’s surface energy reduced chemical treating and increased the gold nanoisland contact angle on the substrate via the thermal annealing process. It was also confirmed that chemical treatment was useful to control the morphology of gold nanoisland arrays on a substrate, particularly when related to tuning their optical property.

Controlling the Nature of Etched Si Nanostructures: High- versus Low-Load Metal-Assisted Catalytic Etching (MACE) of Si Powders

Metal-assisted catalytic etching (MACE) involving Ag deposited on Si particles has been reported as a facile method for the production of Si nanowires (Si NWs). We show that the structure of Si particles subjected to MACE changes dramatically in response to changing the loading of the Ag catalyst. The use of acetic acid as a surfactant and controlled injection of AgNO₃(aq) enhanced Ag deposition. The use of acetic acid and controlled injection of H₂O₂ not only facilitated optimization of the etching step but also allowed us to identify a previously unobserved etching regime that we denote as low-load MACE (LL-MACE). Material produced by LL-MACE exhibits dramatically different yield and structural characteristics as compared to conventionally produced material. We demonstrate the production of Si NWs as well as mesoporous Si nanoparticles from an inexpensive metallurgical-grade Si powder. High loading of Ag (HL-MACE) generates parallel etch track pores created by the correlated motion of Ag nanoparticles. The uniform size distribution (predominantly 70-100 nm) of the Ag nanoparticles is generated dynamically during etching. The walls of these etch track pores are cleaved readily by ultrasonic agitation to form Si NWs. Low loading of Ag (LL-MACE) creates 10-50 nm Ag nanoparticles that etch in an uncorrelated (randomly directed) fashion to generate a bimodal distribution of mesoporosity peaking at ∼4 and 13-21 nm. The use of a syringe pump to deliver the oxidant (H₂O₂) and Ag⁺ is essential for increased product uniformity and yield. Different process temperatures and grades of Si produced significantly different pore size distributions. These results facilitate the production of Si NWs and mesoporous nanoparticles with high yield, low cost, and controlled properties that are suitable for applications in, e.g., lithium-ion batteries, drug delivery, as well as biomedical imaging and contrast enhancement.