Using the coupled-wave method supplemented by the quasi-optical approximation, we study the dynamics of wide-aperture distributed-feedback (DFB) heterolasers with planar resonators having a step of corrugation phase. In configurations where the width of the Bragg structure is several times smaller or equal to the width of the active zone, the diffraction transverse losses provide mode selection with respect to the transverse index and, simultaneously, suppress the excitation of the longitudinal modes with frequencies above the bandgap. Stable single-mode operation of the DFB-heterolaser is possible in the wide range of positions and values of the corrugation phase step, which determine the level and ratio of the values of power radiated from the opposite ends of the DFB heterolaser. Conditions are found that ensure almost unidirectional output of radiation.