Mo Layer Research Articles

Back contact selenization and absorber layer etching for improvement in Schottky diode behavior of [Mo/CIGS/Al] structure

The effect of back contact selenization and absorber layer etching on Cu(In,Ga)Se2 (CIGS) structure for thin-film solar cell applications was investigated. Considering the dependence of the efficiency of solar cells on the performance of the absorber layer, it is essential to find its optimum parameters. This study deals with the influence of selenization of Mo layer and KCN chemical etching of the absorber layer on the structure. The Schottky diode current-voltage curves of [Glass/Mo/CIGS] samples with the selenized Mo layer showed enhanced current intensity which was confirmed by the presence of MoSe2 phase in XRD patterns. The wet KCN etching was used to remove the copper sulfide impurities, thereby higher shunt resistance was achieved in the etched samples. The effectiveness of etching treatment was confirmed by SEM images and EDX analysis. Our results showed that the selenization of the Mo layer and etching of the absorber layer lead to a decrease in the series resistance and an increase in the sheet and shunt resistances. Moreover, the rectifying factor was enhanced about 150% with an ideality factor of 2.87.

Spin valve effect induced by spin-orbit torque switching

The spin valve effect driven by the magnetic field in sandwich structures stands out as a seminal phenomenon in the emerging field of spintronics, but it has not been realized in low-power-dissipation devices operated via electrical means. Here, we investigate spin-orbit torque switching in a perpendicularly magnetized CoFeB/Mo/CoFeB sandwich, where the spin current generated in the Mo layer flows upward and downward to the ferromagnetically coupled CoFeB layers. When scanning the current, two CoFeB layers can be switched from one antiparallel state to a parallel state and then to the other antiparallel state, producing an unprecedentedly current-dependent spin valve effect. Such a spin valve effect is sensitive to the additional magnetic field direction due to the small spin torque efficiency and efficiency difference of the two CoFeB layers. The experimental observations are supported by the dynamics simulation based on the Landau-Lifshitz-Gilbert equation. Besides the fundamental interest, our finding would add a different dimension to energy efficient memory devices and sensors.

Hydrogen interaction with selectively desulfurized MoS2 surface using Ne+ sputtering

By Ne ion sputtering on the single-crystalline MoS2 surface, we demonstrate that the S layers sandwiching Mo in the top layer are sequentially removed by top-down desulfurization, but the intermediate Mo layer is maintained. Selective desulfurization can be used to control the bandgap of MoS2 by switching the polarity from n-type to p-type conductivity and further inducing metallization. Furthermore, the polarity of the MoS2 surface can also be switched by controlling the hydrogen bonding at/around various sulfur vacancy defects. More importantly, we reveal that such desulfurization weakens the hydrogen interaction on the cleaved MoS2 surface by removing the mono-sulfur vacancies (VS). This finding elucidates the important role of the VS defect for high catalytic activity.

Deposition and oxidation behaviour of molybdenum disilicide coating on Nb based alloys substrate by combined AIP/HAPC processes

A MoSi2 coating was prepared on Nb-10Hf-1Ti alloys by a two-step process. The first step was arc ion plating (AIP) of the Mo layer, and the second step was halide-activated pack cementation (HAPC) of Si. The coating is composed of an outer layer of MoSi2 and an inner bond layer. After oxidation testing at 1200 °C for 100 h in a static air atmosphere, the mass gain of the coated specimen was 0.64 mg/cm2. The good oxidation resistance of the MoSi2 coating is attributed to the protective SiO2 scale generated by the oxidation process.

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.

