Interdisciplinary surface studies on porous silicon (PSi)—II. Cathodoluminescence (CD, Auger (AES), electron energy loss (EELS) and Raman spectroscopy of different PSi samples

Abstract

Porous silicon (PSi) samples prepared with different electrochemical procedures and experimental parameters have been studied using AES, EELS, Raman spectroscopy and cathodoluminescence. AES and EELS of anodic oxidized PSi samples indicate the formation of SiO 2, with a band gap of 9 ± 0.5 eV as measured in the loss spectra from the threshold for interband transitions. Oxygen is not detected in freshly etched samples; these PSi layers are characterized by Auger and loss spectra close to that of elemental Si; some differences with respect to bulk Si spectra are attributed to the local chemical surroundings of Si atoms and particularly to the presence of hydrogen resulting from the HF treatment. A red shift of the CL emission with respect to photoluminescence is generally observed in cathodoluminescence experiments. Strong local intensity variations may also be detected; these bright ‘hot spots’ are attributed to a localized electron beam heating, corresponding to the temperature rise of Si clusters embedded in insulating SiO 2. The dimensions of the Si crystallites of a given morphology are estimated from the shift and the half-width of the one-phonon Raman peak detected near 520 cm −1 for Si. Some weak bands detected in the Raman spectra of PSi(p) formed on a p-type substrate are attributed to amorphous silicon, while the Raman spectra of Psi(n) display a crystalline-like character.