BaF2 Thin Films Research Articles

Large-area narrowband Fabry–Pérot interferometers for long-wavelength infrared spectral sensing

This paper presents a proof-of-concept for microelectromechanical system (MEMS)-based fixed cavity Fabry–Pérot interferometers (FPIs) operating in the long-wavelength infrared (LWIR, 8 to 12 μm) region. This work reports for the first time on the use of low-index BaF2 thin films in combination with Ge high-index thin films for such applications. Extremely flat and stress-free ∼3-μm-thick free-standing distributed Bragg reflectors (DBRs) are also presented in this article, which were realized using thick lift-off of a trilayer structure fabricated using Ge and BaF2 optical layers. A peak-to-peak flatness was achieved for free-standing surface micromachined structures within the range of 10 to 20 nm across large spatial dimensions of several hundred micrometers. Finally, the optical characteristics of narrowband LWIR fixed cavity FPIs are also presented with a view toward the future realization of tunable wavelength MEMS-based spectrometers for spectral sensing. The measured optical characteristics of released FPIs agree with the modeled optical response after taking into consideration the fabrication-induced imperfections in the free-standing top DBR such as an average tilt of 15 nm and surface roughness of 25 nm. The fabricated FPIs are shown to have a linewidth of ∼110 nm and a suitable peak transmittance value of ∼50 % , which meets the requirements for their utilization in tunable MEMS-based LWIR spectroscopic sensing and imaging applications requiring spectral discrimination with narrow linewidth.

Surface erosion of BaF2 thin films under SHI irradiation: Angular distribution and role of different substrates

In the present report, the surface erosion of the thin films of barium fluoride (BaF2) deposited on different substrates (Glass, Silicon, and Aluminum) using the electron beam evaporation method, has been investigated under swift heavy ions irradiation employing experimental and theoretical aspects. The experiments have been carried out at room temperature using normally incident 100 MeV Au+28 ions. Rutherford backscattering spectrometry has been employed to estimate the sputtering yield and angular distribution of sputtering yield has been obtained using the unique catcher technique approach. The variation of sputtering yield at different angles shows an isotropic distribution superimposed with enhanced sputtering at surface normal. The sputtering studies in the thin films on different substrates show that the maximum sputtering takes place for glass (insulator) substrate while minimum sputtering occurs for the aluminum (metal) substrate. Simulations based on the inelastic thermal spike model have been performed to compare the observed experimental findings. The temperature spike generated in Al will smear out more efficiently and quickly because of its high thermal conductivity as compared to Si and glass substrates resulting in less contribution to the temperature spike generated in the film and thus causing lower sputtering yield for Al substrate.

Effect of interface grading on the optical performance of distributed Bragg reflector multilayers in Fabry-Pérot optical filters

The design, fabrication, and characterization of abrupt interface and graded interface distributed Bragg reflector (DBR) multilayers is reported. Electron-beam deposited zinc selenide (ZnSe) and Barium Fluoride (BaF2) thin films were chosen as the high and low refractive index dielectric materials respectively, for the DBRs in the mid-wavelength infrared (MWIR) region. In this region, DBRs comprised of 3 stacks of abrupt interface ZnSe and BaF2 thin films with central wavelength of 4 µm were found to suffer from defects and absorption loss that later affect the optical transmission and spectral resolution of assembled Fabry-Perot optical filters. Interfacial grading of the DBRs with a thin layer of amorphous silicon monoxide (SiO) ~ 80 nm, reduced the absorption loss and the interdiffusion of defect atoms at the interface between alternating layers. Fabry-Perot filters assembled from abrupt interface DBRs showed 22% optical transmission, whereas those assembled from graded interface DBRs acheived 26.5% optical transmission. The Full-width at half maximum (FWHM) of the transmission peak for graded interface Fabry-Perot optical filter decreased significantly compared to the abrupt interface optical filter, improving its spectral resolution.

