Relationships between density of total incident hydrogen ions and full peak width at half maximum (FWHM) at (400) diffraction spot in reflection high-energy electron diffraction (RHEED) from vicinal Si (100) surface during plasma irradiation and postannealing are first reported. At the density of total incident hydrogen ions of 6.0 x 10(14) ions/cm(2), the FWHM, which is normalized by that before plasma irradiation, is almost saturated at 1.8. No damaged layer and no defect are observed in cross-sectional transmission electron microscopic (XTEM) images. On the other hand, at the density of 3.6 x 10(15) ions/cm(2), the normalized FWHM is slightly increased to 1.9. However, the drastic degradation of surface crystalline quality, a uniform 50-nm thick damaged layer and a lot of extended planar defects, is observed in XTEM images. During postannealing of Si wafer irradiated at the density of 6.0 x 10(14) ions/cm(2), the FWHM drastically decreases between 700 degrees C and 800 degrees C. Considering the causes of FWHM broadening and thermal stabilities of defects and disordered lattice, it is considered that this drastic improvement of FWHM is caused by annihilation of Si point defects and rearrangement of disordered lattice at Si surface. (C) 1000 Elsevier Science B.V. All rights reserved.