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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]

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. [ABSTRACT FROM AUTHOR]

The effects of GaN quantum barriers with changing growth temperatures on the interfacial characteristics of GaN/InGaN single quantum well (SQW) grown on GaN templates by metalorganic vapour phase epitaxy were in situ investigated by X-ray crystal truncation rod (CTR) scattering and X-ray reflectivity measurements at growth temperature using a laboratory level X-ray diffractometer. Comparing the curve-fitting results of X-ray CTR scattering spectra obtained at growth temperature with that at room temperature, the InxGa1-xN with indium composition less than 0.11 was stabile of the indium distribution at the interface during the whole growth processes. By using several monolayers thickness GaN capping layer to protect the InGaN well layer within temperature-ramping process, the interfacial structure of the GaN/InGaN SQW was drastically improved on the basis of the curve-fitting results of X-ray CTR scattering spectra, and the narrow full width at half-maximum and strong luminous intensity were observed in room temperature photoluminescence spectra. [ABSTRACT FROM AUTHOR]

The oxygen incorporation kinetics of vicinal m(10−10) gallium nitride (GaN) growth during metal‐organic vapor phase epitaxy is clarified using a diffusion equation‐based approach that incorporates diffusion potentials obtained by large‐scale density functional theory (DFT) calculations. A diffusion model based on the Burton, Cabrera and Frank (BCF) theory is proposed, and then, the oxygen concentration in the epitaxial films is calculated quantitatively. The calculation results agree with the experimental tendency that the oxygen concentration in the −c 5° off m‐GaN epilayers is lower than that in the +c 5° off m‐GaN epilayers. Then, the off‐angle dependence of oxygen incorporation in vicinal m‐GaN growth is predicted. [ABSTRACT FROM AUTHOR]

The growth of GaN 3-D microstructures is investigated by SAG-HVPE. Capitalizing on the properties of this kinetically-controlled process, the main experimental parameters and physical mechanisms that control the shaping of 3D GaN prisms and pyramids in SAG-HVPE are highlighted. Growth experiments performed on N-polar AlN/Si(100) and Ga-polar GaN/Si(111) substrates also provide insight into how to switch from a pyramid to a prismatic shape for a given substrate polarity. The aspect ratio of GaN rods could be tuned by playing with the HCl partial pressure additionally introduced during growth. The influence of both mass transport and surface kinetics is discussed, as the crystal growth rate varies with increasing surface area as time goes by. Ammonia treatment prior to the growth, aimed at blocking the r planes thanks to H2 passivation, is proposed to tune the morphology of the GaN rods. Raman spectroscopy performed on individual GaN rods shows no relevant strain field and no structural differences between the rods and state-of-the-art bulk GaN. [ABSTRACT FROM AUTHOR]

We investigate the influence of crystal defects on p-type GaN grown by metalorganic vapor phase epitaxy. Sets of p-type GaN films were grown on sapphire substrates and on free-standing GaN (F-GaN) substrates simultaneously using various Et-Cp2Mg flow rates. Although there is a difference of two orders of magnitude between the threading dislocation densities of p-type GaN grown on sapphire and F-GaN substrates, there is no significant difference in hole concentration. However, there are problems with the surface morphology of p-type GaN grown on sapphire. The deterioration of the surface was caused by the difference in nanopipe density. The electrical properties of a p-n junction diode formed on sapphire with a high density of nanopipes were observed using emission microscopy under both forward- and reverse-bias conditions. Our results demonstrate that the nanopipes are electrically inactive, and that other types of threading dislocation have more influence on the current-voltage characteristics. [ABSTRACT FROM AUTHOR]

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]

Highlights • Ga-polar untwinned (1 0 −1 3) GaN was obtained by directional sputtering. • FWHM values are less than 550 aresec. • Al nucleation layer in directional sputtering affects the GaN layer much. Abstract Directional sputtering of Al and AlN on (1 0 −1 0) sapphire was used to obtain metal-polar (1 0 −1 3) templates. After overgrowth with AlN and GaN using metal-organic vapor phase epitaxy, we obtained untwinned (1 0 −1 3) GaN layers. Full width at half maximum of the X-ray rocking curve of symmetric (1 0 −1 3) GaN is less than 550 arcsec along both [3 0 −3 −2] GaN and [1 −2 1 0] GaN directions. Ga-polarity of the layers was confirmed by high-resolution scanning transmission electron microscopy. Careful optimization of time and temperature of the initial Al sputtered layer was a key parameter to achieve high quality GaN templates. [ABSTRACT FROM AUTHOR]

