Observation of the formation of a carbon-rich surface layer in silicon

Due to its small size, the carbon atom has a very low solubility in a silicon crystal. In spite of this, we show that carbon atoms adsorbed on top of a silicon surface readily migrate into substitutional sites below the surface, leading to a narrow layer of a highly concentrated alloy. This was demonstrated by depositing carbon on a Si (001) surface by molecular beam epitaxy and investigating the changes in the chemical environment by x-ray photoelectron spectroscopy during successive annealing steps at increasing temperatures. Ab initio total-energy calculations were used to interpret the measured spectra.