Growth and characterization of SiGe atomic layer superlattices

Abstract

The MBE growth of ultrathin (Si m Ge n ) p atomic layer superlattices (ALSs) consisting of p periods of alternating m and n monolayers of silicon and germanium is reported. Various structures were prepared on (100) Si, (100) Ge and on a 20-nm Si 0.5Ge 0.5 buffer on (100) Si. The physical properties of the ALSs were investigated by Raman scattering (RS), double-crystal X-ray diffraction, transmission electron microscopy and photoluminescence (PL). The samples were found to have accurate stoichiometry and thicknesses. Substrate-dependent variations in the growth morphology were observed. The microstructures showed some waviness on silicon or germanium substrates and were heavily defected when an alloy buffer was introduced. Phonon peaks due to folding of acoustic modes were seen in the frequency range 20–200 cm −1. The optical modes in RS were calculated using a linear chain model and were compared with the experimental spectra. Good fits were obtained after introducing interdiffusion in the model by adjusting the mass of the interfacial atoms. The PL investigation revealed features related to defects in silicon. However, no strong luminescent features that could be ascribed to a direct band-gap transition were observed.