Devices and methods for forming a device are presented. The method includes providing a substrate prepared with a device region. A fin is formed in the device region. The fin includes top and bottom portions. An isolation layer is formed on the substrate. The isolation layer has a top isolation surface disposed below a top fin surface, leaving an upper fin portion exposed. At least one isolation buffer is formed in the bottom fin portion, leaving the top fin portion crystalline, the top fin portion serves as a body of a fin type transistor. Source/drain (S/D) regions are formed in the top portions of the fin and a gate wrapping around the fin is provided.