A LEED-AES study of the initial stages of oxidation of Fe (001)

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and oxygen at room temperature. The AES oxygen signal was quantified by using a two-dimensional oxide layer as a calibration point. This reproducible oxide layer was prepared by the high temperature reaction of H₂O at 10^?6 torr with Fe(001). The initial oxygen sticking coefficient was observed to be close to unity, which suggests that the chemisorption is non-activated and involves a mobile adsorption step. The rate of chemisorption decreased as (1-Θ) and exhibited a minimum at Θ = 0.5. LEED data indicate that the minimum value of the sticking coefficient corresponded to the completion of a c (2 × 2) surface structure. Upon additional exposure to oxygen, an increase in the sticking coefficient was observed in conjunction with the disappearance of the c (2 × 2) and a gradual fade out of all diffraction features. After mild heating, epitaxial FeO (001) and FeO (111) structures were observed. The simultaneous appearance of a shifted M_2,3M_4,5M_4,5 iron Auger transition with the increase in the sticking coefficient and the disappearance of the c (2 × 2) indicated that oxide nucleated on the surface after the complete formation of the c (2 × 2) structure. The relatively high sticking coefficient during the initial oxidation indicates that formation of a mobile adsorbed oxygen state precedes the formation of oxide.     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

A study of the initial reaction of water vapor with Fe(001) surface

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and water vapor at temperatures from 298 to 473 K. A disordered c(2 × 2) structure was formed at all temperatures, and only 80% of the total number of sites were filled at saturation. The initial sticking coefficient was 0.56 ± 0.03, and the reaction rate increased with increasing temperature. A model was proposed that successfully accounted for these experimental observations. Irreversible chemisorption of water is proposed to take place via a precursor of physically adsorbed water molecules. The precursor, which is adsorbed on both bare surface and surface covered by chemisorbed species, is mobile and retains most of its degrees of rotational freedom. Water molecules in the precursor state can either desorb or dissociate, and the difference in activation energies for these reactions was found to be 5.7 ± 0.5 $\text{kcal}\text{mol}$. Only 80% of the available c(2 × 2) sites are filled and the surface layer is disordered because the chemisorbed species are immobile, and because each one blocks four nearest neighbor sites for further adsorption. The chemisorbed species occupy the fourfold symmetric sites either above the iron atoms or above the interstitial “holes” betweeh iron atoms.     

On the kinetics of oxygen adsorption on a Pt(111) surface

The kinetics of O₂ adsorption on a clean Pt(111) surface were investigated in the temperature range 214–400°C. The oxygen coverage was measured by CO titration as well as Auger electron spectroscopy both of which show the same dependence on O₂ exposure. The initial sticking coefficient on clean Pt(111) is 0.08–0.10 and decreases exponentially with increasing oxygen coverage. For θ > 0.23 a (2 × 2)-O LEED pattern was observed. The highest oxygen coverage obtained was approximately 0.45. A theoretical model was proposed which correlates the coverage dependence of the sticking coefficient with adsorbate interactions in the chemisorbed state. These interactions cause a coverage dependent activation energy of adsorption assuming the existence of a precursor state. Experiments dealing with the effect of carbon contamination on the sticking coefficient showed that the initial sticking coefficient decreases with increasing carbon coverage.     

Condensation of gases on metals: The one-dimensional square well model

Trapping probabilities of gas atoms at surfaces are calculated assuming a classical onedimensional square well potential as a function of gas and surface temperatures. It is shown that initial sticking coefficients of chemisorbed gases on transition metal surfaces can in most cases be fit fairly well by this model using reasonable values of the interaction energy, although the model does not predict observed behavior for surface temperatures>600°K. For some systems the initial sticking coefficients are higher than predicted by this model, indicating that other mechanisms of energy transfer are probably operative. The angular dependent sticking coefficient which would be measured in a molecular beam experiment is also computed.     

