Raman scattering from very thin Si layers of Si/SiO2 superlattices: Experimental evidence of structural modification in the 0.8–3.5 nm thickness region

Raman study of very thin (⩽3.5 nm) Si layers constituting ${\mathrm{Si/SiO}}_2$ superlattices and grown by molecular beam epitaxy is described. The Raman spectra show systematic dependence on thickness of the Si layers, which highlights the variety of disordered microstructures in the ${\mathrm{Si/SiO}}_2$ superlattices. A clear change in the vibrational properties is found to occur in the 0.8–3.5 nm thickness region. In particular, the Raman spectra are typical for amorphous silicon for the thicker layers, and the characteristic phonon band disappears for the thinner layers, presumably representing another form of Si coordination with a small Raman scattering cross section. In addition, absorption of the material changes essentially with the Si-layer thickness. Photoluminescence is detected from the ${\mathrm{Si/SiO}}_2$ superlattices, the superlattices with 1.2 and 1.8 nm Si layers being the most efficient emitters among our samples, and the photoluminescence is blueshifted with the decrease of the Si-layer thickness. The Raman spectra show no sign of nanocrystalline structure at any thickness of the as-deposited Si layers so that the observed photoluminescence cannot be connected with Si nanocrystallities. Annealing strongly changes the Raman and photoluminescence spectra, a well-ordered Si phase appears in the superlattices, but its increase does not correlate with the photoluminescence, which further disregard it as an emitter. Nevertheless, the emitting phase is not identified in the Raman spectra.