To investigate the carrier recombination processes in GaN crystals grown by the low-pressure acidic ammonothermal (LPAAT) method, the photoluminescence (PL) spectra and PL lifetimes of LPAAT GaN crystals grown on acidic ammonothermal (AAT) GaN seed crystals were correlated with the growth polarity and species/concentration of point defects. The PL spectra of LPAAT GaN grown toward the (0001¯) direction (−c region), which provided the highest growth rate, exhibited a predominant near-band edge (NBE) emission. Neither bandgap narrowing nor Burstein–Moss shifts due to high concentration residual impurities were observed in the NBE emissions, indicating higher purity than the previously reported AAT GaN crystals. In addition, strain-induced energy shift or energy broadening of excitonic emission peaks was not observed, indicating excellent crystal coherency. Because of the reduced concentration of midgap recombination centers, a record-long room-temperature PL lifetime for the NBE emission of ammonothermal GaN (40 ps) was obtained from the −c region. Meanwhile, the PL spectra also exhibited the yellow and blue luminescence bands originating from particular deep-state radiative recombination centers. The major vacancy-type defects acting as midgap recombination centers are identified as vacancy complexes comprising a Ga vacancy (VGa) and a few N vacancies (VN), namely, VGa(VN)n buried by H and/or O, where n is an integer. Further reduction of such defect complexes will allow less compensated stable carrier concentration in the LPAAT GaN crystals.

In this presentation, seeded growths of large diameter GaN crystals using the low-pressure acidic ammonothermal (LPAAT) method operated at around 100 MPa will be demonstrated. Nearly bowing- and mosaic-free GaN crystals exhibiting the full-width at half-maximum values for the 0002 X-ray rocking curves below 20 arcsec were achieved on high lattice coherency c-plane AAT seeds with gross dislocation densities in the order of 104 cm−2. The photoluminescence spectra of the grown crystals exhibited a predominant near-band-edge emission at 295 K, of which intensity was one order of magnitude higher than the characteristic deep-state emission called "yellow luminescence band". A nearly bowing-free large diameter c-plane GaN crystal was eventually obtained.

Seeded growth of 2-inch-diameter GaN crystals via low-pressure (∼100 MPa) acidic ammonothermal method is demonstrated. Nearly bowing- and mosaic-free GaN crystals exhibiting full-width at half-maximum values for the 0002 X-ray rocking curves below 20 arcsec were achieved on high lattice coherency c -plane SCAAT TM seeds with gross dislocation densities in the order of 10 4 cm −2 . The photoluminescence spectra of the grown crystals exhibited a predominant near-band-edge emission at 295 K, of which intensity was one order of magnitude higher than the characteristic deep-state emission bands. A nearly bowing-free 60 mm × 60 mm c -plane GaN crystal was eventually obtained.

Thermal boundary conditions for numerical simulations of ammonothermal GaN crystal growth are investigated. A global heat transfer model that includes the furnace and its surroundings is presented, in which fluid flow and thermal field are treated as conjugate in order to fully account for convective heat transfer. The effects of laminar and turbulent flow are analyzed, as well as those of typically simultaneously present solids inside the autoclave (nutrient, baffle, and multiple seeds). This model uses heater powers as a boundary condition. Machine learning is applied to efficiently determine the power boundary conditions needed to obtain set temperatures at specified locations. Typical thermal losses are analyzed regarding their effects on the temperature distribution inside the autoclave and within the autoclave walls. This is of relevance because autoclave wall temperatures are a convenient choice for setting boundary conditions for simulations of reduced domain size. Based on the determined outer wall temperature distribution, a simplified model containing only the autoclave is also presented. The results are compared to those observed using heater-long fixed temperatures as boundary condition. Significant deviations are found especially in the upper zone of the autoclave due to the important role of heat losses through the autoclave head.

The ammonothermal method involves an upper and lower temperature difference that is provided in an autoclave by a baffle plate, and GaN, which is dissolved in supercritical ammonia in the raw material dissolution region and deposited on a seed crystal in the crystal growth region. Because dissolution and deposition can be continuously performed in an autoclave, this is a suitable method for producing large crystals. The growth of bulk GaN single crystals by the ammonothermal method is currently under development and there remain many indeterminate factors. This chapter describes the technological developments to achieve a high-speed growth and high quality of grown crystals.

We describe a facile method for synthesizing high-purity Zn-doped and Mg-doped GaN powders from Ga metal in supercritical ammonia at a moderate reaction temperature using ZnX2 and MgX2 (X = F, Cl, Br, or I) as combined mineralizers and doping agents. Successful doping of Zn or Mg into the GaN matrix was indicated by characteristic bands in the photoluminescence spectra of the crystals at 10 K. The results suggest that the halide ion plays an important role in the transformation of Ga metal to GaN powder.

Seeded ammonothermal growths of a few-mm-thick GaN crystals on a 2 inch diameter c-plane and a 45 mm long m-plane GaN wafers were carried out by using an NH4F mineralizer in a 60 mm diameter Ag-lined autoclave. As a result of dynamic control of the temperature profile, low dislocation density and nearly bowing-free m-plane GaN was grown: i. e. the full-width at half-maximum values for the X-ray rocking curves of the 1010 and 10 1 2 reflections were smaller than 28 arcsec and the radius of curvature was estimated to be 1460 m. In addition, its low temperature photoluminescence spectrum exhibited free and neutral donor-bound exciton emission peaks. (c) 2020 The Japan Society of Applied Physics

The presence of an extra Al metal in an autoclave tremendously improved the overall quality of m-plane GaN single crystals grown by the acidic ammonothermal method using an NH4F mineralizer. Although the growth rate was commonly decreased by adding an extra metal such as Al, Si, Ca. or Ti. the crystal coloration was mostly suppressed and the crystal mosaics were decreased, and the near-band-edge excitonic fine structure was observed in the low-temperature photoluminescence spectrum only when Al was present. The results likely indicate that the extra Al suppressed the incorporation of oxygen into m-planes of GaN owing to the oxygen gettering effect. (C) 2018 The Japan Society of Applied Physics

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 NH 3 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.

We performed ammonothermal synthesis of a self-nucleated GaN seed and grew crystalline GaN on the seed in the presence of an NH4F-based mineralizer. Our results suggest that spontaneously nucleated, high-quality GaN crystals can be obtained by recrystallization of polycrystalline hydride vapor phase epitaxy (HVPE) GaN under acidic ammonothermal conditions. We achieved average growth speeds of up to 410 and 465μm/day on the c- and m-directions, respectively, after four consecutive crystal growths of GaN on a self-nucleated seed. GaN crystals grown on an HVPE seed and on a self-nucleated seed had comparable crystal quality, judged from room-temperature photoluminescence measurements.

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 mu m per day on both Ga and N faces. The GaN powder was synthesized by ammonolysis of Ga metal in the presence of NH(4)l. 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 NH(4)l-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 mm per day on Ga and N faces, respectively. Growth rates up of to 33 mm 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.

NH4F is demonstrated to be a promising mineralizer for the acidic ammonothermal crystal growth of GaN. In comparison with other acidic mineralizers such as NH4Cl, NH4Br, and NH4I, NH4F behaves distinctively different First, NH4F affords a negative temperature gradient for crystal growth of GaN in supercritical NH3 at a temperature range from 550 to 650 degrees C. Second, it enables GaN crystal growth in polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH4F remarkably increases both the growth rate and quality of the GaN crystal. With the aid of NH4F, self-nucleation of GaN and bulk growth of hexagonal GaN crystals from the self-nucleated seed have been realized.