Mo Silicide Research Articles

Fermi-level depinning in Mo/Si junctions by insertion of amorphous Si-rich Mo silicide film formed via gas-phase reactions of MoF6 with SiH4

We demonstrate the reduction of the electron Schottky barrier height (SBH) to 0.48 eV at Mo/n-type Si junctions through the insertion of a semimetal Si-rich Mo silicide (MoSi n , n = 7.9) film. Raman scattering measurements elucidated the persistence of the same amorphous structure within the MoSi n film even when subjected to temperatures as high as 900 °C. This excellent thermal stability yielded a notable result: preservation of the SBH modulation effect even following annealing at 700 °C. Moreover, we investigated the capacitance−voltage characteristics of MOS capacitors, revealing that MoSi n film has a remarkably low effective work function, measuring 4.1 eV when deposited onto SiO2. The deposition of MoSi n film was accomplished with an excellent coverage by using MoF6 and SiH4 gases. Thus, MoSi n film is a promising contact material in advanced CMOS transistors.

Fabrication of Si protrusions by local melting of a narrow current path on a Si wafer via resistive heating

Si protrusions were fabricated by surface melting and resolidifying of a Si(111) wafer covered with a 100 nm thick molybdenum (Mo) layer, in which a narrow region was generated using a microgrinder. This region results in a narrow current path and thus localized resistive heating, leading to specific melting of the Si in the path. The melted Si flowed away from the path to both sides, which were at temperatures lower than that of the path. Consequently, two Si protrusions ∼400 μm in height capped with Mo silicide precipitates were formed on each side of the burned-out path, as revealed by scanning electron microscopy with energy-dispersive X-ray spectroscopy. The formation mechanism was discussed in terms of the non-uniform temperature, surface tension, interdiffusion, and freezing-point depression of the surface. This study revealed the potential of fabricating an array of self-assembled Si protrusions by resistive heating of narrow paths.

Shape and Orientation Controlled Hydrothermal Synthesis of Silicide and Metal Dichalcogenide on a Silicon Substrate.

Shape-controlled MoS2 has been grown directly on a silicon substrate, for the first time, with the use of a facile hydrothermal synthesis approach. The growth morphology is dependent on the substrate orientation. Square, hexagonal, and triangular patterns of MoS2 are grown on Si(100), Si(110), and Si(111), respectively. Detailed studies reveal that Mo silicide is formed at the initial stage, and the formation of silicide patterns is dictated by the different surface energies of Si(100), Si(110) and Si(111). Subsequently, shaped MoS2 patterns are formed following the silicide ones at the thermodynamic equilibrium. The approach for the formation of these patterns can be generalized to other 2D materials and can also be formed on a large scale by a lithography method. The work has shown a new technique to form silicide via solution processing and grow patterned 2D materials directly on silicon substrates, which may have the potential for advancing next-generation electronic devices.

Microstructures and Oxidation Behavior According to Nb:Mo Ratio in a Nb–Mo–Si System with Si Pack Cementation Coatings

Research is being conducted on Mo- and Nb-based alloys that are used in the aerospace sector, including those used for advanced gas turbines and aircraft engines. There is a limit to using Mo, which has a high density among refractory metals, and a few studies exist describing the addition of Nb to Mo–silicide alloys. There is a lack of guidance research on the basic Nb:Mo ratio of alloys, and it is necessary to study how to improve oxidation resistance. Therefore, this study aims to improve oxidation resistance by controlling the ratio of Nb and Mo in (Nbx, Moy)Si2 coating layers with Si pack cementation coatings on Nb–Mo alloys. Static oxidation tests were carried out at 1200 °C for 6 h to confirm the oxidation characteristics. As a result, a SiO2 or SiO2 + Nb2O5 ceramic protective layer was formed on the surface. After the oxidation tests, alloys with a Nb content of less than 35 at.% were found to protect the surface. The ratios of Nb and Mo in the Nb–Mo alloy and silicide coating layer were compared, and the improvement of oxidation resistance is discussed in terms of microstructural evolution.

Processing of Mo silicide powders as filler particles in polymer-derived ceramic coatings for Mo-Si-B substrates

Oxidation resistant Mo5SiB2 powders, which will be used as filler particles in polymer-derived ceramic (PDC) coatings, are processed by mechanical alloying and heat treatment. After microstructural analysis of the silicide particles they are incorporated in a preceramic polymer slurry. Subsequent pyrolysis results in a multi-phase composite consisting of a silicon oxynitride matrix and homogeneously distributed Mo silicide particles. Using cyclic oxidation tests it is shown that the newly developed particle-filled PDCs are promising for future application as protective oxidation barrier coatings on Mo-based materials.

