Metal-assisted Etching Research Articles

Strategies for controlling Si nanowire formation during Au-assisted electroless etching

Dramatic changes in the morphologies of Si nanowires (NWs) ranging from vertical, to hooked, to ground-collapsed shapes are presented depending on the HF/H2O2 ratio in the etch solution during Au-assisted electroless etching. It is shown that the deviations from vertical NWs are not due to capillary action but correlated with the development of nanoporous structures, which is attributed to excess hole (h+) diffusion toward the NWs' sidewalls during the etching process. The role of hole diffusion is made more evident by observation of NWs with the neck formation when transitioning to different etching regimes during SiNW formation. Furthermore, the hooked and ground-collapsed SiNW can be suppressed at the cost of enhanced tapering by adding a strong electrolyte in solution, which is correlated to reduced etching rate resulting from electric screening of the charged Au surfaces. We provide an important finding that common salts such as NaCl, KCl, and CaCl2 can be useful retardants for the metal-assisted electroless etching. The new insights into structural transformation presented in this study afford greater control over the structure of SiNWs produced by metal-assisted electroless etching.

Controllable growth and optical properties of ZnO nanostructures on Si nanowire arrays

In this paper, two types of hybrid semiconductors made up of silicon nanowires and ZnO nanostructures, namely Si/ZnO core–shell nanowire arrays and ZnO quantum dots (QDs)-decorated Si nanowire arrays, have been prepared by combining metal-assisted wet-chemical etching and metal–organic chemical vapor deposition (MOCVD). We demonstrate that ZnO QDs and thin ZnO layers can be grown on Si nanowires in a controlled manner by varying growth parameters including working pressure and growth time. Meanwhile, porous silicon and porous Si/ZnO nanowire arrays have also been fabricated. The morphology and optical properties of both hybrid nanostructures have been carefully investigated for their potential applications in nanowire optoelectronics. A quantum confinement effect in ZnO QDs was confirmed by the blue-shifted photoluminescence. Porous Si/ZnO core–shell nanowires display a very broad emission band throughout the entire visible light range.

利用奈米技术制作砷化镓之微奈米孔洞<br>Properties of Microporous Nanocomposite GaAs by Metal-Assisted Etching Technique

本研究利用金属辅助式蚀刻技术在无通电下制作出具有微米等级的孔洞于n型砷化镓(GaAs)基板，观察其不同时蚀刻时间条件下砷化镓基板表面型态与反射光谱。实验结果显示随着蚀刻时间的增加孔洞结构越明显，并在蚀刻时间20分钟有较低的反射率。使用近0度角入射之反射式光谱，在未蚀刻前抛光GaAs之反射光谱曲线反射率在50%以下，在400~800 nm范围内平均的反射率为33.69%，使用45度角入射之反射式光谱，未蚀刻前抛光GaAs反射光谱与近0度角入射之镜面反射式光谱架构的光谱都低于50%，400~800 nm波段间之平均反射率为33.56%。 This work reports on the photovoltaic properties of n-type microporous GaAs substrates. The microporous morphology on GaAs substrates were fabricated utilizing Ag metal-assisted etching technique under electroless condition. Then, we investigates the surface morphology of microporous GaAs substrates with different etching times.The experimental results indicated the microporous structures of GaAs are larger and clearly defined as increasing etching time. The lower reflectivity of microporous structures was observed in 20 min of etching time. Incident angle near 0 degrees using the reflection spectrum of GaAs polished without etching before the reflectivity curve of reflectance of 50% or less, in the range of 400~800 nm average reflection rate was 33.69%, with 45-degree angle of reflection for incident Spectra, reflectance spectra of GaAs is not polished and near 0° incidence of the specular reflection spectra of spectral structure are less than 50%, 400~800 nm band average reflectance between the rate of 33.56%.

