# Recent Advances in Black Silicon Surface Modification for Enhanced Light Trapping in Photodetectors
## 3 Dry Methods for Black Silicon Fabrication
### Femtosecond Laser Ablation  
Direct femtosecond laser surface structuring was employed to fabricate an array of nanostructure-textured patterns on silicon substrates [67]. [PERSON] et al. created BSi using femtosecond laser pulses, focusing on minimizing charge carrier recombination caused by laser-induced damage while preserving elevated levels of absorption within the bandgap [68]. In their research, the authors conducted a comprehensive examination of the influence of laser parameters, such as focal position, average power, and scanning speed, by evaluating surface morphology, absorptance spectra, and minority carrier recombination lifetime. The results indicated that an average absorptance of approximately 96% within the visible spectrum, along with a minority carrier lifetime of 54 us at an injection level of \(1\times 10^{15}\) cm\({}^{-3}\) can be attained through optimized laser parameters. These findings signify considerable progress in developing high-performance broadband optoelectronic devices utilizing surface-passivated, femtosecond-laser-processed BSi. The BSi microfabrication process employs a variety of dry microfabrication methods to precisely define small features. It typically involves using RIE for controlled material ablation and integrating hierarchical texture and precisely sized Au NPs to achieve exceptionally low broadband reflectivity for BSi, with a reflectivity value below 1% over a wavelength range spanning 220 to 2600 nm. Additionally, using phosphorus oxychloride (POCI3) and O2 gases in RIE results in a nano-texture and superior surface roughness compared to alternative methods such as PIII and wet etching processes. Adjusting the deposition time while maintaining other parameters fixed in the DRIE leads to three distinct etching regimes. ICP-RIE etching of the BSi surface forms an effective passivation layer without the need for a separate passivation step, reducing the number of process steps. This process also allows for the production of highly curved surfaces, such as hemispherical Si lenses with a radius of 12.7 mm, resulting in significantly reduced reflectance. Furthermore, ion implantation introduces dopants into the material, creating doped Si nanowire arrays that significantly absorb light over a wide wavelength range. Plasma etching generates intricate patterns and fabricates BSi surfaces covered with nanoneedles of varying characteristics. Finally, femtosecond laser pulses are effective in achieving ultrafast and extremely accurate ablation, minimizing charge carrier recombination and preserving high bandgap absorption. The nanostructured surface of BSi plays a pivotal role in minimizing reflectance through its anti-reflective properties and photon scattering capabilities. The surface modifications, often in the form of conical spikes or porous textures, create a graded refractive index that gradually transitions from air (-1.0) to silicon (-3.5), significantly reducing light reflection across the visible and near-infrared spectrum. This smooth transition minimizes abrupt refractive index changes that would otherwise cause light to reflect off the surface. Additionally, the rough and porous nanostructured surface promotes photon scattering, effectively extending the optical path length within the material and increasing the chances of light absorption. This mechanism can reduce reflectance to as low as 1%, as demonstrated in reactive ion etching (RIE) processed BSi surfaces [69]. The geometry of the nanostructures also plays a crucial role in enhancing light trapping. High-aspect-ratio structures, such as nanopillars and conical spikes, improve light trapping by inducing multiple internal reflections that prevent light from escaping the surface. Porous and needle-like structures further increase the surface area available for light interaction, which contributes to higher absorption efficiency. For instance, [PERSON] et al.