Epitaxial thin films of NiSi2 on Si(100) have been grown by room-temperature deposition of Ni followed by a high-temperature reaction. Initial stages of epitaxy revealed by transmission electron microscopy show nucleation of crystallographically equivalent islands related by a translation vector a/4〈111〉 via the underlying silicon substrate. Coalescence of islands thus requires the generation of a/4〈111〉 dislocations, which is energetically unfavorable. We find that very thin films (∼60 Å) do not coalesce, but choose to remain as islands leaving trenches of exposed substrate 15±1.5 Å in width between them. We propose that the trench left between islands can be described as a coreless defect in the silicide.