Interaction of slow N2+ ions with the Si(001) surface: A combined photoemission and LEED study.

Abstract

We report results of a combined study of in situ x-ray-photoemission spectroscopy (XPS) and low-energy-electron diffraction (LEED) for silicon nitride films fabricated at room temperature. A variety of nitride films of ${\mathrm{SiN}}_{\mathit{x}}$ (0.2\ensuremath{\le}x\ensuremath{\le}1.33) were formed by bombarding low-energy nitrogen ions (E\ensuremath{\le}600 eV) onto a Si(001) surface without thermal treatment. The results reveal that the Si-N bond nature in the ion-deposited layers (IDL's), derived from the characteristic XPS spectra, strongly resembles that of a typical thermally prepared silicon nitride, \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$. We find the chemical shift per Si-N bond to be 0.62 eV, and a shift of Fermi level due to nitridation less than 0.1 eV. We estimate the thickness of the unannealed IDL's to be less than 18 \AA{}. Upon annealing an IDL, progressive changes of XPS peaks and LEED patterns suggest that nitrogen atoms migrate from initial metastable defect sites to thermally stable sites, and tend to coalesce to form locally ordered microcrystallites. The thermal activation energy barrier in an IDL of E=400 eV, x=1.33 is found to be about 0.21 eV, in agreement with theoretical predictions.