We have reexamined the formation and surface structure of Si(100)-c(4 x 4) obtained by hydrogen exposure using high resolution scanning tunneling microscopy (STM). The observed filled and empty state STM images are in good agreement with a recent carbon incorporated reconstruction model. We found all of Si dimers present in the c(4 x 4) unit cell are perpendicular to the underlying Si dimer row. The observation of long line missing dimer defects and SiC islands indicates that the c(4 x 4) structure observed in this work is also associated to carbon contamination. (C) 2001 Elsevier Science B.V. All rights reserved.