Optical characteristics of porous silicon photonic crystals prepared on the back surface of silicon wafers

Surface-enhanced Raman scattering sensors and optical detection devices often incorporate rough surfaces or porous Si photonic crystals to enhance optical absorption and localized surface plasmon resonances. This study reports a porous Si photonic crystal structure fabricated on the backside of a crystalline Si substrate by room-temperature electrochemical etching. Scanning electron microscopy showed that the rough surface did not affect the vertical etching rate, but influenced the porosity and pore size distribution. X-ray diffraction analysis showed broader and lower-intensity peaks from the sample prepared on the back side, indicating that the backside porous Si had strong light-scattering effects and lattice contraction, which led to greater oxidation because of its higher porosity. A reflectance of ∼1% was obtained over a broad wavelength range (400–700 nm) and the optical reflectance suppression mechanism was analyzed. These optical characteristics showed backside porous Si photonic crystals have significant potential utility in photovoltaic and photonics applications.