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Controlling the surface morphology and composition of the perovskite substrates is a critical aspect in tuning the final properties of the deposited films and of their interfaces. The paper reports on a chemical etching method developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain a well-defined GaO2−x-terminated surface. The etching process is based on a HF + NH4OH solution and includes an annealing step performed in air or under O2 flow at temperatures of 800–1000 °C. In order to obtain the desired composition and surface morphology, the etching procedure was optimized for the vicinal step density at the surface and substrate crystal orientation. Growth nucleation studies of one-unit-cell MeO films (Me = Ti, Sr, Ba) on chemically etched and on only annealed substrates were performed in order to determine the composition of the substrate topmost layer. The results indicate that the chemically etched NdGaO3 substrate surface has a predominantly GaO2−x termination, with a lower free surface energy compared to the NdO1+x termination.

Nanostructured targets showed improved interaction with ultra-intense laser pulses in comparison to planar ones, both in simulations and in experiments. By increasing the surface area, the absorption and conversion efficiency of the laser energy to the accelerated particle energy are enhanced due to volumetric heating, leading to advanced proton acceleration, x-ray emission, ultra-high energy density matter creation, and terabar pressure generation. This work is focused on exploring the limits of the electrodeposition methods for the fabrication of nanostructured targets suitable for ultra-intense laser experiments at focused intensities as high as 1023W/cm2. The geometrical characteristics of the nanostructures are expanded to meet a wide range of experimental requirements: diameter, length, distance between structures, and substrate thickness. Nickel nanotubes and nanowires on few hundreds nanometer thick substrates were fabricated using porous alumina as template, obtained by aluminium anodization in various electrolyte solutions. The resulting structures revealed diameters and spacing of several hundreds of nanometers, with length varying between 1–10 micrometers, covering homogeneous areas of several square centimetres. The influence of temperature on the current density, with two electrolyte mixtures containing oxalic, citric, phosphoric acids used for anodization, is also reported. In the initial testing using high-power lasers, we found an increase in proton energy by 1.5 times and flux at high-energy tail of the spectrum higher by an order of magnitude, from the nanostructured targets.

We review prevalent fabrication techniques for targets used in high-power and high-intensity single-beam laser experiments, emphasizing their applications and performance. Solid targets include free-standing metallic and carbon films of various thicknesses for particle acceleration studies involving proton and carbon beams and micro-/nano-structured surfaces, as well as near-critical-density materials, such as foams, which enhance laser absorption and coupling, and boron targets for proton–boron fusion experiments. The resulting gamma and proton beams enable applications in isomer production, proton therapy, societal applications and fundamental studies. The use of gas targets for electron acceleration via the laser wakefield acceleration mechanism is discussed, as are cryogenic clusters undergoing Coulomb explosion. Finally, liquid, cryogenic and tape target systems are presented as high-repetition-rate and application-oriented solutions, offering high reproducibility and long-term operation.

Controlling the surface morphology and composition of the perovskite substrates is a critical aspect in tuning the final properties of the deposited films and of their interfaces. The paper reports on a chemical etching method developed for (110) and (001) NdGaO[sub.3] single crystal substrates in order to obtain a well-defined GaO[sub.2−x] -terminated surface. The etching process is based on a HF + NH[sub.4] OH solution and includes an annealing step performed in air or under O[sub.2] flow at temperatures of 800–1000 °C. In order to obtain the desired composition and surface morphology, the etching procedure was optimized for the vicinal step density at the surface and substrate crystal orientation. Growth nucleation studies of one-unit-cell MeO films (Me = Ti, Sr, Ba) on chemically etched and on only annealed substrates were performed in order to determine the composition of the substrate topmost layer. The results indicate that the chemically etched NdGaO[sub.3] substrate surface has a predominantly GaO[sub.2−x] termination, with a lower free surface energy compared to the NdO[sub.1+x] termination.

The present article evaluates, in qualitative and quantitative manners, the characteristics (i.e., thickness of layers, crystal structures, growth orientation, elemental diffusion depths, edge, and screw dislocation densities), within two GaN/AlN/Si heterostructures, that alter their efficiencies as positron moderators. The structure of the GaN film, AlN buffer layer, substrate, and their growth relationships were determined through high-resolution transmission electron microscopy (HR-TEM). Data resulting from high-resolution X-ray diffraction (HR-XRD) was mathematically modeled to extract dislocation densities and correlation lengths in the GaN film. Positron depth profiling was evaluated through an experimental Doppler broadening spectroscopy (DBS) study, in order to quantify the effective positron diffusion length. The differences in values for both edge ( ρ d e ) and screw ( ρ d s ) dislocation densities, and correlation lengths (Le, Ls) found in the 690 nm GaN film, were associated with the better effective positron diffusion length (Leff) of L eff GaN 2 = 43 ± 6 nm.

