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The state-of-the-art ammonothermal method for the growth of nitrides is reviewed here, with an emphasis on binary and ternary nitrides beyond GaN. A wide range of relevant aspects are covered, from fundamental autoclave technology, to reactivity and solubility of elements, to synthesized crystalline nitride materials and their properties. Initially, the potential of emerging and novel nitrides is discussed, motivating their synthesis in single crystal form. This is followed by a summary of our current understanding of the reactivity/solubility of species and the state-of-the-art single crystal synthesis for GaN, AlN, AlGaN, BN, InN, and, more generally, ternary and higher order nitrides. Investigation of the synthesized materials is presented, with a focus on point defects (impurities, native defects including hydrogenated vacancies) based on GaN and potential pathways for their mitigation or circumvention for achieving a wide range of controllable functional and structural material properties. Lastly, recent developments in autoclave technology are reviewed, based on GaN, with a focus on advances in development of in situ technologies, including in situ temperature measurements, optical absorption via UV/Vis spectroscopy, imaging of the solution and crystals via optical (visible, X-ray), along with use of X-ray computed tomography and diffraction. While time intensive to develop, these technologies are now capable of offering unprecedented insight into the autoclave and, hence, facilitating the rapid exploration of novel nitride synthesis using the ammonothermal method.

The morphology of GaN substrates grown by hydride vapor-phase epitaxy (HVPE) and by ammonothermal methods has been correlated with reverse-bias stress testing applied to GaN-on-GaN p-i-n diodes. GaN substrates grown by HVPE showed ordered, well-separated arrays of surface features when observed using x-ray topography (XRT). All fabricated diodes that overlapped with these features had reverse-bias voltages typically of less than 100 V before reaching a critical leakage current limit that was set at 10–6 A. In contrast, diodes not overlapping with such features reached reverse-bias voltages exceeding 300 V for the same leakage current limit. After surface etching, the HVPE substrate showed evidence for defect clusters and macro-pits. XRT images of the ammonothermal GaN substrate revealed no visible features. However, some diodes fabricated on the ammonothermal substrate still failed to reach reverse-bias voltages comparable to those of the HVPE-grown samples. Diodes on HVPE and ammonothermal substrates with low breakdown voltage showed crater-like surface damage. Progressive ion milling across such failed devices revealed the presence of voids and threading dislocations that penetrated deep into the substrate (~ 25 µm); these features were not observed in diodes with high reverse-bias voltages and low leakage current. This work emphasizes the potential unsuspected impact of substrate morphology in limiting the performance of vertical GaN devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

GaN and its alloys with InN and AlN are of technological importance for a variety of optical, electronic, and optoelectronic devices due to its high thermal conductivity, wide band gap, high breakdown voltage and high saturation velocity. GaN-based devices now provide superior performance for a variety of high power, high frequency, high temperature, and optical applications. The major roadblock for the full realization of Nitride semiconductor potential is still the availability of affordable large-area and high-quality native substrates with controlled electrical properties. Despite the impressive accomplishments recently achieved by techniques such as hydride vapor phase epitaxy and ammonothermal for GaN growth, much more must be attained before establishing a fully satisfactory bulk growth method for this material. Recent results suggest that ammonothermal GaN wafers can be successfully used as seeds to grow thick freestanding GaN wafers by hydride vapor phase epitaxy. A brief review of defect-sensitive optical and paramagnetic spectroscopy techniques employed to evaluate structural, optical, and electronic properties of the state-of-the-art bulk and thick-film (quasi-bulk) Nitride substrates and homoepitaxial films is presented. Defects control the performance of devices and feeding back knowledge of defects to growth efforts is key to advancing technology. [ABSTRACT FROM AUTHOR]

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.

ON-state conductance properties of vertical GaN n-channel trench MISFETs manufactured on different GaN substrates and having different gate trench orientations are studied up to 200 °C ambient temperature. The best performing devices, with a maximum output current above 4 kA/cm2 and an area specific ON-state resistance of 1.1 mΩ·cm2, are manufactured on ammonothermal GaN substrate with the gate channel parallel to the a-plane of the GaN crystal. The scalability of the devices up to 40 mm gate periphery is investigated and demonstrated. It is found that, in addition to oxide interface traps, the semiconductor border traps in the p-GaN layer limit the available mobile channel electrons and that the channel surface roughness scattering limits the channel mobility. Both strongly depend on the gate trench orientation and on the GaN substrate defect density.

