KINETICS OF DISSOCIATIVE WATER-ADSORPTION ON STEPPED SI(001), ON SI(115), SI(113), SI(5,5,12) AND SI(112)

We present photoelectron spectroscopic and low energy electron diffrac tion measurements of water adsorption on flat Si samples of the orient ations (001), (115), (113), (5,5,12) and (112) as well as on curved sa mples covering continuously the ranges (001)-(117) and (113)-(5,5,12)- (112). On all orientations, water adsorption is dissociative (OH and H ) and non-destructive. On Si(001) the sticking coefficient S and the s aturation coverage Theta(sat) are largest. On Si(001) and for small mi scuts in the [110]-azimuth, S is constant nearly up to saturation whic h proves that the kinetics involves a weakly bound mobile precursor st ate. For (001)-vicinals with high miscut angles (9-13 degrees), the st ep structure breaks down, the precursor mobility is affected and the a dsorption kinetics changed. On (115), (113), (5,5,12) and (112), the v alues of S and Theta(sat) are smaller which indicates that not all sit es are able to dissociate and bind water. For (113) the shape of the a dsorption curves Theta versus exposure shows the existence of two adso rption processes, one with mobile precursor kinetics and one with Lang muir-like kinetics. On (5,5,12), two processes with mobile precursor k inetics are observed which are ascribed to adsorption on different sur face regions within the large surface unit cell. From the correspondin g values of S and Theta(sat), data for structure models are deduced. ( C) 1997 Elsevier Science B.V.