Atomic-layer doping is one of the important processes in order to fabricate the ultra-small structure devices. There were some reports of atomic-layer doping by molecular beam epitaxy (MBE). However, Chemical Vapor Deposition (CVD) offers many advantages over MBE, such as high throughput, in-situ doping, and selective deposition. Atomic-layer doping of P and B in SiGe epitaxy was achieved using rapid thermal CVD. This chapter presents a study in which the P doping process and the thermal stability of the electrical characteristics of the multi-layer P-doped epitaxial Si films have been investigated. On the P formed layer surface, Si deposition is achieved by 220Pa SiH4, although the P reduction by SiH4 occurs. In the 2-layer P-doped epitaxial Si films, P atoms incorporated into Si spacer decrease with decreasing the Si spacer thickness. It is suggested that P segregates to the surface of Si spacer from P layer formed surface during Si deposition. It is considered that more electrically active P atoms exist in the heavily P-doped Si film formed at 450°C compared with those in the P-doped Si films formed at higher temperature at the same P concentration. These results suggest that the very low-resistive heavily P-doped epitaxial Si films can be achieved at very low-temperature around 450°C.