BaF2 Substrates Research Articles

Alkaline Earth Bismuth Fluorides as Fluoride-Ion Battery Electrolytes.

Fluoride-ion batteries have several potential advantages over lithium-ion batteries. Materials development is still needed, however, to realize electrolytes with sufficiently high anion conductivity and compatibility with anode and cathode layers. Fluoride compounds are difficult to synthesize directly as single crystals but can be realized from oxide film precursors via topotactic chemistry techniques. Here, we create crystalline alkaline earth bismuth fluoride films BaBiF5 and SrBiF5 through oxide molecular beam epitaxy and topotactic fluorination. We characterize their ionic conductivities and demonstrate their potential as electrolytes. Finally, we realize epitaxial synthesis of BaBiF5 on BaF2 substrates, providing a route to thin film fluoride-ion battery devices.

Design and fabrication of a liquid crystal retarder for the mid- and far-infrared wavelength ranges

We designed and fabricated a liquid crystal (LC) retarder that operates within mid-infrared (MIR) and far-infrared (FIR) wavelength regions. Fluorine-based mixed LCs and BaF2 substrates were employed to assemble the LC retarder on the basis of their absorption spectra. A thin NiCr film, which served as a semitransparent electrode in the infrared region, was also formed on the BaF2 substrate. The fabricated LC retarder showed transmittances of 40% and 42% at wavelengths of 3.88µm and 9.5µm, respectively. Here, we demonstrated phase shift that can be electrically controlled to exceed π/2. Our LC retarder can be applied to the polarization modulation in the MIR and FIR wavelength regions.

Growth and characterization of epitaxial films based on semimagnetic Pb1−xEuxTe solid solutions on various substrates

This paper presents the results of a study of the growth and structure properties of Pb[Formula: see text]EuxTe, ([Formula: see text]) epitaxial films with 0.5–1[Formula: see text][Formula: see text]m thickness, grown on BaF2 and SiO2 substrates oriented in the (111) direction, by the molecular beam condensation method in a vacuum of 10[Formula: see text] Pa with the use of an additional compensating source of Te vapor during the growth process. The optimal conditions for obtaining structurally perfect ([Formula: see text]–100[Formula: see text]) epitaxial films were established: [Formula: see text]–623[Formula: see text]K; [Formula: see text]K; [Formula: see text]–9 Å/s. It was determined that conductivity inversion occurs at the temperature of the compensating source 410[Formula: see text]K.

Thermodynamic constraints and substrate influences on the growth of high-quality Bi2Te3 thin films by pulsed laser deposition

We investigated the growth of Bi2Te3 thin films on (111) CaF2 and (111) BaF2 substrates by pulsed laser deposition. Stoichiometric Bi2Te3 thin films were grown using targets with excess tellurium. We adopted a combinatorial temperature variation principle, where the growth temperature is monotonically varied during the growth. This growth method is supportive to effectively minimize Te loss in the Bi2Te3 films. We found large differences in growth conditions between Bi2Te3 thin films on the CaF2 and BaF2 substrates. The lattice matched (111) BaF2 substrate is preferred to grow Bi2Te3 for the further development of topological electronics.

Weak antilocalization effect and multi-channel transport in SnTe quantum well

Magnetoresistance measurements were performed on a 30 nm-thick SnTe quantum well (QW) grown by molecular beam epitaxy on the BaF2 substrate in the temperature range of 1.9–50 K. The weak antilocalization (WAL) effect was observed at low temperatures and low magnetic fields as a result of the strong spin–orbit coupling present in the QW. Using the Hikami–Larkin–Nagaoka equation, we analyzed the experimental data and found that the WAL effect is not purely 2D but composed of 2D and 3D channels that exist within the QW structure. The spin–orbit and phase coherence mechanisms are also extracted, and a general view of the transport properties of the QW is also provided.