First principle study of formation mechanism of molybdenum-doped amorphous silica in MoO<sub>3</sub>/Si interface

An amorphous mixing layer (3.5–4.0 nm in thickness) containing silicon (Si), oxygen (O), molybdenum (Mo) atoms, named <i>α</i>-SiO<sub><i>x</i></sub>(Mo), is usually formed by evaporating molybdenum trioxide (MoO<sub>3</sub>) powder on an n-type Si substrate. In order to investigate the process of adsorption, diffusion and nucleation of MoO<sub>3</sub> in the evaporation process and ascertain the formation mechanism of <i>α</i>-SiO<sub><i>x</i></sub>(Mo) on a atomic scale, the first principle calculation is used and all the results are obtained by using the Vienna <i>ab initio</i> simulation package. The possible adsorption model of MoO<sub>3</sub> on the Si (100) and the defect formation energy for substitutional defects and vacancy defects in <i>α</i>-SiO<sub>2</sub> and <i>α</i>-MoO<sub>3</sub> are calculated by the density functional theory. The results show that an amorphous layer is formed between MoO<sub>3</sub> film and Si (100) substrate according to <i>ab initio</i> molecular dynamics at 1500 K, which are in good agreement with experimental observations. The O and Mo atoms diffuse into Si substrate and form the bonds of Si—O or Si—O—Mo, and finally, form an <i>α</i>-SiO<sub><i>x</i></sub>(Mo) layer. The adsorption site of MoO<sub>3</sub> on the reconstructed Si (100) surface, where the two oxygen atoms of MoO<sub>3</sub> bond with two silicon atoms of Si (100) surface, is the most stable and the adsorption energy is －5.36 eV, accompanied by the electrons transport from Si to O. After the adsorption of MoO<sub>3</sub> on the Si substrate, the structure of MoO<sub>3</sub> is changed. Two Mo—O bond lengths of MoO<sub>3</sub> are 1.95 Å and 1.94 Å, respectively, elongated by 0.22 Å and 0.21 Å compared with the those before the adsorption of MoO<sub>3</sub> on Si substrate, while the last bond length of MoO<sub>3</sub> is little changed. The defect formation energy value of neutral oxygen vacancy in <i>α</i>-SiO<sub>2</sub> is 5.11 eV and the defect formation energy values of neutral oxygen vacancy in <i>α</i>-MoO<sub>3</sub> are 0.96 eV, 1.96 eV and 3.19 eV, respectively. So it is easier to form oxygen vacancy in MoO<sub>3</sub> than in SiO<sub>2</sub>, which implies that the oxygen atoms will migrate from MoO<sub>3</sub> to SiO<sub>2</sub> and forms a 3.5–4.0-nm-thick <i>α</i>-SiO<sub><i>x</i></sub>(Mo) layer. As for the substitutional defects in MoO<sub>3</sub> and SiO<sub>2</sub>, Mo substitutional defects are most likely to form in SiO<sub>2</sub> in a large range of Mo chemical potential. So based on our obtained results, the forming process of the amorphous mixing layer may be as follows: the O atoms from MoO<sub>3</sub> bond with Si atoms first and form the SiO<sub><i>x</i></sub>. Then, part of Mo atoms are likely to replace Si atoms in SiO<sub><i>x</i></sub>. Finally, the ultra-thin buffer layer containing Si, O, Mo atoms is formed at the interface of MoO<sub>3</sub>/Si. This work simulates the reaction of MoO<sub>3</sub>/Si interface and makes clear the interfacial geometry. It is good for us to further understand the process of adsorption and diffusion of atoms during evaporating, and it also provides a theoretical explanation for the experimental phenomenon and conduces to obtaining better interface passivation and high conversion efficiency of solar cell.

Исследование формирования омических контактов Au/Mo/Ti с пониженным сопротивлением к эпитаксиальным слоям алмаза p-типа

The formation of ohmic Au/Mo/Ti contacts to epitaxial p-type diamond films is studied. The effect of annealing on the electrical and structural properties of contacts has been investigated. It was shown that during rapid thermal annealing, the outer layer of gold protects the contact system from oxidation up to a temperature of 850°C, unlike the simplified Au/Ti system, which is more common in modern works. In Au/Ti structures without a Mo layer after high-temperature annealing, effective diffusion of titanium into the gold layer occurs, which reduces its protective properties and accelerates the diffusion of oxygen to the boundary with the diamond. Oxidation of the Ti/C contact region blocks the formation of a conductive layer of titanium carbide with high adhesion at the border with diamond. The role of various factors in reducing the contact resistance is compared: annealing for the formation of titanium carbide, heavy doping of diamond with boron atoms, and crystalline perfection of epitaxial diamond substrates. For doped epitaxial films grown on single-sector quality substrates, non-annealed ohmic contacts with a record contact resistance of 4•10<-7> Ω•cm<2> were obtained.

