Thin CaF Research Articles

Driven electronic bridge processes via defect states in Th229 -doped crystals

The electronic defect states resulting from doping $^{229}$Th in CaF$_2$ offer a unique opportunity to excite the nuclear isomeric state $^{229m}$Th at approximately 8 eV via electronic bridge mechanisms. We consider bridge schemes involving stimulated emission and absorption using an optical laser. The role of different multipole contributions, both for the emitted or absorbed photon and nuclear transition, to the total bridge rates are investigated theoretically. We show that the electric dipole component is dominant for the electronic bridge photon. In contradistinction, the electric quadrupole channel of the $^{229}$Th isomeric transition plays the dominant role for the bridge processes presented. The driven bridge rates are discussed in the context of background signals in the crystal environment and of implementation methods. We show that inverse electronic bridge processes quenching the isomeric state population can improve the performance of a solid-state nuclear clock based on $^{229m}$Th.

The neutron component of two high-energy photon reference fields

The 4.4 MeV photon reference field described in ISO 4037 is produced by the (12)C(p,p')(12)C (E(x) = 4.4389 MeV) reaction using a thick elemental carbon target and a proton beam with an energy of 5.7 MeV. The relative abundance of the isotope (13)C in elemental carbon is 1.10%. Therefore, the 4.4 MeV photon field is contaminated by neutrons produced by the (13)C(p,n) (13)N reaction (Q = -3.003 MeV). The ambient dose equivalent H*(10) produced by these neutrons is of the same order of magnitude as the ambient dose equivalent produced by the 4.4 MeV photons. For the calibration of dosemeters, especially those also sensitive to neutrons, the spectral fluence distribution of these neutrons has to be known in detail. On the other hand, a mixed photon/neutron field is very useful for the calibration of tissue-equivalent proportional counters (TEPC), if this field combines a high-linear energy transfer (LET) component produced by low-energy neutrons and a low-LET component resulting from photons with about the same ambient dose equivalent and energies up to 7 MeV. Such a mixed field was produced at the PTB accelerator facility using a thin CaF(2) + (nat)C target and a 5.7 MeV proton beam.

Composition and morphology studies of ultrathin CaF 2 epitaxial films on silicon

Thin CaF 2 films with thicknesses between 1 and 100 nm were grown on Si(1 1 1) at UHV conditions by evaporation from a CaF 2 source. The correlation of film composition and growth morphology with the deposition parameters was studied by various techniques such as heavy-ion elastic recoil detection analysis (ERDA), RBS/channeling, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and auger electron spectroscopy (AES). The composition and morphology of the CaF 2 films strongly depend on the substrate temperature during deposition. Epitaxial growth is observed at deposition temperatures higher than 450°C. The F/Ca ratio of the films in the monolayer range is found to deviate appreciably from the stoichiometric composition (F/Ca=2) suggesting that the interface composition does not obey the rules of bulk equilibrium chemistry. AES results show the formation of different bonding configurations (in particular of silicide-like bonds) at the CaF 2/Si interface.

Ion beam irradiation of thin CaF 2 films: a study of lithographic properties

The technological track to ever smaller devices in very- and ultra-high-scale integration schemes with feature sizes lower than 1 μm, carries photolithographic methods to physical limits, and is claiming a transition to lower wavelength or particle irradiation. New resist materials and exposure techniques are under study. Earth alkaline fluorides, such as CaF 2, BaF 2, and SrF 2, decompose under electron and ion irradiation by desorbing fluor, and oxidizing the remaining earth alkaline metal under the presence of oxygen. The solubility of the oxidized areas is then modified. The present paper deals with resist properties of CaF 2 under ion beam exposition with Ar +-, He +-, Si +- and O +-ions. RBS measurements are applied in order to determine modifications in the film composition. Positive and negative resist behavior is found in water and dilute HCl, respectively. Both the change of chemical composition and the pattern characteristics correlate with the energy deposition of implanted ions. Characteristic lithographic curves are shown.

Study of adsorption on radiation-damaged CaF 2(111) surfaces

Thin CaF 2(111) films (60–130 Å) have been grown in ultra-high vacuum by vapor deposition on Si(111)-(7×7) surfaces. Damage by low-energy (50 eV) electrons and the subsequent adsorption of small molecules (O 2, CO 2, N 2, C 2H 6, and C 2H 4) have been examined using photoemission and electron energy loss spectroscopies. The damaged surface is highly reactive with O 2 and CO 2. The initial rapid uptake of O saturates at exposures of ≤10 Langmuirs (L), leading to the attenuation of a damage-induced defect peak at 1.8 eV in the energy loss spectrum and the appearance of a feature indicating Ca–O bonding in the surface-sensitive Ca 3p photoemission spectrum. The O 1s and valence-band photoemission data indicate two distinct states of chemisorbed O differing in valence charge density. No chemisorption on the damaged surface is seen for ≤10 3 L of N 2 or ≤300 L of C 2H 6. For C 2H 4, deposition of C is shown by the appearance of the C 1s in photoemission and of the 6.6 eV energy loss peak characteristic of graphitic C.

