Lattice constants of GaAs layers grown by molecular beam epitaxy were examined by using the high resolution x‐ray diffractometer. For highly doped samples (up to 9×1018cm−3of free‐electron concentration) we observed an increase of the lattice constant with respect to the undoped layers. Since substitutional silicon atoms decrease the lattice constant of GaAs, the results are explained by the influence of free‐electrons via the deformation potential of the &Ggr; minimum of the conduction band. The best fit to our diffractometric data was obtained for the band‐gap deformation potential equal to −8.5 eV. ©1995 American Institute of Physics.