Atomic layer epitaxy of silicon has been studied by alternating exposures of atomic hydrogen and SiH2Cl2. An ideal growth rate of 1 monolayer per cycle has been achieved with a wide temperature window from 550 degrees C to 610 degrees C under long SiH2Cl2 residence time and high pressure conditions. These requirements seem to come from the generation of dense SiHCl, the desirable precursor, by gas-phase reaction of SiH2Cl2.