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This paper reviews recent development of two-inch gallium nitride (GaN) substrates fabricated by the near equilibrium ammonothermal (NEAT) method. The NEAT method utilizes a low driving force to achieve consistent crystal growth over a long period of time (> 90 days) while maintaining high quality microstructure. Through refinement of growth conditions for the NEAT method and proper preparation of seed crystals we have achieved 2" GaN substrates with excellent microstructure. Currently, 2" GaN wafers sliced from bulk GaN crystals typically have a full width half maximum (FWHM) of the 002 X-ray rocking curve of 50 arcsec or less, a dislocation density of mid-105 cm-2 or less, and an electron density of about 2 x 1019 cm-3. The high electron density is attributed to an oxygen impurity in the crystal. Due to high oxygen concentration, GaN crystal grown in the ammonothermal method tends to show a brownish color. Through process refinement, we successfully reduced oxygen concentration to 7 x 1018 cm-3, which resulted in optical absorption coefficient of 5.6 cm-1 at 450 nm. This progress ensures feasibility of the NEAT method for producing GaN wafers usable for various optoelectronic devices, power transistors and high-frequency transistors. |
