Electron energy-loss spectroscopy investigation of Y2O3 films on Si (001) substrate

Electron energy -loss spectroscopy has been used to investigate the interface between a Y₂O₃ film and the silicon substrate. The chemical composition of the interface layer is revealed to be nearly pure amorphous SiO₂. Yttrium silicates are found at the Y₂O₃/SiO₂ interface region. The formation of the interfacial yttrium silicates has been interpreted by the direct chemical reaction between the deposited Y₂O₃ film and the SiO₂ interface layer. The Si L₂₃ and O K edges of yttrium silicates (Y₂SiO₅ and Y₂Si₂O₇) have been calculated by the first-principle full multiple - scattering method. The theoretical results are consistent with the experimental spectra, which confirms the formation of yttrium silicates.     

Thermal oxidation of transition metal silicides: The role of mass transport

Thin films of almost all transition metal silicides on a silicon substrate oxidize and form SiO₂ on their surface when they are annealed in an oxidizing ambient atmosphere. In applications of these silicides as interconnects in integrated circuits, the oxidation characteristics of the silicide and the SiO₂ growth rate are very important.We review the four major steps controlling silicide oxidation: (1) oxidant transport through the oxide; (2) reaction at the silicide-oxide interface: (3) net transport of silicon atoms with respect to metal atoms in the silicide; (4) reaction at the silicide-silicon interface. The oxidant transport is shown to be the same for all silicides. The reaction at the silicide-oxide interface is explored using equilibrium thermodynamic arguments. The transport through the silicide is discussed and experimental results of inert marker experiments are presented.The diffusing species during the oxidation of PdSi, Pd₂Si, NiSi₂, CoSi₂, PtSi, CrSi₂ and TiSi₂ are discussed. The diffusing species during oxidation correlate with the moving species in silicide formation. A discussion of a mechanism that explains why the oxidation rate of some silicides on a silicon substrate is faster than that of the bare substrate is presented.     

Thermal oxidation of transition metal silicides

A survey has been made of most publications on the oxidation of transition metal silicide films on either silicon or an SiO₂ substrate.On silicon substrates the general trend is that SiO₂ forms on the sample surface in preference to metal oxides, with the silicide layer being morphologically preserved. Thermodynamics, in terms of heats of formation and ternary phase diagrams, has been used successfully to explain the general absence of metal oxides and also to explain the exceptions to that rule. Kinetics also plays a part in the determination of the reaction products. The growth rate of SiO₂ on silicon substrates obeys the linear-parabolic law. The parabolic rate constant of silicide oxidation is essentially the same as that of silicon oxidation, indicating that the oxide and its diffusivity for the oxidant are the same for silicon and silicides. However, the linear rate constant of silicides exceeds that of silicon, and its value varies with the silicide. The differences among silicides might be attributed to differences in the atomic transport processes within the silicide; the enhancement with respect to silicon has been ascribed to the metallic nature of the silicides.On SiO₂ substrates, the oxidation ultimately leads to the formation of metal oxides as well. Instabilities of structure and loss of material can occur.The properties of the grown SiO₂ are reviewed and directions for further studies are outlined.     

Comparison of TiSi2 films prepared by diffusion and by co-evaporation

The effect of the preparation technique of TiSi₂ films on their properties is discussed. Films formed by the diffusion of silicon into a titanium layer evaporated on top of an Si/SiO₂/n⁺-poly-Si structure (where poly-Si is poltcrystalline silicon) have some unfavorable properties for metal/oxide/semiconductor technology. A significant improvement can be achieved using the co-evaporation technique to form the silicide layer. Superior silicides are obtained by sintering the co-evaporated TiSi₂ using short-time annealing.     

