AlN Grains Research Articles

Microstructure evolution during AlSi10Mg molten alloy/BN microflake interactions in metal matrix composites obtained through 3D printing

Utilization of metal/ceramic powders opens new possibilities for 3D printing of metal matrix composites of complex shape with high strength, but it is still a great challenge. In this work, an AlSi10Mg matrix composite embedded with 1 wt% of hexagonal BN phase microflakes (h-BN) was obtained by means of 3D printing. Then the present study elucidated microstructure evolutions occurring at the h-BN/melt interface during selective laser melting (SLM) of an h-BN-AlSi10Mg powder mixture. During short-term (0.15 ms) high-temperature (∼2900 K) processing the BN inclusions partly dissolved in the Al–Si melt. This process was accompanied by the formation of an AlN phase at the BN surfaces. The AlN crystallites, 100–200 nm in size, had spherical/semispherical shape and formed a continuous layer along the BN/metal grain boundaries. The peculiar growth of AlN grains along the metal/BN interfaces was governed by the specific features of localized N diffusion in the vicinity of interfaces. By contrast, B atoms, released from the dissolved BN phase, were randomly distributed over the melt. AlB2 nanocrystallites (∼10 nm in size) precipitated from the supersaturated Al–Si melt during cooling stage. With the addition of h-BN microflakes, the composite hardness and tensile strength increased by 32% and 28%, respectivelly. The observed experimental results were supported by ab initio molecular dynamics simulations. Our study demonstrates the possibility and wide prospects of obtaining a dense BN/AlSi10Mg material reinforced with h-BN, AlN, and AlB2 phases via SLM 3D printing and sheds a new light on fine morphological and microstructural features of thus obtained new composites.

Graphene-induced crystal-healing of AlN film by thermal annealing for deep ultraviolet light-emitting diodes

We achieve stress-free AlN films with low dislocation density by employing Graphene (Gr)-induced crystal-healing via high-temperature annealing (HTA). The N2 plasma-treated Gr effectively facilitates the formation of high-density and small-size AlN grains during the initial quasi-van der Waals epitaxial growth. Thus, it is easier to rotate the inhomogeneous crystal orientation for subsequent AlN epilayers during the HTA process, greatly improving the recrystallization efficiency of AlN. Due to the improved AlN quality, the 283-nm deep ultraviolet light-emitting diode (DUV-LED) on the HT-annealed AlN with Gr shows a significant increase in light output power of 2.3 times at an injection current of 20 mA compared to its counterpart on bare sapphire. This research broadens the applications of Gr in assisting group-III nitride epitaxy and provides a useful strategy for the manufacture of high-quality AlN and high-performance DUV-LEDs.

Thermal conductivities and mechanical properties of AlN ceramics fabricated by three dimensional printing

AlN ceramics were successfully fabricated through a joint process of digital light processing (DLP) 3D printing technology and heat treatment at 1780 °C∼1845 °C. DLP is an addictive manufacturing process, enabling the near net shape fabrication. The AlN grains in this work developed well and there were small amounts of grain-boundary phases at the three-grain junctions. The particle size of AlN became larger and the densification increased with increasing sintering temperature. The pores of AlN ceramics also decreased, which led to the increase of thermal conductivity and flexural strength. The optimal thermal conductivity and flexural strength of AlN ceramic reached 155 W/(m·K) and 265 ± 20 MPa when sintered at 1845 °C.

Effects of pulse cycle number on the quality of pulsed atomic-layer epitaxy AlN films grown via metal organic chemical vapor deposition

The effect of different numbers of Al and N2 pulse cycles on the quality of AlN films prepared via the pulsed atomic-layer epitaxy (PALE) technique and epitaxially deposited on a c-plane sapphire substrate by metal organic chemical vapor deposition were investigated. The characteristics of AlN/sapphire were studied by atomic force microscopy, field emission scanning electron microscopy and Raman spectroscopy. AlN film deposited with the PALE technique with the highest number of pulse cycles (1050) was observed to exhibit highly uniform spherical AlN grains and a dense film surface with a root mean square roughness of 0.46 nm. Transition of the E2 (high) peak from the Raman spectrum shows that the strain compression in PALE AlN films is inversely proportional to the number of pulse cycles.

