Modification of high-doped GaAs surface by the interaction with RF plasma and a monoenergetic beam of similar chemical composition (Ar with a low content of H-2) was investigated via the evolution of optical, electrical and structural properties. A reduction in the free charge concentration in the GaAs surface region of similar to100 nm in thickness is concluded from Raman spectroscopy. In the processed samples, the surface electric field strength induced by a laser beam (photoreflectance technique) continually decreases with both the energy and fluence of impinging particles. The same results were obtained by evaluation of quasistatic C-V curves of corresponding MOS structures. They confirmed that longer exposure caused the formation of similar to100-nm-thick modified near-surface region with a decreased donor concentration. After the plasma irradiation, an similar to10-nm-thick outermost insulating layer was created in situ. The formation of textured polycrystalline grains with (100) orientation in the surface region was observed by X-ray diffraction. (C) 2003 Elsevier Science B.V. All rights reserved.