Before revision: Epitaxial strain in thin films generally arises due to lattice mismatch between the film and its substrate , and can arise either during film growth or due to thermal expansion mismatch.
After revision: Epitaxial strain in thin films generally arises due to lattice mismatch between the film and its substrate and triple junction restructuring at the surface triple junction, which arises either during film growth or due to thermal expansion mismatch.