Understanding silica-supported metal catalysts: Pd/silica as a case study

Supported metal catalysts, particularly noble metals supported on SiO₂, have attracted considerable attention due to the importance of the silica–metal interface in heterogeneous catalysis and in electronic device fabrication. Several important issues, e.g., the stability of the metal–oxide interface at working temperatures and pressures, are not well-understood. In this review, the present status of our understanding of the metal–silica interface is reviewed. Recent results of model studies in our laboratories on Pd/SiO₂/Mo(1 1 2) using LEED, AES and STM are reported. In this work, epitaxial, ultrathin, well-ordered SiO₂ films were grown on a Mo(1 1 2) substrate to circumvent complications that frequently arise from the silica–silicon interface present in silica thin films grown on silicon.     

Fabrication of Schottky barrier MOSFETs using self-assembly CoSi2 nanopatterning and spacer gate technologies

A self-assembly patterning method for generation of epitaxial CoSi₂ nanostructures was used to fabricate 50 nm channel-length MOSFETs. The transistors have either a symmetric structure with Schottky source and drain or an asymmetric structure with n⁺-source and Schottky drain. The patterning technique is based on anisotropic diffusion of Co/Si atoms in a strain field during rapid thermal oxidation. The strain field is generated along the edges of a mask consisting of 20 nm SiO₂ and 300 nm Si₃N₄. During rapid thermal oxinitridation (RTON) of the masked silicide structure, a well-defined separation of the silicide layer forms along the edge of the mask. These highly uniform gaps define the channel region of the fabricated device. The separated silicide layers act as metal source and drain. A poly-Si spacer was used as the gate contact. The asymmetric transistor was fabricated by ion implantation into the unprotected CoSi₂ layer and a subsequent out-diffusion process to form the n⁺-source. I–V characteristics of both the symmetric and asymmetric transistor structures have been investigated.     

Microstructure and properties of MoSi2–MoB and MoSi2–Mo5Si3 molybdenum silicides

MoSi₂-based intermetallics containing different volume fractions of MoB or Mo₅Si₃ were fabricated by hot-pressing MoSi₂, MoB, and Mo₅Si₃ powders in vacuum. Both classes of alloys contained approximately 5 vol.% of dispersed silica phase. Additions of MoB or Mo₅Si₃ caused the average grain size to decrease. The decrease in the grain size was typically accompanied by an increase in flexure strength, a decrease in the room temperature fracture toughness, and a decrease in the hot strength (compressive creep strength) measured around 1200 °C, except when the Mo₅Si₃ effectively became the major phase. Oxidation measurements on the two classes of alloys were carried out in air. Both classes of alloys were protected from oxidation by an in-situ adherent scale that formed on exposure to high temperature. The scale, although not analyzed in detail, is commonly recognized in MoSi₂ containing materials as consisting mostly of SiO₂. The MoB containing materials showed an increase in the scale thickness and the cyclic oxidation rate at 1400 °C when compared with pure MoSi₂. However, in contrast with the pure MoSi₂ material, oxidation at 1400 °C began with a weight loss followed by a weight gain and the formation of the protective silica layer. The Mo₅Si₃ containing materials experienced substantial initial weight losses followed by regions of small weight changes. Overall, the MoB and Mo₅Si₃ additions to MoSi₂ tended to be detrimental for the mechanical and oxidative properties.     

Oxidation behavior of silicide coating on Ti3SiC2-based ceramic

A silicide coating was prepared on Ti₃SiC₂-based ceramic by pack cementation to improve the oxidation resistance of Ti₃SiC₂, which is a technologically important material for high temperature applications. The microstructure, phase composition and oxidation resistance of the coated sample were investigated. The results demonstrated that the silicide coating was mainly composed of TiSi₂ and SiC. A single layer of a mixture of SiO₂ and TiO₂ was formed on the surface of the coated sample during isothermal oxidation at 1100 °C and 1200 °C for 20h. Compared to Ti₃SiC₂, the parabolic rate constant of silicide coated Ti₃SiC₂ decreased by 2~3 orders of magnitude. Furthermore, the coated sample showed much better cyclic oxidation resistance than Ti₃SiC₂ during the cyclic oxidation at 1100 °C for 400 times. However, during the preparation of the coating, a number of fine cracks formed in the outer layer of the coating. When these cracks penetrated the whole coating during the cyclic oxidation, the oxidation rate was accelerated, which degraded the oxidation resistance. Electronic Publication     

High‐performance thin‐film transistor using silicide‐mediated crystallization

Abstract— We studied the silicide‐mediated crystallization of a‐Si for low‐temperature polycrystalline‐silicon (LTPS) on glass. By controling the heating method and Ni density on the a‐Si, the grain size could be increased to 40 μm. Radial grain growth from a NiSi₂ crystalline nucleus gives rise to a large‐grain poly‐Si without amorphous phase inside. A field‐effect mobility of over 200 cm²/V‐sec was achieved by using LTPS.     

