Growth Of CaF2 Research Articles

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.

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.

Structural evolution of CaF2 nanoparticles during the photoinduced crystallization of a Na2O–K2O–CaO–CaF2–Al2O3–ZnO–SiO2 glass

A glass in the system Na2O–SiO2–Al2O3–K2O–CaO–CaF2–ZnO was doped with Ce, Ag, Sn, Sb and Br. Homogeneous and transparent glass ceramics are obtained from this glass by the precipitation of CaF2 nanoparticles. An interface-controlled crystallization mechanism hinders crystal growth after some time. By adding photosensitive agents, a tailored photoinduced CaF2 crystallization was achieved. Structural evolution during irradiation and heat treatment was investigated by different electron microscopic techniques, e.g., scanning transmission electron microscopy (STEM) including energy-dispersive X-ray analysis (EDXS) and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). Agglomerated structures with a size of ca. 35 nm were observed after the first thermal treatment at 530 °C. The Ag nucleation is accompanied by clustering of several components within the glass. CaF2 particles with diameters of ca. 300 nm with a spherulitic structure are observed in the glass ceramic after the second heat treatment step at 560 °C. Calcium and fluoride are depleted from the glass matrix during crystallization, while SiO2 is enriched. From (S)TEM micrographs, supplemented by selected area diffraction (SAD), outward growth of CaF2 from a central point is observed. STEM–EDXS analyses show residual glass within the spherulitic structure.

Sequential growth of CaF2:Yb,Er@CaF2:Gd nanoparticles for efficient magnetic resonance angiography and tumor diagnosis.

It is a significant challenge to develop nanoscale magnetic resonance imaging (MRI) contrast agents with high performance of relaxation. In this work, Gd3+-doped CaF2-based core-shell nanoparticles (CaF2:Yb,Er@CaF2:Gd) of sub-10 nm size were controllably synthesized by a facile sequential growth method. The as-prepared hydrophilic CaF2:Yb,Er@CaF2:Gd nanoparticles modified using PEG-PAA di-block copolymer benefited from the presence of Gd only in the outer CaF2 layer of the nanoparticles, which exhibited r1 as high as 21.86 mM-1 s-1 under 3.0 T, seven times as high as that of commercially used gadopentetate dimeglumine (Gd-DTPA). Low cytotoxicity, no hemolysis phenomenon and no potential gadolinium ion leakage phenomenon of the hydrophilic CaF2:Yb,Er@CaF2:Gd nanoparticles have been observed and confirmed. Clear vascular details can be observed in magnetic resonance angiography and obvious MR signal of 4T1 tumor area could be significantly improved by intravenous injection of the hydrophilic CaF2:Yb,Er@CaF2:Gd nanoparticles at a low dosage in mice. A series of in vivo biological safety evaluations confirmed the good biocompatibility of the hydrophilic CaF2:Yb,Er@CaF2:Gd nanoparticles, which might be employed in clinical blood pool imaging and tumor diagnosis as a safe and efficient MRI probe.

Heteroepitaxy of large grain Ge film on cube-textured Ni(001) foils through CaF2 buffer layer

Cube-textured Ni(001) foils have been considered as a viable alternative substrate to grow high quality functional films for large area optoelectronic devices. In this work, we report the heteroepitaxial growth of CaF2(001) films on cube-textured Ni(001) foils at 350–600°C with in-plane orientation of CaF2[110]//Ni[100] and CaF2[1-10]//Ni[010] with 45° rotation respect to the Ni(001) substrate. Unlike CaF2(111)/Ni(001) films where there exist four independent rotational domains with rotational domain boundaries, CaF2(001)/Ni(001) contains no rotational domains or rotational domain boundaries. This makes CaF2(001)/Ni(001) films better candidates as templates for the growth of high quality functional semiconductors. We also demonstrate that Ge(001) film with no rotational domains and with a grain size of ~50μm similar to that of the Ni substrate can be grown on the CaF2(001) buffered Ni substrate.

