SrF2 Research Articles

Enhancement of Vibrant Up-Conversion in (Ca,Sr)F2:Er,Yb,Al Phosphors via Energy Transfer, Localized Interstitial Distortion, and Au Nanostructures.

A remarkable increase in the luminescent intensity of Er3+-doped CaF2 up-conversion phosphors was achieved, showing an approximate enhancement of over 1100-fold. This enhancement was realized by incorporating Yb3+, Al3+, Sr2+, and gold nanospheres and nanorods. The substantial improvement in up-converting luminescence effectively enhances sensitivity and efficiency at low excitation power densities. The up-conversion phosphors, consisting of (Ca,Sr)F2:Er,Yb,Al, were easily prepared using excess NH4F flux at 950 °C for 30 min. The structural confirmation of interstitial Al3+ ions within the CaF2 lattice was achieved through synchrotron powder X-ray powder diffraction. The significant enhancement of up-conversion emission and their mechanisms in the phosphors were vividly represented through energy transfer, interstitials and local distortions, and localized surface plasmon resonances when excited with a 980 nm diode laser.

Mid-infrared emission properties of erbium-doped fluorite-type crystals

We report on a comparative study of the spectroscopic properties and mid-infrared laser performance of five 5 at.% Er3+-doped fluorite-type crystals MF2, including parent compounds CaF2, SrF2, BaF2, and solid-solution (“mixed”) ones (Ca,Sr)F2 and (Sr,Ba)F2. In the M = Ca → Sr → Ba series, the host matrix phonon energy decreases, the absorption and mid-infrared emission spectra of Er3+ become narrower and more structured, and the luminescence lifetimes of the 4I11/2 and 4I13/2 Er3+ manifolds increase. The Er3+ transition probabilities were calculated using the Judd-Ofelt theory. In the “mixed” compounds, the Er3+ ions tend to reside in the larger / heavier cation environment. The low-temperature (12 K) spectroscopy evidences the presence of a single type of clusters at this doping level; the crystal-field splitting for Er3+ ions in clusters was determined. Continuous-wave low-threshold laser operation at ∼2.8 µm (the 4I11/2 → 4I13/2 transition) was achieved with all five Er3+:MF2 crystals. The maximum achieved laser slope efficiency was 37.9% (Er3+:CaF2), 23.5% (Er3+:SrF2) and 17.2% (Er3+:BaF2).

Conversion efficiency improvement of ELO GaAs solar cell, deposited on water soluble sacrificial buffer

We demonstrate the improvement of power conversion efficiency of an epitaxially-lifted-off single junction GaAs solar cell deposited on a water-soluble sacrificial buffer architecture by introducing an additional germanium (Ge) interlayer between GaAs and the fluoride buffer. The epitaxial lift-off (ELO) technique has been extensively used to separate III-V device layers from their single crystal GaAs substrates. However, conventional ELO requires the use of concentrated hydrofluoric acid (HF) for extended times to etch out the sacrificial layer, subsequently degrading the surface roughness of the parent wafer. As a result, the wafer has to undergo expensive and intensive chemical mechanical polishing (CMP) processes, costing about 25 % of a pristine 6-inch GaAs substrate. In our previous work, we demonstrated a method to eliminating the need for CMP post-processing by using water-assisted ELO (H2O-ELO). A water-soluble, 3-layer buffer architecture was developed using alkaline earth compounds. However, devices suffered low performance due to a high defect density in the GaAs active layers. A Ge interlayer was introduced to provide a more favorable surface energy for GaAs growth, which leads to an improvement of GaAs crystal quality. The Ge deposition conditions were optimized to achieve a high-quality Ge layer on triple-layer fluoride buffer. Single junction GaAs devices fabricated on Ge/(Ca,Sr)F2/BaF2/(Ca,Sr)F2 showed improvement of solar cell performance parameters featuring increases in Voc by 23.7 %, and fill factor (F.F.) by 4.9 %, which results in an overall improvement of power conversion efficiency from 10.3 % to 12.69 %.

