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Black silicon was discovered by accident and considered an undesirable by-product of the silicon industry. A highly modified surface, consisting of pyramids, needles, holes, pillars, etc., provides high light absorption from the UV to the NIR range and gives black silicon its color—matte black. Although black silicon has already attracted some interest as a promising material for sensitive sensors, the potential of this material has not yet been fully exploited. Over the past three decades, black silicon has been actively introduced as a substrate for surface-enhanced Raman spectroscopy (SERS)—a molecule-specific vibrational spectroscopy technique—and successful proof-of-concept experiments have been conducted. This review focuses on the current progress in black silicon SERS biosensor fabrication, the recent advances in the design of the surface morphology and an analysis of the relation of surface micro-structuring and SERS efficiency and sensitivity. Much attention is paid to problems of non-invasiveness of the technique and biocompatibility of black silicon, its advantages over other SERS biosensors, cost-effectiveness and reproducibility, as well as the expansion of black silicon applications. The question of existing limitations and ways to overcome them is also addressed. [ABSTRACT FROM AUTHOR]

In this study, the effects of variations in the height ( h ) and bottom radius ( r ) of black silicon microstructures on their absorptance and photoelectric response efficiency were analyzed. By using the relation cot ⁡ θ 2 = h r to combine the parameters, it was found that changes in morphology affected the absorptance of black silicon microstructures, with h being directly proportional to the absorptance, while r was inversely proportional. A positive correlation was observed between cot ⁡ θ 2 and absorptance. However, the correlation between cot ⁡ θ 2 and photoelectric response efficiency was not significant. Through Raman spectroscopy analysis of the samples, it was concluded that as the laser ablation energy density increased, more lattice defects were introduced, weakening the charge carrier transport efficiency. This study further elucidated the mechanism by which microstructural changes impacted the absorptance and energy density of black silicon, providing valuable insights for optimizing its energy density. [ABSTRACT FROM AUTHOR]

This study investigates the optical properties (light reflectance, absorptance, transmittance, and scattering) of black silicon (b‐Si) layers formed using reactive ion etching method. The corresponding spectra are determined across the visible, near‐infrared, and near‐ultraviolet wavelength ranges (250–1400 nm). It is demonstrated that b‐Si layers reduce the reflectance, and increase the absorptance and light scattering due to the disordered distribution of the nanoneedles. Increasing the etching duration strengthens this trend. It is shown that b‐Si layers etched for 10 min can be considered perfect light scatterers upon reflection at wavelengths less than 700 nm. Based on the obtained results, the possible profits of light scattering from the b‐Si interlayer for perovskite/Si tandem solar cells are analyzed. It is substantiated that the b‐Si interlayer can increase the useful absorptance not only within the bottom Si solar subcell but also in the top perovskite solar subcell. The prospects for future research directions and challenges are also provided. [ABSTRACT FROM AUTHOR]

The morphology of regular and uniform arrays of black silicon structures was evaluated for bactericidal efficacy against gram-positive, non-motile Staphylococcus epidermidis (S. epidermidis). In this study, uniform and regular arrays of black silicon structures were fabricated using nanosphere lithography and deep reactive ion etching. The effects of nanomorphology on bacterial killing were systematically evaluated using silicon nanostructures with pitches ranging from 300 to 1400 nm pitch on spherical cocci approximately 500 to 1000 nm in diameter. Our results show that nanostructure morphology factors such as height and roughness do not directly determine bactericidal efficacy. Instead, the spacing between nanostructures plays a crucial role in determining how bacteria are stretched and lysed. Nanostructures with smaller pitches are more effective at killing bacteria, and an 82 ± 3% enhancement in bactericidal efficacy was observed for 300 nm pitch nanoneedles surface compared to the flat control substrates. [ABSTRACT FROM AUTHOR]

Based on the ultrafast and extremely strong interaction between laser pulses and materials, ultrafast laser irradiation can break the solid solubility constraints and enable hyperdoping of impurities. This process overcomes the bandgap constraints of crystalline silicon, resulting in heightened absorption across a broad spectral range spanning from ultraviolet to infrared wavelengths, therefore commonly referred to as black silicon (b‐Si). The b‐Si demonstrates significant changes in optoelectronic properties, making it highly promising for applications in silicon photonics. Specifically, b‐Si photodetectors exhibit distinct advantages in terms of high photoelectric gain at low voltage, ultrabroadband spectral responsivity, large dynamic range, and suitability for operation over a wide temperature range. These properties address the limitations of traditional silicon photodetectors, showcasing great potential for applications in optoelectronic integration, artificial intelligence, information technology, energy devices, and beyond. This review focuses on b‐Si achieved through ultrafast laser processing, with a special emphasis on its applications in photodetectors. The mechanism of ultrafast laser irradiation and the properties of hyperdoped silicon are discussed. Then, the research progresses and state‐of‐the‐art b‐Si photodetectors are introduced, as well as working mechanism and potential application expansion. Finally, the development prospects of b‐Si photodetectors based on ultrafast laser hyperdoping are predicted. [ABSTRACT FROM AUTHOR]

