Reduction of crack density in ammonothermal bulk GaN growth

This paper reviews the current progress of ammonothermal growth at SixPoint Materials. Growths were performed at T<600 °C and P<300 MPa on GaN seed crystals produced by hydride vapor phase epitaxy (HVPE). For thin boules, <1 mm growth thickness, no cracking is observed. Historically however, SixPoint Materials' ammonothermal growth on HVPE seeds eventually experiences a curvature flip giving extremely high radius of curvature at a critical thickness. As the growth continues the radius of curvature degrades and cracking is observed. Since IWBNSVIII, SixPoint Materials has improved the crack free area for 5 mm thick boules from 5 to 80 mm2 to the complete seed area. This result is repeatable in multiple reactors. Careful selection of the HVPE seeds led to the greatest reduction in cracking. Seed selection combined with an additional technique has allowed boules to be grown crack free. X-ray diffraction was carried out on an ammonothermally grown boule at 90 points along a 44 mm line providing a mean (002) and (201) full width half max (FWHM) reflection of 29 and 35″ respectively using a beam spot of 0.3 mm x 0.3 mm and an open detector. The radius of curvature is typically between 3 and 20 m across the sample. Dislocation densities are routinely low 105 cm−2 .