Effect of aluminum nitride on the properties of cordierite

The low dielectric constant and thermal expansion coefficient of cordierite are two attractive properties for the material to be used in electronic applications. However, compared to some other materials such as alumina, aluminum nitride (AlN), and beryllium oxide, it has a much lower mechanical strength and thermal conductivity. In the present work, AlN-cordierite composite systems are fabricated using different AlN contents, and the effect of AlN content on the mechanical, thermal, and dielectric properties of the AlN-cordierite system are investigated. It was observed that there exists an optimum AlN content for the mechanical and thermal properties while the dielectric property decreases with an increase of AlN content. An explanation for the observed data trend is offered based on an effective void concept.     

Determination of interfacial properties between AlN and aluminum beneath salt at high temperature

Aluminum nitride is of interest as a material for electrolysis cells in the aluminum industry due to its chemical stability when in contact with molten aluminum and/or cryolite-based salt melts. It has also been considered in combination with electrically conductive materials (i.e. AlN/Al-composite) as a material for drained cathode systems in Hall-Heroult processes. Knowledge of the interfacial properties of AlN in contact with molten aluminum and/or cryolite-based melts is therefore important. This paper reports observations of the wettability of AlN by aluminum under salt cover at high temperature using an X-ray technique. Results obtained in this work combined with previously published data are used for the assessment of the work of adhesion of molten aluminum on AlN under vacuum as well as under a cryolitic salt. Scanning electron microscopy examination of metallographic sections was used to confirm the nature of the interfaces. The measured contact angle between AlN and molten aluminum beneath a salt cover at 850 °C is 136° demonstrating the non-wettability of AlN by liquid aluminum under these conditions. The work of adhesion of molten aluminum on AlN is higher under vacuum than under salt. Previously published data allowed the determination of the interfacial properties between liquid aluminum and AlN under a salt cover. The interfacial energy between molten aluminum and salt is 773 mN/m at 850 °C. The work of adhesion of aluminum on AlN is 217 and 1322 mN/m under salt and under vacuum, respectively.     

TEM observation of liquid-phase bonded aluminum–silicon/aluminum nitride hetero interface

Liquid-phase bonded aluminum–silicon/aluminum nitride interface structure was investigated using high-resolution transmission electron microscopy. A textured layer of aluminum formed a stable orientation relationship with aluminum nitride, which showed Al(111) to be tilted by about 4° with respect to the AlN(0001) interface plane. The unique orientation relationship between Al and AlN was predicted as one of the stable orientation relationships using coincidence of reciprocal lattice point method, which surveys the degree of geometrical coherency between two crystals in three-dimensional space. A stable orientation relationship was found to be (001)[1_boxclose \bar{1} 0]Al//(2[`2] \bar{2} 03)[11[`2] \bar{2} 0]AlN.     

Mechanism of in situ formation of AlN in Al melt using nitrogen gas

In situ processing of AlN particle reinforced aluminum composites was investigated using a gas bubbling method with nitrogen gas as the gaseous precursor and pure aluminum as the starting matrix in the temperature range of 1173–1573 K. The products were characterized using XRD, SEM, and EDS techniques. Experimental results showed that it is feasible to synthesize AlN particle reinforced Al composites in situ using purified nitrogen gas. Significant AlN was synthesized by bubbling deoxidized N₂ through Al melt. The AlN particles synthesized in situ were small in size (<10 m) and were enriched in the top part of the product formed in the crucible. Directly bubbling commercial purity nitrogen gas did not lead to formation of significant AlN due to the deleterious effect of the trace oxygen impurities in the bubbling gas. The deleterious effect of trace oxygen impurities on the mechanism of formation of AlN in the Al-N system was critically analyzed from both thermodynamic and kinetic points of view. Chemisorption of O₂ molecules at the gas bubble-Al melt interface is more favorable and much faster than that of N₂, thereby inhibiting chemisorption of N₂ molecules. Significant AlN can be formed only at the content of oxygen below a critical value in the N₂ bubbling gas.     

