Tohru Ishiguro

Time-Resolved Photoluminescence of a Two-Dimensional Electron Gas in an Al0.2Ga0.8N/GaN Heterostructure Fabricated on Ammonothermal GaN Substrates

A characteristic photoluminescence signal is identified for a two-dimensional electron gas (2DEG) confined at an Al0.2Ga0.8N/GaN heterointerface fabricated on an ammonothermal GaN (AT-GaN) substrate. The use of a gas-phase synthesized NH4Cl acidic mineralizer reduced oxygen contamination in AT-GaN by two orders of magnitude, and metalorganic vapor phase epitaxy of atomically smooth, coherent AlGaN/GaN heterostructures was realized. The emission originating from the 2DEG is interpreted using self-consistent Schrodinger–Poisson calculation, taking the interfacial immobile charge due to polarization discontinuity into account. The initial decay time at low temperature was close to that of the bulk free excitons, reflecting the lifetime of photoexcited holes.

Numerical Simulation of Heat and Fluid Flow in Ammonothermal GaN Bulk Crystal Growth Process

A numerical simulation of an ammonothermal process for growing GaN bulk single crystals has been performed. The autoclave and growing crystal are assumed to be axisymmetric and the raw material is assumed to be a porous medium. Heat transfer by natural convection is discussed in terms of the open-space ratio of the baffle and the height of the crystals. Simulation results show that a larger open-space ratio, at least 30%, is better for the crystal growth from the viewpoints of average surface temperature and flow direction.

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.

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.

High quality bulk GaN crystal grown by acidic ammonothermal method

Acidic ammonothermal method is one of the most promising techniques which enable the mass production of large diameter bulk GaN crystal. State-of-the-art high-power light-emitting diodes and laser diodes are usually fabricated on GaN substrates grown by hydride vapor phase epitaxy. However, to realize vertically conducting high-power GaN switching devices, bowing-free large-diameter GaN substrates are essential, because the size of such devices is much larger than that of optical devices. Our group has been studying the characteristics of supercritical NH3 using ammonium halides as mineralizers, and succeeded in growing GaN. The crystal quality and the growth rate strongly depend on mineralizer species. We have also studied the dependence on temperature and pressure, and found it possible to achieve the growth rate faster than 1000μm/day in the optimum growth condition. Based on these studies and optimization, we have successfully demonstrated high speed bulk GaN growth at the pressure condition at 100MPa.