Mo Thin Films Research Articles

Interface studies of Mo/Si multilayers with carbon diffusion barrier by grazing incidence extended X-ray absorption fine structure

Single layer Mo and Si thin films deposited for different deposition times on c-Si substrates by Ion beam sputtering technique have been characterized by Grazing Incidence X-ray Reflectivity (GIXRR) measurements. Subsequently, few Mo/Si multilayers having 5 bi-layers with different Mo to bi-layer thickness ratio (Γ) at a fixed bi-layer thickness have been deposited and characterized by GIXRR measurements. Investigation has been carried out on the different behaviors of the interfaces and also variation of the interface width with Γ has been studied. Finally, 25 bi-layer Mo/Si multilayer samples having the lowest Γ value and with and without carbon diffusion barrier layer at both the Mo/Si interfaces are deposited and the interfaces have been characterized by hard X-ray and soft X-ray reflectivity measurements. The study shows improvement in reflectivities of the multilayer with carbon diffusion barrier layer from 38% to 54% at 130 Å wavelength. To have further insight into the modification of the interface due to introduction of the carbon barrier layer, depth selective study of both the multilayers have been carried out by Grazing Incidence Extended X-ray Absorption Fine Structure measurements using synchrotron radiation and thereby investigating the local structures around Mo atoms in Si bulk region and Si-on-Mo interface. This study confirms the reduction of diffusion of Mo and Si layer into each other when the carbon buffer layers are present at the interfaces, thus reducing the chance of formation of MoSi2.

Vanadium-doped molybdenum disulfide film-based strain sensors with high gauge factor

This paper reports the piezoresistive performance of the two-dimensional (2D) material of vanadium (V)-doped molybdenum disulfide (MoS2) films based on sulfurization of sputtered Mo thin films. I–V characteristics indicate that V atom doping indeed decreases the resistivity of MoS2. Strain sensors based on V-doped MoS2 resistive elements were fabricated. By using a four-point bending method, a gauge factor (GF) of 140 under compressive and tensile strain conditions was obtained. The piezoresistive effect of V-doped MoS2 with different V sputtering conditions was also investigated. The doping method introducing V atoms as dopants is found to play an important role in enhancing piezoresistive performance.

Reaction pathways in the oxidation and pesting of molybdenum disilicide MoSi2 studied with scanning tunneling microscopy and spectroscopy

The oxidation and pesting of Molybdenum Disilicide is a critical limiter in its technical use. The materials disintegration during the pesting reaction in cracks and at grain boundaries prevents direct observation of the process. A model system consisting of silicide crystallites on a Si(001) surface was developed to study the progression of the oxidation reaction in-situ with Scanning Tunneling Microscopy and Spectroscopy. Deposition and annealing of Mo thin films is used to synthesize crystallites in a size range between 10 and 100 nm, and atomic resolution is achieved on several facets. The MoSi2 is oxidized in the temperature range 400–800°C bracketing the pesting regime and beyond, combined with molecular oxygen exposure from 2 to 100 Langmuir. The oxidation process can be subdivided in three distinct regimes defined by their respective oxidation conditions: regime 1 (incubation) where only subtle changes in the electronic structure of the crystallites occurs, to regime 2 with a rapid increase in the crystallite density due to the volume gain of the pesting reaction transforming silicide to oxide, and regime 3 where a silica layer starts to form. The incubation time is attributed to oxygen dissolution and/or formation of a ternary Mo-Si-O phase, which is followed by the release of MoOx molecules. The diffusion of MoOx triggers secondary nucleation events, and contributes to the accelerated oxidation of the surface. Increasing the surface temperature > 650°C promotes desorption and sublimation over diffusion and supports silica formation. This study unraveled the intermediate reaction steps of silicide oxidation, and specifically underscores the critical role of reactant diffusion in the rapid progression of the pesting reaction.

