Metal Assisted Synthesis of Single Crystalline Silicon Nanowires at Room Temperature for Photovoltaic Application

Synthesis of single crystalline silicon nanowires (SiNWs) array at room temperature by metal assisted chemical etching and its optical absorption measurements have been reported in this article. It has been confirmed that, SiNWs were formed uniformly on p-type silicon substrate by electroless deposition of Cu and Ag nanoparticles followed by wet chemical etching in (Hydrogen Fluoride) HF based Fe(NO3)3 solution. Synthesized SiNW structures were analyzed and investigated by Scanning Electron Microscopy (SEM) and Ultraviolet-Visible (UV-VIS) spectrophotometer. Formation of SiNWs is evident from the SEM images and morphology of the structures depends upon the concentration of chemical solution and etching time. The synthesized SiNWs have shown strong broadband optical absorption exhibited from UVspectroscopy. More than 82% absorption of incident radiation is found for Cu treated samples and a maximum of 83% absorption of incident radiation is measured for Ag synthesized samples which is considerably enhanced than that of silicon substrate as they absorbed maximum of 43% of incoming radiation only. light trapping and as we have shown in this paper, high absorption [31,32]. Herein, we have reported the room-temperature synthesis of highly oriented SiNWs array, using simple chemical technique named Electroless Metal Deposition (EMD) and metal-assisted Electroless Etching (EE) of Si wafers in an aqueous HF solution containing Fe3+ ions. We have employed this method because it has several advantages over many other established methods. This technique is cost effective and functions without requiring any vacuum and high temperature requirement. Growth of SiNWs by this technique is slightly depends on the crystallographic orientation which makes them to utilize polycrystalline substrate [33]. This chemical electrolytic process is based on a galvanic displacement reaction. In this method oxidization of metal ions to metallic species such as particles and films, and the dissolution of silicon substrate occur extemporaneously on a silicon surface. After successful synthesis of Silicon nanowires (SiNWs) we have investigated the optical behaviour of synthesized structures by means of Ultraviolet-Visible (UV-VIS) spectroscopy. Silicon nanowire (SiNW) arrays synthesized by wet-chemical etching process have revealed very high absorption for a particular range of wavelength of electromagnetic spectrum. This high broadband optical absorption of SiNWs array makes them an important contestant for photovoltaic solar cell applications. Journal of Nanomedicine & Nanotechnology J o u r n a l o f N an om edicine & Natechn o l o g y ISSN: 2157-7439 Citation: Md Asgar A, Hasan M, Md Huq F, Mahmood ZH (2014) Metal Assisted Synthesis of Single Crystalline Silicon Nanowires at Room Temperature for Photovoltaic Application. J Nanomed Nanotechnol 5: 221. doi: 10.4172/2157-7439.1000221


