BaF2 Films Research Articles

Фоточувствительность полевого транзистора металл-диэлектрик-полупроводник на основе пленки PbSnTe:In с составом вблизи инверсии зон

A prototype of a metal–insulator–semiconductor field-effect transistor based on PbSnTe:In/(111)BaF2 film with an Al2O3 gate dielectric was designed for the first time. With the gate voltage applied in the range – 7.7 V < Ugate < +7.7 V the relative modulation in the drain-source current Ids/Ids attained near five-fold change at T = 4.2 K. When illuminated with rela-tively low (~100 photon/s) fluxes, negative photoconductivity was detected accompanied with a decrease in Ids by ~104 times and a simultaneous decrease in Ids by ~103 times or even more. The estimated detectivity was about 71016 cmHz0.5W-1 at a wavelength  about 25 micron with the accumulation time about 0.5 s. A qualitative model is discussed which assumes the ex-istence of deep traps and a photo-capacitance effect.

Pulsed laser deposition and structural evolution of BaF2 nanolayers in Eu-doped BaF2/Al2O3 layered optical nanocomposite thin films

We have developed a pulsed laser deposition (PLD) geometry for the growth of uniform BaF2 nanoscale thin films, through control of the deposition conditions. Our goal is to use the BaF2 layers with controllable structure as component layers in layered optical nanocomposites. The structure of the BaF2 nanolayer evolves as a function of the layer thickness: BaF2 grows via a layer-by-layer growth mode on Al2O3; the layers are amorphous for a thickness < 3 nm, and then become nanocrystalline as the layer thickness increases. The BaF2 nanocrystals have an FCC crystal structure with a weak <111> texture that becomes stronger for thicker films. We then demonstrate that our BaF2 films can be introduced into layered Al2O3/BaF2/EuOx nanocomposite films, which allows for control of the relative position of the Eu ions and the BaF2 layer. Cross-section samples of the multilayered films show interfacial intermixing between the layers, which is related to implantation during the PLD process. This intermixing enables the incorporation of Eu ions into BaF2 layers and form a thin Eu-doped BaF2 nanolayer at the interface. The layered nanocomposite films show photoluminescence (PL) emission from Eu3+ ions, and the PL intensity changes can be correlated with the crystallinity and crystal size changes in the BaF2 layer. Our results provide guidance for achieving thin film nanocomposite materials with controllable structure and photoluminescence behavior for light emitting diodes, photovoltaics and other optical applications.

Diamond inverter consisted of high mobility and low on-resistance enhancement-mode C[sbnd]H diamond MISFET

Hydrogenated diamond (CH diamond) inverters have been fabricated by normally-off Al/BaF2/C-H diamond metal-insulator-semiconductor field effect transistors (MISFETs) and load resistors. Electron beam evaporation was used to deposit BaF2 film on the diamond surface at room temperature to reduce thermal degradation effect on the CH bond and adsorbates during the deposition. The resulting 4-μm normally-off MISFET achieves the maximum saturation drain current, on-resistance, maximum transconductance and switching ratio of −113.4 mA/mm, 37.1 Ω·mm, 35.4 mS/mm and 108, respectively. Effective hole mobility of more than 200 cm2/V·s is achieved in a large gate voltage range (−0.35 V ≤ VGS -VTH ≤ −4.35 V). Correspondingly, based on the high-performance diamond MISFET, the diamond inverters exhibit good voltage transfer characteristics and dynamic switching characteristics at the frequency of 50 kHz.

