GaN Substrate development through the near equilibrium ammonothermal (NEAT) method and its application to higher performance GaN-based devices

Abstract

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.