Fabrication of photoluminescent nc-Si:SiO₂ thin films prepared by PLD

In the present report, the structural, compositional, morphological, and photoluminescence properties of nanostructured non-stoichiometric silicon oxide (nc-Si:SiO₂ or SiO_x) thin films fabricated by pulsed-laser ablation of silicon in the presence of oxygen pressure, from 10^-4 to 0.5 mbar, are presented. X-ray diffraction spectra and Raman spectra confirmed the formation of nanocrystalline Si within the films while electron diffraction X-ray spectroscopy confirmed the increase in oxygen content with increasing O₂ pressure. Scanning electron microscopy images of the SiO_x films showed spreading of the micron-sized clusters on the otherwise uniform background, while Raman maps confirm the presence of nanocrystalline Si in these clusters embedded in a uniform matrix comprising oxidized amorphous silicon. A systematic blue shift in the band gap energy from 1.55 to 2.80 eV was observed with increasing O₂ pressure in the SiO_x films due to a shift in the stoichiometry of the films from x = 0.03 to 2.14 respectively. The films with higher oxygen content exhibited broad and intense PL emissions with multiple peaks originating from quantum confined (QC) Si nanocrystals as well as oxygen defects like NBOH and V_O centers. The variation in PL intensity as a function of excitation intensity displays an initial linear increase followed by saturation, a characteristic feature of emissions from QC nc-Si.