Several aspects such as the growth relation between the layers of the GaN/AlN/SiC heterostructure, the consistency of the interfaces, and elemental diffusion are achieved by High Resolution Transmission Electron Microscopy (HR-TEM). In addition, the dislocation densities together with the defect correlation lengths are investigated via High-Resolution X-ray Diffraction (HR-XRD) and the characteristic positron diffusion length is achieved by Doppler Broadening Spectroscopy (DBS). Moreover, a comparative analysis with our previous work (i.e., GaN/AlN/Si and GaN/AlN/Al2O3) has been carried out. Within the epitaxial GaN layer defined by the relationship F4¯3m (111) 3C-SiC || P63mc (0002) AlN || P63mc (0002) GaN, the total dislocation density has been assessed as being 1.47 × 1010 cm−2. Compared with previously investigated heterostructures (on Si and Al2O3 substrates), the obtained dislocation correlation lengths (Le = 171 nm and Ls =288 nm) and the mean distance between two dislocations (rd = 82 nm) are higher. This reveals an improved crystal quality of the GaN with SiC as a growth template. In addition, the DBS measurements upheld the aforementioned results with a higher effective positron diffusion length LeffGaN2 = 75 ± 20 nm for the GaN layer.

The use of Fe films as multi-element targets in space radiation experiments with high-intensity ultrashort laser pulses requires a surface structure that can enhance the laser energy absorption on target, as well as a low concentration and uniform distribution of light element contaminants within the films. In this paper, (110) preferred orientation nanocrystalline Fe thin films with controlled morphology and composition were grown on (100)-oriented Si substrates by oblique angle RF magnetron sputtering, at room temperature. The evolution of films key-parameters, crucial for space-like radiation experiments with organic material, such as nanostructure, morphology, topography, and elemental composition with varying RF source power, deposition pressure, and target to substrate distance is thoroughly discussed. A selection of complementary techniques was used in order to better understand this interdependence, namely X-ray Diffraction, Atomic Force Microscopy, Scanning and Transmission Electron Microscopy, Energy Dispersive X-ray Spectroscopy and Non-Rutherford Backscattering Spectroscopy. The films featured a nanocrystalline, tilted nanocolumn structure, with crystallite size in the (110)-growth direction in the 15–25 nm range, average island size in the 20–50 nm range, and the degree of polycrystallinity determined mainly by the shortest target-to-substrate distance (10 cm) and highest deposition pressure (10−2 mbar Ar). Oxygen concentration (as impurity) into the bulk of the films as low as 1 at. %, with uniform depth distribution, was achieved for the lowest deposition pressures of (1–3) × 10−3 mbar Ar, combined with highest used values for the RF source power of 125–150 W. The results show that the growth process of the Fe thin film is strongly dependent mainly on the deposition pressure, with the film morphology influenced by nucleation and growth kinetics. Due to better control of film topography and uniform distribution of oxygen, such films can be successfully used as free-standing targets for high repetition rate experiments with high power lasers to produce Fe ion beams with a broad energy spectrum.

This study assesses the characteristics (edge and screw dislocation density) of a commercially available GaN/AlN/Al2O3 wafer. The heterostructure was evaluated by means of high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), and Doppler-Broadening Spectroscopy (DBS). The results were mathematically modeled to extract defect densities and defect correlation lengths in the GaN film. The structure of the GaN film, AlN buffer, Al2O3 substrate and their growth relationships were determined through HR-TEM. DBS studies were used to determine the effective positron diffusion length of the GaN film. Within the epitaxial layers, defined by a [GaN P 63 m c (0 0 0 2) || P 63 m c AlN (0 0 0 2) || (0 0 0 2) R 3 ¯ c Al2O3] relationship, regarding the GaN film, a strong correlation between defect densities, defect correlation lengths, and positron diffusion length was assessed. The defect densities ρ d e = 6.13 × 1010 cm−2, ρ d s = 1.36 × 1010 cm−2, along with the defect correlation lengths Le = 155 nm and Ls = 229 nm found in the 289 nm layer of GaN, account for the effective positron diffusion length Leff~60 nm.

We describe the status of a project for obtaining an intense beam of polarized slow positrons at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) at Magurele (near Bucharest, Romania) [1]. Positrons will be created via pair production and moderated at a tungsten target using the pulsed brilliant gamma beam which will be produced by Compton back-scattering of circularly polarized laser photons on electrons from a warm linac beam [2]. Simulations of the interaction of circularly polarized γ‑rays of energies up to 3.5 MeV and an intensity of 2.4×1010 γ/s with the target, moderation of created positrons and beam formation are discussed. The optimization of the target design showed that the primary slow positron beam can be obtained with intensity of 1‑2×106 e+/s. The primary beam will be transversally polarized with a degree of polarization of ~30%. We discuss the necessity of changing the e+ beam polarization from transversal to longitudinal by an electrostatic 90˚ bender which is proposed to work in combination with a remoderator. Simulations show that neither the remoderator nor the electrostatic bender will change the degree of e+ beam polarization. The longitudinally polarized e+ can be successfully transported to the sample chambers without depolarization, but with reduced intensity (by approximately one order of magnitude) due to the remoderation. We present a convertor-moderator assembly with a hole which will allow creating positrons in parasitic mode, i.e., simultaneously with the nuclear physics experiments at ELI-NP. The positron spectroscopy laboratory at ELI-NP will be user dedicated and the beam will have the highest intensity of polarized slow positrons for material science in the world and therefore it could become a unique tool for investigation of magnetic samples.