Introduction
Research attention in semiconducting nanostructures has been decoupled after successful synthesis of Carbon Nano-Tubes (CNTs), for their novel size and dimensionality dependent physical properties [1,2]. To date, the most significant semiconducting material in microelectronic industries is silicon (Si), owing to their technical compatibility with existing semiconductor technology. Though silicon cannot emit visible light as being an indirect material, low dimensional Si nanostructures yield visible light because of quantum confinement effect [3][4][5]. As a result, much effort has been made to prepare various silicon nanostructures such as Silicon nanotubes, Silicon Nano-Cones (SiNCs) and Silicon Nano-Wires (SiNWs) etc [6][7][8][9].
Many other applications have been verified primarily in the form of individual nanowire devices, though some devices based on arrays have also been shown [27]. One feature that has been generally overlooked is the interaction of light of varying wavelength with nanowire assemblies on p-type Si substrate, which may have implications for future optical and optoelectronic devices such as Laser, optical sensor [28]. and nanophotovoltaic solar cell [29]. The high aspect ratio of nanowires (typically on the order of 100 to 1000) and sharp dielectric contrast with its surrounding (e.g. air or glass) lead to optical anisotropy as observed in individual silicon nanowires [30]. Moreover, sub-wavelength diameters and proximity, combined with micron-scale lengths, may lead to interesting optical properties such as low reflectance through better light trapping and as we have shown in this paper, high absorption [31,32].
Herein, we have reported the room-temperature synthesis of highly oriented SiNWs array, using simple chemical technique named Electroless Metal Deposition (EMD) and metal-assisted Electroless Etching (EE) of Si wafers in an aqueous HF solution containing Fe 3+ ions. We have employed this method because it has several advantages over many other established methods. This technique is cost effective and functions without requiring any vacuum and high temperature requirement. Growth of SiNWs by this technique is slightly depends on the crystallographic orientation which makes them to utilize polycrystalline substrate [33]. This chemical electrolytic process is based on a galvanic displacement reaction. In this method oxidization of metal ions to metallic species such as particles and films, and the dissolution of silicon substrate occur extemporaneously on a silicon surface.
After successful synthesis of Silicon nanowires (SiNWs) we have investigated the optical behaviour of synthesized structures by means of Ultraviolet-Visible (UV-VIS) spectroscopy. Silicon nanowire (SiNW) arrays synthesized by wet-chemical etching process have revealed very high absorption for a particular range of wavelength of electromagnetic spectrum. This high broadband optical absorption of SiNWs array

Experimental Details
Preparation of 1D silicon nanostructures array was conducted in Teflon lined stainless steel vessel. The fabrication process comprised of three steps. The first step is cleaning of the silicon wafers with cleaning solution-here we sequentially used acetone (5 min), ethanol (5 min), deionized water (2-3 times), and H 2 SO 4 /H 2 O 2 (3:1 H 2 SO 4 (97%)/ H 2 O 2 (30%), 10 min), then the wafers are thoroughly rinsed with deionized water (10 min) and dip into a solution of HF (1 min). The second step of fabrication is electroplating the metal-nanoparticle films onto the cleaned silicon surface. A group of cleaned samples were dipped in HF/ AgNO 3 solution for Silver nanoparticle deposition. Deposition time was varied from 60 seconds to 75 seconds. The concentration of HF and AgNO 3 were 5 M and 0.02 M respectively. Another group of samples were dipped in HF/Cu(NO 3 ) 2 .H 2 O for copper nanoparticles deposition. In this case, deposition time was varied from 60 seconds to 120 seconds. The concentration of HF was 5 M, whereas for Cu(NO 3 ) 2 .H 2 O, it was varied from 0.02 M to 0.05 M. All the above mentioned chemical processes performed in Teflon beaker. After deposition of silver/copper nanoparticles, the silver/copper deposited substrate was immersed in another solution for etching of uncovered areas of the silicon substrate. The solution was HF based Fe(NO 3 ) 3 , where the concentration of HF and Fe(NO 3 ) 3 were 5 M and 0.02 M respectively. Etching was performed at room temperature and etching time was varied from 30 min to 120 min for different samples. After the preparation process, the obtained samples were rinsed copiously in deionized water and dried at room temperature.