Characterization and optimization of AlN nucleation layer for nonpolar a-plane GaN grown on r-plane sapphire substrate

Abstract Nonpolar (11–20) a-plane GaN films with AlN nucleation layer were grown on (10–12) r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The crystalline and surface qualities of a-plane GaN were found to closely depend on the growth conditions of AlN nucleation layer. With decreasing AlN growth temperature, the AlN grains became larger and sparser, which significantly reduced the defects density of a-plane GaN films. The growth time of the low temperature AlN layer was further optimized, and a-plane GaN films with reduced anisotropy in the crystalline quality, surface morphology and in-plane strains were achieved. It was found that the lateral growth lengths along different directions of GaN could be modulated by the growth time of AlN nucleation layer, thus changing the anisotropy of a-plane GaN films.

Ionic interdiffusion as interaction mechanism between Al and Si3N4

AbstractAl‐Si3N4 couples were heat‐treated at 850‐1150°C for 250 hours. The thickness of the interacted area was measured by scanning electron microscopy (SEM) and scanning/transmission electron microscopy (TEM/STEM). The interaction rate increases exponentially with inverse temperature, with an activation energy of 194.23 kJ/mol and diffusion pre‐coefficient of 5 × 10−9 m2/s, indicating that the interaction is diffusion‐dependent. As the results showed, the interfacial area is comprised of Al alloy channels, Si precipitates, and AlN grains. Al‐Si transfer through the solid solution (Si3‐xAlxN4‐y) at the interface of Al alloy and β‐Si3N4 grains controls the kinetic of the interaction. When concentration of Al in solid solution exceeds a certain amount, it undergoes a topotactic phase transformation to form Al1‐xSixN1+y (viz., AlN). Next, the Al1‐xSixN1+y grains detach from the β‐Si3N4 grains and subsequently new Al‐Si3N4 interfaces are established. These interfaces repeat the interaction process, continuing until all the reactant is depleted. Thus, the interaction kinetics consist of a sequence of associated parabolic stages, precluding the observation of parabolic kinetics.

AlN and AlN/Al2O3 seed layers from atomic layer deposition for epitaxial growth of AlN on sapphire

The annealing of amorphous AlN and AlN/Al2O3 seed layers from atomic layer deposition (ALD) on sapphire substrates and their application as starting layers for metal-organic vapor phase epitaxial growth on sapphire was investigated. During annealing in hydrogen, the amorphous ALD layers become crystalline with epitaxial relation to the underlying sapphire substrate. In contrast to the pure AlN ALD seed layers, mixed layers containing Al2O3 help to avoid the formation of polycrystalline material. Additionally, such mixed ALD seeds support void formation at the AlN/sapphire interface resulting in the formation of a smooth AlN surface. This void formation can be seen in situ during AlN growth in the reflectivity at 405 nm. The tilt and twist component of the AlN grains could be decreased by increasing the annealing time at 1290 °C from 1.5 to 40 min.

Influence of 4H-SiC substrate miscut on the epitaxy and microstructure of AlGaN/GaN heterostructures

AlGaN/GaN heterostructures were grown on “on-axis” and 2° off (0001) 4H-SiC substrates by metalorganic vapor phase epitaxy (MOVPE). Structural characterization was performed by transmission electron microscopy. The dislocation density, being greater in the on-axis case, is gradually reduced in the GaN layer and is forming dislocation loops in the lower region. Steps aligned along [11̅00] in the off-axis case give rise to simultaneous defect formation. In the on-axis case, an almost zero density of steps is observed, with the main origin of defects probably being the orientation mismatch at the grain boundaries between the small not fully coalesced AlN grains. V-shaped formations are observed in the AlN nucleation layer, but are more frequent in the off-axis case, probably enhanced by the presence of steps. These V-shaped formations are completely overgrown by the GaN layer, during the subsequent deposition, presenting AlGaN areas in the walls of the defect, indicating an inter-diffusion between the layers. Finally, at the AlGaN/GaN heterostructure surface in the on-axis case, V-shapes are observed, with the AlN spacer and AlGaN (21% Al) thickness on relaxed GaN exceeding the critical thickness for relaxation. On the other hand, no relaxation in the form of V-shape creation is observed in the off-axis case, probably due to the smaller AlGaN thickness (less than 21% Al). The AlN spacer layer, grown in between the heterostructure, presents a uniform thickness and clear interfaces.

