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.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

The structure and chemistry of ethylene adsorbed on platinum (111): CFSO-BEBO calculations and experimental studies

The structure and chemistry of ethylene adsorbed on a platinum (111) surface has been studied. Ethylene adsorbs irreversibly and dissociatively with unity sticking coefficient in the first monolayer, which is ordered in a (2 × 2) array. A second monolayer in registry with the first is adsorbed reversibly. Self-hydrogenation takes place during dynamic adsorption-desorption which poisons the reversible adsorptivity. The reversibly-held ethylene desorbs at the lowest temperatures (E_d = 25 kcal) followed by dissociated hydrogen (E_d = 30 kcal) and the decomposition of hydrogen in the acetylenic first monolayer (E_d = 38 kcal). Residual carbon coalesces to graphite at temperatures in excess of 500°C. CFSO-BEBO calculations qualitatively predict all of the binding states observed. Treating PtC valency as a single adjustable parameter, all four binding energies and four of the five activation energies are in quantitative agreement with experiment.     

The reaction of nitrous oxide with rhodium

A clean rhodium filament at room temperature is highly reactive towards nitrous oxide. The oxygen atom of the N₂O molecule is adsorbed with a sticking probability of 0.45 whilst the nitrogen atoms appear in the gas phase as molecular nitrogen. The room temperature uptake of oxygen is about 5 × 10¹⁴ atom cm^?2 and is independent of nitrous oxide pressure in the range 3.5 × 10^?8 to 1.1 × 10^?6 torr. The adsorption curve is of typical form with an initial region of essentially constant sticking probability. For the first 80% of adsorption at room temperature the shape is satisfactorily accounted for if molecules are able to visit 4–5 adsorption sites whilst held in a weakly-bonded precursor state.     

Adsorption d'hydrogène entre 300 et 873 K sur un catalyseur Pt/MgO

Hydrogen adsorption on MgO-supported platinum was studied by thermal desorption and infrared spectroscopy at 300 and 800 K. For both temperatures, reversibly and irreversibly adsorbed species have been detected. At 300 K, reversible adsorption leads to the appearance of infrared bands at 2120 and 2060 cm^?1, attributed to terminal Pt-H species. Irreversibly adsorbed hydrogen has been detected by thermal desorption, whereas no infrared band was detected in the spectral range 4000–4750 cm^?1 for Pt/MgO sample. For hydrogen adsorption at 800 K, reversibly adsorbed hydrogen gave the same picture as for the 300 K adsorption. An additional form of irreversibly adsorbed hydrogen has been evidenced both by thermal desorption and infrared spectroscopy. This form corresponds to hydrogen strongly adsorbed on platinum and gives an infrared band at 950 cm^?1 which is characteristic of an hydrogen atom in interaction with more than one platinum atom (multicentered) species.     

Interaction of NO with a Ni (111) surface

The interaction of NO with a Ni (111) surface was studied by means of LEED, AES, UPS and flash desorption spectroscopy. NO adsorbs with a high sticking probability and may form two ordered structures (c4 × 2 and hexagonal) from (undissociated) NO_ad. The mean adsorption energy is about 25 $\text{kcal}\text{mole}$. Dissociation of adsorbed NO starts already at ?120°C, but the activation energy for this process increases with increasing coverage (and even by the presence of preadsorbed oxygen) up to the value for the activation energy of NO desorption. The recombination of adsorbed nitrogen atoms and desorption of N₂ occurs around 600 °C with an activation energy of about 52 $\text{kcal}\text{mole}$. A chemisorbed oxygen layer converts upon further increase of the oxygen concentration into epitaxial NiO. A mixed layer consisting of N_ad + O_ad (after thermal decomposition of NO) exhibits a complex LEED pattern and can be stripped of adsorbed oxygen by reduction with H₂. This yields an N_ad overlayer exhibiting a 6 × 2 LEED pattern. A series of new maxima at ≈ ?2, ?8.8 and ?14.6 eV is observed in the UV photoelectron spectra from adsorbed NO which are identified with surface states derived from molecular orbitals of free NO. N_ad as well as O_ad causes a peak at ?5.6 eV which is derived from the 2p electrons of the adsorbate. The photoelectron spectrum from NiO agrees closely with a recent theoretical evaluation.     

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.     

