Optical Properties Of Black Silicon Research Articles

Correction to: Effect of thermal annealing on the structural and optical properties of black silicon

Correction to: Effect of thermal annealing on the structural and optical properties of black silicon

Impacts of Wafer Doping Type on Structural and Optical Properties of Black Silicon Fabricated by Metal-Assisted Chemical Etching

In this work, the impacts of wafer doping type on structural and optical properties of black silicon (b-Si) fabricated by metal-assisted chemical etching (MACE) process are investigated. P-type and n-type mono-crystalline silicon (mono c-Si) wafers are etched in an aqueous solution of hydrofluoric acid (HF), silver nitrate (AgNO3) and deionised water (DI H2O) at room temperature and various durations from 5-20 minutes. Surface morphological results demonstrate the formation of b-Si nanowires (NWs) with average lengths of 0.4-0.8 μm for p-type wafers and 0.8-3.0 μm for n-type wafers. The higher length of the NWs for the n-type wafers is due to the minority charge carriers, which lead to a higher etching rate during the MACE process. Within the 300-1100 nm wavelength region, weighted average reflection (WAR) for the p-type and n-type wafers decreases to 6.6% and 6.4%, respectively, after 20 minutes of etching. The corresponding improvement in broadband light absorption results in maximum potential short-circuit current density (Jsc (max)) of 38.2 and 38.8 mA/cm2 for the p-type and n-type b-Si, respectively, which is an of enhancement of 39.9% and 42.1% when compared to the Jsc (max) of planar c-Si reference.

Effect of sputtered silver thin film thickness towards morphological and optical properties of black silicon fabricated by two-step silver-assisted wet chemical etching for solar cells application

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.

Morphologies and optical properties of black silicon by room temperature reactive ion etching

Black Silicon (BS) nanostructures fabricated by Reactive Ion Etching at Room Temperature (RT-RIE) are presented. We discuss the influence of the plasma process parameters on the silicon etching and their influence on the shape of the nanostructures constituting the BS. Furthermore, we study - both experimentally and using modeling by finite difference time domain (FDTD) method – how to increase absorptance in the visible and NIR regions using light trapping effect based on a fine control of the BS morphology. Absorptance higher than 99 % is reached with a strong angular acceptance. This paper proposes an optimized process to fabricate high aspect ratio and high absorptance BS using a room temperature process.

Modeling of the Optical Properties of Black Silicon Passivated by Thin Films of Metal Oxides

Using the finite difference time domain (FDTD) method, we studied the optical properties of black silicon (BSi) layers passivated with the various metal oxides (Al2O3, TiO2, HfO2, and Sc2O3) films, obtained by atomic layer deposition (ALD) method. The results of FDTD modeling indicate an improvement in the antireflection properties of BSi/ALD film structures in the wide spectral range. The necessity to choose the optimal film thickness is shown.

A cost-effective and facile approach for realization of black silicon nanostructures on flexible substrate

In this paper, we report a cost effective and facile approach to produce uniform nanostructural black silicon over large-area by metal assist chemical etching followed by its transfer over pressure sensitive flexible substrate. Structural and optical properties of black silicon over flexible substrate were investigated. Field emission scanning electronic microscope (FESEM) reveals textured surface and dense morphology of silicon nanostructures; that appeared to be black. An intense and broadband UV and Visible photoluminescence spectra from these nanostructures was observed and suggested the optically active nature of black silicon. Broadening and asymmetric shifting of Raman line shape, corresponding to black silicon, confirmed quantum confinement of phonon. The crystal sizes of silicon (Si) nanostructures calculated using frequency shift in Raman spectra analytical model were 3.4 nm to 4.7 nm. Significant reduction of reflection below (1%) over the wide UV–Vis spectrum was recorded due to presence of textured surface as observed by FESEM. The present work aids our understanding of tuning the optical properties of black silicon and its’ realization on flexible substrate could be beneficial for flexible electronics including bio-sensing and optoelectronics devices. More interestingly low reflectance of black silicon could pave the way for realization of highly efficient flexible solar cells.

