Lithium Fluoride Films Research Articles

Defect generation in low-energy ion-assisted thermal deposited lithium fluoride films

We present first results concerning stable formation of primary electronic defects and lithium nanometer sized clusters in LiF thin films grown by ion-assisted thermal deposition. The optical and morphological properties of the as grown LiF films, dependent on the deposition conditions, such as ion-beam energy and ion species (Xe, Ar), are reported. The experimental results show a larger efficiency of low-energy Xe ions in inducing the formation of lithium nano-clusters. To analyse the role of the deposition conditions, a preliminary interpretation of the lithium nano-cluster formation mechanism based on the spherical and/or cylindrical spike thermal model is given.

Optical absorption and photoluminescence studies on thin films and bulk crystals of LiF under swift heavy ion irradiation

Thermally grown thin film and bulk crystals of lithium fluoride (LiF) were irradiated by swift heavy ions to understand the color center production mechanism and their transformation to more complex defects. These samples have been characterized by optical absorption and photoluminescence (PL) under 442 nm excitation at room temperature by HeCd laser. We observed that bulk crystals show relatively very intense PL bands at around 530 and 665 nm , which correspond to the emission of F 3 + and F 2 centers, respectively, as compared to the thin films at the same electronic excitation and ion fluence. Detailed study on bulk LiF has been performed at different electronic excitations and ion fluences for understanding the ion matter interaction under these dense electronic excitations.

The structure and composition of lithium fluoride films grown by off-axis pulsed laser ablation

Alkali halide coatings have been reported to act as effective dipole layers to lower the surface work function and induce a negative electron affinity of diamond surfaces. Here, the results of the analysis of films grown on silicon and quartz substrates by 193 nm pulsed laser ablation from a commercially available sintered disk of LiF are reported. The morphology, composition and crystallinity of films grown are examined and suitable deposition parameters for optimising the growth are suggested. The ablation was shown to be very efficient at removing a large amount of material from the target, even at relatively low fluence. The morphology of the films produced was poor, however, with a high density of asperities categorised as either particulates produced by exfoliation, or as droplets produced by hydrodynamic sputtering. An improved morphology with smaller droplets and fewer particulates could be produced by mounting the substrate at an angle of 65° to the axis of the ablation plume and using a fluence close to the measured ablation threshold of 1.2±0.1 J/cm 2. The elemental composition of the films was shown to be indistinguishable from that of bulk LiF, despite evidence for significant recondensation of Li back onto the target. Films containing crystal grains oriented with the 〈1 0 0〉 direction normal to the substrate surface were observed at substrate temperatures in excess of 300 ° C. An improved extent of orientation was observed on the quartz substrates.

Concentration of F2 and F3+ defects in He+ implanted LiF crystals determined by optical transmission and photoluminescence measurements

Abstract Lithium fluoride (LiF) crystals and films treated with ionizing radiation are promising materials to realize light emitting devices in the visible spectral range. We studied the ion dose dependence of the concentration of F 2 and F 3 + active defects, both absorbing at about 450 nm and separately emitting in the visible range, in He + implanted LiF crystals. Quantitative information was deduced from transmission measurements by using a specially developed theoretical model to fit them, and was compared with the results obtained from photoluminescence spectra. Their qualitative agreement confirms the validity of our theoretical approach, which provides a simple and versatile tool of investigation of these peculiar active defects.

Leaky modes in lithium fluoride thick films thermally evaporated on glass

Polycrystalline thick lithium fluoride (LiF) films have been grown by thermal evaporation on heated glass substrates. Excellent agreement between the values obtained from effective index measurements carried out by the m-line technique and those provided by computer simulations has shown that leaky modes are supported by these dielectric films, as they have a gradually increasing refractive index distribution along the thickness moving from the substrate interface towards the air interface. In some cases the films present also optical anisotropy which results in TM modes having higher effective indices than TE ones.

Effects of F2 Color Center Spatial Distribution on the Emission Properties of Optical Microcavities Based on LiF Films

Highly stable F2 color centers are very efficiently produced in lithium fluoride (LiF) by electron beam irradiation at room temperature. We have fabricated optical microcavities in which the active medium is a low-energy electron beam irradiated LiF film, whose optical thickness is comparable with the peak wavelength (∼668nm) of the F2 broad photoluminescence band. By selecting the proper electron beam energy, one can control the F2 color center depth distribution. This distribution influences the photoemission angular distribution of the microcavity, whose resonance properties are determined by the coupling of the depth profile of the defects with the pump electromagnetic field and microcavity modes.

