Enhanced Surface Properties of Light-Trapping Si Nanowires Using Synergetic Effects of Metal-Assisted and Anisotropic Chemical Etchings

Metal-assisted chemical etching (MACE) has been widely explored for developing silicon (Si)-based energy and optical devices with its benefits for low-cost and large-area fabrication of Si nanostructures of high aspect ratios. Surface structures and properties of Si nanostructures fabricated through MACE are significantly affected by experimental and environmental conditions of etchings. Herein, we showed that surfaces and interfacial energy states of fabricated Si nanowires can be critically affected by oxidants of MACE etching solutions. Surfaces of fabricated Si nanowires are porous and their tips are fully covered with lots of Si nano-sized grains. Strongly increased photoluminescence (PL) intensities, compared to that of the crystalline Si substrate, are observed for MACE-fabricated Si nanowires due to interfacial energy states of Si and SiOx of Si nano-sized grains. These Si grains can be completely removed from the nanowires by an additional etching process of the anisotropic chemical etching (ACE) of Si to taper the nanowires and enhance light trapping of the nanowires. Compared with the MACE-fabricated Si nanowires, ACE-fabricated tapered Si nanowires have similar Raman and PL spectra to those of the crystalline Si substrate, indicating the successful removal of Si grains from the nanowire surfaces by the ACE process.

oxidations and etchings of the surface and interior of the Si nanowires and substrates if excess holes are created on the catalytic metals and diffused into the Si substrate and nanowires 2,11,16 . Since the oxidant concentration in the etching solution is critical for hole generations on the catalytic metal surface, surface structures and morphologies of fabricated Si nanowires can be significantly affected by the concentration of H 2 O 2 in the etching solution 1,13,20 . By this reason, the concentration of H 2 O 2 needs to be optimized for fabricating Si nanowires through MACE.
In this study, we investigate the influences of H 2 O 2 on Si nanowire surfaces and on their interfacial energy states during the MACE process with electron microscopy, and Raman and photoluminescence (PL) spectroscopy studies. Surfaces of MACE-fabricated Si nanowires are highly porous and their tips are covered with lots of Si nano-sized grains which could be easily oxidized during and after the MACE process. The formation and oxidization of the Si nano-sized grains on the nanowire surfaces are strongly affected by etching procedures and conditions 8,18,[20][21][22] , contributing in significant increases in PL intensities of the nanowires 9,17 . These Si nano-sized grains and their interfaces can be completely removed from the nanowire surface by selective etching of reactive Si atoms with the anisotropic chemical etching (ACE). The Si nanowires are vertically tapered during the ACE processes, to afford sharp tip ends, significantly enhanced light trapping and highly suppressed PL emission 5,6,8,[21][22][23] . To better understand how surface oxidation of the Si nanowires during the MACE process will affect on crystallinities and interfacial energy states of the tapered Si nanowires of the ACE process, which is the post-etching process of the MACE process, changes in surface morphologies and optical properties of the tapered nanowires are compared with those of the MACE fabricated nanowires.

Results and Discussion
The MACE mechanism is generally considered to involve several redox reactions near the catalytic metal surface including H 2 O 2 reduction in the etching solution on the catalyst surface (Eq. 1), injection of holes into the underlying Si substrate, oxidation of Si atoms due to the injected holes (Eq. 2) and etching of the oxidized Si atoms by HF 1,11 . The half reactions of reduction and oxidation involved in MACE with Au catalysts can be described as follows 9,23,24 : Au 2 SiF 6H (2) 6 As described in Fig. 1(a), the holes injected into the underlying Si substrate are mainly consumed by oxidization of Si atoms near the catalytic Au. These oxidized Si atoms are etched by HF in the etching solution. These redox reactions proceed continuously on the Si substrate with the patterned Au catalyst and vertically etched Si nanowires of high aspect ratio are formed by MACE. Although there might be differences depending on the type and concentration of dopants of the Si substrate, holes excessively produced and injected into the Si substrate can be diffused and accumulated in the interior of the substrate or along the nanowires. These excess holes can  (100) substrate. The holes, generated excessively on the interface of the Au mask and the Si substrate during the MACE process, can be diffused and accumulated in the Si substrate and Si nanowires. These accumulated holes can make the nanowire surfaces oxidized and porous with Si nano-sized grains during and after the MACE process. (b) Unstable Si atoms and Si grains can be effectively removed from the nanowire surface by performing multiple rounds of ACE, which etches the nanowires selectively to obtain tapered shapes.
contribute to additional etchings of the structures and could increase the surface porosity of the nanowires with Si nano-sized grain formation, as described in Fig. 1(b) 2,12,13 .