Thin films of rare earth metal silicides

The phase composition, conductance and surface morphology of thin films of silicides of rare earth metals (of the yttrium subgroup) were studied. The silicides were formed by annealing thin film structures of the rare earth metal (Ln) and silicon at 473–673 K for 1–120 min in vacuum. X-ray analysis revealed the formation of crystalline silicide phases of composition LnSi_2-x and of the AlB₂ structural type for all the metals concerned except scandium, gadolinium and lutetium.It was established that the formation of the crystalline silicide phase is determined by the relation between the crystallographic parameters of a rare earth metal hexagonal lattice and silicon; the critical value of the lattice mismatch a is ± 1.3%. The kinetics of formation of a silicide phase were determined by measuring the conductance of thin film structures. A model for the formation of rare earth metal silicides in thin film structures is proposed, which serves as a basis for establishing conditions for the formation of quasi-amorphous, polycrystalline or large-block rare earth metal silicide layers, with a perfect silicide-silicon interface, taking into account the crystallographic orientation and parameter relationship of the substrate and the silicide.     

Formation and characterization of semiconductor Ca2Si layers prepared on p-type silicon covered by an amorphous silicon cap

Formation of calcium silicide on three types of templates: Si(111)7 × 7, 2D Mg₂Si, and 3D Mg₂Si, was studied during Ca deposition at 120 °C in situ by Auger and electron energy loss spectroscopy, and by differential optical reflectance spectroscopy. A continuous Ca₂Si layer is formed on 2D and 3D Mg₂Si templates; but, on an atomically clean silicon surface (Si(111)7 × 7), a mixture of Ca₂Si with another Ca silicide was found. The growth of a Si cap layer over the Ca silicide layers at about 100 °C studied by in situ methods demonstrated the full embedding of Ca silicide in amorphous silicon, independent of the used template. Transmission electron microscopy, Rutherford backscattering spectrometry, atomic force microscopy, and electrical characterization of Schottky junctions revealed the Ca₂Si and Mg₂Si nanoparticles and the redistribution of Mg and Ca during the silicon cap growth and its effect on the electronic properties of the structures. Reproduction of the experiments on higher doped and better purity substrates is needed to understand better the role of Mg- and Ca-related defects, and defects of silicon generated by the growth process.     

Phase stability,microstructure and high-temperature properties of NbSi<Subscript>2</Subscript>- and TaSi<Subscript>2</Subscript>-oxide conducting ceramic composites

NbSi₂- and TaSi₂-based electroconductive ceramic composites with the addition of 40–70 vol% Al₂O₃ and ZrO₂ particles were fabricated by high-temperature sintering (1400–1600 °C) under argon. Their phase stability, microstructural evolution, oxidation kinetics and electrical properties were studied at high temperatures. The densification of the composites was improved by increasing the oxide phase content and sintering temperature. The interaction of the starting metal disilicides with residual oxygen sources resulted in the formation of the hexagonal-structured 5–3 metal silicide (Nb₅Si₃ and Ta₅Si₃) phases. The increasing sintering temperature and volume percentage of the oxide phase reduced the pest oxidation, particularly for the silicide–alumina composites, which exhibited lower oxidation-induced mass changes than their dense monolithic metal silicides. Depending on the silicide–oxide volume percentage, their electrical conductivities ranged from 5.3 to 111.3 S/cm at 900 °C. Their phase stability, reduced oxidation rates and high electrical conductivities at high temperatures show promise for future high-temperature applications in advanced sensing.     

Glass-ceramic coatings based on organoyttroxanealumoxanesiloxanes

Using organoyttroxanealumoxanesiloxanes soluble in organic solvents, we prepared binders and film-forming agents that were employed to produce protective high-temperature antioxidativ xY₂O₃ · yAl₂O₃ · zSiO₂ coatings on carbon and silicon carbide fibers [1].     

Incorporation of Si–O induced valence state variation of cerium ion and phase evolution in YAG:Ce phosphors for white light emitting diodes

