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Pulsed laser deposition (PLD) represents a promising bottom-up methodology for the synthesis and transference of nanoparticles to the surface of a biomedical device. Silver (Ag) nanoparticles directly incorporated on the metallic implant emerge as an alternative strategy for local action against prosthetic joint-associated infections. In the present research, a dual sequential PLD process is proposed to obtain a bilayer coating with (1) a bio-derived calcium phosphate (CaP) layer, to provide osteointegrative properties and (2) the controlled growth of the Ag nanoparticles over it, ranging the number of laser pulses from 100 to 500. The characterization by SEM, EDS, TEM, XPS and AFM revealed the uniform deposit of Ag rounded nanoparticles, with a narrow mean size distribution, in the original non-oxidized metallic state. Moreover, given the evidences from XPS and AFM techniques, the occurrence of a coalescence phenomenon from 400 pulses onwards was proposed together with the expected positive linear relation between the number of pulses and Ag contribution with a deposition rate of 0.05 at. % of Ag per pulse. Conversely, the decrease in roughness as the Ag content increased was also verified. Finally, the expected bacteriostatic activity for these PLD deposited metallic state Ag nanoparticles against the bacterial strain Staphylococcus aureus was confirmed. Moreover, the evaluation of the osteoblast-like MG-63 cells viability on the Ag(100–500)-CaP coatings revealed a significant increased proliferation (p > 0.05) on the Ag100-CaP coating compared to the control (Ag0-CaP). When same coating was evaluated against S. aureus the effect was not significant. The possibility of modulating the amount of nanoparticles in the bilayer coating to obtain a greater or lesser effect in combination with CaP was revealed.

Deposition of Au nanoparticles (NPs) through UV ultra-short-pulse laser ablation in vacuum is a methodology suitable for obtaining high purity supported plasmonic NPs with a superb adherence to both planar and patterned substrates. Furthermore, the peculiarity that upon sub-picosecond laser irradiation NPs form directly within the target leads to narrow and controlled size-distribution. While extensive literature theoretically models phase changes and pressure relaxation of metal targets under sub-picosecond irradiation, the evolution of the nanostructured deposit through different ablation regimes is poorly documented experimentally. In this paper, besides modeling the temporal evolution of electron and lattice temperature following sub-picosecond laser pulse irradiation of Au bulk at the threshold ablation fluence (0.1 J/cm2), we report an accurate statistical, morphological and structural characterization of the AuNP distributions as a function of the laser fluence tuned from 0.1 to 3 J/cm2 to drive the target through two ablation regimes. The occurrence of photomechanical spallation and phase explosion at low and high fluence, respectively, is correlated with the evolution of the NP deposits, which are discussed by providing a physical insight in the interplay between fluence-driven ablation regimes of the target, arrangement kinetics of the deposited species (NPs and vapour species) and (crystalline or amorphous) phase of the NPs. This knowledge is key for controlling the surface plasmon resonance response of AuNPs depending on the designed application.

The ablation of a metal target at laser energy densities in the range of 1–10 TW/cm2 leads to the generation of nanoparticles (NP) of the ablated material. This aspect is of particular interest if the immobilization of NPs on three-dimensional (3D) substrates is necessary as for example in sensing applications. In this work the deposition of Au NP by irradiation of a Au bulk target with a sub-picosecond laser beam (500 fs; 248 nm; 10 Hz) on 2D (silica and Si(100)) and 3D substrates (silica nanowire forests) is reported for different number of laser pulses (500, 1000, 1500, 2000, 2500). A uniform coverage of small Au NPs (with a diameter of few nm) on both kinds of substrates has been obtained using a suitable number of laser pulses. The presence of spherical droplets, with a diameter ranging from tens of nm up to few μm was also detected on the substrate surface and their presence can be explained by the weak electron-phonon coupling of Au. The optical characterization of the samples on 2D and 3D substrates evidenced the surface plasmon resonance peak characteristic of the Au NPs although further improvements of the size-distribution are necessary for future applications in sensing devices.

