Disclosed are methods for forming a localized buried dielectric layer under a fin for use in a semiconductor device. In some embodiments, the method may include providing a substrate comprising a bulk semiconductor material and forming at least two trenches in the substrate, thereby forming at least one fin. The method further includes filling the trenches with an insulating material and partially removing the insulating material to form an insulating region at the bottom of each of the trenches. The method further includes depositing a liner at least on the sidewalls of the trenches, removing a layer from a top of each of the insulating regions to thereby form a window opening at the bottom region of the fin, and transforming the bulk semiconductor material of the bottom region of the fin via the window opening, thereby forming a localized buried dielectric layer in the bottom region of the fin.