Abstract: A substrate for forming a semiconducting layeris provided to grow the semiconducting layer on a major surface thereof, wherein the substrate comprises a single crystal of a chemical formula of XB2 where X contains one of Ti and Zr and the major surface may preferably be substantially parallel to plane (0001) of the single crystal because the plane (0001) of the boride substrate is highly coherent to the lattices of GaN and AlN layers grown eptaxially on the substrate. The single crystal of the substrate may be a solid solution containing impurities of not more than 5%, wherein at least one of the impurities is one selected from Cr, Hf, V, Ta and Nb. Further, a semiconductor device includes the substrate of a single crystal of a chemical formula of XB2 and at least one semiconducting layer which is grown epitaxially on the substrate, the semiconducting layer including a nitride semiconductor of a chemical formula of ZN where Z is one of gallium, aluminum and indium and boron.