In one instance, the seed crystal of this invention provides a nitrogen-polar c-plane surface of a GaN layer supported by a metallic plate. The coefficient of thermal expansion of the metallic plate matches that of GaN layer. The GaN layer is bonded to the metallic plate with bonding metal. The bonding metal not only bonds the GaN layer to the metallic plate but also covers the entire surface of the metallic plate to prevent corrosion of the metallic plate and optionally spontaneous nucleation of GaN on the metallic plate during the bulk GaN growth in supercritical ammonia. The bonding metal is compatible with the corrosive environment of ammonothermal growth.