Yttrium Thin Films Research Articles

Effect of interfacial mixed layer on hydrogen absorption in yttrium thin films

Pd/Y bilayers and Pd/Ti/Y trilayers were prepared in a UHV chamber by DC/RF magnetron sputtering. The characterization of the Pd-Y and Ti-Y interfaces was performed immediately after the deposition using in-situ X-ray photoelectron spectroscopy (XPS). The results indicated significant interface mixing for the Pd/Y bilayers. Further studies showed that the additional Ti layer deposited between the palladium and yttrium layers can significantly reduce the mixing of the Pd-Y layers during the deposition process at room temperature, leading to the formation of a sharp Ti-Y interface. The structure of the obtained layers before and after the hydrogenation was examined by the standard X-ray diffraction method. The hydrogenation kinetics was studied in-situ at room temperature and pressure up to 1 bar using simultaneous measurements of resistivity and optical transmittance. It was found that for a thickness of Ti equal to 5 nm, the shortest transition time to the YH3 phase was obtained while maintaining a short switching time.

Pr3+-doped Y2O3 nanocrystals embedded in Y2O3 thin films as a sandwich-like structure prepared by pulsed laser deposition

Pr3+-doped yttria (Y2O3) nanocrystal layers embedded in between pure yttria thin films were prepared on four different substrates. Pulsed laser deposition was used to fabricate this sandwich-like structure. An exhaustive structural and optical characterization of the initial nanocrystals was performed to study their preservation once incorporated between the deposited thin films. We demonstrate that the prepared Y2O3:Pr3+ nanocrystals can be integrated into the thin films after the pulsed laser deposition process, retaining their original crystal structure and luminescent features regardless of the number of deposition cycles and the nature of the substrate. In this sense, we present a novel method to embed and protect the luminescent material, paving the way for developing future optoelectronic applications.

Effect of Interfacial Mixed Layer on Hydrogen Absorption in Yttrium Thin Films

Effect of Interfacial Mixed Layer on Hydrogen Absorption in Yttrium Thin Films

Effect of thermally grown Al2O3 on electrical insulation properties of thin film sensors for high temperature environments

NiCrAlY thin films are employed as buffer layers for high temperature thin film sensors, and thermally grown alumina (TG Al2O3) scales are formed by vacuum segregation and oxidation of Al processes at high temperature, which could synergistically enhance the insulation properties of the thin film sensors. In this work, nickel-22% chromium-10% aluminum-1% yttrium (NiCrAlY) thin films were deposited on GH3536 Ni-based superalloys by direct current (DC) magnetron sputtering. The samples were vacuum heat-treated at 1050 °C for 6 h and then subjected to isothermal oxidation at 1050 °C for different time. The morphological and electrical insulation properties of the TG Al2O3 scales were then investigated. The X-ray diffraction peaks of TG Al2O3 gradually increased with increasing oxidation time, while the surface became compact, as confirmed by scanning electron microscopy. The gradual structure from metal alloy to oxide was tailored. Several creaks were observed in the cross-section of TG Al2O3 after prolonged oxidation; thermal mismatch stress is mainly responsible for this phenomenon, which is detrimental to the adhesion of thin film sensors. Furthermore, the results of insulation resistance tests on the TG Al2O3 scales showed that the insulation resistance increased by an order of magnitude with increasing oxidation time, reaching a maximum of ~5 kΩ at 800 °C. Finally, a layer of Al2O3 was deposited by electron beam evaporation to form a multilayer insulating structure, in which the TG Al2O3 was oxidized for 6 h. A favorable insulation resistance (>2 MΩ) was obtained at 800 °C; the electrical insulation properties were assessed using a palladium chromium (PdCr) thin film strain gauge, which exhibited excellent repeatability over four thermal cycles from 25 °C to 800 °C.

Influence of Ar-impurities on the wettability of IBS-deposited Y2O3 thin films

The wettability and the control of the contact angle of surfaces are important for various applications. Materials with a water contact angle less than or greater than 90 deg are hydrophilic or hydrophobic, respectively. Most of the binary oxides are hydrophilic, but oxides of rare earth metals and metal atoms with low electronegativity have hydrophobic surfaces. Yttria is one of the materials predicted to have hydrophobic properties. In this work, we investigated the wettability of ion-beam-sputtered Yttria thin films. The measured water contact angles were between 67° and 96°. The concentration of the embedded argon atoms was found to influence the contact angle.

