Role of surface strain in the subsurface migration of adsorbates on silicon

Modification of silicon surfaces through the insertion of atoms or even small molecular fragments of an adsorbate into a silicon-silicon bond can be affected tremendously by the effects of surface strain. This process takes place as either surface insertion or subsurface insertion, depending on whether the inserted species remains within the topmost layer or undergoes migration into subsurface layers, respectively. Using density-functional-theory cluster calculations, we show that insertion can be both thermodynamically and kinetically favorable if it takes place in such a way that surface strain is mitigated by neighboring surface sites. By considering the thermal decomposition of ammonia (NH₃) adsorbed on a Si(100)-2×1 surface, we find that insertion mainly depends on the initial distribution of adsorbates and the orientation taken by inserted species with respect to neighboring structures along the surface. These factors seem to greatly affect the subsurface insertion, which can therefore be considered a long-range process. On the other hand, for surface-insertion processes the factors mentioned above are less influential, and insertion has more of a local character. Understanding the factors governing insertion mechanisms may lead to development of more approaches to surface functionalization, where the adsorbates decorating the surface can decompose in a controllable fashion.