Low-temperature wafer-scale growth of MoS2-graphene heterostructures

In this study, we successfully demonstrate the fabrication of a MoS2-graphene heterostructure (MGH) on a 4 inch wafer at 300 °C by depositing a thin Mo film seed layer on graphene followed by sulfurization using H2S plasma. By utilizing Raman spectroscopy and high-resolution transmission electron microscopy, we have confirmed that 5–6 MoS2 layers with a large density of sulfur vacancies are grown uniformly on the entire substrate. The chemical composition of MoS2 on graphene was evaluated by X-ray photoelectron spectroscopy, which confirmed the atomic ratio of Mo to S to be 1:1.78, which is much lower than the stoichiometric value of 2 from standard MoS2. To exploit the properties of the nanocrystalline and defective MGH film obtained in our process, we have utilized it as a catalyst for hydrodesulfurization and as an electrocatalyst for the hydrogen evolution reaction. Compared to MoS2 grown on an amorphous SiO2 substrate, the MGH has smaller onset potential and Tafel slope, indicating its enhanced catalytic performance. Our practical growth approach can be applied to other two-dimensional crystals, which are potentially used in a wide range of applications such as electronic devices and catalysis.

High-temperature annealing effects on molybdenum–silicon contact substrate for vertically structured silicon quantum-dot solar cells

This paper studies the applicability of sputtered Molybdenum (Mo) thin films on silicon wafers (Mo–Si) to act as contact substrates for vertically structured Si quantum-dot (QD) solar cells. The compatibility of Mo–Si contact substrate with Si QD material under different annealing temperatures is examined. Cross sections of annealed samples show well-defined interfaces without metal penetration into the Si QD bilayer regions. Through comparing samples deposited on Mo–Si substrates with those on fused-silica substrates, we identified from Raman spectra that the presence of Mo is advantageous for the Si-crystallization process and in fact provide beneficial passivation effects on the Si QD material according to changes in photoluminescence intensity. This allows us to conclude that Mo thin films are compatible with the sputter–anneal process, and verify the feasibility of using the proposed Mo–Si contact scheme for future fabrication of Si QD solar cells.

Post-Sputtering Heat Treatments of Molybdenum on Silicon Wafer

This paper investigated the property evolutions of Mo thin films that were subjected to post-sputtering heat treatments from 700 °C to 1100 °C. It was found that, after annealing, the use of Si wafers eliminated crack formations found in previously reported Mo thin films sputtered on fused silica substrates. The recrystallization of the Mo thin film was found to start at 900 °C, which led to rearrangements of the preferred crystalline orientation and enhancement of grain size when the annealing temperature was further increased. The electrical conductivity of the Mo thin films was majorly affected by the increase of Mo crystallite size as the annealing temperature was increased. Overall, the improvement of material sustainability and compatibility in the high temperature annealing process has made it positive to implement a Mo-Si contact-substrate scheme for vertical structured Si QDs solar cells.

DC and RF sputtered molybdenum electrodes for Cu(In,Ga)Se2 thin film solar cells

In this study, molybdenum (Mo) thin films are deposited at different working pressures and different target-substrate distances (Dt-s) (8 cm and 10 cm) under the excitation of the radio frequency (RF) or direct current (DC) power supply for applications in Cu(In,Ga)Se2 (CIGS) thin film solar cells. The various material properties including dynamic deposition rate, electrical and optical properties, crystal structure and morphology are systematically investigated and compared for these RF and DC sputtered Mo thin films. Mo bilayer, i.e. bottom Mo layer prepared at a high working pressure and top Mo thin film sputtered at a low working pressure are employed as back electrodes in CIGS thin film solar cells. CIGS thin film solar cells with DC Mo/DC Mo bilayer have relatively higher efficiency as compared to the others with RF Mo layers, which might be related to a relatively easy migration of sodium (Na) passing through the DC Mo layers and entering into the CIGS absorbers. A high efficiency CIGS thin film solar cell of 16.2% has been achieved in this work.

