AlN Composites Research Articles

Improving hole injection from p-EBL down to the end of active region by simply playing with polarization effect for AlGaN based DUV light-emitting diodes

In this work, we simply take advantage of the polarization effect to efficiently improve the hole injection from the p-type electron blocking layer (p-EBL) to the end of the active region for AlGaN based deep ultraviolet light emitting diodes (DUV LEDs). By properly increasing the AlN composition of AlGaN quantum barriers, a smaller positive polarized charge density at the last quantum barrier/p-EBL interface can be obtained, which correspondingly leads to the suppressed hole depletion and the reduced hole blocking effect in the p-EBL. Meanwhile, we properly increase the quantum well thickness so that the polarized electric field can even more accelerate the holes, and this will homogenize the hole distribution more across the MQWs. Therefore, the external quantum efficiency for DUV LEDs can be enhanced.

AlN and MgO thin-layer coatings on the bendable polymeric substrates as selective filters for IR and THz spectral ranges

Thin AlN and MgO poly- or nano-structured layers were synthesized on bendable polymeric substrates using the helicon-arc ion-plasma method. They can be used as filters to block infrared (IR) radiation from civil and military objects. To suppress IR radiation within the range λ ~ 7.5…25 µm, where most of the radiation power from objects with the temperature T ~ 300…400 K is concentrated, we have used AlN and MgO thin layers (d ~ 1.5…10 μm) that have the Restrahlen bands within the spectral range indicated above. The optical, structural properties and chemical composition of AlN and MgO films on the Mylar or Teflon substrates have been investigated. These filters demonstrate blocking characteristics for masking the thermal image of objects heated to a temperature of 300…400 K in the IR spectral range. At the same time, they are well transparent in the microwave and THz spectral ranges (transparency τ up to ≈ 80…85%). Moreover, thin Mylar films with AlN coatings are transparent in the visible range. Using the combined coatings of AlN with high thermal conductivity (110…140 W/m·K in the polycrystalline form) for heat withdraw to the periphery or air and MgO coatings, one can suppress IR radiation by more than 90% within the spectral range λ = 7.5…25 μm.

Multifractal Characteristics of Hot-Pressed AlВ12–AlN Composites of Nanopowders

Multifractal Characteristics of Hot-Pressed AlВ12–AlN Composites of Nanopowders

Interplay between various active regions and the interband transition for AlGaN-based deep-ultraviolet light-emitting diodes to enable a reduced TM-polarized emission

Al-rich AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have a low light extraction efficiency, especially when the emission wavelength is shorter than 280 nm, and this is partially because of the dominant transverse-magnetic polarized light. Our results show that the transverse-electric (TE) polarized light can be obtained even if the emission wavelength becomes even shorter by reducing the quantum well thickness. The ultrathin quantum well enables the enhanced TE-polarized emission that arises from the redistributed subbands for holes. On the contrary to the common belief, we observe a blueshift for the emission wavelength when the AlN composition in the quantum barrier increases. The internal quantum efficiency (IQE) for DUV LEDs with ultrathin quantum wells is no longer determined by the quantum-confined Stark effect, while quantum barrier with high AlN composition is vitally important to improve the electron injection efficiency and thus enhance the IQE.

Особенности пироэффекта в эпитаксиальных слоях нитрида алюминия, полученных на Si-подложках

The pyroeffect was studied in quasi-bulk AlN layers with a thickness of 10-170 μm obtained by the technology of chloride-hydride vapor phase epitaxy on standard Si sub-strates. The pyroelectric current was measured by the method of thermal exposure to non-stationary (me-ander type) laser radiation, that together with the data of independent non-contact measure-ment of the active layer surface temperature dynamics made it possible to determine the value of the pyroelectric coefficient AlN in the composition of the bimorph AlN / Si structure for different thicknesses of AlN layers. It was found that the mean values are less than those that correspond to the material obtained by the same technology, but on SiC substrates; so, in or-der to achieve the pyrocoefficients of comparable magnitude with those in the case of “AlN on Si”, an increase in the thickness of AlN layers by 60 -70% is re-quired. At the same time, when the thickness of the AlN layer is large (110 μm, 170 μm) after the removal of the silicon substrate, the pyroelectric coefficient increased and reached ~8.6-9.0μC/(m2·K).

