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Abstract: Mg-based films have been prepared by pulsed laser deposition technique for photocathode applications. We have investigated the influence of pulse laser duration on morphology and photoemissive properties. Two laser sources have been used, generating pulses of 30ns at 308nm (XeCl excimer laser), 5ps and 500fs at 248nm (KrF excimer laser) to grow Mg films onto Si and Cu substrates in high vacuum (∼10−7 Pa) and at room temperature. Morphological investigations carried out by scanning electron microscopy (SEM) have revealed that, in our experimental conditions, the number and the mean size of the droplets on the films surfaces decreases as the pulse laser duration shortens. The contamination level of Mg film surfaces have been studied by energy dispersive X-ray spectroscopy (EDX). The photoelectron performances in terms of quantum efficiency (QE) and emission stability have been tested in a UHV DC photodiode cell (10−7 Pa). Measures of the QE of the samples surfaces have revealed a decrease on the initial value for Mg-based photocathodes prepared by fs laser (from 7.8×10−4 to 6.6×10−4) PLD with respect to ps (from 6.2×10−4 to 7.4×10−4) and ns lasers (from 5.0×10−4 to 1.6×10−3). A comparison among Mg-based photocathodes prepared by ns, ps and fs PLD for the production of high brightness electron beams has been presented and discussed. [Copyright &y& Elsevier]

Abstract: Advanced high brightness radio frequency (RF) gun injectors require photocathodes with a fast response, high quantum efficiency (QE) and good surface uniformity. Metal films deposited by various techniques on the gun back wall could satisfy these requirements. A new deposition technique has been recently proposed, i.e. pulsed laser ablation. Several Mg samples have been deposited by this technique: the emission performance and morphological changes induced on the cathode surface during laser activation are compared and discussed. [Copyright &y& Elsevier]

Abstract: Pd-doped-SnO2 thin films were grown on Si (100) substrates using pulsed laser deposition at room temperature. Scanning electron microscopy and atomic force microscopy studies showed that the film morphologies are strongly related to two major parameters: first, high O2 background pressure, namely ≥10 Pa, induces the formation of nanoparticles by condensation in the gas phase; second, when the target–substrate distance d t–s is close to the plume length L p , considering an adiabatic expansion, textured thin films are obtained. Their morphologies depend therefore on the O2 pressure, since by increasing the pressure from 10 to 100 Pa the film morphology changes from a cracked structure to a well developed cauliflower-like-structure with a columnar growth. X-ray photoelectron spectroscopy, energy dispersive X-ray and grazing incidence X-ray diffraction were also employed to analyze, respectively, the chemical bonding state, the chemical composition and the structure in the deposited films. Sub-stoichiometric thin films are also synthesized with a Pd content of about 2.3 at.% under O2 background pressures higher than 10 Pa. [Copyright &y& Elsevier]

Abstract: Nanostructured tin oxide films were deposited on Si(100) substrates at room temperature using the PLAD method. Depositions were achieved by using a XeCl* laser, with a fluence of 10J/cm2 to ablate a SnO2 target either in vacuum or in an atmosphere containing a relatively low partial pressure (10−2 to 100Pa) of O2. A range of spectroscopic diffraction and real space imaging techniques, SEM, EDS, XRD, and XPS were used in order to characterize the surface morphology, structure, and composition of films. XPS results indicate that the film deposited in vacuum comprises a mixture of Sn, SnO and SnO2. In contrast, films deposited in oxygen comprise only SnO and SnO2. This indicates that the deposition of tin oxide films in relatively low partial pressures of O2 prevents the accumulation of unreacted tin metal within the resulting film. SEM studies reveal a sub-micron grain size structure, while XRD analyses indicate that the samples are crystalline, with no evidence for the presence of amorphous material. In this paper we discuss the significance and implications of these findings in terms of the optimization of tin oxide films for use in gas sensing devices and other technological applications. [Copyright &y& Elsevier]

The results of a new geometrical configuration of pulsed laser ablation deposition technique that uses a hemi-cylindrical transparent plastic substrate are presented and discussed. With the new arrangement, two distinct components with different material compositions have been clearly observed. While the first component comprises atoms, ions and molecules and constitutes visible or the so-called “luminous” plume, the second component is formed of fragments, clusters and liquid droplets and is invisible to the naked eye. The latter component constitutes a major part of the ablated material. The luminous plume symmetry axis does not coincide with the mass distribution symmetry axis. In the case of Si ablation and 45° incident beam angle, the maximum deflection angle of the luminous plume reaches 30° with respect to the normal to the target surface, while the deflection angle of the second component is higher than 40°. The change in the target morphology and the dependence of the composition of the ablated material on local fluence, along with the hydrodynamic effects, is sought to explain this previously unexpected and unreported phenomenon. [ABSTRACT FROM AUTHOR]

The conventional configuration (substrate parallel and frontal to the target surface) of the pulsed laser ablation deposition technique has been modified for improving the film uniformity with acceptable deposition rates, leading to the ‘dynamic deposition configuration’. The novelty of this arrangement is the ability to place by computer-control the substrate synchronously in front of the plume axis compensating the plume deflection effect. The new geometrical configuration was used to deposit films of Ti, Al and Cu. All the deposited films exhibited both high thickness uniformity and remarkable deposition rates. Initial results of the experiments are explained and compared with the results obtained in the conventional configuration, in otherwise similar experimental conditions. [Copyright &y& Elsevier]

Abstract: The effects of laser fluence on the morphology of Y films in the regime characteristic of multiple-pulse laser deposition were investigated. Y thin films were deposited on silicon and copper substrates. The samples deposited on silicon substrate were used to deduce the morphology and the thickness of the deposited films. On the contrary, the samples deposited on copper were tested as photocathodes in a DC photodiode cell. The interest to produce Y-based photocathodes is due to the low work function of this metal with the possibility to drive such photocathodes with a visible radiation in the radio-frequency photo-injector. In this way it is possible to reduce the thermal emittance of the photoelectron beam and to increase the photocurrent intensity by utilizing the second harmonic of Ti:Sa driver laser. The quantum efficiency was measured for the first time by using a visible CW laser diode emitting at 406nm. [Copyright &y& Elsevier]

Hastelloy thin films have been deposited on carbon steel substrates at room temperature by pulsed laser ablation of bulk Hastelloy targets. Different numbers of laser pulses were used in order to obtain films of various thicknesses. Scanning electron microscopy (SEM) was used to analyze the surface morphology of the deposited films. Energy dispersion X-ray (EDX) spectroscopy and X-ray diffraction (XRD) have shown that the laser ablation process was almost congruent, and the crystallographic structure was retained in the films. In order to relate the microstructure of the film surface with its corrosion behavior, electrochemical tests in aqueous CO2 environment, such as potentiodynamic curves and linear polarization resistance, were performed. The film characteristics and their dependence on deposition parameters are presented and discussed. [Copyright &y& Elsevier]