The 4×1 (respectively 1×1) (001) GaN surfaces obtained when molecular-beam-epitaxy (MBE) growth is carried out on (001) cubic SiC were exposed to an As background pressure in the MBE chamber: The reconstructions rapidly and irreversibly changed to 2×2 [respectivelyc(2×2)] as usually observed for GaN growth on (001) GaAs. The usual reversible 2×2/c(2×2) transitions were consequently observed when bringing the Ga flux up or down. The respective positions for the 4×1/1×1 and 2×2/c(2×2) transitions were worked out as a function of the growth parameters. These observations indicate that the 2×2 andc(2×2) GaN surface reconstructions are mediated by As atoms which we tentatively assign to a surfactant effect. A simple structural model involving As dimers is proposed that accounts for Ga coverages of 0.5 and 1 monolayer for the 2×2 andc(2×2) growth regimes, respectively. ©1997 American Institute of Physics.