MgF2 Films Research Articles

Optical parameter inversion of thin films based on angular reflectance spectroscopy

This article proposes a method for obtaining the refractive index and thickness of thin films based on reflectivity at different angles combined with optimization algorithms. This method has high computational accuracy for both high absorption films (such as aluminum films, gold films, etc.) and low absorption films (such as MgF2 films). Since this study did not adopt the inversion scheme of refractive index dispersion equation fitting, it can be used for optical constant inversion in the case of unknown composition of coating materials. The results indicate that this method can simultaneously obtain high-precision information on the complex refractive index and film thickness of the coating material on the substrate. The calculation example achieved a thickness inversion error of 0.165 % for a 2nm metal aluminum film layer and 0.3303 % for a 150 nm MgF2 transparent film layer. Therefore, this study may provide further guidance for high absorption and measurement of the complex refractive index and thickness of transparent films.

Utilizing magneto-optical short-range surface plasmon resonance to enhance the performance of optical fiber sensor

Although the short-range surface plasmon resonance (SRSPR) has larger sensitivity than the long-range surface plasmon resonance (LRSPR) excited simultaneously, SRSPR is usually ignored in previous studies of the LRSPR sensors due to its much larger peak width. However, since the resonance peaks of both SRSPR and LRSPR narrow vastly when combined with the magneto-optical (MO) effect, SRSPR has the potential to reach much higher figure-of-merit (FOM) than LRSPR due to its higher sensitivity. In this work, the feasibility of utilizing magneto-optical short-range surface plasmon resonance (MOSRSPR), which is the combination of SRSPR and MO effect, to enhance FOM was verified first by analyzing the prism-based structure sensor with the transfer matrix method (TMM). Then a novel optical fiber sensor based on MOSRSPR was proposed. The structure of the proposed sensor is a D-shaped optical fiber coated with an MgF2 film, a thin silver film, and a MO material film of cerium-doped yttrium-iron garnet (CeYIG). The influence of the structural parameters including the MgF2, silver, and CeYIG film thicknesses was analyzed theoretically by the finite element method (FEM). With the optimal parameters, the FOM of the optical fiber MOSRSPR sensor is greatly enhanced to 10,750 RIU−1 in maximum and 2888 RIU−1 in average, which surpasses all reported optical fiber LRSPR sensors vastly. The miniaturized and simple design of the D-shaped fiber MOSRSPR sensor, coupled with its high performance, offers a significant reference for the study of optical fiber sensors with ultra-high FOM.

Thin film field-effect transistor with ZnO:Li ferroelectric channel

An n-type channel transparent thin film field-effect transistor (FET) using a top-gate configuration on a sapphire substrate is presented. ZnO:Li film was used as a channel, and MgF2 film as a gate insulator. Measurements showed that ZnO:Li films are ferroelectrics with spontaneous polarization [Formula: see text]–5[Formula: see text][Formula: see text]C/cm2 and coercive field [Formula: see text]–10[Formula: see text]kV/cm. The dependences of drain–source current on drain–source voltage at various gate–source voltages in two antiparallel [Formula: see text] states were measured and the values of field-effect mobility and threshold voltage were determined for two [Formula: see text] states are as follows: (a) [Formula: see text][Formula: see text]cm2/Vs, [Formula: see text][Formula: see text]V; (b) [Formula: see text][Formula: see text]cm2/Vs, [Formula: see text][Formula: see text]V. Thus, [Formula: see text] switching leads to a change in FET channel parameters. Results can be used to create a bistable or, more precisely, digital FET.

Achieving effective performance of silicon solar cell through electrosprayed MgF2 antireflection coating

Solar cells technology will be one of the futuristic major resources for global energy demands. One of the major problems in energy harvesting using solar cells was reflection of incident light over solar cell surface. The present research work aims to synthesize the magnesium fluoride through sol-gel process which acts as antireflection layer for achieving better output performance. MgF2 films were deposited over solar cells at varying time periods of 60, 90, 120 and150 min through electrospraying technique. MgF2 electrosprayed solar cells were inspected for evaluating its structural, optical, electrical and morphological characteristics. The coated solar cells were analyzed under dark and light conditions. The precursor materials involved in sol-gel synthesis of MgF2 were magnesium chloride hexahydride, polyethylene glycol and hydrofluoric acid. From the I–V results, it is evident that performance of 120 min MgF2 coated solar cell exerts maximum performance of 19.38 % (open) and 19.41 % (controlled). MgF2 antireflective electrosprayed films were prominent materials for minimizing reflection and ensuring the maximum photon transmission into the coated surface especially in opto-electronic applications.

