Using ab initio calculations, we compare the effects produced on the Al/GaAs(100) Schottky barrier height by Ge and Si interface layers with thickness ranging from 0 to 2 monolayers. The dipole layers, generated by the group-IV atomic substitutions at the interface, increase/decrease the p-type Schottky barrier at the As-/Ga-terminated Al/GaAs(100) junction. Although the trends with Ge and Si interlayers are similar for coverages less than 0.5 monolayers, at higher coverages the Schottky barrier exhibits a stronger nonlinear behavior, and smaller barrier variations, in Al/Ge/GaAs(100) than in Al/Si/GaAs(100). In particular, at a coverage of 2 monolayers, the use of Ge interlayers instead of Si ones reduces by 25% the barrier tunability. These trends are at variance with those predicted by existing macroscopic band-structure models, and our results should help in discriminating between different mechanisms of Schottky barrier tuning.