Decomposition of supercritical ammonia and modeling of supercritical ammonia–nitrogen–hydrogen solutions with applicability toward ammonothermal conditions

Ammonothermal Crystal Growth of Functional Nitrides for Semiconductor Devices: Status and Potential

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.

High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors-A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN

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.

Numerical Simulation of Ammonothermal Crystal Growth of GaN—Current State, Challenges, and Prospects

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.

Approaching Dissolved Species in Ammonoacidic GaN Crystal Growth: A Combined Solution NMR and Computational Study

Solutions of gallium trihalides GaX₃ (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (⁷¹ Ga, ³⁵ Cl, ⁸¹ Br) NMR spectroscopy. To unravel the role of pH, the analyses were done both in absence and in presence of ammonium halides, which are employed as mineralizers during ammonoacidic gallium nitride crystal growth. While gallium trifluoride and its ammoniate were found to be too sparingly soluble to give rise to a NMR signal, the spectra of solutions of the heavier halides reveal the presence of a single gallium-containing species in all cases. DFT calculations and molecular dynamics simulations suggest the identification of this species as consisting of a [Ga(NH₃ )₆ ]³⁺ cation and up to six surrounding halide anions, resulting in an overall trend towards negative complex charge. Quantitative ⁷¹ Ga NMR studies on saturated solutions of GaCl₃ containing various amounts of additional NH₄ Cl revealed a near linear increase of GaCl₃ solubility with mineralizer concentration of about 0.023 mol GaCl₃ per mol NH₄ Cl at room temperature. These findings reflect the importance of Coulombic shielding for the inhibition of oligomerization and precipitation processes and help to rationalize both the low solubility of gallium halides in neutral ammonia solution and, in turn, the proliferating effect of the mineralizer during ammonoacidic gallium nitride formation.

Ammonothermal Synthesis and Optical Properties of Ternary Nitride Semiconductors Mg-IV-N2 , Mn-IV-N2 and Li-IV2 -N3 (IV=Si, Ge)

Grimm-Sommerfeld analogous nitrides MgSiN₂ , MgGeN₂ , MnSiN₂ , MnGeN₂ , LiSi₂ N₃ and LiGe₂ N₃ (generally classified as II-IV-N₂ and I-IV₂ -N₃ ) are promising semiconductor materials with great potential for application in (opto)electronics or photovoltaics. A new synthetic approach for these nitride materials was developed using supercritical ammonia as both solvent and nitride-forming agent. Syntheses were conducted in custom-built high-pressure autoclaves with alkali metal amides LiNH₂ , NaNH₂ or KNH₂ as ammonobasic mineralizers, which accomplish an adequate solubility of the starting materials and promote the formation of reactive intermediate species. The reactions were performed at temperatures between 870 and 1070 K and pressures up to 230 MPa. All studied compounds crystallize in wurtzite-derived superstructures with orthorhombic space groups Pna2₁ (II-IV-N₂ ) and Cmc2₁ (I-IV₂ -N₃ ), respectively, which was confirmed by powder X-ray diffraction. Optical bandgaps were estimated from diffuse reflectance spectra using the Kubelka-Munk function (MgSiN₂ : 4.8 eV, MgGeN₂ : 3.2 eV, MnSiN₂ : 3.5 eV, MnGeN₂ : 2.5 eV, LiSi₂ N₃ : 4.4 eV, LiGe₂ N₃ : 3.9 eV). Complementary DFT calculations were carried out to gain insight into the electronic band structures of these materials and to corroborate the optical measurements.

Ammonothermal Synthesis of Earth-Abundant Nitride Semiconductors ZnSiN2 and ZnGeN2 and Dissolution Monitoring by In Situ X-ray Imaging

In this contribution, first synthesis of semiconducting ZnSiN₂ and ZnGeN₂ from solution is reported with supercritical ammonia as solvent and KNH₂ as ammonobasic mineralizer. The reactions were conducted in custom-built high-pressure autoclaves made of nickel-based superalloy. The nitrides were characterized by powder X-ray diffraction and their crystal structures were refined by the Rietveld method. ZnSiN₂ (a=5.24637(4), b=6.28025(5), c=5.02228(4) Å, Z=4, R_wp =0.0556) and isotypic ZnGeN₂ (a=5.46677(10), b=6.44640(12), c=5.19080(10) Å, Z=4, R_wp =0.0494) crystallize in the orthorhombic space group Pna2₁ (no. 33). The morphology and elemental composition of the nitrides were examined by electron microscopy and energy-dispersive X-ray spectroscopy (EDX). Well-defined single crystals with a diameter up to 7 μm were grown by ammonothermal synthesis at temperatures between 870 and 1070 K and pressures up to 230 MPa. Optical properties have been analyzed with diffuse reflectance measurements. The band gaps of ZnSiN₂ and ZnGeN₂ were determined to be 3.7 and 3.2 eV at room temperature, respectively. In situ X-ray measurements were performed to exemplarily investigate the crystallization mechanism of ZnGeN₂ . Dissolution in ammonobasic supercritical ammonia between 570 and 670 K was observed which is quite promising for the crystal growth of ternary nitrides under ammonothermal conditions.