Synthesis Mechanism of Silicon Nanowires
For the initial electroless deposition of silver from HF/AgNO 3 solution on silicon, simultaneous electrochemical processes including cathodic (silver-deposition reaction) and anodic reactions (silicon oxidation and dissolution) occur on the silicon surface. First, Ag + ions in the vicinity of the silicon surface capture electrons from the Valence Band (VB) of silicon and are deposited in the form of metallic silver nuclei. With the deposition and growth of silver nuclei, SiO 2 is formed simultaneously underneath the silver nanoparticles (Illustrated in Figure  1a and b). As the SiO 2 underneath the silver nanoparticles is etched away as by HF, then pits are formed immediately at the same location (Illustrated in Figure 1c), so the silver nanoparticles sink into the pits as they are formed. This is consistent with our SEM observations, which confirmed that the oxidation and dissolution of the silicon substrate occur from beneath the metal deposits. In the solution of HF/Fe(NO 3 ) 3 , the redox level of the Fe 3+ / Fe 2+ system lies well below the VB of silicon, so active kinetics are expected for the electron-capture process from the VB bonding electrons. However, in contrary to fast reduction of Ag + ions, reduction of Fe 3+ ions and oxidation of silicon proceed very slowly on the silicon surface in the absence of silver nanoparticles. When the silicon surface is covered with silver nanoparticles, reduction of Fe 3+ ions and oxidation of silicon are greatly enhanced. It is generally believed that metal nanoparticles adhered on a silicon surface have a higher electronegativity than silicon, so they could attract electrons from silicon to become negatively charged. These metal nanoparticles could then act as local microcathodes and enhance the cathodic reaction as a result of their catalytic activity [34][35][36]. Therefore, Fe 3+ ions have the strong tendency to preferentially obtain electrons from silver nanoparticles and be reduced to Fe 2+ ions, while the silicon underneath the silver nanoparticles is oxidized to SiO 2 (illustrated in Figure 1d-f). The Ag + /Ag system has more positive redox potential than that of the Fe 3+ /Fe 2+ couple, so silver nanoparticles are stable and can continue to enhance the reduction of Fe 3+ ions and the oxidation of silicon in the HF/Fe(NO 3 ) 3 solution. As the silver nanoparticles are pinned by the pits and cannot move horizontally on the silicon surface, selective etching occurs and deeper pits or pores finally form (depicted in Figure 1g and 1h).
The following equations explain the chemical reactions that occur in the process:

Scanning electron microscopy
Synthesized samples were characterized to scrutinize the structural and optical behaviours. For the structural characterization Scanning Electron Microscope (SEM) image of all prepared samples were taken and the SEM images of metal deposited samples are shown in Figure  2. The solution concentration of HF and AgNO 3 were 5 M and 0.02 M respectively for both samples but the deposition time of the first sample was 60 seconds whereas it is 75 seconds for the second one. Highly dispersed silver particles are evident on the silicon substrate in Figure  2. In case of Figure 2b, dispersed silver particles are also appeared like Figure 2a, however in some locations silver clusters look like branched dendrite. As the deposition time is 15 s longer than that of first sample, the density of cluster is higher results silver dendrites. For both samples no regular geometric shape of deposited particles was available. Figure 2c and d shows the copper nanoparticles that were produced on silicon surfaces by the galvanic displacement reaction on treatment of the silicon wafer in a solution of HF/Cu(NO 3 ) 2 for one minute. Deposited particles were approximately 0.2 µm diameter. From the figure it is also obvious that Ag-nanoparticles were deposited more homogeneously than Cu-nanoparticles.  Figure 3c. Therefore it can be stated that the morphology of the deposited Ag-particles greatly influence the etching morphology of the Si wafer. The density and size of the as-prepared SiNWs also depend on the distribution patterns of the Ag particles on the Si surface. A high density of Ag particles facilitated the formation of Si nanowires. If the Ag particles are smaller caused by short electroless deposition time, for instance less than 15 seconds, their lift-off or Brownian movement might take place during silicon dissolution, resulting in disordered Si structures [37]. In contrast, stable Ag particles would facilitate the formation of 1D Si nanostructures. The size of the SiNWsis closely related to the space between metal particles and also the immersion time in the HF solution.
The role of Ag particle is also applied for the copper (Cu) nanoparticle with a higher electronegativity than that of silicon. In the case of Cu nanoparticle, high density porous silicon nanostructure arrays were formed and confirmed from the SEM micrograph which is shown in Figure 4. From this SEM morphology, it has been also recognized that silicon nanostructures were formed very uniformly. It has been also evident from the SEM micrograph that, density of SiNWs is smaller in case of 1 hour etching with concentration of Cu(NO 3 ) 2 is 0.02 M than that of 2 hours etching through concentration of Cu(NO 3 ) 2 is 0.05 M. This may be occurred because of lower density of Cu particle during electroless metal deposition and shorter etching time. Therefore, it can be concluded that with greater deposition time and concentration of the sample solution high density, well oriented SiNWs will be produced. The Cu-nanoparticle may have disappeared from the aqueous HF/Fe(NO 3 ) 3 solution, because the standard redox potential of Cu/Cu 2+ is lower than that of Fe 3+ /Fe 2+ . Cu-nanoparticles would dissolve into the solution as soluble Cu 2+ ions, according to the following Equation 1 [37]. 2Fe 3+ +Cu(s) → 2Fe 2+ +Cu 2+ (1)

