Dramatic reduction of dislocations on a GaN point seed crystal by coalescence of bunched steps during Na-flux growth

•Threading dislocation density (TDD) in GaN crystals naturally reduced from ~109 cm−2 in a seed to less than ~103 cm−2, just by using the small-sized seed.•Threading dislocations (TDs) in the c-growth sector merged into one only in the case of crystals grown on 250-μm point seeds.•TDD was dramatically reduced after the c face became small (<50×50 μm2) in the habit-change process.•TDs on the c face were led to gather at the center of the c face by coalescence of bunched steps growing from the edge to the center•The size of the c face of the crystal grown on the 1000-μm point seed was too large for coalescence of bunched steps.

In our study, we found that threading dislocation density (TDD) in GaN crystals naturally reduced from ~109 cm−2 in a seed to less than ~103 cm−2, just by using the small-sized seed called a “point seed”. However, the mechanism of the dramatic reduction was unclear. In order to reveal the mechanism of this substantial reduction of TDD, we investigated the relationship between TDD and the crystal habit during the growth. Cathodoluminescence (CL) and scanning electron microscopy (SEM) images showed that TDD was dramatically reduced after the c face became small (<50×50 μm2) in the habit-change process caused by changes of supersaturation during growth, in which bunched steps growing from the edge of the c face coalesced at the center. It is thought that the shrinking of the c face in the growth process enabled the coalescence of bunched steps, which led to the gathering of threading dislocations (TDs), and resulted in the dramatic reduction of TDD. We concluded that the natural reduction of TDs was caused by coalescence of bunched steps, which easily occurs in during the Na-flux growth on small-sized “point seeds”, and which allowed fabrication of low-dislocation-density GaN wafers.