Macro and quasi‐mesoporous silicon by self‐assembling and metal assisted etching

AbstractThe combination of two recent techniques developed in the last years demonstrates the possibility to obtain regular and semi‐ordered macro and mesopores on any type of silicon substrates and with holes diameter ranging from 800 to 60 nm and less. Self‐assembling of polystyrene nanospheres (PSNS) is obtained by floating technique, then the 2D crystal is lifted on a silicon substrate with a 30 nm silver thin film deposited by thermal evaporation or sputtering. The nanospheres are then reduced in diameter by reactive ion etching (RIE) in oxygen plasma, then the samples are exposed to Ar ions sputter‐etching (SE) for thin film structuration. At this point of the process the polystyrene nanosphere mask is removed and a metal assisted etching (MAE) of few minutes is performed. Ordered and regular pores of diameters ranging from 500 to 60 nm have been obtained.magnified image Macropores (450 nm) obtained by self‐assembly nanolithography and metal assisted etching.

Fabrication of Silicon Nanostructured Thin Film and Its Transfer from Bulk Wafers onto Alien Substrates

Various Si nanostructures can be fabricated using a metal-assisted etching technique, which must be applied on bulk Si wafers, limiting its applications and wasting a significant amount of material. Here, we report a technique to form a Si nanostructured thin film created by metal-assisted chemical etching from bulk Si wafers and to transfer it onto alien substrates. To detach the Si nanostructured thin films completely from bulk Si wafers, a second-step metal-assisted chemical etching made the root of the Si nanostructures become fragile. The transferred Si nanostructures are well-aligned along the normal direction of the receiver substrate. The X-ray diffraction spectrum reveals that the transferred Si nanostructured thin films exhibit good crystal orientation and morphology. A strong light trapping effect between the nanostructures causes such films of thickness to exhibit nearly 99% absorption from . This exceeds the theoretically calculated limits of planar Si.

Fabrication of nanostructured silicon by metal-assisted etching and its effects on matrix-free laser desorption/ionization mass spectrometry

A matrix-free, high sensitivity, nanostructured silicon surface assisted laser desorption/ionization mass spectrometry (LDI-MS) method fabricated by metal-assisted etching was investigated. Effects of key process parameters, such as etching time, substrate resistance and etchant composition, on the nanostructured silicon formation and its LDI-MS efficiency were studied. The results show that the nanostructured silicon pore depth and size increase with etching time, while MS ion intensity increases with etching time to 300 s then decreases until 600 s for both low resistance (0.001–0.02 Ω cm) and high resistance (1–100 Ω cm) silicon substrates. The nanostructured silicon surface morphologies were found to directly affect the LDI-MS signal ion intensity. By characterizing the nanostructured silicon surface roughness using atomic force microscopy (AFM) and sample absorption efficiency using fluorescence microscopy, it was further demonstrated that the nanostructured silicon surface roughness was highly correlated to the LDI-MS performance.

Metal-assisted electrochemical etching of silicon

In this paper the metal-assisted electrochemical etching of silicon is introduced. Byelectrochemical measurement and sequent simulation, it is revealed that the potential ofthe valence band maximum at the silicon/metal interface is more negative thanthat of the silicon/electrolyte interface. Accordingly, holes injected from the backcontact are driven preferentially to the silicon/metal interface. Consequently,silicon below metal is electrochemically etched much faster than a naked siliconsurface without metal coverage. Metals such as Ag and Cu have been utilizedto catalyze the electrochemical etching. Feature sizes as small as 30 nm can beachieved by metal-assisted electrochemical etching. Meanwhile, the metal-assistedelectrochemical etching method enables convenient control over the etching direction ofnon-(100) substrates, and facilitates the fabrication of orientation-modulated siliconnanostructures.