Evaluation of microstructure, physical and mechanical properties of Al 7079–AlN metal matrix composites

In the present research, Al7079– AlN composites with different weight proportions of reinforcements ranging from 2%–12% with intervals of 2% were fabricated using stir casting method. Density of the developed composite was determined through Archimedes principle. Basic mechanical properties such as tensile, compressive, hardness and ductility were evaluated. The developed composites were subjected to microstructural studies using Scanning Electron Microscope (SEM). The microstructural study revealed uniform distribution of the reinforcement particles in the matrix. The porosity of the composite increased with addition of reinforcement upto 4% and it was observed that it decreases for further addition of AlN reinforcement. An increase in hardness, compressive strength and tensile strength of the composite by 28%, 41% and 28% respectively was observed as compared to unreinforced alloy.

Material constants of AlN—graphene composites determined by ultrasound method

The aim of the article is to present the elastic properties of AlN composites with addition of 0 to 8 wt% graphene. The composites were prepared using the hot pressing method. Samples prepared in this way show the anisotropy of elastic properties. All material constants i.e. Young’s modulus shear modulus and Poisson’s ratio of the composite including the anisotropy of its elastic properties were determined by the through-transmission method. This method is based on the measurement of the propagation velocities of appropriately polarized ultrasonic waves in the samples. A full description of the elastic properties of the AlN-graphene composite has been presented. In addition, SEM microstructure observations confirmed and allowed to explain the obtained values of elastic properties of ALN graphene—composites. The conducted research showed a significant influence of the amount of graphene addition on the elastic properties of the composites and the anisotropy of their properties. The highest, up to 55% anisotropy of the Young’s modulus is characterized by composite materials with an 8 wt% graphene content in the AlN composite—graphene. Simultaneously, for composites with 8 wt% graphene content the lowest elastic properties were calculated.

Thermal energy harvesting performance in 0.94Bi0.5Na0.5TiO3-0.06BaZr0.2Ti0.8O3: AlN composite ceramics based on the Olsen cycle

The pyroelectric effect provides an efficient route to convert low-grade waste heat into electricity. (Bi0.5Na0.5)TiO3-based ceramic is one of the most attractive lead-free pyroelectric candidates due to its high polarization. However, the low breakdown strength (BDS) restricts its ability to achieve a high polarization, hindering the application for thermal energy harvesting based on the Olsen cycle. In this work, by incorporation of AlN into BNT-BZT ceramics to form the BNT-BZT: AlN composites, the BDS is enhanced from 160 kV cm−1 to 260 kV cm−1, leading to a high energy density of 1.54 J cm−3 which is ∼5 times that of the pristine BNT-BZT. The reason for the high energy harvesting performance is due to the addition of AlN that can depress the conductivity of the matrix, resulting in the high BDS that permits high electric fields to be applied on the samples to induce high polarization that changes pronouncedly with temperature.

Electrochemical Behavior of AlB12–AlN Composites in Natural Environments

Electrochemical processes occurring on ceramic AlB12–AlN composites induced by polarization in a 3% NaCl aqueous solution at 37°C are examined for the first time. Aluminum dodecaboride AlB12 is found to dissolve so slowly that it hardly influences the composites’ corrosion resistance in actual environments. A higher aluminum nitride amount in the composite increases its electrochemical corrosion resistance. The electrochemical behavior of aluminum dodecaboride and ceramic nitride composites generally allows the materials to retain their service properties in natural corrosive environments even at 37°C.

