(0 0 1)-Oriented GaAs metal-insulator-semiconductor (MIS) structures having a silicon interface control layer (Si ICL) were fabricated on surfaces having Ga-rich (4 x 6) reconstructions. Si ICL was grown by molecular beam epitaxy. MIS structures were fabricated by partially converting Si ICL to SiNx by direct nitridation, and further depositing a thick SiO2 layer on top as the main passivation dielectric by plasma-assisted chemical vapor deposition. Reflection high-energy electron diffraction, in situ Xray photoelectron spectroscopy and MIS capacitance-voltage (C-V) techniques were used for characterization. The initial surface reconstruction was found to have a surprisingly strong effect on the degree of Fermi level pinning at the MIS interface. In contrast to the standard As-rich (2 x 4) surface, which results in strongly pinned MIS interfaces, the novel SiO2/SiNx/Si ICL/ GaAs MIS structures formed on "genuine" (4 x 6) surface realized complete unpinning of Fermi level over the entire band gap with a minimum interface state density of 4 x 10(10) cm(-2) eV(-1) range. (C) 2003 Elsevier Science B.V. All rights reserved.