Growth of Si twinning superlattice

We review our attempts to grow a new type of single crystal Si epitaxially on Si(111). The new single crystal Si is a superlattice of layers that have twinned and untwinned orientations with respect to the substrate, and is called a twinning superlattice. The Si twinning superlattices are grown using the fact that Si layers grown epitaxially on Si(111)√3×√3-B have a twinned orientation with respect to the substrate. We investigate conditions for growing twinned epitaxial layers, and clarify that growth of twinned layers requires high surface B concentration and low densities of surface structural defects, such as steps and domain boundaries of the √3×√3 reconstruction. We also investigate the thermal stability of the twinned layers, and clarify that the temperature at which the twinned layers are transformed into untwinned layers strongly depends on the thickness. Additionally, we establish a technique for measuring crystallographic orientations in surface regions in situ during growth. Based on these results, we finally succeed in growing the Si twinning superlattice by repeated growth of Si with a unit thickness and post-growth annealing.