Thin Yttrium Films Research Articles

Features of Formation and Cathodoluminescence of Thin Films of Yttrium and Gadolinium Oxides Activated by Europium

Features of Formation and Cathodoluminescence of Thin Films of Yttrium and Gadolinium Oxides Activated by Europium

The electronic state of thin films of yttrium, yttrium hydrides and yttrium oxide

Thin films of yttrium hydride have for almost 20 years been under investigation for optoelectronic and solar energy applications due to the hydrogen-induced switching in electronic state from the metallic elemental yttrium and yttrium dihydride to the transparent semiconductor material yttrium trihydride. In this study, we investigate the electronic structure of yttrium, yttrium hydride and yttrium oxide by using X-ray photoelectron spectroscopy and kelvin probe measurements. The investigated samples have been prepared by reactive sputtering deposition. We show that the electronic work function of transparent yttrium hydride is of 4.76eV and that the recently discovered photochromic reaction lowers the electronic work function of the transparent hydride by 0.2eV.

Stress development in thin yttrium films on hard substrates during hydrogen loading

Polycrystalline (0002)-textured yttrium (Y) films of 50–500 nm thickness on sapphire substrates were loaded electrolytically with hydrogen (H). The stresses which build up in these films were measured in situ using curvature measurements. The results are compared to the behavior of bulk Y–H. A linear elastic model is used to predict the behavior of clamped thin films. Basic properties of the bulk Y–H phase diagram and elastic constants resemble the measured values of the thin films. Compressive stress builds up during H-loading in the α-Y phase and in the (α-Y+β-YH2) two-phase field, showing an initial stress increase of −1.3 GPa per hydrogen concentration XH (compressive stress). While bulk Y–H samples are known to show a contraction in the β-YH2 phase during H loading, thin films show no evidence for such a contraction during the first loading cycle of the film. The stress remains constant in the bulk β-phase concentration range (ΔXH=0.1 H/Y). This is attributed to the narrow β-phase field (ΔXH=0.02 H/Y) of the thin film during the first loading. Only samples which have been kept at a hydrogen concentration of about 1.5 H/Y for weeks show tensile stress in the concentration range of the bulk β phase. Amazingly a stress increase of about +0.5 GPa/XH (tensile stress) is measured in the β+γ two-phase field. This is attributed to the smaller in-plane nearest-neighbor distance in the γ phase compared to the β phase. In the γ-phase field compressive stress is built up again, compensating the tensile stress. It increases by −1.3 GPa/XH. In total, the net stress in Y–H films remains comparably small. This could be a reason for the good mechanical stability of such Y–H switchable mirrors during H cycling.

Kinetics of the Electrochemical Loading of Hydrogen into Thin Yttrium Films Covered by Palladium

The behavior of a thin-film yttrium electrode (150-200 nm) capped by a palladium layer (20 nm) toward electrochemical hydrogen uptake was studied in 1 M NaOH electrolyte under galvanostatic conditions. The system is of interest for smart optical windows/mirrors in devices with changeable transparency. The electrochemical behavior of the Y/Pd electrode depended on many factors, e.g., whether the loading is performed on a fresh or used electrode, current density, presence of reducible components in the electrolyte, time of spontaneous open-circuit potential relaxation and/or anodic discharge. Residual hydrogen kept by the system and its interaction with some electrolyte components is a reason for this behavior. A decisive role of the palladium layer in electrochemical kinetics was elucidated for the first time. We assume that the kinetics of the hydrogen uptake by underlying yttrium as well as the corresponding hydrogen release process is controlled by diffusion from/through hydrogenated palladium. The constant potential hydrogenation process realized at 0.045 V vs. the reversible hydrogen electrode (RHE) was proposed to be the α-β Pd hydride transition. Formation of the β-Y dihydride phase is completed on the way to the −0.095 V potential level (vs. RHE) and is irreversible. In contrast, the β-γ Y hydride transition realized at this potential level is reversible and could be repeated many times. The last potential drop to −0.345 V (vs. RHE) is a hydrogen evolution process and, simultaneously, a metal-semiconductor (and optical) transition in the phase. In a discharge process, a β-Y dihydride of different stoichiometry and α-Pd hydride are probably the only species that could be obtained. The kinetics depends extremely on the discharge conditions (open-circuit relaxation or anodic polarization), and the process shows two potential levels. Under anodic hydrogen release, surface Pd oxidation could also take place. We have proposed and discussed a scheme for hydrogenation/release in the Y/Pd electrode under different experimental conditions. © 2002 The Electrochemical Society. All rights reserved.