Layering influence on Nd agglomeration and hard magnetic properties of Mo/NdFeB/Mo thick films

The hard magnetic characteristics of the Nd-Fe-B films deteriorate and the c-axis alignment of anisotropic Nd2Fe14B hard magnetic grains tends to be deviated from the out-of-plane orientation when thickness of the films increases over a critical value. At the same time, a Nd agglomeration effect was found in the Nd-Fe-B layer volume, which is accentuated by the increase in the treatment temperature and is stronger when the Nd content is higher. In order to prevent these drawbacks we have divided the Nd-Fe-B thick films in a number of n NdFeB(x nm)/Mo(5 nm) bilayers using molybdenum (Mo) film as interlayer. The variable thickness, x, of the Nd-Fe-B magnetic layers along with the annealing conditions (temperature and time) influences both the microstructure and the hard magnetic characteristics of Nd-Fe-B thick films. Single-layer and multilayer Nd-Fe-B films of various total thickness ranging between 540 nm and 1620 nm, have been deposited by vacuum r.f. sputtering on a (001) silicon (Si) substrate heated at about 470 °C. The resulted multilayer Nd-Fe-B thick films with a total thickness of about 1620 nm (three times higher than 540 nm thin film), stratified in 9 sequences - NdFeB(180 nm)/Mo(5 nm) bilayers and annealed for 40 min at low temperature (550 °C) exhibit coercive field and remanence of the same order of magnitude with the single-layer anisotropic Nd-Fe-B thin films having the thickness of about 540 nm. At the same time, these multilayer Nd-Fe-B thick films preserve the out-of-plane anisotropic character and present a 26% increased maximum energy product as compared to the single-layer Mo/NdFeB(540)/Mo thin films.

Effects of Heating Mode and Temperature on the Microstructures, Electrical and Optical Properties of Molybdenum Thin Films.

In this paper, molybdenum (Mo) thin films are deposited on soda-lime glass (SLG) substrates by direct current magnetron sputtering and heated in three different modes at different temperatures, including substrate heating, annealing treatment, and both substrate heating and annealing treatment. The effects of heating temperature and heating mode on the structures, morphology, optical and electrical properties of Mo thin films were systematically investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM), atomic force microscope (AFM) and UV-visible spectrophotometer (UV-vis spectra). It is shown that as the substrate and annealing temperature increase, the crystallinity of Mo thin films is improved, and the grain sizes become bigger. Especially in the mode of both substrate heating and annealing treatment at higher temperature, the obtained Mo thin films show higher crystallinity and conductivity. Moreover, with the increase of substrate and annealing temperature in different heating modes, both the surface compactness of Mo films and the optical reflectance increase correspondingly. Furthermore, the Mo film, prepared at the substrate heating temperature of 400 °C and annealed at 400 °C, showed excellent comprehensive performance, and the resistivity is as low as 1.36 × 10−5 Ω·cm. Using this optimized Mo thin film as an electrode, copper indium gallium selenium (CIGS) solar cells have a maximum photo-conversion efficiency of 12.8%.

Electro-mechanical behavior of Al/Mo bilayers studied with in situ straining methods

Magnetron sputter deposited Mo thin films are frequently used as part of the metallization of thin film transistors. To ensure technological applications these Mo thin films are often combined with Al thin films, but are inherently brittle compared to the electrical charge carrying ductile Al thin films. To improve the fracture behavior of bilayered stacks containing Mo layers for future flexible devices like rollable or foldable displays, an optimum ductile/brittle layer thickness ratio is necessary. Within this work the electro-mechanical behavior of bilayered Al/Mo thin films grown by dc magnetron sputter deposition on polyimide substrates under tensile strain was investigated. Various Al/Mo layer thickness ratios (1:1, 3:1, 5:1 and 10:1), all with a base Mo film of 30 nm were tested. In situ tensile tests combining electrical 4-point probe resistance and optical measurements revealed an optimum thickness ratio between 5:1 and 10:1 for the crack onset strain. Post mortem analysis on the crack morphology revealed that higher thickness ratios better compensate cracks by leaving wider subsurface pathways for electron conduction of the layer stack than lower thickness ratios.

