Magnetic and structural properties of nitrified FINEMET powder using mechanical milling method

The magnetic and structural properties of FINEMET [Fe_73.5Si_13.5B₉Nb₃Cu₁ wt%] amorphous powder were investigated after nitrification and mechanical milling. Fe-based amorphous powder were nitrified and crystallized simultaneously at 550 °C using by ammonia (NH₃) gas. Nitrified powder exhibited iron nitride phase such as γ′-Fe₄N, Fe₃N, and α″-Fe₁₆N₂. Nitrified particles were more brittle than raw particles. As a result, nanometer-sized nitride powder was fabricated by high-energy ball milling method. The saturation magnetization (M_s) and coercivity (H_c) of nitrified powder were increased due to nitride phase.     

Electronic structure of silicon nitride according to ab initio quantum-chemical calculations and experimental data

Charge transfer ΔQ = 0.35e at the Si-N bond in silicon nitride is determined experimentally using photoelectron spectroscopy, and the ionic formula of silicon nitride Si₃^+1.4N₄^−1.05 is derived. The electronic structure of α-Si₃N₄ is studied ab initio using the density functional method. The results of calculations (partial density of states) are compared with experimental data on X-ray emission spectroscopy of amorphous Si₃N₄. The electronic structure of the valence band of amorphous Si₃N₄ is studied using synchrotron radiation at different excitation energies. The electron and hole effective masses m _e ^* ≈ m _h ^* ≈ 0.5m _e are estimated theoretically. The calculated values correspond to experimental results on injection of electrons and holes into silicon nitride.     

Effect of tellurium on electrical and structural properties of sintered silicon nitride ceramics

The effects of tellurium (Te) additives on electrical conductivity, dielectric constant and structural properties of sintered silicon nitride ceramics have been studied. Different amounts of Te (10% and 20%) were added as sintering additives to silicon nitride ceramic powders and sintering was performed. Microstructure and composition were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrical conductivity and dielectric constant (ε′) increase exponentially with temperature greater than 800 K. The electrical conductivity and dielectric constant increase but activation energy decreases from 0.72 to 0.33 eV with the increase of Te concentration. However, the conductivity increases five orders of magnitude at the concentration of 10% of Te in Si₃N₄. As the Te concentration increases the sintered silicon nitride ceramics become denser. These types of samples can be used as high temperature semiconducting materials.     

Investigations of silicon nitride and boron carbide monolayers and Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/B<Subscript>4</Subscript>C multilayers

Many different coating concepts for improving mechanical properties have been worked out. One of the advanced coating concepts is the multilayer and superlattice concept, mainly with one or two metallic components. Previous work has shown that the mechanical properties of the covalent-bonded Si₃N₄ and SiC could be improved when combining them in a multilayer system. In the present work the silicon nitride monolayer from the earlier work was combined with boron carbide instead of silicon carbide. First, the boron carbide thin films deposited at different substrate temperatures were examined. Then the number of monolayers in the multilayer system with a constant layer thickness was varied in order to investigate the influence of the interfaces on film properties of the multilayer system. PACS 81.05.Je; 81.15.Cd; 82.80.Pv; 87.64.Je     

Ion tracks in amorphous silicon nitride

The morphology of the tracks of swift ions revealed in amorphous silicon nitride after treatment in an HF solution is studied. The Si₃N₄/Si structures were irradiated with Fe, Kr, and W ions in the electron energy loss regime. Discontinuous tracks were recorded upon exposure to W only, with an electron energy loss of 20.4 keV nm⁻¹ being maximal for the conditions of our experiment. The results from calculations of the track formation in Si₃N₄ based on the thermal spike model are presented.     

Capturing properties of a threefold coordinated silicon atom in silicon nitride: Positive correlation energy model

The electronic structure and capturing properties of threefold coordinated silicon atoms (≡Si·) and the Si-Si bond in silicon nitride (Si₃N₄) were studied using the ab initio density functional theory. The results show that the previously proposed negative correlation energy (NCE) model is not applicable to Si₃N₄. The NCE model was proposed for interpreting the absence of the ESR signal for threefold coordinated silicon defects and suggested that an electron can transfer between two silicon defects. We proposed that the absence of this ESR signal is due to the creation of neutral diamagnetic Si-Si defects in Si₃N₄. This model offers the most fundamental theory for explaining the hole localization (memory) effect in silicon nitride.     

