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.