Hydrogen isotope permeation through yttria coatings on Eurofer in the diffusion limited regime

In fusion power plants a tritium permeation barrier is required in order to prevent the loss of the fuel. Moreover, the tritium permeation barrier is necessary to avoid that the radioactive tritium accumulates in the first wall, the cooling system, and other parts of the power plant. Oxide thin films, e.g. Al2O3, Er2O3 and Y2O3, are promising candidates as tritium permeation barrier layers. With regard to the application, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y2O3 layers with thicknesses from 100 nm to 500 nm are deposited on both sides of Eurofer substrates by RF magnetron sputter deposition. Some of the samples are additionally deposited with palladium thin films to analyse the limited regime. During the annealing in the experiments the palladium layers do not show any crack formation or delamination, verified by scanning electron microscopy. After annealing the cubic crystal structure of the Y2O3 layers is verified by X-ray diffraction. The cubic phase contains a small amount of a monoclinic phase, which is eliminated after the permeation measurements. The permeation reduction factors of the samples are determined in gas-driven deuterium permeation experiments. A permeation reduction of 5000 of the yttria thin film is verified. The diffusion limited regime is identified by the pressure dependence of the permeation measurement and by permeation experiments with the palladium top layers on the Y2O3 thin films. Furthermore, the activation energy of the permeation through the yttria thin films is determined. Pre-annealing times for more than 70 h of the Y2O3 thin films and permeation measurements with temperature cycles for 20 days are performed to show the stability of the permeation flux and hence the microstructure of the barrier layers. Measurement times at each constant temperature level of more than 25 h are required for the stabilization of each permeation flux to a constant value. The permeation measurement setup is enhanced to enable a continuously running equipment for these measurement times.

Photoemission studies of yttrium photocathodes by using the visible radiation

The present work reports on Yttrium based photocathodes. A Yttrium (Y) thin film is deposited via pulsed laser deposition (PLD) on the copper (Cu) back flange of a radio frequency (rf) gun for photocathode application. Because of a lower work function with respect to Cu, Y photocathodes are particularly appealing for the possibility to illuminate them with visible laser pulses, with the advantage of a higher energy per pulse, paving the way to high repetition rate photoinjectors, driven by conventional laser sources. In addition, working at $\ensuremath{\lambda}\ensuremath{\sim}400\text{ }\text{ }\mathrm{nm}$ the small energy difference between the Y work function (about 3 eV) and the laser photon energy reduces the contribution of the intrinsic emittance of the material. Photoelectrons, emitted by the thin film Y photocathode driven by the second harmonic of a Ti:Sapphire laser, have been characterized in terms of quantum efficiency and transverse emittance. Results have been compared with the theoretical ones obtained by the three-step model of Spicer for metallic photocathodes.

Enhancement of electrical performance of ZnSe thin films via Au nanosandwiching

Abstract In this work, we report the effect of sandwiching of Au nanosheets on the structural and electrical properties of ZnSe thin films. The ZnSe films which are grown by the thermal evaporation technique onto glass and yttrium thin film substrates exhibit lattice deformation accompanied with lattice constant extension, grain size reduction and increased defect density upon Au nanosandwiching. The temperature dependent direct current conductivity analysis has shown that the 70 nm thick Au layers successfully increased the electrical conductivity by three orders of magnitude without causing degeneracy. On the other hand, the alternating current conductivity studies in the frequency domain of 10 MHz to 1800 MHz have shown that the alternating current conduction in ZnSe is dominated by both of quantum mechanical tunneling and correlated barrier hopping of electrons over the energy barriers formed at the grain boundaries. The Au nanosheets are observed to increase the density of localized states near Fermi level and reduce the average hopping energy by ~5 times. The conductivity, capacitance, impedance and reflection coefficient spectral analyses have shown that the nanosandwiching of Au between two layers of ZnSe makes the zinc selenide more appropriate for electronic applications and for applications which need microwave cavities.

Nanomechanical and microtribological properties of yttrium thin films for photocathode engineering

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.