Initial Stages of High-Temperature CaF&lt;SUB&gt;2&lt;/SUB&gt;/Si(001) Epitaxial Growth Studied by Surface X-Ray Diffraction

Surface X-ray diffraction was applied to study structure of the fluorite-silicon interface forming upon epitaxial growth of CaF2 on Si(001) surface kept at 750 degrees C. Samples with CaF2 coverage of 1.5-4 (110)-monolayers were grown and in-situ characterized using synchrotron radiation. The 3 x 1-like surface reconstruction was observed in agreement with the previous studies by electron diffraction. Interestingly, a well pronounced splitting of the fractional x 1/3 reflections was revealed. This splitting was ascribed to the effect of antiphase domain boundaries in the row-like structure of the interface layer. The in-plane integrated intensities were used to reconstruct two-dimensional atomic structure of the high-temperature CaF2/Si(001) interface.

Formation of nanoscale clusters during the initial stages of CaF2 growth on miscut Si(111)

The initial stages of high temperature CaF2 growth by molecular beam epitaxy on Si(111) substrates with a 3° miscut were characterized using atomic force microscopy and low energy electron diffraction. At a growth temperature of 750 °C, electron diffraction measurements showed that the surface retained the (3×1) surface reconstruction up to a deposition thickness of at least 1.2 nm. The overall topography of the surface was defined by atomically flat terraces decorated with a large number of clusters. These clusters were confined to step edges and were typically 10–20 nm tall. The clusters appear to nucleate at the top of step edges and then grow in size until they extend across the step onto the neighboring terrace below. These results indicate that in this growth regime, the CaF2 molecules diffuse across terraces to aggregate into relatively large nanostructures after the formation of a thin wetting layer. The unusually rounded features and large heights seen in these clusters appear to arise from the topography of substrate terraces.

Study of the effect of initial conditions on the scaling properties of CaF 2 thin films

The growth of thermally deposited CaF 2 films was studied using three different substrates for deposition: glass, gold and silicon. Each substrate was chosen because of its different topography and used to determine the effect of substrate roughness on the growth of CaF 2 films. After thermally depositing a range of CaF 2 film thicknesses on the substrates, the CaF 2 surfaces were imaged using atomic force microscopy. The images were then used to determine the characteristic exponents which described the surface. In each case the Hurst exponent, H was found to rapidly increase from the initial substrate condition to a constant value ( H ≈ 0.85) with increasing CaF 2 film thickness. This rapid crossover is quite remarkable and occurs in films with nominal thicknesses less than ≈20 nm. These data indicate that the roughness of the substrate, or in other words the initial conditions, have little effect on the growth properties of CaF 2 films beyond the crossover at very small values of the film thickness. The scaling of the dynamic exponent, β, is also presented as are measurements of the CaF 2 film porosity.

Room temperature negative differential resistance of CdF2∕CaF2 double-barrier resonant tunneling diode structures grown on Si(100) substrates

The authors have demonstrated the crystal growth of CaF2∕CdF2∕CaF2 multilayered heterostructures on Si(100) substrates as double-barrier resonant tunneling diode structures by a low-temperature growth technique. Current-voltage characteristics were investigated and the authors observed negative differential resistance (NDR) characteristics at room temperature. The peak-to-valley current ratio was 2–8, and 13 at maximum, and peak current density was 80–90A∕cm2. The quantum-well layer thickness dependence of NDR peak voltages is also discussed on the basis of qualitative analytical model using the Esaki-Tsu formula.

Oriented growth of large size calcium fluoride single crystals for optical lithography

We report (1 1 1), (1 1 0) and (1 0 0) growth of CaF 2 by the vertical Bridgman method. Crystals up to 250 mm diameter were grown and various growth parameters such as growth rate, temperature gradient and post-growth cooling rate were studied. It was found that the growth rate and the cooling rate are slower for the larger diameter crystals with a fixed temperature gradient. These growth parameters were optimized for growing the crystals along specific orientation after realizing that CaF 2 has a tendency to grow along an orientation close to 〈1 1 0〉. Degradation in optical transmittance was evaluated by irradiating the crystal to γ -rays up to a dose of 10 5 rad. Optimized scavenger addition resulted in crystals with better radiation resistance and excellent VUV transmittance.

