## 2 Materials and Methods  
The porous layers were fabricated on the surface of one side polished boron-doped p-type crystalline silicon wafers (surface roughness of the order of 0.1 nm), with orientation \(<\)100\(>\). The anodization method was employed to grow porous structures on the polished side, while the other side, coated with aluminum (Al) via electron beam evaporation, serves as the bottom contact. For Al evaporation, the base pressure was 1.25 \(\times\) 10\({}^{-5}\) mbar and the evaporation time was 4 min, resulting in 150 nm-thick Al thin films, measured by the machine thickness monitor. The Al thin films were subsequently annealed by rapid thermal processing (RTP) in nitrogen atmosphere for 5 min to turn the initial rectifying behavior into ohmic. The silicon samples with an area of 1.5 \(\times\) 1.5 cm\({}^{2}\) were mounted in Teflon holder with a circular aperture of 1.23 cm\({}^{2}\). The electrolyte for each case is a mix from HF and ETOH or DMF. The samples properties and the anodization parameters are summarized in Table 1.  
Before the etching process, the substrate was cleaned by immersing it in ethanol to remove any organic contamination, then rinsed with water and dried. The fabrication process of the experimental setup is schematically described in Figure 1. The intended samples were etched using the conventional Pt and heavily doped Si wafer (\(\rho_{s}\) = 0.01-0.02 \(\Omega\)-cm) as counter electrodes. The geometry and electrode type has some effects on the fabricated structure. The geometry of Pt electrode is a wire with 1 mm diameter, forming the following setup: Pt/electrolyte/Si/Al with none symmetric conductivity surfaces exposed to the electrolyte. While the heavily doped Si with the same area and holder aperture used in the second case form the following setup: Al/Si/electrolyte/Si/Al with symmetric conductivity surfaces exposed to the electrolyte. The anodization cell were controlled using potentiostat with constant current mode. Two etching current density values were used for both cell setups to form different nanostructured surface on silicon samples namely 20 mA/cm\({}^{2}\) and 80 mA/cm\({}^{2}\). The selection of the time and the etching current density depends on the resistivity of Si substrate [31; 32; 33; 34]. In the case of low doped silicon (\(\rho_{s}\) = 25-30 \(\Omega\)-cm) we used illumination with halogen lamp (60 W) through a window in the Teflon cell to facilitate the etching process by activating the carriers [34; 35]. After the fabrication process, each sample was rinsed with ethanol, followed by deionized water and finally dried with a mild stream of dry nitrogen. The morphology of the PSi layers was studied by field emission scanning electron microscope (FESEM- XL-40 FEG, Philips, Eindhoven, Netherlands). An energy dispersive spectroscopy X-ray analyzer (EDS, Inca X-sight 7558, Oxford Instruments, London, UK) coupled to the microscope was used to determine the elemental compositions of the samples. The Optical characterization in the UV-visible range (250-900 nm) was performed using a Jasco V-560 double-beam spectrophotometer equipped with an integrating sphere to avoid \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Substrate Resistivity (\(\rho_{s}\))** & **Etching Solution** & **J (mA/cm\({}^{2}\))** & **t (s)** \\ \hline S1 = 0.01–0.02 \(\Omega\)-cm & HF 48\%: ETOH 99\% (1:2) & 80 & 14 \\ \hline S2 = 0.8–2 \(\Omega\)-cm & HF 48\%: ETOH 99\% (1:2) & 20 & 100 \\ \hline S3 = 25–30 \(\Omega\)-cm & HF 48\%: DMF 98\% (1:6) & 20 & 200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of fabrication parameters. Figure 1: Fabrication setup of the electrochemical etching process and samples holder are from Teflon with quartz window in the cell reactor for illumination purpose. The anodization process driven by a programmable potentiostat working at constant current mode. scattering losses.