A semiconductor device and method for fabricating a semiconductor device is disclosed. An exemplary semiconductor device includes a substrate including a first dielectric layer disposed over the substrate. The semiconductor device further includes a buffer layer disposed over the substrate and between first and second walls of a trench of the dielectric layer. The semiconductor device further includes an insulator layer disposed over the buffer layer and between the first and second wall of the trench of the dielectric layer. The semiconductor device also includes a second dielectric layer disposed over the first dielectric layer and the insulator layer. Further, the semiconductor device includes a fin structure disposed over the insulator layer and between first and second walls of a trench of the second dielectric layer.