CaF2 Films Research Articles

Fabrication of Europium-Doped CaF2 Films via Sol-Gel Synthesis as Down-Shifting Layers for Solar Cell Applications.

In the present work, an in-depth study on the sol-gel process for the fabrication of Eu-doped CaF2 materials in the form of thin films has been addressed for the production of down-shifting layers. Fine-tuning of the operative parameters, such as the annealing temperature, substrate nature and doping ion percentage, has been finalized in order to obtain Eu(III)-doped CaF2 thin films via a reproducible and selective solution process for down-shifting applications. An accurate balance of such parameters allows for obtaining films with high uniformity in terms of both their structural and compositional features. The starting point of the synthesis is the use of a mixture of Ca(hfa)2•diglyme•H2O and Eu(hfa)3•diglyme adducts, with a suited ratio to produce 5%, 10% and 15% Eu-doped CaF2 films, in a water/ethanol solution. A full investigation of the structural, morphological and compositional features of the films, inspected using X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX), respectively, has stated a correlation between the annealing temperature and the structural characteristics and morphology of the CaF2 thin films. Interestingly, crystalline CaF2 films are obtained at quite low temperatures of 350-400 °C. The down-shifting properties, validated by taking luminescence measurements under UV excitation, have allowed us to correlate the local environment in terms of the degree of symmetry around the europium ions with the relative doping ion percentages.

Friction films analysis and tribological properties of composite antifriction self-lubricating material based on nickel alloy

Abstract This article analyzes the composition and distribution of chemical elements in friction films and their effect on the tribological properties of the self-lubricating, high-temperature antifriction composite based on EP975 powder nickel alloy with CaF2 solid lubricant. Analysis of the chemical elements by energy-dispersive spectroscopy (EDS) showed their uniform distribution, on both the composite’s surface and the counterface’s surface. The alloying elements’ uniform distribution leads to a uniform distribution of the corresponding phases and structural elements in the antifriction film. This ensures high tribological properties at high temperatures. Analysis of the material’s tribological properties, by means of metallographic and micro-X-ray research confirmed the correctness of the technology for producing the composite. Solid lubricant CaF2, alloying elements, and their corresponding phases form the continuous antiscoring film. The film influences the antifriction properties formation during the friction process and provides a self-lubricating mode under the action of high temperature and oxygen. Antiscoring, self-lubricating CaF2 films minimize wear of the friction pairs and defend the contact surfaces against intensive wear. The dense antifriction films have smooth microtopography, which stabilizes the high-temperature friction unit operation. Thus, the self-lubrication mode is realized for a long exploitation time. Tribological properties analysis allowed us to determine the ranges of rational exploitation modes for the material being studied: a load up to 5.0 MPa, a slide speed from 0.3 to 1.0 m/s, a temperature up to 800°C, in the air. The results obtained opened the opportunity to control the antifriction film formation and the composite’s tribological properties by the choice of the initial ingredients while taking into account the operating conditions.

Atomic layer deposition of calcium fluoride for barrier coating applications

We report on the development of an atomic layer deposition (ALD) process for calcium fluoride thin films using bis(N,N-di-i-propylformamidinato)calcium(II) and anhydrous HF as reactants. Deposition rates were observed to be linear versus the number of ALD cycles performed, varying between 0.4 and 0.3 Å/cycle at substrate temperatures ranging from 175 to 250 °C, respectively. The optical properties of the resulting nanocrystalline CaF2 films were characterized by spectroscopic ellipsometry and show good transparency down to a UV wavelength of 193 nm, and good agreement with reference refractive index parameters. One motivation for the development of this ALD process is for chemical barrier applications in fluorine-containing chemistries. The robustness of the coating was tested by reactive ion etching of CaF2 films deposited on silicon in an SF6 plasma, yielding an Si:CaF2 etch ratio of greater than 2000:1 at a plasma power of 300 W.