Effects of thickness ratios and sputtering mode on the structural, electrical and optical properties of bilayer molybdenum thin films

In this paper, the bilayer Mo films with a constant thickness were deposited by direct current and direct current (DC/DC), radio frequency and direct current mixed (RF/DC) magnetron sputtering, respectively. Changing thickness ratios of bottom layer to total thickness of bilayer film in the range from 10% to 50%, ten types of bilayer Mo thin films were deposited. The purpose is to improve the photo-conversion efficiency of Cu(In, Ga)Se2(CIGS) solar cells by changing the sputtering modes and thickness ratio. The microstructures, electrical and optical properties of the bilayer Mo thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscope (AFM), Hall Effect measurement system, ultraviolet-visible spectrophotometer (UV-vis) and four-point probe resistance system. It was found that with the increase of thickness ratios in two sputtering modes, both the crystallinity and grain size decreased, while the reflectance increased. Especially, when the thickness ratio was 40%, the resistivity of Mo film prepared in RF/DC mode was as low as 3.365 ×10-5 Ω·cm and the highest reflectance was above 60%. Using this optimized Mo thin film as electrode, the highest photo-conversion efficiency for the CIGS thin film solar cells was as high as 11.5%.

Control of physical and microstructural properties in molybdenum by direct current magnetron sputtering deposition producing bilayer thin film

Molybdenum (Mo) thin films are widely used in microelectromechanical systems (MEMS) applications. Mo bilayer deposition by direct current (DC) magnetron sputtering has been proposed in order to attain the desired smooth surface, small in-plane tensile stress and high degree crystallisation of Mo thin film for the fabrication of MEMS actuators and electrodes. The influences of sputtering time (10 min–40 min) and sputtering DC power (100 W–250 W) on the physical and microstructural properties of single layer Mo thin film have been evaluated. The optimised sputtering conditions for bottom and top layer of Mo bilayer have been determined and the individual influence of each layer on the resulting Mo bilayer has been discussed. Our studies reveal that the deposition of top Mo layer with small surface roughness but high in-plane tensile stress has reduced the high surface roughness of bottom Mo layer and simultaneously retained the small in-plane tensile stress. Reducing the sputtering time and/or using similar sputtering power for bottom and top layers improved the crystallinity of Mo bilayer along the preferred 〈110〉 direction. Mo bilayer thin films of total thickness < 1 μm, crystallite size >19 nm, surface roughness <2.5 nm and in-plane tensile stress <500 MPa have been attained.

Strain induced FCC to BCC structural change in sputtered molybdenum thin films

We find direct evidence of stain driven phase transition from face centered cubic (FCC) to body centered cubic (BCC) structure in radio frequency (RF) sputtered molybdenum (Mo) thin films as a function of deposition parameters. The structural, microstructural and electrical behavior of the Mo films are greatly influenced by the sputtering parameters i.e. RF power and argon (Ar) pressure. Although Mo is reported to be stable in BCC structure, our findings reveal that at certain deposition conditions, Mo films crystallize in FCC structure with good adherence to glass substrate; high resistance and strain (~214 Ω/□, ~3.43%). On varying sputtering parameters, strain releases and Mo films crystallize in mixed phase (FCC and BCC) which turns into pure BCC phase on further optimization. The stable BCC structured films have low resistance and strain (~1.29 Ω/□, ~−0.23%). We deposited bi-layer of Mo, where the bottom layer has strong adhesion to the substrate and top layer has low resistance (~0.81 Ω/□). The Mo film that crystallized in pure BCC form showed a strain value of −0.09% (compressive).

Negative substrate bias induced modifications of the physical properties of DC sputter deposited nano-crystalline Mo thin films

Negative bias on the substrate during DC sputter deposition of nano-crystalline molybdenum thin films has been utilized to tune their properties for solar cells and the cryogenic radiation detector applications. Films have been deposited on Si substrates under different out of plane negative biases aiming to improve their physical properties such as the sheet resistance, superconducting transition temperature, width of transition and surface roughness. Significant modifications in the electrical and surface morphological properties of nano-crystalline Mo thin films have been reported. The superconducting transition temperature and crystallite size of the films does not show much improvement but the surface roughness and electrical resistivity of the film have been improved by the application of substrate bias.

