Thermal stability and crystallization kinetics of sputtered amorphous Si3N4 films

The crystallization of thin silicon nitride (Si₃N₄) films deposited on polycrystalline SiC substrates was investigated by X-ray diffractometry as a function of annealing time. The amorphous Si₃N₄ films were produced by means of reactive r.f. magnetron sputtering. Annealing at temperatures between 1300 and 1700 °C led to the formation of crystalline films composed of -Si₃N₄ and β-Si₃N₄. The fraction of β-Si₃N₄ in the films reaches approximately 40% at temperatures above 1550 °C. Both polymorphic modifications were formed simultaneously during the crystallization process. A transformation of -Si₃N₄ to β-Si₃N₄ could not be observed in the time and temperature range investigated. The crystallization process of amorphous Si₃N₄ can be described according to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism, assuming a three-dimensional, interface controlled grain growth from pre-existing nuclei. The rate constants show an Arrhenius behaviour with an activation enthalpy of approximately 5.5 eV.     

Texture of copper films on Ta35Si18N47 and Ti33Si23N44 underlayers

The crystallographic texture and the grain size have been measured by X-ray diffraction techniques for about 200 nm-thick Cu films sputter-deposited on amorphous Ta₃₅Si₁₈N₄₇ and Ti₃₃Si₂₃N₄₄ underlayers, and for comparison also on TiN underlayers and oxidized silicon, all on Si (100) substrates. The (111) texture of the as-deposited Cu films increases in the sequence TiN233Si₂₃N₄₄35Si₁₈N₄₇. Amorphous Ta₃₅Si₁₈N₄₇ and Ti₃₃Si₂₃N₄₄ layers evidently promote quite effectively the growth of highly (111) textured Cu films. After vacuum annealing at 450°C for 30 min the texture of Cu rises on Ti₃₃Si₂₃N₄₄, falls on Ta₃₅Si₁₈N₄₇, while that on TiN and on SiO₂ changes little and the sequence becomes TiN235Si₁₈N₄₇33Si₂₃N₄₄. The grain size of the as-deposited Cu films increases in the sequence Ti₃₃Si₂₃N₄₄235Si₁₈N₄₇47N₅₃ and rises moderately upon annealing, least for TiN and most for SiO₂ and Ti₃₃Si₂₃N₄₄.     

The effect of Si3N4 on the thermal expansion behavior of MoSi2

The effect of Si₃N₄ particulates on the thermal expansion coefficient (CTE) of MoSi₂ was investigated. It was observed that as the volume percent of Si₃N₄ increases, the CTE of the MoSi₂-Si₃N₄ composites decreases. In the temperature range 1000–1500 °C, typical of that required for glass melting, about 30–35 vol% Si₃N₄ particulates are needed in the MoSi₂-Si₃N₄ composites such that the CTE of the composite matches the CTE of Mo.     

WS2/g-C3N4异质结光催化分解水制氢性能及机制

通过溶剂蒸发和二次高温煅烧石墨相碳化氮(g-C₃N₄)纳米片和WS₂纳米片混合物构建WS₂/g-C₃N₄异质结,该异质结保留g-C₃N₄和WS₂主体结构的同时,在界面处形成化学键,确保该异质结的化学稳定性和热稳定性。光催化分解水制氢实验表明,WS₂纳米片含量为3wt%时光催化制氢速率高达68.62 μmol/h,分别是g-C₃N₄纳米片和WS₂纳米片的2.53倍和15.29倍,表明异质结的构建可大幅提升g-C₃N₄的光催化性能,循环实验表明该异质结在5次循环实验后光催化性能没有明显下降,表明该异质结的稳定性较好。光电性能测试表明异质结的构建不仅提高激发电子的转移效率,同时抑制激发电子空穴的复合率,大幅提升激发电子的利用效率,致使光催化分解水制氢速率较g-C₃N₄纳米片和WS₂纳米片大幅提升。      

Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si₃N₄ ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO₂ was used as a sintering additive rather than using Li₂O, since its grain boundary glass is not stable above 1200 °C. LiAlO₂ was synthesised from 42% Li₂CO₃ and 58% Al₂O₃ powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO₂ was obtained. LiAlO₂ was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO₂ containing Si₃N₄ compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si₃N₄ ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.     

