Silicon Nitride

The aim of this study is to understand the effect of the glass frit chemistry used in thick-film Ag pastes on the electrical performance of the silicon solar cell. The study focuses on the physical behavior of the glass frit during heat treatment as well as the resulting Ag−Si contact interface structure. We observe that the glass frit transition temperature (Tg) and softening characteristics play a critical role in the contact interface structure. The glass transition temperature also significantly influences the contact ohmicity of the thick-film metal grid. A high glass frit transition temperature generally results in thinner glass regions between the Ag bulk of the grid and the Si emitter. It was found that a glass frit (with high Tg) that crystallizes fast during the firing cycle after etching the silicon nitride and Si emitter results in smaller Ag crystallite precipitation at the contact interface. This results in smaller junction leakage current density (Jo2) and higher open-circuit voltage (Voc). Using high Tg pastes (with the appropriate Ag powder size), greater than 0.78 fill factors and >17.4% efficiency were achieved on 4 cm2 untextured single crystal Si solar cells with 100 Ω/sq emitters.

Microstructure development and fracture toughness of Si3N4 composites were studied in the presence of seeds and Al2O3 + Y2O3 as sintering aids. The elongated β-Si3N4 seeds were introduced into two different α-Si3N4 matrix powders; one was the ultra fine powder matrix and the other was the coarse powder matrix. The amount of seeds varied from 0 to 6 wt%. The grain growth inhibition and the mechanism of toughening were discussed and correlated with microstructure. The maximum fracture toughness of 9.0 MPa m1/2 was obtained for ultra fine powder with 5 wt% seeds hot pressed at 1,700 °C for 6 h.

The feasibility of using IPAP for the deposition of AIN for 2-stroke engine applications has been tested and the results have proved favourable. The quality of the initial coatings deposited was poor in terms of surface roughness and porosity. However, the micro-hardness of the IPAP coatings is three times greater than conventional nitriding process. Further investigations are being carried out to optimise the IPAP process for coating 2-stroke engine components and obtain a better fundamental understanding of the processing parameters involved. It is also hoped to extend the utility of this process for silicon nitride coatings. The optimum parameters recorded for AIN coatings should facilitate initial trials. The IPAP process has shown several advantages over other plasma processing systems at laboratory scale. Yet, a transfer from a laboratory scale to an industrial scale needs further assessment. The current work on IPAP is directed towards a final achievement at an industrial scale processing system.

A low energy (1.45 kJ) Mather type plasma focus device is used for nitrogen ion implantation in mono-crystalline silicon. The silicon specimens are exposed to different number of focus shots by placing the targets in front of the anode at a fixed distance. Raman spectroscopy and X-ray diffraction are employed to characterize the implanted specimens. The results indicate that mono-crystalline silicon transforms into amorphous structure through micro-crystalline phase on increasing the implantation dose. Further irradiation of the specimens results in the formation of amorphous Si3N4 layers. High temperature annealing in argon ambient transforms the amorphous Si3N4 into β-Si3N4.

The effect of secondary sintering additives and/or a post-sintering heat treatment on the semicrystalline atomic structure of the intergranular phase in silicon nitride ceramics is investigated. Three different Yb-doped Si3N4 ceramic compositions are examined using a scanning transmission electron microscope, whereby the intergranular atomic structure is directly imaged with Ångstrom resolution. The resulting high-resolution images show that the atomic arrangement of the Yb takes very periodic positions along the interface between the intergranular phase and the matrix grains, and that a postsintering 1250 °C heat treatment, as well as a change of the secondary sintering additives (Al2O3 vs SiO2), does not alter the atomic positions of Yb. This result has implications for the understanding of how the mechanical properties of ceramics are influenced by the presence of the nanoscale intergranular phase, and for associated computational modeling of its precise role and atomic structure.