Influence of pre-surface treatment on the morphology of silicon nanowires fabricated by metal-assisted etching

Herein we demonstrate an improved metal-assisted etching method to achieve highly dense and uniform silicon nanowire arrays. A pre-surface treatment was applied on a silicon wafer before the process of metal-assisted etching in silver nitrate and hydrogen fluoride solution. The treatment made silver ion continuously reduce on silver nuclei adherence on the silicon surface, leading to formation of dense silver nanoparticles. Silver nanoparticles acting as local redox centers cause the formation of dense silicon nanowire arrays. In contrast, an H-terminated silicon surface made silver ion reduce uniformly on the silicon surface to form silver flakes. The silicon nanowires fabricated with a pre-surface treatment reveals higher density than those fabricated without a pre-surface treatment. The volume fraction improves from 18 to 38%. This improvement reduces the solar-weighted reflectance to as low as 3.3% for silicon nanowires with a length of only 0.87 μm. In comparison, the silicon nanowires fabricated without a pre-surface treatment have to be as long as 1.812 μm to achieve the same reflectance.

Silicon Porosification: State of the Art

This review is devoted to the analysis of the problems related to fabrication of the Si porous layers. The review was motivated by a great interest to Si-based porous materials from nano- to macro-scale for various applications in electronics, optoelectronics, photonics, chemical sensors, biosensors, etc. The peculiarities of the silicon porosification and the principles of preparing porous layers are considered in the present article. Various methods used for Si porosification such as chemical stain etching, chemical vapor etching, laser-induced etching, metal-assisted etching, spark processing and reactive ion (plasma) etching were analyzed. However, the main attention was focused on electrochemical porosification of Si. The review discusses in detail the influence of parameters such as electrolyte composition and pH, current density, etching time, temperature, wafer doping and orientation, lighting, magnetic field, and ultrasonic agitation on the process of Si porosification. It was shown that the structure of porous silicon strongly depends on both technological parameters of electrochemical etching and the parameters of the semiconductor subject to treatment. This review also addresses the main properties of porous silicon, porous multilayer and 3D structure formation, oxidation of porous Si, release of the porous layer, drying, storage, etching, filling and surface functionalizing of porous Si. Features of III-V compound porosification are also briefly analyzed.

Porous Si layers—Preparation, characterization and morphology

Porous silicon layers (PSL) of nano- and micro-structures were prepared by metal-assisted electroless etching of silicon in HF–oxidizing agent aqueous solutions. The effect of oxidizing agent and HF content on the characteristics of the formed porous layers was investigated. A thin Pt film was electroless deposited on p-Si〈1 0 0〉 prior to immersion in the etching solution. The properties and morphology of the PSL formed by this method were investigated by electrochemical impedance spectroscopy (EIS) scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) technique. The characteristics of the PSL were found to be affected by the constituents of the etching medium and also, the etching time. Potassium bromate (KBrO 3), potassium iodate (KIO 3), and potassium dichromate (K 2Cr 2O 7) have been used as oxidizing agents. Pt-assisted etching of p-Si for 1 h in an etching solution consisting of 22.0 M HF and 0.05 M of KBrO 3, results in the formation of nano- and micro-pores on the Si surface. The use of 0.05 M KIO 3 or K 2Cr 2O 7 as oxidizing agent has led to the formation of a deposit on the silicon surface. At relatively higher concentration [>0.05 M] of K 2Cr 2O 7 the surface deposit becomes clear and was found to consist of an insoluble passive solid-phase of K 2SiF 6 which increases the film impedance and blocks the porous structure formation. The use of higher concentration [>22 M] of HF in the etching electrolyte is accompanied by an increase in the dissolution rate of the insoluble K 2SiF 6 layer and a decrease in the PSL passivity. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model which accounts for the mechanism of the porous film formation at the Si/electrolyte interface.

A strong antireflective solar cell prepared by tapering silicon nanowires

Vertically aligned silicon nanowires (SiNWs) were cost-effectively formed on a four-inch silicon wafer using a simple room temperature approach, i.e., metal-assisted electroless etching. Tapering the NWs by post-KOH dipping achieved separation of each NW from the bunched NW, resulting in a strong enhancement of broadband optical absorption. As electroless etching time increases, the optical crossover feature was observed in the tradeoff between enhanced light trapping (by graded-refractive index during initial tapering) and deteriorated reflectance (by decreasing the areal density of NWs during later tapering). Compared to the bunched SiNWs, tapered NW solar cells demonstrated superior photovoltaic characteristics, such as a short circuit current of 17.67 mA/cm² and a cell conversion efficiency of ~6.56% under 1.5 AM illumination.

