Competition between strain and chemistry effects on adhesion of Si and SiC

Yun Liu, Izabela Szlufarska, Materials Science Program

       As the device size is scaled down to the nanometer regime and the surface-to-volume ratio increases, surface forces can dominate the device performance. In fact, undesired adhesion and wear have been already shown to be prohibitive for successful operation of Si-based micro- and nano-electro-mechanical systems (MEMS and NEMS). One way to improve the tribological properties of Si is by coating the surfaces with a thin layer of hard material, such as SiC.

      In this work, we use ab initio calculations to study adhesion properties of Si and SiC in various chemical environments. Recently we discovered that adhesion is controlled by the interplay between surface strain and chemistry. We determined that work of adhesion of Si depends linearly on surface strain, i.e., compressive surface strain increases adhesion while tensile strain reduces it. This effect explains why bare Si-terminated SiC has larger adhesion than Si. The topmost Si layer of the former surface is under considerable compressive strain because of the 20% lattice mismatch between lattice constants of cubic SiC and cubic Si. We also found that

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with as little as one monolayer of oxygen coverage, adhesion of SiC becomes negligible while adhesion of Si remains finite irrespectively of the specific oxygen configuration. This effect is due to fundamental differences between reactivity of Si and SiC during the early stages of oxidation. Specifically SiC surface can be effectively passivated with O, while such perfect passivation is not possible for O-terminated Si surface. Unsaturated surface bonds form interfacial bridges when surfaces are brought into contact, which results in a larger work of adhesion.

Y. Liu and I. Szlufarska “Competition between strain and chemistry effects on adhesion of Si and SiC”, Physical Review B 79, 094109 (2009)