Properties of Clean Silicon Surfaces by Paramagnetic Resonance

Abstract

Silicon crystals crushed in ultrahigh vacuum (∼10⁻⁹ Torr) display an electron spin resonance signal close to g=2.0055 with a width of 7.0 Oe. The signal is strongly affected by exposure to 10⁻² Torr of molecular hydrogen (increase ≈60%) and oxygen (increase ≈80%), indicating that it is associated with surfaces. From surface area measurements, the ratio of dangling bonds to surface atoms was found to be approximately 1 to 5. High‐vacuum heat treatment causes an irreversible decrease in the surface resonance at 380°C (1‐h heating), but the signal is still increased by gas exposure. Above approximately 610°C the relatively weak remaining signal is now decreased by oxygen exposure, indicating a second surface transformation which correlates with that observed in this temperature region by low‐energy electron diffraction. It is concluded that the surface structures for both cleaved and annealed clean silicon surfaces involve dangling bonds, the concentrations being of order 20% and 2%, respectively. Consequences with respect to surface atom arrangements are discussed.