Slow-release of fluorite and its effect on flotation separation of magnesite from calcite

The flotation separation of magnesite from calcite under Pb-BHA collector system is difficult due to the same action site of CO32– anion. Surface transformation of fluorite particles present great selectivity for different calcium containing minerals, which is due to the selective adsorption of slowly-released fluoride ions (F-). The application of surface transformation and Pb-BHA collectors can achieve the efficient separation of magnesite and calcite. This study suggests that the floatability of magnesite is similar to that of calcite without any depressant, and the presence of fluorite triggers a selectively inhibiting action on calcite but not on magnesite. Zeta potential and X-ray photoelectron spectroscopy measurements indicate that slowly-released F- from fluorite particles strongly is chemisorbed on the surface of calcite instead of magnesite, and the adsorbed F- react with Ca2+ on the calcite surface to generate CaF2. Atomic force microscopy measurement confirms that a CaF2 films is formed to change the surface morphology of calcite and increase the surface roughness. Furthermore, the slow release of fluorite particles can precisely control the adsorption and the surface transformation of F- on calcite, instead on magnesite. Therefore, the slow release and surface transformation of fluorite particles plays an important role in the flotation separation of carbonate minerals.

Radiation-Induced Nucleation and Growth of CaSi2 Crystals, Both Directly during the Epitaxial CaF2 Growth and after the CaF2 Film Formation.

The radiation-induced phenomena of CaSi2 crystal growth were investigated, both directly during the epitaxial CaF2 growth on Si (111) and film irradiation with fast electrons on Si (111) after its formation, while maintaining the specified film thickness, substrate temperature and radiation dose. Irradiation in the process of the epitaxial CaF2 film growth leads to the formation of CaSi2 nanowhiskers with an average size of 5 µm oriented along the direction <110>. The electron irradiation of the formed film, under similar conditions, leads to the homogeneous nucleation of CaSi2 crystals and their proliferation as inclusions in the CaF2 film. It is shown that both approaches lead to the formation of CaSi2 crystals of the 3R polymorph in the irradiated region of a 10 nm thick CaF2 layer.

CaF2: A novel electrolyte for all solid-state electrochromic devices

The energy consumption in building ventilation, air, and heating conditioning systems, accounts for about 25% of the overall energy consumption in modern society. Therefore, cutting carbon emissions and reducing energy consumption is a growing priority in building construction. Electrochromic devices (ECDs) are considered to be a highly promising energy-saving technology, due to their simple structure, active control, and low energy input characteristics. At present, H+, OH- and Li+ are the main electrolyte ions used for ECDs. However, H+ and OH- based electrolytes have a high erosive effect on the material surface and have a relatively short lifetime. Li+-based electrolytes are limited due to their high cost and safety concerns. In this study, inspired by prior research on Ca2+ batteries and supercapacitors, CaF2 films were prepared by electron beam evaporation as a Ca2+-based electrolyte layer to construct ECDs. The structure, morphology, and optical properties of CaF2 films were characterized. ECDs with the structure of ITO (indium tin oxide) glass/WO3/CaF2/NiO/ITO show short switching times (22.8 s for the coloring process, 2.8 s for the bleaching process). Additionally, optical modulation of the ECDs is about 38.8% at 750 nm. These findings indicate that Ca2+ based ECDs have the potential to become a competitive and attractive choice for large-scale commercial smart windows.

Electron passivation in CaF2 on calcium metal anodes.

Current electrolytes in calcium-ion batteries suffer from a lack of stability and degradation caused by reduction from the anode. The solid-electrolyte interphase (SEI) that forms on the anodes during operation stems the flow of electrons from the anode to the electrolyte. CaF2 is a common inorganic compound found in the SEI, and is derived from electrolyte salts such as Ca(PF6)2. CaF2 can exist in crystalline, polycrystalline, and amorphous phases in the SEI, and as recent work has shown, different phases of the same compound can have vastly different electronic conductivities. Using the non-equilibrium Green's function technique with density functional theory (NEGF-DFT), we find that amorphous phase systems enhance electron tunneling in thin CaF2 films by 1-2 orders of magnitude when compared to crystalline and polycrystalline CaF2 systems. Transport through several amorphous structures was considered showing that, despite their random structures, their conductance properties are similar. Through analysis of the decay constant β and the low-bias conductance of each system, we show that crystalline and polycrystalline CaF2 offer greater protection of the electrolyte than amorphous CaF2.

