Production of a GaN bulk crystal substrate and a semiconductor device formed on a GaN bulk crystal substrate

A method of producing a group III nitride crystal in a reaction vessel pressurized to a higher pressure than that of a surrounding, said method comprising the steps of:

• forming a mixed melt comprising a group III metal element and an alkali metal element in the reaction vessel;
• disposing a seed crystal in the reaction vessel;
• supplying a nitrogen source of nitrogen or a compound containing nitrogen to the reaction vessel, the nitrogen source causing a reaction with the mixed melt and forming the group III nitride crystal;
• and
• growing the group III nitride crystal on an outer side of the seed crystal.