The role of Si⁴⁺ in the valence state of Ce and phase control in YAG:Ce phosphors was investigated upon incorporation of Si–O in the form of SiO₂. By varying the amount of SiO₂ addition, the valence state of tetravalent and trivalent Ce ions was identified distinctly by X-ray photoelectron spectroscopy and the phase purity of YAG host phase was examined by X-ray diffraction patterns. The self-reduction phenomena from Ce⁴⁺ to Ce³⁺ in YAG:Ce samples was observed to be featured in a typical 5d → 4f yellow-green transition of Ce³⁺ ion upon firing in air, driven by charge compensation for imbalance substitution of Ce⁴⁺ for Y³⁺. The addition of SiO₂ promotes further reduction of Ce⁴⁺ to Ce³⁺ in an amount up to 7.0 wt%, owing to spontaneous charge compensation, and suppresses the formation of YAP (yttrium aluminate perovskite, YAlO₃) and YAM (yttrium aluminate monoclinic, Y₄Al₂O₉). The results reveal the role of SiO₂ addition in a proper amount to be able to achieve desired luminous center of Ce³⁺ and phase-pure YAG for a series of YAG hosted luminescence materials such as blue-excitable YAG phosphor or laser-pumped YAG-based transparent ceramics or glass ceramics for lighting and display purposes.     

Oxidation resistance of β-Sialon/TiN composites: an ion beam analysis (IBA) study

The oxidation resistance of β-Sialon processed with Y₂O₃ sintering additive and β-Sialon/TiN composites containing 1–10 wt% TiN was studied using ion beam analysis (IBA) techniques, augmented by XRD and SEM measurements. Rutherford backscattering spectrometry was used to monitor the diffusion of Y and Ti in the oxidised samples, and the diffusion of oxygen and nitrogen was observed by particle-induced gamma emission and nuclear reaction analysis. These techniques showed that in the Sialon control sample without TiN, oxygen was the first element to migrate at 1000 °C, followed by Y and N at 1100 °C. At 1200 °C, a N-poor, Y- and O-rich oxidised layer was formed, containing crystalline Y₂Si₂O₇. In the TiN-containing samples, Si, Al, Y and Ti were very mobile even at 1000 °C and the surface nitrogen was depleted by 1250 °C. The combined presence of yttrium aluminium garnet (YAG) and TiN protects the β-Sialon phase by forming an oxygen-rich crystalline barrier layer. The oxidation products of TiN in these composites are TiO₂ and Y₂Ti₂O_7. The details of the oxidation mechanism of the β-Sialon/TiN composites provided by these IBA studies (movement of yttrium and titanium, replacement of nitrogen by oxygen in the glassy yttrium phase and major crystalline and chemical changes in an outer oxidised layer) could not readily have been obtained by any other techniques, and illustrate the value of IBA for oxidation studies of non-oxide ceramics.     

Formation and oxidation mechanisms in two semiconducting silicides

Marker experiments have been used to study the formation and the oxidation of two semiconducting silicides with large band gaps. Both Ru₂Si₃ and Ir₃Si₅ have been shown to form by mechanisms dominated by the motion of the silicon atoms. The same is true for the growth of SiO₂ over these two silicides under conditions which ensure the integrity of the silicide layers. IrSi has also been shown to grow mostly through the motion of silicon.     

Europium-doped yttrium silicate nanophosphors prepared by flame synthesis

Europium-doped yttrium silicate (Y₂SiO₅:Eu³⁺) nanophosphors were successfully synthesized by flame spray pyrolysis method. The effect of silicon concentration on the crystal structure and morphology of the Y₂SiO₅:Eu³⁺ phosphors were investigated. As-prepared phosphor consists of spherical nanoparticles with filled morphology, high crystallinity, narrow size distribution, and intense photoluminescence. The crystal structure and photoluminescence intensity of Y₂SiO₅:Eu³⁺ nanophosphors are strongly affected by the ratio of silicon to yttrium in the precursor solution, and the maximum photoluminescence intensity is obtained from particles prepared from the silicon to yttrium ratio of 1.25. A concentration quenching limit is observed at 30 mol% Eu of yttrium. The photoluminescence intensity also increases with the increase of the concentration of precursor solution. This work demonstrates the advantages of flame spray pyrolysis method for the preparation of multi-component nanophosphor, which can be found potential application in lamp and display industries.     

