Metal-catalyzed Etching Research Articles

Inverted pyramid structures fabricated on monocrystalline silicon surface with a NaOH solution

Low-cost aqueous alkaline etching has been widely adopted for monocrystalline silicon surface texturing in current industrial silicon solar cells. However, conventional alkaline etching can only prepare upright pyramid structures on mono-crystalline silicon surfaces. This study demonstrates for the first time the use of ethylene glycol butyl ether (EGBE) to regulate aqueous anisotropic alkaline etching and prepare inverted pyramid structures on monocrystalline silicon surfaces. Acidic metal-catalyzed etching solutions are not the best choice for monocrystalline silicon due to their inherent disadvantages, such as noble metal pollution and relatively high costs. The one-step method to produce the inverted pyramid structures by using alkaline etch with EGBE additive is simple and inexpensive, does not generate noble metal pollution, and especially compatible with current industrial silicon solar cell production lines. With the use of a sodium hydroxide (NaOH) solution containing a low-cost additive, inverted pyramid structures can be prepared on mono-crystalline silicon surface in a short time. This method is suitable for various types of silicon wafers and has great potential for industrial solar cell applications.

(invited) Metal-Catalyzed Etching of Graphene Layers

We present a comparative investigation on the etching of graphene layers catalyzed by Fe and Cu and demonstrate that their strikingly different etching behaviors are governed by their distinct interactions with carbon. Metal thin films are deposited by sputtering onto mechanically exfoliated graphene, few-layer graphene (FLG), and graphite flakes on a Si/SiO2 substrate. When the sample is rapidly annealed in forming gas or nitrogen, particles are produced due to the dewetting of the metal thin film. The evidence from the low-voltage scanning electron microscopy and Raman spectroscopy reveals that, at 950 oC in forming gas, the particles catalytically etch channels in graphite. No etching is observed on graphite for the Fe thin film annealed in nitrogen, indicating that this graphite etching process is catalyzed by Fe particles through the carbon hydrogenation reaction. Due to the strong Fe-C interactions, graphene and FLG are severely damaged through not only catalytic carbon hydrogenation but also carbon dissolution into Fe alone. By comparing with the etched monolayer and FLG in nitrogen, we identify that, in forming gas, the catalytic etching of monolayer and FLG is through carbon hydrogenation. During carbon hydrogenation, Fe particles are catalytically active in the dissociation of hydrogen into hydrogen atoms and in the production of hydrogenated amorphous carbon through hydrogen spillover. For the Cu case, while no etching takes place for Cu particles on graphite in either gas environment at 1050 oC, Cu particles catalyze the carbon hydrogenation reaction on graphene and FLG. The weak interaction between Cu and graphene, along with the low solubility of carbon in Cu, makes the Cu particles ideal for tracking their etching paths on graphene layers. Further temperature-dependent study shows that no catalytic etching occurs at 850 oC. Both etching from the graphene step edge and on the graphene basal plane are observed for Cu particles at 950 oC and vertical etching takes place in graphite at 1150 oC, resulting in the formation of hexagon pits with all zigzag edges.

Chemical wet etching of germanium assisted with catalytic-metal-particles and electroless-metal-deposition

Ge nanostructures were fabricated by wet processes based on etching assisted with the catalytic-metal-particles and the electroless-metal-deposition under various conditions. In the etching assisted with catalytic-metal-particles, we used HF+H2O2 solutions after forming Ag particles on the Ge wafer surfaces from an AgNO3 solution. We succeeded in fabrication of porous layers on the Ge substrates and found that the pore sidewall inclination angles with respect to the surfaces can be controlled by the H2O2 concentrations. In the etching assisted with electroless-metal-deposition, we fabricated whisker-like nanostructures and demonstrated that their size and morphology can be varied by changing the AgNO3 concentration, deposition/etching time and the substrate resistivity. We have revealed differences between Ge and Si in the metal-catalyzed etching and pointed out problems in Ge nanowire fabrication via a wet chemical etching.

High efficiency silicon nanowire/organic hybrid solar cells with two-step surface treatment.

A simple two-step surface treatment process is proposed to boost the efficiency of silicon nanowire/PEDOT:PSS hybrid solar cells. The Si nanowires (SiNWs) are first subjected to a low temperature ozone treatment to form a surface sacrificial oxide, followed by a HF etching process to partially remove the oxide. TEM investigation demonstrates that a clean SiNW surface is achieved after the treatment, in contrast to untreated SiNWs that have Ag nanoparticles left on the surface from the metal-catalyzed etching process that is used to form the SiNWs. The cleaner SiNW surface achieved and the thin layer of residual SiO2 on the SiNWs have been found to improve the performance of the hybrid solar cells. Overall, the surface recombination of the hybrid SiNW solar cells is greatly suppressed, resulting in a remarkably improved open circuit voltage of 0.58 V. The power conversion efficiency has also increased from about 10% to 12.4%. The two-step surface treatment method is promising in enhancing the photovoltaic performance of the hybrid silicon solar cells, and can also be applied to other silicon nanostructure based solar cells.