The state-of-the-art ammonothermal method for the growth of nitrides is reviewed here, with an emphasis on binary and ternary nitrides beyond GaN. A wide range of relevant aspects are covered, from fundamental autoclave technology, to reactivity and solubility of elements, to synthesized crystalline nitride materials and their properties. Initially, the potential of emerging and novel nitrides is discussed, motivating their synthesis in single crystal form. This is followed by a summary of our current understanding of the reactivity/solubility of species and the state-of-the-art single crystal synthesis for GaN, AlN, AlGaN, BN, InN, and, more generally, ternary and higher order nitrides. Investigation of the synthesized materials is presented, with a focus on point defects (impurities, native defects including hydrogenated vacancies) based on GaN and potential pathways for their mitigation or circumvention for achieving a wide range of controllable functional and structural material properties. Lastly, recent developments in autoclave technology are reviewed, based on GaN, with a focus on advances in development of in situ technologies, including in situ temperature measurements, optical absorption via UV/Vis spectroscopy, imaging of the solution and crystals via optical (visible, X-ray), along with use of X-ray computed tomography and diffraction. While time intensive to develop, these technologies are now capable of offering unprecedented insight into the autoclave and, hence, facilitating the rapid exploration of novel nitride synthesis using the ammonothermal method.

GaN and its alloys with InN and AlN are of technological importance for a variety of optical, electronic, and optoelectronic devices due to its high thermal conductivity, wide band gap, high breakdown voltage and high saturation velocity. GaN-based devices now provide superior performance for a variety of high power, high frequency, high temperature, and optical applications. The major roadblock for the full realization of Nitride semiconductor potential is still the availability of affordable large-area and high-quality native substrates with controlled electrical properties. Despite the impressive accomplishments recently achieved by techniques such as hydride vapor phase epitaxy and ammonothermal for GaN growth, much more must be attained before establishing a fully satisfactory bulk growth method for this material. Recent results suggest that ammonothermal GaN wafers can be successfully used as seeds to grow thick freestanding GaN wafers by hydride vapor phase epitaxy. A brief review of defect-sensitive optical and paramagnetic spectroscopy techniques employed to evaluate structural, optical, and electronic properties of the state-of-the-art bulk and thick-film (quasi-bulk) Nitride substrates and homoepitaxial films is presented. Defects control the performance of devices and feeding back knowledge of defects to growth efforts is key to advancing technology.

Gallium nitride (GaN) substrates grown by different methods were characterized by high-resolution x-ray diffraction and synchrotron x-ray topography. Using the monochromatic beam in the grazing incidence geometry, high-resolution x-ray topographs reveal the various dislocation types present. Dislocation contrasts were correlated with ray-tracing simulation results successfully so that the Burgers vectors of the dislocations could be determined. Ammonothermal-grown GaN substrate wafers show the best quality among all the wafers. These wafers, which are free of basal plane dislocations (BPDs) have threading mixed dislocations (TMDs) dominant among the threading dislocations (TDs). Images of patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities in between a grid of strain centers with higher threading dislocation densities and BPDs. The strain level of regular HVPE GaN substrates is very high, and the dislocation density is around 105–106 cm−2, which is much higher than 104 cm−2 of ammonothermal samples and dislocation-free areas in the patterned HVPE samples. [ABSTRACT FROM AUTHOR]

Abstract Using a mass-loss method, we investigated the solubility change of gallium nitride (GaN) in supercritical ammonia with mixed mineralizers [ammonium chloride (NH4Cl) + ammonium bromide (NH4Br) and NH4Cl + ammonium iodide (NH4I)]. The solubilities were measured over the temperature range 450–550 °C, at 100 MPa. The solubility increased with NH4Cl 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.

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.

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.

