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.

With the ammonothermal method, one of the most promising technologies for scalable, cost-effective production of bulk single crystals of the wide bandgap semiconductor GaN is investigated. Specifically, etch-back and growth conditions, as well as the transition from the former to the latter, are studied using a 2D axis symmetrical numerical model. In addition, experimental crystal growth results are analyzed in terms of etch-back and crystal growth rates as a function of vertical seed position. The numerical results of internal process conditions are discussed. Variations along the vertical axis of the autoclave are analyzed using both numerical and experimental data. During the transition from quasi-stable conditions of the dissolution stage (etch-back process) to quasi-stable conditions of the growth stage, significant temperature differences of 20 K to 70 K (depending on vertical position) occur temporarily between the crystals and the surrounding fluid. These lead to maximum rates of seed temperature change of 2.5 K/min to 1.2 K/min depending on vertical position. Based on temperature differences between seeds, fluid, and autoclave wall upon the end of the set temperature inversion process, deposition of GaN is expected to be favored on the bottom seed. The temporarily observed differences between the mean temperature of each crystal and its fluid surrounding diminish about 2 h after reaching constant set temperatures imposed at the outer autoclave wall, whereas approximately quasi-stable conditions are reached about 3 h after reaching constant set temperatures. Short-term fluctuations in temperature are mostly due to fluctuations in velocity magnitude, usually with only minor variations in the flow direction.

For the fundamental understanding and the technological development of the ammonothermal method for the synthesis and crystal growth of nitrides, an in situ monitoring technique for tracking mass transport of the nitride throughout the entire autoclave volume is desirable. The feasibility of using high-energy computed tomography for this purpose was therefore evaluated using ex situ measurements. Acceleration voltages of 600 kV were estimated to yield suitable transparency in a lab-scale ammonothermal setup for GaN crystal growth designed for up to 300 MPa operating pressure. The total scan duration was estimated to be in the order of 20 to 40 min, which was sufficient given the comparatively slow crystal growth speed in ammonothermal growth. Even shorter scan durations or, alternatively, lower acceleration voltages for improved contrast or reduced X-ray shielding requirements, were estimated to be feasible in the case of ammonoacidic growth, as the lower pressure requirements for this process variant allow for thinned autoclave walls in an adapted setup designed for improved X-ray transparency. Promising nickel-base and cobalt-base alloys for applications in ammonothermal reactors with reduced X-ray absorption in relation to the maximum operating pressure were identified. The applicability for the validation of numerical simulations of the growth process of GaN, in addition to the applicability of the technique to further nitride materials, as well as larger reactors and bulk crystals, were evaluated.

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.

Numerical simulations are a valuable tool for the design and optimization of crystal growth processes because experimental investigations are expensive and access to internal parameters is limited. These technical limitations are particularly large for ammonothermal growth of bulk GaN, an important semiconductor material. This review presents an overview of the literature on simulations targeting ammonothermal growth of GaN. Approaches for validation are also reviewed, and an overview of available methods and data is given. Fluid flow is likely in the transitional range between laminar and turbulent; however, the time-averaged flow patterns likely tend to be stable. Thermal boundary conditions both in experimental and numerical research deserve more detailed evaluation, especially when designing numerical or physical models of the ammonothermal growth system. A key source of uncertainty for calculations is fluid properties under the specific conditions. This originates from their importance not only in numerical simulations but also in designing similar physical model systems and in guiding the selection of the flow model. Due to the various sources of uncertainty, a closer integration of numerical modeling, physical modeling, and the use of measurements under ammonothermal process conditions appear to be necessary for developing numerical models of defined accuracy.

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

Most existing thermal conductivity data for ammonia were obtained before 1988. Data obtained before 1988 may be affected by convection. The potential for these effects needs to be examined by the transient hot-wire method with high-speed sampling. In this study, the thermal conductivity of liquid ammonia was measured by the transient short-hot-wire method and compared with values from REFPROP 10.0. The transient short-hot-wire method was used to measure thermal conductivity at 284–354 K and 2–10 MPa. Compared with the calculated values from REFPROP 10.0, the average absolute deviation was 5.6 % and all the measured values were lower than those from REFPROP 10.0.

Using a mass-loss method, we investigated the solubility change of gallium nitride (GaN) in supercritical ammonia with mixed mineralizers [ammonium chloride (NH 4 Cl) + ammonium bromide (NH 4 Br) and NH 4 Cl + ammonium iodide (NH 4 I)]. The solubilities were measured over the temperature range 450–550 °C, at 100 MPa. The solubility increased with NH 4 Cl mole fraction at 450 °C and 100 MPa. The temperature dependence of the solubility curve was then measured at an equal mole ratio of the two mineralizers. The slope of the solubility–temperature relationship in the mixed mineralizer was between those of the individual mineralizers. These results show that the temperature dependence of the solubility of GaN can be controlled by the mineralizer mixture ratio. The results of the van’t Hoff plot suggest that the solubility species were unchanged over the investigated temperature range. Our approach might pave the way to realizing large, high-quality GaN crystals for future gallium-nitride electronic devices, which are increasingly on demand in the information-based age.

In this study, we report densities and thermal conductivities for 1-butyl-3-methylimidazolium tetrafluoroborate + methanol mixtures. The densities were measured using a vibrating tube densimeter over the temperature range 293-333 K and at pressures up to 20 MPa. The expanded uncertainty in the density measurements was 0.22% (k = 2). The experimental values were correlated with a Tait equation with an average absolute deviation of 0.10%. The transient short hot-wire method was used to measure the thermal conductivities over the temperature range 295-335 K at 0.1 MPa. The expanded uncertainty in the thermal conductivity measurements was 4.4%. The thermal conductivities of the 1-butyl-3-methylimidazolium tetrafluoroborate + methanol mixtures increased in an approximately quadratically with increasing methanol mass fraction. The experimental values were correlated with a Jamieson equation with an average absolute deviation of 0.24%.

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

Ionic liquids (ILs) have attracted great attention as green solvents, heat carriers, and electrolytes. They can be obtained with specific thermophysical properties and functions by changing the kind of species of cations and anions. Knowledge of the fundamental thermophysical properties of ILs, such as their densities, viscosities, and thermal conductivities, is needed to design ILs with desirable thermophysical properties. In this chapter, we will review the various measurement results for the thermal conductivities of the pure components of ILs and methods for predicting the thermal conductivity of an IL, which are based on its structure and physical properties, by conducting correlations between these parameters. In the recent years, the thermal conductivities of IoNano fluids, which comprise of nanoparticles dispersed in an IL, have attracted great attention. Therefore, we will review the unique thermal conductivities of IoNano fluids.