We have investigated a new method of Si selective epitaxial growth (SEG). This is a Cl-2 pulsed molecular flow method in which a Si substrate is alternately irradiated with Si2H6 and Cl-2 in an ultra-high vacuum chemical vapor deposition (UHV-CVD) chamber. The result of the Si etching by Cl-2 reveals that the Cl-2 irradiation has no influence on the Si epitaxial layers at temperatures low enough that the faceting plane does not develop. On the other hand, the Cl-2 irradiation has a large effect on selectivity improvement, when the Si2H6 irradiation is arranged in short periods. This method has realized the selective growth almost indefinitely by the reduction of the Si2H6 irradiation period even at the low temperature. This suggests that for selectivity improvement it is very important that the individual adsorbed Si atoms be removed by Cl-2 in the initial stage of the Si2H6 irradiation because Si atoms cannot be removed at low temperature after they form silicon clusters. We have demonstrated the use of this method in forming a Si-based photodetector and showed that a trench about 2 mu m deep can be filled with selective epitaxial layers having small faceting planes.