Atomic Pathways of Solute Segregation in the Vicinity of Nanoscale Defects

Abstract: This work unravels the atomic details of the interaction of solute atoms with nanoscale crystalline defects. The complexity of this phenomenon is elucidated through detailed atom probe tomographic investigations on epitaxially-strained, compositionally metastable, semiconductor alloys. Subtle variations are uncovered in the concentration and distribution of solute atoms surrounding dislocations, and their dynamic evolution is highlighted. The results demonstrate that crystal defects, such as dislocations, are instrumental in initiating the process of phase separation in strained metastable layers. Matrix regions, close to the dislocations, show clear signs of compositional degradation only after a relatively short time from disrupting the local equilibrium. The solute concentration as well as the density of non-random atomic clusters increases while approaching a dislocation from the surrounding matrix region. In parallel, far from a dislocation the lattice remains intact preserving the metastable structure and composition uniformity. At advanced stages of phase separation, the matrix outside the dislocation reaches the equilibrium concentration, while dislocations act as vehicles of mass-transport, providing fast diffusive channels for solute atoms to reach the surface. Besides, the number of atomic clusters almost doubles and the number of atoms per cluster increases steadily, moving along a dislocation towards the surface. In addition to understanding the atomic features involved in the phase separation of strained metastable alloys, the work also illustrates the thermodynamic and kinetic behavior of solute atoms in the vicinity of a nanoscale defect and describe quantitatively the key processes, thus providing the empirical input to improve the atomic models and simulations.