Molybdenum Thin Films Research Articles

Studies on DC transport and terahertz conductivity of granular molybdenum thin films for microwave radiation detector applications

The morphological, transport, and terahertz optical properties of DC magnetron-sputtered granular molybdenum thin films with nano-grains embedded in an amorphous molybdenum/molybdenum oxide matrix have been studied in their normal and superconducting states. The superconducting transition temperatures of these films are much higher than that of bulk molybdenum. The optical properties of these thin films have been studied using terahertz time-domain spectroscopy. Their properties have been compared with those of the existing materials used for the development of radiation detectors. The films' resistivity lies in the >100 μΩ cm range, ideal for making highly sensitive radiation detectors. Hall measurements indicate holes as the dominant carriers with very small mean free path and mobility. In the normal state, the films are disordered bad metals. However, they have a large superfluid density and stiffness in their superconducting state. The properties of the films in the normal and superconducting states are promising for their use in cryogenic radiation detectors for microwave, terahertz, and far-infrared frequency ranges.

Mapping of the electronic work function anisotropy of RF sputtered molybdenum thin film electrodes for piezoelectric devices

Amplitude modulated Kelvin probe force microscopy was performed on molybdenum (Mo)-thin films deposited on Si(001) substrates by RF magnetron sputtering. Evolution of film microstructure from amorphous to crystalline was observed with increasing RF power from 25 W to 200 W. Spatial mapping of work function across the film surface revealed that the Mo-thin film deposited at 200 W possesses an average work function ~4.94 ± 0.06 eV while higher values were observed at lower RF powers. The genesis of distinct periodic changes in work function is attributed to the formation of the surface dipole layer associated with the adsorbates of different polarities (O2−/OH− or H+). A phenomenological model is also presented to elucidate their effect.

Influence of thickness on physical and optoelectronic properties of DC sputtered bilayer molybdenum thin films for CIGS solar cell

For rear ohmic contact of CIGS solar cell, bilayer Mo thin film on 2 sq- inch glass substrate was deposited using DC sputtering technique. Thickness of bilayer Mo thin film was 300 nm, 500 nm, 1000 nm, and substrate temperature were maintained constant at 215 °C and room temperature. From XRD, FESEM, optical and electrical analysis it was found that Mo bilayer with low thickness (300 nm) have low optical reflectivity 35% and high sheet resistance 2.03 Ω/□ compared to Mo bilayer having high thickness (500 nm and 1000 nm) i.e. about 72% and 0.52 Ω/□. XRD analysis reveals that 300 nm have low crystallite size 6.62 nm while Mo 1000 nm have 12.24 nm. Micro-structure of Mo 500 has closed dense pack dendric structure while 300 nm has columnar growth uneven in size with small circular grains. Slight wide columns appeared for 1000 nm due to interdiffusion of Mo adatoms deposited at pressure 1 mTorr onto 10 mTorr forming homogenous structure.

Stress and Microstructure Evolution in Mo Thin Films without or with Cover Layers during Thermal-Cycling.

The intrinsic stress behavior and microstructure evolution of Molybdenum thin films were investigated to evaluate their applicability as a metallization in high temperature microelectronic devices. For this purpose, 100 nm thick Mo films were sputter-deposited without or with an AlN or SiO2 cover layer on thermally oxidized Si substrates. The samples were subjected to thermal cycling up to 900 °C in ultrahigh vacuum; meanwhile, the in-situ stress behavior was monitored by a laser based Multi-beam Optical Sensor (MOS) system. After preannealing at 900 °C for 24 h, the uncovered films showed a high residual stress at room temperature and a plastic behavior at high temperatures, while the covered Mo films showed an almost entirely elastic deformation during the thermal cycling between room temperature and 900 °C with hardly any plastic deformation, and a constant stress value during isothermal annealing without a notable creep. Furthermore, after thermal cycling, the Mo films without as well as with a cover layer showed low electrical resistivity (≤10 μΩ·cm).