Simultaneously probabilistic and deterministic approach (SPADA) for the materials design: methodology and experimental validation

This manuscript presents a relatively simple simultaneously probabilistic and deterministic approach (SPADA) for the experiment organization. To exemplify the efficacy and flexibility of the method, we optimized the conditions for the synthesis of the Al2O3–SiC–AlN composite. Optimal synthesis parameters were predicted by evaluation of the impact of five variables (four related to mixture composition and one to annealing duration) on four different levels each. Compressive strength of the resulting composites was monitored as an output parameter for the material characterization. Such optimizations are arduous when done by full-factor experimental design (625 experiments necessary), whereas using SPADA design we accomplished the task by doing only 16 experiments. Maximum compressive strength of the composite was increased by 320% as compared to the starting experiment and by 45% as compared to the highest result produced by direct search; additionally, suggestions for the further improvements were outlined on the basis of calculated correlations.

Influence of InN and AlN concentration on the compositional inhomogeneity and formation of InN-rich regions in InxAlyGa1−x−yN

The application of quaternary InxAlyGa1−x−yN active regions is a promising path towards high efficiency UVB-light emitting diodes (LEDs). For the utilization of InxAlyGa1−x−yN, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the trimethylaluminium (TMAl) and trimethylindium (TMIn) flux on the composition and luminescence properties of InxAlyGa1−x−yN layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 μmol min−1 led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 μmol min−1 resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analyzing the emission properties of these InN-rich regions showed that their emission energy is inhomogeneous and varies by ≈150 meV. We provide evidence that the formation of these InN-rich regions is highly dependent on the AlN and InN composition of the layer and that their formation will strongly affect the performance of InxAlyGa1−x−yN LEDs.

Increasing the hole energy by grading the alloy composition of the p-type electron blocking layer for very high-performance deep ultraviolet light-emitting diodes

It is well known that the p-type AlGaN electron blocking layer (p-EBL) can block hole injection for deep ultraviolet light-emitting diodes (DUV LEDs). The polarization induced electric field in the p-EBL for [0001] oriented DUV LEDs makes the holes less mobile and thus further decreases the hole injection capability. Fortunately, enhanced hole injection is doable by making holes lose less energy, and this is enabled by a specifically designed p-EBL structure that has a graded AlN composition. The proposed p-EBL can screen the polarization induced electric field in the p-EBL. As a result, holes will lose less energy after going through the proposed p-EBL, which correspondingly leads to the enhanced hole injection. Thus, an external quantum efficiency of 7.6% for the 275 nm DUV LED structure is obtained.

Synthesis and mechanical properties of TiB<sub>2</sub>/Ti<sub>2</sub>AlN composites fabricated by hot pressing sintering

Synthesis and mechanical properties of TiB<sub>2</sub>/Ti<sub>2</sub>AlN composites fabricated by hot pressing sintering

A study on Mg and AlN composite in microstructural and electrochemical characterizations of extruded aluminum alloy

A combination of composites and metallic materials has been shown to be an effective way to modify the microstructure and thus enhance the corrosion properties. This study focuses on the development of a new aluminum alloy using Mg and AlN composite and the hot extrusion processing. The microstructures and electrochemical properties have been characterized using the advanced surface analysis and electrochemical techniques in a naturally-aerated 0.1 M chloride solution. Open circuit potential and potentiodynamic polarization results indicate that Mg and AlN composite additions could improve corrosion and pitting potentials as well as passive film quality formed on the alloy surface. Electrochemical impedance spectroscopy has confirmed that the addition of Mg and AlN composite also increases the passive film and charge transfer resistances, resulting in an improvement of passive film stability and pitting resistance. X-ray photoelectron spectroscopy suggests that Mg and AlN composite addition promotes the stable passive film formed on the aluminum alloy surfaces due to the existence of Mg oxides/hydroxides and AlN incorporating with Al oxides/hydroxides. The improved pitting resistance could be attributed to the reduction in the grain sizes and defects in the alloy matrix that are observed by electron backscatter diffraction and could directly relate to the alloy dissolution and passive film formation.

Establishment of the relationship between the electron energy and the electron injection for AlGaN based ultraviolet light-emitting diodes.