Numerical Simulation of the Influence of an Sb layer on a Cu(In, Ga)Se2 Solar Cells Performance

A comprehensive study of adding an antimony (Sb) layer on top of the Mo layer in a low cost CIGS solar cell is presented. It was found that adding Sb layer improves the solar cell efficiency which was attributed to the reduction of defects at the CdS/CIGS interface. To elucidate this phenomenon, numerical simulation is used to evaluate a CIGS thin film solar cell figures of merits with and without an Sb layer. The cell performance is evaluated by introducing defects at the interface CdS/CIGS. An improvement of the conversion efficiency from 12.08 to 13.6% is reached. The short circuit current density Jsc improved from 25.74 to 26.46 mAcm-2, the open circuit voltage VOC is reduced from 0.674 to 0.670 V and the fill factor FF increases from 69.57 to 77.13%. The calculated figures of merit are in good agreement with the measurement.

Flexible Crossbar‐Structured Phase Change Memory Array via Mo‐Based Interfacial Physical Lift‐Off

AbstractInorganic phase change memories (PCMs) have attracted substantial attention as a next‐generation storage node, due to their high‐level of performance, reliability, and scalability. To integrate the PCM on plastic substrates, the reset power should be minimized to avoid thermal degradation of polymers and adjacent cells. Additionally, flexible phase change random access memory remains unsolved due to the absence of the optimal transfer method and the selection device. Here, an Mo‐based interfacial physical lift‐off transfer method is introduced to realize a crossbar‐structured flexible PCM array, which employs a Schottky diode (SD) selection device and conductive filament PCM storage node. A 32 × 32 parallel array of 1 SD‐1 CFPCM, which utilizes a Ni filament as a nanoheater for low power phase transition, is physically exfoliated from the glass substrate at the face‐centered cubic/body‐centered cubic interface within the sacrificial Mo layer. First principles density functional theory calculations are utilized to understand the mechanism of the Mo‐based exfoliation phenomena and the observed metastable Mo phase. The flexible 1 SD‐1 CFPCM shows reliable operations (e.g., large resistance ratio of 17, excellent endurance over 100 cycles, and long retention over 104 s) with excellent flexibility. Furthermore, the random access operation is confirmed by addressing tests of characters “KAIST.”

Optimization of Mo/Cr bilayer back contacts for thin-film solar cells.

Molybdenum (Mo) is the most commonly used material as back contact in thin-film solar cells. Adhesion of Mo film to soda–lime glass (SLG) substrate is crucial to the performance of solar cells. In this study, an optimized bilayer structure made of a thin layer of Mo on an ultra-thin chromium (Cr) adhesion layer is used as the back contact for a copper zinc tin sulfide (CZTS) thin-film solar cell on a SLG substrate. DC magnetron sputtering is used for deposition of Mo and Cr films. The conductivity of Mo/Cr bilayer films, their microstructure and surface morphology are studied at different deposition powers and working pressures. Good adhesion to the SLG substrate has been achieved by means of an ultra-thin Cr layer under the Mo layer. By optimizing the deposition conditions we achieved low surface roughness, high optical reflectance and low sheet resistivity while we could decrease the back contact thickness to 600 nm. That is two thirds to half of the thickness that is currently being used for bilayer and single layer back contact for thin-film solar cells. We demonstrate the excellent properties of Mo/Cr bilayer as back contact of a CZTS solar cell.