Fabrication and Characterization of Molybdenum Thin-Film Resistor for Superconducting Quantum Devices

As part of the efforts to establish a reliable fabrication process of superconducting integrated circuits at Center for Excellence in Superconducting Electronics (CENSE) in China, we investigated the influences of various deposition conditions on the properties of molybdenum (Mo) thin films. Thin-film resistors were fabricated based on these films by using a lift-off process. It was found that Mo structures made from films peeled off easily from the substrate after lift-off. We have optimized the deposition conditions to grow Mo films with compressive or weakly tensile stress. Mo thin-film resistors with a sheet resistance of several ohm/square and surface roughness of less than 1 nm were successfully fabricated and were used in the fabrication of superconducting quantum interference devices and small-scale superconducting integrated circuits.

Laser-Induced Tar-Mediated Sintering of Metals and Refractory Carbides in Air.

Refractory metals and their carbides possess extraordinary chemical and temperature resilience and exceptional mechanical strength. Yet, they are notoriously difficult to employ in additive manufacturing, due to the high temperatures needed for processing. State of the art approaches to manufacture these materials generally require either a high-energy laser or electron beam as well as ventilation to protect the metal powder from combustion. Here, we present a versatile manufacturing process that utilizes tar as both a light absorber and antioxidant binder to sinter thin films of aluminum, copper, nickel, molybdenum, and tungsten powder using a low power (<2W) CO2 laser in air. Films of sintered Al/Cu/Ni metals have sheet resistances of ∼10-1 ohm/sq, while laser-sintered Mo/W-tar thin films form carbide phases. Several devices are demonstrated, including laser-sintered porous copper with a stable response to large strain (3.0) after 150 cycles, and a laserprocessed Mo/MoC(1-x) filament that reaches T ∼1000 °C in open air at 12 V. These results show that tar-mediated laser sintering represents a possible low energy, cost-effective route for engineering refractory materials and one that can easily be extended to additive manufacturing processes.

Modulation of microstructural and electrical properties of rapid thermally synthesized MoS2 thin films by the flow of H2 gas

Two dimensional transition metal dichalcogenides (TMDC) have gained potential attention of the researchers, owing to their fascinating electronic and optoelectronic properties. Among the TMDC, the widely studied molybdenum disulphide (MoS2) stands out as a prototypical material. In this work, rapid thermal process (RTP) is adopted for the deposition of MoS2 thin film, using sputtered molybdenum (Mo) thin film and thermally evaporated sulphur (S) film. The microstructural and electrical characteristics of the MoS2 thin films were studied by varying the H2 flow rate in the rapid thermal processing furnace. Field Emission Scanning Electron Microscope images showed the improvement in the uniformity of the MoS2 thin films with the increase in the flow rate of H2 gas. The X-ray diffraction peak has been observed at 14.1°, corresponding to the (002) plane of the hexagonal MoS2 (2H). Raman spectra depict two peaks around 400 cm−1 for A1g and E2g1active modes, which a fingerprint of 2H MoS2. The FTIR studies also depicted the enhancement of MoS2 characteristics peaks with the increase in H2 flow rate. In order to study the effect of substrate, MoS2 films were grown on both Si and SiO2/Si substrates. Among the various H2 flow rates used during the RTP, MoS2 thin films grown with 20 sccm H2 gas at 800 °C has shown better microstructural properties. In order to study the semiconductor properties of MoS2 thin films, the carrier concentration has been calculated from the heterojunction capacitance and is found to be ~1016 cm−3. The comprehensive study on the effect of H2 flow rate on the properties of MoS2 thin films imparts the underlying role of hydrogen atoms in the deposition process, which paves a way for the controlled synthesis of low dimensional materials.