A proposed experiment to measure surface roughness in Si/SiO2 system

The adsorption of oxygen on clean cleaved (111) silicon surfaces has been investigated by high resolution electron spectroscopy (HRES), Auger electron spectroscopy (AES) and ellipsometry. Localized vibrations ($\text{h}\text{?}\text{ω = 94}$, 130 and 175 meV) which are related to the binding state band of oxygen are identified with HRES. AES measures the concentration of adsorbed atoms basically independent of their binding state while ellipsometry refers additionally to the optical properties of the adsorbed layer. The same adsorption kinetics was found with the three methods. Oxygen therefore adsorbs in a single likely molecular state. The sticking coefficient S increases exponentially with the surface step concentration. S is also enhanced by the presence of nude ion gauges. Depending on these parameters sticking coefficients between 2 × 10^?4 and 10^?1 have been obtained. This result might contribute to an explanation of the large differences in earlier works.     

Interaction kinetics of As4 and Ga on {100} GaAs surfaces using a modulated molecular beam technique

A modulated molecular beam technique, using mass spectrometric detection of desorbed species, had been applied to a study of the kinetics of Ga and As₄ interactions on {100} GaAs surfaces. Time domain mass spectrometer signals were processed using fourier transform techniques to provide information on surface lifetimes, sticking coefficients, desorption energies and reaction orders. In the temperature range 300–450 K As₄ is nondissociatively chemisorbed on Ga atoms from a weakly bound precursor state, but above 450 K there is a pairwise dissociation-recombination reaction between As₄ molecules adsorbed on adjacent Ga lattice sites. At temperatures higher than 600 K a temperature dependent Ga adatom population is formed by the desorption of As₂ from the surface. Thus above 450 K it is possible to produce GaAs from beams of the elements, but below this temperature the compound does not form.     

Étude des interactions chimiques gaz-solide par la technique du faisceau moléculaire: V. réactions de l'oxygène et de l'oxyde de carbone avec des rubans polycristallins de tantale

We use the reflection of a noble gas (helium) molecular beam to study the superficial reactions of oxygen and carbon monoxide with polycrystalline tantalum. The fraction of the incident beam which is specularly reflected gives direct information on the formation of an oxygen chemisorbed layer and we observed only a single apparent binding state for oxygen chemisorbed on tantalum. The initial value of the sticking coefficient of oxygen on clean polycrystalline tantalum is 0.86, decreasing rapidly during the formation of a chemisorbed layer. The specularly reflected fraction of the incident beam is also modified by the chemisorption of carbon monoxide and this modification would confirm the dissociative character of the CO chemisorption on tantalum. The influence of the partial pressure of CO on the temperature at which the surface is completely covered by the products of the dissociative chemisorption of CO shows that the coverage becomes equal to unity at temperatures at which the solubility limit of CO is attained and tantalum carbide is formed.     

The interaction of oxygen with the Rh(111) surface a

The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T? 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen $\text{(1,}\text{1}\text{2}\text{) LEED}$ beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is $\text{13.5 ± 0.5}\text{kcal}\text{mole}$. At higher temperatures, the oxygen undergoes a two-step irreversible disordering (T? 280 K) and dissolution (T?400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10^?3 cm² s^?1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.     

Direct inelastic and trapping desorption scattering of N2 from polycrystalline W; elementary steps in the chemisorption of nitrogen

Scattering of N₂ from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N₂-W energytransfer and condensation, allowing inferences to be made about the initial steps of N₂ chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N₂ to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N₂ scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N₂ condensation coefficient as well as an increase in desorption of the N₂, recursor relative to its migration-chemisorption.     

binding states of CO and H2 on clean and oxidized (111)Pt

The binding states and sticking coefficients of CO and H₂ on clean and oxide covered (111)Pt are examined using flash desorption mass spectrometry and Auger electron spectroscopy (AES). On the clean surface at 78 K there is one major binding state of CO with a desorption activation energy which decreases with coverage plus a second smaller state, while H₂ exhibits three binding states with peak temperatures of 140, 230 and 310 K and saturation density ratios of 0.5 : 1 : 1. Desorption kinetics of CO are consistent with a first order state with a normal pre-exponential factor of 10^{13 ± 1} sec^?1, while all three peaks of H₂ are broader than expected. Interpretations in terms of anomalous pre-exponential factors, coverage dependent desorption activation energies, and desorption orders are considered. On the oxidized surface saturation densities of both gases are nearly identical to those on the clean surface, but desorption temperatures are increased significantly and the initial sticking coefficient on the oxide decreases slightly for CO and increases slightly for H₂.     