In this paper, we report on the current status of the different configurations that have been suggested for the preparation of metallic and superconducting photocathodes (MPs and SCPs, respectively) by pulsed laser deposition. These strategic devices are used in the new generation of free-electron lasers and in research and development into plasma-based accelerators. To date, three different types of photocathodes have been used: conventional, hybrid and non-conventional configurations. In the case of the conventional configuration, the photocathode consists of a metallic bulk, typically Cu for normal conducting radio-frequency guns and Nb for superconducting radio-frequency guns. In the hybrid configuration, a thin emitting film with high photoemission performance, like Y or Mg, is located in the centre of the Cu flange of the photoinjector. For hybrid SCPs, the emitting material is a thin film of Pb, which has a quantum efficiency higher than that of Nb. Non-conventional configurations of MPs and SCPs have recently been engineered that consist respectively of a Y disc partially covered by a film of Cu and a Pb disc partially covered by a film of Nb. The central surfaces of the Y and Pb discs are not coated, thus providing an emitting area of the materials with higher quantum efficiency. These last devices inserted into a RF gun offer both the photoemission properties and work function of Y and the advantages of Cu in the case of MPs. In the case of SCPs, this new approach offers the relatively high photo-emissive properties of Pb and at the same time preserves all the advantages of Nb as a superconducting material.

In this work, yttrium (Y) thin films have been deposited on Si (100) substrates by the pulsed laser deposition technique. Ex-situ morphological, structural and optical characterisations of such films have been performed by scanning electron microscopy, X-ray diffractometry, atomic force microscopy and ellipsometry. Polycrystalline films with a thickness of 1.2 μm, homogenous with a root mean square roughness of about 2 nm, were obtained by optimised laser irradiation conditions. Despite the relatively high thickness, the films resulted very adherent to the substrates. The high quality of such thin films is important to the synthesis of metallic photocathodes based on Y thin film, which could be used as electron sources of high photoemission performance in radio-frequency guns.

193 nm Excimer laser assisted growth and crystallization of amorphous Si/Ge bilayer patterns with circular structures of 3 μm diameter and around 25 nm total thickness, is presented. Amorphous patterns were grown by Laser induced Chemical Vapor Deposition, using nanostencils as shadow masks and then irradiated with the same laser to induce structural and compositional modifications for producing crystalline SiGe alloys through fast melting/solidification cycles. Compositional and structural analyses demonstrated that pulses of 240 mJ/cm 2 lead to graded SiGe alloys with Si rich discs of 2 μm diameter on top, a buried Ge layer, and Ge rich SiGe rings surrounding each feature, as predicted by previous numerical simulation.

Among the theoretical models addressing the dark matter problem, the category based on a secluded sector is attracting increasing interest. The PADME experiment, at the Laboratori Nazionali di Frascati (LNF) of INFN, is designed to be sensitive to the production of a low mass gauge boson A of a new U(1) symmetry holding for dark particles. The “dark photon” is weakly coupled to the photon of the Standard Model, and it provides an experimental signature for one of the simplest implementation of the dark sector paradigm. The DAΦNE Beam-Test Facility of LNF will provide a high intensity, mono-energetic positron beam impacting on a low Z target. The PADME detector will measure with high precision the momentum of the photon, produced along with A boson in e+e − annihilation in the target, thus allowing to measure the A mass as the missing mass in the final state. This technique, particularly useful in case of invisible decays of the A boson, will be exploited for the first time in a fixed target experiment. Simulation studies predict a sensitivity on the interaction strength (2 parameter) down to 10 −6, in the mass region 1MeV< MA < 23.7MeV, for one year of data taking with a 550MeV beam. In 2018 the first run will take place, and early data will give the opportunity to compare the detector performance with the design requirements.

We report a design of photocathode, which combines the good photoemissive properties of lead (Pb) and the advantages of superconducting performance of niobium (Nb) when installed into a superconducting radio-frequency gun. The new configuration is obtained by a coating of Nb thin film grown on a disk of Pb via pulsed laser deposition. The central emitting area of Pb is masked by a shield to avoid the Nb deposition. The nanomechanical properties of the Nb film, obtained through nanoindentation measurements, reveal a hardness of 2.8±0.3 GPa, while the study of the electrical resistivity of the film shows the appearance of the superconducting transitions at 9.3 K and 7.3 K for Nb and Pb, respectively, very close to the bulk material values. Additionally, morphological, structural and contamination studies of Nb thin film expose a very low droplet density on the substrate surface, a small polycrystalline orientation of the films and a low contamination level. These results, together with the acceptable Pb quantum efficiency of 2×10−5 found at 266 nm, demonstrate the potentiality of the new concept photocathode. Funding agencies: Italian National Institute of Nuclear Physics (INFN); Italian MIUR through the Project FIRB Futuro in Ricerca [RBFR12NK5K]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