Abstract: In this study we demonstrate a cheap and sustainable ammonothermal approach towards nitrogen-doped porous carbons. Sodium borate (borax) is employed as a catalyst during the synthesis resulting in the formation of small interconnected primary particles of <100 nm in size. Microporosity is created in these nitrogen-doped, ammonothermal carbon samples by a synchronous activation and post carbonization procedure at 850 °C, while the interconnected primary particles offer larger interstitial void spaces including mesopores. Variation of the starting ammonia concentration allows for the facile adjustment of the final nitrogen content, reaching up to 7 wt.% after post carbonization. Electrochemical characterization is carried out in two and three electrode modes by means of cyclic voltammetry and galvanostatic cycling at different scan rates and current densities, respectively. The sample prepared at a high glucose-to-ammonia ratio shows high specific capacitance of 185 and 144 F g−1 at 0.2 and 20 A g−1, respectively (271 F g−1 in a three electrode mode at 1 A g−1). All samples demonstrate a very stable capacitance over the tested 5000 cycles at 10 A g−1 with no degradation and an excellent coulombic efficiency of >99%. Comparison of different pore systems indicates that a continuous pore size distribution may explain improved rate performances.A sol-gel-type ammonothermal carbonization of sugar, catalyzed by Borax was combined with physical CO2-activation to obtain well-performing and sustainable electrode materials for aqueous supercapacitors Porous nitrogen-doped carbon was synthesized by ammonothermal carbonizationHigh capacitances of 185 F g−1 were obtainedVery high cycling stability was obtained [ABSTRACT FROM AUTHOR]

A variety of functional nitride materials, including the important wide bandgap semiconductor GaN, can be crystallized in exceptionally good structural quality by the ammonothermal method. However, the further development of this method is hindered by a lack of access to internal process parameters including fluid temperatures, flow stability and reaction kinetics. Internal temperature measurements are thus introduced as a tool for in situ monitoring of fluid flow stability in ammonothermal reactors as well as chemical reactions associated with enthalpy changes. The temperature change of an internal thermocouple is studied numerically in order to estimate possible errors due to heat conduction along thermocouples as well as due to their heat capacity. Results from otherwise identical experiments conducted with air at ambient pressure and ammonothermal reaction medium, respectively, are compared. The comparison indicates that internal temperature distributions during ammonothermal growth of GaN cannot be determined by measurements using ambient pressure air instead of supercritical ammonia. Even an approximate determination is not feasible, given that the internal temperature gradients differ by a factor of seven, and that the Grashof- and Rayleigh numbers differ by approximately four orders of magnitude. Most importantly, convective heat transfer by supercritical ammonia is found to greatly influence the temperature distribution inside the reaction chamber and its walls, suggesting that it probably needs to be taken into account in numerical simulations of the global thermal field of ammonothermal reactors.

Synchrotron monochromatic-beam x-ray topography observation has been performed on high-quality ammonothermal gallium nitride single crystal to evaluate threading dislocations (TD) in a nondestructive manner. Asymmetric diffractions with six equivalent g-vectors of 11-26, in addition to a symmetric diffraction with g = 0008, were applied to determine the Burgers vectors (b) of dislocations. It was found that pure edge-type TDs with b=11-20/3 did not exist in the sample. A dominant proportion of TDs were of mixed type with b=11-20/3+0001, i.e., so-called c + a dislocations. Pure 1c screw dislocations with b=0001 and TDs with c-component larger than 1c were also observed. [ABSTRACT FROM AUTHOR]

Abstract: Nitrides represent an intriguing class of functional materials with a broad range of application fields. Within the past decade, the ammonothermal method became increasingly attractive for the synthesis and crystal growth of nitride materials. The ammonothermal approach proved to be eminently suitable for the growth of bulk III‐nitride semiconductors like GaN, and furthermore provided access to numerous ternary and multinary nitrides and oxonitrides with promising optical and electronic properties. In this minireview, we will shed light on the latest research findings covering the synthesis of nitrides by this method. An overview of synthesis strategies for binary, ternary, and multinary nitrides and oxonitrides, as well as their properties and potential applications will be given. The recent development of autoclave technologies for syntheses at high temperatures and pressures, in situ methods for investigations of crystallization processes, and solubility measurements by ultrasonic velocity experiments is briefly reviewed as well. In conclusion, challenges and future perspectives regarding the synthesis and crystal growth of novel nitrides, as well as the advancement of autoclave techniques are discussed. [ABSTRACT FROM AUTHOR]

Free-standing gallium nitride (GaN) substrates are in high demand for power devices, laser diodes, and high-power light emitting diodes (LEDs). SixPoint Materials Inc. has begun producing 2” GaN substrates through our proprietary Near Equilibrium AmmonoThermal (NEAT) growth technology. In a single 90 day growth, eleven c-plane GaN boules were grown from free-standing hydride vapor phase epitaxy (HVPE) GaN substrates. The boules had an average X-ray rocking curve full width at half maximum (FWHM) of 33 ± 4 in the 002 reflection and 44 ± 6 in the 201 reflection using 0.3 mm divergence slits. The boules had an average radius of curvature of 10.16 ± 3.63 m. The quality of the boules was highly correlated to the quality of the seeds. A PIN diode grown at Georgia Tech on a NEAT GaN substrate had an ideality factor of 2.08, a high breakdown voltage of 1430 V, and Baliga’s Figure of Merit of >9.2 GW/cm2. These initial results demonstrate the suitability of using NEAT GaN substrates for high-quality MOCVD growth and fabrication of high-power vertical GaN switching devices.

