Method for producing group III nitride crystal and seed substrate

A method for producing a group III nitride crystal, the method comprising:

• seed crystal preparation including disposing a plurality of crystals of a group III nitride as a plurality of seed crystals on a substrate;
• and
• crystal growth including growing a group III nitride crystal on each of the seed crystals by contacting a surface of each of the seed crystals with a melt containing at least one group III element selected from gallium, aluminum, and indium and an alkali metal in an atmosphere containing nitrogen;
• wherein in the seed crystal preparation, the plurality of seed crystals are disposed only inside a substantial regular hexagonal region provided on the substrate;
• wherein the substantial regular hexagonal region is filled with the seed crystals in a form of dots with a diameter of 10 to 1,000 μm and a pitch of dots of 1.5 to 10 times the diameter;
• wherein the crystal growth comprises
• first crystal growth including growing a group III nitride crystal on each seed crystal into a pyramid to obtain a first group III nitride crystal formed of adjacent pyramidal group III nitride crystals partially coupled to each other, and
• second crystal growth including growing second group III nitride crystal with a flat surface on the first group III nitride crystal;
• wherein a distance between an outer periphery of the substrate and a center of a seed crystal the nearest to the outer periphery of the substrate among the seed crystals to be disposed in the seed crystal preparation is larger than a value obtained by dividing a maximum value of film thicknesses of the pyramidal group III nitride crystals to be grown in the first crystal growth by the square root of 3.