Mo Layer Research Articles

Enhanced sulfurization reaction of molybdenum using a thermal cracker for forming two-dimensional MoS2 layers.

We propose a method to fabricate two-dimensional (2D) molybdenum disulfide (MoS2) layers to overcome issues in typical fabrication processes by promoting the sulfurization reaction of molybdenum (Mo). A thin sputtered-Mo layer was sulfurized using a sulfur (S) thermal cracker to form 2D MoS2 layers. The effects of key process parameters such as cracking-zone temperature (TC-zone), thickness of the sputtered-Mo layer, and Ar pressure during deposition of the Mo layer were systematically investigated. The degree of thermal treatment of evaporated S vapor is controlled by varying TC-zone. The higher TC-zone enabled easy formation of thin MoS2 layers at a low substrate temperature of 250 °C due to the greatly enhanced sulfurization reaction. The thickness of the final MoS2 layers was controlled by changing the initial thickness of the sputtered-Mo film. Ultra-thin MoS2 film about 2-layers-thick was obtained by sulfurizing a 2 Å-thick Mo film. The chemical state of the MoS2 layers largely depended on the Ar pressure during the sputtering process of the initial Mo. Lower Ar pressure enhanced MoS2 formation due to more efficient substitution of the MoS2 phase for the MoO3 phase. By using the S thermal cracker, we demonstrate a method to easily fabricate 2D MoS2 layers, excluding some problematic issues such as toxic and expensive reactants, non-vacuum conditions susceptible to contamination, and high substrate temperature.

EUV Bragg reflectors with photonic superlattices

The basic properties in the extreme ultraviolet (EUV) of one-dimensional photonic crystals (Bragg reflectors) with incorporated superlattices are investigated by a numerical study using the multiple scattering method. The superlattice is realized in the “standard” Mo/Si system by periodically replacing certain Mo layers by Si layers. At 13.5 nm–the wavelength of interest for EUV lithography–the superlattice sharpens the reflection peak at normal incidence with only weak reduction of the peak value. Between normal incidence and total reflection at large angles, additional reflection peaks appear at certain angles where the reflection is zero for the “standard” Mo/Si system. By combining different superlattices and depth grading, the range of additional reflection peaks is extended towards all-angle reflection. The effect of interface imperfections is considered for the case of the interdiffusion of Mo and Si. The extension to other frequency ranges is addressed via band structure calculations.

Microstructure and oxidation behavior of Mo-Si-Al coating on Nb-based alloy

A Mo-Si-Al coating was prepared on Nb-based alloy utilizing D-gun spraying of a Mo layer followed by co-deposition of Si and Al. The coating possesses a three-layer structure, containing a Mo(Si,Al)2 outer layer, a middle layer composed of Mo5(Si,Al)3 and Al8Mo3, and an AlMo3 inner layer. Static oxidation test was carried out at 1250 °C and the oxidation behavior of the coating was investigated. The result indicates that the oxidation of the Mo-Si-Al coating obeys a parabolic law. The oxidation resistance of the substrate is remarkably enhanced by applying a Mo-Si-Al coating, owing to the formation of a protective external α-Al2O3 scale.

Emergence of ferrimagnetic half-metallicity in two-dimensional MXene Mo3N2F2

Ferrimagnetic half-metal is more promising in spintronic devices than its ferromagnetic counterpart due to its lower stray fields and favorable robustness of magnetism. In comparison to the three-dimensional counterpart, the realization on two-dimensional ferrimagnetic half-metal remains blank up to date. Here, based on first-principles calculations and Monte Carlo simulations, we predict a ferrimagnetic half-metallicity in two-dimensional MXene Mo3N2F2 with a Curie temperature of 237 K and a considerable magnetic anisotropy energy. The ferrimagnetic coupling is mainly from the interactions of itinerant d electron between different Mo layers, and thus endows a 100% spin-polarization at the Fermi level with a sizable half-metallic gap of 0.47 eV. Such ferrimagnetic half-metallicity is also robust against external strains. Additionally, diverse magnetic and electronic characters can be controlled, depending on a differently terminated Mo3N2F2 surface. These findings provide an ideal platform to design spintronic devices related to two-dimensional ferrimagnetic half-metals.

Record Low Thermal Conductivity of Polycrystalline MoS2 Films: Tuning the Thermal Conductivity by Grain Orientation.