The growth of Nb thin films on Si(100) substrates by pulsed Nd:YAG laser deposition (PLD) under different laser fluences (4–15 J/cm2) was reported. The influence of laser fluence on ablation rate and deposition rate was discussed. X-ray diffraction (XRD) investigations of the deposited films showed an amorphous structure. The droplet density on the film surface observed by scanning electron microscopy (SEM) analyses was extremely low. It was experimentally proved that the droplets on the film surface originated from liquid phase on the target surface. Profilometric measurements of the deposited Nb films revealed a substantial asymmetry in the film thickness related to the plume deflection effect. The measured electrical resistivity of the Nb film was higher than that of high purity Nb bulk. The present investigations of ablation and deposition process of Nb thin films are related to its potential application in superconducting radio-frequency (SRF) cavities.

Pb thin films were prepared by the nanosecond pulsed laser deposition technique on Si (100) and polycrystalline Nb substrates for photocathode application. As the photoemission performances of a cathode are strongly affected by its surface characteristics, the Pb films were grown at different substrate temperatures with the aim of modifying the morphology and structure of thin films. An evident morphological modification in the deposited films with the formation of spherical grains at higher temperatures has been observed. X-ray diffraction measurements showed that a preferred orientation of Pb (111) normal to the substrate was achieved at 30 °C while the Pb (200) plane became strongly pronounced with the increase in the substrate temperature. Finally, a Pb thin film deposited on Nb substrate at 30 °C and tested as the photocathode showed interesting results for the application of such a device in superconducting radio frequency guns.

Polyethylene oxide (PEO) films were grown by pulsed laser deposition using two different lasers: ArF (193 nm, 5 ns) and Nd:YAG (355 nm, 7 ns). Even though very similar experimental conditions have been applied to ablate identical targets, different surface morphologies and structures have been observed. Depending on laser fluence, labyrinthine patterns in PEO films have been formed when using 355 nm laser pulses at fluence values in the range 280–1000 mJ/cm2. The same material ablated by 193 nm excimer laser pulses at 200 mJ/cm2 fluence grows in dendritic morphologies. Both target and laser deposited materials have been thoroughly characterized using infrared spectroscopic, microscopic and X-ray analytical methods. Infrared spectroscopy demonstrated the close similarity of molecular chains for both target and film materials. X-ray diffraction analysis indicates polymer chain scissoring by ultraviolet irradiation, a fact also confirmed by size exclusion chromatography.

Yttrium (Y) thin film was grown by pulsed laser deposition (PLD) on a copper (Cu) polycrystalline substrate. Ex situ morphological and structural characterisations of the circular Y film of 1.2 µm thickness and 3 mm diameter have shown a very low droplet density on the film surface and a crystalline feature with a preferred orientation along the Y (100) plane. Moreover, Y thin film resulted in being very adherent to the Cu substrate and more scratch resistant than Cu bulk. A twin thin film was deposited also on a Cu backflange of a radio-frequency (RF) gun to test the suitability of the metallic thin film as photocathode. It was observed that the Y-coated photocathode was characterised by a quantum efficiency (QE) higher than that of the Cu bulk photocathode even if the presence of space charge effects didn’t allow deriving the absolute maximum value of QE of Y photocathode.

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...

Even if it has been recently assessed that sub-picosecond ablation is not a “melting-free” process, a systematic investigation of the issue of droplets is still lacking. In this paper we discuss the effects of fluence (F = 0.3, 1, 3 J/cm 2 ), target-substrate distance (d TS = 5, 6, 7, 8, 9 cm) and number of laser pulses per site (N p/s = 2, 12, 22, 36, 96) on the morphology in terms of droplets and plasmonic response of gold nanoparticles deposited by sub-picosecond pulsed laser ablation. Among metals, gold is an ideal candidate to study the occurrence of droplets due to its low electron-phonon coupling that favors the appearance of heat affected zones. Our findings demonstrate that N p/s affects the production of large droplets more intensely than F and d TS . In fact, increasing N p/s produced much larger droplets than the ones obtained by tuning the fluence and an increment in the droplet density of 10 times. On the other hand, while the maximum density of small droplets (average diameter ≤ 0.15 μm) occurs at F = 1 J/cm 2 (with respect to 0.3 J/cm 2 and 3 J/cm 2 ), the total droplet density exhibits a minimum at such fluence.

