Mushroom structure of GaN template for epitaxial growth of GaN

In the present study, we show the formation of mushroom morphology produced by a ramp anneal of a low-temperature GaN buffer layer. Structural analysis by transmission electron microscopy indicates that the cap of the mushroom has the stable wurtzitic GaN structure, whereas the stem possesses the metastable zinc-blende structure. With the air gap introduced between the substrate and the cap of the mushroom structure, threading dislocations propagate along its stem. The formation of the mushroom morphology is suggested to result from the nucleation of wurtzitic GaN on the surface of the low-temperature buffer layer during the ramp anneal, followed by mass transport of GaN from the buffer layer, which remains zinc-blende during the anneal, to the surface, because wurtzitic GaN has the lower structure energy than zinc-blende GaN. This study extends limits of the conventional use of the buffer layer, laying the foundation for the development of low-cost recipes for achieving GaN templates with a low density of threading dislocations. ► Novel mushroom structure is formed by a simple anneal of a GaN buffer layer. ► Cap of the mushroom structure is identified as wurtzitic GaN. ► Stem region is observed to remain zinc-blende. ► Structure energy difference between the two phases drives the mushroom formation. ► This study provides a wider perspective on the use of the GaN buffer layer.