L. C. Scavarda Do Carmo

LIF FILMS : PRODUCTION AND CHARACTERIZATION

Abstract LiF films were produced by thermal evaporation onto amorphous substrates kept (during evaporation) at temperatures ranging from ambient up to 350°C. The evolution through two polycrystalline states was analysed by X-ray diffraction techniques, which also revealed that the LiF film lattice parameter is equal to that of LiF crystal. The refractive index of the produced films was measured by transmission optical interferometry. Talisurf-6 profilometer measurements on the film surface show sharp edges and very smooth surfaces, which reflect the substrate surfaces.

LiF films: absorption and luminescence of colour centres

Abstract LiF films 1 μm thick have been produced by thermal evaporation on amorphous silica substrates. X-ray diffraction has shown the existence of a polycrystalline structure. The films were subjected at room temperature to electron beam irradiation of energy 3 keV and current 120 μA on anea of about 0.5 cm2. This irradiation produced F2 and F3+ centres stable at room temperature that were observed by light absorption measurements. Although the concentration of the centres was much higher than that obtained usually in bulk crystals, luminescence was also observed.

Photoluminescence of LiF crystal colored by a focused electron beam

Abstract The absorption, emission and excitation optical spectra of F2 and F+3 color centers generated in LiF through low energy (30 keV) electron beam irradiation were measured at room temperature by pumping with an Ar+ laser the resulting thin colored layer. A quenching of the emission intensity of the F+3 centers versus the pump power was observed, with characteristics similar to bulk colored crystals. The analysis of the photoemission spectra showed a new band, which has been discovered previously also in bulk crystals colored and treated in different ways. The possibility of applications of the colored ribbon for miniaturized laser devices is commented.

Grazing incidence X-ray diffraction analysis of alkali fluoride thin films for optical devices

Abstract Alkali fluoride multilayered thin films are promising systems for different optical applications, including light generation and waveguide. Despite this, some of their physical properties are still not well known. The mechanical behaviour of these materials, when evaporated in the form of thin films, plays a fundamental role for the success of the related optoelectronic device. In this work, LiF, NaF and LiF/NaF polycrystalline thin films were deposited onto different substrates at temperatures varying from 50 to 200°C by physical vapour deposition (PVD). Using the X-ray diffraction technique with grazing incidence geometry (GIXRD), the residual stresses and the microstrains present in films deposited in different experimental conditions were measured. The lattice parameters and the mean domain were also estimated. The results show the dependence of the residual stresses in the films upon the cooling rate and the type of substrates and give, for the first time, an order of magnitude of these quantities for such systems.

NaF films: Growth properties and electron beam induced defects

Abstract Polycrystalline NaF films were grown by e-beam assisted physical evaporation on amorphous silica substrates kept, during film growth, at constant temperatures ranging from 40°C to 400°C. The structural characterization of the films was performed by X-ray diffraction and by scanning electron microscopy. Irradiating the films with a 15 keV electron beam induced the formation of F and F-aggregate colour centres stable at room temperature. Absorption and photoemission measurements were performed and indicated a dependence of coloration on the deposition conditions.

F-aggregate centres in KCl films

Abstract The optical behaviour of colour centres is studied in thin films of KCl obtained by thermal evaporation onto amorphous substrates. Deposition conditions are optimized in order to minimize chemical and thermal effects on films. Concentration of colour centres even higher than in bulk crystals can be induced by low energy electron irradiation. Thermal bleaching of F centres is observed at room temperature and its kinetics, deduced from absorption measurements, consists of two exponential processes with very different time constants. Luminescence is obtained for the first time from aggregate colour centres in KCl films, and typical emissions of F2, F3, F4 defects (but not of F centres because of concentration quenching) are revealed.

Photoluminescence of LiF : NaF films at room temperature

Abstract In this paper we present the results of the photoluminescence spectra of multilayer LiF : NaF films colored by low energy (3 keV) electron beams. We show that dielectric multilayered films can be used in the design of new materials with optical properties that cannot be matched by bulk materials.

Crystalline properties and colour centres in alkali halide thin films

Abstract Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage.

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.