Continuous in situ X-ray reflectivity (XRR) measurements were used to investigate the growth process of an In 0.11 Ga 0.89 N epilayer and its single quantum well grown on c-plane GaN/sapphire templates using an in-house-designed metalorganic vapor phase epitaxy installed in a laboratory-grade X-ray diffractometer. The surface roughening of the epilayer as a function of growth time was calculated from the continuous in situ XRR curve. The growth rate, critical thickness h c(r) for surface roughening, and roughening rate were obtained. The experimental critical thickness h c(r) of the In 0.11 Ga 0.89 N epilayer analyzed from the continuous in situ XRR curve was 14.8±0.4 nm. Based on the calculated theoretical critical thickness h c and the experimental h c(r,2) , Fischer’s model seems to be appropriate for describing the critical thickness of the InGaN/GaN. [ABSTRACT FROM AUTHOR]

Abstract: A novel activation process for Mg-implanted GaN was demonstrated. As opposed to the conventional thermal annealing process, an H2/NH3 alternating supply annealing process achieved better optical activation, stronger near-ultraviolet luminescence and weaker yellow luminescence in the photoluminescence spectroscopy. After this process, small hexagonal hillocks were observed on the surface, which indicated that crystal regrowth was induced by this process, consisting of decomposition of GaN by H2 supplies and re-crystallization by NH3 supplies. It was revealed that the implanted Mg could easily be located at the activation site by means of crystal regrowth by this process. [Copyright &y& Elsevier]

We report on the characteristics of a-plane GaN films directly grown on optimized silicon-dioxide-patterned r-plane sapphire substrates. Various shapes and sizes of silicon dioxide patterns were considered with the aim of achieving fully coalescent a-plane GaN films with a smooth surface and high crystalline quality. The omega full widths at half maximum of the (11–20) X-ray rocking curve values of optimized a-plane GaN films with regular hexagonal patterns of 1μm window width and 6μm mask width were measured to be 597arc sec along the c-axis direction and 457arc sec along the m-axis direction. Atomic force microscopy images revealed a significant reduction in the density of submicron pits in the mask region. Plan-view and cross-sectional transmission electron microscopy images showed that basal stacking faults and threading dislocation densities were reduced from ~5.7×105 cm−1 and ~1×109 cm−2 in the window region to ~1.8×105 cm−1 and ~2.1×108 cm−2 in the mask region, respectively. [ABSTRACT FROM AUTHOR]

Abstract: We investigated unintentionally doped nonpolar a- and m-plane GaN layers grown by metalorganic vapor phase epitaxy under several sets of conditions on freestanding a- and m-plane GaN substrates. Oxygen contamination in a-plane GaN is greatly reduced by increasing the V/III ratio during growth. As a result, a high-resistivity GaN buffer layer for an AlGaN/GaN heterostructure field-effect transistor was realized. [Copyright &y& Elsevier]

Abstract: AlN/diamond heterojunction field effect transistors with p-channel and normally-on depletion mode are developed, and the device performance is improved by reducing the contact resistances at source and drain contacts reported previously. The heterojunction structure is constructed from a c-axis-oriented AlN epilayer grown on oxygen-terminated (111) diamond substrate by metal-organic vapor phase epitaxy at temperatures as high as 1250°C. Thermal treatment in the mixed hydrogen (H2) and ammonia atmosphere just before AlN growth improves the AlN adhesion to diamond surface. In addition, this treatment simultaneously produces a much larger surface hole-carrier density than that obtained by conventional H2-plasma treatment. X-ray photoelectron spectroscopy reveals the existence of carbon-nitrogen bonds at the diamond surface, and these may be responsible for such a large hole density. These results are promising in relation to new opportunities for developing diamond-based power electronic devices. [Copyright &y& Elsevier]

Abstract: The electrical and optical properties of Mg-doped a- and c-plane GaN films grown by metalorganic vapor phase epitaxy were systematically investigated. The photoluminescence spectra of Mg-doped a- and c-plane GaN films exhibit strong emissions related to deep donors when Mg doping concentrations are above 1×1020 cm−3 and 5×1019 cm−3, respectively. The electrical properties also indicate the existence of compensating donors because the hole concentration decreases at such high Mg doping concentrations. In addition, we estimated the N D /N A compensation ratio of a- and c-plane GaN by variable-temperature Hall effect measurement. The obtained results indicate that the compensation effect of the Mg-doped a-plane GaN films is lower than that of the Mg-doped c-plane GaN films. [ABSTRACT FROM AUTHOR]

