# Recent Advances in Black Silicon Surface Modification for Enhanced Light Trapping in Photodetectors
## 3 Dry Methods for Black Silicon Fabrication
### Reactive Ion Etching (RIE)  
The RIE method, commonly employed through fluorine-based processes to create BSi, involves the use of sulfur hexafluoride (\(SF_{6}\)) and oxygen (O\({}_{2}\)) as etching gases [51; 52; 53; 54; 55]. These gases generate fluoride and O\({}_{2}\) radicals as part of the etching process. Specifically, the fluoride radicals are responsible for etching the Si by eliminating the volatile silicon tetrafluoride (SiF\({}_{4}\)) that is produced. Simultaneously, the O\({}_{2}\) radicals become ionized and then interact with both Si and fluoride radicals, resulting in the development of a passivation layer. This passivation layer, also referred to as a polymeric film, serves as a thin protective coating during the etching process. It is partly removed due to continuous energetic ion bombardment. This repeated cycle of passivation and etching ultimately leads to the emergence of randomly distributed nanostructures on the surface that exhibit a needle-like morphology. One of the most crucial characteristics of BSi that makes it suitable for use in optical and photonic applications is its antireflective property. This feature can be attained through intrinsic lattice modifications or the enhancement of additional substances. Numerous studies have delved into further investigation. For instance, [PERSON] et al. successfully created BSi with unique chimney-like hierarchical micro/nanostructures using a two-step RIE process [52]. Following that, gold (Au) NPs were deposited onto the BSi using magnetron sputtering deposition, as shown in the scanning electron microscopy (SEM) images in Figure 2. By combining the hierarchical texture with specific-sized Au NPs, they were able to achieve an exceptionally low broadband reflectivity of BSi, measuring less than 1% across a wavelength spectrum extending from 220 to 2600 nm. The nanopores obtained in this work by [PERSON] et al. exhibit a 6.5 \(\upmu\)m and less than 450 nm average depth and diameter, respectively. The attainment of a notably high aspect ratio for these nanopores was achieved through the deliberate adjustment of the \(SF_{6}\)/O\({}_{2}\) gas flow ratio. Specifically, the \(SF_{6}\)/O\({}_{2}\) gas flow ratio for achieving the desired nanostructures was set to 36:47 (increasing the gas flow of O\({}_{2}\) by 30% than that of \(SF_{6}\)). This selection was predicated upon the abundance of O\({}_{2}\) radicals, which expedited the swift development of a silicon oxide (SiO\({}_{x}\)) layer on the lateral surfaces throughout the etching procedure. Consequently, this induced a highly selective etching predominantly in the downward direction, ultimately resulting in the formation of nanopores characterized by an extremely high aspect ratio. Furthermore, [PERSON] et al. conducted a study in which BSi nanostructures were fabricated at room temperature and characterized by both high aspect ratios and remarkable absorptance using RIE (RT-RIE) [53]. Within the visible and near-infrared (NIR) spectra, the absorption efficiencies exceeded 99%. In this study, four distinct samples were processed with varying etching parameters, resulting in diverse nanostructures within the BSi material. Notably, the sample with the tallest nano-cones that was achieved through specific etching parameters exhibited the highest absorptance across the entire visible spectrum. The key parameters for the optimal BSi etching included an \(SF_{6}\)/O\({}_{2}\) ratio of 1.3:1, a pressure of 5 mTorr, a power of 20 W, an etching duration of 20 min, and a bias of 177 V, which led to the formation of randomly distributed nano-cones of approximately 850 nm in height. The RIE method is also used to fabricate BSi that results in a nano-texture that greatly reduces reflectance across a broad range of wavelengths, approaching zero reflectance [54]. The intricate interplay between the fabrication process and the resultant nanotexture posed significant challenges in improving the electrical characteristics of the phosphorus emitter.