B atomic layer formation on Si1-xGex(100) (x=0, 0.3, 1) using BCl3 gas has been investigated by ultraclean low-pressure chemical vapor deposition. At 450° C, in the case using BCl 3-He-Hz gas mixture, B amount tends to increase beyond one atomic layer (6.8×1014 cm-2) with increasing BCl3 exposure time on Si0.7Ge0.3(100) and Ge(100). On the other hand, in the case using BCl3-He-Ar mixture gas, 1/3-1/2 atomic layer (2.3×1014 cm-2) formation is achieved self-limitedly with high Cl coverage on the surface. The Cl atoms on the B adsorbed Si1-xGex(100) are effectively removed by H2 introduction at 450°C after BCl3 exposure. In this way, atomically-controlled B atomic layer formation on Si1-xGe x(100) was demonstrated by using the self-limited surface reaction of BCl3 at 450 °C. copyright The Electrochemical Society.