Ordered nucleation of Ge islands along high index planes on Si

Lateral ordering of Ge islands along 〈100〉 directions is observed when the growth is performed on Si mesas with crystallographic planes of the (100) zone. The islands nucleate in single one-dimensional rows along the intersection line between {01h} facets with a {011} plane. The high index planes were generated by using selective epitaxial growth of Si for the formation of faceted mesas. On these mesas Ge was deposited at 700 °C with a low growth rate (0.04 ML s^-1) so as to achieve on large areas a monomodal island distribution (medium sized islands known as dome clusters). However, on the (001) part of the Si mesas one observes a transition from monomodal to bimodal distribution. On small areas (small total coverage) only ordered islands of medium size along the edges are present. On larger mesas (higher total coverage) beside the ordered rows with medium size islands also small sized islands (30 nm diameter, 1 nm height, density 8×10⁹ cm^-2) nucleate on the (001) plane. At even higher coverage medium sized islands are also created on the (001) plane. The higher nucleation probability of islands on the high index planes must be due to a lower energy barrier for nucleation on these planes. The ordering on mesas (even as large as 200×200 μm²) must be related with a high surface diffusion length, λ_Ge for the Ge atoms on the strained wetting (001) layer. From λ_Ge to approximately 100 μm an activation energy for Ge migrating on strained Ge (001) E_s to approximately 0.65 eV could be evaluated.