We report a record low thermal conductivity in polycrystalline MoS2 obtained for ultrathin films with varying grain sizes and orientations. By optimizing the sulfurization parameters of nanometer-thick Mo layers, five MoS2 films containing a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal, were grown. From transmission electron microscopy, the average grain size, typically below 10 nm, and proportion of differently oriented grains were extracted. The thermal conductivity of the suspended samples was extracted from a Raman laser-power-dependent study, and the lowest value of thermal conductivity of 0.27 W m-1 K-1, which reaches a similar value as that of Teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains in similar proportion. Analysis by means of molecular dynamics and finite element method simulations confirm that such a grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity.

High-temperature solar-thermal conversion by semiconducting β-FeSi2 absorbers with thermally stabilized silver layers

In-situ high-temperature measurements of optical constants (refractive indices n and extinction coefficients k) for each layer of solar selective absorbing multilayers were carried out to simulate solar-thermal conversion efficiency at high temperatures. Conventional Mo layers showed a significant rise in infrared emissivity and a degradation of selective absorbing behavior with the increasing temperature. In contrast, thermally stabilized Ag layers maintained low emissivity even at temperatures below 700°C. In these layers, agglomeration and vaporization were suppressed by SiNx nano-particles dispersed in the Ag matrix and interfacial W adhesive layers, respectively. The interband absorption of β-FeSi2 layers at higher temperatures revealed that larger n and k in the infrared region due to thermally excited electrons induced no rise in emissivity of the multilayers, and the narrower band gap based on Einstein's model shifted the absorptance cut-off wavelength closer to the ideal value and hence resulted in a higher solar absorptance. The solar-thermal conversion efficiencies of the multilayers consisting of low emissivity Ag layers with β-FeSi2 absorbers were found to be greater than 75.9%, as estimated from the temperature dependence of n and k spectra.

Study on the High Temperature Interfacial Stability of Ti/Mo/Yb0.3Co4Sb12 Thermoelectric Joints

To improve the interfacial stability at high temperatures, n-type skutterudite (SKD) thermoelectric joints with sandwich structures of Ti/Mo/Yb0.3Co4Sb12 were successfully designed and fabricated. In this structure, Mo and Ti were introduced as the barrier layer with the goal of suppressing the interfacial diffusion and the buffer layer with the goal of enhancing the bonding strength, respectively. To evaluate the high temperature interfacial behavior of the Ti/Mo/Yb0.3Co4Sb12 joints, thermal shocking between 0 °C and 600 °C and isothermal aging at a temperature range of 550 °C to 650 °C were carried out in vacuum. During the isothermal aging process, Ti penetrates across the Mo layer, and finally diffuses into the Yb0.3Co4Sb12 matrix. By increasing the isothermal aging time, Ti continuously diffuses and reacts with the elements of Sb and Co in the matrix, consequently forming the multilayer-structured intermetallic compounds of Ti3Sb/Ti2Sb/TiCoSb. Diffusion kinetics was investigated and it was found that the interfacial evolution of the Ti/Mo/Yb0.3Co4Sb12 joints was a diffusion-controlling process. During the diffusion process, the formed Mo-Ti buffer layer acts as a damper, which greatly decelerates the diffusion of Ti towards the Yb0.3Co4Sb12 matrix at high temperatures. Meanwhile, it was found that the increase in the contact resistivity of the joints mainly derives from the inter-diffusion between Ti and Yb0.3Co4Sb12. As a result, the Ti/Mo/Yb0.3Co4Sb12 joint demonstrates the excellent stability of the interfacial contact resistivity. Service life prediction was made based on the stability of the contact resistivity, and it was found that the Ti/Mo/Yb0.3Co4Sb12 joint is qualified for practical applications at 550 °C.

Low-stress and high-reflectance Mo/Si multilayers for extreme ultraviolet lithography by magnetron sputtering deposition with bias assistance.

To explore the potential of achieving low-stress and high-reflectance Mo/Si multilayers deposited by conventional magnetron sputtering with bias assistance, we investigated the effects of varying Ar gas pressure, substrate bias voltage, and a bias-assisted Si ratio on the stress and extreme ultraviolet (EUV) reflectance of Mo/Si multilayers. To reduce the damage of ion bombardments on an Si-on-Mo interface, only the final part of the Si layer was deposited with bias assistance. Bias voltage has strong influence on the stress. The compressive stress of Mo/Si multilayers can be reduced remarkably by increasing bias voltage due to the increase of Mo-on-Si interdiffusion and postponement of Mo crystallization transition. Properly choosing gas pressure and a bias-assisted Si ratio is critical to obtain high EUV reflectance. Appropriately decreasing gas pressure can reduce the interface roughness without increasing interdiffusion. Too much bias assistance can seriously reduce the optical contrast between Mo and Si layers and lead to a remarkable decrease of EUV reflectance. Thus, by appropriately choosing gas pressure, bias voltage, and a bias-assisted Si ratio, the stress values of Mo/Si multilayers can be reduced to the order of -100 MPa with an EUV reflectance loss of about 1%.