In this article we present and discuss the current status of thin film Pb photocathodes deposited by pulsed laser deposition (PLD) with different laser parameters, such as laser fluence, wavelength or pulse duration. The PLD technique appears very efficient for the fabrication of pure Pb photocathodes, providing good adherence and respectable quantum efficiency. The films deposited on the picosecond and subpicosecond regimes are practically free of big droplets and fragments, whereas in the nanosecond regime their presence cannot be neglected. All the films present a granular structure and polycrystalline character with preferential orientation along the (111) crystalline planes, irrespective of the laser pulse duration or wavelength. The main results obtained from the photoemission performance of Pb thin films deposited by PLD demonstrate their chemical stability under vacuum conditions and respectable quantum efficiency with a maximum of 7.3×10 −5 for films deposited on the subpicosecond regime. The photoemission properties confirm that Pb thin films deposited by PLD are a notable alternative for the fabrication of photocathodes for superconductive radio-frequency electron guns.

The authors study the nanomechanical and microtribological properties of yttrium (Y) thin films deposited by pulsed laser deposition on Cu polycrystalline substrates. Nanoindentation tests reveal that such films have a high hardness of H = 2.3 GPa and a reduced elastic modulus of 71.7 GPa with respect to the Cu substrates. The friction coefficient between a diamond tip and the Y film reaches a steady state value of μ ∼ 0.34, lower than that for the Cu (μ ∼ 0.38). Moreover, nanoscratch experiments show that Y films are more scratch-resistant than the Cu substrates, probably due to their greater hardness, higher elastic recovery, and lower friction coefficient. Their results confirm that the mechanical and tribological properties of the Y films are suitable for designing and fabricating scratch-resistant hybrid photocathodes and can reduce instabilities and unwanted discharges in the cavity of the radio-frequency gun. Furthermore, the low surface roughness and the low work function of the material are important characteristics for a photocathode based on the Y thin film for the production of high-brightness electron beams.

In this work yttrium and lead thin films have been deposited by pulsed laser deposition technique and characterized by ex situ different diagnostic methods. All the films were adherent to the substrates and revealed a polycrystalline structure. Y films were uniform with a very low roughness and droplet density, while Pb thin films were characterized by a grain morphology with a relatively high roughness and droplet density. Such metallic materials are studied because they are proposed as a good alternative to copper and niobium photocathodes which are generally used in radiofrequency and superconducting radiofrequency guns, respectively. The photoemission performances of the photocathodes based on Y and Pb thin films have been also studied and discussed. Funding agencies: Italian National Institute of Nuclear Physics (INFN); Italian Ministry of Research [RBFR12NK5 k]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

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...

Silica two-dimensional substrates and nanowires (NWs) forests have been successfully decorated with Au nanoparticles (NPs) through laser ablation by using a pulsed ArF excimer laser, for sensor applications. A uniform coverage of both substrate surfaces with NPs has been achieved controlling the number of laser pulses. The annealing of the as-deposited particles resulted in a uniform well-defined distribution of spherical NPs with an increased average diameter up to 25 nm. The deposited samples on silica NWs forest present a very good plasmonic resonance which resulted to be very sensitive to the changes of the environment (ethanol/water solutions with increasing concentration of ethanol) allowing the detection of changes on the second decimal digit of the refractive index, demonstrating its potentiality for further biosensing functionalities.

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.

This paper reports the fabrication of Nb thin films through pulsed laser deposition at different substrate temperatures, ranging from 300 to 660 K. While the variation of the substrate temperature does not affect significantly the excellent Nb thin film adhesion to the Si(100) substrate surface, the increase of the substrate temperature up to 570 K promotes an improvement of the grown film in terms of morphology and roughness. Such improvement is achieved through the formation of wider columnar structures with a reduced superficial roughness, around 5 nm, as shown by scanning electron microscopy (SEM) and atomic force microscopy. The use of temperatures over 570 K increases the substrate roughness due to the formation of irregular structures inside the film, as observed by SEM cross section analysis, and does not produce a relevant improvement on the crystalline structure of the material.