Fluorite particles as a novel barite depressant in terms of surface transformation

The flotation separation of scheelite and barite is difficult because they have the same active sites. The surface transformation of semi-soluble minerals is inevitable in the flotation of salt minerals, but an effective flotation separation can be achieved through the dissolution and surface transformation of fluorite. In the current flotation test, fluorite particles and sodium fluoride are efficient depressants for barite but not for scheelite. These results suggest that fluorite ions released from fluorite are crucial to the separation of barite and scheelite. Zeta potential measurement indicates that fluorite ions can obviously shift the surface charge of barite, but it cannot influence that of scheelite. X-ray photoelectron spectroscopy confirms that fluoride ions are strongly chemisorbed on the barite surface and generate BaF2. Atomic force microscopy measurement directly illustrates the change in the barite surface morphology and the formed BaF2 film. Thus, surface transformation is clearly important for flotation and provides a new strategy for the separation of salt minerals.

The optical constants of the films MgBaF4 in the range of 1.3–16 micron

A BaF2 films, characterized by low refraction coefficient in the infrared region of the spectrum, are widely used in the production technology of optical structures. Their serious drawback is the low mechanical strength and moisture resistance. This paper presents the results of an experimental study of the optical properties of a new type of MgBaF4 films made from a vacuum-sintered mixture of MgF2 and BaF2 to a homogeneous composition.

Giant magnetoresistance of PbSnTe:In films in the space-charge-limited current regime: Angular features and effect of the surface

In the space-charge-limited current regime at T = 4.2 K, the magnetoresistance of PbSnTe:In/(111)BaF2 films has been studied at various mutual orientation of the magnetic field B (up to 4 T), electric field E (up to ~103 V/cm), and normal to the surface n. At B ‖ n, the reduction of the current reaches a factor of ~105, whereas at B ‖ E, the current increases by a factor of ~103. The angular dependences of the magnetoresistance have been studied at the “rotation” of B in three different planes. The angular dependences of the magnetoresistance for the plane corresponding to the orientation B ⊥ E exhibit local maxima near the orientations B ⊥ n, at which charge carriers are deflected by the magnetic field to one of the boundaries of the film. At the deviation to the free surface, the half-width of maxima is several degrees. At the deviation to the interface with the substrate, the half-width of maxima is about an order of magnitude larger and their amplitude is one or two orders of magnitude smaller. Possible mechanisms of giant positive and negative magnetoresistance, as well as the effect of the boundaries of the film on the angular dependences of the magnetoresistance, have been discussed.

Room temperature persistent photoconductivity in p-PbTe and p-PbTe:BaF2

We investigated the persistent photoconductivity effect observed in p-PbTe:BaF2 and undoped p-PbTe films in the temperature range of T = 100–300 K. It was observed that the PPC effect scales with temperature and that there is a transition in the relaxation time behavior around ∼150 K. We found that the transition is caused by the particular dynamics of the hole carries between the energy barriers that characterize the traps originated from disorder present in the samples. The analysis was performed by comparing the theory of the random potential with the experimental data and revealed the presence of both random local potential fluctuations and localized states, which can be attributed to the presence of disorder due BaF2 doping and Te vacancies.

Role of Ultrathin Metal Fluoride Layer in Organic Photovoltaic Cells: Mechanism of Efficiency and Lifetime Enhancement

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrathin BaF2 layer grows by island growth, so BaF2 layers with a nominal thickness less than that of single-coverage layer (≈3 nm) partially cover the polymeric photoactive layer. As the nominal thickness of the BaF2 layer increased to that of a single-coverage layer, the Voc and power conversion efficiency (PCE) of the organic photovoltaic cells (OPVs) increased but the short-circuit current remained almost constant. The fill factor and the PCE decreased abruptly as the thickness of the BaF2 layer exceeded that of a single-coverage layer, which was ascribed to the insulating nature of BaF2 . We find the major cause of the increased Voc observed in these devices is the lowered work function of the cathode caused by the reaction and release of Ba from thin BaF2 films upon deposition of Al. The OPV device with the BaF2 layer showed a slightly improved maximum PCE (4.0 %) and a greatly (approximately nine times) increased device half-life under continuous simulated solar irradiation at 100 mW cm(-2) as compared with the OPV without an interfacial layer (PCE=2.1 %). We found that the photodegradation of the photoactive layer was not a major cause of the OPV degradation. The hugely improved lifetime with cathode interface modification suggests a significant role of the cathode interfacial layer that can help to prolong device lifetimes.

