Sc2O3 Film Research Articles

Enhancing electrochemical corrosion resistance in Al–6Ni alloys through trace Sc additions

Al–Ni-Sc alloys exhibit superior mechanical properties at both room and elevated temperatures, owing to the high thermal and chemical stability of Al3Ni microfibers, and Al3Sc nanoprecipitates. This study explores the impact of scandium additions (0.2 and 0.4 wt%) in eutectic Al–6Ni casting alloys and the influence of aging treatment on electrochemical corrosion behavior in a 0.05 M NaCl solution. Scandium addition did not alter the polarization curve direction but reduced the anodic current density, slowing corrosion. Scandium strengthened the passive film, evident in Electrochemical Impedance Spectroscopy (EIS) with higher Rp values. Sc addition influences the susceptibility to localized corrosion in Al–Ni alloys by promoting the coherence of passive films through the formation of Al3Sc nanoprecipitates and affecting the distribution of Al3Ni. XPS results show that as-aged Sc-containing increases the density of coherent Al3Sc nanoprecipitates, which could result in the formation of Sc2O3 film along with films like Ni2O3 and Al2O3. These synergistic effects contribute to improving the overall corrosion resistance with Sc additions subjected to aging treatment. The microgalvanic effects between the matrix and reinforcement phases, as a result of the formation of nobler Al3Ni and Al3Sc that serve as cathodes within anode Al matrix, cause severe pitting. Nevertheless, after aged treatment, the corrosion resistance dramatically drops for alloys with 0.2 wt% Sc and remains unchanged for 0.4 wt% Sc, due to higher density of nobler Al3Sc nanoprecipitates phase that can counteract the microgalvanic effects and lead to less extensive pitting than as-aged Al–6Ni-0.2Sc.

Relationship between Oxygen Defects and Properties of Scandium Oxide Films Prepared by Ion-Beam Sputtering

Scandium oxide (Sc2O3) thin films with different numbers of oxygen defects were prepared by ion-beam sputtering under different oxygen flow rates. The results showed that the oxygen defects heavily affected crystal phases, optical properties, laser-induced damage threshold (LIDT) and surface quality of Sc2O3 films. The thin film under 0 standard-state cubic centimeter per minute (sccm) oxygen flow rate had the largest number of oxygen defects, which resulted in the lowest transmittance, LIDT and the worst surface quality. In addition, the refractive index of 0 sccm Sc2O3 film could not be measured in the same way. When the oxygen flow rate was 15 sccm, the Sc2O3 film possessed the best transmittance, refractive index, LIDT and surface roughness due to the lowest number of oxygen defects. This work elucidated the relationship between oxygen defects and properties of Sc2O3 films. Controlling oxygen flow rate was an important step of limiting the number of oxygen defects, which is of great significance for industrial production.

Mechanistic study of the atomic layer deposition of scandium oxide films using Sc(MeCp)2(Me2pz) and ozone

The atomic layer deposition (ALD) of scandium oxide (Sc2O3) thin films is investigated using Sc(MeCp)2(Me2pz) (1, MeCp = methylcyclopentadienyl, Me2pz = 3,5-dimethylpyrazolate) and ozone on hydroxyl-terminated oxidized Si(111) substrates at 225 and 275 °C. In situ Fourier transform infrared spectroscopy reveals that 1 not only reacts with surface hydroxyl groups at 275 °C, as expected but also with the SiO2 layer, as evidenced by losses in the SiO2 longitudinal optical and transverse optical phonon modes, resulting in the partial transformation of near-surface SiO2 to an ScSixOy interface layer. Ozone then combusts the MeCp groups of the O–Sc(MeCp)2 chemisorbed species, yielding surface carbonates, and oxidizes some of the underlying silicon, evidenced by gains in the SiO2 phonon modes. The Me2pz group from the next pulse of 1 reacts with these surface carbonates, leading to Sc–O–Sc bond formation (Sc2O3 deposition) and the restoration of an O–Sc(MeCp)2 surface. The reaction of the SiO2 substrate with 1 and the oxidation of silicon by ozone are temperature-dependent processes that occur during the initial cycles of film growth and directly impact the changes in the intensities of the SiO2 phonon modes. For instance, the intensity of the net gains in the phonon modes following ozone exposure is greater at 275 °C than at 225 °C. As the ALD cycle is repeated, the formation of an ScSixOy interface layer and deposition of an Sc2O3 film result in the gradual attenuation of the reaction of the SiO2 substrate with 1 and the oxidation of the underlying silicon by ozone. In addition to the ALD process, characterized by ligand exchange and self-limiting reactions, there are gas-phase reactions between 1 and residual water vapor near the substrate surface that lead to deposition of additional Sc2O3 and surface carbonates, the extent of which are also dependent on the temperature of the substrate. After 20 cycles of 1/ozone, the film thicknesses derived from ex situ X-ray photoelectron spectroscopy measurements are 2.18 nm (225 °C) and 3.88 nm (275 °C). This work constitutes the first mechanistic study of an Sc2O3 ALD process using ozone as the oxidant and emphasizes the significance of atypical reactions between the substrate and the reactants that influence the growth rate and near-surface stoichiometry during the initial cycles of film deposition.