Structural, infrared optical and mechanical properties of the magnesium fluoride films

In this research, the magnesium fluoride (MgF2) films are deposited by electron beam evaporation at different substrate temperatures. The structural, infrared optical properties, mechanical properties and application of the films are investigated. The films prepared at higher temperatures have better crystallinity. As indicated by SEM and AFM, large grains are formed on the film surface at higher temperatures. Preparing thin films at higher temperatures should effectively improve the microstructure with packing density, resulting in higher refractive index and lower moisture absorption. The infrared optical constants of the films are characterized by Gaussian oscillators, and the relationship between the optical absorption and the substrate temperature is revealed. The film deposited at 300 °C exhibits excellent hardness, which is above 9 GPa. The change of residual stress is mainly determined by thermal stress. By depositing MgF2 films on sapphire substrates, the transmittance of the sapphire is greatly improved in the range of 3 μm–5 μm. Besides, the MgF2 films are found to be excellent antireflective coatings for infrared optical elements.

Corrosion resistance and biocompatibility of the KMgF3 coated AZ31 magnesium alloy

MgF2 film was usually created on magnesium (Mg) alloys to enhance its corrosion resistance and biocompatibility. Nonetheless, MgF2 film was mainly created by immersion of Mg alloys in highly toxic HF solution. In the current work, a potassium magnesium fluoride (KMgF3) film was fabricated on AZ31 Mg alloy through immersion of the MgF2 coated alloy in 1 mol·dm−3 KF solution. The surface/cross-section morphologies and structure of the KMgF3 film coated alloy before and after corrosion were examined. The corrosion resistance, surface properties and biocompatibility of the KMgF3 film coated AZ31 Mg alloy were characterized as well. The results showed that a compact monolayer film consisting of KMgF3 as the major phase and MgF2 and MgO as the minor phases was formed on AZ31 Mg alloy. The KMgF3 film coated alloy showed a reduction in the evolved hydrogen volume by 68% and a corrosion current density by around 49 times as compared to AZ31 Mg alloy. Additionally, The KMgF3 film coated alloy exhibited a better long-term durability in simulated body fluid than the MgF2 coated Mg alloy. Moreover, the KMgF3 film improved the hydrophilicity of AZ31 Mg alloy and stimulated the proliferation of bone marrow mesenchymal stem cells.

Mixed-Ligand Precursors for the Preparation of MgF2 Films: Effect of the Fluorinated Substitute on the Structure and Thermal Properties

Mixed-Ligand Precursors for the Preparation of MgF2 Films: Effect of the Fluorinated Substitute on the Structure and Thermal Properties

Biodegradable Mg alloy modified with bioactive exosomes for cardiovascular stent application

Cardiovascular stent has been widely applied to treat cardiovascular disease (CVD), which is the major disease contribution to mortality in the world wide. Biodegradable magnesium (Mg) alloys are the encouraging materials in cardiovascular stents benefit from absorbability and biocompatibility. While, the ability of degradation is a double-edged sword for manufacture stent, modifying the surface to decrease the excessive degradation rate and promote the surface endothelialization could expand the prospect of the further application. In this work, the biodegradable Mg-Zn-Y-Nd alloy was modified by MgF2 and dopamine polymer film (PDA) as the corrosion resistance layer and the bonding layer respectively, and then the exosome, a natural nanoparticle contains mRNAs and proteins, was tailored to give the surface better biocompatibility. The electrochemical test and weight loss test reflected the MgF2-PDA/exosome coating increase the corrosion resistance of the Mg-Zn-Y-Nd alloy. The cytocompatibility data indicated the novel MgF2-PDA/exosome coating selectively reduced the tumor necrosis factor (TNF-α) expression and ROS release from macrophage, and promoted the α-SMA expression of smooth muscle cells. In addition, the MgF2-PDA/exosome coating also improved the adhesion, proliferation, CD31 expression and nitric oxide (NO) release of vascular endothelial cells (ECs), all of which contribute to the surface endothelialization. And the mechanism experiments showed the exosome released from the coating uptake by the ECs and assemble around the lysosome and mitochondria, and the released rate of the exosome on the coating is around 5 to7 days, indicating excellent multi-functions of MgF2-PDA/exosome coating in cardiovascular stent.

