Group III nitride composite substrate and method for manufacturing the same, and method for manufacturing group III nitride semiconductor device

A group III nitride composite substrate comprising a group III nitride film and a support substrate formed from a material different in chemical composition from said group III nitride film,

• said group III nitride film being joined to said support substrate in one of a direct manner and an indirect manner,
• said group III nitride film having a thickness of 10 μm or more, and
• a sheet resistance of a group III-nitride-film-side main surface of said group III nitride composite substrate being 200 Ω/sq or less, said group III-nitride-film-side main surface being a main surface common to said group III nitride composite substrate and said group III nitride film, wherein
• an area of a joined region joining said group III nitride film and said support substrate is 70% or more relative to an area of the main surface, and
• a non-joined region failing to join said group III nitride film and said support substrate includes at least one non-joined partial region, and said non-joined partial region is a small non-joined partial region having a maximum size in radial direction of less than 20 mm.