This article discusses a method for forming black silicon using plasma etching at a sample temperature range from −20 °C to +20 °C in a mixture of oxygen and sulfur hexafluoride. The surface morphology of the resulting structures, the autocorrelation function of surface features, and reflectivity were studied depending on the process parameters—the composition of the plasma mixture, temperature and other discharge parameters (radical concentrations). The relationship between these parameters and the concentrations of oxygen and fluorine radicals in plasma is shown. A novel approach has been studied to reduce the reflectance using conformal bilayer dielectric coatings deposited by atomic layer deposition. The reflectivity of the resulting black silicon was studied in a wide spectral range from 400 to 900 nm. As a result of the research, technologies for creating black silicon on silicon wafers with a diameter of 200 mm have been proposed, and the structure formation process takes no more than 5 min. The resulting structures are an example of the self-formation of nanostructures due to anisotropic etching in a gas discharge plasma. This material has high mechanical, chemical and thermal stability and can be used as an antireflective coating, in structures requiring a developed surface—photovoltaics, supercapacitors, catalysts, and antibacterial surfaces. [ABSTRACT FROM AUTHOR]

The impact of three different pulse durations (100 fs, 1, and 10 ps) on the formation of laser hyperdoped black silicon with respect to surface morphology, sub‐bandgap absorptance, the sulfur concentration profile, and the effective minority carrier lifetime after Al2O3 surface passivation is investigated. The current flow behavior is compared through the hyperdoped layer by I–V measurements after hyperdoping with different pulse durations. For conditions that give the same absolute sub‐bandgap absorptance, an increase in pulse duration from 100 fs to 10 ps results in a shallower sulfur concentration profile. Findings are explained by an increasing ablation threshold from 0.19 J cm−2 for a pulse duration of 100 fs to 0.21 J cm−2 for 1 ps and 0.34 J cm−2 for 10 ps. The formation of an equally absorbing layer with a shallower doping profile results in a reduction in contact and/or sheet resistance. Despite the higher local sulfur concentration, the samples show no decrease in carrier lifetime measured by quasi‐steady‐state photoconductance decay on Al2O3 surface‐passivated samples. The investigation shows that longer pulses of up to 10 ps during laser hyperdoping of silicon result in advanced layer properties that promise to be beneficial in a potential device application. [ABSTRACT FROM AUTHOR]

The modulation of the surface of silicon stimulates its applications in optics (antireflection surface, solar cells, and photoelectric devices). In this work, an appropriate ratio of the O2 to SF6 plasma through reactive ion etching generated a double-layer Silicon nanograss (upper layerflocculent SiOxFy passivation; lower layerpointy silicon spike) on the surface of the silicon wafer. The height and density of Silicon nanograss can be controlled through process parameters (pressure, O2/SF6 ratio, and RF power), mask spacing, and a variety of assistant substrates. The underlying formation mechanisms of Silicon nanograss disclosed that the sputtering of Al substrate acts as a micromask, i.e., primarily responsible for the formation of Silicon nanograss. Considering an in-depth study, a simple, low-cost, and CMOS-compatible method for wafer-level preparation of Silicon nanograss is provided. Specifically, the Silicon nanograss exhibited excellent antireflection and enhanced absorption properties. This work contributes to micro-nano surface science accompanied by optical applications. [ABSTRACT FROM AUTHOR]

The results of a study of the angular dependence of the reflectivity of black silicon structures with conical and filamentary nanowires, and a silicon surface with a textured pyramidal surface coated with an ITО layer are presented. The possibility to increase the annual electricity generation for solar cells based on black silicon has been demonstrated due to the weak angle dependence of the total reflectance. Compared to the textured pyramidal surface, the increase is 7.34 and 6.33% for the solar nanowires, respectively. [ABSTRACT FROM AUTHOR]

Black silicon (BS) layers coated with passivation films are widely used as antireflective frontal surfaces for solar cells. The most common BS fabrication techniques are reactive ion etching (RIE), metal‐assisted chemical etching, and laser‐induced processing. Herein, the structural and optical properties, as well as the minority carrier lifetime, of BS are compared with and without atomic layer deposited HfO2 passivation films produced by the above formation methods. The antireflection behavior of the samples is discussed based on the light trapping effect and the change in the BS refractive index from air to the bulk of crystalline Si. Finally, test solar cells are manufactured, and their photovoltaic parameters are studied. The comparison results show that RIE is the most preferred in all technical respects. The features of using different BS fabrication techniques from the solar cell manufacturing point of view are analyzed. [ABSTRACT FROM AUTHOR]

The wettability of black silicon (b-Si) layers formed by reactive ion etching, metal-assisted chemical etching, and laser-induced etching has been studied. The wetting contact angles of the prepared samples with deionized water, glycerol, diiodomethane, and ethylene glycol were determined. It has been shown that the silicon oxide surface film and the enlargement area factor of b-Si layers have a significant influence on their wettability, varying from hydrophilic to hydrophobic properties. [ABSTRACT FROM AUTHOR]

The impact of three different pulse durations (100 fs, 1, and 10 ps) on the formation of laser hyperdoped black silicon with respect to surface morphology, sub‐bandgap absorptance, the sulfur concentration profile, and the effective minority carrier lifetime after Al2O3 surface passivation is investigated. The current flow behavior is compared through the hyperdoped layer by I–V measurements after hyperdoping with different pulse durations. For conditions that give the same absolute sub‐bandgap absorptance, an increase in pulse duration from 100 fs to 10 ps results in a shallower sulfur concentration profile. Findings are explained by an increasing ablation threshold from 0.19 J cm−2 for a pulse duration of 100 fs to 0.21 J cm−2 for 1 ps and 0.34 J cm−2 for 10 ps. The formation of an equally absorbing layer with a shallower doping profile results in a reduction in contact and/or sheet resistance. Despite the higher local sulfur concentration, the samples show no decrease in carrier lifetime measured by quasi‐steady‐state photoconductance decay on Al2O3 surface‐passivated samples. The investigation shows that longer pulses of up to 10 ps during laser hyperdoping of silicon result in advanced layer properties that promise to be beneficial in a potential device application.