氮化硅（Si3N4）微粉的超高压烧结研究

 氮化硅（Si₃N₄）是具有优良性能的陶瓷材料，具有广泛的工程应用前景。但由于这种化合物属于共价结合性质并具有很高的熔点，陶瓷的烧结需要在很高的温度下进行。本文探讨了在超高压力下这种材料的烧结行为的变化。对含有AlN、Al₂O₃及少量La₂O₃添加物的Si₃N₄微粉在3~7 GPa，800~2 000 ℃的压力温度范围内进行烧结，并应用X射线衍射、扫描电子显微镜观察及密度测量等方法研究了烧结样品。结果表明高压力可以大大促进Si₃N₄的α→β转变及固相反应，降低烧结温度。高压下烧结过程分为两个阶段：低温下以粉末粒子结合为主要内容的过程及高温下各物相间的相变和固相反应为主的过程。研究表明，超高压力下烧结没有改变所研究组份样品的复合相组成。     

纳米非晶氮化硅键态结构的X射线径向分布函数研究

本文用X射线径向分布函数法研究了室温到1000℃不同退火条件下的纳米非晶氮化硅样品的微结构和键合特征。观察到占庞大体积百分数界面不是“gas-like”结构,而是与非晶纳米粒子不同的新的短程序结构。Si—N键长和最近邻原子配位数(CN)均比传统Si₃N₄小,并存在大量的Si悬键和不饱和键。纳米氮化硅与传统Si₃N₄饱和共价键不同,是含有大量非饱和键和悬键的非典型共价键结构。由于键配位的不饱和特征,纳米非晶氮化硅的分子式应写作Si_3-xN_4-y。纳米非晶氮化硅出现强极性与非饱和键和悬键有密切的关系。 关键词：     

Effect of processing parameters on the deposition rate of Si3N4/Si2N2O by chemical vapor infiltration and the in situ thermal decomposition of Na2SiF6

Silicon nitride (Si₃N₄) and oxynitride (Si₂N₂O) were deposited by chemical vapor infiltration (CVI) through a novel route involving the in-situ thermal decomposition of Na₂SiF₆ in commercial nitrogen precursors containing impurity oxygen. In addition, the quantitative effect of processing time (30, 60, 90, 120 min), temperature (1000, 1100, 1200 and 1300 °C), nitrogen precursor (N₂ or N₂-5%NH₃) and gas flow rate (46.5, 93, 120 and 240 cm³/min) on phase percentage and deposition rate of Si₃N₄ and Si₂N₂O was investigated. Analysis of variance shows that the parameter that most significantly impacts the total amount of deposited phase is the processing temperature, followed by processing time and nitrogen precursor. Regardless of the nitrogen precursor, at 1300 °C, Si₃N₄ and Si₂N₂O depositions follow an S-like and parabolic behavior, respectively. The incubation period shown by Si₃N₄ in N₂-5%NH₃ is associated to a decrease in the O₂ partial pressure during Si₂N₂O formation while the rapid increase at long processing times is attributed to the enhanced effect of hydrogen. PACS 81.15.Gh; 81.05.Je; 81.15.-z; 81.05.Rm; 47.85.L-     

The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si¹⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si¹⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si⁴⁺ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si₃N₄ by molecular beam epitaxy (MBE).     

Excimer laser surface treatment of bulk ceramics

Changes of surface morphology following excimer laser (KrF, ArF) irradiation were investigated for three technical ceramic materials (TZP, PSZ, Si₃N₄). Zirconia samples exhibit a smooth and glass-like surface at the low power density regime, while at higher power densities colour changes were observed due to changes in the chemical composition of the bulk material. In the silicon nitride samples new crystals of specific orientation are formed, which could be crystalline silicon.     

Electrical characteristics of metal-dielectric-semiconductor structures with silicon nitride prepared by reactive cathode atomization

The effect of the technology of preparation of silicon nitride in a low temperature gas discharge plasma upon the volt-ampere and volt-farad characteristics of metal-dielectricsemiconductor (MDS) structures (Al-Si₃N₄-Si-Al) is studied. It is shown that by using a heterogeneous Si₃N₄ formation reaction with ionic purification of the silicon surface, it is possible to obtain MDS structures with lower and more stable surface charge in comparison to similar structures in which the Si₃N₄ is grown by other methods (for example, gas transport reaction methods). The conductivity of the Si₃N₄ film is described approximately by the well known Frankel model, and its value is close to that of Si₃N₄ films prepared by other methods.     

