In order to achieve better surface passivation on technologically important (001) GaAs surfaces, microscopic and macroscopic properties of the clean (001) surface with Ga-rich (4x6) reconstruction, and effects of monolayer level Si deposition on this surface, were investigated. According to scanning tunneling spectroscopy and contactless capacitance -voltage (C-V) measurements, the Ga-rich genuine (G-)(4X6) surface had high-density acceptor-type states above E-C-0.5eV. However, the surface exhibited a much lower and wider surface state density (N-SS) distribution as well as a much stronger band-edge photoluminescence (PL) intensity than the conventional As-rich surfaces. Irradiation of Si molecular beam on the (4X6) surface resulted in epitaxial growth of Si. This completely removed the acceptor-type surface states from the energy gap, and further enhanced the PL intensity. The result indicates that the Ga-rich G-(4 X 6) surface is more ordered and more stable against processing than conventional As-rich surfaces. Metal-insulator-semiconductor capacitors fabricated by further depositing a thick SiO2 layer indicated that the Fermi level was completely unpinned over the entire band gap. (C) 2003 American Vacuum Society.