Deposition chemistry in the Cat-CVD processes of the SiH4/NH3 system

Laser spectroscopic as well as mass-spectrometric techniques were employed to examine the deposition chemistry in the catalytic chemical vapor deposition processes of the SiH₄/NH₃ system. The absolute densities of NH, NH₂ and SiH₃ radicals were measured under various conditions. The densities of the stable products, H₂ and N₂, as well as those of the reactants, NH₃ and SiH₄, were also measured. The NH₂ density is always higher than that of NH and both densities decrease by the addition of SiH₄. The SiH₃ density increases nonlinearly with the increase in the SiH₄ pressure. The SiH₃ density was found to be much higher than that of NH₂ under near practical deposition conditions to fabricate Si₃N₄ films (an NH₃ to SiH₄ flow-rate ratio of 50:1, a total pressure of 20 Pa and a catalyzer temperature of 2300 K). No aminosilane molecules were identified, suggesting that the contribution of aminosilyl radicals to the film deposition is minor. Thus, NH₂ and SiH₃ must be the major deposition species to form Si₃N₄.     

Low temperature plasma-assisted oxidation and thin-film deposition processes for forming device-quality SiO2/Si and composite dielectric-SiO2/Si heterostructures

In the high-temperature thermal oxidation of Si, the SiO₂/Si interface is continuously regenerated as the bulk oxide grows. This paper describes an alternative low temperature, 200–300 °C, plasma-assisted process that optimizes electrical properties of SiO₂/Si interfaces and bulk SiO₂ layers by separately controlling interface formation and bulk oxide deposition. Composite dielectrics, oxide/nitride (ON) and oxide (ONO), have been fabricated by extending the low temperature plasma-assisted processes to include deposition of Si₃N₄ films. The electrical properties of SiO₂/Si structures formed by the two-step, low temperature oxidation-deposition process are essentially the same as those of SiO₂/Si structures formed by high temperature, 850–1050 °C, thermal oxidation. The electrical properties of devices incorporating ON and ONO composite dielectrics are degraded with respect to the SiO₂/Si structures, but are similar to those of composite dielectrics formed by combinations of high temperature processing.     

Auger-free luminescence of the BaF2:Sr, BaF2:MgF2 and CsBr:LiBr crystals under excitation of VUV photons and high-energy electrons

The emission spectrum, the time dependence of the luminescence, the excitation spectrum and the reflectance spectrum have been measured for the pure BaF₂ crystal, the BaF₂:Sr crystals with Sr⁺⁺ concentrations of 2 and 5 mol%, the BaF₂:MgF₂ crystal with a mole mixing ratio of 1:2, and the pure CsBr crystal and the CsBr:LiBr crystal with a mole mixing ratio of 1:1. The measurements have been made by using synchrotron orbital radiation and high-energy electrons produced by gamma-rays. It is shown that the BaF₂, the BaF₂:Sr and the BaF₂:MgF₂ crystals have the Auger-free luminescence having decay times around 0.90 ns. The slow and fast luminescence components of the BaF₂ crystal are suppressed by introduction Sr⁺⁺ and Mg⁺⁺ ions. It is, however, clearly seen that the suppression of the slow component is more effectively occurring than that of the fast component, especially in the BaF₂:MgF₂ crystal. By analyzing the Auger-free luminescence spectrum, it is shown that the valence band widths are 2.7 ± 0.3 eV, 2.7 ± 0.3 eV and 4.2 ± 0.3 eV and the band-gap energies are 11.1 ± 0.3 eV, 11.1 ± 0.4 eV and 11.9 ± 0.4 eV for BaF₂, BaF₂:Sr and BaF₂:MgF₂ crystals, respectively. The introduction of LiBr into CsBr has resulted in a significant enhancement in the luminescence intensity, attaining 40 times larger intensity than that of CsBr crystal at room temperature, due to the suppression of the non-radiative relaxation of the outer-most core holes due to the absorption induced by the Urbach effect. The observations of the increases in the band gap-energy, the exciton energy and the decay time of the CsBr:LiBr crystal compared with those of the CsBr crystal support the argument given in the present study regarding the suppression of the Urbach effect in the CsBr:LiBr crystal. The decay times observed are 0.20 ± 0.05 ns and 0.90 ± 0.06 ns for CsBr and CsBr:LiBr crystals, respectively.     

