Effect of substrate roughness on photoluminescence spectra of silicon nanocrystals grown by off axis pulsed laser deposition

Silicon nanoparticles were prepared by off axis pulsed laser deposition (PLD) technique. The optical properties of Si nanoparticles grown on quartz substrate have been characterized by optical absorption, photoluminescence, Raman, and transmission electron microscopy. TEM has demonstrated that the radius of nanocrystals decreases from 4to0.8nm as the off axis target to substrate distance increases. A blueshift up to 4.2eV is observed in the optical absorption spectra of the Si quantum dots. The slope of log(hν) vs log(α) graph shows that the optical transitions in Si quantum dots are allowed direct, allowed indirect, and direct forbidden transitions which depend on the nature of the substrate used for the growth of silicon. Relaxation of k selection rule is observed in these samples. Photoluminescence (PL) emission consists of an intense broad emission extending over visible to ultraviolet region. The photoluminescence peak energy and intensity are found to be sensitive to the nature of substrate. Possible mechanism of optical absorption and PL origin are discussed in terms of quantum confinement luminescent center model.