Investigation on surface morphological and optical properties of black silicon fabricated by metal-assisted chemical etching with different etchant concentrations

In this study, the surface morphological and optical properties of black silicon (b-Si) fabricated by two-step metal-assisted chemical etching (MACE) process are investigated. The two-step MACE combines low-temperature annealing of silver (Ag) thin film to produce Ag nanoparticles (NPs) and short etching duration of crystalline silicon (c-Si) wafer. The etching is carried out in HF:H2O2:DI H2O solution for 70 s with different etchant concentrations (represented in the form of volume ratio). The MACE process produces b-Si nanopores on the wafer. Compared with planar c-Si reference, broadband reflection (in 300-1100 nm wavelength region) of the b-Si is significantly lower. B-Si wafer with volume ratio of 1:5:10 exhibits the lowest broadband reflection of 3% at wavelength of 600 nm, which is believed to be due to refractive index grading which leads to enhanced light coupling into the b-Si wafer. The best b-Si wafer (with lowest reflection) shows 50 nm average pillar width and 300 nm height. The increased broadband light absorption results in the highest maximum potential short-circuit current density (Jsc(max)) of 40.9 mA/cm2. This represents 55.4% enhancement, if compared with the planar c-Si reference wafer, assuming unity carrier collection.

Effects of annealing temperature towards surface morphological and optical properties of black silicon fabricated by silver-assisted chemical etching

This paper presents investigation on the effects of annealing temperature towards surface morphological and optical properties of black silicon (b-Si) fabricated by a two-step silver-assisted chemical etching process. Crystalline (c-Si) wafers are deposited with a thin film silver (Ag) layer. The c-Si wafers are annealed in nitrogen atmosphere for 40 min at different temperatures (200–260 ℃) to form Ag nanoparticles. The wafers are then etched in the aqueous solution of HF:H2O2:DI H2O at room temperature for 70 s. The results show that the surface morphologies of all samples are rough due to the presence of b-Si nanotextures. The b-Si wafers demonstrate significantly lower broadband reflection compared to the planar reference. The b-Si wafer annealed at 230 ℃ demonstrates the lowest reflection (3%) and highest absorption (97%) at the wavelength of 600 nm. This sample exhibits b-Si nanotextures with average width less than 50 nm and height of 245 nm. The enhanced broadband light absorption in the best b-Si wafer gives the highest maximum potential short-circuit current density (Jsc(max)) of 39.6 mA/cm2, or enhancement 50.6% compared to the planar c-Si reference wafer, calculated for wavelength region of 300–1100 nm.

A theoretical study on the optical properties of black silicon

There is a wide application prospect in black silicon, especially in solar cells and photoelectric detectors. For further optimization of black silicon, it is important to study its optical properties. Especially, the influence of the surface nanostructures on these properties and the light propagation within the nanostructures are relevant. In this paper, two kinds of black silicon models are studied via the finite differences time domain method. The simulated reflectance spectra matches well with the measured curve. Also, the light intensity distribution within the nanostructures shows that near 80% of the incident light are redirected and subjected to internal reflection, which provides powerful support for the good light trapping properties of black silicon.

Wave optical simulation of the light trapping properties of black silicon surface textures.

Due to their low reflectivity and effective light trapping properties black silicon nanostructured surfaces are promising front side structures for thin crystalline silicon solar cells. For further optimization of the light trapping effect, particularly in combination with rear side structures, it is necessary to simulate the optical properties of black silicon. Especially, the angular distribution of light in the silicon bulk after passage through the front side structure is relevant. In this paper, a rigorous coupled wave analysis of black silicon is presented, where the black silicon needle shaped structure is approximated by a randomized cone structure. The simulated absorptance agrees well with measurement data. Furthermore, the simulated angular light distribution within the silicon bulk shows that about 70% of the light can be subjected to internal reflection, highlighting the good light trapping properties.

