Direct imaging of interfacial ordering in ultrathin (${\mathrm{Si}}_{\mathit{m}}$${\mathrm{Ge}}_{\mathit{n}}$${)}_{\mathit{p}}$ superlattices

We present the first atomic-resolution images of interfacial ordering occurring in ultrathin (${\mathrm{Si}}_{\mathit{m}}$ ${\mathrm{Ge}}_{\mathit{n}}$ ${)}_{\mathit{p}}$ superlattices. The observed asymmetric interfacial abruptness is associated with several distinct ordered configurations, which differ from phases considered previously. This is explained by a novel Ge-atom pump mechanism which is chemically driven, and takes place principally at the rebonded edge configuration during type-${\mathit{S}}_{\mathit{B}}$ step propagation, leading to lateral (2×2) compositional ordering.