Disclosing The Origin Of The Postcoalescence Compressive Stress In Polycrystalline Films By Nanoscale Stress Mapping

A method based on atomic force microscopy, which allows mapping the residual stress at nanoscale on the surface of crystalline solids [Polop et al., Nanoscale 9, 13938 (2017)], sheds light on the controversial origin of the intrinsic compression that arises in continuous polycrystalline films under high atomic mobility conditions. The maps of residual stress reveal that the compression is concentrated in narrow strips adjacent and parallel to the grain-boundary triple junctions, but not inside the grain boundary as usually assumed. We explain these findings in the light of Mullins's theory for surface diffusion of adatoms towards grain boundaries. As the surface slope at the grain-boundary triple junctions is constrained by the balance between interfacial tensions, the kinetic surface profile is different from the mechanical equilibrium profile predicted by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The average value of the stress profile is compressive. In turn, the resulting stress induces a Srolovitz-type surface diffusion, which counters the Mullins-type diffusion. The competition between both diffusion mechanisms addresses the main fingerprints of the intrinsic stress behavior in polycrystalline films, namely, the stabilization of the stress profile during growth, its reversibility with the flux, and the kinetics of the stress relaxation and recovery.