LiF Thin Films Research Articles

ESD from LiF thin films

Desorption of Li +, F + and H + ions from LiF thin layers during electron stimulation has been measured by means of a time-of-flight (TOF) spectroscope. It was found that the kinetic energy distributions (KED) of the observed ions peaked at −3.1 and −2.2 eV for Li + and F +, respectively. The half-width of KED for metal, as well as for halogen ions, was about 2 eV. The energy onset of desorbed Li + ions is shifted by about 1.2 eV to a higher energy with respect to that of F + ions and by about 1.4 eV with respect to that of H + ions. The relative positions of the energy onset of separate ions are measured directly for the first time. On the basis of a strong relationship between the mass of emitted ion and its maximum kinetic energy, we suggest that the KED is mainly determined by the relaxation time of the lattice and the inertia of the emitted ion.

Texture Analysis of LiF Thin Films Evaporated onto Amorphous Substrates at Different Temperatures

Polycrystalline thin LiF films thermally evaporated on amorphous substrates show different crystallite orientations depending on the substrate temperature during evaporation. The recording of direct diffraction pole figure shows that the LiF crystallites present a single crystal texture, which can be described as a 〈522〉 [uvw] texture for high temperature deposition (250 to 300 °C) and as 〈16 9 7〉 [uvw] texture for low temperature (room temperature) substrates. Scanning electron microscopy shows the presence of a globular film structure both for high and low temperature depositions with an average grain size of about 200 to 250 and 130 to 150 nm for high and low temperature evaporations, respectively. By rising the deposition temperature the 〈100〉 LiF crystallographic direction approaches the normal to the substrate plane with an increase of the film refractive index. Polykristalline dünne LiF-Schichten, die auf amorphe Substrate thermisch aufgedampft wurden, zeigen verschiedene kristallographische Orientierungen in Abhängigkeit von der Substrattemperatur. Die Polfiguren zeigen eine Einkristalltextur für alle Proben. Die dünnen LiF-Schichten, die bei hohen Temperaturen (250 bis 300 °C) aufgedampft worden sind, lassen sich als 〈522〉 [uvw]-Texturen eines kubischen Gitters beschreiben, während bei niedrigen Temperaturen (Zimmertemperatur) eine 〈16 9 7〉 [uvw]-Textur aufgewiesen wird. Rasterelektronenmikroskopie-Aufnahmen zeigen eine globulare Struktur der Schichten unabhängig von der Substrattemperatur, mit einer durchschnittlichen Keimgröße von 130 bis 150 nm für Zimmertemperatur bzw. 200 bis 250 nm für 250 bis 300 °C. Die Steigerung der Aufdampftemperatur bewirkt eine Orientierungsänderung der LiF-Kristalle in Richtung der 〈100〉 [uvw]-Textur und eine Zunahme der Brechzahl.

Investigation on the color center distribution in LiF thin films

The F2 center depth distribution in LiF thin films irradiated by 15 and 30 keV electron beams was obtained experimentally by means of optical absorption measurements of multilayered LiF/KBr films. Monte Carlo simulations of the incidence of electron beams in LiF were also carried out to provide the dissipated energy and the fluor ionization depth profiles for comparison with the experimental data. The ionization and the dissipated energy profiles have essentially the same shape, with their maxima at a depth of 41±5% of the electron range. The F2 distributions could be described by Gaussian distributions centered at depths of 1.5 and 1.6 μm for 15 and 30 keV irradiation, respectively.

Production and characterization of multilayer KCl:LiF thin films on glass

Alkali halide films containing color centers have important applications in information storage, lithography, radiation dosimetry, and solid state lasers. KCl:LiF multilayers have been grown by thermal evaporation on glass substrates at p<10−4 Pa under continuous monitoring of the substrate temperature (Ts=30–350 °C) and deposition rate (Vd=0.3–0.7 nm/s). Optically active point defects have been produced by low energy (3 keV) electron irradiation in these thin layers of dielectric materials. The characterization of the samples has been performed using depth profile x-ray photoelectron spectroscopy for the profile composition, x-ray diffraction and scanning electron microscopy for the structural analysis, and optical absorption for defect formation. At substrate temperatures ranging from 30 to 350 °C the LiF and KCl layers grow with (200) crystallite planes parallel to the substrate surface.

Color centres in crystalline Lif films

Abstract Policrystalline LiF thin films were produced on amorphous substrates at different temperatures. Preliminary optical measurements on F2 and F3 + aggregate color centres produced by electron irradiation were performed.

Experimental study of the relationships between the near-normal reflectance, the optical constants, and the roughness of thin silver films

Independent measurements of the rms roughness and the reflectance have been made on silver films deposited on CaF2, LiF, and MgF2 thin films of various degrees of roughness. These measurements show a linear relationship between the dip in the magnitude of the near-normal reflectance minimum at the plasma energy and the rms roughness δ of the various samples. The autocovariance length of the rough surface turns out to have little influence on the relationship between the magnitude of the reflectance and δ. The relationships between the optical constants of silver films and δ are also investigated.

