One of the main requirements for ultta-Iarge-scale integration (ULSI) is atomic-order control of process technology. the concept of atomically controlled processing for group IV semiconductors is based on atomic-order surface reaction control in Si-based CVD epitaxial growth. On the atomic-order surface nitridation of a few om-thick Ge/about 4 nm-thick Sio.5Geo.s/Si(100) by NH3, it is found that N atoms diffuse through nm-order thick Ge layer into Sio.sGeo.s/Si(100) substrate and form Si nitride, even at 500°C. By subsequent H2 heat treatment, although N atomic amount in Ge layer is reduced drastically, the reduction of the Si nitride is slight. It is suggested that N diffusion in Ge layer is suppressed by the formation of Si nitride and that Ge/atomic-order N layerISii_xGe)Si (100) heterostructure is formed. These results demonstrate the capability of CVD technology for atomically controlled nitridation of group IV semiconductors for ultta-large-scale integration.