Search Results (34)

This study presents a computational fluid dynamics (CFD) simulation to investigate fluid flow and heat transfer within the growth zone of gallium nitride crystals synthesized via the alkaline ammonothermal method, with particular emphasis on the influence of seed crystal arrangement and installation geometry. The model analyzes temperature and velocity distributions, highlighting how seed configuration affects turbulent and transitional flow behavior. Key findings demonstrate the effectiveness of CFD in evaluating and optimizing growth zone design. Both simulation and experimental results show that achieving more uniform flow and temperature fields leads to more consistent growth rates and improved structural crystal quality. Furthermore, the study underscores the critical role of installation geometry in shaping flow characteristics such as velocity distribution, temperature gradients, and their transient fluctuations, factors essential for optimizing the ammonothermal crystallization process. Full article

A computational fluid dynamics simulation was developed for the growth zone of gallium nitride crystallized using the alkaline ammonothermal method, considering the geometry of the seed crystals and the installation setup. The model focuses on temperature and velocity distributions, revealing turbulent and transient flow characteristics. Significant findings include the effect of crystal thickness on temperature and velocity changes, as well as the relationship between temperature distribution and growth rate. The results indicate that transient variations in flow and thermal fields affect the uniformity of growth and structural quality of the crystals. The paper contributes to optimizing ammonothermal crystallization processes by addressing critical parameters such as turbulence, thermal mixing, and crystal geometry. Full article

We investigated the carrier dynamics of ammonothermal Mn-compensated gallium nitride (GaN:Mn) semiconductors by using sub-bandgap and above-bandgap photo-excitation in a photoluminescence analysis and pump–probe measurements. The contactless probing methods elucidated their versatility for the complex analysis of defects in GaN:Mn crystals. The impurities of Mn were found to show photoconductivity and absorption bands starting at the 700 nm wavelength threshold and a broad peak located at 800 nm. Here, we determined the impact of Mn-induced states and Mg acceptors on the relaxation rates of charge carriers in GaN:Mn based on a photoluminescence analysis and pump–probe measurements. The electrons in the conduction band tails were found to be responsible for both the photoconductivity and yellow luminescence decays. The slower red luminescence and pump–probe decays were dominated by Mg acceptors. After photo-excitation, the electrons and holes were quickly thermalized to the conduction band tails and Mg acceptors, respectively. The yellow photoluminescence decays exhibited a 1 ns decay time at low laser excitations, whereas, at the highest ones, it increased up to 7 ns due to the saturation of the nonradiative defects, resembling the photoconductivity lifetime dependence. The fast photo-carrier decay time observed in ammonothermal GaN:Mn is of critical importance in high-frequency and high-voltage device applications. Full article

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. Full article

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 10¹⁸ and 2 × 10¹⁹ cm⁻³ and resistivities of ~10¹¹ Ωcm and ~10⁶ Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed. Full article

One of the key issues in GaN-based devices is the resistivity and technology of ohmic contacts to n-type GaN. This work presents, for the first time, effective intentional oxygen doping of sputtered GaN films to obtain highly conductive n⁺-GaN:O films. We have developed a novel and simple method to obtain these films. The method is based on the room temperature magnetron sputtering of a single crystal bulk GaN target doped with oxygen. The n⁺-GaN:O films exhibit a polycrystalline structure with a crack-free surface and a free electron concentration of 7.4 × 10¹⁸ cm³. Ohmic contact to GaN:Si with n⁺-GaN:O sub-contact layer achieves specific contact resistance of the order of 10⁻⁵ Ωcm² after thermal treatment. The obtained results are very promising for the development of the technology of a whole new class of ohmic contacts to n-GaN. Full article

Gallium nitride continues to be a material of intense interest for the ongoing advancement of electronic and optoelectronic devices. While the bulk of today’s markets for low-performance devices is still met with silicon and blue/UV LEDs derived from metal–organic chemical vapor deposition gallium nitride grown on foreign substrates such as sapphire and silicon carbide, the best performance values consistently come from devices built on bulk-grown gallium nitride from native seeds. The most prominent and promising of the bulk growth methods is the ammonothermal method of high-pressure solution growth. The state-of-the-art from the last five years in ammonothermal gallium nitride technology is herein reviewed within the general categories of growth technology, characterization and defects as well as device performance. Full article

A detailed analysis of morphology of gallium nitride crystal growth obtained by ammonothermal and halide vapor phase epitaxy methods was carried out. The work was conducted to determine the source of triangular planar defects visible in X-ray topography as areas with locally different lattice parameters. It is shown that the occurrence of these defects is related to growth hillocks. Particular attention was paid to analyzing the manner and consequences of merging hillocks. In the course of the study, the nature of the mentioned defects and the cause of their formation were determined. It was established that the appearance of the defects depends on the angle formed between the steps located on the sides of two adjacent hillocks. A universal growth model is presented to explain the cause of heterogeneity during the merging of growth hillocks. Full article