Effect of the microstructure of Si3N4 on the adhesion strength of TiN film on Si3N4

The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si₃N₄ was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase ( or β) of starting Si₃N₄ powder, and sintering temperature. The microstructure of Si₃N₄ was characterized in terms of grain size, aspect ratio of the elongated grain, and β-to- phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si₃N₄ were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si₃N_4, such as critical loads for yield (P_y) and for ring cracking (P_c). The effect of the microstructure of Si₃N₄ on adhesion strength evaluated by scratch test is discussed. TiN films on Si₃N₄ showed high adhesion strengths in the range of 80–140 N. Hardness and the P_y of Si₃N₄ substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si₃N₄, substrates with finer grain sizes and higher phase ratios, which show high hardness and high P_y, were suitable for higher adhesion strength of TiN film.     

Si_3N_4原位增强Cu基复合材料的制备

采用感应炉熔炼及水雾化工艺制得了Cu-Si合金粉末,经N2、H2混合气体选择氮化和真空放电等离子烧结(SPS)成型,制备得到了Si3N4原位增强Cu基复合材料(Si3N4/Cu),利用萃取法研究了选择性氮化产物及其晶体结构。结果表明:复合粉末中N含量随氮化温度的升高和氮化时间的延长而增大。在1 000℃下氮化,持续时间大于60h时,粉末中的N含量明显提高;Cu的衍射峰出现整体向大角度方向的明显偏移,同时晶格常数变小,表明Si从Cu基体中脱溶,与N反应生成Si3N4;Si3N4/Cu复合材料的增强体以β-Si3N4为主;随着氮化温度的升高和氮化时间的延长,Si3N4/Cu复合材料的电导率和硬度逐步提高。     

Chemical and electrical characteristics of low temperature plasma enhanced CVD silicon oxide films using Si2H6 and N2O

Silicon oxide films have been deposited at low temperatures in the range of 30–250 °C using Si₂H₆ and N₂O by conventional plasma enhanced chemical vapor deposition technique. The dependencies of deposition temperatures on the film properties are studied. The leakage current and the etch rate of these low temperature films compare favorably to films deposited by silane and TEOS at higher temperatures, respectively.     

Investigation of the physical and mechanical properties of hot-pressed machinable Si3N4/h-BN composites and FGM

A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si₃N₄ ceramics. Silicon nitride/hexagonal boron nitride (h-BN) ceramic composites and functionally graded materials were fabricated by hot pressing at 1750 °C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si₃N₄/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. The texture of h-BN and β-Si₃N₄ was observed during hot pressing sintering. Physical and mechanical properties of Si₃N₄/h-BN with different content of h-BN were investigated, such as bulk density, Vickers's hardness, flexural strength, and elastic modulus. All of these properties are important for the design of the machinable Si₃N₄/h-BN FGM (Functionally Graded Materials).     

Sensitivity enhancement for H2 gas detection using a SnO2–Ag2O–PdOx nanocrystalline system

Thick film H₂ sensors were fabricated using SnO₂ loaded with Ag₂O and PdO_x. The composition that gave highest sensitivity for H₂ was in the wt.% ratio of SnO₂:Ag₂O:PdO_x as 93:5:2. The nano-crystalline powders of SnO₂–Ag₂O–PdO_x composites synthesized by sol–gel method were screen printed on alumina substrates. Fabricated sensors were tested against gases like H₂, CH₄, C₃H₈, C₂H₅OH and SO₂. The composite material was found sensitive against H₂ at the working temperature 125 °C, with minor interference of other gases. H₂ gas as low as 100 ppm can be detected by the present fabricated sensors. It was found that the sensors based on SnO₂–Ag₂O–PdO_x nanocrystalline system exhibited high performance, high selectivity and very short response time to H₂ at ppm level. These characteristics make the sensor to be a promising candidate for detecting low concentrations of H₂.     