In this study, the effects of variations in the height (h) and bottom radius (r) of black silicon microstructures on their absorptance and photoelectric response efficiency were analyzed. By using the relation cot⁡θ2=hr to combine the parameters, it was found that changes in morphology affected the absorptance of black silicon microstructures, with h being directly proportional to the absorptance, while r was inversely proportional. A positive correlation was observed between cot⁡θ2 and absorptance. However, the correlation between cot⁡θ2 and photoelectric response efficiency was not significant. Through Raman spectroscopy analysis of the samples, it was concluded that as the laser ablation energy density increased, more lattice defects were introduced, weakening the charge carrier transport efficiency. This study further elucidated the mechanism by which microstructural changes impacted the absorptance and energy density of black silicon, providing valuable insights for optimizing its energy density.

Black silicon was discovered by accident and considered an undesirable by-product of the silicon industry. A highly modified surface, consisting of pyramids, needles, holes, pillars, etc., provides high light absorption from the UV to the NIR range and gives black silicon its color—matte black. Although black silicon has already attracted some interest as a promising material for sensitive sensors, the potential of this material has not yet been fully exploited. Over the past three decades, black silicon has been actively introduced as a substrate for surface-enhanced Raman spectroscopy (SERS)—a molecule-specific vibrational spectroscopy technique—and successful proof-of-concept experiments have been conducted. This review focuses on the current progress in black silicon SERS biosensor fabrication, the recent advances in the design of the surface morphology and an analysis of the relation of surface micro-structuring and SERS efficiency and sensitivity. Much attention is paid to problems of non-invasiveness of the technique and biocompatibility of black silicon, its advantages over other SERS biosensors, cost-effectiveness and reproducibility, as well as the expansion of black silicon applications. The question of existing limitations and ways to overcome them is also addressed.

For uncooled infrared cameras based on microbolometers, silicon caps are often utilized to maintain a vacuum inside the packaged bolometer array. To reduce Fresnel reflection losses, anti-reflection coatings are typically applied on both sides of the silicon caps.This work investigates whether black silicon may be used as an alternative to conventional anti-reflective coatings. Reactive ion etching was used to etch the black silicon layer and deep cavities in silicon. The effects of the processed surfaces on optical transmission and image quality were investigated in detail by Fourier transform infrared spectroscopy and with modulated transfer function measurements. The results show that the etched surfaces enable similar transmission to the state-of-the-artanti-reflection coatings in the 8–12 µm range and possibly obtain wider bandwidth transmission up to 24 µm. No degradation in image quality was found when using the processed wafers as windows. These results show that black silicon can be used as an effective anti-reflection layer on silicon caps used in the vacuum packaging of microbolometer arrays. [ABSTRACT FROM AUTHOR]

Superhydrophilic/superhydrophobic patterned surfaces can be used to create droplet microarrays. A specific challenge with the liquids needed for various biomedical applications, as compared to pure water, is their lower surface tension and potential for contaminating the surfaces through adsorption. Here, a method is shown to create biofluid droplet microarrays using discontinuous dewetting of pure water, an oil protective layer, and finally biofluid exchange with the water droplet array. With this method, a droplet array of a viscous nucleic acid amplification solution can be formed with a low surface tension of 34 mN m−1 and a contact angle of only 76° with the used hydrophobic coating. This droplet array is applied for nucleic acid detection of SARS‐CoV‐2 virus using strand invasion‐based amplification (SIBA) technology. It is shown that by using an array of 10 000 droplets of 50 µm diameter the limit of detection is 1 RNA copy µL−1. The results demonstrate that SIBA on droplet microarrays may be a quantitative technology. [ABSTRACT FROM AUTHOR]

A technique is proposed for the formation of a developed regular structure on the surface of a polished single-crystal silicon wafer by ion-beam etching. It is shown that when single-crystal silicon is etched by a beam of accelerated Ar+ ions with normal ion incidence on the sample surface, a surface region with a developed relief is formed, which can serve as a reflection-reducing layer. The reflection of radiation with wavelengths of 532, 633, 780, and 980 nm from a sample of single-crystal silicon with the orientation of the surface cut {100}, subjected to ion-beam treatment with Ar+ ions with an energy of Eion = 400 eV for 10 hours (material removal was 7.5 µm) is studied. The specular-reflection curves of s-polarized radiation are obtained as functions of the angle of incidence. A decrease in the reflection coefficient relative to a polished silicon wafer is found at all investigated wavelengths. The largest decrease (more than 4 times) is recorded for a wavelength of 532 nm. It is shown that the range of heights of inhomogeneities formed during ion-beam etching increases linearly with an increase in the depth (time) of etching, which can be used to optimize the relief for a given wavelength. [ABSTRACT FROM AUTHOR]

The structural and optical properties of tandem silicon/perovskite structures with a hierarchical antireflective surface at the interface have been studied. The surface consisted of micro-sized pyramids and nano-sized short needles formed respectively by chemical and reactive ion etching of silicon substrates. It is shown that a perovskite film without pores and voids covers a hierarchical surface with high conformity. The manufactured samples are characterized by low reflectance over a wide range of radiation wavelengths. [ABSTRACT FROM AUTHOR]

