X-ray and synchrotron studies of porous silicon

The results of comprehensive studies of layers of porous silicon of different conductivity types, grown by anodizing standard Si(111) substrates in an electrolyte based on fluoric acid and ethanol with the addition of 5% of iodine and kept in air for a long time, are discussed. Measurements are performed by scanning electron microscopy, high-resolution X-ray diffraction, and ultrasoft X-ray spectroscopy using synchrotron radiation. The structural parameters of the layers (thickness, strain, and porosity) and atomic and chemical composition of the porous-silicon surface are determined. It is found that an oxide layer 1.5–2.3-nm thick is formed on the surface of the silicon skeleton. The near-edge fine structure of the Si $2p$ absorption spectrum of this layer corresponds to the fine structure of the $2p$ spectrum of well coordinated SiO$_2$. In this case, the fine structure in the Si $2p$-edge absorption region of the silicon skeleton is identical to that of the $2p$ absorption spectrum of crystalline silicon.