Interfacial reaction barriers during thin-film solid-state reactions: The crystallographic origin of kinetic barriers at the NiS2/Si(111) interface

Epitaxial NiSi₂ islands have been grown on Si(111) substrates by the direct reaction of nickel vapour with the silicon substrate in ultra-high vacuum at 400° C. Growth kinetics was shown to depend on the orientation of the islands: A-oriented islands grow about ten times faster than B-oriented ones, with the ratio of the advance rates of the main growth fronts even reaching 30. Applying plan-view transmission electron microscopy and high-resolution electron microscopy of cross sections, a corresponding difference was found in the structure of the NiSi₂/Si(111) growth front: Steps at the B-oriented growth front were of three or six interplanar (111) spacings in height, whereas at the A-oriented growth front step-like defects of less than one interplanar (111) spacing in height were observed. These observations are explained by an atomic-scale model of the solid-state reaction, which involves the diffusion of nickel to the interfaces and the nucleation and subsequent lateral propagation of interfacial steps. The difference in the reaction kinetics originates from the presence of kinetic reaction barriers at the NiSi₂/Si(111) growth fronts, the barrier at the B-front being higher owing to the lower formation rate of steps of triple atomic height than that of steps of lower height at the A-NiSi₂/Si(111) growth front.