Growth and mid-infrared emission properties of "mixed" fluorite-type Er:(Ca,Sr)F2 and Er:(Ba,Sr)F2 crystals

Fluorite-type 5 at.% Er3+:(M1,M2)F2 (M1 = Ca, Ba; M2 = Sr) crystals were grown by the conventional Bridgman technique and a comparative study of their spectroscopic properties was performed. The vibronic properties of the fluorite-type crystals were studied by Raman spectroscopy. The Er:(M1,M2)F2 crystals exhibited a slightly inhomogeneous broadening of mid-infrared luminescence spectra as compared to the ‘parent’ compound, Er:SrF2. The luminescence lifetimes of the 4I11/2 and 4I13/2 manifolds were measured, e.g., for the Er:(Ca,Sr)F2 crystal, the luminescence lifetimes were estimated to be 8.64 ms and 5.64 ms, respectively, representing a favorable ratio for mid-IR laser operation.

Study on the vacuum ultraviolet transmittance of barium fluoride crystals at different temperature

Two VUV-grade BaF2 windows with 0.5 mm-thick and 1 mm-thick respectively were selected to study the transmittance variety with the temperature. The results show that the cutoff wavelength of BaF2 crystals will shift towards the long wave with the increase in temperature. In a certain temperature range, BaF2 crystals can depress 130.4 nm radiation well, and also has a high transmittance at 135.6 nm. Compared with the reported method in which SrF2 crystals can be applied to suppress 130.4 nm stray light by heating, BaF2 crystal can inhibit the 130. 4 nm emission line completely, and thus reduce the power consumption of the device at the same time. This indicates that BaF2 crystals can play an important role in the ionosphere optical remote sensing detection.

Progress in fluoride laser ceramics

AbstractA variety of Er3+‐, Tm3+‐, and Yb3+‐activated CaF2‐, SrF2‐, (Ca, Sr)F2‐ and (Sr, Ba)F2‐based fluoride laser ceramics have been prepared by the hot‐forming technique. Lasing of the SrF2:Yb3+, (Ca, Sr)F2:Er3+, (Ca, Sr)F2:Tm3+, (Ba, Sr)F2:Tm3+ and (Ba, Sr)F2:Yb3+ specimens with diode pumping has been observed for the first time. Thermal conductivity data for the above fluorite‐type laser materials are also presented and discussed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural Studies of (Ca,Sr)F2 Single Crystals with Raman and NMR Spectroscopies

Single crystal CaF2 is the material of choice for optics in the vacuum UV, due in part to its high transmission at these wavelengths. Unfortunately, as the wavelengths decrease, intrinsic birefringence (also referred to as spatial dispersion) can become larger than the target birefringence for precision optics at 157 nm, even for a cubic material such as CaF2. One elegant solution to this problem is to capitalize on opposite signs for this intrinsic birefringence, such as mixing SrF2 or BaF2 with CaF2. If the proper fluorides are mixed with the correct concentration, the intrinsic birefringence of the mixed single crystals can be minimized. It is also important that the resulting mixed single crystal maintain the high symmetry of CaF2, which requires that the cations mix randomly in the CaF2 structure. We show through Raman and NMR spectroscopic studies that this random mixing of cations can be achieved for mixed (Ca,Sr)F2 single crystals. A single Raman band is observed for all mixed crystals, which changes in both position and width upon addition of SrF2 to CaF2. 19F NMR spectra also reflect a random distribution of cations around the different fluorine environments in the mixed crystals.

Longitudinal optical and transverse optical modes of (Ca, Sr) F2 films grown on Si(111)

Abstract The transmission of IR radiation through single-crystal thin films of (Ca, Sr) F2 grown on Si(111) was investigated. The longitudinal optical and transverse optical modes have been found at frequencies predicted by a linear variation in the frequency with concentration. The potential of IR spectroscopy for determining the composition of very thin fluoride layers is discussed.