Modeling of Optical Properties of Black Silicon/Crystalline Silicon

Black Silicon, a surface modification of silicon has low reflectance and correspondingly high absorptance in the visible range of wavelengths making it more viable than crystalline silicon for applications in silicon optoelectronics. Like crystalline Si, black silicon has potential applications in solar cells, sensors, antibacterial surfaces and offers new opportunities. In this study, the optical properties of black silicon, in the visible, near and short wavelength infrared region, are simulated based on the effective medium approximation. The application of Helmholtz?s law of optical reciprocity to the two layer medium of black silicon on crystalline silicon is examined. In the long wavelength infrared region, the reported values of emissivity of black silicon are compared with those in the literature. In general, the emissivity of black silicon is higher than that of crystalline silicon.

The structural and optical properties of black silicon by inductively coupled plasma reactive ion etching

Black Silicon nanostructures are fabricated by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) in a gas mixture of SF6 and O2 at non-cryogenic temperatures. The structure evolution and the dependency of final structure geometry on the main processing parameters gas composition and working pressure are investigated and explained comprehensively. The optical properties of the produced Black Silicon structures, a distinct antireflection and light trapping effect, are resolved by optical spectroscopy and conclusively illustrated by optical simulations of accurate models of the real nanostructures. By that the structure sidewall roughness is found to be critical for an elevated reflectance of Black Silicon resulting from non-optimized etching processes. By analysis of a multitude of structures fabricated under different conditions, approximate limits for the range of feasible nanostructure geometries are derived. Finally, the technological applicability of Black Silicon fabrication by ICP-RIE is discussed.

Optical properties of black silicon prepared by wet etching

In this paper, the optical properties of black silicon have been studied. The black silicon samples were fabricated by alkaline etching and metal assisted etching. The micro-columns and nanopores on the silicon surface were obtained in KOH and Au-induced HF/H2O2 solution, respectively. The height and diameter of micro-columns prepared by KOH etching is about 470 nm and 2 μm. In the Au-induced HF/H2O2 etching, the metallic nuclei behave as a cathode and their surrounding area acts as an anode, resulting in nanopores with diameters ranging from 80 to 120 nm. These microstructures formed in the etching process directly affect the optical properties of black silicon such as reflectance, transmittance and absorptance. According to the measurement of integrating sphere detector, the absorptance of the black silicon produced by wet etching remains roughly 90% from 250 to 1,000 nm wavelength, which is almost 150% of the absorptance of conventional silicon. However, the reflectance of black silicon is less than 13% and the transmittance is less than 4%.

Conformal Al2O3 Coatings on Black Silicon by Thermal ALD for Surface Passivation

Upon inductive coupled plasma reactive ion etching (ICP-RIE) of Si surfaces needle-like nanostructures with aspect ratios up to 10 emerge showing excellent anti-reflection and light-trapping properties with absorption over 97% throughout the UV and VIS spectral range. In addition, the absorption at the band edge of silicon is enormously enhanced due to scattering. In this work we report on the feasibility to deposit conformal Al2O3 dielectric layers on these very rough silicon surfaces which enable adequate surface passivation. Lifetimes over 170μs have been measured on deep structured b-Si substrates. The optical properties of black silicon (b-Si) and the possible influence of applied alumina passivation layers as well as their passivation performance are discussed.

Optical Properties of Black Silicon with Precipitated Silver and Gold Nanoparticles

Black porous silicon is a new material with low light reflectance and high light absorbance values. Black porous silicon layers are especially useful and important in solar energy conversion. In this work black porous silicon plates were prepared by wet chemical metal-assisted method. Silver and gold nanoparticles were precipitated from colloidal silver nitrate and chloroauric acid solutions. Obtained black porous silicon samples with precipitated nanoparticles were investigated by scanning electron microscope combined with energy dispersive X-ray spectrometer, ultraviolet and visible light spectrometer, the Fourier transformation infrared spectrometer. Scanning electron microscopy and energy dispersive X-ray analysis have revealed the presence of silver and gold nanoparticles on black porous silicon surface. Silver nanoparticles size varied from 30 to 100 nm, gold — from 20 to 70 nm. During UV‐VIS analysis significant changes in reflectance of processed black porous silicon samples were obtained. Reflectance of black porous silicon samples was lower than 10%. The Fourier transform infrared analysis has revealed decrease in reflectance in far infrared region. Changes in the Fourier transform infrared spectra in “fingerprint” zone prove modification of the surface of black porous silicon layers after precipitation of metal nanoparticles.