Semiconductor devices including fin-shaped isolation structures and methods of forming the same are disclosed. In an embodiment, a semiconductor device includes a fin extending from a semiconductor substrate; a shallow trench isolation (STI) region over the semiconductor substrate adjacent the fin; and a dielectric fin structure over the STI region, the dielectric fin structure extending in a direction parallel to the fin, the dielectric fin structure including a first liner layer in contact with the STI region; and a first fill material over the first liner layer, the first fill material including a seam disposed in a lower portion of the first fill material and separated from a top surface of the first fill material, a first carbon concentration in the lower portion of the first fill material being greater than a second carbon concentration in an upper portion of the first fill material.