Quantum confinement effects in the soft X-ray fluorescence spectra of porous silicon nanostructures

Soft X-Ray Emission Studies of the Electronic Structure in Silicon Nanoclusters

“…This effect has been modeled by the absorption edge of bulk silicon shifted in energy to simulate the average quantum shift and broadened by convolution of a Gaussian determined by the size distribution measured in the AFM images [7]. We note that the L-edge absorption of the Si nanoclusters is very similar to that found in porous silicon [8,9]. Changes in the electronic structure of the VB were monitored by SXF spectroscopy on the same samples that there previously investigated by NEXAFS.…”

“…Yet x-ray diffraction and TEM studies of these clusters have shown they are crystalline in nature [4]. It is interesting to note that the peak at 91.5 is observed in the SXF spectra of hydrogenated porous silicon [9,11].…”

“…The SXF spectra were excited at 100 eV for the bulk silicon and the 1.6 nm clusters and 100.2 eV for the 2nm clusters. The bulk silicon SXF spectrum exhibits the three characteristic peaks, one at 89 eV associated with low-lying 3s states, another due to a density of states (DOS) maximum at 91.5 eV with strong s-p hybridization and a high DOS at 96 eV which is dominated by p-type states [9][10][11]. The intense peak at approximately 100 eV is due to reflected light from the undulator beamline into the spectrometer.…”

Optical Properties of Nanostructured Silicon

“…Therefore, in contrast to a bulk silicon sample with the same surface composition, a film of Si NCs with (limited) surface oxidation presents surface oxide throughout the film thickness, thus increasing the oxide signal relative to a conventional solid silicon sample. A broadening of absorption onset was observed by Eisebitt et al [87,88] in XAS from porous Si, and attributed to the crystallite size distribution in their samples. However, in the spectra shown in Figure 18, which are from a sample with a welldefined and narrow diameter distribution [71], a clear 0.61 eV spin-orbit splitting [89] is observed in the PPY from the NCs (magnified region of Figure 18, curve b), while such splitting may be washed out by the size inhomogeneity in the porous Si crystallites [87,88].…”

“…A broadening of absorption onset was observed by Eisebitt et al [87,88] in XAS from porous Si, and attributed to the crystallite size distribution in their samples. However, in the spectra shown in Figure 18, which are from a sample with a welldefined and narrow diameter distribution [71], a clear 0.61 eV spin-orbit splitting [89] is observed in the PPY from the NCs (magnified region of Figure 18, curve b), while such splitting may be washed out by the size inhomogeneity in the porous Si crystallites [87,88]. Single-dot luminescence spectroscopy has been used to study the emission line width of individual Si NCs and these studies have confirmed that oxidized NCs exhibit discrete energy levels rather than continuous bands [90].…”