Low pressure sulfurization and characterization of multilayer MoS2 for potential applications in supercapacitors

The emergence of two-dimensional molybdenum disulfide (MoS2) has prompted enormous research attention as supercapacitor electrode materials due to their exciting electrochemical, electrical and mechanical properties. In this work, we present a low-pressure technique to grow multilayer MoS2 by sulfurization of molybdenum thin films at three different temperatures (700, 750 and 800 °C) to study the effect of growth temperatures on surface morphology, film thickness and film quality. Large area growth of MoS2 throughout the Si/SiO2 substrate was achieved at all the sulfurization temperature. The as-prepared MoS2 films were characterized by several techniques including X-ray diffraction, atomic force microscopy, scanning electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. Although MoS2 is successfully grown at different temperatures, the surface morphology, thickness and crystallite size have varied significantly with temperature. Our results presented here will contribute towards the reproducible growth of large area MoS2 at the desired temperature for energy storage applications.

Influences of substrate temperature and Ar flow on the properties of RF sputtered Mo thin films

We have reported the influences of substrate temperature and the rate of Argon (Ar) flow on the properties of Molybdenum (Mo) thin films. Different Ar flow rates (40, 60, 80, 100 SCCM) and different substrate temperatures (30, 100, 200, 300, 400 °C) were used for the substrate of Mo thin films by radio frequency (RF) magnetron sputtering technique. Structural properties studied by X-ray diffraction technique revealed cubic crystal structure with (110) prominent reflection. The electrical properties, viz. carrier concentration, resistivity and mobility were studied with the help of Hall measurement system. It has been revealed that substrate temperature and Ar flow significantly affect on the thickness, carrier concentration, resistivity and mobility of Mo layer. Upon increasing the substrate temperature, thickness increases, whereas thicker samples were grown at 80 and 100 SCCM for temperature 400 °C. The sample grown at 300 °C with 80 SCCM Ar flow measured high carrier concentration, whereas the resistivity and microstrain were found decreased systematically upon increasing the substrate temperature from room temperature 30 °C to 400 °C. Percentage reflectance examined with UV–Vis spectrometer revealed the sample grown with 80 SCCM at 400 °C measured higher reflectance. Highly dense, compact, uniform, and void-free growth of Mo layer were observed by AFM images. The effect of substrate temperature and Ar flow rate on surface morphology as well as RMS roughness is clearly observed.

Temperature-dependent negative friction coefficients in superlubric molybdenum disulfide thin films

We obtain resistance characteristics for molybdenum disulfide and temperature-dependent coefficient of friction (COF) : 0.0005 which could be a superlubricating state for molybdenum disulfide coatings or thin films. Considering molybdenum disulfide lubrication we adopt the transition-state approach to calculate the temperature-dependent friction behavior. We have performed intensive calibrations via verified models considering previous experimental friction tests on some thin-solid-film samples which measured the COF in 4.2 K, 77 K and room temperature conditions to accommodate the actual working condition. To improve the performance of molybdenum disulfide we predict its better COF (=0.0005 via fine tuning of the microscopic activation volume) which could be a superlubricating state (COF <0.001) for molybdenum disulfide thin films. Our results also have negative friction coefficients for superlubricity.

DC Magnetron-Sputtered Mo Thin Films with High Adhesion, Conductivity and Reflectance

The main challenge in the deposition of molybdenum thin films for high efficiency in copper indium gallium selenide (CIGS) modules lies in gaining an adherent coating without compromising conductivity and reflectance characteristics. In this study, Mo thin films were deposited on soda-lime glass by DC magnetron sputtering at different deposition power (55, 100, 200 and 300 W) and with high working gas pressure (2 and 4 Pa). Analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall effect were employed to analyze the structure, morphology and electrical resistivity of the deposited films. Ultraviolet–visible (UV–Vis) spectrometry was performed to measure the reflectance and a cross-hatch adhesion tape test was employed to determine the adhesion behavior of deposited films. With higher sputtering power and reduced gas pressure, an increase in the crystallite size of the deposited films was observed. Films deposited at higher gas pressure were found with tensile stresses and higher adhesion with the substrate. The van der Pauw method reveals an increase in conductivity at high power and low gas pressure. Improved reflectance was achieved at moderate sputtering power and low gas pressure.