SHI induced evolution of surface and wettability of BaF2 thin films

AbstractWe report evolution of the surface and wetting behavior of Barium fluoride (BaF2) thin films under the effect of swift heavy ion (SHI) irradiation at different fluences. The analogy of this study may be used may be used for the development of dust resistant fabric technology for rural area. The ion irradiation has been performed at normal incidence on the films with Au ions having 100 MeV energy. Further, the wettability of irradiated surfaces is studied through contact angle of water droplet. The value of contact angle of droplet changes with irradiation, it increases from 111° to 123° with the increase in fluence from 5×1011 to 1×1013 ions/cm2. The mechanism of wettability of BaF2 is explained on the basis of increase in contact area of water droplet with surface. SHI irradiation deposits a huge amount of energy in materials due to extreme electronic excitation and it causes a large increase in the temperature of material around the ion track. Ion beam irradiation leads to the large ejection of atoms from the surface which is one of the major factors in increasing the roughness of the surface and thus for the change in contact angle.

Substrate effect on the evolution of surface morphology of BaF2thin films: A study based on fractal concepts

Fractal concepts are used to explore how different substrates affect the morphology of deposited barium fluoride (BaF2) thin films. BaF2 thin films of thickness 20 nm are prepared by electron beam evaporation technique at room temperature on glass, silicon and aluminum substrates. The structural properties and surface morphologies are investigated using glancing angle X-ray diffraction (GAXRD) and atomic force microscopy (AFM), respectively. Crystallite size, calculated from the XRD peaks, using the Debye Scherrer’s formula, is found to be significantly less for the case of Si substrate than for Al and glass substrates. Higuchi’s algorithm is applied to extract the fractal dimension of the horizontal and vertical sections of AFM images of the films. From the fractal dimension, the values of Hurst exponent (H) are inferred. For Al and glass substrates we find that H>0.5, which shows that the height fluctuations at neighboring pixels are positively correlated. However, for Si substrate, it is found that H<0.5 indicating that the height fluctuations at neighboring pixels are negatively correlated.

Fractal and multifractal characteristics of swift heavy ion induced self-affine nanostructured BaF2 thin film surfaces.

Fractal and multifractal characteristics of self-affine surfaces of BaF2 thin films, deposited on crystalline Si ⟨1 1 1⟩ substrate at room temperature, were studied. Self-affine surfaces were prepared by irradiation of 120 MeV Ag(9+) ions which modified the surface morphology at nanometer scale. The surface morphology of virgin thin film and those irradiated with different ion fluences are characterized by atomic force microscopy technique. The surface roughness (interface width) shows monotonic decrease with ion fluences, while the other parameters, such as lateral correlation length, roughness exponent, and fractal dimension, did not show either monotonic decrease or increase in nature. The self-affine nature of the films is further confirmed by autocorrelation function. The power spectral density of thin films surfaces exhibits inverse power law variation with spatial frequency, suggesting the existence of fractal component in surface morphology. The multifractal detrended fluctuation analysis based on the partition function approach is also performed on virgin and irradiated thin films. It is found that the partition function exhibits the power law behavior with the segment size. Moreover, it is also seen that the scaling exponents vary nonlinearly with the moment, thereby exhibiting the multifractal nature.

Dynamic scaling of swift heavy ion induced surface restructuring of BaF2 thin film

Dynamic scaling of swift heavy ions induced cracks in the nanodimensional BaF2 thin films is studied using 100MeV Au+8 ions at different ion fluences. Atomic force micrographs show that, up to a fluence of 1×1012 ions/cm2, the surface morphology is almost unchanged from virgin film, but thereafter, the formation of cracks is seen. The dimensions and density of the cracks is found to increase with increase in ion fluence. Dynamic scaling of surfaces demonstrates that the surface roughness exponent (α) values are in the range of 0.2–0.5. The growth exponent values are found to be β=0.10±0.03 and 1.1±0.1 for continuous and cracked surfaces, respectively. Shift of X-ray diffraction peaks after irradiation indicates contribution of tensile stress in the crack evolution with ion fluence.

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.

