Structural transition in Ge growth on Si mediated by sub-monolayer carbon

Ge growth on Si mediated by sub-monolayer (ML) carbon (C) covered directly on Si surface was studied. C and Ge layers were grown on Si(100) substrates by using solid-source molecular beam epitaxy system. After Si surface cleaning by heating up to 900 °C, C up to 0.45 ML was deposited and then 10 to 15-nm-thick Ge were deposited. Reflection high energy electron diffraction patterns after sub-ML C deposition changed from streaks to halo depending on C coverage. The Ge dots were formed at low C coverage of 0.08-0.16 ML. Octagonal dots had three same facet planes of (001), (111), and (113) and consisted of the mixture of single crystals with dislocations along [111]. This is due to the event that the incorporation of small amount of C into Si surface gave rise to a strain. As a result, Si surface weaved Si(100) 2 × 1 with SiC c(4 × 4) and Ge atoms adsorbed selectively on Si(100) 2 × 1 forming dome-shaped dots. A drastic structural transition from dots to films occurred at C coverage of 0.20 ML. The Ge films, consisting of relaxed poly- and amorphous-Ge, formed at C coverage of 0.20-0.45 ML. This is because a large amount of SiC bonds induced strong compressive strain and surface roughening. In consequence, the growth mode changed from three-dimensional (3D) to 2D due to the reduction of Ge diffusion length.