In the last decade, UV-Excimer laser systems have been dramatically improved in terms of beam quality, power stability and gas lifetime. The benefits of using them for group IV semiconductor processing have, up to now, principally been recognized for some few processes like laser marking, UV-lithography and Excimer Laser Crystallization (ELC). Although several other techniques like Laser induced Chemical Vapor Deposition (LCVD), Excimer Laser Annealing (ELA) and Pulsed Laser Induced Epitaxy (PLIE) have been extensively studied 30 years ago for producing silicon coatings, little is known about the prospects of up-scaling those processes to other group IV semiconductors. This contribution presents UV-Laser processes combining LCVD of amorphous Si and of SiGeC alloys with their subsequent laser, in particular PLIE, treatment for modifying their composition and structure in order to engineer lattice parameters.

Research on epitaxial crystaline silicon (c-Si) and silicon-germanium (Si1-xGex) alloys growth and annealing for microelectronic purposes, such as Micro- or Nano-Electro-Mechanical Systems (MEMS or NEMS) and Silicon–On-Nothing (SON) devices is continuously in progress. Laser assisted annealing techniques using commercial ArF Excimer Laser sources are based on ultra-rapid heating and cooling cycles induced by the 193 nm pulses of 20 ns, which are absorbed in the near surface region of the heterostructures. During and after the absorption of these laser pulses, complex physical processes appear that strongly depend on sample structure and applied laser pulse energy densities. The control of the experimental parameters is therefore a key task for obtaining high quality alloys. The Finite ElementsMethod (FEM) is a powerful tool for the optimization of such treatments, because it provides the spatial and temporal temperature fields that are produced by the laser pulses. In this work, we have used a FEM commercial software, to predict the temperatures gradients induced by ArF excimer laser over a wide energy densities range, 0.1

A Finite Element Method (FEM) study of the coupled thermal-stress during the heteroepitaxial growth induced by excimer laser radiation of patterned amorphous hydrogenated silicon (a-Si:H) and germanium (a-Ge:H) bi-layers deposited on a Si(100) wafer is presented. The ArF (193 nm) excimer laser provides high energy densities during very short laser pulse (20 ns) provoking, at the same time, melting and solidification phenomena in the range of several tenths of nanoseconds. These phenomena play an important role during the growth of heteroepitaxial SiGe structures characterized by high Ge concentration buried under a Si rich surface. In addition, the thermal-stresses that appear before the melting and after the solidification processes can also affect to the epitaxial growth of high quality SiGe alloys in these patterned structures and, in consequence, it is necessary to predict their effects. The aim of this work is to estimate the energy threshold and the corresponding thermal-stresses in the interfaces and the borders of these patterned structures.

In this manuscript, a 3-D axisymmetric model for the heteroepitaxial growth induced by irradiating thin patterned amorphous hydrogenated silicon (a-Si:H) and germanium (a-Ge:H) bi-layers on Si (100) with pulsed UV-laser radiation, is presented. For reducing optimization steps, an efficient simulation of the laser induced processes that include rapid heating and solidification phenomena in the range of several tenth of nanoseconds, must be performed, if alloy composition and quality has to be adjusted. In this study, the effects of various laser energy densities on different amorphous Si/Ge bi-layer structures has been predicted and adjusted to obtain the desired Ge concentration profiles for applications as sacrificial layers, i.e. a Ge containing film buried under a Si rich surface layer. The numerical model includes the temperature dependent variations of the thermophysical properties and takes the coupled effects of temperature and hydrodynamic phenomena for a Boussinesq fluid, to estimate the element interdiffusion during the process and predicting the concentration profiles.

This manuscript presents a numerical analysis of the Excimer Laser Annealing and Crystallization induced in a set of amorphous hydrogenated silicon (a-Si:H) and germanium (a-Ge:H) bi-layers deposited on crystalline (c-) Si(100), with the aim to select the adequate range of energy densities for growing SiGe alloys of definite graded compositions, that might be useful as sacrificial layers for the fabrication of Si based Micro-Electro-Mechanical Systems and Silicon-On-Nothing devices. We have been able to predict the energy threshold to get an adequate epitaxial SiGe alloy growth with a thickness ranging from 10 to 30 nm. Temperature distribution, melting depth as well as the Ge concentration gradient have been calculated and compared with experimental results.