UV spectroscopy
The optical properties of SiNWs include reflection, transmission and absorption. Here, optical absorption measurement of SiNWs was carried out over a range of wavelengths 200 nm to 1000 nm, which covers most of the spectrum that is useful for silicon nanowires (SiNWs) based solar cells. Figure 5 shows typical total optical absorption spectra of Cu treated samples: type-A sample (sample-b, sample-c and sample-d) along with absorption of bulk silicon (sample-a). The absorbance of the solid silicon film shows typical behaviour as expected for silicon. Here we have reported that, absorption of bulk Si begins to increase at 400 nm and remains almost steady up to 1000 nm and might be decrease somewhere around 1100 nm because of back reflected loss for long wavelength from the top surface [38]. Solid Si film absorbs maximum of 47% of incident radiation but SiNW arrays absorb maximum 82% of radiation. It has also been observed that the absorbance of the Cu deposited silicon nanowire films increases continually with increasing etching time. The length of SiNWs is directly proportional to the etching time at a constant concentration of sample solution. Hence, the length of silicon nanowire arrays increase and diameter decrease as the etching time elapses. Therefore, superior quantum confinement of photon occurs and for this reason higher absorption takes place. Previous studies also confirmed that porous silicon, occurs in case of Cu nanoparticle, can reduce back reflection up to 5.8% in the wavelength range of 400-1000 nm, which replace the surface texture to high absorption of photon energy [39][40][41]. From the Figure 5, it has been observed that the absorbance of the Cu deposited silicon nanowires films is much greater than that of the silicon wafer. It has also looked into that with increasing treating time and concentration, higher absorption can be accomplished. However, if the concentration of Cu(NO 3 ) 2 is very high then metal nanoparticles would cover the most part of the substrate. SiNWs grow only in the uncovered area of the Si substrate as discussed in Figure 1. So if the metal particle covers almost all surface area of the bulk substrate then the possibility of formation of SiNWs array would be very low and nanowires may not develop if the nanoparticles cover all of the surface area. Otherwise, if the density of the deposited particles is very low then the space between them will be large. As a result, silicon microwires (SiMWs) would be produced (similar way as Figure 1) in place of silicon nanowires. Again, if etching time becomes very high then the length of the free standing SiNWs would be fair enough to break apart. Therefore, it can be stated that, an optimum condition of etching time and density of catalytic metal is required for the formation of free standing SiNWs array.
Absorption measurement was also carried out for Ag-synthesized samples: type-B sample (sample-e, sample-f, and sample-g) with different etching time for same wavelength range of 200 nm to 1000 nm which is shown in Figure 6. It has been found that, the absorption of sample-e was very low for wavelength range of 350 nm-1000 nm whereas absorbance increased up to 84% in the range of 200 nm-350 nm. For sample-f and sample-g, absorption of light energy was greater than the silicon wafer in 350 nm to 1000 nm but it was increased tremendously up to 83% in 200 nm to 350 nm.
Based on the above evidence it is clarified that large area single crystalline silicon nanowires (SiNWs) have revealed superior absorption measurement compared to bulk silicon. This high absorption characteristic can be utilized to fabricate absorber in photovoltaic solar cell by preparing a p-n junction on fabricated p-type SiNWs by means of thermal diffusion.

Conclusion
In conclusion, we have demonstrated that Si nanowire films possess unique macroscopic and optical properties. The nanowires yield significantly increased optical absorption over the full spectrum above the bandgap, as well as reduced transmission for wavelengths of