Molecular beam epitaxy of Nd-doped CaF2 and CaSrF2 layers on Si and GaAs substrates

The incorporation of Nd3+ in CaF2 layers grown on Si and GaAs substrates by molecular beam epitaxy is studied by photoluminescence spectroscopy. The results are in qualitative agreement with those obtained on CaF2:Nd homoepitaxial layers. A lower emission intensity (∼70%) at λ=1.0475 μm is attributed to residual stress and crystalline defects. Concentration quenching of photoluminescence appears at concentrations higher than 3.6 wt % Nd. The use of (Ca,Sr)F2 for lattice matching to GaAs leads to a significant inhomogeneous broadening of Nd3+ emissions due to disorder in the cationic sublattice.

MBE-grown (Ca, Sr) F2 layers on Si (111) and GaAs (111) : Electronic structure of interfaces

MBE-grown (Ca, Sr) F2 layers on Si (111) and GaAs (111) : Electronic structure of interfaces

Atomic-size effects on the growth of SrF2 and (Ca,Sr)F2 on Si(111).

Strain-induced lattice relaxation during overlayer growth is investigated by {ital in} {ital situ} monitoring of SrF{sub 2} and Ca{sub {ital x}}Sr{sub 1{minus}{ital x}}F{sub 2} growth on Si(111) with x-ray standing-wave fluorescence and low-energy electron diffracton. Submonolayer films show both intrinsic lateral disorder and variable interface stoichiometry with discommensuration parallel to the surface after completion of the first monolayer. In contrast, local commensuration is observed for Ca-rich alloy films. In the alloy, Sr and Ca occupy different sites in the first monolayer, but have similar average positions in subsequent layers.

Generalization of Bragg reflector geometry: Application to (Ga,Al)As-(Ca,Sr)F2 reflectors

Bragg reflectors based on (Ca,Sr)F2 and (Ga,Al)As are studied. Modeling and fabrication of these structures by molecular-beam epitaxy were performed. Quarter-wave Bragg reflectors were found to present an excellent reflectance around the wavelength of interest, 870 nm, for only three periods of bilayers. However, the structures grown exhibited cracks after epitaxy due to thermal stress between both materials. To alleviate this problem, other reflector geometries were investigated consisting of deviations from the classical Bragg reflector. The new geometries enable one to reduce the absolute or relative fluoride thickness within the structure. The results obtained show that the use of adequate geometries allows one to overcome the stress problem, and good heteroepitaxial reflectors with a crack-free surface morphology were obtained.

Influence of off-oriented substrates on the crystallinity of (Ca,Sr)F2 layers grown on Si (100)

Off-oriented Si (100) substrates had considerable influence on the crystallinity of a (Ca,Sr)F2 layer, which is lattice matched with GaAs and grown on the substrates by molecular beam epitaxy. The (Ca,Sr)F2 layer grown on the Si (100) substrate, off oriented toward 〈011〉, was not a single crystal but had a columnar structure. The (Ca,Sr)F2 layer grown on the exact Si (100) susbstrate, however, was a single crystal in which the crystallinity was quite good with a minimum channeling yield (χmin ) of the mixed fluoride layer in the Rutherford backscattering spectra of about 0.1. To obtain such good crystallinity, the temperature range was limited to around 500 °C. The influence of off-oriented substrates was opposite that found in GaAs/Si (100) structures.

Influence of Off-Oriented Substrates on Heteroepitaxial Growth of (Ca, Sr)F2 Layers on Si (100)

AbstractHeteroepitaxial growth of CaxSr1-xF2 layers on Si (100) substrates by molecular beam epitaxy has been investigated for GaAs / fluoride / Si structures. The Si (100) substrates off-oriented toward [011] show no considerable influence on crystallinity of CaF2 and SrF2 epitaxial layers. The off - oriented substrates, however, influence a remarkable effect on CaxSr1-xF2 layers, resulting in poor crystallinity rather than good one. This influence of off-oriented Si (100) substrates on the heteroepitaxy of CaxSr1-xF2 layers was the opposite of that found in GaAs/Si (100) structures.