Design, fabrication and characterization of piezoelectric cantilever MEMS for underwater application

This work shows a preliminary microfabrication route for a novel directional hydrophone based on a cross-shaped design of piezoelectric cantilevers. A thin layer of aluminum nitride (AlN) using Molybdenum (Mo) thin film as electrodes will be exploited as piezoelectric functional layer for the microfabrication of a cantilever-based ultrasonic micro electro mechanical system (MEMS) hydrophone. A parameterized simulation based on length of these cantilevers between 100 and 1000 μm allowed to set the first resonant mode between 20 kHz and 200 kHz, the desired underwater ultrasonic acoustic range. The microsystem was designed with cantilevers facing each other in a cross configuration in order to have novel MEMS hydrophone with an omnidirectional response. In order to investigate the first resonance frequency mode and displacement measurements, a Laser Doppler Vibrometer was used and good agreement between simulations and experimental results was achieved. Responsivity and directionality measurements of the piezoelectric MEMS cantilevers were performed in water. Maximum sensitivity up to −153 dB with omnidirectional directivity pattern was achieved by fabricated MEMS sensor. • We show the microfabrication of a novel directional hydrophone based on a cross-shaped design of piezoelectric cantilevers. • Cantilevers facing each other in a cross configuration allow to have an omnidirectional response to underwater ultrasounds. • Aluminum nitride (AlN) using Molybdenum (Mo) thin film as electrodes is exploited as piezoelectric functional layer. • Maximum sensitivity underwater up to -153 dB with omnidirectional directivity pattern was achieved.

Impact of Water Ingress on Molybdenum Thin Films and Its Effect on Cu(In,Ga)Se2 Solar Cells

Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.

Зондовая микроскопия и электронно-транспортные свойства тонких эпитаксиальных пленок Mo на сапфире

The surface and electronic transport properties of epitaxial thin molybdenum films were studied. The experimental results were compared with the known quantum models of the effect of the surface relief of the films on their resistance.

Formation of buried superconducting Mo2N by nitrogen-ion-implantation.

Nitrogen ion implantation is a useful technique to put nitrogen ions into lattices. In this work, nitrogen ion implantation into epitaxial Mo films is performed to create a buried superconducting γ-Mo2N. Atomically flat epitaxial (110) Mo films are grown on (0001) Al2O3. By impinging nitrogen ions, where the beam energy is fixed to 20 keV, we observe (111) γ-Mo2N diffraction and the formation of a γ-Mo2N layer from X-ray reflectivity. Magnetization and transport measurements clearly support a superconducting layer in the implanted film. Our strategy shows that formation of a buried superconducting layer can be achieved through ion implantation and self-annealing.

Microstructure and Magnetron Sputtering Properties of Molybdenum Target Prepared by Low-Pressure Plasma Spraying

Planar molybdenum targets were fabricated by low-pressure plasma spraying (LPPS) under 2.6 × 104 Pa pressure. A lamellar structure consisting of vertical columnar grains and compact interlamellar contacts was found in the LPPS deposits. The oxygen content by mass, porosity, average grain size, microhardness, and ultimate tensile strength of the LPPS target were about 0.18%, 1.1%, 0.4 μm, 361.8 HV0.025, and 373.2 MPa, respectively. Electron backscattered diffraction (EBSD) analysis of the LPPS sample showed proportions of -, -, and -oriented grains of about 12.0%, 16.9%, and 9.2% of the total, respectively. The molybdenum target exhibited excellent magnetron sputtering performance, since most of the grains with size less than 1.0 μm were irregularly distributed without preferred orientation. Rapid sputtering and uniform thinning on the surface of LPPS molybdenum targets took place during magnetron sputtering. Smooth and continuous molybdenum thin film with thickness of about 700 nm could be deposited by magnetron sputtering from the LPPS samples. The x-ray diffraction (XRD) spectra of molybdenum thin film with body-centered cubic structure showed that the intensity of (110) diffraction peak was much higher than that of (220) one.