Abstract Superhydrophilic/superhydrophobic patterned surfaces can be used to create droplet microarrays. A specific challenge with the liquids needed for various biomedical applications, as compared to pure water, is their lower surface tension and potential for contaminating the surfaces through adsorption. Here, a method is shown to create biofluid droplet microarrays using discontinuous dewetting of pure water, an oil protective layer, and finally biofluid exchange with the water droplet array. With this method, a droplet array of a viscous nucleic acid amplification solution can be formed with a low surface tension of 34 mN m−1 and a contact angle of only 76° with the used hydrophobic coating. This droplet array is applied for nucleic acid detection of SARS‐CoV‐2 virus using strand invasion‐based amplification (SIBA) technology. It is shown that by using an array of 10 000 droplets of 50 µm diameter the limit of detection is 1 RNA copy µL−1. The results demonstrate that SIBA on droplet microarrays may be a quantitative technology.

Silicon nano/microstructures are widely utilized in the semiconductor industry, and plasma etching is the most prominent method for fabricating silicon nano/microstructures. Among the variety of silicon nano/microstructures, black silicon with light-trapping properties has garnered broad interest from both the scientific and industrial communities. However, the fabrication mechanism of black silicon remains unclear, and the light absorption of black silicon only focuses on the near-infrared region thus far. Herein, we demonstrate that black silicon can be fabricated from individual flower-like silicon microstructures. Using fluorocarbon gases as etchants, silicon flower microstructures have been formed via maskless plasma etching. Black silicon forming from silicon flower microstructures exhibits strong absorption with wavelength from 0.25 μm to 20 μm. The result provides novel insight into the understanding of the plasma etching mechanism in addition to offering further significant practical applications for device manufacturing.

Herein, it is investigated if black diamond is useful in a bactericidal surface. Black diamond is derived from black silicon, a silicon surface structured into nanosized needles. Black diamond is obtained by coating black silicon with a thin diamond film, rendering the nanostructures more robust. The bactericidal and antibacterial properties of fluorine‐terminated and hydrogen‐terminated black diamonds with those of black silicon and for flat surfaces of diamond (on silicon) with the same terminations are studied. The ability to repel and kill Gram‐positive Staphylococcus aureus and Staphylococcus epidermidis is evaluated, which have a thicker cell wall and are more mechanically robust than the bacteria that are studied before. The initial adhesion as well as long‐term 24 h biofilm formation is studied. The number of bacteria that initially adhere to the fluorine‐terminated black diamond surface is reduced and has the highest dead bacterial ratio. Biofilm formation after 24 h shows that while all surfaces outperform glass over the long term, diamond‐coated surfaces with both fluorine and hydrogen termination have a significant inhibiting biofilm formation effect. In conclusion, fluorinated and hydrogenated diamond‐coated surfaces with and without nanoneedles have repelling, bactericidal, and biofilm‐inhibiting effects on Gram‐positive bacterial strains and are promising antimicrobial surfaces. [ABSTRACT FROM AUTHOR]

Ion-flow-stimulated roughening transition is a phenomenon that may prove useful in the hierarchical structuring of nanostructures. In this work, we have investigated theoretically and experimentally the surface texturing of single-crystal and multi-crystalline silicon wafers irradiated using ion-beam flows. In contrast to previous studies, ions had relatively low energies, whereas flow densities were high enough to induce a quasi-liquid state in the upper silicon layers. The resulting surface modifications reduced the wafer light reflectance to values characteristic of black silicon, widely used in solar energetics. Features of nanostructures on different faces of silicon single crystals were studied numerically based on the mesoscopic Monte Carlo model. We established that the formation of nano-pyramids, ridges, and twisting dune-like structures is due to the stimulated roughening transition effect. The aforementioned variety of modified surface morphologies arises due to the fact that the effects of stimulated surface diffusion of atoms and re-deposition of free atoms on the wafer surface from the near-surface region are manifested to different degrees on different Si faces. It is these two factors that determine the selection of the allowable "trajectories" (evolution paths) of the thermodynamic system along which its Helmholtz free energy, F, decreases, concomitant with an increase in the surface area of the wafer and the corresponding changes in its internal energy,   U ( d U > 0 ), and entropy,   S ( d S > 0 ), so that d F = d U   –   T d S < 0 , where T is the absolute temperature. The basic theoretical concepts developed were confirmed in experimental studies, the results of which showed that our method could produce, abundantly, black silicon wafers in an environmentally friendly manner compared to traditional chemical etching. [ABSTRACT FROM AUTHOR]

Nitrogen dioxide (NO2) is a serious environmental pollutant and can cause negative consequences on both human health and vegetation growth. Herein, the possibility and promise of using a resistive sensor based on pristine black silicon (BSi) to detect NO2 at room temperature are investigated. BSi is prepared using plasma‐based reactive ion etching. Scanning electron microscope (SEM) studies show that BSi consists of vertically standing nanoneedles and has a large surface area, which facilitates gas adsorption. The gas‐sensing properties of the BSi‐based sensor are tested for low NO2 concentrations (1–5 ppm). It is found that the prepared sensor samples without posttreatments of BSi exhibit high sensitivity, fast transient response, and good repeatability. In particular, the response and recovery time of the sensor are ≈35 and ≈25 s for 4 ppm NO2 gas exposure, respectively. Besides, the results indicate that the BSi‐based sensor has excellent selectivity. Finally, the sensing mechanism is discussed, the dominating factors of which are the large active sensing area and the vertical arrangement of the BSi nanoneedles. It is believed that with further optimization, BSi has good potential as a functional material for gas sensors. [ABSTRACT FROM AUTHOR]