This work establishes the relationship between the electron energy and the electron concentration within the multiple quantum wells (MQWs) for AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The electron energy of different values can be obtained by modulating the Si doping concentration in the n-AlGaN layer and/or engineering the polarization induced interface charges. The modulated Si doping concentration in the n-AlGaN layer will cause the interface depletion region within which the electric field can be generated and then tunes the electron energy. The polarization induced charges and the polarization induced electric field can be obtained by stepwisely reducing the AlN composition for the n-AlGaN layer along the [0001] orientation. We find that the electron concentration in the MQWs can be increased once the electron energy is reduced to a proper level, which correspondingly improves the external quantum efficiency (EQE) for DUV LEDs. According to our investigations, it is more advisable to adopt the n-AlGaN layer with the stepwise AlN composition, which can make both the EQE and the wall plug efficiency high.

Structural design and optimization of deep-ultraviolet light-emitting diodes with AlxGa1−xN/AlyGa1−yN/AlxGa1−xN(x > y) p-electron blocking layer

This work reports AlGaN-based deep ultraviolet light-emitting diode (DUV LED) possessing a specifically AlxGa1 − xN / AlyGa1 − yN / AlxGa1 − xN (x > y) structured p-electron blocking layer (p-EBL) to achieve the high external quantum efficiency (EQE). The impact of the p-EBL with AlyGa1 − yN insertion layer at different positions and with different AlN compositions on the hole and electron injection is systematically investigated. Our results show that, for the DUV LED structure in this work, both electrons and holes can be most efficiently injected into the active region by keeping the AlyGa1 − yN insertion layer near to the p-region. The AlN composition for the AlyGa1 − yN insertion layer has also to be optimized for maximizing the carrier injection.

Thermoelectric properties and thermal stress simulation of pressureless sintered SiC/AlN ceramic composites at high temperatures

This paper provides a detailed study of the thermoelectric properties of SiC/AlN composites in a wide temperature range. Evaluation of thermal stress durability of the composites was simulated and analyzed. SiC/AlN composites with 0–30 wt% of AlN were produced by the pressureless sintering method. Phase analysis, densification, and microstructure of the obtained composites were inspected. SiC/AlN ceramics had a porous structure with porosity ranging from 23.77% to 31.74%. Complete 2Hss SiC/AlN solid solution was formed and its content increased with the increment of AlN wt%. Addition of AlN to SiC ceramics gave a harmonic structure with elongated grains morphology. Electrical resistivity and thermoelectric properties of the investigated ceramics were evaluated in the temperature range of 25–1000 K. The results were essentially dependent on AlN content, microstructure and temperature. SiC/AlN composites gave identical behavior of the p-type semiconductors with outstanding thermoelectric properties at high temperatures. The lowest electrical resistivity of 1.5 × 105 µOhm × cm and the highest Seebeck coefficient of 370 µ V/K were recorded for 30% AlN composite at 1000 K. Figure of merit (ZT) increased dramatically with increasing the AlN content. 30% AlN composites recorded ZT value 130 times larger than that of the 0% AlN composite. Thermoelectric power factor was markedly enhanced by increasing AlN wt%. Thermal stress analysis of the different ceramics was simulated by finite element method up to a temperature of 1773 K. SiC/AlN composites gave splendid thermal stress durability with uniform transition and distribution of heat. SiC/AlN composites are strongly nominated to be used effectively in thermoelectric power generation and high-temperature applications.

Growth of High‐Quality AlN and AlGaN Films on Sputtered AlN/Sapphire Templates via High‐Temperature Annealing

We investigated the fabrication and characterization of AlN and AlGaN epilayers grown on sputtered and annealed AlN/sapphire templates by metal organic vapor phase epitaxy. The surfaces of the AlN homoepilayers exhibited clear step‐terrace structures with minimal root mean square roughness values. X‐ray rocking curve (XRC) measurements showed that the AlN homoepilayers had smaller (102)‐plane full width at half maximum (FWHM) values (192 arcsec) than the AlN/sapphire templates did (214 arcsec), and that they had larger (0002)‐plane FWHM values (97 arcsec) than the AlN/sapphire templates (22 arcsec). The latter results may be due to the wafer curvature increasing as the AlN layer thickness increases. We also present results for the growth of AlGaN heteroepilayers with different AlN compositions on AlN/sapphire templates that either did or did not contain AlN regrowth layers. High‐AlN mole fraction AlGaN heteroepilayers had larger (0002)‐plane FWHM values and smaller (102)‐plane FWHM values than low‐AlN mole fraction AlGaN. Furthermore, the presence of an AlN regrowth layer tended to have a negative effect on the high‐AlN mole fraction AlGaN layers. The above results are discussed using lattice mismatch, wafer curvature, surface condition, and growth temperature parameters.