Surface roughness and fractal nature of thin films of MgF2 and Ag/MgF2

An extensive study of magnesium fluoride as an underlayer is reported. Its behavior and characteristics are compared to those of the calcium and lithium fluorides previously studied. In addition, the possible fractal nature of the MgF2 films is investigated and compared to that of CaF2 and LiF thin films.

Primary ion dependence of LiF direct recoil intensities and ion fractions

Time-of-flight (TOF) spectra of the scattered and recoiled particles resulting from 1–10 keV He+, Ne+, Ar+, Kr+, and Xe+ ions impingent on surfaces of LiF thin films have been obtained. Measurements of directly recoiled (DR) neutrals plus ions and neutrals alone are used to calculate positive and negative ion fractions Y+,− from DR events. The oppositely charged ion fractions have a distinctly different behavior as a function of kinetic energy. The Y+ values exhibit a threshold at low energy followed by a plateau region at higher energy while the Y− values are maximum in the low energy region followed by a decreasing yield as energy increases. The energy dependence of Y+,− is interpreted in terms of the recently developed model [J. Chem. Phys. 85, 3615 (1986)] for electronic charge exchange in keV ion/surface collisions which considers electron promotions in the close atomic encounter and resonant and Auger transitions along the outgoing trajectory. The ionization potential of the primary ion relative to the energy levels of the target atom is shown to have a large influence on charge exchange in the close encounter. The ratio of direct recoil to scattering particle flux increases by a factor of >102 from He to Xe; scattering and recoil cross sections are used to model this process.

Ionic conductivity of LiF thin films containing Di- or trivalent metal fluorides

Evaporated LiF thin films containing M IIF 2 (M II=Mg, Ca, Ni, Cu, Zn, Sr) or M IIIF 3 (M III=Al, Ti, V, Cr, Ga, Y, Ce) of less than 20 mol% are prepared and their electrical conductivity determined. It is found that fluorides of Cu, Ni, Zn, Al, Ti, V, Cr, Ga and Y increase the ionic conductivity of these thin films by up to the order of six, while those of Mg, Ca, Sr and Ce do not. These experimental results are discussed in terms of tendency to form intermediate compounds.

Optical modes of vibration in non-stoichiometric LiF thin film

Thermal emission spectra due to the virtual modes in LiF thin films are observed at high temperature up to 1053K. Anomalous softening of the mode frequency and increase of the mode width are observed above 1000K. The results of the ion micro-analysis indicates that the film heat-treated in nitrogen atmosphere at 1053K is largely shifted from stoichiometry, which is caused by the sublimation of fluorine ions from the film. The spectrum analysis using an infrared dielectric function in which the effects of defects is considered, results in the abrupt increase of the high-frequency dielectric constant with elevating temperature as well as the softening of the frequency and the increase of the damping for both the transverse and the longitudinal optical phonons.

High-temperature optical modes of vibration in LiF thin films (thermal emission spectra)

The thermal emission of a metal-backed thin film of LiF is observed at the emission angle of 20 degrees . The observed spectra measured at elevated temperatures are analysed by using the response function based on the virtual-mode theory for the ionic film in order to derive the temperature dependence of the lattice dynamical quantities, such as phonon frequency and damping. The dielectric function derived from a theory based on the thermodynamic Green function technique is considered in the spectrum analysis. The estimated temperature dependence is discussed in terms of the phonon anharmonicity. The LST relation including the phonon anharmonicity is derived from the virtual-mode theory.

Determination of the optical parameters and packing density of non-absorbing columnar thin films by means of immersion reflectometry: Application to LiF thin films

In this paper a method is described which enables us to perform a complete optical analysis of non-absorbing columnar thin films. the columnar refractive index, packing density and thickness of these films can be determined by means of an interpretation of the spectral dependences of the reflectance of the films. These dependences must be measured in suitable ambients. The method is demonstrated using LiF films deposited onto single-crystal silicon. The reflectance of LiF films was measured in air, toluene and acetone. Using the reported method it is found that the columnar refractive indices of LiF films are identical with those of bulk LiF. Furthermore, the packing densities of LiF films reach those expected. In this paper we also present the surprising fact that a small amount of water is contained in the pores of LiF films, at least at several tens of minutes after deposition.

The aging effect on the dispersion of the refractive index of lithium fluoride in the wavelength range of 460–1000 nm

Transmittance measurements were carried out for LiF thin films directly after preparation, then one day, one week, four weeks and six weeks later. The aging effect on the optical properties of the film was thus studied. This showed that the aged LiF films have refractive indices larger than those of fresh films.