Observation of vertical honeycomb structure in InAlN∕GaN heterostructures due to lateral phase separation

The microstructure of InxAl1−xN∕GaN heterostructures (where x∼0.13–0.19), grown by molecular beam epitaxy, was investigated by transmission electron microscopy. Observations in the cross-section and plan-view geometries show evidence for lateral phase separation originating at the GaN surface that results in a vertical honeycomblike structure within the InAlN layers. The lateral dimensions of the honeycomb cells are ∼5–10nm. The vertical walls are In rich with a width of ∼1–2nm and align roughly perpendicular to ⟨112¯0⟩ and ⟨11¯00⟩ directions. The phase separation is attributed to random compositional fluctuations during the early stages of growth, possibly associated with misfit-strain relaxation.