We demonstrate and compare two approaches for reducing processing time for ultrashort pulse laser surface functionalization with application to femtosecond laser hyperdoping of silicon with a laser pulse duration of 800 fs and an irradiation wavelength of 1030 nm. In the first, we use a Gaussian intensity distribution and increase the repetition rate from 1 kHz to 1002 kHz while keeping all other parameters and thus the accumulated fluence constant. We find that the sub-bandgap absorptance of the material, which we take as target measure, decreases above a repetition rate of 250 kHz. This suggests an inherent limitation of this approach. The second approach is characterized by the use of a line-shaped intensity distribution which is achieved by diffractive beam shaping using a phase-only spatial light modulator. This process proves to be suitable for laser hyperdoping of silicon with a 22-fold enhanced area processing rate while maintaining a sub-bandgap absorptance of above 80 %abs. [ABSTRACT FROM AUTHOR]

This article discusses a method for forming black silicon using plasma etching at a sample temperature range from −20 °C to +20 °C in a mixture of oxygen and sulfur hexafluoride. The surface morphology of the resulting structures, the autocorrelation function of surface features, and reflectivity were studied depending on the process parameters—the composition of the plasma mixture, temperature and other discharge parameters (radical concentrations). The relationship between these parameters and the concentrations of oxygen and fluorine radicals in plasma is shown. A novel approach has been studied to reduce the reflectance using conformal bilayer dielectric coatings deposited by atomic layer deposition. The reflectivity of the resulting black silicon was studied in a wide spectral range from 400 to 900 nm. As a result of the research, technologies for creating black silicon on silicon wafers with a diameter of 200 mm have been proposed, and the structure formation process takes no more than 5 min. The resulting structures are an example of the self-formation of nanostructures due to anisotropic etching in a gas discharge plasma. This material has high mechanical, chemical and thermal stability and can be used as an antireflective coating, in structures requiring a developed surface—photovoltaics, supercapacitors, catalysts, and antibacterial surfaces.

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The results of FDTD modeling of the spectral reflection characteristics of periodic systems with unit cells in the form of cylinders and cones corresponding to the morphology of the porous and black silicon layer presented. The antireflection behavior of these systems with a characteristic size on the order of several hundreds of nanometers is analyzed. The properties of reflection depending on the geometric parameters of unit cells and the angle of incidence of light rays are investigated. It is shown that periodic systems with cones are superior in antireflection properties to systems with cylinders. [ABSTRACT FROM AUTHOR]

Effect of sputtered silver (Ag) thin film thickness towards morphological and optical properties of black silicon (b-Si) fabricated by two-step silver-assisted wet chemical etching for solar cells applications is investigated. The method involves low temperature annealing of crystalline silicon (c-Si) coated with Ag thin films of 10 nm, 15 nm, and 25 nm. This is followed by an etching in a solution of HF:H2O2:DI H2O (1:5:10 volume ratio) at room temperature for 70 s. Dense and spherical Ag NPs with an average diameter of 203 ± 17.8 nm and surface coverage of about 72.5% are achieved on a sample with Ag film thickness of 15 nm prior to the annealing process. After the etching, the average nanopores’ height of ~420 nm with an average diameter of ~200 nm owing to denser Ag NPs on the c-Si surface before the etching are obtained. Optical absorption enhancement due to low weight average reflection (WAR) within wavelength region of 300–1100 nm is observed on the b-Si wafers. Sample with 15 nm of Ag thin film prior to annealing, demonstrates WAR of 7.7% compared 40.0% of the reference planar c-Si. The low WAR is due to the efficient light coupling effect of the b-Si nanopores. The fabricated b-Si nanopores can be used in b-Si solar cells for enhanced optical absorption and high photocurrent in the future.

Structural anti-reflective coating and bactericidal surfaces, as well as many other effects, rely on high-aspect-ratio (HAR) micro- and nanostructures, and thus, are of great interest for a wide range of applications. To date, there is no widespread fabrication of dense or isolated HAR nanopillars based on UV nanoimprint lithography (UV-NIL). In addition, little research on fabricating isolated HAR nanopillars via UV-NIL exists. In this work, we investigated the mastering and replication of HAR nanopillars with the smallest possible diameters for dense and isolated arrangements. For this purpose, a UV-based nanoimprint lithography process was developed. Stability investigations with capillary forces were performed and compared with simulations. Finally, strategies were developed in order to increase the stability of imprinted nanopillars or to convert them into nanoelectrodes. We present UV-NIL replication of pillars with aspect ratios reaching up to 15 with tip diameters down to 35 nm for the first time. We show that the stability could be increased by a factor of 58 when coating them with a 20 nm gold layer and by a factor of 164 when adding an additional 20 nm thick layer of SiN. The coating of the imprints significantly improved the stability of the nanopillars, thus making them interesting for a wide range of applications. [ABSTRACT FROM AUTHOR]

