Chiaki Yokoyama

Acidic ammonothermal growth of GaN crystals using GaN powder as a nutrient

Ammonothermal crystal growth of gallium nitride (GaN) was realized for the first time using GaN powder as a nutrient with growth rates of about 61 μm per day on both Ga and N faces. The GaN powder was synthesized by ammonolysis of Ga metal in the presence of NH4I. The crystal quality of the as-grown GaN wafers was characterized by scanning electron microscope (SEM), X-ray rocking curve (XRC) and photoluminescence (PL) measurements. We examined the effect of different nutrients, including polycrystalline GaN, Ga metal, and GaN powder, on the acidic ammonothermal crystal growth of GaN using an NH4I-based mineralizer. Our results suggest that growth rate and crystal quality of GaN depend largely on the type of nutrient. Polycrystalline GaN as a nutrient can afford high growth rates of up to 150 and 237 μm per day on Ga and N faces, respectively. Growth rates up of to 33 μm per day were achieved using Ga metal as a nutrient. However, SEM, XRC, and PL measurements of GaN crystals grown using different nutrients indicate that Ga metal and GaN powder nutrients can provide better crystal quality of GaN in terms of surface morphology and crystal uniformity than polycrystalline GaN.

Convection patterns and temperature fields of ammonothermal GaN bulk crystal growth process

The natural convection heat transfer in an ammonothermal process for growing GaN bulk single crystals has been examined numerically. We consider only one crystal to simplify the calculation and discuss the relationship between convection patterns and temperature fields. Two types of convection patterns are observed owing to the difference in the crystal radius. When the convection pattern is transformed, the crystal surface temperature decreases as the crystal radius increases.

Powder synthesis and ammonothermal crystal growth of GaN from metallic Ga in the presence of NH4I

An efficient and mild method to synthesize high-purity hexagonal GaN powder using NH4I as a catalyst was developed. The method makes it possible to use Ga metal for ammonothermal crystal growth of GaN at a rate of 20 μm per day, which is comparable to its growth rate using polycrystalline GaN grown by hydride vapor phase epitaxy as a nutrient.