Influence of ion-plasma treatment on the surface morphology of epitaxial lead-tin telluride films

The paper discusses the physical aspects of surface modification of Pb1-xSnxTe (x = 0.00-0.80) epitaxial films during ion-plasma treatment in argon plasma. Lead–tin telluride films 1−2 μm thick were grown on (111) BaF2 substrates by molecular beam epitaxy. The ion-plasma treatment of the samples was carried out in a high-density low-pressure radio frequency (RF) inductively coupled plasma at an ion energy of 75 eV. The duration of the process is 240 s. The evolution of the surface morphology of the films and the formation of micro- and nanostructures at different ratios of lead and tin are studied.

Features of Raman scattering in lead sulfide and lead sulfide-selenide epitaxial films

Raman scattering spectra of 1-2 μm thick n-PbS(111) epitaxial films grown by molecular beam epitaxy on BaF2(111) substrates were obtained and analyzed. The spectra were recorded at a low excitation level of 0.36 mW/μm2, which did not cause photo- and thermal degradation of the films. It is shown that, in accordance with the symmetry selection rules, the bands in the spectra correspond to overtone or combination tones of phonon modes of PbS at special points of the Brillouin zone. The analysis of the bands of oxides and oxysulfates of lead, which can mask the bands of lead sulfide, was carried out. The obtained data were used in the analysis of the recorded Raman scattering spectra by epitaxial films of a ternary solid solution PbS0.5Se0.5. Keywords: Raman scattering, low excitation level, photooxidation, epitaxial films, lead sulfide, lead sulfide-selenide.

Особенности комбинационного рассеяния света эпитаксиальными пленками сульфида и сульфида-селенида свинца

Raman scattering spectra of 1-2 µm thick n-PbS(111) epitaxial films grown by molecular beam epitaxy on BaF2(111) substrates were obtained and analyzed. The spectra were recorded at a low excitation level of 0.36 mW/µm2, which did not cause photo- and thermal degradation of the films. It is shown that, in accordance with the symmetry selection rules, the bands in the spectra correspond to overtone or combination tones of phonon modes of PbS at special points of the Brillouin zone. The analysis of the bands of oxides and oxysulfates of lead, which can mask the bands of lead sulfide, was carried out. The obtained data were used in the analysis of the recorded Raman scattering spectra by epitaxial films of a ternary solid solution PbS0.5Se0.5.

ДОСЛІДЖЕННЯ КУТОВИХ ТА ПОЛЯРИЗАЦІЙНИХ ХАРАКТЕРИСТИК ТЕПЛОВОГО ВИПРОМІНЮВАННЯ ОДНОМІРНИХ ФОТОННИХ СТРУКТУР НА СКІНЧЕННІЙ ПІДКЛАДИНЦІ

One-dimensional photonic structures (PS) are the layered metamaterials, which optical properties are due to interference effects within the structure. Due to the interaction of electromagnetic waves with the periodic structure, the thermal radiation (TR) of such PS has the features of coherent: it has a selective spectrum and a petal pattern. The relevance of TR PS research is due to the search for new materials to create sources of infrared (IR) radiation in the middle and far IR range (with a wavelength > 5 μm). The second important area of application of the radiating properties of the PS is the creation of non-radiating coatings to remove unwanted TR from the heated surfaces of the IR devices. Despite the fact that the irradiative properties of PS have been studied quite well, in the literature insufficient attention is paid to the influence of the finite substrate on the parameters of TR structures. The influence of a finite incoherent substrate on the angular and polarization characteristics of the thermal radiation of the system (photonic structure)/substrate has been investigated. Systems consisting of plane-parallel Ge or Si plates as PS and incoherent BaF2, BaF2/Al and Al substrates were used in experimental studies. It has been established, the contrast and amplitude of the directional diagram petals of the TR system depend on the ratio of the optical characteristics of the photonic structure and the substrate. The presence of a strongly reflective substrate eliminates the difference between the angular dependences of the s- and p-polarized components of the TR. In the case when the PS is placed on a transparent incoherent substrate with a lower refractive index, there is an increase in the p-polarized component of the TR and the suppression of the s-polarized. The thermal imaging picture of the TR angular distribution of the Si/Al system has been recorded and it was experimentally proved that the TR of photonic systems exhibits circular patterns. The results of the work can be used in the development of narrowband emitters in the middle and far infrared range, coatings to increase radiation energy exchange and coatings with minimal emissivity that are invisible in the thermal range.