Two-fluid wetting behavior of a hydrophobic silicon nanowire array.

The two-fluid wetting behavior of surfaces textured by an array of silicon nanowires is investigated systematically. The Si nanowire array is produced by a combination of colloidal patterning and metal-catalyzed etching, with control over its roughness depending upon the wire length. The nanowires are made hydrophobic and oleophobic by treatment with hydrocarbon and fluorinated self-assembled monolayers, respectively. Static, advancing, and receding contact angles are measured with water, hexadecane, and perfluorotripentylamine in both single-fluid (droplet on solid in an air environment) and two-fluid (droplet on solid in a liquid environment) configurations. The single-fluid measurements show wetting behavior similar to that expected by the Wenzel and Cassie-Baxter models, where the wetting or non-wetting behaviors are amplified with increasing roughness. The two-fluid systems on the rough surface exhibit more complex configurations because either the droplet or the environment fluid can penetrate the asperities depending upon the wettability of each fluid. It is observed that, when the Young contact angles are significantly increased or reduced from single-liquid to two-liquid systems, the effect of roughness is relatively minimal. However, when the Young contact angles are similar, roughness has almost identical influence on apparent contact angles in single- and two-liquid systems. The Wenzel and Cassie-Baxter models are modified to describe various two-fluid wetting states. In cases where metastable behavior is observed for the droplet, advancing and receding measurements are performed to suggest the equilibrium state of the droplet.

Metal-catalyzed etching of graphene governed by metal–carbon interactions: A comparison of Fe and Cu

We present a comparative investigation on the etching of graphene catalyzed by Fe and Cu. When Fe or Cu thin film deposited on graphene is rapidly annealed in either N2 or forming gas (10% H2/90% N2), particles are produced due to the dewetting of thin films. Low-voltage scanning electron microscopy reveals different morphology for Fe and Cu particles and their strikingly different catalytic etching behaviors. For the Fe thin film on graphene annealed at 950°C in either gas environment, graphene is severely damaged, suggesting that the etching could occur through catalytic carbon hydrogenation or carbon dissolution into Fe due to the strong Fe–C interactions. In contrast, while no etching takes place for Cu particles on graphene at 1050°C in N2, Cu particles catalytically etch channels in graphene in forming gas through carbon hydrogenation, and the width of the channel is much narrower than the diameter of Cu particle due to the non-wetting behavior of Cu on graphene. The weak interactions between Cu and graphene, along with the low solubility of carbon in Cu, make Cu particles ideal for tracking their etching paths on graphene. This work provides new insights into the metal-catalyzed etching of graphene.

Low-Contact-Resistance Graphene Devices with Nickel-Etched-Graphene Contacts

The performance of graphene-based transistors is often limited by the large electrical resistance across the metal-graphene contact. We report an approach to achieve ultralow resistance metal contacts to graphene transistors. Through a process of metal-catalyzed etching in hydrogen, multiple nanosized pits with zigzag edges are created in the graphene portions under source/drain metal contacts while the graphene channel remains intact. The porous graphene source/drain portions with pure zigzag-termination form strong chemical bonds with the deposited nickel metallization without the need for further annealing. This facile contact treatment prior to electrode metallization results in contact resistance as low as 100 Ω·μm in single-layer graphene field-effect transistors, and 11 Ω·μm in bilayer graphene transistors. Besides 96% reduction in contact resistance, the contact-treated graphene transistors exhibit 1.5-fold improvement in mobility. More importantly, the metal-catalyzed etching contact treatment is compatible with complementary metal-oxide-semiconductor (CMOS) fabrication processes, and holds great promise to meet the contact performance required for the integration of graphene in future integrated circuits.

Solid–liquid–vapor metal-catalyzed etching of lateral and vertical nanopores

Etching is an essential tool for the creation of nanostructures, where patterned metal structures can be used as masks. Here, we investigate HCl gas etching of InP substrates decorated with Au nanoparticles, and find that the etch rate is strongly increased at the Au–InP interfaces. The {111}A facets of the InP are preferentially etched. The metal nanoparticles follow in the etch direction, thereby creating nanopores. The size and position of the pores is controlled by the Au nanoparticles, and we measure nanopores as thin as 20 nm with an aspect ratio of 25:1. The direction of the nanopores is influenced by the temperature and the substrate orientation, which we use to demonstrate lateral, vertical and inclined nanopores. We explain the process by a solid–liquid–vapor model, in which the liquid metal particle catalyzes the dissolution of the solid InP.