Synthesis of aluminum nitride fibres from aluminum silicate fibres by carbothermal reduction method

Aluminum nitride (AlN) fibres were synthesized by carbothermal reduction of aluminum silicate fibres and carbon black in a flowing nitrogen atmosphere. XRD and SEM were employed to study the phase structure, chemical composition and morphologies of the products. It is suggested that aluminum silicate fibres can be converted into AlN fibres by carbothermal reduction. SEM confirmed that the fibres maintained the morphologies of aluminum silicate fibres during reduction, but their details of surface have been altered. A carbothermal nitriding model of the aluminum silicate fibres has been developed.     

微波碳热还原法制备氮化铝粉末的工艺研究

采用微波碳热还原法制备了氮化铝粉末,研究了铝源、碳源和添加剂对制备氮化铝粉末的影响. 通过对所合成的产物进行XRD检测分析表明,氢氧化铝和乙炔黑是最合适的铝源和碳源、单质添加剂的氮化催化效果最明显. 以氢氧化铝和乙炔黑为原料,加入单质添加剂,在氮气气氛下反应温度为1300℃、反应时间为1h时能获得完全氮化的氮化铝粉末,可见微波碳热还原工艺能够大大降低碳热还原法制备氮化铝粉末的反应温度,并缩短反应时间.     

AlN颗粒在不同铝合金中的增强行为

LD2为了解AlN颗粒对不同强度等级的铝合金的增强效果及机制 ,对 40 %体积分数的AlN颗粒增强 10 70、10 6 1、LY12铝合金复合材料拉伸前后的微观组织进行了观察 ,发现拉伸前在基体中存在由热错配引起的高密度位错 ,在AlN颗粒的内部也存在大量的位错 ,拉伸后基体中的位错增殖 ,同时 ,AlN颗粒中的位错亦增多 .力学性能的测试结果表明 ,AlN颗粒对低强度、高塑性的L3纯铝增强率最高 ,中等强度、较高塑性的LD2铝合金不仅有较高的增强率 ,而且保持了一定的塑性 .AlN颗粒对基体的这种选择性主要与AlN颗粒在拉伸过程中产生微量变形 ,从而松弛部分界面应力有关 .LY12基体的塑性较低 ,易产生低应力断裂 ,因此 ,AlN颗粒的增强作用难以得到充分发挥 .     

Preparation of Aluminum Nitride Thin Films by CVD

AlN films were prepared by CVD using aluminum halide (AlCl₃) and aluminum alkyl ((CH₃)3Al) precursors. The appropriate deposition conditions to obtain polycrystalline AlN films using A1C1₃ precursor were found at T_dep = 1173 K, P_tot = 66.6 Pa and N₂/NH₃ = 0.75. It was found that AlN films of different crystallinity can be obtained from (CH₃)₃Al precursor at T_de = 973-1023 K, P_tot = 1.99 kPa, only under a H₂ atmosphere. AlN films can be grown highly oriented in the (210) direction on amorphous quartz substrates depending on their thicknesses. The 0.1 -0.2 μm thick AlN films were transparent and their refractive indexes were about 1.4-1.6.     

超高真空电子束蒸发合成晶态AlN薄膜的研究

用超高真空蒸发Ａｌ膜，结合氮化合处理工艺在Ｓｉ（１００）衬底上制备了ＡｌＮ晶态薄膜，用Ｘ射线衍射，傅立叶变换红外光谱和Ｘ射线光电子能谱等测试分析技术研究了薄膜的微结构特征。结果表明：经过１０００℃分钟氮化处理后，能形成具有（００２）择优取向的ＡｌＮ薄膜。     

The adhesion of copper films deposited onto aluminum nitride

Good adhesion between copper film and AlN substrate is obtained when the surface of AlN is laser-irradiated prior to copper film deposition and post deposition annealing is conducted. Surface chemistry of AlN substrates before and after laser irradiation and the interfacial reactions of copper film/AlN couples were studied with Auger Electron Spectroscopy (AES) to understand the adhesion mechanisms. The surface of as-received AlN substrates was covered with a thin sheath of Al₂O₃. Laser irradiation removed the surface Al₂O₃ layers, smoothened the surface, and decomposed AlN leaving metallic aluminum on the surface. The interfacial reactions in the copper film/AlN couple are affected by the amounts of oxygen and metallic aluminum available at the interface. The adhesion mechanism is the formation of a Cu-O-Al compound at the interface of copper film/AlN couple. Since copper does not react with AlN, laser induced decomposition of AlN seems to be the driving force for the formation of the compound. This revised version was published online in September 2006 with corrections to the Cover Date.     