On the mode of growth of CaF2 and (Ca,Sr)F2 on GaAs and Si(111) substrates

The early stages of molecular-beam epitaxy of (Ca,Sr)F2 and CaF2 on GaAs and Si(111) substrates have been studied by means of in situ Auger electron spectrometry and reflection high-energy electron diffraction. The results obtained show (i) that the growth proceeds in all cases via a two-dimensional mechanism that preserves the atomic flatness of the growth front and (ii) that this growth has a nonpseudomorphic character. This behavior is related to the properties of the fluorides involved. The general trends in fluoride-semiconductor heterostructure growth are discussed.

Laser-modified molecular beam epitaxial growth of (Al)GaAs on GaAs and (Ca,Sr)F2/GaAs substrates

We report results on the effect of a 193 nm ArF excimer laser on molecular beam epitaxial growth of (Al)GaAs on GaAs substrates and GaAs on lattice-matched (Ca,Sr)F2/GaAs heterostructures. For growth on GaAs substrates, regions exposed to the laser show photoluminescence and excellent channeling as determined by Rutherford backscattering spectroscopy, whereas regions outside the laser show no photoluminescence. For growth on (Ca,Sr)F2 surfaces, laser irradiation inhibits the growth of GaAs for fluences above a critical value of 0.12 J/cm2 pulse because of laser-induced desorption of absorbed Ga atoms, which are relatively weakly bound to (Ca,Sr)F2, compared to GaAs surfaces.

Characterization of epitaxial group II fluoride films on (100)‐oriented GaAs substrates by ion backscattering and channeling

AbstractLattice matched (Ca, Sr)F2 and related fluoride films grown on (100)GaAs substrates were investigated by Rutherford backscattering/Channeling. This technique allows the characterization of the 30–250 nm thick heteroepitaxial layers with respect to chemical composition, film thickness, interface roughness and crystalline perfection. The first flash evaporated GaAs/fluoride/GaAs structure are briefly reported.

Growth and Characterization of Compositionally Graded (Ca, Sr)F2 Layers on Si(111) Substrates

Epitaxial insulator films composed of compositionally fixed (Ca, Sr)F2/compositionally graded (Ca, Sr)F2/CaF2 multilayers have been grown on Si(111) substrates by the coevaporation of CaF2 and SrF2 from two separate crucibles The dependence of the channeling minimum yields on the energy of incident ions in Rutherford backscattering spectroscopy revealed that defects in the top fluoride layers were mainly dislocations. The dislocation density could be reduced by decreasing the compositional gradient of the underlying graded layers and decreasing the growth temperature. As a result of the reduced dislocation density, the generation of cracks in fluoride films could be suppressed.

Crystalline defects in heteroepitaxial GaAs/(Ca,Sr)F2 grown by molecular-beam epitaxy

The crystallinity of GaAs/(Ca,Sr)F2 structures grown by molecular-beam epitaxy on (100) and (111) B-oriented GaAs substrates is studied by transmission electron microscopy. The main defects found in the GaAs heteroepitaxial layers were microtwins and dislocations. The geometry and the origin of these defects are discussed in terms of growth and stress relaxation mechanisms, and placed within the framework of published results on similar heteroepitaxial systems. The results obtained on (111)-oriented layers suggest the preeminent role played by stress at growth temperature.

High-resolution electron microscopy study of (Ca,Sr)F2/GaAs grown by molecular-beam epitaxy

GaAs/(Ca,Sr)F2/GaAs structures have been studied by means of high-resolution electron microscopy; two substrate orientations, (100) and (111)As, were considered. The (Ca,Sr)F2 layers were observed to be monocrystalline, while their crystalline orientation was found to be the same as that of the substrate (A type). The surface morphology of the (Ca,Sr)F2 layers appeared to be highly sensitive to the orientation: it was observed to be atomically flat for the (111) case and to consist of facets, {111} oriented for the (100) orientation. For both orientations, the transmission electron microscopy observations show that the interface seems to be composed of coherent and incoherent structures. These results are discussed and compared to those obtained on similar heteroepitaxial systems.