The adsorption of nitrogen (N) radicals (nitridation) on InAs (001) surfaces has been studied by ultrahigh-vacuum scanning tunneling microscopy and x-ray photoelectronic spectroscopy. The nitridation proceeds as Langmuir adsorption. The N adsorption at350 °Cis faster than that at100 °C,but N adsorption rates at100 °Con InAs and GaAs are almost the same. These results are explained as follows: at350 °C,N radicals bond mainly with the topmost In atoms on an In-stabilized surface, and at100 °C,N radicals replace As atoms in the topmost layer on an As-stabilized surface, and subsequently bond with In atoms in the second layer. The amorphous100 °Cnitrided surface layer is found to have an insulating characteristic without surface states. ©1998 American Institute of Physics.