Na-flux Method Research Articles

Analysis of GaN crystal growth mechanism in liquid-phase epitaxial Na-flux method

Analysis of GaN crystal growth mechanism in liquid-phase epitaxial Na-flux method

Studying the effect of temperature and pressure on GaN crystals via the Na-flux method

GaN single crystals (15 × 15 mm) were epitaxially grown using the Na-flux method, and substrate self-peeling was achieved. The effect of temperature and pressure on the growth rate and the quality of GaN single crystals are investigated.

Suppression of polycrystal nucleation by methane addition at moderate timing to maintain GaN crystal growth on point seeds in the Na-flux method

We have grown low-dislocation, large-diameter GaN substrates using small seed crystals, called as point seeds (PSs) in the Na-flux method. To further reduce dislocation density and improve wafer bowing, reducing the PS diameter is effective. However, it was found that crystals did not start to grow on a small diameter PS. In this study, we also found that a growth on a small diameter PS can be carried out in the condition without graphite addition. However, graphite is conventionally added to suppress polycrystals, so those could not be avoided at the latter process to planarize crystal surface by raising and lowering the crystal, called the flux film coated (FFC) process, in which polycrystals are likely generated. Thus, in this study, we newly proposed to suppress polycrystals by adding methane gas before the FFC process, instead of graphite which cannot be added during the middle of growth.

Preparation of a freestanding GaN substrate in the Na-flux method by laser-assisted separation

We have fabricated large-diameter, high-quality gallium nitride (GaN) substrates by the multipoint seed (MPS) technique in the Na-flux method. To obtain crack-free freestanding GaN crystals, in this study we employed laser-assisted separation (LAS) as a new technique to separate GaN from a sapphire substrate. In LAS, the GaN crystal around the GaN-sapphire interface is partially decomposed by irradiating a laser onto the seed substrate before growth. Since this technique reduces the contact area between the sapphire and GaN, separation occurs spontaneously during the cooling process after growth. We found an appropriate LAS processing pattern for separation and successfully obtained freestanding GaN crystal without cracks. By combining LAS with the MPS method, we succeeded in growing crack-free crystals even in film thicknesses in which cracks occur in conventional MPS substrates.

Reducing GaN crystal dislocations through lateral growth on uneven seed crystal surfaces using the Na-flux method

In recent years, we have achieved low threading dislocation density in GaN wafers by using the Na-flux multi-point seed technique. However, the resulting wafers exhibit regions of high dislocation density, exceeding 105 cm−2 at the coalescence boundary where pyramidal crystals merge. In this study, we discovered that annealing seed crystals at 900 °C generated an uneven surface with approximately 50 μm of GaN decomposition, and growing GaN on the thus-generated uneven surfaces induced lateral growth composed of facets. We then investigated the effect of the uneven surface on dislocation reduction and found that the average threading dislocation density of the grown crystal was reduced from cm−2 in the seed crystal to cm−2. We confirmed that the reduction in threading dislocation density was due to the termination of dislocations by means of inclusions, and to mergers or annihilation as they encountered one another during facet growth.

Numerical Simulation of Temperature Field Optimization to Enhance Nitrogen Transfer in GaN Crystal Growth by the Na-Flux Method.

Improved nitrogen transport is crucial for enhancing the growth rate of GaN crystals using the Na-flux method. This study investigates the nitrogen transport mechanism during the growth of GaN crystals by the Na-flux method using a combination of numerical simulations and experiments. The results indicate that the temperature field affects the effect of nitrogen transfer, and we propose a novel bottom ring heating approach to optimize the temperature field and enhance nitrogen transfer during the growth of GaN crystals. The simulation results demonstrate that optimizing the temperature field improves nitrogen transfer by causing convection within the melt to float up from the crucible wall and sink at the crucible center. This enhancement improves the nitrogen transfer from the gas-liquid interface to the GaN crystal growth surface, thereby accelerating the growth rate of GaN crystals. Additionally, the simulation results indicate that the optimized temperature field substantially reduces polycrystalline generation at the crucible wall. These findings are also a realistic guide to the growth of other crystals in the liquid phase method.

Monitoring of GaN crystal growth rate in the Na flux growth via electroresistometry of Ga-Na solution

Monitoring the GaN crystal growth process in the Na-flux method has been difficult due to the challenging growth process, namely high temperature and high pressure and corrosive nature of Ga and Na. This paper describes the unique method to monitor GaN crystal growth by measuring the electrical resistance of Ga-Na solution. We found that the electrical resistance of Ga-Na solution decreased along with the crystal growth. Furthermore, the decreasing rate of resistance is proportional to growth pressure, which influences the growth rate. During the growth process, Ga in the solution is consumed, reducing the Ga/Na ratio of the Ga-Na solution. The compositional shift toward Na-rich resulted in the decrease of electrical resistance.

Promotion of GaN Crystal Growth with Pre-stirring Using the Na-Flux Method

Promotion of GaN Crystal Growth with Pre-stirring Using the Na-Flux Method

Yellow-Green Luminescence Due to Polarity-Dependent Incorporation of Carbon Impurities in Self-Assembled GaN Microdisk.