Improvements to the growth of nitride crystals in ammonothermal growth environments can be achieved through improved autoclave designs, purity, and use of in situ monitoring techniques. Given the limited data available on the stability of materials in supercritical ammonia solutions, this study intends to broaden the known dataset by experimentally investigating the mechanical and chemical stability of 35 bulk metals, 2 bulk metalloids, and 17 bulk ceramics while identifying suitable materials for future in-depth corrosion studies. This was performed by exposing each material to three different supercritical ammonia solutions in nickel-chromium superalloy autoclaves held at an external wall temperature of 575 °C for 4–12 days. The solutions were formed by initially filling the autoclave with pure ammonia (NH 3 ), ammonia and sodium (NH 3 -Na), or ammonia and ammonium chloride (NH 3 -Cl) to achieve total system pressures of 100–250 MPa. Zirconia, silicon carbide, tungsten carbide, molybdenum and its alloys, tungsten and its alloys, and a cobalt-tungsten-aluminum alloy (Co 80 W 10.6 Al 9.4 ) appeared stable in all three environments. Vanadium, niobium, and tantalum appeared chemically stable in all environments, though these samples embrittled and gained weight. Oxides containing high concentrations of aluminum, silicon, and/or zirconium were most stable in NH 3 -Cl while magnesium oxide was not stable in any environment. Silicon nitride corroded in NH 3 -Na, yet appeared stable in NH 3 and NH 3 -Cl. Silver and a platinum-ruthenium alloy (Pt 91 Ru 9 ) appeared to be the only noble metals stable in NH 3 -Na while all of the noble metals investigated, except for silver, appeared to be stable in NH 3 -Cl. Copper and its alloys were not stable in any environment except for Constantan (Cu 53 Ni 47 ) in NH 3 . Exposed thermocouples of type K and N remained functional post-run in NH 3 and NH 3 -Na. Iron and nickel and their respective alloys appeared stable in NH 3 and NH 3 -Na but not in NH 3 -Cl. Most other metals and metalloids investigated in this study were not stable in any of the three solutions and lost weight. Results in this study agree with literature data, where available. [ABSTRACT FROM AUTHOR]

Ammonothermal growth of nitrides occurs at temperatures in excess of 800 K and pressures greater than 150 MPa. For this region, no experimentally verified equation of state (EOS) exists for ammonia, nor is there any accurate description available for the equilibrium constant of the ammonia decomposition reaction for pressures in excess of 100 MPa. To fill this void, experimental P -v- T data was collected for pure ammonia at T > 700 K and compared to extrapolated data from the reference EOS for ammonia. For the extrapolated region, the reference EOS provided excellent agreement to within error of the collected experimental data. A simplified EOS based on the Beattie–Bridgeman (BB) EOS was derived and fit to calculated and, for ammonia, extrapolated reference EOS data for ammonia, hydrogen, and nitrogen ( T < 1000 K, P < 300 MPa). By applying mixing rules with separated contributions for polar and non-polar interactions, an EOS was derived for NH 3 –N 2 –H 2 mixtures. With these expressions, an accurate description for the equilibrium constant for the ammonia decomposition reaction as a function of pressure and temperature was derived and verified against experimental data determined for total system pressures of 92, 151 and 210 MPa at T ∼810 K. Coupling of the EOSs with the equilibrium constant permitted accurate modeling of a sealed autoclave filled with pure ammonia, after incorporating corrections for the expansion of the internal free volume due to thermal expansion and elastic strain response of the autoclave walls due to internal pressure buildup. Calculated total system pressure and equilibrium ammonia density at various temperatures and initial ammonia fill densities are in very good to excellent agreement with experimental data. This paper thus provides a simple EOS for ammonia, hydrogen, nitrogen and NH 3 –N 2 –H 2 mixtures accurate to within approximately 1–2% in pressure for temperatures greater than 700 K. The equilibrium constant for the ammonia decomposition reaction includes non-ideal mixing contributions from the second virial coefficient with a resulting accuracy in the equilibrium mole fraction of ammonia of approximately 2% for T < 850 K. [ABSTRACT FROM AUTHOR]