Growth of Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> films on Si with AlO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and SiO<Subscript>2</Subscript> buffer layers by ion beam sputtering

Amorphous yttrium iron garnet films ranging in thickness from 100 to 600 nm have been produced on single-crystal silicon substrates by sputtering a polycrystalline target with the composition Y₃Fe₅O₁₂ (yttrium iron garnet) by a mixture of argon and oxygen ions. Before film growth, AlO _x or SiO₂ buffer layers up to 0.8 μm in thickness were grown on the Si surface. The heterostructures were crystallized by annealing in air at a temperature of 950°C for 30 min. The properties of the films were studied by magneto-optical techniques, using Kerr effect and ferromagnetic resonance measurements. The Gilbert damping parameter reached 2.8 × 10^–3 and the effective planar magnetic anisotropy field was independent of the nature of the buffer layer. This suggests that the thin-film heterostructures obtained in this study are potentially attractive for use in spin-wave semiconductor devices.     

Effects of Mechanochemical Treatment on Yttrium Oxyapatite Formation

Structural changes of Y₂O₃-SiO₂ solid system during a fine milling in air were investigated. The milling of a mixture in a molar ratio Y₂O₃/SiO₂ = 7/9 affect the structure of the solids. After heating the samples at T > 1000°C new phases were obtained by a thermal reaction between Y₂O₃ and SiO₂, as a result of the mechanochemical activation. The extent of the mechanochemical reaction was higher when using hydrous yttria instead of Y₂O₃. It was found that the yttrium oxyapatite is unstable and attempts to prepare this silicate by a solid-state reaction always results in the formation of a mixture containing Y₂SiO₅, Y₂Si₂O₇, and yttrium oxyapatite.     

Microstructure and electrical properties of rare-earth oxides doped ZnO-based linear resistance ceramics

The ZnO-based linear resistance ceramics doped with rare-earth oxides (Y₂O₃, La₂O₃) have been prepared by the conventional ceramics method. The effect of the different doping amounts of rare-earth oxides on the microstructure, electrical properties and aging behavior were investigated in detail. The electrical properties analysis shows that the La₂O₃ addition on ZnO-based linear resistance ceramics has more obviously influence than the addition of Y₂O₃. The samples with the La₂O₃ content of 0.25 mol% sintered at 1,320 °C exhibits excellent electrical properties with the resistivity of 253.5 Ω cm and the resistance temperature coefficient of 3.62 × 10^?4/ °C, which are improved by 60 and 90 %, respectively. The nonlinear coefficient of voltage decreases to 1.16, decreases by 4.3 % and the stability of ?1.2 % in the variation rate of the resistivity for aging temperature.     

Initial stages of silicon-iron interface formation

The formation of a silicon-iron (Si/Fe) interface has been studied in situ by the method of high-resolution photoelectron spectroscopy using synchrotron radiation. The experiments were performed under ultrahigh vacuum conditions (at a residual pressure of 3 × 10^?10 Torr) in a range of Si coating thicknesses within 0.04–0.45 nm. It is established that the process begins with the formation of a FeSi silicide and Fe-Si solid solution on the iron substrate surface. As the Si coating thickness increases, the solid solution converts into ferromagnetic (Fe₃Si) and nonmagnetic (FeSi) silicides. It is shown that thermostimulated solid-state reactions leading to the transformation of FeSi and Fe₃Si silicides into a semiconducting β-FeSi₂ silicide start at a temperature close to 600°C.     

A novel Y<Subscript>3</Subscript>ZnAl<Subscript>3</Subscript>SiO<Subscript>12</Subscript> microwave dielectric ceramic for microwave communication application as substrates

Novel microwave dielectric ceramic Y₃ZnAl₃SiO₁₂ were synthesized by solid-state route. The crystal structure of synthesized samples was characterized by X-ray diffraction and refined with Rietveld method. Microstructure and microwave dielectric properties of Y₃ZnAl₃SiO₁₂ ceramics were investigated using Scanning Electron Microscopy and Hakki–Coleman method. X-ray data display that major phase of Y₃ZnAl₃SiO₁₂ is isostructural to Y₃Al₅O₁₂ with a cubic garnet structure and space group of Ia-3d, which is composed of (Al/Si)O₄ tetrahedron, (Zn/Al)O₆ octahedron and YO₈ dodecahedron, besides minor Y₂SiO₅ secondary phase. The distribution of grain sizes is closer to Gauss distribution. Bulk density of samples has a similar variation curve with Q*f of samples. Y₃ZnAl₃SiO₁₂ ceramics exhibit excellent microwave dielectric properties: ε_r?=?10.2, Q*f?=?37938.2 (@9.47 GHz), τ _f ?=??31.7 ppm/°C at sintering temperature of 1500 °C. Our results indicate Y₃ZnAl₃SiO₁₂ could be a potential material for millimeter wave communication systems as microwave substrates.     

