Study on excimer laser irradiation for controlled dehydrogenation and crystallization of boron doped hydrogenated amorphous/nanocrystalline silicon multilayers

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- 0122Spanish 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

Commissioning of the PADME experiment with a positron beam

The PADME experiment is designed to search for a hypothetical dark photon A' produced in positron-electron annihilation using a bunched positron beam at the Beam Test Facility of the INFN Laboratori Nazionali di Frascati. The expected sensitivity to the A'-photon mixing parameter ϵ is 10-3, for A' mass ≤ 23.5 MeV/c 2 after collecting ∼1013 positrons-on-target. This paper presents the PADME detector status after commissioning in July 2019. In addition, the software algorithms employed to reconstruct physics objects, such as photons and charged particles, and the calibration procedures adopted are illustrated in detail. The results show that the experimental apparatus reaches the design performance, and is able to identify and measure standard electromagnetic processes, such as positron bremsstrahlung and electron-positron annihilation into two photons

In-situ and ex-situ investigations of pulsed laser ablation of Y target

High purity yttrium was ablated by using frequency quadrupled ultra-violet pulses of a Nd:YAG laser (λ=266 nm, τFWHM=7 ns) with power density of about 1 GW/cm2. 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

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 at high vacuum (10-5 Pa) using the forth 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

Overview on development of metallic and superconducting photocathodes by the PLD technique for linear accelerator sources

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

Highlights on photocathodes based on thin films prepared by pulsed laser deposition

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 H2O, H2, and O2 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