Sputtering and nitridation of transition metal surfaces under low energy, steady state nitrogen ion bombardment

Transition metal surfaces exposed to low-energy reactive ions undergo dynamic changes in composition and density due to implantation and compound formation. We report measurements of nitrogen ion induced sputter yields for transition metals relevant to fusion and optics applications. Thin films of molybdenum, ruthenium, palladium and tungsten are bombarded by nitrogen ions of kinetic energies in the range of 50–500 eV at steady state fluences (1 × 1018ions/cm2). Measured sputter yields are explained through energy and momentum transfer under the binary collision approximation using the Monte Carlo code TRIDYN. X-ray Photoelectron spectroscopy (XPS) studies showed the nitrogen content in the films at the end of ion exposure is independent of incoming ion energy. This occurs due to competing implantation and preferential surface nitrogen sputtering processes within the XPS probing depth. All metals investigated showed evidence of a nitride formed due to energetic nitrogen impact. The combination of XPS and TRIDYN simulations were applied to extract effective reaction cross-sections for each metal.

Cathodoluminescence enhancement and quenching in type-I van der Waals heterostructures: Cleanliness of the interfaces and defect creation

Van der Waals heterostructures give access to a wide variety of new phenomena that emerge thanks to the combination of properties brought in by the constituent layered materials. We show here that owing to an enhanced interaction cross section with electrons in a type I van der Waals heterostructure, made of single layer molybdenum disulphide and thin boron nitride films, electrons and holes created in boron nitride can be transferred to the dichalcogenide where they form electron-hole pairs yielding luminescence. This cathodoluminescence can be mapped with a spatial resolution far exceeding what can be achieved in a typical photoluminescence experiment, and is highly valuable to understand the optoelectronic properties at the nanometer scale. We find that in heterostructures prepared following the mainstream dry transfer technique, cathodoluminescence is locally extinguished, and we show that this extinction is associated with the formation of defects, that are detected in Raman spectroscopy and photoluminescence. We establish that to avoid defect formation induced by low-energy electron beams and to ensure efficient transfer of electrons and holes at the interface between the layers, flat and uniform interlayer interfaces are needed, that are free of trapped species, airborne ones or contaminants associated with sample preparation. We show that heterostructure fabrication using a pick-up technique leads to superior, intimate interlayer contacts associated with significantly more homogeneous cathodoluminescence.

Design of Metal-Dielectric Filters in Transmitted Ultraviolet Light

A metal-dielectric narrow-band filter for a wavelength of 270 nm is designed within the framework of the conductive surface model to characterize the properties of thin metal films. It is demonstrated that inclusion of four thin molybdenum films into a multilayer dielectric structure ensures effective cut-off of the transmission at <1.5% in the visible range of the spectrum with the maximum transmittance more than 50% for the selected wavelength and the spectral width of the filter <5 nm.

Characterization of confocal sputtered molybdenum thin films for aluminum nitride growth

Molybdenum is commonly deposited as a seed layer for piezoelectric Aluminum Nitride (AlN) growth, whose properties are dependent on the quality of the Mo thin film, which includes the crystal orientation and stress. This paper investigates the morphology effects of varying sputtering parameters using confocal sputtering of Mo thin films. DC and pulsed DC sputtered Mo and bilayers of AlN/Mo on Si were deposited using confocal sputtering techniques by varying power and temperature. Films were characterized by measuring crystallinity properties of the film, electrical resistivity, and stresses. Results demonstrated that controlling parameters of pulsed DC deposition produced lower compressive film stress (−0.13 GPa) compared to DC sputtered film (−1.6 GPa). The crystal structure of the films was analyzed by measuring the full-width-half-maximum values on the 2θ X-ray diffraction scans. Increasing surface mobility by increasing power and temperature resulted in an increased crystallinity of (110) Mo with increased crystallite size and lower electrical resistivity and stress. Pulsed DC sputtered Mo further enhanced the crystallinity of the film. It was determined that depositing pulsed DC AlN as an interlayer for Mo growth resulted in an increase in crystallinity and lower stress compared to DC sputtered Mo.