A combination of ArF-Excimer laser assisted techniques has been used for depositing and modifying ultra thin amorphous Si/Ge bi-layer structures. The first step consisted in producing, at low substrate temperatures, thin bi-layer coatings through Laser induced Chemical Vapour Deposition (LCVD) in both, large areas as well as in small regions of Si(1 0 0) wafers. In the second step, these bi-layer structures have been modified through Pulsed Laser Induced Epitaxy (PLIE) for obtaining heteroepitaxial SiGe alloys with a thin buried Ge rich layer, while keeping a shallow upper Si rich surface with good crystalline quality. Threshold for epitaxial alloy formation has been determined by Raman spectroscopy and estimated to be above 200 mJ/cm 2 . Optical profilometry has been used for evaluating the thickness of the structures and the lateral dimensions of patterned features. SEM, TOF-SIMS and XPS have been used to corroborate the results. For testing IC compatibility, some samples have been overgrown with epitaxial Si and etched through conventional IC processing techniques, revealing that the laser processed layers are suitable to be used as sacrificial layers for producing Micro-Electro-Mechanical Systems (MEMSs) or Silicon-on-Nothing (SON) devices.

The application of the striking electrical and optical properties of amorphous and nano-crystalline silicon in photovoltaic, photonic and nano-electronic devices is attracting increasing attention. In particular, its use both on polymeric substrates and in Integrated Circuit technology for the development of enhanced new devices has shown that processing techniques to produce amorphous hydrogenated and nano-crystalline silicon films avoiding high substrate temperatures are of great importance. A promising strategy to achieve this purpose is the combination of Hot-Wire Chemical Vapor Deposition at 150 °C with Excimer Laser Annealing, thus maintaining the substrate at relatively low temperature during the complete process. In this work we present a numerical analysis of Excimer Laser Annealing, performed at room temperature, of a multilayer structure of thin alternating a-Si:H and nc-Si films deposited on glass and grown by Hot-Wire Chemical Vapor Deposition. A set of two different layer thicknesses a-Si:H (25 nm)/nc-Si (100 nm) and a-Si:H (30 nm)/nc-Si (60 nm) were analysed for a total structure dimension of 900 nm. The aim is to determine the probable temperature profile to achieve controlled localized in depth dehydrogenation. Temperature distribution has been calculated inside the multilayer during the irradiation by a 193 nm Excimer laser, 20 ns pulse length, with energy densities ranging from 50 to 300 mJ/cm2. Calculations allowed us to estimate the dehydrogenation effect in the different layers as well as the structural modifications of the same layers as a function of the applied laser energy. The numerical results have been compared to the experimental ones obtained in similar multilayer structures that have been analysed through Raman spectroscopy and TOF-SIMS in depth profiling mode.

In this work, the deposition of Cu thin films on Y substrates for photocathode applications by pulsed laser deposition employing picosecond laser pulses is reported and compared with the use of nanosecond pulses. The influence of power density (6–50 GW/cm 2 ) on the ablation of the target material, as well as on the properties of the resulting film, is discussed. The material transfer from the target to the substrate surface was found to be rather efficient, in comparison to nanosecond ablation, leading to the growth of films with high thickness. Scanning electron microscope analysis indicated a quasi-continuous film morphology, at low power density values, becoming granular with increasing power density. The structural investigation, through X-ray diffraction, revealed the poly-crystalline nature of the films, with a preferential growth along the (111) crystallographic orientation of Cu cubic network. Finally, energy-dispersive X-ray spectroscopy showed a low contamination level of the grown films, demonstrating the potential of a PLD technique for the fabrication of Cu/Y patterned structures, with applications in radiofrequency electron gun technology.

Time-integrated and-resolved mass spectrometric investigations were carried out during the laser ablation of yttrium target in high vacuum. The main aim of these experiments was to study the implications of the target surface chemistry and of the residual gas on the deposition process quality. The effect of the laser energy density on the ionic species formation during the ablation process was investigated. It was found that at a moderate laser fluence of 4.5 J/cm2 none hydrides, oxides nor nitrides of yttrium were observed; on the contrary, at a higher laser fluence of 7.6 J/cm2, the above chemical species were observed. During and after the laser ablation process, a getter effect was observed due to the absorption of the residual gas by the Y layer deposited on the internal wall of the vacuum system. The results of these mass spectrometric analyses will be useful to improve the performances of photocathodes based on Y thin films grown by pulsed laser ablation technique.