Fabrication of W and Mo layers with multi-modal structures on Ti sheets through intense plastic deformation induced by ball collisions

Tungsten (W) and molybdenum (Mo) layers were fabricated onto Ti surfaces through intense plastic deformation induced by ball collisions initiated through a mechanically vibrated vial. The Ti disk was affixed to one side of the vial with either W or Mo targets affixed to the opposite side. During processing, ball collisions continually introduced the target material to the Ti surface, yielding mechanical alloying and formation of the desired alloyed layer. The as-fabricated W and Mo layers contained Fe component that was introduced from the milling media. The composition of the as-fabricated W layer corresponded to tungsten–iron heavy metal alloys. The introduction of Fe components could be favorable since the interaction between components resulted in the formation of complex multi-modal heterogeneous structures. There was an observed co-existence of alternating nanolaminated amorphous/crystalline structures, elemental flakes with polycrystalline nanograins, and nanocrystalline grains under 10 nm isolated by the amorphous phase. Structural formation in the W layer developed slowly compared to the Mo layer, which was related to the mechanical characteristics of the initial materials and the atomic reactivity with the Fe component. The as-fabricated surface alloyed W and Mo layers exhibited very high hardness values (883 HV and 933 HV, respectively) that were comparable to the hardness of tempered steel.

Current-Induced Spin-Orbit Torque and Field-Free Switching in Mo -Based Magnetic Heterostructures

Magnetic heterostructure Mo/CoFeB/MgO has strong perpendicular magnetic anisotropy and thermal stability. Through current-induced hysteresis loop shift measurements, we show that the dampinglike spin-orbit torque (SOT) efficiency of Mo/CoFeB/MgO heterostructure is $\xi_{DL}\approx -0.003\pm 0.001$ and fairly independent of the annealing temperature from 300$^\circ$C to 400$^\circ$C. Though $|\xi_{DL}|$ is small while compare to those from Ta or W-based heterostructures, reversible current-induced SOT switching of a thermally-stable Mo/CoFeB/MgO heterostruture can still be achieved. Furthermore, we observe field-free current-induced switching from a Mo/CoFeB/MgO structure with the Mo layer being wedge-deposited. Our results indicate that even for a weak spin-orbit interaction 4d transition metal such as Mo, it is still possible to generate sufficient spin current for conventional SOT switching and to realize field-free current-induced switching by structural engineering.

Electronic properties of Janus MoSSe nanotubes

Motivated by the successful experimental approach of Janus MoSSe monolayer, which consists of a Mo layer sandwiched between S and Se atomic layers, we systematically study its nanotube. Its properties are dependent on the tube’s diameter, chirality and outer layer. The electronic properties modulated by the strain are also investigated. The results show that armchair MoSSe nanotubes are mainly indirect semiconductors, while zigzag ones are often direct semiconductors. The band gap mainly decreases with the decrease of the diameter. The nanotubes with S outer layers have much smaller band gaps than the ones with Se outer layers. The calculations also show that the MoSSe nanotube with Se outer layer has similar electronic behavior to the MoS2 nanotube, while MoSSe nanotube with S outer layer is quite different from MoS2 nanotube. After applying the external strain, the nanotubes’ band gaps mainly increase first and then decrease with the increase of the strain, especially the one with Se outer layer. The semiconductor-to-metal transition could appear in some Janus MoSSe nanotubes by the control of external strain. Our investigations could help to design new electronic devices based on Janus MoSSe nanotubes.

Intercalation of alkali metals (Li, Na, and K) in molybdenum dinitride (MoN2) and titanium dinitride (TiN2) from first-principles calculations