Lithium fluoride films and crystals containing metallic colloids studied by scanning near-field optical microscopyPresented at the LANMAT 2001 Conference on the Interaction of Laser Radiation with Matter at Nanoscopic Scales: From Single Molecule Spectroscopy to Materials Processing, Venice, 3–6 October, 2001.

LiF films and crystals containing lithium colloids have been studied by scanning near-field optical microscopy (SNOM). Scattering phenomena and possible wave-guided modes have been observed studying the physical mechanism of the interaction between the LiF samples and an external laser source.

Miniaturized structures based on color centers in lithium fluoride: Optical properties and applications

Research activities concerned with color centers in alkali halide films started recently. The use of versatile, well-assessed, and low-cost fabrication techniques consisting of physical vapor deposition of Lithium Fluoride (LiF) films combined with direct writing lithographic processes allows the realization of miniaturized structures, like broad-band emitters, channel waveguides, optical microcavities and point-light sources emitting in the visible spectral range. Promising results have been obtained in the generation, amplification and waveguiding of visible light in LiF treated by low energy electron beams, where the efficient formation of stable primary and aggregate color centers also induces a local modification of the refractive index. A brief overview of the investigated optical properties is presented together with a short discussion about their perspectives of applications in optoelectronics.

Luminescent patterns based on color centers generated in lithium fluoride by extreme ultraviolet radiation and soft X-rays

Primary and aggregate color centers in lithium fluoride (LiF) crystals and polycrystalline LiF films were produced by an innovative irradiation technique using extreme ultraviolet radiation and soft X-rays generated by a laser-plasma source. This irradiation facility allowed the efficient formation of active color centers on luminescent patterns with submicron spatial resolution on large areas and short exposure times. The method looks promising for the realization of low-dimensionality photonic devices. The optical characterization of the colored structures was performed by means of absorption and photoluminescence measurements on LiF samples colored under different irradiation conditions.

Broad band emitting color centers in lithium fluoride films for optical microcavities

We have fabricated for the first time different optical microcavities made of two Bragg mirrors sandwiching a lithium fluoride film treated by low-energy electrons to create color centers. Among the so-formed defects, we focused our attention on the F2 centers, which can be optically excited around 450 nm and give rise to an efficient broad-band luminescence centered at ∼ 670 nm in the red spectral range at RT. We have modified the design of the upper reflector of the microcavities in order to shift their resonant wavelength from λres ≈ 665 nm to λres ≈ 640 nm. We get evidence of the different optical behaviors from measurements of the photoluminescence spectra of the radiation emitted perpendicularly to the multilayer surface.

Investigation of visible laser active colour centres in LiF films by confocal fluorescence microscopy

Low energy electron irradiation of lithium fluoride (LiF), in the form of bulk crystals and films, gives rise to the stable formation of primary and aggregated colour centres in a thin layer located at the surface of the investigated material. A confocal light scanning microscope in fluorescence mode was used to reconstruct the depth distribution of visible emitting aggregate defects in narrow stripe-like coloured regions induced by electron beam lithography on LiF films thermally evaporated on glass. The formation of the F3 + and F2 aggregate defects appears restricted to the electron penetration and proportional to their energy depth profile, as obtained from Monte Carlo simulations. The influence of the polycrystalline nature of the investigated structure is briefly discussed.

Spontaneous emission properties of color centers based optical microcavities

Optical microcavities based on lithium fluoride films treated by low-energy electrons to create visible-emitting F 2 color centers have been fabricated, and their radiative properties characterized for the first time. By tuning the photon cavity mode to the maximum of the luminescence band for the F 2 centers (∼670 nm), spectral narrowing, peak-intensity enhancement of the emission band as well as a highly directional radiation pattern have been observed comparatively for experiments performed on a half-cavity and a full microcavity. Spontaneous emission decay times have been measured, and a shortening of lifetime by the cavity has been observed.

Broad-band active channels induced by electron beam lithography in LiF films for waveguiding devices

For the first time, a confocal light scanning microscope (CLSM) in fluorescence mode was used to reconstruct the depth distribution of efficiently emitting laser-active color centers (CCs) in a stripe-like region induced by 12 keV electrons on lithium fluoride (LiF) films thermally evaporated on glass. The formation of the F+3 and F2 aggregate defects appears restricted to the electron penetration and proportional to their energy depth profile, as obtained from a Monte Carlo simulation.

Infrared Spectroscopy of LiF on Ag and Si

We present the results of infrared optical studies of thin lithium fluoride films evaporated on silver and silicon substrates. Polarized oblique-incidence reflectance and multiple-angle-of incidence FTIR ellipsometry has been used to study the spectral range of longitudinal optic (LO) lattice vibrations. We discuss in detail the lineshapes, and use the pronounced difference of the optical response of metallic and insulating substrates to elucidate their origin. In particular, we model the field distribution in the stratified structures, and specify the spatial distribution of the absorbed energy.

