Group III-nitride semiconductor crystal and manufacturing method thereof, and group III-nitride semiconductor device

Group III-nitride semiconductor crystal manufactured with a method of manufacturing group III-nitride semiconductor crystal including the steps of accommodating an alloy containing at least a group III-metal element and an alkali metal element in a reactor, introducing a nitrogen-containing substance in said reactor, dissolving the nitrogen-containing substance in an alloy melt in which said alloy has been melted, and growing group III-nitride semiconductor crystal, wherein said reactor is evacuated at a degree of vacuum of at most 1330 Pa and is subjected to heat treatment at a treatment temperature of 100 to 500° C. and for a treatment duration of 0.1 to 5 hours after said alloy containing at least the group III-metal element and the alkali metal element is accommodated in said reactor and before said nitrogen-containing substance is introduced in said reactor.