Пленки фторида кальция толщиной 2-10 нм на кремнии-(111): выращивание, диагностика, изучение сквозного токопереноса

Epitaxial calcium fluoride (CaF2) layers with a nominal thickness up to 10 nm on the (111)-oriented Silicon (Si) are obtained. Surface topographies of the fluoride films are recorded and the current-voltage characteristics of the Au/CaF2/Si structures are studied. On a qualitative level, these structures exhibited all the features usual for metal-insulator-semiconductor systems. The current-voltage curves of the samples were reproduced by modeling considering a finite (0.1–1 nm) value of the standard thickness deviation of the dielectric CaF2 film.

Flotation separation of apatite from calcite based on the surface transformation by fluorite particles

The flotation separation of apatite from calcite is gordian due to the similar surface properties and calcium active sites, which make a poor selectivity in traditional fatty acid system. Pb-BHA coordination collector present great selectivity, and surface transformation by fluoride ions plays an important role in the flotation separation of salt minerals. The application of surface transformation combining with lead nitrate-benzohydroxamic acid (Pb-BHA) coordination collector can achieve the flotation separation of different calcium minerals. This study suggest that the floatability of apatite and calcite is similar without any depressants, and fluoride ions has a strong inhibiting effect on calcite but not on apatite under the Pb-BHA collector system. Zeta potential measurement determine that the surface charge of calcite obviously shift to a negative value with the adsorption of fluoride ions. X-ray photoelectron spectroscopy measurement indicates that fluoride ions is easier to adsorb on surface of calcite than that of apatite, and generate CaF2 films. Atomic force microscopy measurement illustrates the surface morphology of calcite becomes more uneven and the roughness increases obviously after the treatment of F−. Therefore, the surface transformation is meaningful to study and provides a new method for the flotation separation of calcium minerals.

Calcium fluoride films with 2-10 nm thickness on Silicon-(111): growth, diagnostics, study of the through current transport

Epitaxial calcium fluoride (CaF2) layers with a nominal thickness up to 10 nm on the (111)-oriented Silicon (Si) are obtained. Surface topographies of the fluoride films are recorded and the current-voltage characteristics of the Au/CaF2/Si structures are studied. On a qualitative level, these structures exhibited all the features usual for metal-insulator-semiconductor systems. The current-voltage curves of the samples were reproduced by modeling considering a finite (0.1-1 nm) value of the standard thickness deviation of the dielectric CaF2 film. Keywords: calcium fluoride, thin films, MIS structure, leakage current.

Ion transport in nanocrystalline CaF2 films

This work investigates the conductivity and structure of nanocrystalline CaF2 films grown at 200 °C by thermal evaporation. The in-plane conductivity is enhanced by several orders of magnitude compared to lightly doped bulk samples of CaF2, which independently confirms the finding of a previous work [Modine et al., J. Appl. Phys. 74, 2658 (1993)]. Upon heating above 200 °C, the enhancement is partially annealed out, and the activation energy increases continuously from 0.7 to 1.0 eV, which contradicts the annealing model proposed previously. The enhancement is seen only in an ∼20-nm thick region adjacent to the substrate, but this may be because the films show substantial porosity outside this region. A 5–10 nm grain size and a high density of grain boundaries are observed by high-resolution transmission electron microscopy. A 2–4 nm interfacial amorphous layer is seen in films grown on Al2O3, but such a layer is absent on MgO and evidently not responsible for the enhanced conduction. Overall, the evidence points to grain boundaries and/or dislocations as providing fast transport pathways. These results help to reconcile previous reports of enhanced ion transport in CaF2, and they are also relevant for understanding the enhancement mechanism in CaF2-based composites and antifluorite-structured materials such as Li2O.

Spectroscopy of Tm3+-doped CaF2 waveguiding thin films grown by Liquid Phase Epitaxy

Thulium-doped calcium fluoride (Tm:CaF2) single-crystalline films are grown on (100)-oriented undoped bulk CaF2 substrates by Liquid Phase Epitaxy (LPE) using LiF as a solvent. Their spectroscopic properties are studied in details. For ~2 at.% Tm-doped films, the active ions are predominantly isolated and are located in oxygen-assisted trigonal sites, C3v(T2). With increasing the Tm doping level, ion clustering is promoted but it is slowed down as compared to bulk crystals. For ~6 at.% Tm-doped films, the majority of active ions form clusters resulting in a “glassy-like” spectroscopic behavior (smooth and broad spectral bands). For such layers, the maximum stimulated-emission cross-section for the 3F4 → 3H6 transition is 0.14 × 10−20 cm2 at 1856 nm, the luminescence lifetime of the 3F4 state is 21.70 ms and the emission bandwidth exceeds 180 nm. The transition probabilities of Tm3+ ions are determined using the Judd-Ofelt theory. The waveguiding properties of the Tm:CaF2 films are confirmed. Highly-doped Tm:CaF2 epitaxial films are promising for waveguide lasers at ~2 μm.

