An epitaxial structure includes a substrate, a first epitaxial layer and a second epitaxial layer. The substrate has a surface, and the first epitaxial layer is disposed over the substrate and defines a plurality of slanting air voids tapering away from the substrate and an opening over each of the slanting air voids. The second epitaxial layer is disposed on the first epitaxial layer and collectively defines the slanting air voids in a shape of trapezoid with the surface and the first epitaxial layer.