## 3 Results and Discussion  
### Evaluation of Structure and rReflectance of SiNWs on Si Wafers  
To measure the length and reflectance of the SiNWs, the SiNW arrays were fabricated on Si wafers. Figure 2 shows the SEM images of Ag particles fabricated via electroless plating from solutions with different AgNO\({}_{3}\) concentration onto c-Si wafer substrates. When the AgNO\({}_{3}\) concentration was 0.0025 M (Figure 2A), the Ag particles were deposited onto the Si wafer. When the AgNO\({}_{3}\) concentration was increased to 0.005 M (Figure 2B), Ag was formed over the entire substrate, with an island structure, rather than a particle structure; large gaps were observed between the Ag islands. When 0.0025 M and 0.005 M AgNO\({}_{3}\) (Figure 2C,D) were used, Ag particles formed with non-uniform size and shape, and with large gaps among them. When 0.01 M AgNO\({}_{3}\) was used (Figure 2E), Ag nano-particles were formed along with Ag islands. For AgNO\({}_{3}\) concentrations >0.02 M, many dendrites formed, precluding the control of the density of Ag particles, as shown in Figure 2E. Figure 3 shows the top-view SEM images of the SiNW arrays, fabricated with different AgNO\({}_{3}\) concentrations. When AgNO\({}_{3}\) concentrations of 0.0025 M and 0.005 M were used (Figure 3A,B), Si  
Figure 1: (**A**) Fabrication process for silicon nanowire (SiNW)-array-based solar cell on an silicon-on-insulator (SOI) substrate (**B**) Schematic of a SiNW-array-based solar cell etched on the SOI substrate. Figure 2: SEM images of Ag particles fabricated via electroless plating onto p-type c-Si wafers with AgNO\({}_{3}\) concentrations of (**A**) 0.0025 M, (**B**) 0.005 M, (**C**) 0.01 M, (**D**) 0.015 M and (**E**) 0.02 M.  
nano-walls were formed, rather than a SiNW structure. These nano-wall formations strongly resembled the gaps among the Ag particles observed in Figure 2A,B, indicating that the density of Ag particles had likely increased. Accordingly, when the AgNO\({}_{3}\) concentration was 0.01M, a SiNW structure emerged, and Si nano-walls were divided into several nanowires (Figure 3C). The images in Figure 3C-E indicate, that Ag particles were deposited in the gaps between the Ag islands, with increasing AgNO\({}_{3}\) concentration because higher AgNO\({}_{3}\) concentrations led to the involvement of Ag\({}^{+}\) ions in dendrite growth. Figure 4A-C shows cross-sectional SEM images of SiNW arrays formed with AgNO\({}_{3}\) concentrations of 0.0025 M, 0.005 M, and 0.01 M. These images indicate, that the SiNW arrays were fabricated vertical to the substrate, and that the concentration of AgNO\({}_{3}\) affected only the horizontal structure of the SiNW arrays. Therefore, we selected an AgNO\({}_{3}\) concentration of 0.01 M to form SiNW arrays. The SiNW array length increased with increasing etching time, and a 12 \(\upmu\)m-long SiNW array was obtained after 60 min. However, the length of the SiNW array did not linearly increase with respect  
Figure 4: Cross-sectional SEM images of SiNWs fabricated via metal-assisted chemical etching with different concentrations of AgNO\({}_{3}\): (**A**) 0.01 M, (**B**) 0.015 M, and (**C**) 0.02 M.  
Figure 3: Top-view SEM images of etched samples fabricated using electroless plating with AgNO\({}_{3}\) concentrations of (**A**) 0.0025 M, (**B**) 0.005 M, (**C**) 0.01 M, (**D**) 0.01 M, and (**E**) 0.02 M.  
to the etching time. The SiNW formation mechanism can be interpreted according to the following electrochemical reactions of Si in an HF and H\({}_{2}\)O\({}_{2}\) solution:  
\[4\text{Ag}^{+}+\text{Si6F}^{-}\to 4\text{Ag}+\text{SiF}_{6}^{2-}, \tag{1}\]  
\[2\text{Ag}+\text{H}_{2}\text{O}_{2}+2\text{H}^{+}\to 2\text{Ag}^{+}+2\text{H}_{2} \text{O}. \tag{2}\]  
After the consumption of HF and H\({}_{2}\)O\({}_{2}\), new HF and H\({}_{2}\)O\({}_{2}\) must diffuse to the bottom of the forming SiNWs. However, the length of etchant migration increases with increasing etching time because the lengths of the SiNWs increase.