RHEED studies of MnF 2 epitaxial growth on Si(111) substrates

Molecular beam epitaxy was used to grow MnF 2 films on a Si(111) substrate with a thin CaF 2 buffer at room temperature. It was found that films thinner than three molecular layers have a cubic fluorite crystal structure inherited from CaF 2 . The MnF 2 film is coherent to the substrate and has an interlayer spacing of 0.305 ± 0.005 nm, which is 3% less than for bulk CaF 2 . Thicker films have the tetragonal rutile structure, so the film surface is the (110) plane of MnF 2 with its [001] axis parallel to (110) directions of the substrate.

Non-local effects near the longitudinal optical frequency in thin CaF 2 films

The transmission of infrared radiation through nanometer-thick layers epitaxially grown on Si(111) substrates has been measured near the longitudinal optical frequency ω LO. We show that a correct interpretation of the experimental spectra must take into account the spatial distribution of the surface charges and that the spectra obtained through non-local calculations including the dispersion of the longitudinal optical (LO) mode in the Γ– L direction compare reasonably well with the experiments.

Development of a high-resolution alpha camera

We have developed and tested a high-resolution alpha camera. The alpha camera consists of a thin CaF 2(Eu) scintillator, a tapered fiber optics plate (tapered fiber), a position-sensitive photomultiplier tube (PSPMT) and a position calculation circuit. Though this configuration was originally planned as a high-resolution beta camera for direct measurement of positron distribution over brain surface of animals, we found that, with some modifications, it could also image alpha particles. The scintillator is covered by a thin aluminized mylar to protect it from light while allowing the transmission of alpha particles. The tapered fiber is a key component of the camera. The alpha-induced scintillation image is magnified by the tapered fiber with little light loss. Resolution of the alpha camera can be improved by approximately the magnification factor of the tapered fiber, though the field of view of the camera is decreased. We developed a 20 mm diameter field of view alpha camera. Energy resolution of the camera was 30% full-width at half-maximum (FWHM) for 241Am alpha particles (5.5 MeV). Spatial resolution for 241Am alpha particles was better than 0.6 mm. We confirmed that this alpha camera can be used for high-resolution alpha autoradiography or other alpha imaging applications.

Molecular beam epitaxial growth of thin CaF 2 films on vicinal Si(111) surfaces

The growth of CaF 2 on Si(111) 7 × 7 surface at ∼ 770°C and ∼ 250°C on both ∼ 2° and ∼ 0.5° off-normal, vicinal substrates (titled towards the [11 -2] azimuth) has been studied using RHEED, AFM and SEM. For growth at ∼ 770°C on both substrates, the CaF 2 grows in a layer-by-layer fashion for the first two monolayers which gives rise to RHEED intensity oscillations. Beyond two monolayers, relatively thick CaF 2 islands nucleate at the Si step edges (which are bunched into step bands) and grow laterally with constant height (∼ 5 nm for ∼ 2° and ∼ 2 nm for ∼ 0.5° miscut substrates) along the Si step edges and eventually form a flat overlayer. The height of the CaF 2 islands appears to be determined by the height of the Si step bunches which, in turn, is determined by the substrate miscut angle. For growth at ∼ 250°C, the integral diffraction spot RHEED intensity decreases exponentially for the first ∼ 2 monolayers of growth and then oscillates for ∼ 6 periods. In addition, the non-integral diffraction spots, corresponding to the Si(111) 7 × 7 pattern, are not fully removed until the integral diffraction spot intensity starts to oscillate at ∼ 3 monolayers. These results indicate that the CaF 2 initially grows by nucleation and coalescence of 2D islands which are 2 to 3 monolayers high, followed by a multilayer-by-multilayer mode which leads to a featureless, flat surface morphology.