Influence of the grain structure of yttria thin films on the hydrogen isotope permeation

Steel components are required in the infrastructure and the facilities of the hydrogen economy. The high hydrogen pressures in the hydrogen economy lead to embrittlement and surface corrosion of the steels. For the functionality of the facilities it is necessary to suppress the embrittlement and the surface corrosion of the steels by protective layers, e.g. ceramic thin films. With regard to fusion power plants ceramic thin films on the structural steel materials are also required. These thin films work as a tritium permeation barrier that is necessary to prevent the loss of the radioactive fuel inventory. Oxide thin films, e.g. Al2O3, Er2O3, and Y2O3, are promising candidates as tritium permeation barrier layers. In terms of the application in the first wall, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y2O3 layers with thicknesses of 0.5 μm–1 μm are deposited on both substrate sides by RF magnetron sputter deposition. Since the microstructure of the barrier layer plays an important role for the permeation reduction, layers with three different magnetron process modes and thus three different microstructures are prepared. After annealing the cubic crystal structure of all thin films is verified by X-ray diffraction and the different microstructures are investigated by scanning electron microscopy and transmission electron microscopy. The Y2O3 stoichiometry of all thin films and a chromium oxide material segregation at the interface are verified by analysis methods such as X-ray photoelectron spectroscopy. The permeation reduction factors of all thin films are determined in gas-driven deuterium permeation experiments. Corresponding to the three different microstructures, reduction factors of 25, 45, and 1100 are identified. Thus, the permeation reduction is strongly dependent on the Y2O3 microstructure. The measurement results suggest that a high density of grain boundaries leads to a high hydrogen permeation.

Nano-machining, surface analysis and emittance measurements of a copper photocathode at SPARC_LAB

R&D activity on Cu photocathodes is under development at the SPARC_LAB test facility to fully characterize each stage of the photocathode “life” and to have a complete overview of the photoemission properties in high brightness photo-injectors. The nano(n)-machining process presented here consists in diamond milling, and blowing with dry nitrogen. This procedure reduces the roughness of the cathode surface and prevents surface contamination introduced by other techniques, such as polishing with diamond paste or the machining with oil. Both high roughness and surface contamination cause an increase of intrinsic emittance and consequently a reduction of the overall electron beam brightness. To quantify these effects, we have characterized the photocathode surface in terms of roughness measurement, and morphology and chemical composition analysis by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Atomic Force Microscopy (AFM) techniques. The effects of n-machining on the electron beam quality have been also investigated through emittance measurements before and after the surface processing technique. Finally, we present preliminary emittance studies of yttrium thin film on Cu photocathodes.

Pulsed laser deposition of yttrium photocathode suitable for use in radio-frequency guns

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.

Tight comparison of Mg and Y thin film photocathodes obtained by the pulsed laser deposition technique

In this work Magnesium (Mg) and Yttrium (Y) thin films have been deposited on Copper (Cu) polycrystalline substrates by the pulsed laser ablation technique for photocathode application. Such metallic materials are studied for their interesting photoemission properties and are proposed as a good alternative to the Cu photocathode, which is generally used in radio-frequency guns. Mg and Y films were uniform with no substantial differences in morphology; a polycrystalline structure was found for both of them. Photoemission measurements of such cathodes based on thin films were performed, revealing a quantum efficiency higher than Cu bulk. Photoemission theory according to the three-step model of Spicer is invoked to explain the superior photoemission performance of Mg with respect to Y.

Deposition of Y thin films by nanosecond UV pulsed laser ablation for photocathode application

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 1.2μm, homogenous with a root mean square roughness of about 2nm, 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.

Nanomechanical and optical properties of yttrium thin films by magnetron sputtering.

This Letter reports on nanomechanical and optical properties of yttrium thin films deposited on an Si (100) wafer. Elemental depth profiling by a secondary ion mass spectrometer revealed absence of formation of yttrium hydride, both on the surface and beneath. The optical properties were investigated by spectroscopic ellipsometry, and the refractive indices extracted after suitable modeling were found to be 2.51 at 546nm. Hardness and elastic modulus of these films were found to be 7 and 142GPa, respectively. These studies indicate that yttrium thin films are suitable for x-ray mirrors, photocathode emitters in e-beam lithography, electron microscopes, and free-electron lasers.

