A method includes forming a first semiconductor strip on a substrate, the first semiconductor strip including a first crystalline semiconductor material on a substrate and a second crystalline semiconductor material above the first crystalline semiconductor material. A first portion of the first crystalline semiconductor material in first semiconductor strip is converted to a dielectric material, where a second portion of the first crystalline semiconductor material remains unconverted. Gate structures are formed over the first semiconductor strip and source/drain regions are formed on opposing sides of the gate structures.