Both the hexagonal and cubic GaN phases were synthesized in GaAs (001) by 50 keV N ion implantation at 380 °C and subsequent furnace annealing at 850–950 °C for 10 min–2 h. For a fluence of 1.5×1017cm−2, transmission electron microscopy revealed that cubic GaN epitaxially crystallizes as precipitates in the GaAs matrix. A cubic‐to‐hexagonal GaN phase transition was observed for extended thermal anneals. By increasing the N fluence to 3×1017cm−2, a continuous buried layer of randomly oriented hexagonal‐GaN nanocrystals was produced. ©1995 American Institute of Physics.