Experimental setup for the measurement of the electrical resistivity and thermopower of thin films and bulk materials

An experimental setup was developed for the simultaneous measurements of the electrical resistivity and the thermopower in the temperature range going from 77 up to 330K. The system combines accuracy, rapidity, few efforts for the sample mounting, and is fully automated. Pressure contacts are used for contacting the thermocouples to the sample and the sample to the heat sink, making possible the measurement of both thin films and bulk materials with a great flexibility on their geometrical dimensions. Tests of the system have been made on a 99.98% pure Ni bulk sample and on PbTe∕BaF2 thin films for illustration.

Preparation and characterization of volatile alkaline-earth metal complexes with multiply coordinated aminoalkoxide ligands

The aminoalkoxide complexes Sr(amak)2 (1) and Ba(amak)2 (2) were prepared by treatment of a multiply-chelating fluorinated aminoalcohol ligand HOC(CF3)2CH2N(CH2CH2OMe)2, (amak)H, with the Group 2 metal reagents Sr(OPri)2 and BaH2 respectively. Single crystal X-ray diffraction studies indicate that complex 1 possesses an 8-coordinate distorted bicapped octahedral geometry with all O and N atoms coordinated to the central Sr cation. However, complex 2 adopts a unique 10-coordinate bicapped square antiprismatic structure, of which the coordination number is increased by two fluorine-to-barium dative interactions. Variable temperature 19F NMR studies show the existence of two inter-convertible isomers in solution and their possible molecular structures are proposed according to their structures in the solid-state. Preliminary investigation suggests that these complexes are good CVD source reagents for depositing SrF2 and BaF2 thin films.

A study of X-ray reflectivity data analysis methods for thin film thickness determination

The determination of thin film thickness by four X-ray reflectivity methods (namely, the peak separation, the Fourier transform, the modified Bragg equation, and the curve-fitting methods) has been studied. An analysis of SrS and BaF2 thin films showed thickness values determined by the methods agreed to within 4%. The curve-fitting method had the highest accuracy but was time-consuming. The peak separation, the Fourier transform, and the modified Bragg equation methods are considerably faster and, on average, gave 2.8%, 0.9%, and 0.2% larger thicknesses than those of the curve-fitting method.

Molecular beam epitaxial growth of Eu-doped CaF2 and BaF2 on Si

The growth of Eu-doped CaF2 and BaF2 thin films on Si(100), (110), and (111) substrates has been realized by molecular beam epitaxy using elemental Eu evaporation. Very bright blue emissions from Eu-doped CaF2 and yellow emissions from Eu-doped BaF2 were obtained in the wavelength range of 400–850 nm at 10 K. Depending on the Si substrate orientation, the zero-phonon line of Eu2+ in the CaF2 thin films was shifted by different amounts relative to that of bulk CaF2 due to residual strain in these epilayers.

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.

Epitaxial BaF2 films on InP(100) deposited by r.f. magnetron sputtering

Epitaxial BaF2 thin films were deposited onto InP(100) at a substrate temperature ranging from 450 to 480°C by r.f. magnetron sputtering. X-ray diffraction analysis revealed a pure c-axis-oriented film perpendicular to the substrate surface. The X-ray rocking curve of the full width at half-maximum for BaF2(200) was less than 0.8°. The X-ray pole figure investigation showed good in-plane alignment of the crystals. A scanning electron microscopy surface scan indicated a highly textured film with grid growth morphology.

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.

Reactive evaporation of BaF2 in water vapour for in situ growth of YBa2Cu3Ox thin films

The conversion of BaF2 thin films to BaO during deposition in an oxidizing ambient was investigated as a first step towards the in situ growth of superconducting thin films of YBa2Cu3Ox from Y, Cu, and BaF2 source materials. This conversion was found to occur at a temperature of 800 °C when BaF2 was evaporated in a partial pressure of H2O. The observed conversion temperature agrees well with an equilibrium thermodynamic model. Methods to reduce the growth temperature to the 600– 700 °C range are proposed.