Oxidation Rate Effect on the Direction of Metal-Assisted Chemical and Electrochemical Etching of Silicon

Assisted by noble metal particles, non-(100) Si substrates were etched in solutions with different oxidant concentrations at different temperatures. The etching directions of (110) and (111) Si substrates are found to be influenced by the concentration of oxidant in etching solutions. In solutions with low oxidant concentration, the etching proceeds along the crystallographically preferred ⟨100⟩ directions, whereas the etching occurs along the vertical direction relative to the surface of the substrate in solutions with high oxidant concentration. These phenomena are found for both n- and p-type substrates as well as in experiments with different oxidants. The experiments on metal-assisted chemical etching are complemented by additional experiments on metal-assisted electrochemical etching of (111) Si substrates with different current densities. As a function of current density, a change of etching directions is observed. This shows that the change of the etching directions is mainly driven by the oxidati...

Fabrication of silicon nanopillar-based nanocapacitor arrays

We report the fabrication of silicon nanopillar-based nanocapacitor arrays using metal-assisted etching in conjunction with electrodeposition. The high aspect ratio made possible by the catalyzed etching provides for an increased effective electrode area and hence a significant improvement in the capacitance density. Electroplated Ni electrode forms a conformal layer over the silicon nanopillars. Capacitance measurements show the expected trend as a function of pillar height and array period. The fabrication approach is simple, compatible with integration into standard silicon technology, and easily scalable.

Silicon Surface Structure-Controlled Oleophobicity

Superoleophobic surfaces display contact angles >150 degrees with liquids that have lower surface energies than does water. The design of superoleophobic surfaces requires an understanding of the effect of the geometrical shape of etched silicon surfaces on the contact angle and hysteresis observed when different liquids are brought into contact with these surfaces. This study used liquid-based metal-assisted etching and various silane treatments to create superoleophobic surfaces on a Si(111) surface. Etch conditions such as the etch time and etch solution concentration played critical roles in establishing the oleophobicity of Si(111). When compared to Young's contact angle, the apparent contact angle showed a transition from a Cassie to a Wenzel state for low-surface-energy liquids as different silane treatments were applied to the silicon surface. These results demonstrated the relationship between the re-entrant angle of etched surface structures and the contact angle transition between Cassie and Wenzel behavior on etched Si(111) surfaces.

Ordered nanopore boring in silicon: Metal-assisted etching using a self-aligned block copolymer Au nanoparticle template and gravity accelerated etching

Metal-assisted etching into Si (1 0 0) surfaces can be performed in a highly defined and regular arrangement using self-organized patterns of single-size gold catalyst particles that are block polymer templated on Si surfaces. We show that small size catalyst particles (diameter ≈10 nm) can be forced to maintain straight etch tracks perpendicular to the surface using adequate centrifugal gravity force. This allows the creation of highly ordered uniform and synchronized pore boring into the substrate with single pore diameters in the 10 nm range.

Sub-100 nm silicon nanowires by laser interference lithography and metal-assistedetching

By combining laser interference lithography and metal-assisted etching we were able to producearrays of silicon nanowires with uniform diameters as small as 65 nm and densities exceeding2 × 107 mm − 2. Thewires are single crystalline, vertically aligned, arranged in a square pattern and obey strict periodicity overseveral cm2. The applied technique allows for a tailoring of nanowire size and density. Using acontrolled and scalable process to fabricate sub-100 nm silicon nanowires is an importantstep towards the realization of cost-effective electronic and thermoelectric devices.