Surface Phenomena in the Process of Thin Yttrium Hydride Film Formation

Surface phenomena which occur in the process of thin yttrium hydride film YHx (0 < x < 2.62−2.80) formation were studied through simultaneous measurements of work function Φ and hydrogen pressure P during H2 interaction “in situ” with thin yttrium films, deposited under ultrahigh vacuum conditions. These studies were correlated with the results of parallel measurements of thin yttrium film electrical resistance and hydrogen pressure and optical observations. The experiments were performed over the pressure and temperature intervals: 10-8−10 Pa and 78−298 K, respectively. Thin YHx films (0 < x < 2.62−2.80) were easily obtained within the temperature interval 176−298 K, while at 78 K the atomic ratio x = H/Y did not exceed 0.14. At low temperatures, consumption of hydrogen occurred without any measurable work function changes. This suggests generation of a symmetrical H−Y−H adspecies on the surface, reassembling the α‘ phase known for the bulk of YHx. This adsorbate is stable at low temperature and inhibit...

Hydride films display mirror-window changes

Dutch researchers have achieved spectacular changes in optical properties of hydride films—switching them between reflective, mirrorlike state and transparent, windowlike state. room-temperature, completely reversible process may find wide application in such fields as architecture, optical communication, laser technology, and even photography. Scientists at the Condensed Matter Physics & Spectroscopy (COMPAS) Institute at Vrije University in Amsterdam exposed thin films of yttrium and lanthanum hydrides to hydrogen atmospheres. They found that mirrorlike dihydride films coated with protective layers of palladium absorb hydrogen to form transparent, windowlike films of the corresponding trihydrides [Nature , 380, 231 (1996)]. discovery was a rather unexpected spin-off' of research project on hydrogen metallization funded by the Foundation for Fundamental Research on Matter, in Utrecht, notes team leader Ronald Griessen, who is COMPAS's scientific director. The first observation of optica...

Yttrium and lanthanum hydride films with switchable optical properties

IN many substances, changes in chemical composition, pressure or temperature can induce metal-to-insulator transitions1. Although dramatic changes in optical and electrical properties accompany such transitions, their interpretation is often complicated by attendant changes in crystallographic structure2. Yttrium, lanthanum and the trivalent rare-earth elements form hydrides that also exhibit metal–insulator transitions3–5, but the extreme reactivity and fragility of these materials hinder experimental studies5,6. To overcome these difficulties, we have coated thin films of yttrium and lanthanum with a layer of palladium through which hydrogen can diffuse. Real-time transitions from metallic (YH2 or LaH2) to semiconducting (YH3 or LaH3) behaviour occur in these films during continuous absorption of hydrogen, accompanied by pronounced changes in their optical properties. Although the timescale on which this transition occurs is at present rather slow (a few seconds), there appears to be considerable scope for improvement through the choice of rare-earth element and by adopting electrochemical means for driving the transition. In view of the spectacular changes in optical properties—yttrium hydride, for example, changes from a shiny mirror to a yellow, transparent window—metal hydrides might find important technological applications.