To improve the absorption of light in two‐junction tandem perovskite–silicon solar cells, antireflective textures are often created on the front surface in the form of micronsized pyramids using wet‐chemical etching. It is known that single‐junction silicon solar cells with nanotextured black silicon (BS) formed by dry‐etching techniques are more efficient than pyramidal microtextures. Herein, experimental evidence of enhanced efficiency of structures using BS between the perovskite layer and the silicon substrate for the first time is demonstrated. BS is fabricated using plasma‐based reactive‐ion etching on the front surface of the crystalline silicon. The deposition of mixed‐halide perovskite layers is performed by vacuum coevaporation and sequential evaporation. The structural and optical properties of evaporated perovskite layers with various thicknesses are investigated. It is found that the structural‐phase quality (viz., surface coverage, chemical composition, dimensionality, crystallinity, and phase purity) of perovskite layers on planar and nanotextured substrates is similar. However, the root mean square roughness of the perovskite layers on nanotextured substrates is much higher, which reduces the reflection significantly, especially, in the range of radiation incidence angle up to 60°. These results pave the way for the development of high‐performance perovskite–silicon tandem solar cells for improved efficiency. [ABSTRACT FROM AUTHOR]

Silicon microneedle (Si-MN) systems are a promising strategy for transdermal drug delivery due to their minimal invasiveness and ease of processing and application. Traditional Si-MN arrays are usually fabricated by using micro-electro-mechanical system (MEMS) processes, which are expensive and not suitable for large-scale manufacturing and applications. In addition, Si-MNs have a smooth surface, making it difficult for them to achieve high-dose drug delivery. Herein, we demonstrate a solid strategy to prepare a novel black silicon microneedle (BSi-MN) patch with ultra-hydrophilic surfaces for high drug loading. The proposed strategy consists of a simple fabrication of plain Si-MNs and a subsequent fabrication of black silicon nanowires. First, plain Si-MNs were prepared via a simple method consisting of laser patterning and alkaline etching. The nanowire structures were then prepared on the surfaces of the plain Si-MNs to form the BSi-MNs through Ag-catalyzed chemical etching. The effects of preparation parameters, including Ag+ and HF concentrations during Ag nanoparticle deposition and [HF/(HF + H2O2)] ratio during Ag-catalyzed chemical etching, on the morphology and properties of the BSi-MNs were investigated in detail. The results show that the final prepared BSi-MN patches exhibit an excellent drug loading capability, more than twice that of plain Si-MN patches with the same area, while maintaining comparable mechanical properties for practical skin piercing applications. Moreover, the BSi-MNs exhibit a certain antimicrobial activity that is expected to prevent bacterial growth and disinfect the affected area when applied to the skin. [ABSTRACT FROM AUTHOR]

Black silicon (b-Si)-assisted photovoltaic cells have textured b-Si surfaces, which have excellent light-trapping properties. There has been a limited amount of work performed on the theoretical modelling of b-Si photovoltaic cells, and hence, in this work, a coupled optical-electrical-thermal model has been proposed for the simulation of b-Si photovoltaic cells. In particular, the thermal aspects in b-Si photovoltaic cells have not been discussed in the literature. In the proposed model, the finite-difference time-domain (FDTD) method was used to study the optical response of the b-Si photovoltaic cell. Semiconductor equations were used for the electrical modelling of the cell. For the thermal model, the Energy Balance Transport Model was used. The developed model was used to simulate b-Si photovoltaic cells under thermophotovoltaic sources. The impacts of heat generation on the electrical performance of thermophotovoltaic cells are discussed. Simulation results from this study showed that black silicon layer improved efficiency and power output in thermophotovoltaic cells compared to thermophotovoltaic cells with no surface texture. In addition, heat generation due to Joule heating and electron thermalization in b-Si-assisted thermophotovoltaic cells reduced the open-circuit voltage and electrical performance. [ABSTRACT FROM AUTHOR]

A theoretical model of reflection of macroporous silicon and arrays of silicon nanowires on a monocrystalline substrate is presented. Macroporous silicon and silicon structured by nanowires are considered as a two-layer structure of silicon with an effective medium. The analytical model of the reflection from a twolayer silicon structure with an effective medium takes into account the absorption of light by the structure and the multiple reflections of light from the surfaces of the sample and the interface between the effective medium and the monocrystalline substrate. The reflection coefficient from a structured surface, which is the boundary between two media, contains the complex index of refraction of silicon. The effective index of refraction of the effective medium is found from the expression for mixing two media. The reflection of light falling on flat surfaces at different angles is calculated according to Fresnel's formulas. The frontal structured surface and the second structured surface were considered as Lambert surfaces. Total internal reflection from a flat surface between silicon and air is given by Snelius' law, and from structured surfaces between silicon and the effective medium and between the effective medium and air is accounted for by coefficients. The reflection spectra from macroporous silicon and arrays of silicon nanowires on a monocrystalline substrate are calculated according to analytically derived formulas. It is shown that the magnitude of reflection spectra from macroporous silicon and arrays of silicon nanowires on a monocrystalline substrate decreases when the volume fraction of pores increases. The reflectance begins to increase again when the pore volume fraction is high. Reflection from the surface between the effective medium and air is observed at a high volume fraction of pores. [ABSTRACT FROM AUTHOR]