The growth of polycrystalline Pb thin films on Si substrates at room temperature by near infrared laser ablation of pure Pb target is presented. Several diagnostic techniques were used to deduce the structure, the morphology and the thickness of the deposited films. We investigated the above properties as a function of laser fluence (1–8 J/cm 2 ). Detailed morphological studies were also carried out on the irradiated target surface to deduce the laser ablation process. According to our results, the increase of laser fluence improves the crystallinity of the deposited films but not the deposition rate and the droplet density. The measured deposition rate was extremely low and varied between 0.04 and 0.09 A per pulse. The final goal of this research activity is the realization of photocathodes based on Pb-thin-film to be used in superconducting Nb radio-frequency cavity.

We review the current status of metallic photocathodes based on thin films prepared by pulsed laser deposition (PLD) and we explore ways to improve the performance of these devices. PLD seems to be a very efficient and suitable technique for producing adherent and uniform thin films. Time-resolved mass spectrometric investigations definitively suggest that the deposition of high-purity metallic thin films should be carried out in ultrahigh vacuum systems and after a deep and careful laser cleaning of the target surface. Moreover, the laser cleaning of the target surface is highly recommended not only to remove the first contaminated layers but also to improve the quality of the vacuum by reducing the partial pressure of reactive chemical species as H_{2}O, H_{2}, and O_{2} molecules. The challenge to realize high-purity Mg and Y thin films is very interesting for the photocathode R&D due to the good photoemission properties of these metals. Photocathodes based on Mg and Y thin films have been characterized by scanning electron microscopy and x-ray diffraction techniques to derive the morphological and structural features, respectively. They were also tested in a photodiode cell to deduce the photoelectron properties. The quantum efficiency of such photocathodes was systematically improved by in situ laser cleaning treatments of the surface in order to remove the contaminated layers reaching, in this way, the quantum efficiency of the corresponding bulk materials.

High purity yttrium was ablated by using frequency quadrupled ultra-violet pulses of a Nd:YAG laser ( λ = 266 nm, τ FWHM = 7 ns) with power density of about 1 GW/cm 2 . Laser ablation process was studied using in-situ mass spectrometry of the ablated species in combination with ex-situ analyses of both target surface and deposited films. An increase on the Y ablation rate was found at the beginning, followed by a significant drop with increasing of the number of laser pulses per site until it reaches a constant value after 40 pulses per site. Initial topographic changes on the target surface, observed by scanning electron microscope investigations, and plasma shielding effect could be the origin of these changes on the ablation rate. Careful time-integrated and -resolved mass spectrometric studies of the laser ablated material indicate evident hydridation and oxidation processes in gas phase of ablated yttrium. These results clearly suggest that high purity metallic thin films can be deposited only after a deep and prolonged laser cleaning treatment of the target surface. The present parametric studies are aimed and tailored to prepare photocathodes based on Y thin films to be used in RF photoinjectors.

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 Å/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.

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. © 2014 American Vacuum Society.

Copper (Cu) thin films were deposited on yttrium (Y) substrate by sputtering. During the deposition, a small central area of the Y substrate was shielded to avoid the film deposition and was successively used to study its photoemissive properties. This configuration has two advantages: the cathode presents (i) the quantum efficiency and the work function of Y and (ii) high electrical compatibility when inserted into the conventional radio-frequency gun built with Cu bulk. The photocathode was investigated by scanning electron microscopy to determine surface morphology. X-ray diffraction and atomic force microscopy studies were performed to compare the structure and surface properties of the deposited film. The measured electrical resistivity value of the Cu film was similar to that of high purity Cu bulk. Film to substrate adhesion was also evaluated using the Daimler–Benz Rockwell-C adhesion test method. Finally, the photoelectron performance in terms of quantum efficiency was obtained in a high vacuum photodiode cell before and after laser cleaning procedures. A comparison with the results obtained with a twin sample prepared by pulsed laser deposition is presented and discussed.

