Wavelength-tunable blue photoluminescence of <2 nm Si nanocrystal synthesized by ultra-low-flow-density PECVD

Strong blue photoluminescence (PL) spectra from Si nanocrystals (Si-ncs) embedded in Si-rich SiO_x grown by PECVD at ultra-low N₂O flow density and N₂O/SiH₄ flow ratios are characterized. The N₂O flow density is kept as low as 25 sccm to restrict oxygen desorption and to completely facilitate SiH₄ decomposition, thus minimizing the oxygen adsorption and suppressing the hydrogen-passivation on dangling bonds at the Si-nc surface. In contrast to the as-grown SiO_x, the blue-shifted PL of the annealed SiO_x is attributed to the small-size Si-ncs rather than to oxygen vacancy defects. High-resolution transmission electron microscopy analysis reveals dense Si-ncs with diameters of 1.7 ± 0.2 nm in annealed SiO_x, contributing to a minimum PL wavelength of 370 nm. X-ray photoelectron spectroscopy indicates a critical O/Si composition ratio of >1.44 for precipitating small Si-ncs with significant Si-Si binding energy peak at 99.0 eV. The weakened hydrogen-passivation effect for precisely manipulating the Si-nc size blue PL is confirmed by Fourier transform infrared spectroscopy.