Decomposition routes and strain evolution in arc deposited TiZrAlN coatings

Phase, microstructure, and strain evolution during annealing of arc deposited TiZrAlN coatings are studied using in situ x-ray scattering and ex situ transmission electron microscopy. We find that the decomposition route changes from nucleation and growth of wurtzite AlN to spinodal decomposition when the Zr-content is decreased and the Al-content increases. Decomposition of Ti0.31Zr0.24Al0.45N results in homogeneously distributed wurtzite AlN grains in a cubic, dislocation-dense matrix of TiZrN consisting of domains of different chemical composition. The combination of high dislocation density, variation of chemical composition within the cubic grains, and evenly distributed wurtzite AlN grains results in high compressive strains, −1.1%, which are retained after 3 h at 1100 °C. In coatings with higher Zr-content, the strains relax during annealing above 900 °C due to grain growth and defect annihilation.

Orientation of AlN Grains Nucleated on Different Diamond Substrates by Magnetron Sputtering

For AlN/diamond structure, the crystal quality of diamond substrates is of great importance for the epitaxial growth of AlN. In this paper, c‐axis‐oriented AlN films are deposited on diamond with different crystal quality by radio frequency (RF) reactive magnetron sputtering. The effects of the microstructure of diamond substrates on the orientation of AlN are studied. The results show that the crystal quality of the diamond films greatly affects the growth behavior of AlN. The surface defects of diamond, such as the amorphous phase, will cause the growth direction of AlN to deviate from the c‐axis, resulting in the disorder of grain orientation. Meanwhile, diamond crystal orientation also has influence on the growth of AlN. The (111) oriented grains of diamond are beneficial to the c‐axis growth of AlN. The interface of AlN nucleation and initial growth plays a vital role in final performance of the films, where there is a transition zone from randomly oriented AlN grains to c‐axis‐oriented grains. The suitable diamond substrate can eliminate the transition zone, and can lead to AlN films with high‐degree c‐axis orientation.

Effects of Y2O3 and yttrium aluminates as sintering additives on the thermal conductivity of AlN ceramic substrates

AlN ceramic substrates are prepared by tape casting and presureless sintering at 1850 °C for 10 h, with 8 wt% Y2O3 and yttrium aluminates of Y3Al5O12 (YAG), YAlO3 (YAP), and Y4Al2O9 (YAM) as sintering additives. The effects of the sintering additives on the phase assemblage, microstructure, thermal conductivity, and bending strength of the sintered AlN ceramics are discussed. With Y2O3, YAG, YAP, and YAM as sintering additives, the secondary phases in the sintered AlN ceramics are YAM, YAG, YAP + YAG, and YAM + YAP, and the average sizes of AlN grains are 5.5, 3.6, 4.4, and 4.9 µm, respectively. In the order of sintering additives with decreasing Y2O3/Al2O3 ratio, viz. Y2O3 > YAM > YAP > YAG, both the thermal conductivity and bending strength of the sintered AlN ceramics decrease. With Y2O3 as the sintering additive, the sintered AlN sample shows the maximum thermal conductivity of 205 W/(m K) and bending strength of 295 MPa.

Study of dual nitridation processes in growth of non-polar a-plane AlGaN epi-layers

The effect of dual nitridation processes for both r-plane sapphire and low temperature-grown AlN (LT-AlN) nucleation layer on non-polar a-plane AlGaN epi-layer was studied intensively. A root-mean-square value as small as 1.54 nm for a-plane Al0.53Ga0.47N epi-layer was achieved. It was revealed that the generation of AlN grains as well as the coalescence and recrystallization of LT-AlN islands were the key factors for growing a-plane AlGaN epi-layers with smooth surface morphology. Meanwhile, the evolution of surface morphology with varied nitridation processes and the mechanisms for improving surface morphology of a-plane AlGaN epi-layers were also investigated.

