Low-curvature and large-diameter GaN wafers are in high demand for the development of GaN-based electronic devices. Recently, we have proposed the coalescence growth of GaN by the Na-flux method and demonstrated the possibility of enlarging the diameter of high-quality GaN crystals. In the present study, 2 in. GaN wafers with a radius of curvature larger than 100 m were successfully produced by the Na-flux coalescence growth technique. FWHMs of the 002 and 102 GaN X-ray rocking curves were below 30.6 arcsec, and the dislocation density was less than the order of 102 cm−2 for the entire area of the wafer.

Centimeter-sized bulk GaN single crystals with large dislocation-free areas were fabricated by the Na-flux method with a necking technique. This necking, which is the key technique in Czochralski growth of dislocation-free Si ingots, was realized using a newly developed GaN point seed. Structural properties of grown crystals were investigated using panchromatic cathodoluminescence (CL) measurements and X-ray diffraction. Prism-shape and well-faceted bulk GaN crystals with dimensions as large as 0.85 cm (width) and 1 cm (length) were grown by this technique. The GaN single crystal grown for 400 h have full-width at half-maximum values for c// and c⊥ as narrow as 42.8 and 32.5 arcsec, respectively, indicating an extremely high quality. Panchromatic CL images of (0001) GaN wafers sliced from grown crystals revealed that large areas of the wafers were dislocation free. We concluded that the necking technique in Na flux GaN growth may be a major breakthrough for fabricating large dislocation-free GaN ingots.

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Dramatic reduction of dislocations on a GaN point seed crystal by coalescence of bunched steps during Na-flux growth, Journal of Crystal Growth, http://dx. Abstract:In our study, we found that threading dislocation density (TDD) in GaN crystals naturally reduced from ~10 9 cm -2 in a seed to less than ~10 3 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. Cathodeluminescence (CL) and scanning electron microscopy (SEM) images showed that TDD was dramatically reduced after the c face became small (< 50 × 50 μm 2 ) 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 2 (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. High lights We found that dislocation density in GaN crystals naturally reduced from ~10 9 cm -2 in a seed to less than ~10 3 cm -2 , just by using the small-sized seed. Dislocations merged into one only in the case of crystals grown on 250-μm point seeds, while dislocations remained in case of 1000-μm point seeds  Dislocations were dramatically reduced after the c face became small in the habit-change process, which is thought to occur due to the changes of flux composition and supersaturation in a closed system.

We have recently shown that dislocation-free GaN crystals could be grown on a GaN point seed by the Na-flux method. To enlarge the diameter of dislocation-free GaN crystals, we propose here the coalescence of GaN crystals grown from many isolated point seeds. In this study, we found that two GaN crystals grown from two point seeds arranged along the a-direction coalesced without generating dislocations at the coalescence boundary, and the c-axis misorientation between two crystals around the coalescence boundary gradually diminished as the growth proceeded. These results indicate that coalescence growth may become a key technique for fabricating large-diameter dislocation-free GaN crystals.