Nitrogen chemisorption by iron

The conversion of molecularly adsorbed nitrogen present on ion surfaces to the dissociatively chemisorbed state has been observed by X-ray photoelectron spectroscopy at low temperature. The molecularly adsorbed nitrogen reversibly adsorbed at 80 K is also immediately displaced when exposed to molecular hydrogen at the same temperature. A possible explanation is given for the low sticking probability (～10^?7) of nitrogen chemisorption observed on iron above room temperature.     

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.     

Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt(111) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 × 10¹⁴ S atoms cm^?2 ($\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ structure) oxygen exposures of 5 × 10^?5 Torr s do not result in the adsorption of oxygen nor in the formation of SO₂. At concentrations lower than 3.8 × 10¹⁴ S stoms cm^?2 ((2 × 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO₂ and O₂. With decreasing sulfur concentration the amount of desorbing O₂ increases and that of SO₂ passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 × 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO₂ is adsorbed with high sticking probability and can be desorbed nearly completely as SO₂ with desorption maxima occurring at 400, 480 and 580 K. The adsorbed SO₂ is highly sensitive to hydrogen. Small H₂ doses remove most of the oxygen and leave adsorbed sulfur on the surface. After adsorption of SO₂ on an oxygen predosed surface small amounts of SO₃ were desorbed in addition to SO₂ and O₂ during heating. Preadsorbed oxygen produces variations of the SO₂ peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.     

The adsorption and decomposition of methanol on tungsten. II

A clean tungsten filament adsorbs methanol rapidly at room temperature, the initial sticking probability being 0.8. At saturation, the composition of the adsorbed layer is roughly CO:H = 1:1 and it is suggested that the hydrogen may be in the form of a surface complex. The continuous decomposition of methanol by the hot filament under steady-state conditions, or when the filament had been previously oxygenated, followed a different course from that previously reported for the newly-cleaned filament. Rather than a rapid rise in the rate of decomposition (to CO + H₂) for 600 < T_fil < 1300 K to a high plateau above 1300 K, decompositon to formaldehyde, carbon monoxide and methane was observed. The rates at which these products appeared passed through low maxima between 900 and 1100 K. The change in the relative importance of formaldehyde and carbon monoxide production with filament temperature within this range is attributed to a temperature-dependent life-time of formaldehyde molecules on the oxygenated surface. At the highest temperature (> 1500 K) the reactivity increased rapidly to join that of the clean surface, probably due to the desorption of surface oxygen.     

Adsorption of oxygen on a Cu{110} surface with and without the influence of A keV Ne+ beam: Stage I: Coverages up to half a monolayer

Low Energy Ion Scattering has been used to study the interaction of molecular oxygen with a Cu{110} surface. The amount of adsorbed atomic oxygen was monitored by the 4 keV Ne⁺¦O reflection signal. In the first adsorption stage (coverage less than half a monolayer) the sticking probability varied proportional to the number of empty adsorption sites: S = S₀ (1 ? $\text{}\text{?}$gq). It turned out not to be influenced by the Ne⁺ bombardment. The initial sticking probability S₀ was found to be ≈ 0.24. In this first adsorption stage the oxygen-covered surface is reconstructed according to the “missing row” model, leading to a (2 × 1) LEED pattern.     

Adsorption of ethylene on stoichiometric and reduced NiO(100)

The adsorption of ethylene has been studied on stoichiometric NiO(100) and on surfaces reduced to 40% of the stoichiometric oxygen content. The adsorption process was followed with XPS, Auger spectroscopy and LEED at substrate temperatures of 200 to 500 K and at ethylene pressure of 5 × 10^?7 Torr. At 200 K, two distinct ethylene species are observed on stoichiometric NiO(100). The first species saturates at 0.02 ML after 200 L and is adsorbed molecularly, interacting with both nickel and oxygen sites. A condensed species then forms which does not saturate for exposures up to 2100 L. Both adsorb reversibly with all traces of carbon absent by 270 K. At 200–300 K, reduced NiO(100) also adsorbs two molecular ethylene species, although with a preference for nickel sites. However, the uptake of ethylene increases only slightly with surface reduction. Adsorption is no longer reversible for the reduced surface and increasing the substrate temperature causes fragmentation of the adsorbed ethylene with a concomitant reduction in lattice oxygen content.     

