We report on the bimodal distribution and long-range ordering of GeAs nanodots obtained in strain-relaxed epitaxial Si0.5Ge0.5 alloy layers after arsenic implantation and rapid thermal annealing. GeAs dots of two different average sizes around 15 and 55 nm are found after high temperature rapid thermal annealing. The larger dots are of elliptical shape and located at the surface region; they are distributed preferably along <110> directions which correlates well with the observed cross-hatch pattern. The origin of the bimodal precipitate distribution as well as of the long-range ordering effect of the GeAs nanodots is discussed in terms of strain-induced nucleation and diffusion-limited growth. © 2001 American Institute of Physics.