We propose a simple, fast, and low-cost method for producing Au-coated black Si-based SERS-active substrates with a proven enhancement factor of 106. Room temperature reactive ion etching of silicon wafer followed by nanometer-thin gold sputtering allows the formation of a highly developed lace-type Si surface covered with homogeneously distributed gold islands. The mosaic structure of deposited gold allows the use of Au-uncovered Si domains for Raman peak intensity normalization. The fabricated SERS substrates have prominent uniformity (with less than 6% SERS signal variations over large areas, 100 × 100 μm2). It has been found that the storage of SERS-active substrates in an ambient environment reduces the SERS signal by less than 3% in 1 month and not more than 40% in 20 months. We showed that Au-coated black Si-based SERS-active substrates can be reused after oxygen plasma cleaning and developed relevant protocols for removing covalently bonded and electrostatically attached molecules. Experiments revealed that the Raman signal of 4-MBA molecules covalently bonded to the Au coating measured after the 10th cycle was just 4 times lower than that observed for the virgin substrate. A case study of the reusability of the black Si-based substrate was conducted for the subsequent detection of 10–5 M doxorubicin, a widely used anticancer drug, after the reuse cycle. The obtained SERS spectra of doxorubicin were highly reproducible. We demonstrated that the fabricated substrate permits not only qualitative but also quantitative monitoring of analytes and is suitable for the determination of concentrations of doxorubicin in the range of 10–9–10–4 M. Reusable, stable, reliable, durable, low-cost Au-coated black Si-based SERS-active substrates are promising tools for routine laboratory research in different areas of science and healthcare. [ABSTRACT FROM AUTHOR]

Black silicon (b‐Si) has many attractive properties and is currently being adopted in various fields of semiconductor technology. It is shown here that b‐Si during thermal activation may serve as an external gettering layer to remove metallic impurities and associated defects from bulk Si. The gettering efficiency by b‐Si is estimated according to the results of studying the resistivity, defect density, interstitial iron concentration, and effective minority carrier lifetime on gettered test and ungettered reference Si samples. It is shown that in the thermal oxidation process, the b‐Si layer serves as an effective drain for excess iron atoms and other fast‐diffusing impurities, thereby significantly reducing the density of oxidation‐induced stacking faults in Si. In addition, the resistivity of the substrates decreases, and the bulk carrier lifetime increases significantly. Finally, gettering by b‐Si is introduced into the solar cells fabrication process and its effect on solar cell current–voltage characteristics is investigated. It has been shown that all electronic parameters of the solar cells are improved. The conversion efficiency of ungettered solar cells is 16.8%, and for gettered solar cells, depending on the oxidation temperature, it increases by 1.36–1.96%. [ABSTRACT FROM AUTHOR]

Black silicon (bSi) is a highly absorptive material in the UV-vis and NIR spectral range. Photon trapping ability makes noble metal plated bSi attractive for fabrication of surface enhanced Raman spectroscopy (SERS) substrates. By using a cost-effective room temperature reactive ion etching method, we designed and fabricated the bSi surface profile, which provides the maximum Raman signal enhancement under NIR excitation when a nanometrically-thin gold layer is deposited. The proposed bSi substrates are reliable, uniform, low cost and effective for SERS-based detection of analytes, making these materials essential for medicine, forensics and environmental monitoring. Numerical simulation revealed that painting bSi with a defected gold layer resulted in an increase in the plasmonic hot spots, and a substantial increase in the absorption cross-section in the NIR range. [ABSTRACT FROM AUTHOR]

The technology of vacuum co-deposition and sequential deposition of the mixed-halide perovskite films (CH3NH3PbI3–xClx) has been optimized. The possibility of improving the thermal stability of films by adding cesium atoms to an inorganic precursor is demonstrated. The structural and optical properties of perovskite films on black silicon are synthesized and investigated. It is shown that films of high quality in structure and phase composition are formed, which are characterized by a low reflection coefficient in a wide range of radiation wavelengths. Vacuum-deposited perovskite films on black silicon have great potential for the development of high-efficiency tandem solar cells. [ABSTRACT FROM AUTHOR]

For uncooled infrared cameras based on microbolometers, silicon caps are often utilized to maintain a vacuum inside the packaged bolometer array. To reduce Fresnel reflection losses, anti-reflection coatings are typically applied on both sides of the silicon caps.This work investigates whether black silicon may be used as an alternative to conventional anti-reflective coatings. Reactive ion etching was used to etch the black silicon layer and deep cavities in silicon. The effects of the processed surfaces on optical transmission and image quality were investigated in detail by Fourier transform infrared spectroscopy and with modulated transfer function measurements. The results show that the etched surfaces enable similar transmission to the state-of-the-artanti-reflection coatings in the 8–12 µm range and possibly obtain wider bandwidth transmission up to 24 µm. No degradation in image quality was found when using the processed wafers as windows. These results show that black silicon can be used as an effective anti-reflection layer on silicon caps used in the vacuum packaging of microbolometer arrays.