颗粒级配对Si_(3)N_(4)w/Si_(3)N_(4)复合材料弯曲强度与介电性能的影响EI北大核心CSCD

为了获得一种弯曲和介电性能良好的氮化物陶瓷材料,本工作首先以氮化硅晶须(Si_(3)N_(4w))为原料,采用喷雾造粒工艺制备3种具有不同粒径分布的Si_(3)N_(4w)球形颗粒粉体,研究雾化盘转速对Si_(3)N_(4w)球形颗粒粉体粒径分布的影响。然后以喷雾造粒得到的Si_(3)N_(4w)球形颗粒为原料,采用干压法制备3种颗粒级配的Si_(3)N_(4w)预制体,研究颗粒级配Si_(3)N_(4w)预制体的孔径分布。采用化学气相渗透(CVI)和先驱体浸渍裂解(PIP)工艺在3种颗粒级配的Si_(3)N_(4w)预制体中进一步制备Si_(3)N_(4)基体,研究Si_(3)N_(4w)/Si_(3)N_(4)复合材料制备过程中的物相和微结构演变以及颗粒级配对Si_(3)N_(4w)/Si_(3)N_(4)复合材料的微结构、密度、弯曲强度和介电性能的影响。结果表明:3种颗粒级配的Si_(3)N_(4w)预制体均具有二级孔隙特征,其中小孔孔径均约为0.7μm,大孔孔径分别为45.2,30.1μm和21.3μm。在制备的3种颗粒级配的Si_(3)N_(4w)/Si_(3)N_(4)复合材料中,S13样品的颗粒级配效果最好,复合材料的弯曲强度达到81.59 MPa。此外,该样品的介电常数和介电损耗分别为5.08和0.018。良好的弯曲强度和介电性能表明制备的Si_(3)N_(4w)/Si_(3)N_(4)复合材料有望应用于导弹天线罩领域。     

Si3N4/FePd/Si3N4薄膜的晶体结构和磁性能研究

采用磁控溅射的方法制备了Si₃N₄/FePd/Si₃N₄三层膜, 研究了非磁性材料Si₃N₄作为插入层对磁记录FePd薄膜结构与磁性能的影响。结果表明, 热处理后Si₃N₄分布在FePd纳米颗粒之间, 抑制了FePd晶粒的生长, 与纯FePd薄膜相比, Si₃N₄/FePd/Si₃N₄薄膜的颗粒明显得到细化; 通过添加Si₃N₄层, FePd薄膜的晶体学参数c/a从0.960减小到0.946, 表明Si₃N₄可以有效促进FePd薄膜的有序化进程, 同时提升了矫顽力和剩磁比, 分别提高到249 kA/m、0.86; 随着600℃退火时间的进一步延长, 添加Si₃N₄的薄膜磁性没有迅速下降, 在较宽的热处理时间范围内磁性能保持在比较高的水平, 提高了抗热影响的能力。Si₃N₄作为插入层对FePd薄膜的磁性能具有较大的提升作用, 这对磁记录薄膜的发展具有重要意义。     

CHEMICAL VAPOR DEPOSITION OF TITANIUM CARBIDE ON AISI M2 TOOL STEELS AND Si3N4-TiC CERAMIC COMPOSITE

TiC deposition experiments were performed on high speed steel and on Si₃N₄-TiC composite ceramic cutting tools through chemical vapor deposition (CVD) using gaseous mixture of TiCl₄, CH₄, and H₂. The effects of the deposition temperature and the composition of reactant gases on deposition rate, structure and microhardness of the TiC film were investigated. Experimental apparatus and deposition procedures are also presented. Deposition at 1323K with (CH₄/TiCl₄) ratio of 1.2 gives the optimum mechanical properties of the film for AISI M2 steel substrate while Si₃ N₄-TiC composite shows its maximum strength at 1373K and 1.3(CH₄/TiCl₄).     

