Effect of grain boundaries on the Raman spectra, optical absorption, and elastic light scattering in nanometer-sized crystalline silicon

The intensity of the Raman-active 25 mode of nanometer-sized crystalline silicon, nc-Si, normalized to that of calcium fluoride, CaF2, at 322 cm-1 was measured for samples deposited under controllably varied conditions. Changes of the intensity by a factor of up to approximately 6.7 were found. These are correlated with the lattice expansion and with the compressive stress in thin films of the material. It is suggested that the enhancement of the scattering cross section, which scales with the observed optical-absorption coefficient and diffuse elastic light scattering, is due to enhanced coupling of the electromagnetic field of the incident light to the charge-density fluctuations at the grain boundaries of the quasi-isolated crystallites.