Epitaxial growth of CaF 2(1 1 1) on Cu(1 1 1) visualized by STM

The initial stages of the growth of CaF 2 on Cu(1 1 1) have been studied by STM as a function of deposition temperature and coverage. No evidence for a reaction between the CaF 2 molecules and the Cu substrate has been found. The islands formed at 700 K and above show uniform heights, triangular shapes with two orientations rotated by 60° and flat top surfaces. They can not be imaged at voltages below +3 V indicating their insulating character. The LEED pattern shows that the film is epitaxial, (1 1 1)- oriented and in registry with the substrate. The observations indicate that epitaxial growth of twinned crystallites of CaF 2(1 1 1) takes place on Cu(1 1 1).

Metastable He deexcitation at semiconductor interfaces

A review is given of the application of metastable deexcitation spectroscopy (MDS) to the study of the interface formation between semiconductors and different materials. In particular we present an overview of the results obtained on nanostructured interfaces, where strain and reaction between the substrate and the overlayer atoms drive the growth mode and the morphology of the system. As prototypical examples we discuss the growth of CaF2 on silicon and rare earths (Yb,Er) on silicon and gallium arsenide. The mechanisms and chemical reactions which bring to interface formation are examined on the basis of MDS results and their comparison with photoemission.

Scaling behavior of island density in submonolayer growth of CaF 2 on vicinal Si(1 1 1)

We have studied the scaling behavior of two-dimensional island density during submonolayer growth of CaF 2 on vicinal Si(1 1 1) surfaces using scanning tunneling microscopy. We have analyzed the morphology of the Si(1 1 1) surfaces where CaF 2 partial monolayers with coverages of about 0.1 monolayer are deposited at ∼600 °C. The number density of terrace nucleated islands increases with substrate terrace width l as ∼ l 4 in a low island density regime. This scaling behavior is consistent with predictions for the case of the irreversible growth of islands.

Growth of CaF 2 block and dome by gradient freeze technique

Calcium fluoride (CaF 2) crystals were grown by gradient freeze technique (GF), and especially a novel shaped crystal growth method was presented to grow dome-shaped crystal. A calcium fluoride dome was successfully grown directly in the shaper we designed. The obtained CaF 2 block and dome demonstrate a high integrity and optical quality. The defects and crystallographic properties were characterized by chemical etching and the FWHM of the rocking curve respectively.

Initial Stage of CaF2/Si(111) Epitaxy Investigated by Lateral/Atomic Force Microscopy

The initial stages of the high temperature (~ 700°C) MBE growth of CaF 2 on well-oriented Si(111) substrates have been studied by atomic force microscopy (AFM) and lateral force microscopy (LFM) in the atmosphere. At these temperatures CaF 2 molecules react with the silicon surface and form a CaF 1–Si interface layer. The CaF 1 layer covers the silicon surface completely before CaF 2 islands start to form, and it is stable in the atmospheric environment for some days. The subsequent growth on this interface layer results in multilayer CaF 2 islands (about 5 TL in height). The friction force experienced by the Si tip was found to be larger on the CaF 1 interface layer than that on the CaF 2 layer and allows a direct distinction between the CaF 1–Si interface layer and CaF 2. Two different types of islands are observed. One has a regular shape with a flat top surface; the other has an irregular shape and exhibits a 0.3–0.6-nm-high corrugation. We suppose that the formation of disturbed islands can be attributed to the dissociation of the CaF 1 interface at the growth temperature of 700°C. These results suggest that high growth temperatures (&gt; 600°C) do not always result in the best CaF 2 film.

Initial stages of CaF 2/Si(1 1 1) epitaxy investigated by friction force microscopy

The initial stages of high temperature growth of CaF 2 on Si(1 1 1) substrates have been studied by in situ scanning tunnelling microscopy and ex vacuo atomic force microscopy. At these temperatures, CaF 2 molecules react with the silicon surface and form a CaF 1–Si interface layer, which completely covers the surface before CaF 2 start to grow. Using the material contrast ability of lateral force microscopy (LFM) a direct distinction between the CaF 1–Si interface layer and CaF 2 can be achieved. CaF 2 decorates substrate steps from both sides in a non-uniform way; some terraces are nearly completely covered, some not at all. Additionally, isolated islands are observed on the terraces. The LFM signal shows a contrast between CaF 2 and two types of CaF 1. The CaF 1 phase with the highest friction appears rough in AFM topography. This roughness contrast is also observed in UHV by STM. Thermal decomposition of parts of the CaF 1 layer could be responsible for this effect.