RBS analysis of Langmuir-Blodgett films

Rutherford backscattering spectrometry (RBS) has been used to characterize thin films of yttrium and erbium arachidate deposited on a silicon substrate by a Langmuir-Blodgett technique. RBS can measure film thickness as well as the stoichiometry. Beam effects do not influence these results at moderate beam currents and fluences, although the film's visual appearance was very sensitive to beam exposure. Beam-induced hydrogen desorption is also expected, and evidence for this is presented at high doses. RBS has also been used to characterize LB films during thermal decomposition and to analyze the metal-oxide end products (Y 2O 3 and Er 2O 3 films). An example is also given where a mixed YBaCu-oxide film is deposited by this method.

Solid State Structures, Decomposition Pathways, and Vapor Phase Byproducts of Y(acac)3 Type OMVPE Precursors for thin films of Yttrium -Containing Ceramic Materials

ABSTRACTSeveral useful ceramic materials target compositions contain yttrium, Y2O3and YBa2Cu3O7-δbeing the two most widely employed. One known CVD precursor for yttrium-containing thin films is Y(tmhd)3H2O (tmhd = 2,2,6,6-tetramethylheptane-3,5-dionato). We have determined the structure and examined the vapor phase decomposition of this species. A related compound, [Y(tmod)3]2(tmod = 2,2,7-trimethyloctane-3,5-dionato), has been prepared, structurally characterized, and studied as an organometallic vapor phase epitaxy (OMVPE) precursor for Y and Y2O3films. Mechanisms of vapor phase decomposition are discussed in terms of solid film deposits and vapor phase by-products. The two precursors are compared to each other with respect to their stability windows, defined as being bound by source volatility on the low side and source stability on the high side.

Investigation of yttrium and yttrium hydride by AES and REELS

The results of combined investigations by Auger electron spectroscopy (AES) and reflection electron energy loss spectroscopy (REELS) of thin films of yttrium and yttrium hydride are presented. The hydride phase was prepared by exposing evaporated metal films to hydrogen in a high pressure reaction chamber connected to the main UHV vessel. Special attention was paid to the effects of surface contamination by oxygen. The specific spectroscopic characteristics of the metal and the hydride phase are compared and analysed in terms of the known electronic structures of yttrium and yttrium hydrides.

On the effect of substrate temperature, deposition rate and annealing on the electrical resistivity of thin yttrium films

On the effect of substrate temperature, deposition rate and annealing on the electrical resistivity of thin yttrium films

The effect of substrate temperature, deposition rate and annealing on the electrical resistivity of thin yttrium films

The effect of substrate temperature, deposition rate and annealing on the electrical resistivity of thin yttrium films in the thickness range 10 to 80 nm is reported. The resistivity of films decreases at higher deposition rates and substrate temperatures. These experimental results are analysed using the Fuchs—Sondheimer and Mayadas—Shatzkes theories. The annealing behaviour of yttrium films is in agreement with the Vand theory.

Thin film studies of yttrium, yttrium hydrides and yttrium oxide

Electron diffraction work has been carried out on yttrium films prepared in a vacuum of 5*10-7 Torr during evaporation. The crystal structure of thin yttrium films (300 AA thick or less) is mainly face-centred-cubic with a0=5.25+or-0.05 AA which can be attributed to cubic yttrium dihydride. The amount of the FCC phase decreases with increasing film thickness and films about 600 AA thick are mainly HCP alpha yttrium. When air and hydrogen are admitted to the system, the films react to form oxide and hydride respectively. The time dependence of the electrical resistance of these films has been studied and it is found that in the case of hydrogen contamination there is a minimum in the resistance-time curve. Electron diffraction observations have also been made. All the films transform to Y2O3 when heated in an intense electron beam inside the electron microscope. The resistivity results are discussed in terms of the known electrical resistivities of compounds of yttrium.

A new f.c.c. phase in yttrium films

A new polymorphic phase, characterised by a face-centered cubic structure, with lattice parameter 5.83 ± 0.02 A ̊ , has been identified in thin films of yttrium. Electron diffraction evidence and electrical resistivity measurements have been carried out in order to detect the new f.c.c. phase.