An epitaxial-deposition composite substrate, of more than about 50 mm diameter, in which a nitride-compound semiconductor first substrate is bonded together with a second substrate of either identical or different material. The first substrate is ion-implanted, and on its nitrogen-face side is coated with a special film of thickness within a predetermined range. On a bonding side of the second substrate a special coating of thickness within the predetermined range is formed. The join created by the coated nitrogen-face side of the first substrate being bonded to the coated bonding side of the second substrate occupies at least 90% of the surface area where the two substrates meet.