Oxidation kinetics of thin titanium films grown on tungsten

Growth of thin Ti films on (100)W and the kinetics of their oxidation are studied using thermal-desorption spectroscopy and Auger electron spectroscopy. Titanium films grow nearly layer by layer on the (100)W face at room temperature. The activation energy for desorption of Ti atoms decreases from 5.2 eV for coverage θ=0.1 to 4.9 eV in a multilayer film. Oxidation of a thin (θ=6) titanium film starts with dissolution of oxygen atoms in its bulk to the limiting concentration for a given temperature, after which the film oxidizes to TiO, with the TiO₂ oxide starting to grow when exposure of the film to oxygen is prolonged. The thermal desorption of oxides follows zero-order kinetics and is characterized by desorption activation energies of 5.1 (TiO) and 5.9 eV (TiO₂).     

Adsorption of gold on oxidized tungsten

The kinetics of adsorption and desorption of gold atoms from the surface of a thin (<2 nm) oxide film grown on a textured W ribbon with the preferred emergence of the (100) face is studied using termal desorption spectrometry in a wide range of coatings. A single desorption phase is observed in the spectra of termal desorption of Au atoms from oxidized W. The activation energy of desorption of Au atoms from tungsten oxides is lower than the gold sublimation heat (it amounts to E = 3.1 eV for the concentration of adsorbate atoms on the surface corresponding to coverage θ _s = 2.38). The gold film on oxidized tungsten at room temperature grows in the form of 3D islands. The sticking coefficient for gold atoms at T = 300 K is close to unity in the coverage range 0.5 < θ _s < 2.5.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

Electron-stimulated desorption of rare-earth metal atoms

The yield of europium and samarium atoms in electron-stimulated desorption from layers of rare-earth metals (REMs) adsorbed on the surface of oxidized tungsten has been measured as a function of the incident electron energy, surface coverage by REMs, degree of tungsten oxidation, and substrate temperature. The measurements were performed using the time-of-flight method with a surface-ionization-based detector within the substrate temperature interval 140–600 K. The yield studied as a function of electron energy has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, and those of Eu atoms, at 36 and 41 eV. These energies correlate well with the REM 5p and 5s core-level excitation energies. The REM yield is a complex function of the REM coverage and substrate temperature. The peaks due to REM atoms are seen at low REM coverages only, and their intensity usually passes through a maximum with increasing coverage and substrate temperature. The concentration dependence of the REM atom yield is affected by the deposition of slow Ba⁺ ions, but only if they are deposited after the REM adsorption. At higher REM coverages, additional peaks are observed at electron energies of 42, 54, and 84 eV, which originate from excitation of the 5p and 5s tungsten levels and result from desorption of SmO and EuO molecules. The temperature dependence of the intensity of these peaks is explained to be due to the order-disorder phase transition. The desorption of REM atoms is the result of their reversed motion through the adsorbed REM layer, and the SmO and EuO molecules desorb due to the formation of an antibonding state between the REM oxide molecules and the tungsten ions.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten: Concentration dependence of the low-energy peak

The nature of electron-stimulated desorption of europium atoms Eu⁰ at low incident electron energies E _e (～30 eV) and the specific features of the dependence of the yield of europium atoms Eu⁰ on their concentration on the surface of oxidized tungsten are discussed. The crucial stage is found to be the primary event of vacancy creation in the inner 5p shell of the europium adatom. As follows from estimates, only the first of the two possible ionization scenarios (intratomic electron transfer to the outer shell of the Eu adatom or ejection of the knocked-out electron into vacuum) results in Eu⁰ desorption. The concentration threshold of the Eu⁰ yield is determined.     

Growth of germanium thin films on the W(100) surface

The growth of Ge thin films on the surface of a textured predominantly (100)-oriented tungsten ribbon is studied by thermal desorption spectrometry at different substrate temperatures over a wide range of coverages. The mechanism of growth of the Ge films at T = 300 K is similar to a layer-by-layer mechanism. For T > 300 K, the films grow through the Stranski-Krastanov mechanism, according to which the completion of the monolayer coverage is followed by the formation of three-dimensional crystallites; as a result, the desorption kinetics changes. For small coverages (i.e., in the absence of lateral interactions), the activation energy of Ge desorption from W(100) is E = 4.9 ± 0.2 eV. In a monolayer, this activation energy decreases to E = 3.9 ± 0.2 eV due to the repulsive lateral interactions. The energy of pairwise lateral interactions is determined to be ω = 0.3 eV.     