Atomic layer deposited (ALD) Al2O3 coatings were applied on black silicon (b-Si) structures. The coated nanostructures were investigated regarding their reflective and transmissive behaviour. For a systematic study of the influence of the Al2O3 coating, ALD coatings with a varying layer thickness were deposited on three b-Si structures with different morphologies. With a scanning electron microscope the morphological evolution of the coating process on the structures was examined. The optical characteristics of the different structures were investigated by spectral transmission and reflection measurements. The usability of the structures for highly efficient absorbers and antireflection (AR) functionalities in the different spectral regions is discussed. [ABSTRACT FROM AUTHOR]

Phosphorous dopant diffusion profiles feature in many silicon semiconductor devices, including the vast majority of silicon solar cells. Accurate spatially resolved dopant profiling is crucial for understanding the performance of these diffused regions, however, it is very challenging to obtain such profiles in non‐planar samples. Scanning electron microscopy for dopant contrast imaging (SEMDCI), where the secondary electron (SE) image contrast is used to determine the dopant level of a semiconductor sample, is an ideal candidate for Si dopant profiling, especially for silicon samples with surface nanotexturing or black silicon (BSi) technology. However, in previous SEMDCI studies, the dopant concentration of heavily doped n‐type layers in silicon samples have shown a poor correlation with the SE signal contrast. In this work, 1) good contrast for n‐type diffused silicon without contrast‐enhancing techniques; 2) a new contrast definition to account for imaging non‐uniformities; 3) clear correlations between SE contrast and sample work function for phosphorus‐diffused planar silicon specimens across a wide range of emitter profiles; 4) implementation of an empirical baseline correction to normalize scanning electron microscopy image condition variations, are presented. This SEMDCI method is subsequently used for the first time to obtain 2D electron concentration maps for both planar and BSi samples. [ABSTRACT FROM AUTHOR]

The optical properties of a tandem three-layered structure of crystalline silicon-black siliconperovskite have been theoretically studied for using it in solar energy conversion. The transfer matrix method is used for obtaining the analytical expressions for the reflection, transmission, and absorption coefficients. It is shown that the transfer matrix method can be successfully applied to the more complicated case of three layers with complex refractive indexes. Numerical calculations performed at an angle of incidence of radiation of 60° showed that in the region of short wavelengths the reflection coefficient takes on low values (several percent), which gradually increase and become > 10% at wavelengths above 0.8 µm. The theoretically modeled results of the reflection coefficient are in good agreement with experimental data carried out for the structures crystalline silicon-black silicon-TiO2-perovskite. The discrepancy between the experimental and model data slowly increases with increasing wavelength. The transmittance was extremely low and increased slowly with the increasing wavelength. It is found that cheaper and easier created tandem crystalline silicon-black siliconperovskite structure may have the potential for solar energy conversion. [ABSTRACT FROM AUTHOR]

Atomic layer deposited (ALD) Al2O3 coatings were applied on black silicon (b-Si) structures. The coated nanostructures were investigated regarding their reflective and transmissive behaviour. For a systematic study of the influence of the Al2O3 coating, ALD coatings with a varying layer thickness were deposited on three b-Si structures with different morphologies. With a scanning electron microscope the morphological evolution of the coating process on the structures was examined. The optical characteristics of the different structures were investigated by spectral transmission and reflection measurements. The usability of the structures for highly efficient absorbers and antireflection (AR) functionalities in the different spectral regions is discussed. [ABSTRACT FROM AUTHOR]

Ion-flow-stimulated roughening transition is a phenomenon that may prove useful in the hierarchical structuring of nanostructures. In this work, we have investigated theoretically and experimentally the surface texturing of single-crystal and multi-crystalline silicon wafers irradiated using ion-beam flows. In contrast to previous studies, ions had relatively low energies, whereas flow densities were high enough to induce a quasi-liquid state in the upper silicon layers. The resulting surface modifications reduced the wafer light reflectance to values characteristic of black silicon, widely used in solar energetics. Features of nanostructures on different faces of silicon single crystals were studied numerically based on the mesoscopic Monte Carlo model. We established that the formation of nano-pyramids, ridges, and twisting dune-like structures is due to the stimulated roughening transition effect. The aforementioned variety of modified surface morphologies arises due to the fact that the effects of stimulated surface diffusion of atoms and re-deposition of free atoms on the wafer surface from the near-surface region are manifested to different degrees on different Si faces. It is these two factors that determine the selection of the allowable “trajectories” (evolution paths) of the thermodynamic system along which its Helmholtz free energy, F, decreases, concomitant with an increase in the surface area of the wafer and the corresponding changes in its internal energy, U (dU>0), and entropy, S (dS>0), so that dF=dU – TdS<0, where T is the absolute temperature. The basic theoretical concepts developed were confirmed in experimental studies, the results of which showed that our method could produce, abundantly, black silicon wafers in an environmentally friendly manner compared to traditional chemical etching.

Black silicon (BSi) is a branch of silicon material whose surface is specially processed to a micro/nanoscale structure, which can achieve ultra‐low reflectance or ultra‐high electrochemical reactivity. The diversity and complex surface structures of BSi make it challenging to commercialize BSi devices. Modeling and simulation are commonly used in the semiconductor industry to help in better understanding the material properties, predict the device performance, and provide guidelines for fabrication parameters' optimization. The biggest challenge for BSi device modeling and simulation is obtaining accurate input surface morphological data. In this work, the 3D models of challenging BSi textures are compared as obtained by atomic force microscopy (AFM) and plasma focused ion beam (PFIB) tomography techniques. In previous work, the PFIB tomography workflow toward the application of surface topography is optimized. In this work, the 3D models obtained from both AFM and PFIB are comprehensively compared, by using the surface models as inputs for finite‐difference time‐domain‐based optical simulation. The results provide strong evidence that PFIB tomography is a better choice for characterizing highly roughened surface such as BSi and provides surface 3D models with better reliability and consistency. [ABSTRACT FROM AUTHOR]