The invention relates to new improvements in a process for crystal growth in the environment of supercritical ammonia-containing solution, which are based on use of specific azide mineralizers and result in the improved bulk Group XIII element nitride monocrystals, in particular balk monocrystalline gallium-containing nitride, intended mainly for variety of nitride-based semiconductor products such as various opto-electronic devices. The invention further relates to a mineralizer used for supercritical ammonia-containing solution which comprises at least one compound selected from the group consisting of LiN3, NaN3, KN3, and CsN3.