Solvothermal Synthesis, Gas‐Sensing Properties, and Solar Cell‐Aided Investigation of TiO2–MoOx Nanocrystals

AbstractTitania anatase nanocrystals were prepared by sol‐gel/solvothermal synthesis in oleic acid at 250 °C, and modified by co‐reaction with Mo chloroalkoxide, aimed at investigating the effects on gas‐sensing properties induced by tailored nanocrystals surface modification with ultra‐thin layers of MoOx species. For the lowest Mo concentration, only anatase nanocrystals were obtained, surface modified by a disordered ultra‐thin layer of mainly octahedral MoVI oxide species. For larger Mo concentrations, early MoO2 phase segregation occurred. Upon heat treatment up to 500 °C, the sample with the lowest Mo concentration did not feature any Mo oxide phase segregation, and the surface Mo layer was converted to dense octahedral MoVI oxide. At larger Mo concentrations all segregated MoO2 was converted to MoO3. The two different materials typologies, depending on the Mo concentration, were used for processing gas‐sensing devices and tested toward acetone and carbon monoxide, which gave a greatly enhanced response, for all Mo concentrations, to acetone (two orders of magnitude) and carbon monoxide with respect to pure TiO2. For the lowest Mo concentration, dye‐sensitized solar cells were also prepared to investigate the influence of anatase surface modification on the electrical transport properties, which showed that the charge transport mainly occurred in the ultra‐thin MoOx surface layer.

Ferroelectric of HfO2 dielectric layer sputtered with TiN or ZrN for sandwich-like metal-insulator-metal capacitors

In this study, metal-insulator-metal (MIM) capacitors, which use HfO2 as the intermediate insulating layer, and TiN or ZrN as the upper/lower capping layers were fabricated to form the sandwich-like structures, i.e. Al/TiN/HfO2/TiN/Mo/p-Si and Al/ZrN/HfO2/ZrN/Mo/p-Si. The crystallization of high-k HfO2 thin-film is induced by high power impulse magnetron sputtering (HIPIMS) during the deposition of TiN and/or ZrN capping layers. The ferroelectric performance became better in both two structures as the thickness of metal nitride layer decreased. As the rapid thermal annealing (RTA) temperature after TiN layer deposition increased, the ferroelectric performance of the structure with TiN layer became better. On the other hand, the structure with ZrN layer exhibited opposite trends. According to X-ray diffraction (XRD) measurement, both TiN and ZrN layers offered stress and produced orthorhombic phase on HfO2 layer. Both two layers at any thicknesses also protected Mo layer from the invasion of Hf atoms. The structure with TiN layer exhibited a higher remanent polarization (Pr) value as the roughness of TiN layer increased. However, the higher the roughness that the structure with ZrN layer had, the worse the ferroelectric performance obtained.

Investigation of the resistivity of molybdenum diselenide prepared with varied sodium content

Thin film solar cells made of Cu(In,Ga)Se2 or Cu2ZnSnSe4 tend to form a chalcogen phase with the molybdenum back contact during absorber processing. The impact on solar cell performance is manifold and depends on the MoSe2 layer properties. In this study, the characteristics of MoSe2 layers produced from Mo layers with varied Na content were investigated. The formation of MoSe2 for all samples was confirmed by Raman spectroscopy and X-ray diffraction (XRD). Scanning electron microscopy (SEM) shows a decrease in thickness of the MoSe2 layers with increased Na content. Results of glow discharge optical emission spectroscopy (GDOES) demonstrate that Na is incorporated in the Mo layers of the samples, while Na accumulates near the surface of the samples and at the MoSe2/Mo interfaces. Measurements of the electrical resistivity of the MoSe2 layers give a range of 270–600 Ωcm for all samples, confirming a low impact of Na on the electrical characteristics of the MoSe2 layers.

Evolution with thermal annealing of magnetic anisotropy in FeCoB thin film interfaced with Mo layers

Evolution of magnetic properties of thin FeCoB films sandwiched between two Mo layers has been studied as a function of thermal annealing at different temperatures. The as-deposited film exhibits a uniaxial magnetic anisotropy. With thermal annealing up to 300°C, anisotropy gradually decreases while coercivity exhibits a significant increase. Uniaxial anisotropy reappears after annealing at 400°C. In order to understand the origin of magnetic anisotropy in the film, in-plane and out-plane X-ray diffraction measurements have been done using Hard X-ray beamline BL-11 of INDUS-2. It is found that with thermal annealing the film exhibits significant changes in grain texture as well as internal stresses. Variation in grain texture in the film is expected to be the main cause for observed magnetic anisotropy.

