We have previously reported a process for low temperature selective silicon epitaxy using Si2H6, H-2, and Cl-2 in an ultrahigh vacuum rapid thermal chemical vapor deposition reactor.(1) Selective deposition implies that growth occurs on the Si surface but not on any of the surrounding insulator surfaces. Using this method and process chemistry, the level of Cl species required to maintain adequate selectivity has been greatly reduced in comparison to SiH2Cl2-based, conventional CVD approaches.(2,3) In this report, we have extended upon the previous work and provide information regarding the selectivity of the silicon deposition process to variations in the growth conditions. We have investigated the selectivity of the process to variations in disilane flow/partial pressure, growth temperature, and system contamination. We demonstrate that increases in either the Si2H6 partial pressure or flow rate, the process temperature, pr the source contamination levels can lead to selectivity degradation. In regard to the structural quality of the selective epitaxial layers, we have observed epitaxial defects that have appeared to be a strong function of two basic conditions : the contamination level of the process and the chlorine flow rate or chlorine partial pressure. Overall, the results in this study indicate several process conditions that can inhibit the quality of a selective silicon deposition process developed for single-wafer manufacturing.