Amorphous MoSi Research Articles

Optimizing the growth conditions of superconducting MoSi thin films for single photon detection

We investigate the growth of amorphous MoSi thin films using magnetron co-sputtering and optimize the growth conditions with respect to crystal structure and superconducting properties (e.g., critical temperature ). The deposition pressure, Mo:Si stoichiometry and substrate temperature are systematically varied to achieve a transition temperature of 8.4(3) K for films with a thickness of 17.7(8) nm and 6.2(9) K for a 4.3(4) nm thick film. For Mo concentrations above 81% the crystalline phase Mo3Si is observed in grazing incidence X-ray diffraction measurements. The same phase appears when the working pressure during deposition is reduced below 3.1×10-3mbar and when the substrate temperature during deposition is increased above C. By choosing a sufficient Si concentration and optimum deposition pressure we identify deposition conditions that ensure a homogeneous amorphous growth of the superconducting thin film. We then fabricate superconducting nanowire single-photon detectors which exhibit an unitary internal efficiency to single photons at an operational temperature of 1.2 K while simultaneously having a dark count rate below 1 Hz. Our results establish the link between MoSi film deposition, morphology and the performance of SSPD.

Magnetoconductivity of amorphous films based on molybdenum and tungsten silicide and germanide

Abstract Amorphous materials have shown remarkable properties for the fabrication of superconducting single photon detectors in different wavelength range. In this work, we have deposited four films with a thickness of 5 nm, from different amorphous materials, namely WGe, WSi, MoGe, and MoSi. Through comprehensive analyses of the properties of the materials, we investigate their fundamental parameters and characteristic interaction times, derived from magnetoconductivity measurements in a physical property measurement system. By comparing them, we aim to provide a deeper understanding of their suitability for superconducting strip photon detectors applications. Specifically, our findings highlight similarities between WGe and MoGe with their counterparts based on silicon, which are already established materials to produce superconducting detectors capable of operating at extended wavelengths.

Computational approach to identify structural and elastic relationship in metastable crystalline and amorphous alloy thin films: Mo1-xNix and Mo1-xSix case studies

Previously well-established experimental trends of ground-state properties of crystalline and amorphous Mo1-xSix and Mo1-xNix alloys with 0 ≤ x ≤ 1 are predicted by first-principles calculations and inter-relationship with sound velocities and elastic properties are identified. The free energy of mixing, calculated at 300 K provides accurate values of the critical Si and Ni concentrations leading to the crystalline-to-amorphous transition. Specifically, a transition from BCC to amorphous state is predicted for xSi ~ 0.14 and xNi ~ 0.32, while FCC to amorphous transition is observed for xNi ~ 0.77. These structural transitions are accompanied by modifications of the out-of-plane longitudinal and shear elastic constants. In the crystalline region, a pronounced softening of elastic moduli is predicted for both solid solution alloys. While for amorphous Mo-Ni, the elastic constants do not undergo significant changes, for amorphous Mo-Si, they exhibit two distinct behaviors depending on the electronic properties and bonding character. For 0.2 ≤ xSi ≤ 0.7, the metallic character of the amorphous alloys is maintained and the elastic properties are remarkably stable. For xSi > 0.7, a progressive increase in the atomic volume is observed and the amorphous alloys acquire a covalent character and a reduced coordination number. Surprisingly, during this transition, both the longitudinal and shear acoustic velocities continuously increase, despite a progressive softening of the elastic stiffness constants. These calculations provide deep insight at the atomic-scale and reproduce satisfactorily the earlier experimental works of magnetron co-sputtered Mo-Si films, while some disagreement on elastic properties remain for more energetic ion beam sputtered Mo-Ni films, likely partially attributed to point-defects.

Suppression of superconductivity dominated by proximity effect in amorphous MoSi nanobelts

A traditional concept proposes that the suppression of the transition temperature ${T}_{c}$ in an amorphous nanobelt is driven by enhanced disorder, which accounts for localized Cooper pairs. However, in this paper, we observe ${T}_{c}$ suppression in an amorphous molybdenum-silicide (MoSi) nanobelt, which scales as the inverse square of the width but contradicts disorder theory. Instead, the transition regime can be well described by Cooper pair diffusion in the proximity effect. Both the nonlinear reduction of the switching current density and the abnormal increase of the effective retrapping current density with the reduction of the width further verify the proximity-induced relation. Therefore, we attribute the main size dependence of the suppressed superconducting properties in the MoSi nanobelt to the proximity effect rather than disorder. We speculate that the competition between superconductivity and disorder only appears at the two narrow edge bands rather than the entire nanobelt. Subsequently, the reduction in width does not produce a significant impact on superconductivity for disorder, and only the proximity effect plays an overwhelming role in the MoSi nanobelt.

