Growth Of GaN Crystals Research Articles

Nucleation mechanism of GaN crystal growth on porous GaN/sapphire substrates

This paper describes the nucleation mechanism of GaN crystal growth on porous GaN/sapphire substrates. The growth behavior of epitaxially grown GaN on porous substrates is studied in detail for the first time at the nucleation stage.

Effect of additional N2O gas on the suppression of polycrystal formation and high-rate GaN crystal growth by OVPE method

One of the challenges in the oxide vapor phase epitaxy (OVPE) method is to suppress polycrystal formation for the high-rate growth of thick GaN crystals. In this study, we present the addition of N2O gas as the method to suppress the polycrystal formation in the OVPE-GaN growth. We confirmed that N2O gas could remove the Ga droplet, the origin of polycrystalline GaN. Results showed that the polycrystal density decreased by adding N2O gas. The decline in growth rate with the increase of N2O gas flow rate was smaller compared to that with H2O vapor, which was consistent with the fact that N2O would not affect directly the GaN growth reaction. Furthermore, we obtained a 512-µm-thick OVPE-GaN layer at the growth rate of 128 µm/h, which is the highest value for growing thick GaN layers in the OVPE method.

Floral design GaN crystals: low-resistive and low-dislocation-density growth by oxide vapor phase epitaxy

GaN crystal growth mode in the oxide vapor phase epitaxy (OVPE) method, which simultaneously provides low electrical resistance and low threading dislocation density (TDD), has been investigated in detail. The results clarified that these qualities can be achieved by the expression of numerous inverted pyramidal pits, called three-dimensional (3D) growth mode. This mode reduced TDD from 3.8 × 106 cm−2 to 2.0 × 104 cm−2 for 1 mm thick growth because the threading dislocations (TDs) converged to the center of each pit. Moreover, when the crystal surface after polishing was observed by photoluminescence measurement, peculiar floral designs reflecting the distribution of oxygen concentration were observed over the entire surface. In addition, the etch pits exhibited TDs in the center of each floral design. On the basis of our results, we proposed that the 3D-OVPE-GaN will serve as a key material for improving the performance of vertical GaN devices.

Two Intermediates in Ammonothermal InN Crystal Growth: [In(NH3)5Cl]Cl2 and InF2(NH2)

AbstractThe ammonothermal synthesis is a promising instrument for synthesis and crystal growth of the metastable semiconductor InN. Two novel intermediates within the ammonothermal InN crystal growth have been synthesized, namely pentaamminechloridoindium dichloride, [In(NH3)5Cl]Cl2, and indium amide difluoride, InF2(NH2). Both intermediates were obtained under ammonothermal conditions (Tmax=663 K, pmax=118.3 MPa, and Tmax=753 K, respectively) in a near‐ammononeutral milieu as established for InN crystal growth, using the respective indium trihalide together with potassium amide as mineralizer system. The crystal structure of [In(NH3)5Cl]Cl2 shows strong structural relations to the corresponding intermediate [Ga(NH3)5Cl]Cl2 within ammonoacidic GaN crystal growth. At ambient pressure, it decomposes above 450 K under release of ammonia. InF2(NH2) shows attributes of a classical MX3 crystal structure, realizing the motif of a hexagonal close packing of (NH)2− together with F− ions and indium occupying one third of the octahedral holes between every layer of the packing. Vibrational spectroscopy for both solid intermediates is reported.

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.

Boundary Conditions for Simulations of Fluid Flow and Temperature Field during Ammonothermal Crystal Growth—A Machine-Learning Assisted Study of Autoclave Wall Temperature Distribution

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.

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.

High-rate OVPE-GaN growth by the suppression of polycrystal formation with additional H2O vapor in a high-temperature condition

The oxide vapor phase epitaxy (OVPE) method enables the fabrication of low-resistivity GaN crystals. However, polycrystal formation is detrimental to the growth of thick GaN crystals at the high growth rate by the OVPE method. In this study, we focused on H2O additive under the high-rate and high-temperature growth condition to suppress generation of Ga droplet, the origin of polycrystalline GaN. The polycrystal density dramatically decreased by adding H2O, which can convert Ga droplet into Ga2O vapor. We obtained a 431 μm thick OVPE-GaN layer at 108 μm h−1, which is the highest value in GaN crystals grown by the OVPE 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.