Comprehensive Investigation on the Nanovoid Heterogeneity in Mo Thin Films Used for Solar Cells Applications

ABSTRACTThe purpose of this research work is to gain a better understanding of the nanostructural properties of Molybdenum (Mo) thin films’ porosity, nanovoid heterogeneity and provide detailed quantitative data on voids volume fractions, sizes, shapes, and their preferred orientations as the growth sputtering pressure changes systematically. This knowledge shall assists in optimizing Mo film nano- and micro-structural properties as desired for solar cells applications. Therefore, two separate series of Mo thin films (∼ 0.7 μm thick) were deposited on high purity (99.999) Al-foil (10 μm thick), and Si/SiO2 substrates using direct-current (DC) planar magnetron sputtering. The sputtering pressure was varied from 0.8 mT to 12 mT, with a sputtering power density of 1.2 W/cm2. High-Resolution Scanning Electron Microscopy (HRSEM) was used to examine the Mo films’ morphology. Whereas, the Mo films’ bulk resistivity was calculated from the films’ thickness and average sheet resistance measurements using Dektak Surface Profilometer, and Four Point Probe method, respectively. Small Angle X-Ray Scattering (SAXS) technique was applied to examine the existence of nanovoids and its heterogeneity in the Mo-coated Al foils (Al/Mo). Moreover, the porosity of the Mo films as a function of sputtering pressure was studied by Transmission Electron Microscopy (TEM) on Mo-coated Si/SiO2 (Si/SiO2/Mo) substrates.

Structure, morphology and selected mechanical properties of magnetron sputtered (Mo, Ta, Nb) thin films on NiTi shape memory alloys

In this study, pseudo-elastic Ni 50.8 at.%-Ti alloy sheets of 1000 μm thickness were coated with 10 μm thick refractory metal thin films (Mo, Ta and Nb thin films, respectively), by nonreactive d.c. magnetron sputtering. These thin films were characterized with regard to their microstructure, morphology and selected mechanical properties. Microstructural characterization of the thin films included X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses. SEM images show, that Mo thin films grow in a densely packed structure with columnar grains. Pole figure analyses reveal a distinct (110) texture of the deposited Mo thin films on the NiTi substrate. The Mo thin films exhibit relatively large crystallite sizes, 290–300 nm. Ta and Nb thin films show XRD reflections of various lattice planes, i.e. polycrystalline b.c.c. structures, with preferential (110) orientation. Both of these thin films grow with much smaller crystallite size in comparison to the Mo thin films, 30–40 nm.The Young's Modulus of both the thin films and thin film/substrate composites were investigated on various scales. For this purpose, indentation experiments were performed using both nano- and microindentation techniques. A comparison of the values of the Young's Modulus of the thin film materials (in bulk, literature data) and the results obtained from nanoindentation shows a very good agreement between the measured data and bulk data. Progressive scratch tests were used to evaluate the adhesion of the thin film materials on the NiTi substrate. These scratch tests suggest that the metallic thin films undergo plastic deformation at larger normal forces. However, no thin film delamination is observed, independent of the material combinations and testing conditions applied.

Electrical, structural, and topographical properties of direct current (DC) sputtered bilayer molybdenum thin films

