We investigated the cleaning process of Si(100) surfaces by annealing in H-2 gas ambient following chemical treatments by scanning tunneling microscopy. We observed the monohydride Si structure: Si(100):2 x 1-H on the surfaces annealed at 1000 degrees C in 2.5 x 10(4) Pa H-2 gas ambient without conspicuous contaminants. On the sample annealed for 10 min or longer times, well-defined Si(100) structures with alternating S-A and S-B steps were observed, whereas the initial roughness still remained on the surfaces annealed for only 5 min. (c) 2005 Elsevier B.V. All rights reserved.