Abstract
Formation of intermetallic compounds in the field of a sharp concentration gradient has complicated thermodynamic driving forces and kinetic constraints. We have compared the formation of Cu3Si at a gradual interface to that at an abrupt one by complementary high resolution methods (APT, HR-TEM and XRD). The gradual transition leads to the immediate formation of an amorphous interlayer during deposition. This reduces the nucleation barrier for the crystalline Cu3Si to practically nil. However, in the case of the abrupt transition, phase formation is delayed. First an intermediate mixture must be formed. This can happen at the Cu side of the interface, but the diffusion of Si into Cu is slow. Therefore, an alternative route is also observed: 1) Wetting of Cu grain boundaries by Si. 2.) Stress relaxation by heterogeneous penetration of Cu into the a-Si matrix. 3.) Formation of Cu precipitates in a-Si. 4) Dissolution of Si into the pure Cu precipitates. 5) Transformation of Cu (Si) precipitates into stoichiometric Cu3Si layer. These findings contrast with the traditional view that the first silicide phase always forms nucleation-independently. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.