Boron‐doping spikes in Si have been grown by fast gas switching chemical vapor deposition at 800 or 850 °C using Si2H6in 0.03 or 0.1 atm H2, respectively. The B2H6doping gas was added for 2 s in two ways, viz. during growth, or as a flush while the Si2H6was interrupted. High‐resolution secondary‐ion mass spectrometry (HR‐SIMS) analysis has revealed the sharpest as‐measured SIMS dopant profiles reported for Si grown by deposition from the gas phase. Electrical measurements show the sheet resistivity of the B spikes to be as low as 580 &OHgr;/&laplac;.