A performance study on a phoswich detector consisting of an inner NE213 scintillator and an outer CaF 2(Eu) crystal wall

We developed a new phoswich detector. The detector consists of an inner NE213 organic liquid scintillator and an outer thin CaF 2(Eu) crystal which are coupled to a photomultiplier. The charged particles produced in the NE213 scintillator by the interaction of radiation with the detector either stop in the NE213 scintillator or pass through both the NE213 scintillator and the outer CaF 2(Eu) crystal. The fast output pulses of a few ns from the NE213 scintillator can easily be separated from the slow output pulses of about 1 μs from the outer CaF 2(Eu) scintillator. From this difference, the events that the charged particle stops inside NE213 scintillator can be separated out from the events that the charged particle escapes from the inside NE213 detector. The phoswich detector response functions for gamma rays and neutrons measured with the radioactive gamma-ray sources and the neutrons produced by a Be + Cu target bombarded by protons from the cyclotron, and compared with Monte Carlo calculations. It was found that the main characteristics of the NE213 scintillator were not degraded by assembling as a phoswich detector using the CaF 2(Eu) crystal wall. The n-γ discrimination property is slightly degraded but neutrons can be separated from gamma rays. Further we used to test the phoswich detector for neutron measurements under the intense charged particles, mainly protons, with considering the possibility of neutron detection in space crafts. It was shown that the phoswich detector is capable of detecting neutrons in the mixed field.

Kinetics of chemical beam epitaxy for high quality ZnS film growth

A comprehensive study is reported of the chemical and metalorganic molecular beam epitaxy ( CBE MOMBE ) growth of ZnS on Si and GaAs(100) substrates for applications in advanced electroluminescent displays and for optoelectronic device integration on Si. Growth kinetics studies of conventional and photoassisted MOMBE and CBE using diethylzinc (DeZn) with H 2S, di-isopropylsulfide (DipS) and a novel precursor, t-butyl mercaptan (t-BuSH), are reported. The results obtained indicated that the low temperature growth of ZnS using cracked DeZn and DipS was hindered. This was attributed to the reattachment of the alkyl radicals to surface growth sites, which resulted in a lower growth rate. However, by using uncracked DeZn and H 2S or cracked DeZn and t-BuSH, this growth hindrance was removed by the surface reaction of hydrogen with the alkyl radicals. The addition of a thin CaF 2 buffer layer was found to remarkably improve the growth of ZnS on Si.

Photovoltaic IV-VI on Si infrared sensor arrays for thermal imaging

Photovoltaic narrow-gap IV-VI (lead chalcogenide) infrared sensor arrays on Si substrates have the potential for low-cost infrared focal-plane arrays. The arrays can be bump bonded to readout multiplexers, or be grown on prefabricated active Si substrates containing the whole readout circuits. Sensitivities are similar to that of Hg_1-<i>x</i>Cd_<i>x</i>Te, but processing procedures are much less demanding. This is because the structural quality of even heavily lattice-mismatched IV-VI layers is adequate to fabricate devices with good sensitivities, because 2- to 4-μm layer thickness suffices, and because good homogeneity in ternary Pb_1-<i>x</i>Sn_<i>x</i>Se for the 8- to 12-μm range is much easier to obtain than in Hg_1-<i>x</i>Cd_<i>x</i>Te. New results are presented on the molecular beam epitaxal growth of the layers, including a very thin CaF₂ buffer needed for compatibility reasons, and a new photolithographic patterning technique suited for full wafer processing has been developed to fabricate the sensor arrays. First thermal images using these chips are demonstrated.

Study of CaF 2 growth on Si, a-Si O 2 by in-situ spectroscopic ellipsometry

Ellipsometric measurements have been performed in-situ on very thin CaF 2 films at fixed wavelength λ during deposition or vs. wavelength at different deposition steps. In modelling tan ψ and cos Δ spectra as a function of λ at different stages of growth, the Bruggeman effective medium theory is used to describe the interfaces. The value of the index of refraction n depends strongly on the nature and the temperature of the substrate, and on the thickness of CaF 2 films. For films deposited at 400°C on Si wafers covered with their native oxide, cross-sectional high resolution electron microscopy shows that, in spite of the presence of a SiO 2 layer (2 nm thick), CaF 2 grows on Si through this layer. A model including interfacial layers, which takes into account this interaction with the Si substrate, is proposed for reproducing ellipsometric data. For CaF 2 films deposited on SiO 2, no evidence for chemical interaction has been found.

The determination of the absolute response function of a deuterated benzene total energy detector to 6.13 MeV γ-rays

The γ-ray response function of a 0.4 1 deuterated benzene liquid scintillation detector, used in the measurement of total neutron capture cross sections has been measured at 6.13 MeV and compared with spectra generated by the comprehensive Monte Carlo electron—gamma tracking code EGS-4. The experiment was performed in a nearly monoenergetic photon calibration field generated using the 19F(p, αγ) 16O reaction by bombarding a thin CaF 2/Ta target with protons from the Harwell 6 MV Van de Graaff accelerator. The absolute yield of photons was measured using a 101 cm 3 HPGe γ-ray spectrometer whose efficiency had been previously determined using a combination of radionuclide and thermal neutron capture γ-ray sources. A secondary calibration, based on measurements with a standard 3 in. × 3 in. NaI(Tl) scintillator, gave consistent results. A particularly open irradiation geometry was used so as to minimise the importance of scattered radiation and to enable the geometry to be accurately represented in the computer simulations. Satisfactory agreement between the experiment and the Monte Carlo simulations was only obtained when the transport of secondary radiations produced in the target assembly and other materials close to the detector were included in the calculation. The calculated spectra exhibit rather more fine structure when was actually observed. Subsidiary measurements however, made using intensity calibrated radionuclide sources, indicate that below 1.5 MeV agreement between the predicted and observed response is excellent.

