Titanium silicide nanoparticles on silicon substrates catalyze the decomposition of a silicon-containing gas, resulting in accelerated growth of silicon in one direction to form nanowires. Under some processing conditions, however, the Ti-catalyzed growth results in tapered silicon nanowires. The tapered nanowires are typically several microns long with diameters of tens of nanometers near the base and less than 10 nanometers at the tip. We show that the tapering is caused by uncatalyzed deposition of silicon on the sidewalls of the growing nanowires. We demonstrate that introducing chlorine-containing species in the gas phase greatly inhibits the uncatalyzed silicon deposition rate, thus yielding nanowires with uniform diameter along their length. Controlling the nanowire diameter along its length is essential for novel nanowire-based electronic, optical, and optoelectronic applications. (C) 2004 Elsevier B.V. All rights reserved.