Surface analysis of β-FeSi2 layer epitaxially grown on Si(100)

The epitaxial growth process of β-FeSi₂ on Si(100) surface under ultrahigh vacuum condition has been studied by low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The LEED pattern of Si (100)-2×1 changes into amorphous structure with Fe deposition of about 10 Å at room temperature. With annealing at 540 °C, the LEED pattern shows 2×2 structure corresponding to the formation of the epitaxial β-FeSi₂ (100) template layer. The α-scan in Li⁺-LEIS and X-ray diffraction (XRD) study strongly suggest that the topmost surface of the 2×2 structure is terminated by Si atoms. By XRD, it is shown that the β-FeSi₂ develops with characteristic orientation even if iron reactant is deposited onto the template surface.     

Ultra-high temperature oxidation resistance of a novel (Mo,Hf, W,Ti)Si2 ceramic coating with Nb interlayer on Ta substrate

In order to solve the challenge of recyclability of tantalum substrates in high temperature oxidation environments, a novel MoSi₂-WSi₂-HfSi₂-TiSi₂ composite ceramic coating containing an Nb interlayer was prepared on the surface of tantalum substrate by a three-step method. The mix ceramic silicide coating exhibited superior performance and effective protection for 10.2 h at 1800 °C, possibly due to the formation of an outer SiO₂-HfO₂-HfSiO₄ composite oxide film with low oxygen permeability, moderate viscosity and thermal expansion coefficient, as well as good self-healing ability. Furthermore, the coating successfully passed 537 thermal cycles from room temperature to 1800 °C. The presence of Nb interlayer significantly mitigated the thermal mismatch between the ceramic coating and the tantalum substrate, and the bidirectional diffusion of Nb element during the high temperature oxidation and thermal shock process further reduced the tendency of the coating to crack.     

Hot wire chemical vapor deposition: limits and opportunities of protecting the tungsten catalyzer from silicide with a cavity

Hot Wire Chemical Vapor Deposition (HW-CVD) is one of the most promising techniques for depositing the intrinsic microcrystalline silicon layer for the production of micro-morph solar cells. However, the silicide formation at the colder ends of the tungsten wire drastically reduces the lifetime of the catalyzer, thus limiting its industrial exploitation. A simple but interesting strategy to decrease the silicide formation is to hide the electrical contacts of the catalyzer in a long narrow cavity which reduces the probability of the silane molecules to reach the colder ends of the wire. In this paper, the working mechanism of the cavity is elucidated. Measurements of the thickness profile of the silicon deposited in the internal walls of the cavity have been compared with those predicted using a simple diffusion model based on the assumption of Knudsen flow. A lifetime study of the protected and unprotected wires has been carried out. The different mechanisms which determine the deterioration of the catalyzer have been identified and discussed.     

钼离子注入硅薄层硅化钼的合成

用大束流密度的钼金属离子注入硅，能够直接合成性能良好的薄层硅化物。随着束流密度的增加，硅化钼生长，薄层硅化物的方块电阻Ｒｓ明显下降，当束流密度为０．５Ａ／ｍ＾２时，Ｒｓ达到上值９０Ω，说明连续的硅化物已经形成。Ｘ衍射分析结果表明，注入层中形成了３种硅化钼Ｍｏ３Ｓｉ、Ｍｏ５Ｓｉ３和ＭｏＳｉ２。经过９００℃退火后，Ｒｓ下降至４Ω，电阻率可小到０．１６μΩ．ｍ，说明硅化钼薄层质量得到了进一步的改善。大束     

化学汽相淀积耐熔金属及其硅化物

本文综述了化学汽相淀积耐熔金属及其硅化物研究方面的最新进展,并对它们进行了讨论。