Temperature and concentration effects in electron-stimulated desorption of sodium atoms from sodium layers adsorbed on tungsten coated with a gold film

The yield and energy distributions of sodium atoms upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a gold film are investigated for the first time as functions of the thickness of the gold film, the concentration of deposited sodium, and the surface temperature. It is found that the energy distributions exhibit two peaks, namely, a narrow peak with a maximum at about 0.15 eV, whose intensity continuously increases with increasing temperature, and a broad peak with a maximum at about 0.35 eV, whose intensity either decreases or remains constant with increasing temperature. It is shown that both peaks arise as a result of the same excitation, which gives rise to different channels of electron-stimulated desorption of sodium atoms. Possible mechanisms of electron-stimulated desorption and the kinetics of destruction of the surface coating are discussed.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Initial stages of the interaction of sodium and cesium with gold

The desorption of Cs and Na atoms from the corresponding layers applied to a gold film deposited on textured tungsten ribbon with a preferred orientation of the (100) surface is studied by thermal desorption spectroscopy with the products of thermal desorption scanned on a pulsed time-of-flight mass spectrometer. The Cs atoms evaporated at T = 300 K are desorbed by two phases, one of which can be identified with the filling of a monolayer and the other can be attributed to the formation of the CsAu compound. The Na atoms evaporated at T = 300 K are desorbed by three phases associated with the formation of a monolayer coating, a sodium compound of with gold, and a multilayer sodium film.     

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The molecular dynamics simulation of interaction between CH⁺ with various energy and fusion material tungsten is conducted. The simulated results show that in the incident process, the sputtering rates of C and H atoms change suddenly at the different exposure doses when the incident energy is 50, 100 and 150eV respectively, a few of W atoms are sputtered in the interaction process, but the sputtering rate is less than 0.24%. When the exposure dose is about 3.92×10¹⁶cm⁻², the incident energy is 50eV, a hydrocarbon firm without W atom is formed on the sample surface bombarded by the ions. A mixed film of W, C and H is formed at the other energy. The deposited rates of C and H atom first decrease then increase with the incident energy increament, the minimum deposited rats appear at 250 and 200eV respectively. The density profiles of C, H atoms, C−H, C−C, W−C bonds in the sample after bombardment move towards the inside of sample, and the C sp³ dominated the sample.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Electron-stimulated desorption of cesium atoms from cesium layers deposited on a germanium-coated tungsten surface

The yield and energy distribution of Cs atoms from cesium layers adsorbed on germanium-coated tungsten were measured, using the time-of-flight technique with a surface-ionization-based detector, as a function of the energy of bombarding electrons, germanium film thickness, the amount of adsorbed cesium, and substrate temperature. The threshold for the appearance of Cs atoms is ～30 eV, which correlates well with the germanium 3d-level ionization energy. As the electron energy increases, the Cs atom yield passes through a broad maximum at ～120 eV. For germanium film thicknesses from 0.5 to 2 monolayers, resonance Cs yield peaks were observed at electron energies of 50 and 80 eV, which can be related to the tungsten 5p and 5s core-level ionization energies. As the cesium coverage increases, the Cs atom yield passes through a flat maximum at monolayer coverage. The energy distribution of Cs atoms follows a bell-shaped curve. With increasing cesium coverage, this curve shifts to higher energies for thin germanium films and to lower energies for thick films. The Cs energy distribution measured at a substrate temperature T = 160 K exhibits two bell-shaped peaks, namely, a narrow peak with a maximum at ～0.35 eV, associated with tungsten core-level excitation, and a broad peak with a maximum at ～0.5 eV, deriving from the excitation of the germanium 3d core level. The results obtained can be described within a model of Auger-stimulated desorption.     

Electron-stimulated cesium atom desorption from the Cs/CsAu/Au/W system

This paper reports on a continuation of the investigation of electron-stimulated Cs-atom desorption from a tungsten surface on which cesium and gold films had been adsorbed at T = 300 K. Earlier studies revealed that Cs atoms start to desorb only after more than one monolayer of gold and more than one monolayer of cesium had been deposited on the tungsten surface. In this case, a coating consisting of a gold adlayer on tungsten, a CsAu compound possessing semiconducting properties, and a cesium monolayer capping CsAu (Cs/CsAu/Au/W) is formed on the tungsten surface at 300 K. The yield of atoms from this system exhibits a resonant dependence on the incident electron energy E _e , with an appearance threshold of 57 eV and a maximum at 64 eV. In this case, Cs atoms desorb in two channels, with one of them involving Cs desorption out of the cesium monolayer, and the other, from the CsAu monolayer. The Cs yield at E _e = 64 eV has been investigated in both desorption channels, with an additional cesium coating deposited on the already formed Cs/CsAu/Au/W layered system, as well as of the effect annealing produces on the yield and energy distributions of Cs atoms. It has been demonstrated that Cs atoms evaporated at 300 K on a layered coating with a cesium monolayer atop the CsAu layer on tungsten capped with a gold adlayer, rather than reflected from the cesium monolayer or adsorbing on it, penetrate through the cesium monolayer into the bulk of CsAu even with one CsAu layer present. The desorption yield does not vary with increasing cesium concentration at 300 K, but falls off gradually at 160 K. Annealing within the temperature range 320 K ≤ T _H ≤ 400 K destroys the cesium monolayer and the one-layer CsAu coating, but the multilayer CsAu compound does not break up in this temperature range even after evaporation of the cesium monolayer. It is shown that Cs atoms escape from the multilayer CsAu compound primarily out of the top CsAu layer.     