Superhard amorphous/nanocrystalline multilayer coatings for carbide inserts used for intermittent cutting

Ti-Al-Si-N coatings deposited by filtered cathodic vacuum arc deposition method are characterized by multilayer structure that corresponds to alternating layers of crystalline TiN with variable stoichiometry and amorphous Si3N4 with inclusions of hexagonal AlN grains. The thickness of the layers was 5–20 nm. The structure of coatings was regulated by rotation speed of the samples coated (n) in relation to evaporated cathodes and substrate bias voltage (Ub). The multilayer structure was not observed at the minimal energy of evaporated particles which is determined by the minimal Ub = − 60 V. Its separate areas were only observed in a limited volume of deposited coating. The grain structure of coatings was extremely heterogeneous. The distinct multilayer structure of coating with clear interphase boundary was formed when increasing Ub up to − 120 V. The reducing of layer thickness and corresponding grains were observed. The subsequent increase of Ub up to − 140 V leads to formation of sublayers 2–3 nm thick within the layers ~ 15 nm thick. The increasing of layer boundary length and refinement of coating grain structure are accompanied by increase of hardness from 33 to 56 GPa. The friction coefficient of the coatings obtained on carbide inserts is 0.60 as compared to 0.75 for the uncoated substrate (sliding wear tests against 100Cr6 balls under dry condition using a ball-on-disk method at high speed). During scratch test, the coatings were destructed according to the cohesion mechanism. The complete abrasion of the coating to the substrate was not observed up to load 90 N.

Influence of Transient Liquid Phase Promoting Additives upon Reactive Plasma Spraying of AlN Coatings and Its Properties

The feasibility of using a transient liquid phase promoting agent upon reactive plasma spraying (RPS) of AlN coatings in atmospheric ambient is successfully investigated. Several sprayable fine AlN–Al2O3–Y2O3 mixtures with different compositions are prepared by spray‐drying. Upon plasma spraying in N2/H2 plasma, thick and homogeneous AlN based coatings (h‐AlN, c‐AlN, γ‐Al2O3, Al5O6N, α‐Al2O3, and YAG) are fabricated. Fine Y2O3 acts as transient liquid phase promoting additive, and reacts with Al2O3 phase (from feedstock mixture and on AlN particles surface) to create wetting agent of yttrium aluminate Y–Al–O around solid AlN particles. However, due to rapid solidification rates of plasma, it is difficult to react with the interspersed oxygen in AlN grains. The Y2O3 addition significantly reduces the coatings oxide content due to the consumption of starting Al2O3 to form the YAG. Influence of Y2O3 significantly observes with increasing AlN feedstock and high AlN content coatings with small oxides are fabricated by RPS of 80AlN–10Al2O3–10Y2O3 mixture. Thermal conductivity is improved with using Y2O3 and increasing AlN feedstock. However, the conductivity is not so high compared to the AlN compacts, due to increased porosity, incomplete sintering of the formed initial liquid phase state, remaining oxide content, and low density of the coatings.

On the AlN precipitation and grain refinement in the Al(N)-added medium C–Mn steels

The precipitation of AlN particles and grain refinement in the Al(N)-added medium C–Mn steel have been studied using a Gleeble hot-deformation simulator and extraction replica TEM technique. No significant grain refinement occurs during either hot-rolling or hot-deformation/isothermal-holding, which is due to the absence of AlN precipitation; the lack of precipitation is caused by an unusually high-energy barrier for nucleation. Abrupt grain refinement occurs on reheating because of the copious precipitation of AlN during reheating. The copious precipitation of AlN during reheating is possible because thermally unstable Fe-rich sulphides act as precursors for AlN precipitation during reheating.

Influence of Al Preflow Time on Surface Morphology and Quality of AlN and GaN on Si (111) Grown by MOCVD**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0400200.

We investigate the influence of Al preflow time on surface morphology and quality of AlN and GaN. The AlN and GaN layers are grown on a Si (111) substrate by metal organic chemical vapor deposition. Scanning electron microscopy, atomic force microscopy, x-ray diffraction and optical microscopy are used for analysis. Consequently, we find significant differences in the epitaxial properties of AlN buffer and the GaN layer, which are dependent on the Al preflow time. Al preflow layers act as nucleation sites in the case of AlN growth. Compact and uniform AlN nucleation sites are observed with optimizing Al preflow at an early nucleation stage, which will lead to a smooth AlN surface. Trenches and AlN grain clusters appear on the AlN surface while melt-back etching occurs on the GaN surface with excessive Al preflow. The GaN quality variation keeps a similar trend with the AlN quality, which is influenced by Al preflow. With an optimized duration of Al preflow, crystal quality and surface morphology of AlN and GaN could be improved.