Metal diffusion properties of ultra-thin high-k Sc2O3 films

The diffusion barrier properties of Sc2O3 against metal diffusion were studied. Tin and ruthenium were used as probe materials to study the barrier properties of Sc2O3 in thickness ranges that are of relevance for gate materials. Tin deposition and hydrogen radical etching from Sc2O3 layers of 0.5-1.5 nm thickness, deposited on Ru, show that these Sc2O3 layers effectively block the diffusion of Sn into Ru. We show that Sn adhesion and etching depends strongly on the thickness of the Sc2O3 film. The etch-rate is found to be inversely proportional to the Sc2O3 layer thickness, which we attribute to Sc2O3 becoming a more effective charge transfer barrier at larger thicknesses.

Control of scandium diffusion in LiNbO3 single crystal by co-diffusion of titanium

The mutual influence of co-diffused Ti4+ and Sc3+ in LiNbO3 single-crystal was studied. Sc3+/Ti4+ co-doped LN plates were prepared by annealing (at 1060 °C in wet O2) Z-cut congruent LiNbO3 substrates coated with stacked Sc2O3 and Ti films, which have different thicknesses and coating sequences. After the annealing, the refractive index and composition at the undoped and Sc3+-only doped surface parts were characterized by prism coupling technique. The Sc3+ and Ti4+ profiles were analyzed by secondary ion mass spectrometry. The results show that the Sc3+-doping has little contribution to the LiNbO3 refractive index. Li2O out-diffusion is not measurable. In the only diffusion case, the Sc3+ diffuses five times slower than the Ti4+. In the Sc3+/Ti4+ co-diffusion case, the Ti4+ assists and controls the Sc3+ diffusion. The Sc3+ diffusivity increases linearly with the initial Ti-metal thickness. In contrast, the Sc3+ affects the Ti4+ diffusion little. Neither the Sc2O3 film thickness nor the coating sequence influences the diffusivity of the two ions. The co-diffusion features are explained. A model for the co-diffusion system is suggested and verified experimentally. The Ti:Sc:LiNbO3 planar and strip waveguide are optically characterized. The results show that the Sc3+ presence does not affect the waveguide characteristics and the waveguide is optical-damage-resistant.

Diffusion Properties of Scandium in Lithium Niobate Crystal

Diffusion properties of Sc3+ in LiNbO3 crystal were studied, together with other two related issues: Sc3+‐doping effect on LiNbO3 refractive index and Li2O out‐diffusion arising from Sc3+ in‐diffusion. To reach the goal, some Sc3+‐doped LiNbO3 crystal plates were prepared by in‐diffusion of Sc2O3 film coated onto Z‐cut congruent substrates in air at different temperatures ranging from 1030°C to 1130°C. After diffusion, the refractive indices at the doped and undoped parts of surface were measured by prism coupling technique and the surface composition was evaluated from the measured index. The results show that the Sc3+ doping has little contribution to the substrate index and the Li2O out‐diffusion is not measurable. The Sc3+ profile was analyzed by secondary ion mass spectrometry. Some characteristic parameters such as temperature‐dependent diffusivity, diffusion constant, activation energy, and surface concentration are obtained. The Sc3+/Ti4+ co‐diffusion characteristics were also studied. Comparison shows that in the single‐diffusion case, the Sc3+ diffusivity is considerably smaller than the Ti4+ diffusivity. In the Sc3+/Ti4+ co‐diffusion case, the Ti4+ co‐diffusion results in substantial increase in Sc3+ diffusivity while the Ti4+ diffusivity changing little. The diffusion characteristics are qualitatively explained. The Ti4+‐assisted Sc3+ diffusion would be utilizable in shortening the fabrication period of an optical‐damage‐resistant Ti‐diffused LiNbO3 waveguide doped with Sc3+, and some considerations for the fabrication are given.