Broadband antireflection and hydrophobic CaF2 film prepared with magnetron sputtering

The main aim of the work was to develop an effective antireflective and hydrophobic film, which could have potential application in outdoor environment. Here, a facile magnetron sputtering method was proposed to prepare micro-nano structured CaF2 film on the glass substrate, of which morphology and properties could be controlled with the flow ratio of SF6/Ar during the process of sputtering. The surface of the film was transformed into porous morphology composed of nano-sized particles when the flow ratio of SF6/Ar reached 10%, and the roughness got the maximum value of 144 nm. Moreover, the 10% CaF2 film showed a low refractive index of 1.33–1.31, and the average transmittance in the wavelength range of 300–1100 nm of coated glass was 95.2%. Apart from the broadband antireflective property, 10% CaF2 film also exhibited a lower incidence angle dependence, and the average reflectance remained below 6% as the θ varied from 45° to 90°. Besides, the water contact angle of the film reached 139.4°, demonstrating its good self-cleaning capability. Furthermore, when 10% CaF2 film was used as the cover glass, the short-current density of the solar cell was 5.77, 11.21, and 15.50 mA/cm2, respectively, corresponding to 30°, 60°, and 90° incident angle. Meanwhile, the solar cell’s photoelectric conversion efficiency was as high as 9.13%, 19.35%, and 28.45%, respectively.

Crystal structure of thin CaSi2 films grown by radiation-induced epitaxy

The electron-beam irradiation (electron energy 20 keV, current density 50 μA/m2) effects on the CaF2 films epitaxially grown on a Si(111) substrate have been studied with Raman spectroscopy (RS). It was found that the radiation-induced phase transition of CaF2 into CaSi2 films takes place at electron-beam irradiation both as during the epitaxial CaF2 film growth and as after completed growth of the film. The temperature dependence studies of the observed radiation-induced phase transition show needs to use substrate heating above 300 °C. The crystal structure of CaSi2 films was found to depend on the thickness of the grown CaF2 film. For thin films (less than 20 nm) crystal structure of CaSi2 films belongs to the trigonal rhombohedral modification 3R (space group R3̅m with a three-layer translational period of silicon substructures in the unit cell 3R). The transition of CaSi2 crystal structure to the trigonal rhombohedral modification a six-layer period material 6R was found for thicker films (>20 nm).

Optical Properties of CaF2 Thin Film Deposited on Borosilicate Glass and Its Electrical Performance in PV Module Applications

Calcium fluoride (CaF2) is deposited via vacuum thermal evaporation on borosilicate glass to produce an anti-reflection coating for use in solar modules. Macleod’s essential simulation is used to optimize the thickness of the CaF2 coating on the glass. Experimentally, a 120 ± 4 nm-thin CaF2 film on glass shows an average increase of ~4% in transmittance and a decrease of ~3.2% in reflectance, respectively, when compared to that of uncoated glass (Un CG), within the wavelength spectrum of approximately 350 to 1100 nm. The electrical PV performance of CaF2-coated glass (CaF2-CG) was analyzed for conventional and lightweight photovoltaic module applications. An improvement in the short-circuit current (Jsc) from 38.13 to 39.07 mA/cm2 and an increase of 2.40% in the efficiency (η) was obtained when CaF2-CG glass was used instead of Un CG in a conventional module. Furthermore, Jsc enhancement from 35.63 to 36.44 mA/cm2 and η improvement of 2.32% was observed when a very thin CaF2-CG was placed between the polymethyl methacrylate (PMMA) and solar cell in a lightweight module.

