Correlations of electrochemical behavior, microstructure and chemical composition of porous silicon

Porous silicon (PSi) layers were prepared by anodic polarization of heavily doped n-type Si in hydrofluoric acid–ethanol solutions under varying current densities and etching time. PSi nanocrystallites formed were examined under a high-resolution transmission electron microscope and analyzed by the broadening and red shifting of characteristic Si Raman band. The Raman data were used to calculate the average sizes and distortion angles of crystallites. The chemical compositions of PSi were studied by Fourier transform infrared spectroscopy, and the amounts of Si hydrides and oxides present on PSi were semi-quantitatively evaluated through the integrations of the corresponding bands. The polarity of Si–H and Si–O bonds caused apparent lattice distortions. The chemical species formed on the steps and (1 1 1) surface of PSi nanocrystallites were well distinguished in infrared bands. The amount of SiySiHx species per unit PSi thickness was found to decrease linearly with the increasing current density. The observed relationships among the electrochemical behavior, microstructure and composition are discussed, and a simplified nanocrystallite model is suggested.