Microscopic symmetry properties of (001) Si/Ge monolayer superlattices

Short-period Si/Ge superlattices have special symmetry properties due to the tetrahedral bonding. In [001]-oriented throughout (Si${)}_{\mathit{m}}$/(Ge${)}_{\mathit{n}}$ (m:n) superlattices (m monolayers Si,n monolayers Ge) the primitive unit cell is determined by a multiple of two monolayers. Thus, for m+n odd the periodicity is doubled to 2(m+n) monolayers. Low-temperature molecular-beam epitaxy is used to realize atomically sharp Si/Ge superlattices. The symmetry properties have been studied by selected area diffraction with the transmission electron microscope and by Raman spectroscopy. Clear evidence is observed for the ten-monolayer periodicity in a (Si${)}_2$/(Ge${)}_3$ superlattice.