NANOSTRUCTURAL AND NANOCHEMICAL INVESTIGATION OF LUMINESCENT PHOTOELECTROCHEMICALLY ETCHED POROUS N-TYPE SILICON

Porous silicon obtained on n-type silicon by photoelectrochemical etching in HF, is formed of a macroporous silicon layer beneath a nanoporous silicon layer. Microstructural investigations and chemical analysis at the atomic level of the nanoporous silicon film (obtained from a highly doped (111) oriented Si substrate) have been done by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) using a scanning transmission electron microscope (STEM). We have found that the nanoporous Si consists of a regular Si macroarray with triangular geometry. Nanometer-size tangled wires are contained within and attached to the macroarray. HRTEM images clearly demonstrate the existence of quantum-sized Si wires made of a crystalline core covered with an amorphous layer. Electron energy loss spectra (EELS) have been recorded for different positions of the incident probe across the quantum-sized Si wires. The results obtained in the low-loss region and at the Si L23 edge have been compared with those recorded on reference specimens (Si/SiO2 interface and hydrogenated Si sample). Although they do not exclude the presence of one or a few monolayers of foreign species, of hydrogen in particular, on the outer surface, our results generally support the quantum-confinement model to interpret the observed photoluminescence in nanoporous Si.