Preparation of Spherical Mo5Si3 Powder by Inductively Coupled Thermal Plasma Treatment

A method was developed to fabricate spherical Mo5Si3 powder by milling and spheroidizing using inductively coupled thermal plasma. A Mo5Si3 alloy ingot was fabricated by vacuum arc melting, after which it was easily pulverized into powder by milling due to its brittle nature. The milled powders had an irregular shape, but after being spheroidized by the thermal plasma treatment, they had a spherical shape. Sphericity was increased with increasing plasma power. After plasma treatment, the percentage of the Mo3Si phase had increased due to Si evaporation. The possibility of Si evaporation was thermodynamically analyzed based on the vapor pressure of Mo and Si in the Mo5Si3 liquid mixture. By this process, spherical Mo silicide powders with high purity could be fabricated successfully.

Environmentally Resistant Mo-Si-B-Based Coatings

High-temperature applications have demonstrated aluminide-coated nickel-base superalloys to be remarkably effective, but are reaching their service limit. Alternate materials such as refractory (e.g., W, Mo) silicide alloys and SiC composites are being considered to extend high temperature capability, but the silica surfaces on these materials require coatings for enhanced environmental resistance. This can be accomplished with a Mo-Si-B-based coating that is deposited by a spray deposition of Mo followed by a chemical vapor deposition of Si and B by pack cementation to develop an aluminoborosilica surface. Oxidation of the as-deposited (Si + B)-pack coatings proceeds with partial consumption of the initial MoSi2 forming amorphous silica. This Si depletion leads to formation of a B-saturated Mo5Si3 (T1) phase. Reactions between the Mo and the B rich phases develop an underlying Mo5SiB2 (T2) layer. The T1 phase saturated with B has robust oxidation resistance, and the Si depletion is prevented by the underlying diffusion barrier (T2). Further, due to the natural phase transformation characteristics of the Mo-Si-B system, cracks or scratches to the outer silica and T1 layers can be repaired from the Si and B reservoirs of T2 + MoB layer to yield a self-healing characteristic. Mo-Si-B-based coatings demonstrate robust performance up to at least 1700 °C not only to the rigors of elevated temperature oxidation, but also to CMAS attack, hot corrosion attack, water vapor and thermal cycling.

Environmental Resistant Coatings for High Temperature Mo and Nb Silicide Alloys

ABSTRACTThe challenges of a high temperature environment impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. In metallic systems the multiphase microstructures that can be developed in the Mo-Si-B system and Nb silicide alloys offer useful options for high temperature applications. Since the alloy compositions that exhibit the lowest oxidation rate will most likely not yield optimum mechanical properties performance, it is important to develop robust and compatible oxidation resistant coatings. An effective strategy to achieve the necessary environmental resistance is based upon the use of an integrated Mo-Si-B based coating that is applied by a pack cementation process to develop an aluminoborosilica surface and in-situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. The environmental performance requires resistance not only to high temperature oxidation, but also resistance to water vapor, CMAS (calcia-magnesia-aluminosilica) attack, hot corrosion and thermal cycling. Under these extended environmental conditions the Mo-Si-B based coating exhibits robust performance.

Microstructure and electrical properties of SiC dispersed molybdenum silicide composite

In this study, a core–shell composite powder with Mo as the shell material and β-SiC as the core material was produced via chemical vapour transport caused by the hydrogen reduction of Mo oxide. Using this core–shell composite powder, a Mo silicide composite with uniformly dispersed β-SiC was fabricated by pressureless sintering at 1400°C. The relative density of specimens sintered for 10 hours was 96.1% and the electrical resistivity was 4.68 × 10−5 Ω m. It was concluded that a β-SiC dispersed molybdenum silicide composite with enhanced electrical resistivity was successfully fabricated at a relatively low sintering temperature using the core–shell composite powder.

Synthesis of silicon and molybdenum–silicide nanocrystal composite films having low thermal conductivity

We synthesized semiconducting composite films of Si and Mo–silicide nanocrystals (NCs) by phase separation from amorphous Mo–Si alloy (Si:Mo=12:1). Raman measurements show that the crystallization occurred at ≥700°C. Transmission electron microscope images show that Si and Mo silicide NCs were grown to average diameters of 8nm and 11nm, respectively, by annealing at 800°C. The electrical resistivity of the composite film is semiconducting (0.2–1.2kΩcm at room temperature depending on preparation conditions) and decreases with temperature increase. The thermal conductivity was reduced to 2.2Wm−1K−1 by enhancement of phonon scattering at NC interfaces, suggesting that the composite film is promising as a high-efficiency Si-based thermoelectric material.