Swift heavy-ions induced sputtering in BaF2 thin films

In our present experiment a series of barium fluoride thin films of different thicknesses have been deposited by electron beam evaporation technique at room temperature on silicon substrates. The effect of film thickness on the electronic sputter yield of polycrystalline BaF2 thin films has been reported in the present work. Power law for sputtered species collected on catcher grids has also been reported for film of lowest thickness. Sputtering has been performed by 100MeV Au+28 ions. Atomic force microscopy (AFM) has been done to check the surface morphology of pristine samples. Glancing angle X-ray diffraction (GAXRD) measurements show that the pristine films are polycrystalline in nature and the grain size increases with increase in film thickness. Rutherford backscattering spectrometry (RBS) of pristine as well as irradiated films was done to determine the areal concentration of Ba and F atoms in the films. A reduction in the sputter yield of BaF2 films with the increase in film thickness has been observed from RBS results. The thickness dependence sputtering is explained on the basis of thermal spike and the energy confinement of the ions in the smaller grains. Also transmission electron microscopy (TEM) of the catchers shows a size distribution of sputtered species with values of power law exponent 1/2 and 3/2 for two fluences 5×1011 and 1×1012ions/cm2, respectively.

Spectral-splitter coatings in laser systems for the visible and infrared regions

This paper describes examples of the synthesis and fabrication of optical coatings that operate in the mid-IR region and the visible region simultaneously. In particular, it tells how to separate the radiations of CO2 and He-Ne lasers by means of interference coatings that transmit (or reflect) the radiation of a CO2 laser and simultaneously reflect (or transmit) pilot radiation in the visible region, including the radiation of a He-Ne laser. Structures based on doped BaF2 films are considered that reflect the visible range and simultaneously operate as antireflection coatings in the IR region. The use of metal-dielectric coatings makes it possible to implement structures that reflect radiation in the IR region and simultaneously transmit radiation in the visible and near-IR regions of the spectrum. Examples are given of the synthesis and practical implementation of this type of coatings.

Optical properties of ZnO nanocrystals embedded in BaF2 film fabricated by magnetron sputtering

ZnO nanocrystals (NCs) embedded in a BaF2 matrix have been prepared by a radio-frequency magnetron sputtering technique followed by a thermal annealing process from 400 to 1100 °C. X-ray diffraction results show that ZnO NCs with a hexagonal wurtzite structure are formed in a BaF2 matrix. The average grain sizes are estimated ranging from 7.5 to 33 nm, corresponding to fabricated samples annealed at temperatures from 400 to 800 °C. Photoluminescence showed a very strong ultraviolet near-band-edge (NBE) emission located in the 364–383 nm spectral region. The emission intensity is enhanced and the linewidth is narrowed as the annealing temperature increases. The intensity ratio of ultraviolet NBE emission to deep-level visible is also increased with annealing temperature indicating that deep-level defects in ZnO are suppressed.

Smoothing, compaction, mixing, sputtering and dewetting of semimetal- and halogenide-coatings by swift heavy ion irradiation

We have investigated the response of thin Bi, BaF2 and CaF2 films (100–150nm) deposited onto Si and SiO2 to the bombardment with swift heavy ions (230MeV Xe, 350MeV, 600MeV Au) at 80K. The morphological and compositional properties of the films were characterized by means of Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy and mechanical surface profilometry. The as-deposited films showed rough surfaces and an enhanced porosity. Upon irradiation compaction of the films and smoothing of the surfaces occurs. The halogenide films on SiO2 showed a distinct interface mixing, while the halogenide/Si interface did not. In case of the Bi films, no mixing was observed with both substrates. Besides smoothing, compaction and interface mixing also sputtering of the halogenides takes place with sputtering yields of about 2500 for BaF2 and 1000 for CaF2. The ratio of 2.5 appearing for the sputter yields was also found for mixing and smoothing, which occur about 2.5 times faster for BaF2 as compared to CaF2. Astonishingly, perforation of the fluoride films was found at very high fluences, indicating dewetting by irradiation induced plastic or viscous flow of the coating, driven by capillary forces, as recently reported also for oxide coatings.

