F. Gontad

Microstructural, nanomechanical, and microtribological properties of Pb thin films prepared by pulsed laser deposition and thermal evaporation techniques

In this work, the authors compare the morphological, structural, nanomechanical, and microtribological properties of Pb films deposited by thermal evaporation (TE) and pulsed laser deposition (PLD) techniques onto Si (111) substrates. Films were investigated by scanning electron microscopy, surface probe microscopy, and x-ray diffraction in θ-2θ geometry to determine their morphology, root-mean-square (RMS) roughness, and microstructure, respectively. TE films showed a percolated morphology with densely packed fibrous grains while PLD films had a granular morphology with a columnar and tightly packed structure in accordance with the zone growth model of Thornton. Moreover, PLD films presented a more polycrystalline structure with respect to TE films, with RMS roughness of 14 and 10 nm, respectively. Hardness and elastic modulus vary from 2.1 to 0.8 GPa and from 14 to 10 GPa for PLD and TE films, respectively. A reciprocal friction test has shown that PLD films have lower friction coefficient and wear rate...

Fabrication of Nb/Pb structures through ultrashort pulsed laser deposition

This work reports the fabrication of Nb/Pb structures with an application as photocathode devices. The use of relatively low energy densities for the ablation of Nb with ultrashort pulses favors the reduction of droplets during the growth of the film. However, the use of laser fluences in this ablation regime results in a consequent reduction in the average deposition rate. On the other hand, despite the low deposition rate, the films present a superior adherence to the substrate and an excellent coverage of the irregular substrate surface, avoiding the appearance of voids or discontinuities on the film surface. Moreover, the low energy densities used for the ablation favor the growth of nanocrystalline films with a similar crystalline structure to the bulk material. Therefore, the use of low ablation energy densities with ultrashort pulses for the deposition of the Nb thin films allows the growth of very adherent and nanocrystalline films with adequate properties for the fabrication of Nb/Pb structures to be included in superconducting radiofrequency cavities.

Evolution of morphology and structure of Pb thin films grown by pulsed laser deposition at different substrate temperatures

Pb thin films were prepared by pulsed laser deposition on a Si (100) substrate at different growth temperatures to investigate their morphology and structure. The morphological analysis of the thin metal films showed the formation of spherical submicrometer grains whose average size decreased with temperature. X-ray diffraction measurements confirmed that growth temperature influences the Pb polycrystalline film structure. A preferred orientation of Pb (111) normal to the substrate was achieved at 30 °C and became increasingly pronounced along the Pb (200) plane as the substrate temperature increased. These thin films could be used to synthesize innovative materials, such as metallic photocathodes, with improved photoemission performances.

Growth of lead thin films on silicon and niobium substrates by sputtering technique

In this paper, the authors report the growth of Pb thin films on both Si and Nb substrates by radio-frequency sputtering technique. Deposited films were characterized and tested to deduce the structure, the morphology, the nanomechanical properties, and also the quantum efficiency. Granular structures and large presence of voids were observed by scanning electron microscopy; moreover, the roughness and grain size of the film surface, investigated by surface probe microscopy, increased with the film thickness. Crystallographic orientation, studied by x-ray diffraction, showed the growth of polycrystalline Pb thin films and the presence of weak diffraction peaks related to penta-lead oxide (Pb5O8). The nanomechanical analysis reveals a film hardness with a value (∼1.5 GPa) well beyond the hardness of Pb bulk (0.04 GPa). Finally, twin Pb thin films deposited on Nb substrates were tested as photocathodes showing its great potentiality to be used in superconducting radio-frequency guns with a quantum efficienc...

Deposition of MgF2 Thin Films by Pulsed Laser Ablation Technique

In this paper, we report the successful growth of MgF2 thin films on Si and sapphire (Al2O3) substrates at room temperature by direct laser ablation of a pure MgF2 target. The irradiations were performed in high vacuum (10-5 Pa) using the fourth harmonic of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (λ= 266 nm, τFWHM = 7 ns) with energy density of about 10 J/cm2. Uniform films, with a good adhesion on the substrate were obtained. The average ablation and deposition rates resulted to be 1.1 µg/pulse and 0.03 A/pulse, respectively. Different diagnostic techniques were used to study the morphology and chemical composition of deposited films. Time-resolved mass spectrometry studies of the laser ablated material indicate a strong correlation between the chemical species present in the plume and the residual gas. Present interest in the deposition of MgF2 thin films by laser ablation technique is related to their potential application as protecting coating in photocathodes.