Structural properties of subnanometer thick Y layers in extreme ultraviolet multilayer mirrors

We studied the structure and optical properties of B(4)C/Mo/Y/Si multilayer systems. Using extended x-ray absorption fine structure measurements at the Y and Mo K-edge, the structure of the subnanometer thick Y layer and the underlying Mo layer were analyzed. It was found that even a 0.2 nm thick Y layer significantly reduced silicon diffusion toward Mo, thus reducing Mo silicide formation. Hard x-ray reflectometry showed that the difference in average interface roughness of the B(4)C/Mo/Y/Si multilayer structure compared to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures was negligible. Soft x-ray reflectometry showed optical improvement of B(4)C/Mo/Y/Si with respect to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures.

Compositional regions of single phases at 1800 °C in Mo-rich Mo–Si–B ternary system

Single-phase regions in Mo-rich Mo–Si–B alloys at 1800°C were experimentally determined using a field-emission (FE) gun-type electron probe micro-analyzer (EPMA) in this study. A quantitative analysis by FE-EPMA was conducted with the calibration method we applied in our previous study to improve the accuracy in the B measurements. The compositional ranges of the determined Mo solid solution, silicide and boride phases were in good agreement with those of Mo–Si and Mo–B binary phase diagrams. On the other hand, the determined solubility of B in Mo solid solution, Mo3Si and T1 (Mo5Si3) were quite different from that indicated in previously reported ternary phase diagrams. The compositional region of the T2 (Mo5SiB2) single-phase ranges from 9.7 to 13.3at% for Si and from 23.5 to 26.8at% for B. The following two points were made clear by the ternary phase diagram as determined in this study. One is that no stoichiometric T2 composition is in the T2 single-phase region, confirming no T2 single-phase material can be obtained at the stoichiometric composition at 1800°C. The another is that the T2 region of this study expands to a Si-rich area from the stoichiometric composition.

Recent Advances in the Development of Mechanically Alloyed Mo Silicide Alloys

We review the current development status of Mo-Si-B alloys consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 which could take advantage of the beneficial oxidation resistance of the silicide phases and of the outstanding mechanical properties of molybdenum. For adequate low temperature toughness a continuous Mo solid solution matrix should be established in the microstructure. Besides, wrought processing of such alloys at elevated temperatures requires the presence of an ultra-fine grained (UFG) microstructure. Both the prerequisites can be fulfilled using mechanical alloying (MA) as the crucial processing step which even yields nanostructured supersaturated powders after milling. However, values for the ductile-to-brittle transition temperature (DBTT) close to room temperature are unlikely due to grain boundary embrittlement by Si segregation. The possibility of reducing this segregation tendency by various micro-alloying additions will be demonstrated. Finally, the high temperature deformation behaviour of these UFG materials will be comparatively assessed against state-of-the-art Nickelbase single-crystalline superalloys.

Improvement of Charge-Storage Characteristics of Mo Nanocrystal Memory by Double-Layer Structure

An oxygen-incorporated Mo silicide layer was explored to form the Mo nanocrystals after rapid thermal annealing. Transmission electron microscopy showed the nanocrystals embedded in . Charge-storage characteristics of Mo nanocrystals influenced by Mo oxide and its surrounding oxide were investigated through X-ray photoelectron spectroscopy and electrical measurement. X-ray photoelectron spectral analyses revealed a redox reaction in the oxygen-incorporated Mo silicide layer after rapid thermal annealing at a critical temperature. The memory window and retention were improved because of the reduction of Mo oxide. Furthermore, the double-layer nanocrystal structure was fabricated through the annealed stacked oxygen incorporated Mo silicide layer. A larger memory window and long retention were found for the double-layer nanocrystal structure. We used an energy band diagram to explain the difference in retention characteristics between single- and double-layer structures.

Charge storage characteristics of Mo nanocrystal dependence on Mo oxide reduction

An oxygen incorporated Mo silicide was explored to form the Mo nanocrystals after rapid thermal annealing. Transmission electron microscopy showed the nanocrystals embedded in SiOx. Charge storage characteristics of Mo nanocrystals influenced by the Mo oxide and the surrounding oxide were investigated through x-ray photoelectron spectroscopy and the electrical measurement. X-ray photoelectron spectra analyses revealed the redox reaction in the oxygen incorporated Mo silicide layer after rapid thermal annealing. The memory window and retention were improved due to reduction in Mo oxide.