As mentioned in the introduction, the structures of the MACE-fabricated Si nanowires can be affected by several experimental factors such as temperature, humidity, materials and structures of catalysts, and composition of the etching solution 1,11,14 Fig. 2(d). As the HF composition is increased and the H 2 O 2 concentration is decreased in the etching solution, the hole-generation and injection into the Si substrate would be decreased during the MACE processes. As a result, the etching rate, determined from the length of the etched nanowires, is decreased to 0.1, 0.08 and 0.06 µm/min with an increase in the ρ value, respectively. In this study, the nanowires are vertically etched to obtain lengths of 3−4 µm for effective light trapping without agglomeration during drying. The reaction times are controlled within 30 to 70 min with ρ values increased from 0.85 to 0.95. Lengths of fabricated nanowires are measured as 3.0 ± 0.3, 3.6 ± 0.2, and 3.6 ± 0.2 µm for each etching conditions. As shown in Fig. 1(b), the ACE processes are performed with these MACE-fabricated Si nanowires to make the nanowires having pencil-like sharpened tip ends for their enhanced light trapping 3,6,22-24 . The ACE process involves reduction of Ag ions (Eq. 3) and oxidation and etching of Si atoms (Eqs. 4 and 5) and these redox reactions are proceeded selectively on reactive Si atoms of sharp edges or Si nano-sized grains of the nanowires 8,18,22 . Ag nanoparticles are grown on the Si nanowire surface during the reduction and completely removed from the Si surface completely in the HNO 3 solution. These multiple processes are repeated seven times, sequentially to form tapered Si nanowires as shown in Fig. 2(e-g).
Besides changes in the tip ends of the Si nanowires, the lengths of the nanowires are decreased in the ACE process and are measured to be 1.7 ± 0.1, 2.2 ± 0.2, and 2.7 ± 0.1 µm, with an increase in ρ values. Compared the lengths of flat and tapered Si nanowires fabricates with the MACE and ACE processes, tapered nanowires of ACE are shorter than flat nanowires of MACE, showing that length ratios of the tapered nanowires relative to the flat nanowire increase from 0.57 (ρ = 0.85) to 0.73 (ρ = 0.95) with an increase in the ρ value. This dependence can be attributed to that the nanowire surface, etched by MACE of a higher ρ value, i.e., lower H 2 O 2 concentration, is stable and less oxidized during the MACE process.
As expected, due to anti-reflection enhanced by their sharpened tip structures, the surface reflection of tapered Si nanowires is measured as much lower compared to those of the flat Si nanowires, as shown in Fig. 3(a) 25,26 . Compared to the average reflectance in the wavelength region of 450-900 nm (R avg ) of 35% of the bulk c-Si substrate, R avg of the flat Si nanowires etched by MACE is significantly decreased to less than 7% for all nanowires. With an increase in ρ values from 0.85 to 0.95, R avg of the flat nanowires decrease from 6.1% to 4.1%. The R avg values of all tapered nanowires fabricated by ACE, further decrease to less than 2% showing a minimum value of 1.1% for the nanowire of ρ = 0.95. Figure 3 Figure S1 for detail).
For further understanding of the light trapping of the Si nanowires, we investigate the cross-sections of the time-averaged Poynting vectors, <S>, near the flat and tapered nanowires of ρ of 0.95 at wavelengths ranging from 400 to 900 nm, as shown in Fig. 3(c). <S> represents the power distribution near the structures 29 . Cross-sections of power distributions near the nanowires show significant light trapping of shorter wavelength of <700 nm near the tip of the Si nanowire, indicating that the tip structure of the Si nanowire is critical for its light trapping. Figure 3(d) shows the averaged values of the simulated (red) and experimental (black) reflectance over the entire wavelength range from 450 to 900 nm of flat (square) and tapered (triangle) Si nanowires with respect to their ρ values. As described above, the significant decrease in the surface reflection, and thus, the enhancement of light trapping of the nanowires through a change of their tip structures is confirmed. The surface reflections of Si nanowires fabricated with MACE and ACE show a slight decrease with an increase in ρ values and this slight change can be explained with increased light trappings of porous nanowires due to the surface structures in their length directions.  www.nature.com/scientificreports www.nature.com/scientificreports/ tapered nanowires of ρ = 0.95 are observed to exhibit lower PL intensities, similarly to the PL intensity of the bulk c-Si substrate. These EDS line profiles (red) and PL spectra (black) of the flat (■) and tapered (▲) Si nanowires are compared in Fig. 4(c). To compare the O and Si compositions of the flat and tapered Si nanowires fabricated   Fig. 4(c). The PL spectra taken from the flat and tapered nanowires are integrated over all wavelength regions, and their integration values are compared in Fig. 4(c) as well.
As described above, hole generations on catalytic Au surfaces during MACE are strongly influenced by H 2 O 2 concentration in the etching solution and the excess holes injected into the Si substrate are diffused into the substrate and on the nanowires, causing secondary etchings of the structures. In particular, holes diffused into the nanowires are accumulated at their tip end, such that the tip ends become more porous with lots of Si nano-sized grains which can be easily oxidized during and after MACE due to their higher surface energies. Therefore  Raman analysis of fabricated Si nanowires of Figure S2 were performed to support our TEM observations that MACE-fabricated Si nanowires have porous tip surfaces covered with lots of nano-sized Si grains and these grains can be effectively removed from the nanowires by the ACE process. As shown by Figure S2(a), contributions of amorphous Si and SiO 2 , which have characteristic broad Raman peaks below 500 cm −1 , are negligible in Raman spectra of fabricated Si nanowires compared to contributions of c-Si and Si nanocrystals 33,34 . We assumed that phonon modes of nano-sized Si grains can be described with phonon modes of Si nanocrystals, which have phonon modes at lower energy regions compared to the bulk c-Si (centered at 520.7 cm −1 ) due to their phonon locations 20,38,39 . The size and composition of Si nanocrystals of fabricated Si nanowires of ρ = 0.90 and 0.95 were analyzed with fitting analysis, considering shifts to lower energies and broadenings of Raman peaks as shown in Figure S2(b,c) 20,30 . (see description for Figure S2 in Supporting Information) The comparison in Figure S2 (c) indicates significant decrease in the composition of Si nanocrystals in the nanowires fabricated by the MACE processes with the increase in ρ values and by the ACE processes.