Зависимость топологии эпитаксиальных слоев PbSnTe:In от концентрации In

The topology of the surface of epitaxial films of lead tin telluride solid solution, including those with the addition of indium (Pb1-xSnxTe:In), grown on single-crystal BaF2 (111) substrates and a CaF2/BaF2 buffer layer on Si (111) was studied by atomic force microscopy. It is shown that the characteristic statistical indicators of the relief are due to the peculiarities of film growth and the mechanism of incorporation of Indians, the excess content of which was registered on the surface ex situ by X-ray photoelectron spectroscopy.

Strain Engineering at Heterointerfaces: Application to an Iron Pnictide Superconductor, Cobalt-Doped BaFe2As2.

We propose a unique strategy to apply stronger strain at heterointerfaces than conventional epitaxial strain methods to extract hidden attractive physical/chemical properties in materials. This strategy involves precisely accounting for the epitaxial strain induced by lattice mismatch as well as the differences in the thermal expansion coefficients and compressibilities of epitaxial films and substrates. We selected optimally cobalt-doped BaFe2As2(Ba122:Co), an iron-based superconductor with a bulk critical temperature (Tc) of 22 K, as a model material and four types of single-crystal substrates. Ba122:Co was selected because its Tc is robust to hydrostatic pressure but sensitive to epitaxial strain (i.e., one of the anisotropic strains), and the selected substrates entirely cover the positive/negative lattice mismatches, thermal expansion coefficients, and compressibilities with respect to Ba122:Co. With strong anisotropic strain successfully induced by film growth, external hydrostatic pressurizing, and cooling processes, we observed unique carrier transport properties in Ba122:Co epitaxial films on CaF2 and BaF2 substrates including (i) upturn behavior in the temperature dependence of the longitudinal resistivity, (ii) negative magnetoresistance, (iii) large enhancement of anomalous Hall effects in the epitaxial films on CaF2, and (iv) enhancement of Tc to 27 K in the epitaxial films on BaF2. These results demonstrate the effectiveness of our strategy, and this approach can be further extended to other inorganic materials in thin-film form.

Molecular beam epitaxy of antiferromagnetic (MnBi2Te4)(Bi2Te3) thin films on BaF2 (111)

The layered van der Waals compounds (MnBi2Te4)(Bi2Te3) were recently established as the first intrinsic magnetic topological insulators. We report a study on the epitaxial growth of (MnBi2Te4)m(Bi2Te3)n films based on the co-deposition of MnTe and Bi2Te3 on BaF2 (111) substrates. X-ray diffraction and scanning transmission electron microscopy evidence the formation of multilayers of stacked MnBi2Te4 septuple layers and Bi2Te3 quintuple layers with a predominance of MnBi2Te4. The elemental composition and morphology of the films is further characterized by x-ray photoemission spectroscopy and atomic force microscopy. X-ray magnetic circular and linear dichroism spectra are comparable to those obtained for MnBi2Te4 single crystals and confirm antiferromagnetic order in the films.

Study of CaF<sub>2</sub>- Doped PbTe Thin Films Grown by Molecular Beam Epitaxy

We present here a study on the electrical and structural properties of p-type PbTe films doped with CaF2. The layers were grown by molecular beam epitaxy on freshly cleaved (111) BaF2 substrates. The doping level was monitored by the CaF2 solid source cell temperature (TCaF2), which varied from 500 to 1150 °C. The films with low doping level, TCaF2 ≤ 1010 °C, exhibited flat surfaces with crystalline quality close to the undoped PbTe sample. In contrast, samples with high levels of doping (TCaF2 > 1010 °C) presented CaF2 agglomerates on the surface and a worse crystal quality. The hole density at 77 K versus TCaF2 oscillated between 1.3 × 1017 and 3.6 × 1017 cm-3 and did not exhibit a systematic behavior as the fluoride supply is raised. The results indicate that CaF2 is not an effective p-type dopant for PbTe, due to the abscence of a resonant level close to the valence band or to compensation of extrinsic dopant levels.