Characterization of a Magnesium Fluoride Conversion Coating on Mg-2Y-1Mn-1Zn Screws for Biomedical Applications.

MgF2-coated screws made of a Mg-2Y-1Mn-1Zn alloy, called NOVAMag® fixation screws (biotrics bioimplants AG), were tested in vitro for potential applications as biodegradable implants, and showed a controlled corrosion rate compared to non-coated screws. While previous studies regarding coated Mg-alloys have been carried out on flat sample surfaces, the present work focused on functional materials and final biomedical products. The substrates under study had a complex 3D geometry and a nearly cylindrical-shaped shaft. The corrosion rate of the samples was investigated using an electrochemical setup, especially adjusted to evaluate these types of samples, and thus, helped to improve an already patented coating process. A MgF2/MgO coating in the µm-range was characterized for the first time using complementary techniques. The coated screws revealed a smoother surface than the non-coated ones. Although the cross-section analysis revealed some fissures in the coating structure, the electrochemical studies using Hanks' salt solution demonstrated the effective role of MgF2 in retarding the alloy degradation during the initial stages of corrosion up to 24 h. The values of polarization resistance (Rp) of the coated samples extrapolated from the Nyquist plots were significantly higher than those of the non-coated samples, and impedance increased significantly over time. After 1200 s exposure, the Rp values were 1323 ± 144 Ω.cm2 for the coated samples and 1036 ± 198 Ω.cm2 for the non-coated samples, thus confirming a significant decrease in the degradation rate due to the MgF2 layer. The corrosion rates varied from 0.49 mm/y, at the beginning of the experiment, to 0.26 mm/y after 1200 s, and decreased further to 0.01 mm/y after 24 h. These results demonstrated the effectiveness of the applied MgF2 film in slowing down the corrosion of the bulk material, allowing the magnesium-alloy screws to be competitive as dental and orthopedic solutions for the biodegradable implants market.

Design of Quadruple‐Layer Antireflection Coating for AlGaInP/AlGaAs/GaAs Three‐Junction Solar Cell

A combination of AlGaInP/AlGaAs/GaAs is commonly used in world‐record five‐ and six‐junction solar cells for their easily tunable bandgap and nearly matched lattice constants. Wide‐range low reflectivity is of significant importance in this three‐junction solar cell (3JSC), but also difficult to achieve particularly in 300–400 nm due to the optical properties of III–V alloys. Therefore, quadruple‐layer antireflection coatings (QLAR) composed of ZnS and MgF2 films are studied in this article for both performance and practicability. Optical constants of ZnS and MgF2 films are extracted by variable angle spectroscopic ellipsometry. 150 °C is determined as the best deposition temperature to decrease parasitic absorption in ZnS films. The antireflection coating is designed by the transfer‐matrix method considering both low reflectivity in a wide range of 300–900 nm and balanced subcell current density. Fabricated 3JSC with optimized MgF2/ZnS/MgF2/ZnS QLAR is demonstrated to possess low reflection losses in a broadband spectral range, with an average of only 1.59% in 300–900 nm. This boosts the integral current density to 11.58, 11.35, and 11.25 mA cm−2 for AlGaInP, AlGaAs, and GaAs subcells, respectively.

Increased durability of sol–gel derived MgF2 antireflective coatings capped by vapor deposition

Porous MgF2 antireflective λ/4 films were prepared by sol–gel processing and coated with an additional top layer by electron beam evaporation. Scanning Electron Microscopy was applied to characterize the microstructure of the bilayer assembly. It can be shown that the top layer has a protective effect in terms of abrasion resistance and reduced solubility in water. In a second step the thickness of the two film systems has been matched to achieve optimum antireflection properties.Porous antireflective MgF2 coatings were prepared by sol–gel processing. On these samples dense MgF2 films were deposited by evaporation in order to increase the stability of the system.

Investigation of deposition temperature effect on spatial patterns of MgF2 thin films.