The results of an ongoing collaborative project between the New Jersey Institute of Technology (NJIT) and SEMATECH on the temperature-dependent emissivity of silicon-related materials and structures are presented in this study. These results have been acquired using a spectral emissometer. This emissometer consists of a Fourier Transform Infra-Red (FTIR) spectrometer designed specifically to facilitate simultaneous measurements of surface spectral emittance and temperature by using optical techniques over the near- and mid-IR spectral range and temperatures ranging from 300 K to 2000 K. This noncontact, real-time technique has been used to measure radiative properties as a function of temperature and wavelength for a wide range of silicon-related materials and structures. The first results of the temperature and wavelength dependent emissivity and hence refractive index of silicon nitride, in the literature, is presented in this study

Electrical insulating polymers usually incorporate inorganic fillers to achieve specific electrical, mechanical, thermal properties and reduce cost. In this exploratory work, we have studied the thermal properties, at 40degC, of epoxy resin composites with different filler such as: boron nitride, alumina, diamond, silicon carbide, and silicon nitride, with particle size ranges from micro to nano. The results show that BN is the best material in enhancing the thermal conductivity of the epoxy composites despite the various fillers having comparable or higher thermal conductivities than BN. Our current model shows that large disparities in mechanical modulus - a metric for interfacial compatibility - can cause thermal contact resistance due to increased phonon scattering resulting from the large mismatch between filler particle and resin matrix.

The first attempts to introduce carbon into glass date back to 1951. But up until recently, the use of carbon or carbide raw materials, and the oxidation, volatilization and decomposition that accompany high temperature melting, have limited the synthesis of true silicon oxycarbide glasses. Here, the term silicon-oxycarbide refers specifically to a carbon-containing silicate glass wherein oxygen and carbon atoms share bonds with silicon in the amorphous, network structure. Thus, there is a distinction between black glass, which contains only a second-phase dispersion of elemental carbon, and oxycarbide glasses which usually contain both network carbon and elemental carbon. In addition to exploring the unique properties and applications of these glasses, per se, they are also of interest for developing models of the residual amorphous phases in polymer-derived silicon-carbide and silicon-nitride ceramics. The application of sol/gel techniques to glass synthesis has significantly advanced the development and characterization of silicon oxycarbide glasses. In this approach, alkyl-substituted silicon alkoxides, which are molecular precursors containing oxygen and carbon functionalities on the silicon, can be hydrolyzed and condensed without decomposition or loss of the carbon functional group. A low-temperature ( It will be shown that the silicon oxycarbide network is stable up to 1000–1200°C. The network carbon is terminated with hydrogen (i.e., CH, =CH2 and –CH3), and with polyaromatic carbon (i.e., nC6Hx) wherein most of the elemental carbon resides. These glasses can be described as molecular composites of polyaromatic graphene-rings dispersed in a silicon oxycarbide network. After heating to temperatures in excess of 1000–1200°C, the oxycarbide network decomposes through the loss of hydrogen, and a two- or three-phase glass-ceramic consisting of nanocrystalline graphite, silicon carbide, and amorphous silica or cristobalite, is created. Some of the properties and applications of these glasses/glass-ceramics for coatings, composites and porous solids are summarized.

We investigate the breakdown (Vbr) enhancement potential of the field plate (FP) technique in the context of AlGaN/GaN power HEMTs. A comprehensive account of the critical geometrical and material variables controlling the field distribution under the FP is provided. A systematic procedure is given for designing a FP device, using two-dimensional (2-D) simulation, to obtain the maximum Vbr , with minimum degradation in on-resistance and frequency response. It is found that significantly higher Vbr can be achieved by raising the dielectric constant (εi) of the insulator beneath the FP. Simulation gave the following estimates. The FP can improve the Vbr by a factor of 2.8-5.1, depending on the 2-DEG concentration (ns) and εi. For n s=1×1013/cm2, the Vbr can be raised from 123 V to 630 V, using a 2.2 μm FP on a 0.8 μm silicon nitride, and 4.7 μm gate-drain separation. The methodology of this paper can be extended to the design of FP structures in other lateral FETs, such as MESFETs and LD-MOSFETs

This volume contains 65 papers from a symposium entitled "Chemical Aspects of Electronic Ceramics Processing," held over four days at the 1997 MRS Fall Meeting in Boston. The common themes in all of the research presented in this volume are the creative use of chemistry principles for ceramic fabrication and a multidisciplinary approach to materials research. Inorganic chemistry, solid-state chemistry, chemical engineering, materials science and engineering, and electrical engineering have all been skillfully combined to produce materials which will play an increasingly more important part in our lives. As in prior years, chemical vapor deposition (CVD) continues to be a popular area of research and was the subject of approximately half of the papers in this volume. Particularly "hot" areas of research are new and improved precursors, delivery systems for low-vapor pressure precursors, and improved processing and materials properties. Papers are evenly divided between oxide ceramics and non- oxide ceramics.