Vortex dynamics in amorphous MoSi superconducting thin films

Vortex dynamics in superconductors have received a great deal of attention from both fundamental and applied researchers over the past few decades. Because of their critical role in the energy relaxation process of type-II superconductors, vortex dynamics have been deemed a key factor for the emerging superconducting devices, but the effect of irradiation on vortex dynamics remains unclear. With the support of electrical transport measurements under external magnetic fields and irradiation, the photon effect on vortex dynamics in amorphous MoSi (a-MoSi) superconducting thin films is investigated in this work. The magnetic-field-dependent critical vortex velocity v* derived from the Larkin–Ovchinnikov (LO) model is not significantly affected by irradiation. However, vortex depinning is found to be enhanced by photon-induced reduction in the potential barrier, which mitigates the adverse effect of film inhomogeneity on superconductivity in the a-MoSi thin films. A thorough understanding of the vortex dynamics in a-MoSi thin films under the effect of external stimuli is of paramount importance for both further fundamental research in this area and the optimization of future superconducting devices.

Microstructure Evolution and Mechanical Behavior of Mo–Si–N Films

The molybdenum silicon nitride (Mo–Si–N) films were deposited by a radio frequency (RF) magnetron reactive dual-gun co-sputtering technique with process control on input power and gas ratio. Composition variation, microstructure evolution, and related mechanical and tribological behavior of the Mo–Si–N coatings were investigated. The N2/(Ar + N2) flow ratios were controlled at 10/20 and 5/20 levels with the tuning of input power on the Si target at 0, 100, and 150 W. As the silicon contents increased from 0 to 33.7 at.%, the film microstructure evolved from a crystalline structure with Mo2N and MoN phases to an amorphous feature with the Si3N4 phase. The analysis of selected area electron diffraction patterns in TEM also indicated an amorphous feature of the Mo–Si–N films when Si content reached 20 at.% and beyond. The hardness and Young’s modulus changed from 16.5 to 26.9 and 208 to 273 GPa according to their microstructure features. The highest hardness and modulus were attributed to nanocrystalline Mo2N and MoN with Si solid-solution. The crystalline Mo–Si–N films showed a smooth tribological track and less wear failure was found. In contrast, the wear track with severe failures were observed for Mo–N and amorphous Mo–Si–N coatings due to their lower hardness. The ratios of H/E and H3/E2 were intensively discussed and correlated to the wear behavior of the Mo–Si–N coatings.

Ultra-High Oxidation Resistance of Nano Structured Thin Films

Diffusion driven high-temperature oxidation is one of the most important failure mechanisms of protective thin films in industrial applications. Within this study, we investigated the diffusion of oxygen at 800 to 1100 °C through nano-laminated crystalline Ti Al N and amorphous Mo Si B based multilayer coatings. Combining 18O tracer diffusion with atom probe tomography was used for identifying the most prominent oxygen diffusion pathways and hence weakest points for oxidation. An oxygen inward diffusion along column boundaries within Ti-Al-N layers in front of a visually prevalent oxidation front could be proven, highlighting the importance of these fast diffusion tracks. Furthermore, the amorphous Mo Si B layers act as barriers and therefore mitigate the migration of oxygen by accumulating attacking O species at a nanoscale range. Preventing oxygen diffusion along column boundaries - through the utilization of amorphous interlayers - lead to paralinear oxidation behavior and stable scales even after 7 h at 1100 °C. Our results provide a detailed insight on the importance of morphological features such as grain and column boundaries during high-temperature oxidation of protective thin films, in addition to their chemistry.

Strong suppression of the resistivity near the superconducting transition in narrow microbridges in external magnetic fields

We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths w ranging from 2 um to 1000 um and film thicknesses d ~ 4-6 nm and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of the films, the resistivity curves R(B,T) separate at a well-defined and field-dependent temperature T*(B) with decreasing the temperature, resulting in a dramatic suppression of the resistivity and a sharpening of the transitions with decreasing bridge width w. The associated excess conductivity in all the bridges scales as 1/w, which may suggest the presence of a highly conducting region that is dominating the electric transport in narrow bridges. We argue that this effect can only be observed in materials with sufficiently weak vortex pinning.

Influence of N2 Gas Flow Ratio and Working Pressure on Amorphous Mo–Si–N Coating during Magnetron Sputtering

In this study, Mo–Si–N coatings were deposited on Si wafers and tungsten carbide substrates using a reactive direct current magnetron sputtering system with a MoSi powder target. The influence of sputtering parameters, such as the N2 gas flow ratio and working pressure, on the microstructure and mechanical properties (hardness (H), elastic modulus (E), and H/E ratio) of the Mo–Si–N coatings was systematically investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), nanoindentation, and transmission electron microscopy (TEM). The gas flow rate was a significant parameter for determining the crystallinity and microstructure of the coatings. A Mo2N crystalline coating could be obtained by a high N2 gas flow ratio of more than 35% in the gas mixture, whereas an amorphous coating could be formed by a low N2 gas flow ratio of less than 25%. Furthermore, the working pressure played an important role in controlling the smooth surface and densified structure of the Mo–Si–N coating. For the amorphous Mo–Si–N coating deposited with the lowest working pressure (1 mTorr), the hardness, elastic modulus, and H/E ratio reached from 9.9 GPa, 158.8 GPa, and 0.062 up to 17.9 GPa, 216.1 GPa, and 0.083, respectively.