The interference of an electron beam with the surface reaction between oxygen and germanium

Electron beam assisted adsorption and desorption of oxygen was studied by Auger electron spectroscopy (AES). Beam assisted adsorption was observed on clean as well as on oxidized surfaces. After an oxygen exposure of 1000 × 10^?7 Torr min and continuous irradiation with beam voltage of 1.5 kV and beam current density 2 microA mm^?2, the oxygen 510 eV signal amplitude from the point of beam impact was 2.5 times greater than the signal from the non-irradiated region. The Ge 89 eV signal showed a corresponding decrease. Enhanced adsorption occurred at beam energies as low as 16.5 eV. After irradiation, the oxidized surface was not carbon contaminated. Following an oxygen exposure of 30 min at 0.1 Torr and 550°C and subsequent additional beam assisted exposure of 1000 × 10^?7 Torr min, the maximum oxide thickness was about 18 Å. Beam assisted desorption did not occur from thin oxygen layers (0–510 eV signal strength less than 5 units, calculated oxide thickness about 6 Å), but occurred from thick oxides and stopped after the signal amplitude had decreased to 5 units. Based on these results, a model for the structure of the oxygen layer covering the Ge(111) surface is proposed. Mechanisms for adsorption and desorption are discussed. The implications of beam assisted adsorption and desorption on electron beam operated surface measurements (LEED, AES, ELS, APS etc.) are stressed.     

Molecular binding states on clean and oxidized surfaces: SiO(g) + W(clean) and SiO(g) + W(oxidized)

The interactions between a molecular beam of SiO(g) and a clean and an oxidized tungsten surface were examined in the surface temperature range 600 to 1700 K by mass spectrometrically determined sticking probabilities, by flash desorption mass spectrometry (FDMS) and by Auger electron spectroscopy (AES). The sticking probability, S, of SiO has been determined as a function of coverage and of surface temperature for the clean and the oxidized tungsten surface. Over the temperature range studied and at zero coverage S = 1.0 and 0.88 for the clean and oxidized tungsten surfaces respectively. The results are consistent with both FDMS and AES. For coverage up to one monolayer there is one major adsorption state of SiO on the clean tungsten surface. FDMS shows that T_m = constant (T_m is the surface temperature at which the desorption rate is maximum) and that desorption from this state is described by a simple first order desorption process with activation energy, E_d = 85.3 kcal mole^?1 and pre-exponential factor, ν = 2.1 × 10¹⁴ sec^?1. AES shows that the 92 eV peak characteristic of silicon dominates. In contrast on the oxidized tungsten surface, T_m shifts to higher temperatures with increasing coverage. The data indicate a first order desorption process with a coverage dependent activation energy. At low coverage (θ ? 0.14) there is an adsorption state with E_d = 120 kcal mole^?1 and ν = 7.6 × 10¹⁹, while at θ = 1.0, E_d = 141 kcal mole^?1. This variation is interpreted as due to complex formation on the surface. AES shows that on oxidized tungsten, in contrast to clean tungsten, the dominant peaks occur at 64 and 78 eV, and these peaks are characteristic of higher oxidation states of silicon. Thus, it is concluded that SiO exists in different binding states on clean and oxidized tungsten surfaces.     

Adsorption-desorption studies of Zn on GaAs

By employing a wide range of techniques to study adsorption-desorption behaviour in the ostensibly simple system of the metal Zn on the semiconductor GaAs it has been found that many complicating factors can occur, and reliance on any one of the techniques would have given a totally misleading picture. The methods used were temperature programmed thermal desorption, modulated atomic beam adsorption measurements, AES, high resolution UHV SEM combined with AES and high resolution (300 Å) AES, and RHEED. GaAs substrate surfaces were cleaned in-situ by thermal treatment or inert gas ion bombardment followed by annealing. It was established by SEM and RHEED observations that surface topographic and compositional changes could occur at this stage. Zn sticking coefficient measurements by modulated beam and AES techniques showed that it could vary widely for minor changes in surface composition, and that it was also a strong function of deposition time and substrate temperature. However, initial growth of the Zn deposit was always two dimensional (within the resolution limits of the SEM) and epitaxial, for the range of substrate conditions used. Thermal desorption spectra were also found to depend rather critically on the substrate surface, with very pronounced difference between the Ga and As stabilized forms. An attempt has been made at a systematic interpretation of the kinetic data based on reasonably simple models, and also to relate it to previously published work on this system, but the profound influence of substrate surface effects makes a fully quantitative evaluation extremely difficult. Nevertheless, the value, and perhaps the necessity, of employing a wide range of techniques to investigate metal-semiconductor systems is clearly demonstrated.     