Renewable energy sources offer a viable green energy solution to the increasing global energy demands, and the photovoltaics offers a means of tapping into the abundance of the solar energy. Molybdenum (Mo) thin film has been used as a back-contact electrode in high-efficiency solar cells mainly due to its ohmic nature with the absorber material (especially with chalcopyrites and kesterites), low resistivity, adequate reflectance, and porosity to alkali materials. Mo thin films used in solar cells are deposited by direct current (DC) sputtering under argon ion atmosphere. The present work deals with the effects of the sputtering deposition conditions (i.e., working pressure and power) on the electrical properties of Mo thin films deposited on soda-lime glass using DC magnetron sputtering at different working pressures (5–20 mTorr) and deposition powers (100–800 W). Optimized deposition power and sputtering pressure were used to deposit a Mo bilayer thin film intended for solar cells application. X-ray diffractometric studies showed the (110) plane corresponding to the body-centered cubic structure of the Mo material. Calculated values were used to estimate other parameters of the deposited films (e.g., stress, dislocation density), as these properties affect the potential diffusion of sodium (Na) through the soda-lime glass into the absorber layer that is important as the moderate inclusion of Na improves the cell efficiency. The cross-section of the deposited film was analyzed using a field-emission scanning electron microscopy, to confirm the successful deposition of the bilayer. The surface of the films plays a vital role in the Mo interface with the absorber layer. Atomic force microscopy was used to acquire the topographical information from the sputtered bilayer film. The resistivity, as measured using a four-point probe, of the Mo films deposited at different conditions was in the order of 10−6 Ω m while the resistivity of the bilayer was 0.60 × 10−6 Ω m.

Indentation size and loading strain rate dependent creep deformation of nanocrystalline Mo

The creep behavior of nanocrystalline body-centered cubic Mo thin films was evaluated using nanoindentation testing. The creep rate and the corresponding strain rate sensitivity (m) were found to be strongly dependent upon indentation size and loading strain rate (LSR). Other than the mechanism of diffusion along indenter tip-sample interface previously proposed for face-centered cubic metals and metallic glasses, alternative mechanisms involving surface effect enhanced screw dislocation activities, inverse indentation size effect of hardness and LSR dependent microstructural and stress states, were proposed to interpret the observed dependence of penetration depth and LSR of m.

Synthesis and Characterization of Molybdenum Back Contact Using Direct Current-Magnetron Sputtering for Thin Film Solar Cells

In present work, we report synthesis of Mo thin films by DC-magnetron sputtering method. The structural, optical, morphological and electrical properties were investigated as a function of target-to-substrate distance. From the results, it is evident that with increase in target-to-substrate distance the thickness of films decreases while its sheet resistance and electrical resistivity increases, which is confirmed by van der Pauw method. Low angle XRD analysis revealed that with increase in target-to-substrate distance preferred orientation of Mo crystallites changes from (211) to (110) and its size decreases. The FE-SEM analysis revealed a significant change in surface morphology with increase in target-to-substrate distance. UV-Visible spectroscopy analysis showed that Mo films deposited at high target-to-substrate distance have more reflection than those deposited at lower target-to-substrate. Finally, adhesion test was performed using scotch hatch tape adhesion test which show all Mo films have excellent adhesion over the entire range of target-to-substrate distance studied. The employment of such Mo films as back contact can be useful to improve efficiency of CZTS solar cells.

Evaluation of thermal stability in spectrally selective few-layer metallo-dielectric structures for solar thermophotovoltaics

The thermal stability of spectrally selective few-layer metallo-dielectric structures is evaluated to analyze their potential as absorber and emitter materials in solar thermophotovoltaic (STPV) systems. High-efficiency (e.g., STPV) systems require materials with spectrally selective properties, especially at high temperatures (>1273 K). Aiming to develop such materials for high-temperature applications, we propose a few-layer structure composed of a refractory metal (i.e., Mo) nanometric film sandwiched between the layers of a dielectric material (i.e., hafnium oxide, HfO2) deposited on a Mo bulk substrate. In vacuum conditions (<5 × 10–2 Pa), the few-layer structure shows thermal stability at 1423 K for at least 1 h. At 1473 K, the spectral selectivity was degraded. This could have been caused by the oxidation of the Mo thin film by the residual oxygen through the grain boundaries of the upper HfO2 layer. This experiment showed the potential stability of few-layer structures for applications working at temperatures greater than 1273 K as well as the degradation mechanism of the few-layer structure. This characteristic is expected to help improve the thermal stability in few-layer structures further.