Imaging pinning and expulsion of individual superconducting vortices in amorphous MoSi thin films

We use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image individual vortices in amorphous superconducting MoSi thin films. Spatially resolved measurements of the magnetic field generated by both vortices and Meissner screening satisfy the Pearl model for vortices in thin films and yield values for the Pearl length and bulk penetration depth at 4.2 K. Flux pinning is observed and quantified through measurements of vortex motion driven by both applied currents and thermal activation. The effects of pinning are also observed in metastable vortex configurations, which form as the applied magnetic field is reduced and magnetic flux is expelled from the film. Understanding and controlling vortex dynamics in amorphous thin films is crucial for optimizing devices such as superconducting nanowire single photon detectors (SNSPDs), the most efficient of which are made from MoSi, WSi, and MoGe.

High-detection efficiency and low-timing jitter with amorphous superconducting nanowire single-photon detectors

Recent progress in the development of superconducting nanowire single-photon detectors (SNSPDs) made of amorphous materials has delivered excellent performances and has had a great impact on a range of research fields. Despite showing the highest system detection efficiency (SDE) ever reported with SNSPDs, amorphous materials typically lead to lower critical currents, which have impacts on their jitter performance. Combining a very low jitter and a high SDE remains a challenge. Here, we report on highly efficient superconducting nanowire single-photon detectors based on amorphous MoSi, combining system jitters as low as 26 ps and a SDE of 80% at 1550 nm. We also report detailed observations on the jitter behaviour, which hints at intrinsic limitations and leads to practical implications for SNSPD performance.

UV superconducting nanowire single-photon detectors with high efficiency, low noise, and 4 K operating temperature.

For photon-counting applications at ultraviolet wavelengths, there are currently no detectors that combine high efficiency (> 50%), sub-nanosecond timing resolution, and sub-Hz dark count rates. Superconducting nanowire single-photon detectors (SNSPDs) have seen success over the past decade for photon-counting applications in the near-infrared, but little work has been done to optimize SNSPDs for wavelengths below 400 nm. Here, we describe the design, fabrication, and characterization of UV SNSPDs operating at wavelengths between 250 and 370 nm. The detectors have active areas up to 56 μm in diameter, 70 - 80% efficiency at temperatures up to 4.2 K, timing resolution down to 60 ps FWHM, blindness to visible and infrared photons, and dark count rates of ∼ 0.25 counts/hr for a 56 μm diameter pixel. These performance metrics make UV SNSPDs ideal for applications in trapped-ion quantum information processing, lidar studies of the upper atmosphere, UV fluorescent-lifetime imaging microscopy, and photon-starved UV astronomy.

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

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

Reduction of thermal conductivity in semiconducting composite films consisting of silicon and transition-metal silicide nanocrystals

ABSTRACTWe observed significant reduction of thermal conductivity in semiconducting composite films of Si and molybdenum (Mo)-silicide nanocrystals (NCs). These films were synthesized by phase separation due to annealing at 700 -1000°C from sputtered amorphous Mo–Si alloy. Transmission electron microscope images showed that the NCs were grown to diameters of∼10 nm in the films by annealing at 800°C. Raman scattering spectra showed lower shift of peak positions of Si transverse optical (TO) phonon due to the confinement effect and the tensile stress. The electrical resistivity of the films was 0.17- 9 Ωm at room temperature and showed a semiconducting temperature dependence at 20-400 K. Thermal conductivity of the film was reduced to 4.4 W/mK by enhancement of phonon scattering at NC interfaces, suggesting that the composite film is promising as a high-efficiency Si-based thermoelectric material.

Thin film absorbers for visible, near-infrared, and short-wavelength infrared spectra

Two thin film absorbers are presented in this paper: one for the visible (VIS) and the near-infrared (NIR) spectra in the wavelength range of 350…1000 nm, the other for the short-wavelength infrared (SWIR) spectrum in the wavelength range of 1200…2000 nm. First, the refractive indices were determined for Al 2O 3 films prepared with atomic layer deposition (ALD), and amorphous Mo–Si–N films prepared with reactive sputter deposition. The measurements were made by spectroscopic reflectometry, ellipsometry, gonioreflectometry, and double-beam transfer standard spectrometry. The results were utilised in the design of the absorbers. The absorbers were manufactured as well, and they proved to have high absorption over their whole working spectra varying from 93.4% at the minimum to 99.9% at the maximum. The absorbers are applicable, e.g. to MEMS thermopile detectors.

