Correlation between luminescence and structural properties of Si nanocrystals

Strong room-temperature photoluminescence (PL) in the wavelength range 650-950 nm has been observed in high temperature annealed (1000-1300 degrees C) substoichiometric silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapor deposition. A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiOx films, as well as the annealing temperature. The integrated intensity of the PL peaks spans along two orders of magnitude, and, as a general trend, increases with the annealing temperature up to 1250 degrees C. Transmission electron microscopy analyses have demonstrated that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1 nm, are present in the annealed samples. Each sample is characterized by a peculiar Si nc size distribution that can be fitted with a Gaussian curve; by increasing the Si content and/or the annealing temperature of the SiOx samples, the distributions become wider and their mean value increases. The strong correlation between structural (nanocrystal radius and width of the size distributions) and optical (wavelength and width of the PL peaks) data indicates that light emission from the annealed SiOx films is due to carrier recombination in the Si nc, and it can be interpreted in terms of carrier quantum confinement. The possible reasons for the quantitative discrepancy between the experimentally measured luminescence energy values and the theoretical calculations for the enlargement of the band gap with decreasing the crystal size are also discussed.