RETRACTED: Restorations of fresh surfaces for topological materials by de-capping Te

Abstract The topological categorization originates from a depiction of the sequencing of bands around the band gap of the material in comparison to the normal vacuum type band ordering. The structure can either be normal or inverted, and thus topologically trivial or non-trivial. An odd number of Dirac fermions on the surface occurs in this type of material. Time-reversal symmetry is responsible to preserve the surface bands. On the other hand, topological crystalline insulator is protected by mirror symmetry; this system has an even number of Dirac surface state per BZ. Capping layer of Te or Se is necessary due to the high sensitivity of films surfaces to atmospheric pressure. The capping layer works as a protection from oxidation. In Bi2Te3 samples, the bonds between Bi and Te are covalent bonds while the bonds between Te-Te at a certain position are van der Waals bonds. Thereby, the bonds between amorphous Te cap and sample surface is weaker than van der Waals bonds within the sample structure. However, the top boost cleaving method was successful. But, for Pb1-xSnxTe samples the top boost cleaving method unsuccessfully results. The bonds between the metallic Pb1-xSnxTe sample surface and amorphous Te cap are stronger than van der Waals bonds in BaF2 substrate, i.e. F-F van der Waals bonds. In this paper, we present different techniques to recover fresh surfaces, from classical methods (such as sputtering and annealing) working for all types of films and top boost methods, which could be less used due to the strong bonds in some films. Moreover, we were able to recover the immaculate surfaces for non-capped samples. We present here in a systematic study by low energy electron diffraction (LEED), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES).

Novel Perspective Coatings for the Optoelectronic Elements: Features of the Carbon Nanotubes to Modify the Surface Relief of BaF2 Materials

It is well known that the optimization of the basic properties of materials is related not only to changes of the substance of the material itself, but can also predict the change of their surface. In this regards, the search for, and study of, new nanostructured coatings based on the laser deposition method becomes extremely promising. Here, we used a laser-oriented deposition technique in order to place carbon nanotubes in a vertical position on the BaF2 surface to modify it. Such modification affected the increasing material transparency, connected with a decrease of the reflection via change the Fresnel losses; hydrophobicity and microhardness as well. Characteristics of the obtained material were studied via spectral analysis, AFM-method, wetting-angle measurements, microhardness estimations to support the possible covalent bonding between the carbon atoms and the interface materials atoms. Moreover, the quantum–chemical calculations completely confirmed the experimental results of the changes of electronic properties of BaF2 substrate after deposition of the CNTs. As the results novel optimized structure based on BaF2 is presented to be used in general optoelectronics, cosmos and laser technique as well.

The epitaxial growth behavior of thin PbMnTe film on BaF2(1 1 1): Influence of Mn doping density and substrate temperatures

PbMnTe film attracted intense scientific attention because of its promising properties such as enhanced magneto-optical effects, huge negative magneto-resistance effect, and Curie-Weiss paramagnetism. The electro-magnetic properties of PbMnTe film were intensively studied in the past decades. In contrast, the fabrication of PbMnTe film was still less investigated and its growth behavior was needed to be better understood. In this work, we study the growth behaviors of Pb1−xMnxTe as a function of Mn content (x) and temperatures of BaF2(111) substrate. The structure and growth behavior of Pb1−xMnxTe film changed substantially when the Mn content is larger than the criteria of x = 0.026. Mn will replace the position of Pb2+ for cases of x <0.026, leading to the formation of (111)-orientated NaCl-type Pb1−xMnxTe spirals. In contrast, MnTe phase segregated from the Pb1−xMnxTe phase and aggregates at positions of the threading dislocations for cases of x >0.026, resulting in the formation of pit-like domains on the spiral islands of PbMnTe alloy film. The pit-like MnTe grains aggregated nearby the threading dislocations due to the existence of the stress field at these defect regions. The criteria of Mn to replace Pb increases when deposited PbMnTe at higher substrate temperature. This indicates that the quality of Pb1−xMnxTe film would be improved if the substrate was kept at high temperature while in deposition.

