Chemisorption of atomic hydrogen on the Si(1 1 1)root3xroot3-B surface has been studied using scanning tunneling microscopy (STM). Two kinds of specimens with different surface defect density are examined and compared. Upon hydrogen exposure of 120 Langmuir (L), cluster formation is observed on the root3xroot3-B surface with high defect density of similar to50%. The number of clusters increases with exposure and an entire surface is covered with the clusters up to 500 L. In contrast, no cluster formation is observed on the surface with low defect density of similar to3% even after exposure of 4000 L. This result indicates the inhibition of hydrogen etching of the surface Si layer, as reported previously. Hydrogen-reacted species, which cannot be explained by previous theoretical results, are clearly resolved in STM images. (C) 2004 Elsevier B.V. All rights reserved.