THE ADSORPTION OF PH3 ON THE SI(111)-(7X7) SURFACE - AN EXAMPLE OF AUTOCATALYTIC DISSOCIATIVE CHEMISORPTION

The dissociative chemisorption of phosphine, PH3, on the Si(111)-(7 X 7) surface has been examined employing supersonic molecular beam techn iques. The initial probability of reaction, S-R,S-0, has been found to be sensitive to substrate temperature, T-s, where S-R,S-0 increases s harply by approximately a factor of 4-5 as T-s is increased above 800 degrees C, which corresponds well with the (7 X 7) <-> ''(1 X 1)'' pha se transition. The reaction probability, S-R, measured as a function o f dose for PH3 reacting on Si(111)-(7 X 7) at T-s < 800 degrees C, exh ibits a dramatic increase as the surface is exposed to the PH3 molecul ar beam. This unique autocatalytic behavior is consistent with a mecha nism in which submonolayer coverages of P(a) are capable of lifting th e (7 x 7) reconstruction thus giving rise to a more reactive ''(1 X 1) -like'' phase. The reaction probability of Si2H6 on Si(111)-(7 x 7) is also observed to pass through a maximum with increasing P(a) coverage s, and can be explained by considering similar changes in surface stru cture and reactivity.