Electroless porous silicon formation applied to fabrication of boron–silica–glass cantilevers

This work describes the characterization and optimization of anisotropic formation of porous silicon in large volumes (0.5–1 mm3) of silicon by an electroless wet etching technique. The main goal is to use porous silicon as a sacrificial volume for bulk micromachining processes, especially in cases where etching of the full wafer thickness is needed. The porous silicon volume is formed by a metal-assisted etching in a wet chemical solution composed of hydrogen peroxide (30%), hydrofluoric acid (40%) and ethanol. This paper focuses on optimizing the etching conditions in terms of maximizing the etching rate and reproducibility of the etching. In addition to that, a study of the morphology of the pore that is obtained by this technique is presented. The results from the characterization of the process are applied to the fabrication of boron–silica–glass cantilevers that serve as a platform for bio-chemical sensors. The porous silicon volume is formed in an early step of the fabrication process, allowing easy handling of the wafer during all of the micromachining processes in the process flow. In the final process step, the porous silicon is quickly etched by immersing the wafer in a KOH solution.

Metal-assisted etching of p-type silicon under anodic polarization in HF solution with and without H2O2

Pore formation under anodic polarization of a lightly doped p-type Si wafer previously loaded with Pt, Pd and Ag nanoparticles was investigated in HF solution with and without H 2 O 2 . In HF solution without H 2 O 2 , a microporous layer was formed in p-Si loaded with Pt or Pd. However, Ag metal nanoparticles yielded pores due to their intrusion in the Si wafer. The addition of H 2 O 2 to the etching solution leads to different pore morphologies depending on the metals. Particles of Ag were found at the bottom of most pores. In the presence of Pt nanoparticles, cone-shaped macropores were produced, and the pore depth and diameter increased with increasing H 2 O 2 content. Current density influenced the pore morphology. For a sample loaded with Pt or Ag, an increase in applied current density widened the pore diameter. The mechanism of the metal-assisted pore formation was discussed by considering a competitive process between the formation of a microporous layer under polarization and metal-assisted chemical oxidation of the microporous layer by a dissolved oxidizing agent.

Ordered Arrays of Vertically Aligned [110] Silicon Nanowires by Suppressing the Crystallographically Preferred <100> Etching Directions

The metal-assisted etching direction of Si(110) substrates was found to be dependent upon the morphology of the deposited metal catalyst. The etching direction of a Si(110) substrate was found to be one of the two crystallographically preferred 100 directions in the case of isolated metal particles or a small area metal mesh with nanoholes. In contrast, the etching proceeded in the vertical [110] direction, when the lateral size of the catalytic metal mesh was sufficiently large. Therefore, the direction of etching and the resulting nanostructures obtained by metal-assisted etching can be easily controlled by an appropriate choice of the morphology of the deposited metal catalyst. On the basis of this finding, a generic method was developed for the fabrication of wafer-scale vertically aligned arrays of epitaxial [110] Si nanowires on a Si(110) substrate. The method utilized a thin metal film with an extended array of pores as an etching catalyst based on an ultrathin porous anodic alumina mask, while a prepatterning of the substrate prior to the metal depostion is not necessary. The diameter of Si nanowires can be easily controlled by a combination of the pore diameter of the porous alumina film and varying the thickness of the deposited metal film.

Using Self-Assembled Nanoparticles to Fabricate and Optimize Subwavelength Textured Structures in Solar Cells

This paper reports the preparation of optimal textured structures on silicon surfaces through metal-assisted etching using gold nanoparticles (Au NPs) as catalysts in HF/H 2 O 2 solution. The size and density of the NPs on the Si substrates were readily and rapidly determined using spectroscopic ellipsometry. We obtained uniformly textured Si layers containing cylindrical, conical, and bowl-shaped features after immersing the Au-deposited Si substrates in a mixture of HF and H 2 O 2 under various etching conditions. A textured surface possessing close-packed pyramidal features with dimensions on the subwavelength scale exhibited the lowest reflectance (<0.5%) over the entire visible and near-IR spectrum. This low reflectance arose from the refractive index gradient of the Si surface and light trapping phenomena. We used a 3D finite-difference time domains simulation to examine the distribution of light propagating on the textured structures within the near-field regime.