Yellow-green luminescence (YGL) competes with near-bandgap emission (NBE) for carrier recombination channels, thereby reducing device efficiency; yet uncovering the origin of YGL remains a major challenge. In this paper, nearly stress-free and low dislocation density self-assembled GaN microdisks were synthesized by Na-flux method. The YGL of GaN microdisks highly depend on their polar facets. Variable accelerating voltage/power CL, variable temperature PL, and Raman spectroscopy are further performed to clarify the origin of polarity dependence of GaN microdisk YGL behavior, which indicates its independence of dislocations, surface effects, stress, crystalline quality, and gallium vacancies. It was found that the incorporation ability of carbon impurities in the polar (0001) facet is greater than that in the semipolar (101̅1) facets, producing higher content of CN or CNON defects, resulting in a more pronounced YGL in the polar (0001) facet of GaN.

Effect of crucible location on heat transfer in GaN crystal growth using Na flux method

A global model of heat transfer, considering convection, conduction, radiation as well as mass transfer, is established for GaN single crystal growth system employing the Na flux method. Global simulations are carried out to investigate the effects of crucible locations on the heat and mass transfer during growing. It was found that the crucible location has a significant effect on the heat transfer in the melt. When the crucible moves down, the flow in the upper region of melt is enhanced, while that near the crucible bottom is weaken. This flow structure leads to high supersaturation at the crucible bottom, which is favorable to accelerate crystallization. Therefore, it can be concluded that the crucible location should be lowered properly to achieve a better condition for crystallization. The research provides directions to the design of GaN growing systems.

Suppression of newly generated threading dislocations at the regrowth interface of a GaN crystal by growth rate control in the Na-flux method

Abstract In a previous study, we successfully obtained a large-diameter, low-dislocation-density GaN wafer using the flux-film-coated and the multi-point-seed technique (FFC-MPST). As a production cost-cutting strategy, we are aiming to reuse a part of grown GaN crystals and produce thicker films by the Na-flux regrowth. Recently, however, it was found that threading dislocations (TDs) were generated at the growth interface in homoepitaxial growth of GaN crystals by the Na-flux method. In this study, we found that rapid growth in the regrowth contributes to the formation of inclusions causing the generation of TDs at the regrowth interface. Hence, we succeeded in suppressing the generation of TDs by a low growth rate, realized by a low-pressure condition at an initial growth stage. These findings are valuable for the productivity enhancement of high-quality GaN wafers and help the widespread of GaN-based devices.

A study on the growth process for liquid phase epitaxy of GaN crystal using Na–Li–Ca flux

The liquid epitaxial growth process on the HVPE-GaN seed of GaN crystal was investigated using the Na–Li–Ca flux method. The phase structure and morphology of as-grown GaN crystals were characterized by X-ray diffraction (XRD) and the scanning electron microscope (SEM), respectively. Under the growth time of 70h, a large area of dissolution morphology could be observed accompanied by a decrease in the thickness of the HVPE-GaN seed. At the bottom of the crucible, the thinning of the seed crystal thickness was the most obvious. For the growth time of 150h, there were two kinds of appearances along the height direction, presenting gathered ridges in the upper part and cellular organizations in the middle and bottom parts. For 420h, the as-grown crystal presented a typical hexagonal layered morphology and the side face developed outward forming regular hexagonal crystallographic planes. With prolonging time from 150h to 420h, the LPE growth shifted from non-equilibrium growth mode with an unstable cellular growth surface morphology to near-equilibrium growth mode with a typical hexagonal layered growth morphology. The thickness of as-grown crystal showed a decrease trend from the gas-liquid interface to the bottom of crucible. To get near-equilibrium growth mode for the whole crystal plate in Na-flux method, the promotion of N dissolving, GaN growth unit diffusion is the key issue.

Growth behavior and stress distribution of bulk GaN grown by Na-flux with HVPE GaN seed under near-thermodynamic equilibrium

We used Na-flux method to grow Ga-polar GaN (Ga-GaN) and N-polar GaN (N-GaN) on a double-sided epitaxially polished HVPE-GaN seed. The growth behavior and stress distribution of Ga and N-polar GaN grown by Na-flux method was systematically studied for the first time. Under near-thermodynamic equilibrium growth conditions, the growth rate of the N-polar plane is higher than that of the Ga-polar plane, which is related to the high surface energy of the N-polar plane. The lower overall stress level of Ga-GaN/N-GaN bulk single crystals is confirmed by Raman spectroscopy and low-temperature PL. Three-dimensional atom probe tomography (3D-APT) directly proves that C and O impurities are enriched at the atomic level at the growth interface, resulting in the mismatch of the interface lattice, resulting in compressive stress and reduction of free carriers. Under near-thermodynamic equilibrium growth conditions, the dislocation density of GaN bulk single crystals can be further reduced. Ga-GaN and N-GaN dislocation density values were 4 × 105/cm2 and 8 × 105/cm2, respectively. The mechanism of dislocation reduction is discussed. In addition, we can achieve control of stress and free carriers by adjusting the behavior of impurity incorporation.