Effect of Solution Treatment on Precipitation Behaviors, Age Hardening Response and Creep Properties of Elektron21 Alloy Reinforced by AlN Nanoparticles.

In the present study, the solution and ageing treatments behavior of Mg-RE-Zr-Zn alloy (Elektron21) and its nano-AlN reinforced nanocomposites have been evaluated. The properties of the thermal-treated materials were investigated in terms of Vickers hardness, the area fraction of precipitates, microstructure and phase composition. The solution treatments were performed by treating at 520 °C, 550 °C and 580 °C in argon atmosphere. The outcomes show that the hardness of the solutionized alloys was slightly affected by the solution temperature. X-ray diffraction and image analysis revealed that the complete dissolution of precipitates was not possible, neither for Elektron21 (El21) nor for its AlN containing nanocomposites. The ageing treatment of El21 led to a significant improvement in hardness after 20 h, while for longer times, it progressively decreased. The effect of ageing on the hardness of El21–AlN composites was found to be much less than this effect on the hardness of the host alloy. Electron backscatter diffraction (EBSD) analysis of El21 and El21–1%AlN after solution treatment confirm the random orientation of grains with a typical texture of random distribution. The as-cast creep results showed that the incorporation of nanoparticles could effectively improve the creep properties, while the results after solution treatment at 520 °C for 12 h followed by ageing treatment at 200 °C for 20 h confirmed that the minimum creep rate of T6-El21 was almost equal to the as-cast El21–AlN.

STRUCTURE PECULIARITIES OF AL–HF–SC MASTER ALLOYS

The microstructure peculiarities of the new Al–Hf–Sc master alloys were studied using the methods of optical and scanning electronic (SEM) microscopy in combination with EDX analysis. The alloys studied included the meta-stable intermetallic compounds (aluminides) having cubic lattices identical to those in the matrix of aluminum alloys. Binary and ternary alloys were melted in graphite crucibles at a carbon-resistance furnace under an argon atmosphere. Al–0,96at.%Hf (5,98 wt.% Hf) and Al–0,59at.%Hf (3,77 wt.% Hf) alloys were prepared by superheating above the melting point up to about 200 and 400 degrees respectively. Melts were poured into a bronze casting form where crystallization rate was ~103 degrees/sec. Besides stable aluminides with tetragonal lattices, Al3Hf metastable aluminides with cubic lattices were formed only in the melt superheated by 400 degrees above the melting point. The degree of superheat for ternary alloys where Aln(Hf1–xScx) meta-stable aluminides were formed was 240, 270 and 370 degrees. The hafnium fraction in the Aln(Hf1–xScx) aluminides changed from 0,46 to 0,71 depending on the Hf : Sc ratio in the alloy. The master alloys produced (at.%): Al–0,26Hf–0,29Sc and Al–0,11Hf–0,25Sc (wt.%: Al–1,70Hf–0,47Sc and Al–0,75Hf–0,42Sc) demonstrate fine grain structures with meta-stable aluminides of Aln(Hf0,58Sc0,42) and Aln(Hf0,46Sc0,54) compositions respectively. Aluminide sizes are less than 12 and 7 μm. Their crystal lattice mismatch with the aluminum alloy matrix lattice is less than for Al3Sc. This fact allows us to expect high modifying effects of the experimental Al–Hf–Sc master alloys in their further application. In addition, replacement of expensive scandium with hafnium in the master alloys can reduce scandium consumption considerably.