One of the main requirements for Si-based ultrasmall device is atomic-order control of process technology. Here, we show the concept of atomically controlled processing based on atomic-order surface reaction control. The main idea of the atomic layer approach is the separation of the surface adsorption of reactant gases from the reaction process. Self-limiting formation of 1-3 atomic layers of group IV or related atoms in the thermal adsorption and reaction of hydride gases on Si(100) and Ge(100) are generalized based on the Langmuir-type model. Moreover, Si or SiGe epitaxial growth over N, P or B layer already-formed on Si(100) or SiGe(100) surface is achieved. Furthermore, the capability of atomically controlled processing for advanced devices is demonstrated. These results open the way to atomically controlled technology for ultra-large-scale integrations.