Effect of heat treatment on photoluminescence behavior of BaMgAl10O17:Eu phosphors

The blue phosphor of BaMgAl₁₀O₁₇:Eu²⁺ (BAM) powders were prepared by solid-state reaction. The thermal degradation of BAM phosphor significantly reduces the intensity of the blue emission. BAM is reduced by an amount of 50% after heating at around 800 °C for 1 h. Photoluminescence (PL) excitation and emission spectra showed that the blue emission of 450 nm peak decreased with increasing annealing temperature. The ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ were observed at 590 and 615 nm emission lines over 1100 °C. Electron paramagnetic resonance (EPR) spectrum also detected two signals of Eu²⁺, corresponding to g=3.7156(9) for 88 mT, and g=2.9507(9) for 133 mT. X-ray absorption near edge structure (XANES) spectrum decreased the intensity of Eu²⁺ for 6977 eV with increasing annealing temperature, while high-energy peak of Eu³⁺ for 6984 eV was increased. The combined use of X-ray and neutron data by the Rietveld refinement appears to support that the secondary phase of EuMgAl₁₁O₁₉ magnetoplumbite structure in BAM may be formed by heat treatment.     

Fabrication of a depletion mode GaAs MOSFET using Al2O3 as a gate insulator through the selective wet oxidation of AlAs

A 1 μm thick undoped GaAs buffer layer, a 1500 Å thick n-type GaAs layer, an undoped 500 Å thick AlAs layer and a 50 Å thick GaAs cap layer were consecutively grown by molecular beam epitaxy (MBE) on a [100] oriented semi-insulating GaAs substrate. The AlAs layer was oxidized in a N₂ bubbled H₂O vapor ambient at 400°C for 3 h and fully converted to Al₂O₃ for use as a gate insulator. The I–V characteristics, having a maximum drain current of 10.6 mA, a current cut-off voltage of −4.5 V and a maximum transconductance value of 11.25 mS/mm, indicate that the selective wet thermal oxidation of AlAs/GaAs was successful in producing a depletion mode GaAs MOSFET.     

Creep behavior of SiC fiber-reinforced hot-pressed Si3N4 composites

The creep response of SiC fiber-reinforced Si₃N₄ composites has been measured using four-point flexural loading at temperatures of 1200–1450°C and stress levels ranging from 250 to 350 MPa. Parameters characterizing the stress and temperature dependence of flexural creep strain rates were determined. A numerical analysis was also performed to estimate the power-law creep parameters for tensile and compressive creep from the bend test data. The incorpoporation of SiC fiber into Si₃N₄ resulted in substantial improvements in creep resistance even at very high stresses. The steady-state creep deformation mechanism, determined to be subcritical crack growth in the unreinforced matrix, changed to a mechanism in the composites of repeated matrix stress relaxation-fiber rupture-load dispersion by the matrix. Multiple fiber fracture rather than multiple matrix cracking resulted. The tertiary creep in the composite resulted from the rapid growth of the microcracks which initiated from the fiber rupture sites. Fiber strength, matrix cracking stress and interfacial shear strength have been identified as the key microstructural parameters controlling the creep behavior of the composite.     

Porous SnO2 sputtered films with high H2 sensitivity at low operation temperature

Undoped and Pd-doped SnO₂ films were deposited at various substrate temperatures and discharge gas pressures using reactive magnetron sputtering. Structural factors of the films, such as crystallite size, grain size, and film density, were systematically investigated. The main objectives of this study are to clarify the operation temperature dependence of the H₂ sensitivity of these films as well as to clarify the dominant structural factor in the determination of the sensitivity. The operation temperature at which the sensitivity defined by (R_a−R_g)/R_g, where R_a and R_g are the resistances before and after exposure to H₂, showed a maximum decreased with decreasing film density. The highest sensitivity of 4470 was obtained for a Pd-doped film with the lowest density of 3.1 g/cm³ at 100 °C. It was found that the sensitivity correlated with film density rather than with crystallite size and grain size. The high sensitivity of a Pd-doped porous film at a low temperature was discussed in relation to the Schottky-barrier-limited transport as well as the chemical and electronic effects of Pd.