Study on the stress and mechanism of self-separated GaN grown by Na-flux method

A 2 inch free-standing c-plane GaN wafer was fabricated through in situ self-separation using HVPE-seed crystal etching back (HCEB) by intentionally adjusting the nitrogen pressure in the Na-flux growth process of GaN. First, adjust the nitrogen pressure in the reactor to a lower level to facilitate HCEB to form a large number of voids at the interface between the c-plane HVPE seed and the c-plane Na-flux GaN. After regrowth of approximately 340 μm thick Na-flux GaN, self-separation was achieved during the cooling process. The free-standing GaN wafer was almost stress-free as a result of strain relief by the in situ self-separation process, which was confirmed by room-temperature Raman and low-temperature photoluminescence measurements. It is supposed that the HCEB process can be applied to fabricate high-quality free-standing GaN wafers in the future.

Analysis on the dislocation of GaN on different substrate

Though there are still disputes on whether the threading dislocation density (TDD), to produce high quality Gallium Nitride ( GaN ) with low TDD is needed. In this passage, we would first show the impact of dislocation and review the latest methods to reduce dislocation on Si, GaN, and sapphire. The Si substrating with serpentine channels could produce GaN by using Si substrate with low density dislocations. The method that use the markless-3D provides a new way to reduce the dislocation from big off-angle. The multiple-step growth technique controls the conditions of crystallization and use two steps of crystallizing to grow with both low dislocation density and 2D growth shape. The last method is to use the Na-flux method to exclude cracks with sapphire in crystal.

Fabrication of High-Quality and Large-Diameter GaN Wafer by the Na-Flux Method with a Point Seed Technique

Fabrication of High-Quality and Large-Diameter GaN Wafer by the Na-Flux Method with a Point Seed Technique

Fe-doped GaN grown on stirring-assisted Na-flux process GaN thick film and its application on Ohmic contact UV detector

Abstract Na-flux method is the most promising approach to grow GaN thick film substrate for GaN homoepitaxy. Here, the influence of solution stirring on grown GaN crystal morphology, crystal quality, dislocation density and impurity concentration was assessed for GaN crystal growth by Na-flux method. It is proved that the crystallinity, surface roughness and dominated orientation of the GaN thick layers are greatly influenced by stirring process, revealing a direct and correlated growth mechanism for the growth of GaN using Na-flux method. Notably, the surface roughness was reduced by three orders of magnitude under stirring (compared to un-stirred sample). This stirring-assisted GaN thick film was then used as seed layer to grow high resistance Fe-doped GaN epitaxial layer by applying HVPE method. On which Ohmic contact was successfully fabricated and an UV-detector with a high detectivity of ~1013 Jones, which was among the highest values, and fast response (rise time of 100 ms, decay time of 160 ms) was obtained.

Stress evolution in different growth mechanism of GaN grown by Na-flux method

The study found that homoepitaxial Na-flux GaN has a large stress at the interface, and the stress is released to a certain extent within 50 μm in the growth direction of the Na-flux GaN. After passing through the columnar growth region, the Na-flux GaN tends to a stress-free state finally. The columnar growth mode is produced by GaN island growth, the islands nucleate and coalescence to produce tensile stress. The Na-flux GaN undergoes a columnar growth to generate tensile stress, which offsets the residual compressive stress at the interface, which is conducive to stress release.

Anomalous dislocation annihilation behavior observed in a GaN crystal grown on point seeds by the Na-flux method

We recently invented a method called the flux-film-coated technique for purifying a GaN wafer with low dislocation density grown from point-seed crystals. In this study, we investigated the mechanism behind the reduction of dislocation density in the GaN wafer by evaluating the three-dimensional behavior of dislocations using multiphoton-excitation photoluminescence images. We made the surprising discovery that dislocations more than 50 μm away disappeared by annihilating each other as growth proceeded, and this is one of the mechanisms underlying the dislocation density reduction. The moving distance of dislocations before annihilation is uncommon and a unique phenomenon in the Na-flux method.

High‐Quality GaN Crystal Growth Using Flux‐Film‐Coated LPE with Na Flux

AbstractFlux‐film‐coated liquid phase epitaxy (FFC‐LPE), in which a liquid Ga–Na film is coated on a GaN surface, is used for the synthesis of GaN crystals. The addition of a minor amount of additives of group II elements (Ca and Sr) shows the effects that the inclusions are eliminated from a GaN crystal while significantly reducing dislocation density during FFC‐LPE. By performing FFC‐LPE growth with Ca and Sr additives, the wettability of the seed crystal and the flux is greatly improved, resulting in the achievement of the single molecule step growth mode. Inclusions that are inevitably included in the GaN crystal through the conventional Na flux method can be prohibited by the realization of this growth mode. The dislocation density of the seed–GaN decreases to 2.5 × 106 cm−2 (<1/100th of the seed crystal) after the growth thickness of 10 µm by means of the FFC‐LPE method.