Interface morphology of Mo/Si multilayer systems with varying Mo layer thickness studied by EUV diffuse scattering.

We investigate the influence of the Mo-layer thickness on the EUV reflectance of Mo/Si mirrors with a set of unpolished and interface-polished Mo/Si/C multilayer mirrors. The Mo-layer thickness is varied in the range from 1.7 nm to 3.05 nm. We use a novel combination of specular and diffuse intensity measurements to determine the interface roughness throughout the multilayer stack and do not rely on scanning probe measurements at the surface only. The combination of EUV and X-ray reflectivity measurements and near-normal incidence EUV diffuse scattering allows to reconstruct the Mo layer thicknesses and to determine the interface roughness power spectral density. The data analysis is conducted by applying a matrix method for the specular reflection and the distorted-wave Born approximation for diffuse scattering. We introduce the Markov-chain Monte Carlo method into the field in order to determine the respective confidence intervals for all reconstructed parameters. We unambiguously detect a threshold thickness for Mo in both sample sets where the specular reflectance goes through a local minimum correlated with a distinct increase in diffuse scatter. We attribute that to the known appearance of an amorphous-to-crystallization transition at a certain thickness threshold which is altered in our sample system by the polishing.

Reactive brazing Cf/SiC to itself and to Mo using the NiPdPtAu-Cr filler alloy

The NiPdPtAu-Cr filler alloy was proposed for joining Cf/SiC composites. The wettability on Cf/SiC composite was studied by the sessile drop method at 1200°C for 30min. Under the brazing condition of 1200°C for 10min, the Cf/SiC-Cf/SiC joint strength was only 51.7MPa at room temperature. However, when used a Mo layer, the Cf/SiC-Mo-Cf/SiC joint strength was remarkably increased to 133.2MPa at room temperature and 149.5MPa at 900°C, respectively. At the interface between Cf/SiC and Mo, Mo participated in interfacial reactions, with the formation of Cr3C2/Mo2C reaction layers at the Cf/SiC surface. The improvement in the joint strength should be mainly attributed to the formation of MoNiSi. The Cf/SiC-Mo joint strength was 86.9MPa at room temperature and 73.7MPa at 900°C, respectively. After 10 cycles of thermal shock test at 900°C the Cf/SiC-Mo joint strength of 71.6MPa was still maintained.

Bonding and electronics of the MoTe2/Ge interface under strain

Understanding the interface formation of a conventional semiconductor with a monolayer of transition-metal dichalcogenides provides a necessary platform for the anticipated applications of dichalcogenides in electronics and optoelectronics. We report here, based on the density functional theory, that under in-plane tensile strain, a 2H semiconducting phase of the molybdenum ditelluride $({\mathrm{MoTe}}_{2})$ monolayer undergoes a semiconductor-to-metal transition and in this form bonds covalently to bilayers of Ge stacked in the [111] crystal direction. This gives rise to the stable bonding configuration of the ${\mathrm{MoTe}}_{2}$/Ge interface with the $\ifmmode\pm\else\textpm\fi{}\mathrm{K}$ valley metallic, electronic interface states exclusively of a Mo $4d$ character. The atomically sharp Mo layer represents therefore an electrically active (conductive) subsurface $\ensuremath{\delta}$-like two-dimensional profile that can exhibit a valley-Hall effect. Such system can develop into a key element of advanced semiconductor technology or a novel device concept.

Deuterium retention in the divertor tiles of JET ITER-Like wall

Divertor tiles removed after the second JET ITER-Like Wall campaign 2013–2014 (ILW-2) were studied using Secondary Ion Mass Spectrometry (SIMS). Measurements show that the thickest beryllium (Be) dominated deposition layers are located at the upper part of the inner divertor and are up to ∼40µm thick at the lower part of Tile 0 exposed in 2011–2014. The highest deuterium (D) amounts (>8 · 1018at./cm2), in contrast, were found on the upper part of Tile 1 (2013–2014), where the Be deposits are ∼10µm thick. D was mainly retained in the near-surface layer of the Be deposits but also deeper in tungsten (W) and molybdenum (Mo) layers of the marker coated tiles, especially at W–Mo layer interfaces. D retention for the ILW-2 divertor tiles is higher than for the first campaign 2011–2012 (ILW-1) and probable reasons for the difference are that SIMS measurements for the ILW-2 samples were done deeper than for the ILW-1 samples, some of the tiles were exposed during both ILW-1 and ILW-2 and therefore had a longer exposure time, and the differences between ILW-1 and ILW-2 campaigns e.g. in strike point distributions and injected powers.

