A method for forming a semiconductor device includes forming a fin device structure in a buffer layer on a substrate. The fin device structure includes a lower portion extending over the silicon substrate and a fin structure protruding above the lower portion. The method also includes forming a sacrificial layer disposed over the fin device structure and forming a device semiconductor layer disposed over a surface of the sacrificial layer. A gate dielectric layer is then formed and is disposed over a surface of the device semiconductor layer. A gate electrode layer is formed and disposed over a surface of the gate dielectric layer. The method includes removing a portion of the sacrificial layer to form a cavity surrounding the fin structure and performing an oxidation process to form a thermal oxide layer in the cavity surrounding the side surface of the fin structure.