STRUCTURE AND FORMATION OF POROUS SI LAYERS AS STUDIED BY INFRARED-ABSORPTION AND RAMAN-SCATTERING

The microscopic structure and formation processes of porous Si layers (PSLs) have been studied by infrared absorption and Raman scattering. How the concentration of HF used during formation affects the vibrational spectra of PSLs has been examined. The IR spectra of as-anodized PSL in the Si-H(x) vibration region are quite similar to those of Si(111) or (100) surfaces etched in HF solutions of pH approximately 2. The amount of dihydrogen adsorbed on the surface increases for lower HF concentrations relative to that of monohydrogen. Fluorine and oxygen are found on PSLs anodized for a long period. The size of the Si particle (or column) is a few nanometers for samples anodized with the solution of HF:H2O=1:1 as estimated from Raman data, and it decreases with a decrease in the HF concentration. No Si nanostructures are detected in the Raman data for the case of HF:H2O:ethanol=1:1:8. The HF concentration dependence of the IR spectra is explained in terms of the change in the size of columns or particles in the PSL. The Si-Si back bond for the surface Si atom terminated by dihydrogen is more stable against oxidation in air than the bond for the atom terminated by monohydrogen. The HF concentration dependence of the electrochemical process is discussed in connection with the relative contribution of electropolishing and pore formation processes. The relation between the photoluminescence intensity and the surface structure is also discussed.