Abstract: c-Axis-oriented wurtzite (0001) AlN layers are grown on a (111) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using X-ray diffractometry and transmission electron microscopy. At the initial stage of AlN growth, AlN particles with a two-domain structure are nucleated on the (111) diamond surface. The orientation is either 〈112¯0〉AlN ∥ [11¯0] diamond [named the AlNI domain] or 〈101¯0〉AlN ∥ [11¯0] diamond with a rotation angle of 30° from AlNI around the c-axis [named the AlNII domain]. The initial AlN particles are predominantly composed of AlNII. Subsequently, the AlNII grains incorporate the quite-low-density AlNI grains upon further growth. Finally, a continuous AlN layer dominated by the AlNII domain is obtained on the diamond. The growth dynamics is significantly different from that of the AlN layer grown on (111) Si. The growth mechanism is understood by considering the periodic atomic arrangement of AlN and (111) diamond. We conclude that AlNII on (111) diamond has a geometrically appropriate epitaxial relationship that reduces the stress energy of the AlN lattice. [Copyright &y& Elsevier]

Abstract: Wurtzite AlN layers are grown on a (001) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using atomic force microscopy, X-ray diffractometry, and transmission electron microscopy. At the initial stage of AlN growth, AlN crystalline particles with various orientations are randomly nucleated on the (001) diamond surface. At the second step of growth, AlN grains predominantly oriented along the c-axis grow, incorporate the randomly oriented AlN grains, and then grow further. At the final step of growth, a continuous AlN layer with a c-axis-oriented two-domain structure is obtained on the (001) diamond substrate. Microstructural analysis reveals that either the AlN domain or AlN domain is aligned on the [110] diamond direction. The growth mechanism governs by the higher growth rate of the AlN grains along the [0001] direction than along other directions at high growth temperatures up to 1270°C. [Copyright &y& Elsevier]

Abstract: We have demonstrated In x Ga1− x N epitaxial growth with InN mole fractions of x=0.07 to 0.17 on an m-plane ZnO substrate by metalorganic vapor phase epitaxy for the first time. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m-plane SiC substrate. [Copyright &y& Elsevier]

Abstract: The selective regrowth of GaN during sidewall-seeded epitaxial lateral overgrowth was performed. In addition to adjusting the V/III ratio, control of offset angle of the sidewall was found to be effective for realizing one-sidewall-seeded a-plane (112¯0) GaN on r-plane (11¯02) sapphire. The number of coalescence regions on the grooves was reduced, and threading-dislocation and stacking-fault densities as low as 106–107 cm−2 and 103–104 cm−1, respectively, were successfully realized. [Copyright &y& Elsevier]

Abstract: Ultraviolet light-emitting diodes (UV-LEDs) with a peak wavelength of 335nm were fabricated on AlN/sapphire templates. As templates for the fabrication of UV-LEDs, planar AlN and epitaxial laterally overgrown (ELO) AlN on sapphire (0001) substrates were compared. The output power of UV-LEDs grown on ELO-AlN was 27 times higher than that of UV-LEDs on the planar AlN template. [Copyright &y& Elsevier]

Abstract: We investigated impact of high-temperature growth by metal-organic vapor phase epitaxy on crystallographic quality of AlN on 6H-SiC (0001)Si substrate. With increasing growth temperature (T g), growth rate became high and residual impurities decreased. For T g=1600°C, AlN with atomically flat surface and low dislocation density were reproducibly obtained under the high growth rate at 6μm/h. The narrowest distribution of tilt and twist were 76 and 360arcsec, respectively, and the average dislocation density was measured to be 7×108 cm−2. Most of dislocations were of edge-type, generated by the misfit between the AlN and the SiC substrate. For increases of T g up to 1600°C, the number of dislocations at AlN/SiC interface decreased, and many of the dislocations were annihilated by the formation of a loop structure. [Copyright &y& Elsevier]

Abstract: Epitaxial lateral overgrowth (ELO) of high-quality AlN layers on trench-patterned AlN underlying layers has been demonstrated by high-temperature metal-organic vapor phase epitaxy. Under optimized conditions, a high growth rate and an atomically flat surface of thick ELO-AlN without cracks were obtained. Fully coalesced ELO-AlN epilayers were obtained on AlN templates having trenches. AlN grown above the groove had low dislocation density due to filtration caused by the grooves. In addition, most of the dislocations threading from the AlN template were annihilated by the formation of a loop structure during growth with increasing thickness of AlN. The average dislocation density of the ELO-AlN was calculated to be 4×107 cm−2. [Copyright &y& Elsevier]