We studied ternaries of nitrogen-rich titanium and molybdenum compounds combined with alkali metals (Li, Na, and K) as potential layered materials. LiMoN2 has already been synthesized with layered structure corresponding to intercalated 3R-MoS2, however efforts to completely deintercalate Li from LiMoN2 were unsuccessful. We studied ternaries MAN2 (A: Ti, Mo and M: Li, Na, K) having layered crystal structures, their deintercalation, and the layered binaries TiN2 and MoN2 within density-functional theory. We find that LiMoN2 and NaMoN2 have a layered structure isotypic to 2H-MoS2 with Li and Na ions filling interlayer spaces whereas LiTiN2 and NaTiN2 are isotypic to α-NaFeO2. Both KMoN2 and KTiN2 have a different kind of structure isotypic to SrTiN2 which differentiates K from Li and Na. Ternaries Li(Na)MoN2 and Li(Na)TiN2 are all metals and the alkali metal atoms are present as ions in these structures. Partially deintercalated ternaries suggest that layers can interact strongly and the material can loose its layered form. Furthermore, we find that binary MoN2 having a layered structure is not stable since monolayer MoN2 has a positive formation energy and N atoms belonging to neighboring layers interact and form N2 dimer in between Mo layers in which case formation energy becomes negative indicating that structure becomes more stable. These results can explain the decomposition of LiMoN2 during the experimental trials of complete deintercalation. In contrast, TiN2 has a negative formation energy already without interaction of N atoms belonging to neighboring layers.

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.

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.

Thickness Dependence on the Magnetism in Mo-Capped Epitaxial Fe Films

When magnetic metal films are oxidized, in many cases, their saturation magnetization values decrease. The importance of high magnetization is well-known because it is directly related to the maximum energy product. Thus, prevention of oxidation in magnetic metal films via capping is important not only for studying the magnetism in magnetic metal films but also for developing new packaging technology for such films. In this research, we successfully grow epitaxial (110) Fe films on (0001) Al2O3 substrates by using radio-frequency (RF) magnetron sputtering. We capped 10-nm-thick Mo layers on the Fe films to prevent oxidation. By varying the thickness of the films, we systematically observed the changes in both the coercivity and the saturation magnetization. Especially, when the film’s thickness was below 8.5 nm, the coercivity of the film started to decrease. We believe the drastic change in the coercivity appeared in the Fe films when the film’s thickness approached the critical domain size for a magnetic domain transition.

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.

Alkali incorporation into Cu(In,Ga)Se2 determined by crystal orientation of Mo back contact: Implications for highly efficient photovoltaic devices

The structural, optical, and electrical properties of molybdenum (Mo) layers play a major role in Cu(In,Ga)Se2 (CIGS) solar cell performance. The Mo layer works as a transport gate for diffusion of alkali ion from the soda-lime glass substrate to the back contact in CIGS solar cells. In the present work, Mo back contacts are controlled to exhibit two different orientations: (110)-oriented and randomly oriented. The influence of these orientations on CIGS absorbers and resulting solar cells is investigated. In situ thermo-Raman spectroscopy and secondary ion mass spectrometry results indicate that the greater amount of alkali ions are found in the CIGS absorber with randomly oriented Mo back contact than in the (110)-orientated Mo back contact. The resulting different Na incorporations significantly affect the performance of the resulting devices. Devices with the randomly oriented Mo back contact exhibit superior device performances to the devices with (110)-oriented Mo back contact. With comprehensive device characterizations, an alkali ion release determined by the orientation of the Mo back contact affects the recombination mechanism in the CIGS bulk and the back contact properties at the CIGS/Mo interface.

Pre-incorporation of Na into flexible Cu(In,Ga)Se2 thin film solar cells

Incorporation of sodium into the Cu(In,Ga)Se2 (CIGS) absorber layer is a key step for the growth of high conversion efficiency CIGS solar cells. However, the doping of Na into CIGS layer on a flexible substrate, i.e. Titanium foil, is still a challenge. In this paper, we report our results on comparing two different Na pre-incorporation methods in constructing flexible CIGS solar cells on Ti substrates: i) by sputtering a soda-lime glass thin film (SLGTF) onto Ti substrates prior to the growth of CIGS and Mo back contact layers and ii) by evaporating a NaF precursor layer on the Mo layer before the CIGS growth. By carefully optimizing the growth conditions, the conversion efficiencies of 13.47% and 15.02% are achieved for the SLGTF sample and the NaF sample, respectively. The NaF precursor method shows a better effect. The diffusion of Na and the gradient of Ga play important roles in the performance of the solar cells.