Raman study of Ni and Ni silicide contacts on 4H- and 6H-SiC

Ni₂Si, NiSi and NiSi₂ contacts were prepared on n-type 4H- and 6H-SiC(0001) by deposition of Ni and Si multilayers in the respective stoichiometry after high-temperature annealing, as well as pure Ni contacts. After annealing, the individual contacts were analyzed by Raman spectroscopy and electrical property measurements. Contact structures were then etched-off and subsequently observed by means of AFM (Atomic Force Microscopy). Ni reacted with SiC, forming Ni₂Si and carbon. At Ni_xSi_y/SiC contact structures the respective silicides were already formed at low annealing temperatures, when only Schottky behavior of the structures was observed. The intended silicides, once formed, did not change any further with increasing annealing temperature. All contact structures provided good ohmic behavior after being annealed at 960 °C. By means of combined AFM and Raman analysis of the etched-off contacts we found that the silicide contact structures very probably did not react with SiC which is in accordance with the thermodynamic assumptions. After annealing the silicide contact structures at such temperature, when Schottky behavior changed to ohmic, a certain form of interaction between the SiC substrate and the silicide contact structures must have occurred.     

Impurity effects in molybdenum silicide formation

The reaction between molybdenum thin films and single-crystal Si〈111〉 substrates was studied as a function of the concentrations of impurities (mainly oxygen) in the metal film. At a low oxygen concent (1–2 at.%), only the silicide phase MoSi₂ was observed, and a thickness proportional to the square root of time corresponding to an average activation energy of 3 eV in the temperature range 545–600 °C was found. During the formation of the silicide the oxygen originally present in the molybdenum films accumulates at the interface between the silicon and the MoSi₂.In contrast, a higher oxygen content (4–5 at.%) prevents the formation of any silicide phases in the above temperature range and leads to the formation of MoSi₂ and Mo₅Si₃ phases at temperatures near 800 °C. MoSi₂ was always observed at the inner interface with Mo₅Si₃ on the surface.The oxygen segregates from the silicides and accumulates at the Si-MoSi₂ and MoSi₂-Mo₅Si₃ interfaces to form a non-uniform layer of SiO_x(x ? 2).     

Thermal-stress stability of yttrium oxide as a buffer layer of metal-ferroelectric-insulator-semiconductor field effect transistor

The effect of thermal stress on the electrical properties of ferroelectric/semiconductor structure was investigated when introducing Y₂O₃ film as a barrier layer in the structure. Two different thermal stress states could be obtained by fast (400 °C/min) or slow (30 °C/min) cooling of sputter-deposited Y₂O₃ films on silicon wafer from 800 °C. The formation of interfacial layer containing Y-Si-O and SiO₂ layers while annealing could be characterized by using a spectroscopic ellipsometry. The introduction of strain-induced defects from thermal stress of the fast cooled sample showed a soft breakdown at low applying voltage. In the capacitance-voltage relation, a flat band voltage shift, hysteresis, and stretch-out phenomena were also observed. Nd₂Ti₂O₇ was spin deposited using sol-gel procedure on the Y₂O₃/Si to form a metal-ferroelectric-insulator field effect transistor structured sample. These Nd₂Ti₂O₇/Y₂O₃/Si samples were also furnace-annealed at 800 °C and cooled down to room temperature fast or slowly. One order lower value of leakage current, 1E−8 A/cm² was observed with these samples when comparing with the Y₂O₃/Si samples. A soft breakdown of the fast cooled sample seemed to have the same origin as the fast cooled Y₂O₃/Si sample, i.e., the strain-induced defects in the interfacial layer containing Y-Si-O and SiO₂ phases. Hysteretic gaps of the Nd₂Ti₂O₇/Y₂O₃/Si samples showed a possibility to be used as a memory window for ferroelectric gate.