Optical properties of porous silicon on an insulator layer

N-type silicon wafers, consisting of 280 μm upper and 20 μm thick lower layers, were electrochemically etched in a hydrofluoric acid (HF) ethanol solution. The resistivity of the upper layer was 0.015 Ω cm, while the lower layer was a much worse conductor with a resistivity of 2 Ω cm. Porous silicon (PS) samples were produced by etching the rough (upper) side of single-side polished wafers at a constant current density. The process of etching was monitored at different HF concentrations. The samples were investigated by Raman spectroscopy, photoluminescence (PL) and scanning electron microscopy (SEM). Due to the roughness of the unpolished surface, different surface orientations were exposed to electrochemical etching, which resulted in different etching speed and consequently a different morphology (plateaus, valleys) produced by etching. The porous plateaus showed the most intensive PL observable, even by optical microscope. PL spectra exhibited a decrease of peak intensity and the blue shift of maximum with an increase of HF concentration. The presence of nanometer-size Si structures was confirmed by the broadening and red shift of the transversal optical (TO) phonon band in the Raman spectra. The quantum confinement model was used to determine the average size of these structures. SEM images showed pores of different morphology and several nanometers in diameter. The largest pores and thinnest walls were obtained when etched with the lowest HF concentration.