S+ ions with well-controlled kinetic energy were used to bombard n- and p-type GaAs (100) surfaces. 50 eV S+ bombardment formed a maximum of 1.5 monolayers of sulfide on a GaAs (100) surface and did not change the nonordered surface to an ordered surface. After the bombardment, the surface Fermi levels for both sulfur ion-bombarded n- and p-type samples were moved towards the midgap. After the samples were further annealed at 400 degreesC, the surface Fermi level of n- and p-type samples gave a separation larger than that of the samples after the UV/O-3 and HF treatment. A sharp (1 x 1) low-energy electron diffraction pattern was observed and the sulfide coverage was about one monolayer. Under such well-controlled conditions, effective surface passivation by reducing surface states and formation of an ordered surface structure can be achieved.