Phase-dependent distribution of Fe-rich nanocrystals in MOVPE-grown (Ga,Fe)N

摘要

We present a detailed transmission electron microscopy study of the structure and distribution of self-organized Fe-rich nanocrystals in (Ga,Fe)N grown via metal-organic vapor phase epitaxy (MOVPE). In samples with a concentration of magnetic ions above the solubility limit-x ≥ 0.4%-of Fe in GaN at the growth conditions we can identify three different phases of the Fe-rich enclosures, namely: (a) α-Fe segregated in proximity of the surface, (b) ε-Fe_3N in the volume beneath, though well above the nominal interface between the undoped GaN buffer and the Fe-doped overlayer, and (c) nanocrystals at an intermediate location showing the presence of adjacent α-Fe and ε-Fe_3N. These observations suggest that nitrogen evaporation at the growing surface play an important role in the formation of Fe-rich phases. The occurrence of α-Fe and ε-Fe_3N in the same nanocrystal indicates that the two phases are actually correlated: phase separation may first lead to the precipitation of ε-Fe_3N, which inherits the hexagonal structure of GaN but generates misfit dislocations, and then a further depletion of nitrogen promotes the conversion of ε-Fe_3N into α -Fe.