离子注入形成AIN埋层的分析与计算机模拟

以离子背散射和二次离子质谱分析了大剂量N~+离子注入A1形成的A1/A1N/A1多层结构。基于离子注入形成A1N埋层的机理,利用计算机程序动态模拟A1N层的形成过程及终态结构。研究发现,N/A1原子比饱和于A1N化学组分比,模拟与实验结果符合得很好;低能注入能降低形成A1N层的临界剂量,且A1/A1N界面更陡。     

Evaluation on fatigue strength of AISI 4340 steel aluminum coated by electroplating and IVD processes

In spite of toxicity, hydrogen embrittlement susceptibility, and environmental issues, cadmium electroplating is usually applied on high strength AISI 4340 aeronautical steel due to its efficient protection against electrochemical corrosion. Ion vapor deposition (IVD) process with pure aluminum also offers good protection against corrosion with the advantages of decreasing hydrogen embrittlement susceptibility and improving the fatigue strength of metallic components. In this research, the effects of aluminum electroplating and IVD aluminum coating on the rotating bending fatigue strength of AISI 4340 steel were evaluated in comparison with cadmium electroplated specimens. Experimental fatigue results showed that both aluminum electroplating and IVD aluminum coatings are possible alternatives to cadmium electroplating.     

A model for longitudinal piezoelectric coefficient measurement of the aluminum nitride thin films

This paper proposes a new model for the longitudinal piezoelectric coefficient (LPC) measurement of the aluminum nitride (AlN) thin film on (100) Si substrate, the AlN thin film is fabricated by the direct-current magnetron sputtering and the piezoelectricity of the AlN thin film is measured by the piezoresponse force microscopy (PFM) in contact mode. In this model, the electric field distribution is taken into account, and the electrostriction displacement caused by the local field concentration is excluded from the measured displacement by the PFM. A LPC value of 4.22 ± 0.34 pm/V is obtained for the clamped AlN thin film by this model, and the deviation between this value and that measured under homogenous field condition is <5.7 %. Therefore, it is reasonable to apply our model to the piezoelectricity characterization of AlN thin films when using the PFM. Furthermore, piezoelectricity of other thin films could also be characterized using this model, which could simplify the measurement process.     

The characterization of sputtered polycrystalline aluminum nitrideon silicon by surface acoustic wave measurements

Polycrystalline aluminum nitride films were deposited on Si₃ N₄ coated (100) silicon substrates by the reactive sputtering method. We have carried out experiments to evaluate the effect of AlN material parameters on the SAW characteristics. The SAW transducers were fabricated by forming interdigitated Al electrodes on top of the AlN films and transmission measurements made over the frequency range from 50 MHz to 1.5 GHz. The SAW characteristics were correlated with material parameters of crystallite orientation, grain size, surface morphology and oxygen concentration. A key material parameter affecting the SAW characteristics was found to be the preferred degree of crystallite orientation with the c-axis normal to the plane of the substrate. The better oriented the AlN grains, the stronger the SAW response, the higher the SAW phase velocity, and the lower the insertion and propagation losses over the entire frequency range of measurement. Above 500 MHz the propagation losses of the well oriented films followed a frequency squared dependence only slightly higher than the reported values for the best epitaxial films. The coupling factors deduced from the transducer characteristics are in the upper range of values reported by Tsubouchi for epitaxial AlN films deposited on the basal plane of sapphire. There was a strong correlation between the X-ray diffraction intensity from the (002) planes and the oxygen content in the films     

Solar-blind Al x Ga1–x N (x > 0.45) p–i–n photodiodes with a polarization-p-doped emitter

Technical Physics Letters - Polarization-induced p-type doping of AlGaN layers with high aluminum content during plasmaassisted MBE growth has been studied. It is shown that a gradient of the AlN...     