Incorporation of Europium in Bi2Te3 Topological Insulator Epitaxial Films

In the field of topological materials, the interaction between band topology and magnetism remains a current frontier for the advancement of new topological states and spintronic functionalities. Doping with rare-earth elements with large magnetic moments is a current approach to exploit the phenomenology of such interaction. However, dopant solubility into the main matrix plays a major role. In this sense, the present work is focused on elucidating how Eu incorporates into Bi2Te3 lattice as a function of doping. This work reports a systematic investigation of the structural and electronic properties of bismuth telluride epitaxial layers doped with Eu. Bi2Te3 films were grown by molecular beam epitaxy on (111) BaF2 substrates with nominal Eu doping ranging from 0% up to 9%. X-ray diffraction analysis and scanning transmission electron microscopy reveal that Eu atoms enter substitutionally on Bi sites up to 4% of Eu doping. In contrast, the 9% Eu-doped sample contains epitaxially oriented nanoclusters of EuTe. X-ray photoelectron and absorption spectroscopies show that Eu atoms enter the Bi2Te3 crystal matrix in the divalent Eu2+ state for all Eu concentrations. Angle-resolved photoemission experiments indicate that the topological surface state is preserved in the presence of the local magnetic moments introduced by the Eu impurities.

Ternary Lead Chalcogenide Alloys for Mid-Infrared Detectors

We demonstrate thin films of PbSe1−xSx and PbSe1−xTex lead chalcogenide ternary alloys as infrared detectors. The films were deposited on single crystal BaF2 substrates using physical vapor deposition. Detectivity in the wavelength range from 1 μm to 5 μm was measured at −40°C, and all films showed photoresponse signals more than an order of magnitude larger than their noise. The detectivity spectra were used to assess the tunability of the band gap from mixing the lead chalcogenide binaries. The PbSe1−xSx system showed tunability that followed Vegard’s law, while the PbSe1−xTex system showed tunability with a bowing parameter of −0.096 eV. Comparisons to measurements from the literature taken at room temperature suggest that the bowing parameter decreases with decreasing temperature, and the band gap temperature coefficient with respect to composition also shows bowing.

Quasichemical modeling of defect subsystem of tin telluride thin films

By the method of modeling via quasichemical reactions the formation of native point defects in tin telluride films deposited on monocrystalline BaF2 substrates by thermal evaporation using physical vapor deposition technique has been described. Based on the analysis of electroneutrality condition, concentration dependences of charge carriers and point defects on substrate temperature, evaporation temperature and partial vapor pressure of tellurium have been found. The constants of corresponding reactions have been specified.

Modification of the Surface Properties of PbSnTe〈In〉 Epitaxial Layers with Composition near Band Inversion

High-resistance Pb1 –xSnxTe〈In〉 layers grown by molecular beam epitaxy on BaF2(111) substrates with compositions close to band inversion have been investigated. The I–V characteristics and relaxation dependences of the photocurrent of the structures in dependence on the chemical surface treatment and subsequent exposure of the samples in air have been examined. It has been observed that the characteristics significantly transform depending on the physicochemical surface state. It has been found that the chemical surface treatment in the hydrochloric acid solution in isopropyl alcohol leads to an increase in the current by up to four orders of magnitude with the subsequent recovery of the I–V characteristics upon exposure of the samples in air for several days.