氧化 HF 溶液中硅的金属辅助化学蚀刻

Widespread deployment of solar photovoltaics (PVs) is crucial to meeting the world’s growing energy demand and  mitigating climate change in the future; however, solar PVs  remains expensive and unreliable compared with methods,  such as fossil fuel-based electricity generation.It is widely  accepted that low-cost high-efficiency solar PVs capture more  solar energy and make solar energy economically competitive  with fossil fuels. In the past decades, crystalline siliconbased PVs dominated commercial PVs owing to their low-cost,  earth-abundance, and reliability; however, the poor infrared absorption owing to its indirect bandgap,  as well as the high reflectivity resulting  from surface texturing processes, hurts  the cell efficiency, and hence poses a  daunting challenge to the large-scale  deployment of silicon-based PV modules.[8,9] Furthermore, new ideas striving  for more efficient utilization of sunlight  have also been under development.A  promising high-efficiency silicon PV technology is the use of black silicon (b-Si)  solar cells, which are based on crystalline  silicon (c-Si) wafers with micro/nanostructured surfaces that appear black to human  eyes because it can efficiently capture sunlight across a broad range of wavelengths  and angles of incidence. The excellent light trapping capability of b-Si allows  significant reduction in wafer thickness  even without the application of an antireflection coating; hence, it is a promising  material candidate for cost-efficient silicon PVs.

The fabrication of luminescent porous silicon by metal  enhanced silicon etching was first demonstrated by Zhang  et  al. in 1993. They produced a porous silicon layer on an  n-type silicon polished surface when the n-silicon in contact  with a gold was illuminated in an HF solution containing  oxygen without an externally applied potential. In 1997,  Dimova-Malinovska et  al. reported the fabrication of porous  silicon by etching an aluminum coated silicon substrate in an  HF–HNO3 aqueous solution. They claimed that the incubation time necessary for the formation of porous silicon was  decreased dramatically owing to the presence of the Al film. In  1999, inspired by the discovery of Zhang et al., Kelly et al. demonstrated that a luminescent porous silicon layer can be made  on p-type silicon surface in a similar manner without illumination. In their experiment, as shown in Figure 2a, a silicon  wafer was short-circuited to an inert metal by evaporating an  inert Au/Cr film onto the back side of the wafer, thus forming a  galvanic cell. The resulting porous silicon layer with a thickness  of 7 µm is shown in Figure 2b. They proposed that the porous  silicon is obtained by galvanic etching owing to the formation  of metal/silicon galvanic cells in which the silicon acts as anode  and metal as cathode in oxidizing HF aqueous solutions. The  galvanic etch rate can be controlled by the metal/silicon area  ratio and oxidant concentration of the HF solutions.

It is desirable to move vertical SiNW arrays from silicon substrates to a different substrate, such as a flexible one for flexible devices. Weisse and Zheng et al. report an exciting and  simple method to vertically transfer SiNW arrays with uniform  length onto different substrates by creating horizontal crack  in the SiNWs. The crack is formed by soaking an Ag-MacEtch  SiNWs sample in water and the subsequent Ag-assisted etching  processes of SiNW arrays, as shown in Figure 5. The authors  suggested that the crack formation is related to the delamination and reattachment of an Ag film during water soaking;  however, this simple water-soaking crack method does not work  for the delamination and reattachment of gold film. The crack  method facilitates embedding SiNWs within polymers and integrating SiNWs into channels, thereby enabling the realization  of vertical SiNW devices that are mechanically flexible and scalable to large dimensions.

Fig2(a)

Silicon dissolution during the MacEtch of silicon in HF  solutions consisting of divalent and tetravalent dissolution processes was proposed on the basis of experimental results, and  the two half-cell reactions that include cathodic and anodic reactions are expressed in the following equations (Equations (1–4)).  The additional cathodic reaction expected to occur on the metal  or silicon surface is the reduction of noble metal ions since  their metallic form may dissolve in the oxidizing HF solution,  while the strong oxidizing solution environment does not favor  the cathodic reduction of hydrogen ions. Clearly, besides the  localized silicon dissolution at the metal–silicon interface, the  overall electrochemical reactions involved in MacEtch of silicon are identical to those in electrochemical etching or stain  etching of silicon in HF aqueous solution containing strong  oxidants.