Study on microstructure and thermal conductivity of Spark Plasma Sintering AlN ceramics

Dense aluminum nitride (AlN) ceramics were prepared by Spark Plasma Sintering with rare-earth oxide and CaF₂ as sintering additives. The effect of sintering additives on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that those sintering additives not only promoted densification through liquid-phase sintering but also improved thermal conductivity by decreasing oxygen impurities. Thermal conductivities of samples sintered with optimum proportion of rare-earth oxide and CaF₂ were higher than those of other samples. During the Spark Plasma Sintering process, the microstructures, especially the content and distribution of secondary phases, played important roles on the thermal conductivity of AlN ceramics.     

Preparation of highly oriented aluminum nitride thin films on molybdenum bottom electrodes using metal interlayers

We have investigated the influence of metal interlayers on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films prepared on molybdenum (Mo) bottom electrodes. The interlayres were prepared between the Mo bottom electrodes and silicon substrates. Although the sputtering conditions of AlN films and Mo electrodes were the same, the Au/Ti interlayer drastically increased the XRD intensity of the (0002) AlN and (110) Mo planes, and decreased the full width at half maximum (FWHM) of the rocking curves of the (0002) AlN peaks from 9.22 to 3.02. The Au/Ti interlayers were effective for the improvement in the crystallinity and crystal orientation of AlN films deposited on Mo bottom electrodes. Furthermore, we clarified that the crystallinity and orientation of AlN films and Mo electrodes strongly depend on those of the Au interlayers, and the Au interlayers influence the morphologies of the Mo electrodes.     

Influence of AlN buffer layer thickness and deposition methods on GaN epitaxial growth

A gallium nitride (GaN) epitaxial layer was grown by metal-organic chemical vapor deposition (MOCVD) on Si (111) substrates with aluminum nitride (AlN) buffer layers at various thicknesses. The AlN buffer layers were deposited by two methods: radio frequency (RF) magnetron sputtering and MOCVD. The effect of the AlN deposition method and layer thickness on the morphological, structural and optical properties of the GaN layers was investigated. Field emission scanning electron microscopy showed that GaN did not coalesce on the sputtered AlN buffer layer. On the other hand, it coalesced with a single domain on the MOCVD-grown AlN buffer layer. Structural and optical analyses indicated that GaN on the MOCVD-grown AlN buffer layer had fewer defects and a better aligned lattice to the a- and c-axes than GaN on the sputtered AlN buffer layer.     

Deposition and characterization of reactive magnetron sputtered aluminum nitride thin films for film bulk acoustic wave resonator

This work studies the relationship between the deposition process parameters and the properties of sputtered c-axis-oriented aluminum nitride (AlN) thin films. AlN films were deposited on a Pt electrode by reactive magnetron sputtering under various deposition conditions. The films were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). A polycrystalline AlN film with highly c-axis-preferred orientation was achieved. The XRD rocking curve was 2.7°. The FESEM photographs also show that the AlN film has a dense hexagonal surface texture with uniform grain size and a highly ordered column structure.     

Growth of columnar aluminum nitride layers on Si(111) by molecular beam epitaxy

Single crystalline aluminum nitride (AlN) thin films are deposited by molecular beam epitaxy (MBE) using thermally evaporated aluminum and RF-plasma excited nitrogen gas. In this paper we report on films grown on Si(111) at substrate temperatures of 800° with growth rates between 65 and 350 nm h⁻¹. All layers consist of hexagonal and exactly c-axis oriented AlN crystals with column-like structure. For the smoothest layers surface roughness (rms) around 1 nm is obtained. In the XRD-spectra (ω-scan) we have achieved a minimum FWHM of 0.4° (=25′) for the AlN(00.2) reflex. At maximum growth rates (350 nm h⁻¹) for AlN a transition zone of about 200 nm is formed with high defect density compared to the subsequent growth. For lower growth rates (65 nm h⁻¹) no transition zone exists. Application of a substrate nitridation leads to a partial loss of epitaxial relation between AlN layer and Si(111)-substrate.