The relation between the structural quality of Si:O layers grown by chemical‐vapor deposition at low temperatures (700–750 °C) and electrical properties was determined. Transmission electron microscopy (TEM), secondary‐ion‐mass spectroscopy, and resistivity measurements were used for this study. An oxygen concentration in these layers was in the range of 6×10−19–6×10−20cm−3. TEM studies have shown that amorphous SiOxprecipitates were formed at the lower interface (between the Si buffer and Si:O layer) when silane was used for the layer growth. Slightly smaller precipitates were distributed through the entire layer. In the layers with higher oxygen concentration, high density of stacking faults originated at the same interface and propagated through the entire layer. The Si capping layer grown on top of Si:O was monocrystalline with the density of stacking faults two orders of magnitude lower than in the Si:O layer. For the lower oxygen concentration the stacking faults were not formed and the size of precipitates at the lower interface and in the layer was much smaller. The resistivity of ∼105and ∼106Ω cm was measured in the layers with lower and higher oxygen content, respectively. Only for the Si:O layers grown in the same range of temperatures (700 °C) using dichlorosilane oxygen induced stacking faults were formed at the upper interface (between Si:O and the capping layer). Some small precipitates were formed at the lower interface but no visible precipitates were present in the Si:O layer. These layers were not semi‐insulating. It was concluded that the mechanism to explain semi‐insulating properties might be related to the presence of the SiOxprecipitates or structural defects present in these layers.