In this work, the atomic force microscopy (AFM) technique was used to characterize 3D MgF2 thin film surfaces through advanced analysis involving morphological, fractal, multifractal, succolarity, lacunarity and surface entropy (SE) parameters, consistent with ISO 25178-2: 2012. Samples were synthesized by electron beam deposition, grown in three different temperatures. Three different temperatures of 25°C (laboratory temperature), 150 and 300°C were chosen. The temperature of 300°C is usually the highest temperature that can be deposited with the electron beam evaporation coating system. The substrates were made of glass (diameter 16 mm, thickness 3 mm), and the samples were prepared at a pressure of 5 × 10-5 Torr. The statistical results from the AFM images indicate that topographic asperities decrease with increasing deposition temperature, showing a decrease in roughness values. Regardless of the deposition temperature, all surfaces have a self-similar behavior, presenting a very linear PSD distribution, and, according to our results, the sample deposited at 300° had the highest spatial complexity. On the other hand, surface percolation is increasing when temperature increases, indicating that its low roughness and high spatial complexity play an important role on the formation of their most percolating surface microtexture. Our results demonstrate that the lower deposition temperature promoted the formation of less discontinuous height distributions in the MgF2 films.

Simple Design of Double-Layer Antireflection Coating for Er-Doped Glass Laser Application

In an Erbium-doped glass laser resonator, parasitic light oscillations (yielding a lowering of the output laser beam power) may be avoided by deposition of well-adapted antireflection coatings on the edges of the active glass medium. However, towards laser application, efficient double-layers are scarce in literature. Here, we propose a simple design of double-layer (total thickness < 490nm) composed of thin films of MgF2 and Al2O3, materials that are easy to deposit by electron beam evaporation. Such coating design allows a calculated reflectance to be lower than 0.01% in the considered 1530-1570nm Near-Infrared range.

Comparison of measurement results of MgF2–Nb2O5 distributed Bragg reflectors with different periods

In this study, by using the Corning 1737 glass as a substrate and an e-beam system, the optical properties of the deposit MgF2 and Nb2O5 single-layer film were investigated in terms of extinction coefficients ([Formula: see text] values) and refractive indices ([Formula: see text] values). The equation [Formula: see text]/(4[Formula: see text] was used to calculate the thickness ([Formula: see text] of 1/4 wavelength ([Formula: see text] for each layer of MgF2 and Nb2O5 layer in the MgF2–Nb2O5 bilayer films of the designed distributed Bragg reflectors (DBRs) with a reflective wavelength at blue light ([Formula: see text]450 nm). Each pair of MgF2–Nb2O5 bilayer film was defined as a period, and the glass substrates were used to deposit the films with two, four, and six periods for the fabrication of the DBRs. The field emission scanning electron microscope equipped with a focused ion beam was used to measure the thickness of each MgF2–Nb2O5 layer in the DBRs with different periods. The measured maximum reflective ratios were compared with Sheppard’s approximate equation, which calculates only the maximum reflective ratio at a specific wavelength. We would show that the measured results of the fabricated DBRs matched the results simulated using Sheppard’s approximate equation.

A high-Q mid-infrared Tamm plasmon absorber using MgF2 and Ge aperiodic tandem films designed by the genetic algorithm

A high-Q mid-infrared Tamm plasmon (TP) absorber using MgF2 and Ge aperiodic tandem films is designed by the genetic algorithm and fabricated by electron beam evaporation and magnetron sputtering. Such a design circumvents the need for the distributed Bragg reflector grating, and the thickness of individual MgF2 films is much smaller than a quarter of wavelength in films (λ/4n). The fabricated TP absorber exhibits a high and sharp absorption peak at 2.73 μm with a quality factor (Q) of 144, while the simulated counterpart has the absorption peak at 2.72 μm with a Q of 155. The profiles of an electromagnetic field in the tandem films are also simulated to reveal the mechanism of narrow thermal emission. Such a high-Q TP absorber is very promising in the mid-infrared simulator and detection.

Effect of porosity on thickness uniformity of MgF2 films on spherical substrates.

Simulation based on Knudsen's law shows that film thickness uniformity above 99% can be realized on spherical substrates with optimized profiles of shadowing masks. However, a type of optical thickness nonuniformity is revealed when the masks are applied for thickness correction of MgF2 films experimentally. The optical thickness nonuniformity depends on steepness of the spherical surfaces and reaches 5% approximately for surfaces with CA/RoC = 1.22. Porosity of the MgF2 film is superimposed on Knudsen's law to interpret the optical thickness nonuniformity. For theoretical simulation, the influence of porosity on optical thickness distribution is characterized by a new parameter that describes nonlinear dependence of deposition rate on cosine function of molecular injection angles in Knudsen's law. Utilizing the optimized deposition model, optical thickness uniformity of MgF2 films approaching to or above 99% has been achieved for surfaces of different steepness in a single coating run.

