a nitride semiconductor substrate having a rugged surface being lapped by whetting granules to roughness between rms5 nm and rms200 nm , which has a function of reducing dislocations of a gan , ingan or algan layer epitaxially grown on the lapped substrate by gathering dislocations in the epi - layer to boundaries of holes , pulling the dislocations to bottoms of the holes . higher roughness of the nitride substrate degrades morphology of an epitaxially - grown layer thereon but reduces dislocation density to a lower level . morphology of the epi - layer contradicts the dislocation density of the epi - layer . the nitride semiconductor substrate can reduce dislocation density and can be low cost and useful substrates .