The transition from dilute aluminium delta-structures to an AlAs monolayer in GaAs and a comparison with Si delta-doping

Superlattices of Al delta layers embedded in GaAs have been grown by molecular beam epitaxy at 400 degree Celsius on (001) GaAs. Infrared absorption measurements revealed a shift of the Al localized vibrational mode (LVM) from 361.4 to 358.3 cm high -1 as the doping was increased from 0.25 monolayers (ML) to 1.4 ML per delta plane, while Raman scattering demonstrated a clear transition from the TO-like LVM to a longitudinal LO-like mode. X-ray measurements showed that the Al atoms were confined to a thickness of 6 A for the highest areal concentrations. The LVM behavior is compared with that of Si delta layers where the absorption tends to zero as the areal concentration increases up to 0.5 ML. It is concluded tha Si sub Ga atoms move off their lattice sites and/or change their charge states as the coverage approaches 0.5 ML