Patent ID: 7250640

Claim:
An optical semiconductor device comprising: a bulk crystal substrate of GaN; lower and upper cladding layers formed epitaxially on said bulk crystal substrate of GaN; and an active layer formed epitaxially between said lower and upper cladding layers, said bulk crystal substrate of GaN comprising a slab of a GaN single crystal produced by a process comprising the steps of: forming a molten flux of a volatile metal element in a pressurized reaction vessel confining therein said molten flux together with an atmosphere containing N (nitrogen), such that said molten flux contains Ga in addition to said volatile metal element; growing GaN in the form of a single crystal body in said molten flux; and supplying a compound containing N directly into the atmosphere in said reaction vessel from a source located outside said reaction vessel during growth of said GaN single crystal, wherein said bulk crystal substrate of GaN has a defect density of 10 3 cm −2 or less.