Recent progress in basic ammonothermal GaN crystal growth

Progress in basic ammonothermal gallium nitride crystallization is reported. In order to prevent the appearance of cracks in growing GaN a new shape of native seeds is proposed. Crystallization results on irregular (old) and round (new) shapes are compared in terms of growth rate and structural quality. The new type of seeds allowed to grow crystals twice as fast without losing the very high structural quality. Etch pit density in the new-grown material is examined and compared to that in the seeds. Material deposited in vertical <000–1> and lateral <11–20> directions is analyzed. In the latter case the etch pit density is as low as 102 cm−2. An influence of growth process parameters and crystal growth configuration on the structural quality of crystallized GaN is discussed.

Effects of Cooling Process on GaN Crystal Growth by Na Flux Method

The effects of cooling on the yield (percentage of Ga in the original melt that is incorporated into the GaN crystal) and morphology of GaN crystals grown by the Na flux method have been investigated. Yields of pyramidal GaN crystals of up to 94.9% were obtained by spontaneous crystallization using the slow cooling method. A lower cooling rate contributed to higher yield of GaN crystals. Scanning electron microscopy observations indicated that the crystalline perfection increased as the cooling rate was decreased. Instabilities such as macrosteps, cellular structures, and hopper crystals (a form of crystal defined by its “hoppered” shape) were observed after crystal growth at higher cooling rates. These results suggest ways to improve the yield and control the growth of perfect GaN crystals.

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

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

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

Solutions of gallium trihalides GaX 3 (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (71Ga, 35Cl, 81Br) 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 71Ga NMR studies on saturated solutions of GaCl3 containing various amounts of additional NH4Cl revealed a near linear increase of GaCl3 solubility with mineralizer concentration of about 0.023 mol GaCl3 per mol NH4Cl 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.

Modeling of bulk GaN crystal growth from gallium-sodium solution

Abstract A modeling approach for bulk GaN growth from gallium-sodium liquid solutions is suggested. The approach combines the thermodynamic theory of non-ideal solutions with kinetics of the chemical reactions proceeding at the free surface of the solution and at the GaN growth interface. The atomic nitrogen solubility in the solution is verified using available experimental data. The N-solubility and GaN growth rate are studied as a function of the sodium fraction in the solution and the nitrogen gas pressure. Growth of laboratory-scale (

Tungsten carbide layers deposited on graphite substrates via a wet powder process as anti-parasitic-reaction coatings for reactor components in GaN growth

Catalytic tungsten carbide coatings provide multi-functional (w/ anti-parasitic-reaction and protective functions) reactor components for growth of large GaN crystals.

Hydride Vapor‐Phase Epitaxy Reactor for Bulk GaN Growth

An hydride vapor‐phase epitaxy reactor for the growth of bulk GaN crystals with a diameter of 50 mm is developed. Growth rate nonuniformity of 1% is achieved using an axisymmetric vertical gas injector with stagnation point flow. Chemically resistant refractory materials are used instead of quartz in the reactor hot zone. High‐capacity external gallium precursor sources are developed for the nonstop growth of the bulk GaN layers. A load‐lock vacuum chamber and a dry in situ growth chamber cleaning are implemented to improve the growth process reproducibility. Freestanding GaN crystals with a diameter of 50 mm are grown with the reactor.

(Invited) Basic Ammonothermal Growth of Bulk GaN Single Crystal Using Sodium Mineralizers