Hydrothermal‐carbothermal synthesis of highly sinterable AlN nanopowders

AbstractAluminum nitride (AlN) nanopowders have been synthesized by a novel method which combines the hydrothermal and carbothermal techniques. The phase of the nitridation products can be controlled through adjusting the carbon content of precursor by adding different amount of urea (U). The nitridation product synthesized from the precursor (U/Al=2) was comprised of spherical and well‐distributed particles with sizes ranging from 50 to 100 nm. The nitridation procedure was investigated in detail withTG/DSC,XRD, andTEMmeasurements. It was found that the core‐matrix structured Al2O3/C mixture presents after pyrolysis, favoring the nitridation completed at low temperature, and brings in temperature independency for AlN grain size. Finally, the additive‐free AlN ceramic with 99.08% relative density has been achieved by pressureless sintering at 1850°C.

Y2O3가 1 wt% 첨가된 AlN 세라믹의 전기절연성에 미치는 TiO2첨가의 효과

The effects of <TEX>$TiO_2$</TEX> addition on the electrical insulation of AlN ceramics with 1 wt% <TEX>$Y_2O_3$</TEX> as a sintering aid have been investigated. Some of <TEX>$TiO_2$</TEX> has reacted with AlN powders and transformed to fine TiN particles during sintering, which was uniformly dispersed along grain boundaries of AlN. At a high electrical field (500 V/mm), the resistivity of AlN ceramics with <TEX>$TiO_2$</TEX> addition of 0.2 wt% increased about 1000 times from <TEX>$3{\times}10^{10}{\Omega}cm$</TEX> to <TEX>$3.1{\times}10^{13}{\Omega}cm$</TEX>. Based on the impedance spectroscopy measurement, it was found that <TEX>$TiO_2$</TEX> addition increased dramatically electrical resistivity of AlN grains much more than that of grain boundaries. Thus, <TEX>$TiO_2$</TEX> was believed to dissolve inside AlN grains to suppress ionic conduction of Al vacancies. This suppressed ionic conduction by Ti incorporation into AlN grains seems to contribute to more electrically insulating AlN ceramics.

Graphene-assisted growth of high-quality AlN by metalorganic chemical vapor deposition

High-quality AlN films were directly grown on graphene/sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The graphene layers were directly grown on sapphire by atmospheric-pressure chemical vapor deposition (APCVD), a low-cost catalyst-free method. We analyzed the influence of the graphene layer on the nucleation of AlN at the initial stage of growth and found that sparse AlN grains on graphene grew and formed a continuous film via lateral coalescence. Graphene-assisted AlN films are smooth and continuous, and the full width at half maximum (FWHM) values for (0002) and reflections are 360 and 622.2 arcsec, which are lower than that of the film directly grown on sapphire. The high-resolution TEM images near the AlN/sapphire interface for graphene-assisted AlN films clearly show the presence of graphene, which kept its original morphology after the 1200 °C growth of AlN.

Residual stress in AlN films grown on sapphire substrates by molecular beam epitaxy

Residual stress in AlN films grown by molecular beam epitaxy (MBE) has been studied by Raman scattering spectroscopy. A strain-free Raman frequency and a biaxial stress coefficient for E2(high) mode are experimentally determined to be 657.8 ± 0.3 cm−1 and 2.4 ± 0.2 cm−1/GPa, respectively. By using these parameters, the residual stress of a series of AlN layers grown under different buffer layer conditions has been investigated. The residual compressive stress is found to be obviously decreased by increasing the Al/N beam flux ratio of the buffer layer, indicating the generation of tensile stress due to stronger coalescence of AlN grains, as also confirmed by the in-situ reflection high energy electron diffraction (RHEED) monitoring observation. The stronger coalescence does lead to improved quality of AlN films as expected.