# Recent Advances in Black Silicon Surface Modification for Enhanced Light Trapping in Photodetectors
## 2 Wet Methods for Black Silicon Fabrication
### Electrochemical Etching  
The electrochemical etching method involves using Si as the anode, which is then submerged in a solution of HF, H\({}_{2}\)O, and ethanol. A cathode made of platinum is also employed for its conductivity and resistance to HF. The aim of this method is to reduce reflectivity by precisely controlling both the etching time and current density. [PERSON] et al. created a porous Si photonic crystal architecture on a c-Si via electrochemical etching conducted at ambient temperature [40]. This was conducted to enhance surface properties for Raman scattering sensors and optical detection devices. The surface roughness affected porosity and pore size distribution but not the vertical etching rate. The obtained results indicate significant light scattering effects and lattice compression. A reflectance of approximately 1% across a wide range of wavelengths (400-700 nm) was recorded, highlighting the significant potential of rear-side porous Si photonic crystals for use in photonics applications. Furthermore, low-reflective samples have been prepared on a p-type <100> Si substrate using various electrochemical etching currents/durations, which are suitable for optoelectronic applications [41]. The BSi was analyzed by FESEM, revealing layer formation with porosity percentages ranging from 10 to 40%. The reflectivity assessment demonstrated a reduction in intensity and a shift towards longer wavelengths with increasing etching current, ultimately resulting in peak photoluminescence at 360 nm. Overall, wet chemical etching techniques are utilized for BSi surface modification. Cost-effective and versatile methods like MACE, KOH etching, and electrochemical etching, which are commonly used to create nanostructured BSi surfaces, are discussed. The MACE technique uses metal elements (Ag, Cu, Ni, and Al) to produce metal nanoparticles on silicon surfaces. Thus, it enhances light trapping and reflectivity. Each metal influences the surface differently, where Ag affects porosity, and the etching rate in their proximity is significantly elevated. Cu improves light absorption and inverted pyramid formation; Ni NPs disappear during the etching process, the Ni NPs cleaning step is eliminated, and, thus, the etching process steps are decreased. Al NPs have high control over the nanopores' morphology. The KOH etching technique exhibits high controllability by manipulating concentration, temperature, and etching duration. This method facilitates the straightforward fabrication of BSi surfaces. The anisotropic etching process forms a textured surface on the BSi samples, significantly reducing reflectance. Electrochemical etching created a porous Si through ambient temperature for porosity and pore size distribution but not vertical etching rate to reduce reflectivity by precisely controlling the current density and etching time and recording an extremely high light trapping with a reflectance of approximately 1% across the visible range and controlling the porosity percentages of layer formation. A comprehensive review of the recent developments in BSi using wet etching methods is shown in Table 1.  
Recent advancements in photodetector fabrication techniques have demonstrated significant performance improvements, particularly through surface modification strategies. Black silicon, known for its superior light-trapping capabilities, has been widely used to enhance photodetector efficiency, especially in the infrared region. Wet etching techniques, particularly metal-assisted chemical etching (MACE), have been central to creating nanostructured black silicon surfaces that improve light absorption. In parallel, water-assisted transfer printing has emerged as a novel method for fabricating organic photodetectors, offering fast rise times and a high on/off ratio due to the use of a sacrificial ethylene glycol-doped PEDOT layer. This method has proven effective in overcoming surface adhesion challenges, thus potentially opening pathways for integrating organic layers with black silicon surfaces.