This article provides an overview of the state-of-the-art chemistry and processing technologies for silicon nitride and silicon nitride-rich films, i.e., silicon nitride with C inclusion, both in hydrogenated (SiN x :H and SiN x :H(C)) and non-hydrogenated (SiN x and SiN x (C)) forms. The emphasis is on emerging trends and innovations in these SiN x material system technologies, with focus on Si and N source chemistries and thin film growth processes, including their primary effects on resulting film properties. It also illustrates that SiN x and its SiN x (C) derivative are the focus of an ever-growing research and manufacturing interest and that their potential usages are expanding into new technological areas.

A comprehensive study was carried out to establish the effects of controlled stirring during solidification on the microstructure and mechanical properties of aluminium alloys, in comparison to conventionally gravity chill cast material. A novel device comprising a grooved reaction bonded silicon nitride rod rotating in a tube-like crucible was used to process aluminium alloys in the mushy state. The stir casting device was specially designed to also enable rheometric study of the alloys in this condition. A factorial design of experiments was used to determine the effect of the process variables shear rate (), shear time (ts), and volume fraction solid during shear (fs) on microstructure and both static and dynamic mechanical properties of the stir cast alloy. Investigation of the microstructure consisted of computer-aided image analysis of the primary phase morphology. A more globular primary phase was achieved at low values of fs, but this was not the optimum morphology for mechanical properties. In all cases, improved mechanical properties and reduced porosity were obtained in the stir cast condition in comparison with conventional casting and in comparison with previous work on stir casting. Comparison with alloy commercially rheocast via electromagnetic stirring, however, showed that the latter had superior mechanical properties. It is proposed that the mechanical stir casting process be considered as an alternative to gravity die casting in cases where very simple and thick walled shapes are required.

Many space systems such as satellite mirrors and their supporting structures require to be made from very low-thermal expansion materials combining both high hydrostability and relatively high mechanical properties. In this study, we have applied the “composite concept” in order to explore the possibility of fabricating near zero thermal expansion silicon nitride based ceramics. Consequently, a negative thermal expansion material belonged to the lithium aluminosilicate family (LAS powder crystallized under de β-eucryptite structure) was introduced in an alpha-silicon nitride fine powder (5 and 20 vol% of LAS) and the resulting composite system was sintered by Spark Plasma Sintering (SPS) at 1400 and 1500 °C. In the case of 20 vol% LAS compositions, relatively well-densified ceramics (94.4% of the theoretical density) were produced without adding any further sintering additive. The addition of yttria and alumina oxides allowed enhancing the densification level up to 98.2% (20 vol% LAS compositions) or from 62.3% up to 96.7% of the theoretical density in 5 vol% LAS materials. Nevertheless, it was impossible to full consolidate silicon nitride/LAS composite ceramics at temperatures lower than the temperature at which β-eucryptite melts, even by using SPS technology. Moreover, because of the relatively low temperatures involved in SPS, the α to β-Si3N4 transformation was avoided, resulting in microstructures composed of fine equiaxed α-Si3N4 grains (<200 nm) and of a glassy phase. Even if the effect of having a very large negative thermal expansion material was lost during the sintering step (because of the β-eucryptite melting), ceramics containing only 20 vol% of LAS-based phase exhibited very interesting values as regards of mechanical properties (strength, hardness, toughness, and Young's modulus), thermal conductivity and thermal expansion coefficient. We discuss in this work why we are so interested in developing dense silicon nitride/LAS ceramics sintered without any further additive addition, even though β-eucryptite is melted during the process and the transformation to the β phase is avoided.