Adsorption stage in the formation of Eu-Si(111) thin-film structures

The adsorption stage in the formation of the Eu-Si(111) interface has been studied within a broad temperature range by thermal and isothermal desorption spectroscopy, low-energy-electron diffraction, Auger electron spectroscopy, and the contact potential difference method. It is shown that the ordering of an adsorbed europium film is accompanied by silicon surface reconstruction throughout the coverage range studied, 0<θ≤1.8. This self-organized process is also shown to be thermally activated. Ordered adsorbed europium layers have been found to be made up of 2D islands, whose structure depends on the amount of the metal deposited on the surface. The energy required to remove atoms from an island to vacuum has been determined. This energy decreases with decreasing 2D lattice constant of the islands. This pattern of its variation is accounted for, in the final count, by the decrease of the number of the Si surface atoms not bound directly to Eu atoms.     

Interaction of silver atoms with iridium and with a two-dimensional graphite film on iridium: Adsorption,desorption, and dissolution

The initial stages in the interaction of silver with the (111)Ir surface and with a two-dimensional graphite film (2D GF) on (111)Ir were studied by high-resolution electron Auger spectroscopy in ultrahigh vacuum. The growth mechanisms of silver films and the desorption fluxes of Ag atoms were determined, and their desorption energies estimated. It was found that the Ag desorption fluxes from a 2D GF on Ir and from a thick silver film on the pure metal are similar and considerably (an order of magnitude) smaller than the sublimation fluxes from bulk silver at the same temperatures. The activation energy for desorption from a submonolayer film varies from 3.2 eV for coverage θ=1 to 3.7 eV at θ ～ 0. It was shown that silver atoms do not penetrate into the substrate bulk throughout the temperature range covered (300–1800 K).     

Growth of an Eu-Si(111) thin film structure: The stage of silicide formation

Silicide formation in thin films produced by depositing Eu atoms on the Si(111) surface is studied using LEED, Auger electron spectroscopy, contact potential difference, and isothermal thermal-desorption spectroscopy. It is shown that if Eu is deposited on a substrate at room temperature, the growing film is disordered and consists of almost pure Eu. At high temperatures (T≥500 K), the Eu-Si(111) system forms through the Stranski-Krastanow mechanism; namely, first a two-dimensional transition layer (reconstruction) with the (2×1) structure forms and then three-dimensional silicide crystallites grow on it. A specific feature of this system is a low rate of diffusion of Si atoms in the europium silicides. This feature accounts for the concentration gradient of Si atoms across the silicide film thickness and, as a consequence, the multiphase film composition.     

Active oxidation: Silicon etching and oxide decomposition basic mechanisms using density functional theory

The etching of silicon atom from the Si(1 0 0)-p(2 × 2) surface, i.e. the desorption of SiO molecules from this surface, either clean or pre-oxidized, is investigated at the density functional theory level. The reaction paths for desorption are given as a function of the initial oxidation state of the extracted silicon atom. The associated activation energies and the atomic configurations are discussed. Particularly, it is shown that desorption of SiO molecules takes place during conventional thermal oxide growth (∼2 eV activation) via non-oxidized silicon atoms. Further SiO extraction mechanisms of higher silicon oxidation states required higher temperatures. In particular, doubly oxidized silicon atoms (Si²⁺) are able to decompose with an activation of ∼4 eV which corresponds to the actual temperature where decomposition of oxides is observed. This confirms the statement that decomposition of oxide layer nucleates at the interface with silicon where Si²⁺ has been detected thanks to XPS experiment.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten

An analysis of the yield q of europium atoms is made, and scenarios of electron-stimulated desorption are put forward. Expressions are obtained for the dependence of q on the coverage of oxidized tungsten by europium atoms.     

Electron-stimulated desorption of sodium atoms from germanium-coated tungsten

The yield of sodium atoms and energy distribution upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a germanium thin film are measured under variations in the electron energy, the sodium coverage, and the surface temperature by the time-of-flight method with the use of a surface ionization detector. It is revealed that the electron-stimulated desorption of sodium atoms occurs via three channels, namely, a channel involving ionization of adsorbed sodium; the most efficient channel, which is produced by the germanium ionization; and a channel associated with the formation of tungsten excitons, which brings about desorption of NaGe molecules.