Self-assembled Ge dots and dashes on SiGe/Si superlattices

This study explores the self-assembly of Ge nanostructures on SiGe/Si superlattices grown on vicinal Si substrates with surface normals tilted from (001) towards (111) by up to 22°. The superlattices consist of twenty periods of 2.5 nm Si(0.55)Ge(0.45)/10 nm Si bilayers, and Raman scattering indicates that the superlattices are strained to the Si substrate. Prior studies have found 2D ordering of Ge dots on nominally flat Si(001) surfaces with a very homogeneous size distribution. Our results show that the Ge islands are less ordered for tilted Si(001) substrates. For substrates with a miscut of 22°, Ge dots nucleate on top of ripples that form approximately perpendicular to [110] (i.e. perpendicular to step direction). Additionally, we observe the formation of Ge dashes (i.e. elongated islands of ∼200 x 1000 nm), which align preferentially along the [110] direction.