Turning Low-Nanoscale Intrinsic Silicon Highly Electron-Conductive by SiO₂ Coating

Abstract

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal–oxide–semiconductor-compatible materials.

Absorption and Photoluminescence of Silicon Nanocrystals Investigated by Excited State DFT: Role of Embedding Dielectric and Defects

“…However, delocalization of electronic states can only occur if their binding energy decreases. [14,15] With such delocalization being present for Si 3 N 4 -embedding, the quasifree hole and electron constituting the exciton have less electrostatic force to overcome for decreasing their distance, that is, decreasing a exc .…”

“…The strong localization of electronic charge at O atoms leads to the opposite effect, namely, increasing a exc by strong spatial separation of hole (being located in the center of the NC), and electron (being located at the interface to SiO 2 ), see Figure 6 in ref. [15]. In the case of Si 3 N 4 -embedded Si-NCs, the exciton is arguably selflocalized, which, at a first glance, is counterintuitive, given the lower confining potentials of the NC embedded in Si 3 N 4 versus SiO 2 .…”

“…been shown by synchrotron spectroscopic methods for ultrathin Si nanowells. [11,14,15] When an exciton is generated in an embedded Si-NC, its optical phonon emission affects not just the polar bonds at its interface but extends further into the respective dielectric. This strong electron-/hole-phonon coupling is brought about by the Fröhlich interaction, [85] which grows with bond polarity and couples mostly to optical phonons (diatomic vibrational modes).…”

“…To this end, shows that the exciton radius increases significantly for Si‐NCs in 1 ML of , while it stays rather constantly small for embedding in 1 ML . The origin of this seemingly unexpected difference in exciton localization is given by the NESSIAS effect [ 15 ] briefly mentioned at the end of Section 3. Both anions, O atoms of embedding , and N atoms of embedding , induce a nearly identical electron transfer (interface charge transfer – ICT) from the Si‐NC into the respective dielectric.…”

“…is an interface-mediated effect, its impact increases with decreasing d NC , practically being at a maximum for d NC ¼ 20 Å. The influence of the annealing ambient on E PL gap[10] is due to the NESSIAS effect [15]. This effect comes to full bearing when small Si nanovolumes are fully embedded in SiO 2 versus Si 3 N 4 , as hash h…”