Achieving composition-controlled Cu2ZnSnS4 films by sulfur-free annealing process

Cu2ZnSnS4 (CZTS) films were firstly prepared by the nonvacuum spin-coating method, and then annealed at 550 °C in N2 atmosphere. A graphite box was used to inhibit the volatilization of gaseous SnS and S2 to suppress the CZTS decomposition and generation of MoS2 during annealing. The sulfur supplementation carried out in a conventional annealing process was not applied in this work. It was found that Sn loss was overcome and the compositions of postannealed films were close to that of precursor solution. Thus, by this method, the compositions of CZTS films can be controlled by adjusting the elemental ratios of the precursor solution. Besides, the increase in inert atmosphere pressure could further minimize the Sn loss and improve the crystallinity of CZTS films. Furthermore, the resistive MoS2 layer between the CZTS film and the Mo layer was suppressed because sulfur was not used and CZTS decomposition was suppressed.

Electrodeposition and characterization of metallic molybdenum from aqueous electrolytes containing high acetate concentrations

Metallic molybdenum coating was deposited from an aqueous electrolyte containing a high concentration of acetate and molybdate ions in pH of 6.8±0.2. It is complicated to deposit molybdenum from an aqueous electrolyte because of its high reactivity with oxygen. The mole ratios of water to acetate were in the range of (2.4±0.2):1. It was found that the change in bath stirring rate can result in the deposition of molybdenum oxide insisted of Mo coating. Also the morphological differences of metallic Mo and molybdenum oxide layer were investigated. In the present study, a successful attempt was made metallic Mo coating with a thickness of 3–7μm that was dependent on the DC current density.The XRD investigation indicated that the crystal structure of Mo coating was amorphous. The surface composition of deposits was ascertained by energy dispersive X-ray microanalysis (EDS) that showed the presence of high Mo content (90.7%) and low content of oxygen (9.3%). In addition, different EDS analysis was obtained from the top surface of coating and subsurface section. The surface morphology of coating was studied by scanning electron microscopy (SEM). Also, XPS analysis confirmed deposition of Mo(0) metal.

On nanostructured molybdenum\u2013copper composites produced by high-pressure torsion

Nanostructured molybdenum–copper composites have been produced through severe plastic deformation of liquid-metal infiltrated Cu30Mo70 and Cu50Mo50 (wt%) starting materials. Processing was carried out using high-pressure torsion at room temperature with no subsequent sintering treatment, producing a porosity-free, ultrafine-grained composite. Extensive deformation of the Cu50Mo50 composite via two-step high-pressure torsion produced equiaxed nanoscale grains of Mo and Cu with a grain size of 10–15 nm. Identical treatment of Cu30Mo70 produced a ultrafine, lamellar structure, comprised of Cu and Mo layers with thicknesses of sim 5 and sim 10{-}20,hbox {nm}, respectively, and an interlamellar spacing of 9 nm. This microstructure differs substantially from that of HPT-deformed Cu–Cr and Cu–W composites, in which the lamellar microstructure breaks down at high strains. The ultrafine-grained structure and absence of porosity resulted in composites with Vickers hardness values of 600 for Cu30Mo70 and 475 for Cu50Mo50. The ability to produce Cu30Mo70 nanocomposites with a combination of high-strength, and a fine, oriented microstructure should be of interest for thermoelectric applications.

Comparison of Ag(In,Ga)Se2/Mo and Cu(In,Ga)Se2/Mo Interfaces in Solar Cells.

The structural and electrical properties of the junction at Ag(In,Ga)Se2AIGS/Mo, and Cu(In,Ga)Se2 CIGS/Mo layers were characterized. The region between the CIGS and Mo featured a MoSe2 layer with a layered hexagonal structure and thickness of 10-15 nm. The c-axis of the MoSe2 was oriented perpendicular to the Mo layer, and the c -value was 12.6 Å. However, no such layer was observed at the interface between AIGS and Mo. This result was also confirmed by energy-dispersive X-ray spectrometry and X-ray diffraction measurements of the MoSe2 layer. The CIGS/Mo with a MoSe2 layer formed an ohmic contact, while the AIGS/Mo without the MoSe2 layer formed a Schottky contact. This Schottky contact showed a barrier height of 0.8 ± 0.02 eV, a nonideality factor of 1.5 ± 0.1, and a series resistance of 370 ± 8 Ω. A schematic band diagram of the AIGS/Mo junction was constructed on the basis of the above results.