Scanning transmission electron microscopy studies of the microstructure and chemistry of the YBa2Cu3O7−δ/BaF2 interface

A scanning transmission electron microscope (STEM) is used to examine the structural and chemical quality of the interface between thin films of YBa2Cu3O7−δ (YBCO) and BaF2. The STEM images indicate that it is possible to grow a uniform thin film of BaF2 that is relatively free of gross defects via a thermal evaporation method on top of a YBCO thin film. The diffraction contrast results also suggest that a three-dimensional or “island” growth mechanism is involved in BaF2 films grown by this method. An oxygen map across the YBCO/BaF2 interface produced using core loss energy filtered imaging is used to address a concern regarding oxygen diffusion from YBCO into BaF2. It was found that a good quality BaF2 film can play a significant role in reducing outdiffusion of oxygen from YBCO. Quantitative evaluation of the specimen oxygenation suggests that beam spreading effects may be important. The effect of an ultra-high vacuum (UHV) STEM environment on the specimen oxygenation is also considered.

Orientation of vacuum–evaporated BaF2 films in the presence of nitrogen

BaF 2 films were vacuum deposited on borosilicate glass substrates at room and higher temperatures. The films were polycrystalline with different orientations. Introduction of N2 at 3×10−4 mbar pressure during evaporation resulted in a dramatic improvement in orientation along a given direction, which itself was a function of the substrate temperature. Thus, at room temperature, the orientation was along the (111) direction, while at 100 °C it was along the (220) direction.

Study of photocathode protection with thin dielectric films

Long-term stability of air-sensitive solid photocathodes operating in gas could be attained by coating their surface with a thin solid dielectric film. The protective film should have good electron transmission properties. We present data on LiF, NaF, CsF, MgF2, BaF2, SiO2 and Al2O3 films. The electron escape length and the film's protection properties were studied on relatively stable photocathodes, namely CsI, CuI and Al. We observed a reduction of the electron affinity due to adsorption of polarized water molecules on alkali fluorides. We report on the stability of CsI photocathodes coated with thin films, under exposure to air and under gas multiplication conditions. The possible protection of visible photocathodes is discussed.

Growth and interfacial chemistry of insulating (100) barium fluoride on gallium arsenide

The epitaxial growth of insulating BaF2 films on (100) and (111)-oriented GaAs substrates has been investigated. (100)-oriented BaF2 was successfully deposited on (100) GaAs at temperatures as low as room temperature, in contrast to a previous report. This was accomplished by first establishing a chemically reacted template layer a few monolayers thick at the BaF2/GaAs interface. These films consistently exhibited epitaxial reflection high-energy electron diffraction patterns with three-dimensional growth modes for a wide range of incident BaF2 flux rates. The epitaxial quality of the (100) films was, however, temperature dependent. A film deposited on a (111) wafer at 600 °C was (111) oriented and showed two-dimensional growth. X-ray photoelectron spectroscopy studies of the interface chemistry indicate the existence of a Ba state other than that of the BaF2, authenticating the relevance of the template layer. The (100) BaF2 films are insulating, with a breakdown field of ∼1×106 V/cm.