Structural properties and photoluminescence spectra of coloured LiF films on SiO 2

Lithium fluoride films, about 2.6 μm of thickness, were thermally evaporated onto a 10 μm thick SiO 2 film grown on silicon 〈1 0 0〉 substrate. Coloured stripes in the LiF films were created by low energy electron (12 keV) beam lithography. Intense photoluminescence (PL), extending from green to red in the visible spectral range, was measured at room temperature due to the presence of the laser active F 2 and F 3 + colour centres. Atomic force microscopy (AFM) analysis was used to study the film morphology inside and outside the irradiated areas. A confocal microscope was used to observe the surface layer of the film.

On the growth of LiF films by Pulsed Laser Deposition

The production of Lithium fluoride (LiF) films by Pulsed Laser Deposition is reported for the first time. The influence of several deposition parameters such as the laser energy density, the presence of a gas pressure (10 −1 mbar of He) and the substrate temperature on the film quality is studied by using in-situ reflectivity measurements, Scanning Electron Microscopy and X-ray diffraction. Films deposited in vacuum are polycrystalline and fully textured along the 〈100〉 orientation, whereas those grown in He pressure present a more complicated structure. Films are generally rough, the roughness decreasing as the substrate temperature increases or the laser energy density decreases. The origin of this roughness is discussed in terms of the ablation mechanism taking place at the target.

Photoemission spectroscopy of LiF coated Al and Pt electrodes

Thin lithium fluoride (LiF) interlayers between the low work function electrode and the electron transport layer in organic light emitting diodes (OLED) result in improved device performance. We investigated the electronic structure of LiF coated Al and Pt electrodes by x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Thin LiF films were grown in several steps onto Ar+ sputtered Al and Pt foils. After each growth step the surfaces were characterized in situ by XPS and UPS measurements. After evaluating band bending, work function and valence band offset for both samples, their band lineups were determined. Our measurements indicate that despite the insulating character of LiF in both samples, band bending is present in the LiF layer. The difference in band bending between the samples allows the conclusion that the driving force for the development of the band bending results from the contact potential between the metal and the LiF overlayer. The band bending is most likely caused by a redistribution of charged Frenkel or Schottky type defects within the LiF layer. The work function of both samples after LiF deposition was dramatically lowered compared to the values obtained on the clean sputtered metal surfaces.

Intense visible photoluminescence from coloured LiF films on silicon

Lithium fluoride films, about 1.5 μm thick, thermally evaporated onto silicon <100> substrates were subjected to low energy electron bombardment. Intense photoluminescence, extending from green to red in the visible spectral range was measured at room temperature in the irradiated areas, due to the presence of the laser active F2 and F3+ colour centres.

Thin reactive LiF films for nuclear sensors

Lithium fluoride (LiF) films are deposited by thermal evaporation on silicon substrates. The substrate temperature and deposition rate's influence on the morphology and structure of the films are investigated. It is shown that it is necessary to work with a low deposition rate to have crack-free layers. Also, the substrate temperature has a great influence on coating morphology: the grain size increases with temperature. At the same time, the crystallization state increases up to 300°C. For higher temperatures, recrystallization seems to appear. The grain size increases as well as the grain boundary size.

Ion divergence in magnetically insulated diodes

Magnetically insulated ion diodes are being developed to drive inertial confinement fusion. Ion beam microdivergence must be reduced to achieve the very high beam intensities required to achieve this goal. Three-dimensional particle-in-cell simulations [Phys. Rev. Lett. 67, 3094 (1991)] indicate that instability-induced fluctuations can produce significant ion divergence during acceleration. These simulations exhibit a fast growing mode early in time, which has been identified as the diocotron instability. The divergence generated by this mode is modest, due to the relatively high-frequency (≳1 GHz). Later, a low-frequency low-phase-velocity instability develops with a frequency that is approximately the reciprocal of the ion transit time. This instability couples effectively to the ions, and can generate unacceptably large ion divergences (≳30 mrad). Linear stability theory reveals that this mode has structure parallel to the applied magnetic field and is related to the modified two-stream instability. Measurements of ion density fluctuations and energy-momentum correlations have confirmed that instabilities develop in ion diodes and contribute to the ion divergence. In addition, spectroscopic measurements indicate that lithium ions have a significant transverse temperature very close to the emission surface. Passive thin-film lithium fluoride (LiF) anodes have larger transverse beam temperatures than laser-irradiated active sources. Calculations of the ion beam source divergence for the LiF film due to surface roughness and the possible loss of adhesion and fragmentation of this film are presented.