Silicon cluster formation in the laser ablation of SiO at 308 nm

Neutral silicon cluster formation in the laser (308 nm) ablation of silicon monoxide was investigated through the analysis of composition and dynamics of the ablation plume under different laser fluence conditions. The neutral species were ionized by a second laser (193 nm) and the positionized species detected by TOF-MS (time-of-flight mass spectrometry). At low laser fluences, plume composition is dominated by SiO; above 0.6 J/cm² Si, SiO and Si₂ have comparable intensity and Si_n (n≤7) clusters are observed. Flow velocities and temperatures of the ejected species are nearly mass-independent, indicating that the plume dynamics are close to the strong expansion limit, implying a collisional regime. Through the relation between the estimated values of terminal flow velocity and surface temperature, u_T²∝T_S, it is found that, at low laser fluences, the surface temperature increases linearly with laser fluence, whereas, at the laser fluence at which Si_n clusters are observed, the increase of temperature is below the linear dependence. The population distribution of the ejected Si_n provides some indication of a formation mechanism based on condensation. Analogies between the ablation behavior of silicon monoxide and silicon targets are considered. PACS 82.30.Nr; 81.05.Gc; 78.70.-g     

n-type conductivity in Si-doped amorphous AlN: an ab initio investigation

We report the electronic structure and topology of a heavily Si-doped amorphous aluminium nitride (Al_37.5Si_12.5N₅₀) using ab initio simulations. The amorphous Al_37.5Si_12.5N₅₀ system is found to be structurally similar to pure amorphous aluminium nitride. It has an average coordination number of about 3.9 and exhibits a small amount of Si–Si homopolar bonds. The formation of Si–Al bonds is not very favourable. Electronic structure calculations reveal that the Si doping has a negligible effect on the band gap width but causes delocalization of the valence band tail states and a shift of the Fermi level towards the conduction band. Thus, amorphous Al_37.5Si_12.5N₅₀ alloys show n-type conductivity.     

Controllable off-plane deflection of cantilevers for multiple scanning proximity probe arrays

The presented work reports the fabrication of 4×32 arrays of fully addressable proximity probes, which are initially and controllably off-plane deflected (bent). Such a deflection is required for the simultaneous approach and scanning of all cantilevers. It is realized by deposition of the silicon cantilevers with Si₃N₄ film inducing tensile stress. ANSYS simulations are used to calculate the off-plane deflection for different thickness of the cantilever and the Si₃N₄ layer and compared with experimentally obtained values. Cantilever arrays with set bending up to 50 μm, employing LPCVD silicon nitride film with a tensile stress of 750 MPa are fabricated.     

氮化硅薄膜的微结构

利用TEM,STM和PDS显微光度计研究了ECR-PECVD技术制备的Si₃N₄薄膜的微结构.结果表明：在较低沉积温度下，ECR-PECVD制备的Si₃N₄薄膜是一种纳米α-Si₃N₄薄膜，其晶粒粒度在14—29nm间，而且这种薄膜具有较好的表面平整度.初步分析了ECR-PECVD制备的Si₃N₄在较低沉积温度下形成晶态薄膜的机理. 关键词：     

Solid-state NMR investigations of Si-29 and N-15 enriched silicon nitride

We compare ²⁹Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra from the two modifications of silicon nitride, α-Si₃N₄ and β-Si₃N₄, with that of a fully (²⁹Si, ¹⁵N)-enriched sample ²⁹Si₃¹⁵N₄, as well as ¹⁵N NMR spectra of Si₃¹⁵N₄ (having ²⁹Si at natural abundance) and ²⁹Si₃¹⁵N₄. We show that the ¹⁵N NMR peak-widths from the latter are dominated by J(²⁹Si–¹⁵N) through-bond interactions, leading to significantly broader NMR signals compared to those of Si₃¹⁵N₄. By fitting calculated ²⁹Si NMR spectra to experimental ones, we obtained an estimated coupling constant J(²⁹Si–¹⁵N) of 20 Hz. We provide ²⁹Si spin-lattice (T₁) relaxation data for the ²⁹Si₃¹⁵N₄ sample and chemical shift anisotropy results for the ²⁹Si site of β-Si₃N₄. Various factors potentially contributing to the ²⁹Si and ¹⁵N NMR peak-widths of the various silicon nitride specimens are discussed. We also provide powder X-ray diffraction (XRD) and mass spectrometry data of the samples.     

Electronic band structure and effective masses of electrons and holes in the α and β phases of silicon nitride

The electronic band structure of the two main crystallographic modifications of silicon nitride, namely, the α-Si₃N₄ and β-Si₃N₄ phases, is calculated from the first principles. The estimates obtained for the effective charges of silicon and nitrogen atoms and those for the effective masses of electrons and holes in the α-Si₃N₄ phase are in good agreement with the available experimental data for amorphous silicon nitride a-Si₃N₄. The calculations performed demonstrate that the effective mass tensor determined for the β-Si₃N₄ phase differs substantially from the effective mass tensor obtained for the α-Si₃N₄ phase.