Conclusions
In summary, we fabricated vertically aligned flat and tapered Si nanowires through chemical wet etchings of the MACE and ACE processes and investigated the influences of oxidants in the MACE etching solutions on surface structures, morphologies and interfacial energy states of fabricated Si nanowires. We showed that the Si nanowire surfaces are significantly affected by the concentration of H 2 O 2 in the etching solution, which causes excessive injections of holes into the Si nanowires in its higher concentration, increasing the surface porosities of the nanowires with lots of Si nano-sized grains formed on the nanowire surfaces. These porous Si nanowires can be easily oxidized during and after the MACE process and their interfacial energy states are strongly affected by surface structure and oxidation of the nanowires, showing significant changes in the PL spectra with MACE etching conditions. The surface of these porous Si nanowires can be completely cleaned by ACE, which selectively removes unstable Si atoms from the nanowire surface. Flat Si nanowires could be engineered for tapered Si nanowires where Si grains are completely removed from the nanowire surface by the ACE process, showing enhanced light trapping as well as significant suppression of PL intensities. Our study can indicate that the surface oxidation which Si nanowires have during the MACE process can be critical in the surface crystallinities and interfacial energy states of the tapered Si nanowires of the post-MACE etching process. We believe that our study of enhancing surface properties including structural morphologies, crystallinities and interfacial energy states of Si nanowires with the MACE and ACE processes would contribute to the application and fabrication of Si nanowires for efficient Si-based energy harvesting and catalysis.
www.nature.com/scientificreports www.nature.com/scientificreports/ Method Fabrication of vertically aligned Si nanostructure array. In this study, p-type (boron)-doped Si substrates (100) with the resistivity of 1-10 Ω·cm and thickness of 550 µm were used to fabricate vertically etched Si nanowires through MACE. Si substrates were cut into 1.5 × 1.5 cm 2 samples and treated with a pure piranha (H 2 SO 4 :H 2 O 2 = 3:1 v/v) solution and O 2 plasma to remove impurities remaining on the substrates. Close-packed monolayers of polystyrene (PS) beads of 500 nm diameter (Thermo-Scientific, Inc.) were transferred on precleaned Si substrates by the nanosphere lithography method and the size of PS beads was reduced to 350 nm by the inductively coupled plasma reactive ion etch (ICP-RIE) 40 . Au of 30 nm thickness was deposited on these substrates by using the E-beam evaporator at the deposition rate of 3 Å/s and the substrates were stored in a vacuum desiccator prior to the MACE process. Characterization. The structures and surface morphologies of vertically etched Si nanowires were characterized during MACE and ACE processes by using the scanning electron microscope (SEM, Hitachi S-4800) and the transmission electron microscope (TEM, Tecnai G2 F30). The Raman and PL spectra of the Si nanowires were measured by using the microscope system (a high resolution Raman/PL spectrophotometer, Horiba LabRAM HR-800) with a 100 × objective (NA = 0.9). The Ar ion laser of λ = 514 nm and ~25 µW was used for PL and Raman studies of the nanowires. The PL spectra of the nanowires were measured in the wavelength region of 550-900 nm and Raman spectra of the nanowires were measured in the wavenumber region of 480-560 cm −1 . Reflectance of the nanowires were measured in the wavelength region of 450-900 nm.
Finite-difference time-domain simulation. The simulations were performed using the commercial finite-difference time-domain (FDTD) software package of Lumerical Solution 8.15 41 . The shapes and structures of flat and tapered Si nanowires used in the simulation were estimated from SEM images. All nanowires in the simulations were assumed to have a diameter of 350 nm and a pitch of 500 nm. The lengths of the flat nanowires (the tapered ones) were set as 3.0 (1.7), 3.6 (2.2) and 3.6 (2.7) µm, similarly to the lengths of the nanowires estimated from their SEM images. The reflectance spectra and time-averaged Poynting vector distribution of the nanowires, <S>, were simulated in the range of 400-900 nm. All simulations were performed with perfectly matched boundary conditions on the z-axis and with symmetric and anti-symmetric boundary conditions in the x-and y-directions. Simulated reflectance spectra of MACE-fabricated Si nanowires were treated with the low-pass filter to reduce large fluctuations in spectra which result from overestimated interferences between periodically spaced Si nanowires and coherent simulation light source 42,43 . The refractive index values of n and k of Si taken from the literature were considered in the simulation 44 .