The Na-induced surface structures on Ge(100) at concentrations below 0.1 monolayer (ML) have been investigated with low-energy electron diffraction and with scanning tunneling microscopy (STM). Annealing and partial desorption of higher coverages at temperatures between 650 and 680 K lead to the formation of characteristic chain structures, alternating between uncovered Ge dimer or double dimer rows and Na chains. The Na atoms have a preferential distance of three Ge lattice constants along the chains, but there is little correlation between different chains. Locally, only commensurate distances between Na chains of 4, 6 and 8 Ge lattice constants have been detected, with average distances being either commensurate or incommensurate, depending on the Na concentration. STM suggests complete removal of Ge-dimer rows below adsorbed Na chains. These alkali induced chainlike and well ordered self-organized modulations can be used as templates to impose these structures onto adsorbed ultra-thin insulating films. © 2003 The American Physical Society