Sсandium(III) Beta-diketonate Derivatives as Precursors for Oxide Film Deposition by CVD

Abstract Complexes with acetylacetone Sc(acac)3, dipivaloylmethane Sc(thd)3, 2,2,6,6–tetramethyl-4-fluoro-3,5-heptanedione Sc(tfhd)3, pivaloyltrifluoroacetone Sc(ptac)3, trifluoroacetylacetone Sc(tfac)3, and hexafluoroacetylacetone Sc(hfac)3 were synthesized, purified and identified by elemental analysis, m.p., IR and NMR spectroscopy, and mass spectrometry. The thermal behaviour of the synthesized compounds in the solid state was investigated by the method of difference-scanning calorimetry in vacuum. As a result the thermodynamic characteristics of the melting processes were determined. The temperature dependences of saturated and unsaturated vapour pressure of complexes under study were measured by static method with membrane-gauge manometers. The average molecular weight of gas calculated from the experimental data on unsaturated vapours using ideal gas law was close to the molecular weight of monomer for all investigated compounds. Decomposition temperatures of compounds under study were defined as the temperature above that pressure changes became irreversible. The information about melting and decomposition processes were taken into account at measuring saturated vapour pressure of complexes. The row of volatility Sc(hfac)3 > Sc(ptac)3 > Sc(tfac)3 > Sc(thd)3 ≥ Sc(tfhd)3 > Sc(acac)3 was determined from the p–T dependences obtained. Above information about thermal behaviour of complexes enabled one to use it as a guide for CVD experiments aimed at achieving oxide films with high optical properties. Precursor chosen for film deposition was Sc(thd)3 The Sc2O3 film deposition conditions were following: the gas-carrier rate 1 l/h, He gas-reagent rate 10 l/h, total pressure 10 Torr, evaporator temperature 105-110С, substrate temperature 450- 650С, substrate Si(100). Ellipsometry was applied to characterize the film thickness and refractive index. The morphology and the composition of the films were determined with XPS and SEM.

Atomic layer deposition of Sc2O3 for passivating AlGaN/GaN high electron mobility transistor devices

Polycrystalline, partially epitaxial Sc2O3 films were grown on AlGaN/GaN substrates by atomic layer deposition (ALD). With this ALD Sc2O3 film as the insulator layer, the Sc2O3/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors showed excellent electrical performance with a high Ion/Ioff ratio of over 108 and a low subthreshold slope of 75 mV/dec. The UV/NH4OH surface treatment on AlGaN/GaN prior to ALD was found to be critical for achieving these excellent figures. In addition, the Sc2O3 dielectric is found to be negatively charged, which facilitates the enhancement-mode operation. While bare Sc2O3 suffers from moisture degradation, depositing a moisture blocking layer of ALD Al2O3 can effectively eliminate this effect.

Effect of surface condition on the kinetics of scandium-deuteride formation

We studied the effects of surface oxidation on the kinetics of deuterium (D2) absorption by Sc. A Sieverts apparatus was used to measure the D2 absorption kinetics and X-ray photoelectron spectroscopy (XPS) was used to determine the surface oxidation state during heat treatments in ultra-high vacuum. In the as-received condition, the bulk Sc sample was covered by an Sc2O3 film. However, the Sc2O3 phase decomposed dramatically at 573 K due to the fast diffusion of oxygen from the surface to the bulk. The initial surface of Sc was cleaned of most oxygen by heating up to 873 K whereas the obtained fresh surface at temperatures over 873 K was contaminated by oxygen again when cooled down to room temperature. Due to fresh surfaces available for D2 absorption, the sample at temperatures over 873 K started to absorb D2 as soon as it was introduced. The absorption rate at these temperatures appears to be governed by D diffusion in the Sc bulk. However, the sample deuterated at room temperature presented an incubation time before the absorption was observed. And this stage was followed by a surface controlled process that changed gradually to a stage related to deuterium diffusion into the bulk.

Single crystalline Sc2O3/Y2O3 heterostructures as novel engineered buffer approach for GaN integration on Si (111)

The preparation of GaN virtual substrates on Si wafers via buffer layers is intensively pursued for high power/high frequency electronics as well as optoelectronics applications. Here, GaN is integrated on the Si platform by a novel engineered bilayer oxide buffer, namely, Sc2O3/Y2O3, which gradually reduces the lattice misfit of ∼−17% between GaN and Si. Single crystalline GaN(0001)/Sc2O3(111)/Y2O3(111)/Si(111) heterostructures were prepared by molecular beam epitaxy and characterized ex situ by various techniques. Laboratory-based x-ray diffraction shows that the epitaxial Sc2O3 grows fully relaxed on the Y2O3/Si(111) support, creating a high quality template for subsequent GaN overgrowth. The high structural quality of the Sc2O3 film is demonstrated by the fact that the concentration of extended planar defects in the preferred {111} slip planes is below the detection limit of synchrotron based diffuse x-ray scattering studies. Transmission electron microscopy (TEM) analysis reveal that the full relaxation of the −7% lattice misfit between the isomorphic oxides is achieved by a network of misfit dislocations at the Sc2O3/Y2O3 interface. X-ray reflectivity and TEM prove that closed epitaxial GaN layers as thin as 30 nm can be grown on these templates. Finally, the GaN thin film quality is studied using a detailed Williamson–Hall analysis.