We have examined the performance of gallium nitride (GaN) high-power Schottky diodes fabricated on unintentionally doped (UID) metalorganic chemical vapor deposition (MOCVD) films grown simultaneously on four substrates ranging in threading dislocation density from 5 × 10 cm to 10 cm. The substrates were an intentionally doped and a UID freestanding hydride vapor phase epitaxy substrate, an MOCVD GaN template grown on a sapphire wafer, and a bulk GaN substrate grown via an ammonothermal method. Capacitance-voltage ( C- V) results showed the carrier concentration was ∼2 × 10 cm for films grown on each of the four substrates. With that doping level, the theoretical breakdown voltage ( V) is ∼1600 V. However, measured V for the devices tested on each of the four substrates fell short of this value. Also, the breakdown voltages across each of the four substrates were not substantially different. This result was especially surprising for films grown on bulk GaN substrates, because of their superior crystal quality, as determined from their x-ray rocking curve widths. Simple probability calculations showed that most of the diodes tested on the bulk substrate did not cover a single threading dislocation. Although optimization of edge-termination schemes is likely to improve V, we believe that point defects, not threading dislocations, are the main reason for the reduced performance of these devices. [ABSTRACT FROM AUTHOR]

In this work we present structural, optical and electrical properties of GaN epilayers and AlGaN/GaN MOCVD heterostructures obtained on polar and non-polar 10x10 mm2, 1 and 1.5 inch bulk ammonothermal GaN substrates. The results were obtained based not only on a single-point measurement but also on mapping across the sample area. In contactless electroreflectance (CER) spectra CER resonances related to optical transitions between excited states were clearly observed for GaN/AlGaN QWs grown on c -plane GaN substrates, whereas such features were not noted in the case of GaN/AlGaN QWs grown on m -plane GaN substrates. This experimental result clearly shows that the polarization-related electric field leads to the quantum confinement of some extra states in the polar QW system. To check the advantage of homo-epitaxy using ammonothermal GaN, high electron mobility transistors (HEMTs), and light emitting diodes (LEDs) structures were grown on (0001) Ga-polarity, semi-insulating and conductive GaN substrates respectively (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: Tantalum nitride (Ta3N5) was treated under ammonothermal conditions with the ammonia pressure of 10 to 100MPa at 823K for 24h to enhance the photocatalytic activity for hydrogen evolution from aqueous methanol solution under visible light irradiation. The rates of hydrogen evolution for all treated Ta3N5 were faster than those for untreated Ta3N5. Treated Ta3N5 showed higher enhanced activity for hydrogen evolution when treated with high-pressure ammonia at more than 50MPa. The photocatalytic activity for hydrogen evolution of treated Ta3N5, related to the amount of defects, is due to reduced tantalum species. The enhancement in the activity is brought about by a decrease in surface defects, which contributes to improved electron transfer from the surface of Ta3N5 to the loaded Pt co-catalyst. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Free-standing gallium nitride (GaN) substrates are in high demand for power devices, laser diodes, and high-power light emitting diodes (LEDs). SixPoint Materials Inc. has begun producing 2" GaN substrates through our proprietary Near Equilibrium AmmonoThermal (NEAT) growth technology. In a single 90 day growth, eleven c-plane GaN boules were grown from free-standing hydride vapor phase epitaxy (HVPE) GaN substrates. The boules had an average X-ray rocking curve full width at half maximum (FWHM) of 33 ± 4 in the 002 reflection and 44 ± 6 in the 201 reflection using 0.3 mm divergence slits. The boules had an average radius of curvature of 10.16 ± 3.63 m. The quality of the boules was highly correlated to the quality of the seeds. A PIN diode grown at Georgia Tech on a NEAT GaN substrate had an ideality factor of 2.08, a high breakdown voltage of 1430 V, and Baliga's Figure of Merit of >9.2 GW/cm2. These initial results demonstrate the suitability of using NEAT GaN substrates for high-quality MOCVD growth and fabrication of high-power vertical GaN switching devices. [ABSTRACT FROM AUTHOR]