In this study, the native oxide desorption process at GaAs(1 0 0) surface triggered by direct Ga molecular beam irradiation was investigated at a temperature of 440degreesC in order to maintain surface flatness prior to molecular beam epitaxy (MBE). The oxide desorption process associated with the change in compositional and structural surface properties was characterized by in situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM). With an increase in irradiated Ga dose, it is shown that an initial halo RHEED pattern from the native oxide layer transforms to a streaky one, indicating the desorption process taking place in a two-dimensional way with a decrease in oxide layer thickness. At a critical dose of 5.5 x 10(15) cm(-2), which is confirmed by the change in RHEED intensities, the oxide layer is fully removed in conjunction with the smoothest surface morphology. Then the surface turns into a dark RHEED pattern corresponding to the accumulation of excess Ga in the form of droplets. The development of those surface properties monitored by RHEED is in good agreement with the change in root mean square (RMS) value of surface roughness obtained from AFNI images. The minimum RMS roughness obtained is at the critical Ga dose, which is accurately controlled by RHEED measurements. The surface morphology treated at the critical Ga dose at 440degreesC is maintained even after high-temperature annealing at 600degreesC without causing deep pit. Therefore, the direct Ga-beam irradiation on GaAs oxide at 440degreesC is considered to be an effective method providing a deoxidized smooth surface prior to MBE growth without the need of a buffer layer. (C) 2003 Elsevier Science B.V. All rights reserved.