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. Full article

This paper presents low-temperature measurements of magnetoresistivity in heavily doped n-type GaN grown by basic GaN growth technologies: molecular beam epitaxy, metal-organic vapor phase epitaxy, halide vapor phase epitaxy and ammonothermal. Additionally, GaN crystallized by High Nitrogen Pressure Solution method was also examined. It was found that all the samples under study exhibited negative magnetoresistivity at a low temperature (10 K < T < 50 K) and for some samples this effect was observed up to 100 K. This negative magnetoresistivity effect is analyzed in the frame of the weak localization phenomena in the case of three-dimensional electron gas in a highly doped semiconductor. This analysis allows for determining the phasing coherence time τ_φ for heavily doped n-type GaN. The obtained τ_φ value is proportional to T^−1.34, indicating that the electron–electron interaction is the main dephasing mechanism for the free carriers. Full article

In this paper, we investigate, using X-ray Bragg diffraction imaging and defect selective etching, a new type of extended defect that occurs in ammonothermally grown gallium nitride (GaN) single crystals. This hexagonal “honeycomb” shaped defect is composed of bundles of parallel threading edge dislocations located in the corners of the hexagon. The observed size of the honeycomb ranges from 0.05 mm to 2 mm and is clearly correlated with the number of dislocations located in each of the hexagon’s corners: typically ~5 to 200, respectively. These dislocations are either grouped in areas that exhibit “diameters” of 100–250 µm, or they show up as straight long chain alignments of the same size that behave like limited subgrain boundaries. The lattice distortions associated with these hexagonally arranged dislocation bundles are extensively measured on one of these honeycombs using rocking curve imaging, and the ensemble of the results is discussed with the aim of providing clues about the origin of these “honeycombs”. Full article

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. Full article

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. Full article

This paper reviews the near-equilibrium ammonothermal (NEAT) growth of bulk gallium nitride (GaN) crystals and reports the evaluation of 2″ GaN substrates and 100 mmbulk GaN crystal grown in our pilot production reactor. Recent progress in oxygen reduction enabled growing NEAT GaN substrates with lower residual oxygen, coloration, and optical absorption. The oxygen concentration was approximately 2 × 10¹⁸ cm⁻², and the optical absorption coefficient was 1.3 cm⁻¹ at 450 nm. Maps of full-width half maximum (FWHM) of X-ray diffraction rocking curveswere generated for grown crystals and finished wafers. The X-ray rocking curve maps confirmed high-quality and uniform microstructure across the entire surface of the bulk crystals and substrates. The average FWHM of the 50 best bulk crystals from the recent batch was 28 ± 4 arcsec for the 002 diffraction and 34 ± 5 arcsec for the 201 diffraction, with an average radius of curvature of 20 m. X-ray topography measured on both sides of the bulk crystals implied that the density of dislocations wasreduced by one order of magnitude during the NEAT growth. A typical NEAT GaN substrate shows dislocation density of about 2 × 10⁵ cm⁻². Full article

Titanium zirconium molybdenum (TZM) is a high strength at high temperature alloy with favorable properties for use in high temperature structural applications. Use of TZM in high pressure, gas-containing autoclave systems was recently demonstrated for the ammonothermal method. Use of indium (In) in the system is desired, though there is a general lack of literature and understanding on the corrosion and impact of In on the mechanical properties of TZM. This study reports for the first time the mechanical properties of TZM after exposure to metallic In at temperatures up to 1000 °C. Static corrosion testing of TZM in In were performed at 750 °C and 1000 °C for 14 days. A microstructure analysis was performed suggesting no visible alteration of the grain structure. Differential thermal analysis (DTA) was performed to investigate compound formation between In and the primary constituents of TZM yielding no measurable reactions and hence no noticeable compound formation. X-ray energy dispersive spectroscopy (EDS) line scans across the TZM-In interface revealed no measurable mass transport of In into the TZM matrix. These results were confirmed using X-ray diffraction (XRD). Given the apparent inertness of TZM to In, mechanical properties of TZM after exposure to In were measured at test temperatures ranging from 22 °C to 800 °C and compared to unexposed, reference TZM samples from the same material stock. Tensile properties, including ultimate tensile strength, yield strength and total elongation, were found to be comparable between In-exposed and unexposed TZM samples. Impact fracture toughness testing (Charpy) performed at temperatures ranging from −196 °C to 800 °C showed that TZM is unaffected upon exposure to In. Tensile testing indicated ductile behavior at room temperature (slow strain rate) whereas impact testing (high strain rate) suggested a ductile to brittle transition temperature between 100 °C and 400 °C. Given these results, TZM appears to be a promising candidate for use as a force bearing material when exposed to In at high temperature. Full article