The European ULTMAT Project: Properties of New Mo- and Nb-Silicide Based Materials

AbstractThe development, in the frame of the European ULTMAT project, of alloys offering at least 150°C surface temperature increase above the Ni-based superalloys' capability is presented. The expected achievement of the project is a thorough evaluation of the capability of refractory metal (Nb and Mo) silicide based multiphase materials to withstand enhanced temperature turbine service conditions (up to 1300°C). This is based on microstructural, mechanical, physical and environmental investigations in close connection with industrial scale material processing and component fabrication technologies. The paper presents an overview of the project results. Base materials are the metal/intermetallic ductile/brittle composites in the Nb-Nb5Si3 and Mo-Si-B systems. Improvements in high temperature creep resistance (up to 1300°C) as well as oxidation resistance (700°C to 1300°C) have been obtained. Processing routes have been developed (ingot and powder metallurgy) that allowed the manufacture of complex shaped parts.

Formation of Mo Silicide Nanodot Memory by Rapid Thermal Annealing Dual Electron-Gun Evaporated Mo–Si Layer

In this study, distributed charge trapping centers molybdenum silicide nanodots, are demonstrated. The Mo silicide nanodots were formed in the rapid thermal annealed mixed Mo-Si layer deposited by a dual electron gun system. A significant memory effect is observed through the electrical measurements, which is attributed to the formation of Mo silicide nanodots. In addition, the memory window is large enough to be identified as logic level 0 and 1 for the application on nonvolatile memory. The manufacture processes are simple and compatible with the conventional semiconductor manufacturing processes.

High Temperature Deformation Behavior of a Mechanically Alloyed Mo Silicide Alloy

AbstractA 3-phase Mo-Si-B alloy consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) was manufactured employing mechanical alloying (MA) as the crucial processing step. After consolidation via cold compaction, sintering in hydrogen atmosphere and final hot isostatic pressing (HIP) at 1500°C, one obtains an ultra-fine microstructure with a nearly continuous Mo(ss) matrix and the sizes of all phases being in the 1 micron range. Tensile tests were carried out in vacuum at initial strain rates ranging from 10-4 to 10-2 s-1 and the temperature varied between n1200 an 1400 °C. With a stress exponent of about 2 and the activation energy being close to that of Mo-self diffusion the material exhibits superplasticity at temperatures as low as 1300°C and tensile strain to failures up to 400%, thus, making sound wrought processing on industrial-scale facilities at temperatures typical for refractory metals and alloys feasible. To enhance creep resistance at high temperatures the alloys were annealed at 1700°C for 10h for a coarsening of the microstructure. While, still, the average sizes of all phases were below 10 microns, a considerable reduction in minimum creep rate was noted. This finding also demonstrates the extraordinary high thermal stability of this 3-phase Mo-silicide alloy.

Effect of milling time on phase transition and grain growth during the annealing process of MA powders

The phase transition procedure of the mechanic alloy (MA) molybdenum disilicide (MoSi 2) was investigated. The influences of the milling time on the phase transition induced by annealing of the MA powders milled for different time were observed also. Prolong milling time restrained the β-MoSi 2 growth. The amorphous phase first transformed to the intermediate Mo silicide (MoSi 0.65) during the crystallization of the amorphous substance at the annealing process. The sequence of phase transition may be MoSi(amorphous) → MoSi 0.65 → β-MoSi 2 → α-MoSi 2. A critical crystalline size during the phase transition of β → α was observed to be approximately 20 nm in this experiment.

Multilayer diffusional growth in silicon–molybdenum interactions

Growth kinetics of the Mo–silicide layers formed by chemical vapor deposition of Si on a Mo substrate from the SiCl 4–H 2 gas mixtures at 1000 °C was investigated using the ‘Wang’ analysis of multilayer diffusional growth. All of the three Mo–silicide phases, tetragonal-MoSi 2, Mo 5Si 3 and Mo 3Si in the Mo–Si binary phase diagram were observed by cross-sectional transmission electron microscopy, and obeyed a parabolic rate law indicating diffusion-controlled growth. The intrinsic growth rates of the Mo 5Si 3 and Mo 3Si layers were estimated from their apparent growth rates measured in the Si/Mo diffusion couple. Good agreement was found with the reported values measured from apparent growth rates at the MoSi 2/Mo and Mo 5Si 3/Mo diffusion couples, respectively.