Abstract: A substrate for producing a gallium nitride compound-semiconductor (Al.su Ga.sub.1-x N; X=0 inclusive) device in vapor phase on a sapphire substrate using gaseous organometallic compound, and a also blue light emitting diode produced by using the substrate. The buffer layer comprising aluminium nitride (AlN) and having a crystal structure where microcrystal or polycrystal is mixed in amorphous state, is formed on the sapphire substrate. The buffer layer is formed at a growth temperature of 380.degree. to 800.degree. C. to have a thickness of 100 to 500 .ANG.. Further, on the buffer layer is formed the layer of gallium nitride compound-semiconductor (Al.sub.x Ga.sub.1-x N; X=0 inclusive). The layer of gallium nitride compound-semiconductor (Al.sub.x Ga.sub.1-x N; X=0 inclusive) comprising at least two layers having different conductive types and being sequentially layered on the buffer layer, functions as a light emitting layer.