Characteristics of an Optical Fiber Hydrogen Gas Sensor Based on a Palladium and Yttrium Alloy Thin Film

To improve the hydrogen sensor characteristics, the choice of doping atoms in Pd is crucial. A novel palladium (Pd) and yttrium (Y) thin film is prepared and utilized to design a reflected optical fiber hydrogen gas sensor. The performance and mechanical properties of the hydrogen sensor are examined. Experimental results show that the response value of the sensor decreases evidently and rapidly with increasing hydrogen concentration, the first drift of zero point of sensors increases as hydrogen concentration increases, and frequent use can reduce the drift of the sensor and increase repeatability. The sensor with Pd-Y alloy film possesses smaller zero point drift and better resistance to hydrogen embrittlement than that with pure Pd film after several hydrogen loading/unloading cycles. Several thin film samples with different thicknesses are fabricated and tested. The sensitivity of the sensor is higher at low-concentration hydrogen and thick film.

Erbium-doped yttria thin films prepared by metal organic decomposition for up-conversion

Er:Y2O3 thin films have been obtained by spin coating process. Precursor solutions were prepared using nitrates as metal precursors and water as solvent. Citric, malic, and lactic acids were used as complexant. Investigations on resin compositions and on their coating parameters have been made, leading to crack-free thin films with citric and malic acids after direct deposition under standard room conditions (temperature, pressure and atmosphere). The films are homogeneous with a low root mean square roughness, less than 2.5nm. We demonstrated that the nature of the carboxylic acid is the key point to obtain high quality thin films on silicon substrates from 20nm up to 230nm thick, while the film porosity is related to the number of carbon in the acid molecule. All films exhibit up-conversion luminescence in the near infrared and in the visible range, under 1.54μm laser excitation. Furthermore, the up-conversion luminescence intensity increases with the applied annealing temperature on the films, due to an improvement of their crystallinity and to the total decomposition of organics.

Detailed Studies of Yttrium Thin Films Deposited by Laser Radiation of Different Pulse Durations

Parametric studies have been performed on yttrium films deposited by laser ablation deposition at three different laser pulse durations. Morphological investigations of the film surface have confirmed the strong influence of pulse durations on the density, size and shape of the deposited droplets. The average ablation rate and deposition rate in ns regime resulted to be one order of magnitude higher than those obtained in ps and sub-ps regimes. Present interest in the deposition of Y thin films by laser ablation technique starts from the well known photoemission properties of this metal.

Y thin films grown by pulsed laser ablation

The effects of laser fluence on the growth characteristics and surface morphology of yttrium films grown by pulsed laser deposition are investigated. The presence of droplets in the deposited films, which is the main drawback of pulsed laser deposition technique, was studied at different laser fluences. The morphology and the structure of the grown films were studied by scanning-electron microscopy and x-ray diffraction, respectively. Careful scanning-electron microscope investigations obtained by tilting the samples show that the droplets arrive to the substrate in the molten phase. The ablation rate measured at five different laser fluences (0.9–7.6 J/cm2) shows a nonlinear trend correlated with the presence of the plasma-shielding effect. The present interest in the deposition of yttrium thin films by laser ablation is due to the well-known photoemission characteristics of this metal. Depositing good-quality thin films with high adhension and low droplet density will improve the performance of photocathodes by reducing both the dark current and the thermal emittance of the photoemitted current.

Structure and microstructure of EB-PVD yttria thin films grown on Si (111) substrate

Structure and microstructure of yttria thin films grown by electron beam physical vapour deposition on a stationary Si (111) substrate at room temperature (RT), 500° and 700 °C, were investigated by the grazing-incidence X-ray diffraction and scanning electron microscopy, respectively. X-ray photoelectron spectroscopy provided information on the surface contamination from the atmosphere and the Y oxidation state. A strong effect of the deposition temperature and the vapour flux incidence angle was found. The film deposited at RT is polycrystalline with very fine grains of the body-centered cubic (bcc) crystallographic symmetry. An increase of deposition temperature results in a rapid growth of bcc grains with an improved crystalline structure. Moreover, the based-centered monoclinic phase appears for the deposition temperature of 700 °C. Preferred grain orientation (texture) with two main components, (400) and (622), was observed in the films deposited at 500 °C whereas no texture was found for 700 °C. The microstructure exhibits the columnar feather-like structure of different degrees of perfection which can be explained by the shadowing effects caused by an oblique vapour flux incidence angle. Surface morphology of the films is governed by a combination of the triangular and four-sided (square) columns. All films were found to be dense with a little porosity between the columns.