Potential of amorphous Mo–Si–N films for nanoelectronic applications

The properties of amorphous metallic molybdenum–silicon–nitrogen (Mo–Si–N) films were characterised for use in nanoelectronic applications. The films were deposited by co-sputtering of molybdenum and silicon targets in a gas mixture of argon and nitrogen. The atomic composition, microstructure and surface roughness were studied by RBS, TEM and AFM analyses, respectively. The electrical properties were investigated in the temperature range 80 mK to 300 K. No transition into a superconductive state was observed. Nanoscale wires were fabricated using electron beam lithography with their properties measured as a function of temperature.

Analysis of milling energy in synthesis and formation mechanisms of molybdenum disilicide by mechanical alloying

The synthesis and formation of MoSi 2 by mechanical alloying (MA) of the Mo–Si powder mixtures, with the molar ratio of Mo:Si being equal to 1:2, has been investigated. Ball-milling experiments were conducted under different conditions such as rotation speed, ball-powder weight ratio, milling time and charging volume. The mechanically alloyed powders were characterized by X-ray diffraction. The milling energy was calculated by using a collision model and the milling energy maps were attained. Results showed that when the effective extensive factory is above 1.06 J/g s, the formation mechanism of MoSi 2 by mechanical alloying is SHS (self-propagating high-temperature synthesis). Otherwise, when it is below 0.917 J/g s, the formation mechanism of MoSi 2 is a diffusion solution. For the synthesis of MoSi 2, the milling energy maps indicated the total milling energy required for the beginning, ending of crystalline MoSi 2 phase formation and the beginning of amorphous MoSi 2 phase formation.

Periodic Multilayers MoSi2/Si with a Large Number of Periods

Different approaches to decreasing mechanical stresses developed in MoSi 2 /Si multilayers with increase of number of periods due to structural reconstruction in layers of amorphous silicon and nanocrystalline MoSi 2 were studied by scattering CuKα X-ray radiation at small and large angles, cross-sectional electron microscopy and micrometry of substrate sag. It was shown that effective relaxation of mechanical stresses in MoSi 2 /Si multilayers is achieved by annealing them at ∼320 °C during 1 hour, or by deposition of layers at substrate temperature ∼320 °C, or by increasing sputtering gas pressure up to 7 x 10 -3 Torr in case of argon. Optimal conditions for deposition of MoSi 2 /Si multilayers with periods N > 10 3 and high reflectivity of X-rays with wavelengths 12.4-20.0 nm are: substrate temperature T s = 220 °C, argon pressure P Ar = 3 x 10 -3 Torr, layer deposition rate 1 nm/s.

High temperature oxidation and pesting of Mo(Si,Al)2

Molybdenum aluminosilicide, Mo(Si,Al) 2 with the C40 structure is a promising material at elevated temperatures. High temperature oxidation of Mo(Si,Al) 2 gives a protective Al 2 O 3 scale at temperatures below 1868 K of which is the eutectic temperature in the SiO 2 -3Al 2 O 3 2SiO 2 system. Above the eutectic temperature, the scale is a liquid of SiO 2 –Al 2 O 3 with the Al 2 O 3 content being richer than the eutectic composition. The cooling of the hypereutectic liquid scale suppresses the formation of β -crystobalite and gives an amorphous scale with an excellent adherence to intermetallics. The disintegration of MoSi 2 (pesting) due to oxidation is severe around 773 K. The addition of Al markedly reduces pesting. Amorphous Mo–Si–Al–O forms in initial cracks and voids. This amorphous oxide is probably more plastic than the amorphous Mo–Si–O formed on MoSi 2 and decreases the stress generation at the crack which leads to disintegration.

Effect of 690-keV Xe ion irradiation on the microstructure of amorphous MoSi 2/SiC nanolayer composites

The effect of 690-keV Xe ion irradiation at three different dosage levels, 1 × 10 15, 5 × 10 15 and 10 × 10 15 /cm 2, on the microstructure of amorphous-MoSi 2/amorphous-SiC nanolayer composites has been studied using transmission electron microscopy. Results show that the depth of radiation damage in this multilayer material is ∼ 80 nm, which agrees qualitatively well with the calculated damage depth calculated by TRIM [1]. A diffraction ring corresponding to the (10 l 1) plane of C40 MoSi 2 was found in the electron diffraction pattern taken from the irradiated regions; the C40 phase is also found after thermal annealing of amorphous MoSi 2 at 500°C or above. In the damaged regions SiC layers were found to spheroidize while the nanocrystalline grains in the MoSi 2 layers appeared to coarsen with increasing dose.