Investigation of vibrational and photoluminescence spectra of nanocrystalline silicon by micro-Raman spectroscopy using various laser powers

Abstract

The Raman spectra of silicon nanocrystals embedded in silicon oxide and in porous silicon were measured at various laser powers. It was found that the Si–Si stretching Raman peak shifts to lower wavenumbers and broadens when the laser power increases. The effect is significant and reversible, i.e. the peak returns to its former position when the laser power is decreased to the initial level. It was found that this reversible phenomenon is caused by an increase in bond length due to the heating effect of the laser. In addition, the appearance of weak bands on both sides of the main Si–Si stretching mode peak at 155, 332, 620, 940, 2087, 2114 and 2145 cm−1 was observed at high laser power. It was found that the first four bands could be attributed to one- and multi-phonon Raman scattering and the last three bands are the expected SiH2, SiH and SiH3 stretching vibration modes. The photoluminescent spectrum was measured with the use of 1.959 eV illumination from an 11 mW helium–neon laser. It was found that the photoluminescence spectrum consists of a broad line centered at 1.79 eV with a full width at half-maximum of 0.21 eV and a tail located in an energy region higher than the laser incident excitation energy. Based on the analysis of Raman scattering spectra and the assumption that the luminescence spectrum arises from the ensemble of various sized particles distributed in the material, it is suggested that the later band of photoluminescence should be excited by the Raman scattering light in silicon nanocrystals. Copyright © 1999 John Wiley & Sons, Ltd.