Atomically controlled processing, such as layer-by-layer growth, is attractive for creating novel nano-structures and superlattice structures in Si technology. Silicon nitride films under atomic-order thickness control can be applied to tunneling barrier and very thin gate insulator. This chapter presents a study in which Si epitaxial growth conditions on the atomic-order nitrided Si(100) surface have been clarified using the low-pressure Chemical Vapor Deposition (CVD) system. It also investigates the thermal stability of the N atoms in the N delta-doped Si films. In the study, the epitaxial growth of the N delta-doped Si film with the total amount of N atoms as high as 3.0x1014cm-2 for a delta-doped region, has been realized at 500°C by ultraclean low-temperature low-pressure CVD. On the atomic-order nitrided Si(100), an incubation period for Si deposition is observed, while the deposition rate after the incubation period is the same as that on the Si surface without the nitridation. The binding energy of N 1 s for the surface agrees with that for Si3N4. It is suggested that N atoms delta-doped in the film do not have Si3N4 structure.