Anodic and optical characterisation of stain-etched porous silicon antireflection coatings

Porous silicon (PS) antireflection coatings have been obtained by stain etching of crystalline Si in HF:HNO3:H2O solutions at different etching times and HNO3 concentrations. The morphology of the PS samples was characterised by anodic oxidation and revealed the increase of the porous surface area with the HNO3 concentration. Upon a certain critical concentration of HNO3, the surface of crystalline Si immersed in the solution becomes polished, and a decrease of its surface area is observed. The samples were also characterised by a spectrophotometer in the range 300–900 nm, showing an effective reflectance below 4% in most of the spectral range for samples obtained in an etching solution, where the HNO3 is below its critical concentration. Anodic oxidation after etching can be used as an useful tool for the qualitative and quantitative analysis of porous surface areas.