A promising chemical surface preparation technique, which consists in the treatment of GaAs(100) in HCl-isopropyl alcohol (HCl-iPA) solution under nitrogen atmosphere, is further developed. It was shown earlier [Tereshchenko et al., J. Vac. Sci. Technol. A 17 (1999) 2655] that HCI-iPA treatment and subsequent anneals in vacuum yielded atomically clean GaAs(100) surface with the whole range of surface reconstructions characteristic of this crystal face. In the present work the mechanisms of the passivation of GaAs(100) surfaces by arsenic overlayers as a result of HCI-iPA treatment are experimentally studied by X-ray photoelectron spectroscopy, low-energy electron diffraction and atomic force microscopy. The HCI-iPA treatment of clean As-stabilized GaAs(100) surfaces results in chemical passivation of the surface by submonolayer amount of excess arsenic. For the initially oxidized surfaces the treatment leads to the formation of 1-3 monolayers of amorphous arsenic on the surface, with the major part of the arsenic originating from the surface oxides dissolved in HCI-iPA. The HCI-iPA treatment preserves the atomic flatness of the GaAs(100) surface, keeping the mean roughness on a very low level of approximately similar to0.1 nm. (C) 2004 Elsevier B.V. All rights reserved.