Growth and characterization of hydrophobic anti-reflection CaF2 films

CaF2 films of thicknesses in the range of 50–200 nm were deposited by thermal evaporation on glass substrates. A series of CaF2 films were also grown by oblique angle deposition in which the orientation of the substrate with the horizontal direction was varied from 0° to 75°. CaF2 films are crystalline and show X-ray reflections corresponding to (111), (220), and (311) crystal planes of the cubic phase. The surface morphology of the films was analyzed by atomic force microscopy and it is found that surface roughness increases with an increase in the deposition angle. The water contact angle studies on CaF2 films show that the wettability properties of the films depend on the film thickness and hydrophobicity enhances with the angled deposition. Optical studies found that CaF2 films have high transmittance (> 85%) and good anti-reflection properties in the wavelength range of 400–1000 nm. The study shows that the anti-reflection and hydrophobic properties of CaF2 films can be modified by the oblique angle thermal evaporation.

Radiation-Induced epitaxial CaSi2 film growth at the molecular-beam epitaxy of CaF2 on Si

We report on the studies of the 20 keV electron-beam irradiation (current density 50 μA/m2) effects on the epitaxial CaF2 film growth on a Si surface. It was found that, during the CaF2 growth on Si, the area exposed to the electron beam suffers strong modifications, such as in the surface morphology and film chemical composition. With reflection high-energy electron diffraction, atomic force microscopy and Raman spectroscopy, it is shown that the electron beam action leads to the CaSi2 layer synthesis at the interface of the silicon substrate and epitaxially growing CaF2 film.

Metallic band structure of CaF2 thin films grown on silicon(1 1 1): Possible formation of CaSi2

We grew epitaxial 10 monolayer (ML) CaF2 films on a Si(111) substrate and measured its electronic structure with angle-resolved photoemission spectroscopy. Reflection high-energy electron diffraction measurements showed the 1×1 streaks of the grown film with no sign of the formation of the Ca-Si surface superstructure at the growth temperature of 400–500 °C. The band dispersion of the films was metallic in contrast to the highly insulating nature of bulk CaF2. The metallic band dispersion did not change with additional Si deposition (1–10 ML) on the CaF2 film. We suggest that CaSi2 may be formed near the surface layers and this shows that the CaF2/Si is not an ideal insulator/semiconductor heterostructure for application in atomically thin electronic devices.

Effect of calcium incorporation on properties of Cu(InGa)Se2 thin film and solar cell

Cu(InGa)Se2 (CIGS) thin films with Ca doping from CaF2 film synthesized onto quartz glass substrates via a two-step process were studied. XRD peak splitting ascribed to graded compositional distribution existed in pristine CIGS film but disappeared in those with Ca doping. Depth profiles verified that the Ca element mainly existed in the near surface region of CIGS film with CaF2 post-deposition treatment (PDT) while diffused throughout the absorber with CaF2 pre-deposition. The grain sizes in surface of CIGS films without doping and with CaF2 PDT were much larger than those with CaF2 pre-deposition. The Fermi level of film with pre-deposited CaF2 showed no change while that of film with CaF2 PDT exhibited lower energy compared to the pristine film. Performance degradation due to inferior Mo/CIGS interface was observed in device with CaF2 pre-deposition. Inversely, PDT of CaF2 enhanced the device performance with an absolute efficiency of 2.2 %. SCAPS-1D was used to simulate the CIGS devices for further study. It showed a shallower defect position in solar cell with CaF2 PDT, giving explanation for its better performance.

Liquid Phase Epitaxy growth of Tm3+-doped CaF2 thin-films based on LiF solvent

Single-crystalline Tm3+-doped calcium fluoride (Tm3+:CaF2) films were grown onto (100) oriented undoped bulk CaF2 substrates by Liquid Phase Epitaxy method using LiF as a solvent. The growth was performed at the temperatures of 853–865 °C. Crack-free transparent films with a high doping concentration of ∼6 at.% and a thickness up to ∼100 μm were obtained. The film structure and morphology were characterized by confocal microscopy, X-ray diffraction and scanning electron microscopy. High resolution X-ray diffraction and reciprocal space mapping confirmed the high crystallinity and the low mosaicity of the films. Energy-Dispersive X-ray spectroscopy revealed a uniform distribution of Tm3+ ions in the films. The Tm3+:CaF2 films exhibit a broadband emission at ∼2 μm and are of interest for waveguide lasers. The use of CaF2–LiF binary system is also promising for fabrication of other rare-earth-doped CaF2 layers.