GaN is a promising semiconductor of wide band gap of 3.4 eV at room temperature and can be used as a substrate material for blue/green/UV light-emitting diode (LED) and laser diode (LD) [1]. Sapphire has been the most widely used substrate for manufacturing GaN-based LEDs. However, GaN films grown on sapphire substrates include many critical defects owing to the large lattice mismatch (~15%) and large difference of thermal expansion coefficients (~25%) between GaN and sapphire. Thus, GaN substrates are needed for fabrication of the high power LEDs and LDs. Two typical routes, ammono-basic and ammono-acidic, have been studied in the ammonothermal growth of GaN crystals. Addition of alkali (Li, Na, and K) elements to ammonia typically results in a basic ammonia solution, whereas addition of halogens (F, Cl, Br, and I) to ammonia typically results in an acidic ammonia solution. The acidic ammonothermal method has a fast crystal growth rate but requires installation of Pt liner due to the corrosive environment. Thus, the basic ammonothermal method is regarded as more appropriate for mass production of GaN crystals [2]. Only a few types of basic mineralizer including NaNH2 [3] and KNH2 [4] have been used previously for the crystal growth of GaN. The reports of comparison with other sodium-based mineralizers for ammonothermally grown GaN crystals have not to be achieved. Potassium mineralizers were not included in this study because they are more sensitive to oxygen and moisture and corrosive to autoclave compared to sodium mineralizers. In this study, we demonstrate the growth rate, crystal quality, and surface morphology of the as-grown GaN single crystals according to sodium amide, sodium azide, and sodium metal as mineralizer. Also, we describe the morphology and properties of approximately 2 inch GaN crystals grown by ammonothermal method using sodium metal mineralizer. Reference [1] S. Nakamura, G. Fosol, The Blue Laser Diode, Springer, Berlin, 1997, chap. 1. [2] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 311 (2009) 3015. [3] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, J. Cryst. Growth 310 (2008) 3907. [4] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 310 (2008) 3911.

Computational fluid dynamics simulation study of the gas flow balance in a vertical HVPE reactor with a showerhead for low cost bulk GaN crystal growth

In order to make low cost bulk GaN single crystal with a high growth rate, we have built a vertical HVPE reactor with a showerhead configuration. The flow from the showerhead of the reactor is independently controlled by inner/outer two sets of gas supply lines. This special showerhead design makes the flow model different from the conventional showerhead reactors. We have employed a finite element-based simulator to study the fluid dynamic and crystal growth of this inner/outer flow independently controlled showerhead. Two simulation experiments were performed. One experiment has demonstrated that both growth rate and Ga yield can be improved without compromising the uniformity by controlling the different input parameters of inner and outer flow on the showerhead, as compared with the conventional showerhead. The other experiment has demonstrated that the showerhead can be further optimized by changing the inner/outer area ratio and input parameters.

The morphologies of GaN crystals grown on Ga- and N-face of HVPE seeds by the Na flux liquid phase epitaxial method

GaN crystal growth on Ga-face and N-face of HVPE-GaN seeds was investigated using Na flux liquid phase epitaxial (LPE) method, respectively. The phase structure and morphology of as-grown GaN crystals were characterized using X-ray diffraction (XRD) and the scanning electron microscope (SEM), respectively. Under the growth temperature of 850 °C and keeping nitrogen pressure 3.0 MPa for 150 h, the typical pyramidal GaN poly-crystals with wurtize structure grew by spontaneous nucleation growth on the Ga-face of GaN -HVPE seeds. In contrast, GaN single crystals could be obtained by epitaxial growth on the N-face of GaN-HVPE seeds. From the observation of the side view of as-grown GaN crystals grown on the N-face, there were two apparent stages along the growth direction (-c direction) during the crystal epitaxial growth process. The typical hexagonal layered structure of as-grown GaN crystals occurred in the initial stage, and then the typical hexagonal pyramidal structure of it occurred in the second growth stage. In addition, from the observation of optical microscope, the as-grown GaN crystal with the initial growth stage only presented different morphology from that with the two-growth stage. The continuous lateral equilibrium epitaxial growth on N-face of GaN crystal seed may be of great significance for the growth of large size GaN single crystal.

GaN crystals growth in the Na-Li-Ca flux by liquid phase epitaxy (LPE) technique

10 × 10 mm2 area GaN crystals were successfully grown on HVPE-GaN seed by LPE technique in Na-Li-Ca flux. Many growth hillocks with hexagonal shape were formed on the surface of crystal when the growth time was 150 h. Due to the lateral growth, the smooth Ga-face could be obtained after growth 300 h. The dislocation density could be decrease from 107 cm−2 to 103 cm−2 with increasing GaN crystal growth thickness. The reduction mechanism of dislocation was discussed. The PL spectra showed that the relative intensity of the yellow band luminescence peak of GaN crystals was obviously reduced comparing with HVPE-GaN seed, which indicated that the impurity defects in the crystal could be effectively suppressed.