Abstract: To realize high-performance UV-light-emitting diodes (UV-LEDs), thick, crack-free and high-crystalline-quality AlGaN films with a low threading dislocation density and a device structure without any absorbing layer such as GaN are essential. Crack-free and low-dislocation-density AlGaN is developed using facet-controlled technology, and does not contain any GaN layer. As a result, the density of the threading dislocations in the overgrown AlGaN is as low as 5×107cm-2 over the entire wafer. [Copyright &y& Elsevier]

Abstract: We have developed GaN/AlGaN multi-quantum-well (MQW) UV-laser diode (UV-LD) grown on sapphire substrate. The combination of a low-temperature-deposited AlN interlayer technology and lateral seeding epitaxy yielded crack-free and low-dislocation-density AlGaN. The lasing wavelength under pulsed current injection at room temperature was 350.9nm, which is the shortest wavelength ever reported. [Copyright &y& Elsevier]

We propose a new approach to the direct growth of low-dislocation-density AlxGa1−xN single crystal films on highly mismatched substrates. Four different substrates, basal-plane sapphire (0001) and (112¯0), 6H–SiC(0001)Si and Si(111), were used. The surface of the substrates was patterned with periodic grooves. In this technique, neither a selective growth mask nor a patterned GaN single crystal film were used. Low-dislocation-density areas were observed above the trench area and at the boundaries of the terrace and trench areas. The dislocation density was reduced to mid 106 cm−2 which is lower than that of the films grown directly on the planar substrates by at least two orders of magnitude. [Copyright &y& Elsevier]

Abstract: We investigate the microstructures of domain boundaries in an AlN layer grown on a (001) diamond substrate by metal-organic vapor phase epitaxy. The AlN layer has a two-domain structure with crystal orientation along either <112̄0> AlN [named by AlNI domain] or <101̄0> AlN [named by AlNII domain] parallel to [110] direction of diamond. The AlNI and AlNII domains are not atomically bonded at two-domain boundary from initial to final step of growth, while an edge-type dislocation is generated at single-domain boundary (SDB). In addition, an inversion AlNI domain [named by AlNI⁎] is randomly-ordered at the initial stage of the coalescence between the AlNI domains. The AlNI⁎ is easily terminated with increasing the thickness of AlNI domain. The inversion domain boundary changes to the edge-type dislocation at the SDB with further growth, which reduces the defect density in the AlNI domains. [Copyright &y& Elsevier]

Abstract: We succeeded in growing high-crystalline-quality thick (101¯1¯) Ga0.92In0.08N films on a grooved (101¯1¯) GaN/(101¯2¯) 4H-SiC underlying layer. We also fabricated GaInN/GaN multiple quantum wells (MQWs) with a peak wavelength of 580nm on a high-crystalline-quality thick GaInN film. The photoluminescence intensity of the MQWs is about six times higher than that of MQWs grown on planar GaN and twice as high as that of MQWs grown on a GaN underlying layer having the same grooved structure. [Copyright &y& Elsevier]

Abstract: The processes as in title of relaxation of the lattice mismatch and the recovery of crystalline quality in thick Al x Ga1− x N on high-temperature-grown AlN were investigated. When x=0.3, rapid lattice relaxation occurred over a few microns, then the crystalline quality gradually recovered over 10μm. In contrast, when x=0.7, relaxation of the lattice mismatch gradually occurred over 5μm. [Copyright &y& Elsevier]

Abstract: We investigated the electrical properties of Mg-doped a-plane Ga1− x In x N (0

Abstract: We succeeded in growing almost completely relaxed, high-quality, thick GaInN films on an m-plane GaN template with grooves along the 〈0001〉 direction, using lateral-growth technology. Reciprocal space mapping of asymmetrical X-ray diffraction confirmed almost complete relaxation. By transmission electroscopic characterization, the growth of GaInN film with a threading dislocation density of approximately 1×108 cm−2 was confirmed. [Copyright &y& Elsevier]

Abstract: Crack-free and low-dislocation-density Al x Ga1− x N with x higher than 0.2 was successfully grown on a grooved AlN template prepared on a sapphire substrate. Efficient UV-B-light-emitting diodes can be fabricated on such an AlGaN layer. [Copyright &y& Elsevier]