This work deals with the deposition of lead (Pb) thin films by the UV pulsed laser ablation technique, for their further use as photocathode devices in superconducting radio frequency guns. Scanning electron microscopy and atomic force microscopy analyses were performed to study the morphological features of Pb thin films deposited on Si (100) and Nb substrates. The films showed a granular structure with a nearly fully covered surface only for that one deposited on Nb substrate. X-ray diffraction measurements indicate the growth of polycrystalline Pb thin films with a preferential orientation along (111) planes. Results of the photoemission performance of Pb thin film deposited on Nb substrate showed a very encouraging average value of quantum efficiency of 6 x 10(-5) through a single-photon absorption process, promoting further studies in the realisation of Pb photocathodes by this technique

Pb thin films were deposited on Nb substrates by means of pulsed laser deposition (PLD) with UV radiation (248 nm), in two different ablation regimes: picosecond (5 ps) and subpicosecond (0.5 ps). Granular films with grain size on the micron scale have been obtained, with no evidence of large droplet formation. All films presented a polycrystalline character with preferential orientation along the (111) crystalline planes. A maximum quantum efficiency (QE) of $7.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ (at 266 nm and 7 ns pulse duration) was measured, after laser cleaning, demonstrating good photoemission performance for Pb thin films deposited by ultrashort PLD. Moreover, Pb thin film photocathodes have maintained their QE for days, providing excellent chemical stability and durability. These results suggest that Pb thin films deposited on Nb by ultrashort PLD are a noteworthy alternative for the fabrication of photocathodes for superconductive radio-frequency electron guns. Finally, a comparison with the characteristics of Pb films prepared by ns PLD is illustrated and discussed.

We report on the excimer laser annealing (ELA) induced temperature gradients, allowing controlled crystal-lization and dehydrogenation of boron-doped a-Si:H/nc-Si:H multilayers. Depth of the dehydrogenation and crystallization process has been studied numerically and experimentally, showing that temperatures below the monohydride decomposition can be used and that significant changes of the doping profile can be avoided. Calculation of temperature profiles has been achieved through numerical modeling of the heat conduction differential equation. Increase in the amount of nano-crystals, but not in their size, has been demonstrated by Raman spectroscopy. Effective dehydrogenation and shape of the boron profile have been studied by time of flight secondary ion mass spectroscopy. The relatively low temperature threshold for dehydrogenation, below the monohydride decomposition temperature, has been attributed to both, the large hydrogen content of the original films and the partial crystallization during the ELA process. The results of this study show that UV-laser irradiation is an effective tool to improve crystallinity and dopant activation in p+-nc-Si:H films without damaging the substrate., Fundação para a Ciência e Tecnologia (FCT), CRUP Spanish–Portuguese bilateral agreement HP2006- 0122, Spanish national and regional research contracts: MAT-2000-1050, MAT-2003-04908MAT-2011-24077, PGIDIT03-04908, PGIDT-01PX130301PN, XUGA- Infra 93, XUGA-Infra 94-58, SB93-A0742819D and INFRA 99-PR 405a-46

Pb thin films were prepared at room temperature and in high vacuum by thermal evaporation and pulsed laser deposition techniques. Films deposited by both the techniques were investigated by scanning electron microscopy to determine their surface topology. The structure of the films was studied by X-ray diffraction in θ–2θ geometry. The photoelectron performances in terms of quantum efficiency were deduced by a high vacuum photodiode cell before and after laser cleaning procedures. Relatively high quantum efficiency (>10−5) was obtained for all the deposited films, comparable to that of corresponding bulk. Finally, film to substrate adhesion was also evaluated using the Daimler–Benz Rockwell-C adhesion test method. Weak and strong points of these two competitive techniques are illustrated and discussed. Funding Agencies|Istituto Nazionale di Fisica Nucleare (INFN)||Swedish Government Strategic Research Area Grant in Materials Science (SFO Mat-LiU)

Ultraviolet (UV) Excimer laser assisted processing is an alternative strategy for producing patterned silicon germanium heterostructures. We numerically analyzed the effects caused by pulsed 193 Excimer laser radiation impinging on patterned amorphous hydrogenated silicon (a-Si:H) and germanium (a-Ge:H) bilayers deposited on a crystalline silicon substrate [Si(100)]. The proposed two dimensional axisymmetric numerical model allowed us to estimate the temperature and concentration gradients caused by the laser induced rapid melting and solidification processes. Energy density dependence of maximum melting depth and melting time evolution as well as three dimensional temperature and element distribution have been simulated and compared with experimentally obtained results.