A LEED and AES study of the structure,debye temperature,and oxidation of the (111) crystal face of thorium

The structure, and reactivity towards O₂ and CO, of the (111) crystal face of a single crystal of high purity thorium metal was studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After the sample was cleaned in vacuum by a combination of ion bombardment and annealing, a (1 × 1) LEED pattern characteristic of a (111) surface was obtained. Extended annealing of the cleaned sample at 1000 K produced a new LEED pattern characteristic of a (9 × 9) surface structure. A model of a reconstructed thorium surface is presented that generates the observed LEED pattern. When monolayer amounts of either O₂ or CO were adsorbed onto the crystal surface at 300 K, no ordered surface structures formed. Upon heating the sample following these exposures the (111) surface structure was restored accompanied by a reduction in the amount of surface carbon and oxygen. With continued exposure to either O₂ or CO and annealing, a new LEED pattern developed which was interpreted as resulting from the formation of thorium dioxide. Debye-Walter factor measurements were made by monitoring the intensity of a specularly reflected electron beam and indicated that the Debye temperature of the surface region is less than it is in bulk thorium. Consequently, the mean displacement of thorium atoms from their equilibrium positions was found to increase at the surface of the crystal. The presence of chemisorbed oxygen on the crystal surface affected the Debye temperature, raising it significantly.     

Electron beam induced effects on gas adsorption utilizing auger electron spectroscopy: Co and O2 on Si: I. Adsorption studies

The effect of electron beam monitored gas adsorption on the clean Si surface is studied using Auger electron spectroscopy. It is shown that the beam affects the AES adsorption signal of CO and O₂ on Si by dissociating the adsorbed molecules on the surface and subsequently promoting diffusion of atomic oxygen into the bulk. A qualitative explanation of the adsorption data is presented and the initial sticking probability of O₂ on Si (111) surface is estimated to be S0 = 0.21.     

Decomposition of carbon monoxide on a (110) nickel surface

New investigations of the (110) nickel/carbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1×10^?7 torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.     

Oxygen chemisorption on Cr(110): II. Evidence for molecular O2(ads)

Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O₂(ads) vibrational frequency of 1020 cm⁻¹ and a six beam ESDIAD pattern are suggestive of super-oxo O₂(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.     

The oxidation of aluminum studied by sims at low energies

The adsorption of oxygen by aluminum was studied using secondary ion mass spectrometry at low primary ion energies (? 500 eV) and low primary ion fluxes (? 5 × 10¹⁰ions/cm²/sec) in an ultrahigh vacuum system. The SIMS characteristics of a cleaned aluminum surface were measured and yield changes of positive and negative ions were measured as a function of oxygen exposure. The results strongly support a model of oxygen uptake in two stages, the first monolayer equivalent being mostly incorporated into the aluminum, probably in the second layer and the next half-monolayer equivalent being superficially chemisorbed. Selection of the correct model is important in calculations of the resonances in the electronic (valence band) structure of free-electron-like metals caused by oxygen adsorption.     

Messung der Verweilzeit von He-Atomen an Glasoberflächen

Mean sticking times of helium on a glass surface are determined at very low pressures from nonstationary molecular flow through glass capillaries. The temperature range covered is 13.8 °K to 20.4 °K. Resulting sticking times are of the order of 10^?7 to 10^?5 sec. They show a characteristic dependence on temperature and pressure. These measurements can be interpreted by means of a simple model: He-atoms mostly are bound to the surface with an adsorption energyE of 229 cal/mol?0.01 eV (±20%). However with a probability of 10^?4 the energy is 530 cal/mol?0.023 eV (±6%). In both cases sticking times τ follow the equation τ=τ₀exp(E/RT) where τ₀ is about 10^?9 to 10^?10 sec.