Silicon incorporation in GaAs. From delta-doping to monolayer insertion

Raman spectroscopy was used to study the incorporation of Si into doping layers in GaAs, grown by molecular beam epitaxy at a temperature of 400 degrees C, for Si concentrations ranging from the delta -doping level to a ML coverage. The strength of the scattering by local vibrational modes of substitutional Si was almost constant for Si areal concentration (Si)A in the range 5*1012<(Si)A<5*1013 cm but then decreased, dropping below the detection limit for (Si)A>3*1014 cm-2. At these concentrations a new vibrational band emerged at a frequency close to 470 cm-1 and developed into the optic zone center phonon of a coherently strained epitaxial layer of Si embedded in GaAs when a coverage of approximately=1.5 ML (9.3*1014 cm-2) was reached. These findings strongly indicate that the observed saturation and the eventual decrease of the concentration of substitutional silicon is caused by an increasing incorporation of deposited Si into two-dimensional islands of covalently bonded Si.