We investigated the change of optical second harmonic generation (SHG) intensity in the course of Si(1 1 1)root 3 x root 3-(Au,In) surface formation and during subsequent In island growth on this surface. The Si(1 1 1)root 3 x root 3-(Au,In) surface is essentially a modified Si(1 1 1)-alpha-root 3 x root 3-Au surface in which the domain-wall network (characteristic of the original surface) is completely eliminated due to In adsorption. This surface shows up as the one having homogeneous highly ordered atomic structure. Its formation is accompanied by the extremely high (namely, tenfold) increase of the SHG signal. It was recognized that this increase is associated with an extraordinary high long-range atomic ordering of the Si(1 1 1)root 3 x root 3-(Au,In) surface, while its other peculiar features, developing the 2D electron gas system and occurrence of the 2D gas of mobile adatoms on it, produce minor effects. Growth of random In islands on Si(1 1 1)root 3 x root 3-(Au,In) at RT In deposition leads to the decrease of SHG signal indicating its sensitivity to the surface inhomogeneity not only in atomic scale but in the mesascopic scale also. (C) 2012 Elsevier B.V. All rights reserved.