Tunnel magnetodielectric effect: Theory and experiment

The recently discovered tunnel magnetodielectric (TMD) effect—the magnetic field-induced increase in the dielectric permittivity (ε′) of nanogranular composites caused by the spin-dependent quantum mechanical charge tunneling—is of interest for both the scientific value that combines the fields of magnetoelectric and spintronics and multifunctional device applications. However, little is known about how large the maximum dielectric change Δε′/ε′ can achieve and why the Δε′/ε′ variations obey the dependence of square of normalized magnetization (m2), which are critically important for searching and designing materials with higher Δε′/ε′. Here, we perform approximate theoretical derivation and reveal that the maximum Δε′/ε′ can be estimated using intrinsic tunneling spin polarization (PT) and extrinsic normalized magnetization (m), that is, Δε′/ε′ = 2PT2m2. This formulation allows predicting over 200% of theoretical limit for m = 1 and accounts for the observed m2 dependence of Δε′/ε′ for a given PT. We experimentally demonstrate that x-dependence of Δε′/ε′ in (CoxFe100−x)–MgF2 films is phenomenologically consistent with this formulation. This work is pivotal to the design of ultra-highly tunable magnetoelectric applications of the TMD effect at room temperature.

Laboratory-based reflectometer using line spectra of an RF-induced gas-discharge lamp in 30- to 200-nm wavelength range

A laboratory-based reflectometer designed for characterizing the reflectivity of optical coatings in 30- to 200-nm wavelength range was recently developed at IPOE. An RF-produced gas-discharge light source is applied to generate characteristic lines. The light source is mounted on a grazing incident monochromator with a 146-deg deviation angle between the incident and diffracted arms. By precisely adjusting the toroidal grating inside the monochromator chamber, monochromatic lights are acquired through the exit slit. A collimator mirror and two sets of collimation slits with 2 mm × 2 mm dimension are utilized for reducing the divergence of the beam incident on the sample. A high-precision six-axis translation stage, which allows a heavy sample with a maximum diameter of 100 mm, is used to control positions of the samples and the detector. A chopper disk used both for incident light intensity monitor and signal modulation is placed with an incidence angle of 70 deg relative to the incident light beam. The configuration, adjustment process, and test results of the reflectometer are presented in detail. The experimental reflectivity results for Al / LiF / MgF2 film obtained from our laboratory and BESSY-II Synchrotron as well as Hefei Synchrotron Light Source are given and compared for demonstrating the reliability of the system.

Ion beam assisted electron gun deposition of MgF2 thin films

SummaryA traditional method for increasing the density of the optical films, is heating the substrate. An alternative way to increase the density of the film, is bombardment of substrate by ions during film growth. This method is known as ion assisted deposition (IAD). In this paper, magnesium fluoride, MgF2, thin film has been produced on a glass substrate by ion assisted deposition method. The effects of ion energy have been studied on density and refractive index film. Reflection, transmission and scattering spectra of the samples were measured by a spectrophotometer. Moreover, Atomic force microscope (AFM) is prepared. Using these measurements, optical and physical properties of the films are analyzed. A sample of MgF2 film has been produced without ion bombardment so that one can compare the effect of ion bombardment.

Effect of humidity on the performance of Al/LiF/eMgF2 mirrors in the far ultraviolet spectrum

The far ultraviolet (FUV) spectrum is critical for the observation of phenomena in space, among various other applications. Hence, the aim of our study was to investigate the effect of the humidity of Al mirrors coated with LiF and enhanced MgF2 within the FUV spectrum. The samples were first coated with Al and LiF films on a super-polished silicon wafer at room temperature, heated to 220°C, and then coated with an enhanced MgF2 film. All materials were deposited by thermal evaporation. The samples were stored in environments with 20% relative humidity (RH), 40% RH, 80% RH, and 90% RH. The reflectivity remained stable when the environment was 20% RH, which was the optimal storage environment. The reflectivity of the Al / LiF / eMgF2 mirrors stored in high RH decreased over time, the roughness increased over time. Furthermore, the decrease rate of reflectivity and increase rate of roughness decreased over time. The degradation of reflectivity can be attributed to the presence of oxygen in environments with a high RH.