Properties of lattice mismatched IIa-fluorides on silicon

Films of BaF 2 have been grown by molecular beam epitaxy on Si(100) and Si(111) which was covered with thin CaF 2 and/or SrF 2 to obtain perfect epitaxy. The BaF 2 (100) layers grown on SrF 2 /Si(100) using a two-temperature method do not show (111)-facetted surfaces, but smooth BaF 2 (100) surfaces can be obtained despite the high lattice mismatch and preferred (111) growth mode. This indicates that the large free (100)-surface energy of fluorides reduces to a value below 3 1 2 times that of the (111) surface at high growth temperatures. The depth distribution of strains in (111)-oriented stacks has been determined by Rutherford backscattering channelling angular scans. The thermal mismatch strain relaxation leads to a small residual strain across the BaF 2 layers, while steps in the magnitude of the strains occur at fluoride-fluoride and silicon-fluoride interfaces. The layers are intended as epitaxial buffers for heteroepitaxy of lattice and thermal expansion mismatched II–VI and IV–VI compound semiconductors on silicon.

Interface formation and epitaxy of CaF 2 on CoSi 2(111)Si(111)

XPS, UPS and LEED have been used to study the interface formation and epitaxy of very thin CaF 2 films (⩽63 A ̊ ) on thin layers of CoSi 2 (∼ 70 A ̊ ) , formed by solid phase epitaxy on Si(111). XPS core levels (Si 2s, Co 2p, Ca 2p, F 1s) and Si(KLL) Auger transition indicate that both Ca and F are bonded to Si atoms only, whether the CoSi 2 surface is Si or Co-rich. At room temperature CaF 2 grows layer-by-layer on CoSi 2(111), and no ordered surface structure is observed by LEED. XPS valence bands reveal that the electronic structure of CoSi 2 is not affected by the formation of the CaF 2 upper layer. Upon annealing thin layers of CaF 2 in the 550–650°C range, LEED shows good (1 × 1) patterns of CaF 2 (111). The thin epitaxial films are stable up to ∼ 650° C. Above this temperature the CaF 2 valence band undergoesrapid changes indicating a degradation of the insulator layer.

Electron-beam-induced decomposition and oxidation of thin CaF 2-layers on Si(111) Studied by auger electron spectroscopy

The interaction of a 2.5 keV electron beam with MBE-grown CaF 2layers on Si(111) was studied by AES at ambient temperature. The desorption of fluorine is induced by electron bombardment of the films under UHV conditions, leaving behind metallic Ca, which in the presence of oxygen in turn is oxidized to CaO. The kinetics of the reaction is described in terms of a three-step-model involving CaF 2 decomposition, yield of metallic Ca, and a following oxidation. As concluded from the pressure dependence of the reaction rate, the latter is determined by an equilibrium between gaseous and adsorbed oxygen. Auger electron spectroscopy reveals slight differences in the calcium Auger peak in irradiated and oxidized CaF 2 which is attributed to their distinctive electronic structures.

Electron irradiation effects on thin CaF 2 films : F A Koch and R W Vook,J Vac Sci Technol, 10 (1), Jan.Feb 1973, 313–315

Electron irradiation effects on thin CaF 2 films : F A Koch and R W Vook,J Vac Sci Technol, 10 (1), Jan.Feb 1973, 313–315

Störung der 197 keV-γ-wimWinkelverteilung von F − durch innere felder in gasförmigen und metallischen medien

Using the recoil associated with the excitation of the 197 keV level of F 19 by protons, perturbations of the 197 keV γ-angular distribution in tifferent media were explored. The excited nuclei recoiling from a very thin CaF 2 target were stopped in various gases (Ne, Ar, air) or solids (Cu, Au, Ag, Pt, Ni, C). The angular distribution in the experiments with gases was found to be isotropic. This strong perturbation is attributed to a time-dependent hyperfine structure interaction. The time dependence is caused by transitions in the fluorine electron shell during atomic collisions in the gas. The attenuation coefficient G 2 for F 19 nuclei stopped in metals were found to be larger than 0.20 except for nickel.