Environmental stability of candidate dielectrics for GaN-based device applications

The thermal and chemical stability of potential dielectrics for GaN-based devices in a GaN metal organic chemical vapor deposition (MOCVD) environment is investigated, and their suitability for use as a gate dielectric and as a regrowth mask for the selective area growth of GaN is discussed. Thin films of MgO, Sc2O3, and Sc2O3/MgO were grown by molecular beam epitaxy on GaN/sapphire substrates and then annealed in a MOCVD reactor under GaN growth conditions except for the lack of trimethylgallium. All films were processed into metal-oxide-semiconductor diodes and were characterized before and after being annealed using atomic force microscopy, x-ray reflectivity, x-ray photoelectron spectroscopy, current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Gibbs free energy of all possible reactions was calculated, and their probability and possible influence on the characterization results is examined. After being annealed, the atomic force microscopy of the oxide films showed some degree of roughening for all of them. Despite the surface roughening, all the oxide films examined showed potential for use as a regrowth mask. The MOCVD anneal caused the electrical properties of the MgO film to degrade considerably, and the Sc2O3 films were unable to be electrically characterized after annealing due to shorting, which is believed to be caused by the formation of a ScN layer on the surface. The effect of the thickness of the Sc2O3 cap for MgO films was investigated. The characterization results indicate that the Sc2O3 film dissolved into MgO during annealing and that a ScN film did not form on the surface. Of all the oxide films examined in this study, the Sc2O3/MgO stack with the thinner Sc2O3 cap exhibited the greatest stability with respect to its electrical properties.

Conduction band energies and hot-electron transport characteristics of epitaxial Sc2O3∕Si (111) studied by ballistic electron emission microscopy and internal photoemission

Ballistic electron emission microscopy (BEEM) and internal photoemission measurements on a 20-nm-thick epitaxial Sc2O3 film on Si (111) show the existence of a lower “tail state” conduction band (CB) extending ∼0.9eV below the upper CB (similar to that reported for amorphous Sc2O3 films), indicating that these states are not simply due to disorder in amorphous films. This lower CB is also found to support elastic hot-electron transport even against an applied electric field, indicating transport via extended rather than localized states.

Structure of Sc2O3 films epitaxially grown on α-Al2O3 (0001)

The crystal structure of scandium oxide films epitaxially grown on α-Al2O3 (0001) under an ultrahigh-vacuum is studied by single-crystal x-ray diffraction. The Sc2O3 film grows in bixbyite phase on the basal (0001) surface of the sapphire substrate with its ⟨111⟩ axis aligned parallel to the substrate normal. In-plane orientation of the film, however, exhibits two distinct growth directions that are defined by the two possible surface orientations of the stepped α-Al2O3 substrate. The atomic structure of the high-quality epitaxial film is fully relaxed and the film has unusual thickness uniformity.

High-quality nanothickness single-crystal Sc2O3 film grown on Si(111)

High-quality single-crystal Sc2O3 films a few nanometer thick have been grown epitaxially on Si (111) despite a huge lattice mismatch. The films were electron-beam evaporated from a Sc2O3 target. Structural and morphological studies were carried out by x-ray diffraction and reflectivity, atomic force microscopy, high-resolution transmission electron microscopy, and medium-energy ion scattering, with the initial epitaxial growth monitored by in situ reflection high-energy electron diffraction. The films have the cubic bixbyite phase with a remarkably uniform thickness and high structural perfection. The film surfaces are very smooth and the oxide/Si interfaces are atomically sharp with a low average roughness of 0.06nm. The films are well aligned with the Si substrate with an orientation relationship of Si(111)‖Sc2O3(111), and an in-plane expitaxy of Si[1¯10]‖Sc2O3[1¯01].

Surface Passivation of AlGaN terminated and GaN Terminated HEMT Structures Studied by XPS

ABSTRACTSamples of both AlGaN and GaN terminated HEMT structures were studied using x-ray photo spectroscopy (XPS). It was found that the XPS spectra of both AlGaN and native oxide surfaces were shifted by a surface charge of 0.5 to 1.0 eV. The samples were then oxidized using an UV-ozone treatment for 25 minutes at room temperature. The ozone oxide XPS spectra of the AlGaN terminated surface was found to have the same 0.5 to 1.0 eV shift while the ozone oxide XPS spectra of the GaN terminated surface was found to have a 5.0 eV shift, indicating that the native GaN oxide is more insulating. Processed HEMT devices using both surface terminations were given the same UV-ozone treatment followed by a 10nm MBE grown Sc2O3 film for passivation. The GaN terminated HEMT structures showed on average a 10% increase in channel current (from gate-lag measurements, pulse mode) over the AlGaN terminated HEMT structures.