Radiation damage effects in far-ultraviolet filters, thin films, and substrates

Advances in vacuum ultraviolet thin-film filter technology have been made through the use of filter designs with multilayers of materials such as Al(2)O(3), BaF(2), CaF(2), HfO(2), LaF(3), MgF(2), and SiO(2). Our immediate application for these filters will be in an imaging system to be flown on a satellite where a 2 × 9 R(E) orbit will expose the instrument to approximately 250 krad of radiation. Because to our knowledge no previous studies have been made on the potential radiation damage of these materials in the thin-film format, we report on such an assessment here. Transmittances and reflectances of BaF(2), CaF(2), HfO(2), MgF(2), and SiO(2) thin films on MgF(2) substrates, Al(2)O(3) thin films on fused-silica substrates, uncoated fused silica and MgF(2), and four multilayer filters made from these materials were measured from 120 to 180 nm beforeand after irradiation by 250 krad from a (60)Co gamma radiation source. No radiation-induced losses in transmittance or reflectance occurred in this wavelength range. Additional postradiation measurements from 160 to 300 nm indicates 2-5% radiation-induced absorption near 260 nm in some of the samples with MgF(2) substrates. From these measurements we conclude that far-ultraviolet filters made from the materials tested should experience less than 5% change from exposure to up to 250 krad of high-energy radiation in space applications.

Two Step Growth and Characterization of BaF2/Si(III) Heterostructures

AbstractThe influence of growth conditions on structural perfections and epitaxial relation in BaF2 films on Si(111) are considered. It is shown that rotational-twin-free BaF2 films (A-type orientation) can be grown on Si(111) by two step growth method. It is also shown that a mixed (A+B)-type orientation, which usually observed in BaF2 films grown by conventional one step growth method, can be converted into A-type orientation by postgrowth annealing. The correlation between type of epitaxial orientation in BaF2 films and structures of interface is discussed.

Electron Microscopy Studies of The Chemistry And Microstructure of BaF2 / YBa2Cu3O7-x Thin Films

BaF2 thin films are being investigated as candidates for use in YBa2Cu3O7-x (YBCO) / BaF2 thin film multilayer systems, given the favorable dielectric properties of BaF2. In this study, the microstructural and chemical compatibility of BaF2 thin films with YBCO thin films is examined using transmission electron microscopy and microanalysis. The specimen was prepared by using laser ablation to first deposit an approximately 2500 Å thick (0 0 1) YBCO thin film onto a (0 0 1) MgO substrate. An approximately 7500 Å thick (0 0 1) BaF2 thin film was subsequendy thermally evaporated onto the YBCO film.Images from a VG HB501A UHV scanning transmission electron microscope (STEM) operating at 100 kV show that the thickness of the BaF2 film is rather uniform, with the BaF2/YBCO interface being quite flat. Relatively few intrinsic defects, such as hillocks and depressions, were evident in the BaF2 film. Moreover, the hillocks and depressions appear to be faceted along {111} planes, suggesting that the surface is smooth and well-ordered on an atomic scale and that an island growth mechanism is involved in the evolution of the BaF2 film.

Fabrication and characterization of Ti-Ba-Ca-Cu-O superconducting films on LaAlO3 substrates

A multi-step process is used to fabricate Tl2Ba2Ca1Cu2O8 films on (100) LaA103 substrates. Submicron thick precursor films of Ba-Ca-Cu-O are rf magnetron sputter deposited from a single target. Film stoichiometry is measured by ion beam backscattering spectroscopy. Deficiencies of the alkaline earths that are found in the precursor films are then compensated for by the addition of appropriate CaF2 and/or BaF2 films onto the surface of the precursor film. Post deposition annealing of the films is then done in an atmosphere of thallium oxide and oxygen in order to form the superconducting phases. The annealed films are examined using x-ray diffraction (XRD), an ac inductance technique, and critical current in an external magnetic field. XRD shows the c-axis length of the superconducting phase to increase as the overall film stoichiometry approaches 2212. The transition widths measured by inductive coupling weakly correlate with 77 K critical current measurements. Our best critical current results are 1.5*106 amps/ cm2 for a film measured at 4 K in an 8 T magnetic field (parallel to the films' c-axis).