Substrate-step-induced effects on the growth of CaF 2 on Si (111)

The growth of CaF2 on vicinal Si (111) surfaces was studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for the temperature range from 300 to 750 °C. In the submonolayer range a transition from terrace to step nucleation is observed for increasing temperature. The first monolayer grows in the step-flow growth mode since second layer islands are not nucleated before completion of the wetting layer. For the subsequent growth of CaF2 on the CaF interface layer, substrate-induced steps do not act as preferential nucleation sites, but rather as growth barriers. Thus CaF2 films grow very inhomogeneously at high temperatures. At lower deposition temperatures, the film homogeneity increases due to the increased (homogeneous) nucleation rate on terraces. The lattice mismatch leads to (lateral) relaxation of thicker CaF2 film close to substrate steps. In addition, CaF2 self-decoration effects are caused by the relaxed regions close to the film steps at temperatures above 500 °C.

Epitaxy of atomically flat CaF 2 films on Si(1 1 1) substrates

The growth of CaF 2 films with a thickness of approximately 3–4 nm on well-oriented Si(1 1 1) substrates by molecular beam epitaxy at temperatures between 410 and 560 °C were investigated by ex vacuo atomic force microscopy. Layer-by-layer growth producing atomically flat CaF 2 surfaces has been observed in a very narrow growth temperature window between approximately 430 and 470 °C. Perfect triangular shaped islands of one CaF 2 layer height are found on the surface with all corners aligned with the Si 11 2 ̄ directions, indicating a pure B-stacking of the CaF 2 film. Surprisingly, also the substrate steps have been overgrown without visible defects. Below 410 °C, two different island orientations revealed a mixture of A- and B-stacking areas in the films. Above ∼520 °C non-wetting of the CaF interface layer leads to epitaxial films with a rough surface morphology.

Formationof CaF2 nanostructures on Si(001)

Initial stages of epitaxial growth and formation ofCaF2 nanostructures on Si(001) were studied. A variety ofnanostructures were grown including ultrathin two-dimensional layersat 750 °C, quasi-one-dimensional stripes at 650 °C andwell-ordered dots at lower growth temperatures. Atomic forcemicroscopy and reflection high-energy electron diffraction were usedto measure the nanostructure shape and lattice orientation. Theevolution of the surface electronic structure under different growthconditions was studied by ultraviolet photoelectron spectroscopy andmetastable de-excitation spectroscopy. The leading role of thewetting layer in high-temperature formation of the fluorite-siliconinterface was established.

MBE growth of calcium and cadmium fluoride nanostructures on silicon

Molecular beam epitaxy (MBE) growth of CdF2 and CaF2 layers and nanoscale heterostructures on Si(111) and Si(001) substrates has been investigated. The surface morphology of CaF2 and CdF2 layers on Si(111), studied by atomic force microscopy (AFM) was found to depend strongly on the growth temperature. The high temperature 2D growth mode would produce atomically flat surface. In contrast to that, well-pronounced pyramids indicating the 3D growth mode were observed on the layers grown at low temperature. Short-period CdF2–CaF2 superlattices (SLs) were also studied. Analysis of reflection high energy electron diffraction (RHEED) intensity oscillations showed asymmetry of the “CaF2 on CdF2” and “CdF2 on CaF2” interfaces resulting in SL surface smoothening. The possible mechanisms relevant to this phenomenon have been discussed.Atomic force microscopy (AFM), and RHEED have been used to study formation of CaF2 epitaxial nanostructures on Si(001). Different types of nanostructures have been grown including ultra-thin 2D wetting layer, quasi 1D wires at high growth temperature and well-organized dots at lower temperatures. An important role of the wetting layer in the transformation of the surface morphology at the initial stages of CaF2 growth on Si(001) at high temperatures has been demonstrated.