Atomic-order nitridation by NH3 on Si(100) and subsequent Si growth by SiH4 were investigated using an ultraclean low-pressure chemical vapor deposition system with a Xe flash lamp. In thermal nitridation on Si(100), it is found that nitridation occurs even at 260 degreesC with flash heating, and the N atom concentration tends to saturate to about 2.4 X 10(15) cm(-2). In Si deposition on the ultrathin Si nitride, it is found that N desorption from the Si nitride films hardly occurs, and Si grew on Si nitride at 385 degreesC in an SiH4 environment with and without the flash lamp light irradiation. An incubation period of Si growth is observed and increases with increasing N atom concentration of the Si nitride film. On Si with N atom concentration of about 2 X 10(14) cm(-2), the incubation period is hardly observed and the reflection high energy electron diffraction patterns indicates that Si epitaxially grew. Layer-by-layer growth control of Si nitride is proposed by combining atomic-order nitridation on Si and atomic-layer growth of Si on the Si nitride.