Electron Paramagnetic Resonance from Clean Single-Crystal Cleavage Surfaces of Silicon

EPR measurements have been made on aligned cleavage faces of Si, prepared and studied in high vacuum (<${10}^{-9\mathrm{}}$ Torr). The signal, observable after accumulation, is a single line at $g=2.0055\mathrm{}$ with width 6 G, similar to that from vacuum-crushed powders. It is unaffected by oxygen exposures below ${10}^{-3\mathrm{}}$ Torr min, known to affect the work function and surface-region conductivity, but is increased in height (45%) and number of spins (20%) by heavy oxygen exposures in the range ${10}^{-1\mathrm{}}$ Torr min and above. Hyperfine structure or $g$ anisotropy is not resolved. The surface density of spins is approximately 8×${10}^{13}$ ${\mathrm{cm}}^{-2\mathrm{}}$. Comparisons with other measurements of effects of oxygen on surface-region conductivity and work function show that the resonance centers are located on the surface. Analysis of the resonance, in particular the limited hyperfine structure and $g$ isotropy, shows that the unpaired electrons are largely nonlocalized and in a conduction band of large effective mass whose maximum anisotropy in reciprocal lattice space is little more than that along a single axis of the bulk conduction band. A model for the surface structure is proposed which is consistent with both the EPR data and low-energy electron diffraction measurements of surface-structure symmetry. Alternate rows of close-spaced atoms are raised with $s$-type dangling bonds which overlap about 80%, forming conduction rows. The remaining alternate rows are depressed, having $p$ dangling bonds which overlap fully and do not contribute to the resonance. The rows have a preferred direction related to the progression of the crack that caused the cleavage. The on-surface conductance is predicted to be anisotropic, the highest value being along the rows. A possible identification of these $s$ and $p$ bands, with surface-state bands is suggested.