Interaction of oxygen and CO with thin Ag films at AgTi2 substrates

Clean Ag overlayers with a thickness of about three atomic layers are prepared at polycrystalline AgTi₂ surfaces. Adsorption of oxygen and CO at these overlayers is studied with AES, UPS, and TDS methods. UP spectra reveal that oxygen adsorbs dissociatively on the surface of the Ag overlayers at 120 K, similar as at the surfaces of bulk Ag crystals. At higher temperatures, O atoms diffuse into the overlayer and get trapped at Ti at the interface underneath the Ag layer. Unlike at bulk Ag, oxygen adsorption proceeds with high sticking probability, which is attributed to a reduced activation barrier. CO adsorbs at the Ag overlayers in a weakly chemisorbed state with an adsorption energy of ca. 9 kcal/mol, whereas at bulk Ag true physisorption has been observed. Accordingly, UP spectra of CO adsorbed at 120 K exhibit well separated 4σ, 1π and 5σ emission peaks at energetic positions which are essentially different from the CO/Ag physisorption system. This observation is interpreted as a demonstration of the ligand effect, i.e., the activation of weak CO bonding at Ag by strong bonding Ti.     

The adsorption of chlorine and chloridation of Ag(111)

The adsorption of chlorine on the Ag(111) surface has been studied using LEED, Auger and temperature programmed desorption. Chlorine adsorbs dissociately with an initial sticking probability of ~ 0.4, and a precursor state is implicated in the chemisorption process. The chlorine appears to form a close-packed monolayer with the same packing density as in AgCl(111), and is epitaxially related to the substrate mesh. Chlorine continues to adsorb above a monolayer in coverage, though the sticking probability drops precipitately, being ~ 0.01 after the adsorption of 5 monolayers at 300 K. There is little increase in the chlorine Auger signal above one monolayer coverage at 300 K, but when adsorption is carried out at 240 K the chlorine signal is more than doubled. This is interpreted as being due to the formation of a layer structure of alternate Cl and Ag layers at the lower temperature, while adsorption at 300 K results in dissolution of subsurface Cl into the bulk of the crystal. Upon heating, the low temperature layer structure is destroyed, the chlorine signal diminishes to a limiting value at 450 K equivalent to the value for one adsorbed monolayer — apparently due to the dissolution of the near surface Cl layers into the bulk. However, the chlorine re-emerges at the surface at ~ 600 K, probably due to an exothermic heat of solution of Cl in the silver lattice. Desorption from the multilayers peaks at 670 K and both AgCl and Ag are desorbed coincidently with kinetics identical to those for the sublimation of bulk AgCl (ΔH = 235 kJ mol^?1, ΔS = 90 JK^?1 mol^?1). After the multilayers have desorbed, the final Cl layer desorbs in a higher temperature peak ( ~ 760 K) as AgCl (no silver desorption) which shows complex desorption kinetics indicative of the strong influence of a precursor state in the desorption process.     

Contact potential measurements on “clean” CdS surfaces

Contact potential difference (cpd) measurements were carried out on {112&#x0304;} surfaces of high conductivity CdS cleaved in ultrahigh vacuum. A modified Kelvin method was employed. A change in cpd upon illumination with white light (surface photovoltage) was observed, indicating the presence of intrinsic surface states in these surfaces. Long exposure (about 24 hr) to water vapor (2 × 10⁻⁸ torr) caused an increase of about 1.0 V in cpd and a reduction in surface photovoltage. This increase in cpd was attributed primarily to a decrease in the electron affinity of the surfaces brought about by the molecular dipole moment of physically adsorbed water molecules. The “clean” surfaces exhibited no appreciable affinitity for spectroscopically pure oxygen, except under white light illumination apparently due to the increase of the density free electrons at the surface.     

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.     

Simultaneous and sequential adsorption of deuterium and nitrous oxide by rhenium

The addition of nitrous oxide to a stream of deuterium passing over a rhenium filament reduced the initial sticking probability of the latter gas from 0.24 to 0.09 when the proportion of N₂O exceeded 40%. For the addition of deuterium to nitrous oxide the equivalent figures were 0.45 and 0.30 when deuterium exceeded 30% of the gas phase. These results are attributed to a competition between the two gases for places in the precursor state on the surface. The replacement of adsorbed deuterium from a saturated layer by the oxygen atom of nitrous oxide proceeded initially with a high probability, 0.27, at room temperature and with each oxygen atom replacing one deuterium atom. However, the reaction was incomplete, about 2 × 10¹⁴ atoms cm^?2 of deuterium remaining on the surface. It is suggested that kinetic rather than thermodynamic factors are responsible for the incomplete reaction, possibly as the result of a high activation energy for the migration of deuterium atoms over an oxygenated rhenium surface.     

Photoelectron spectroscopy study of oxygen adsorption on Mo2C(0 0 0 1)

Oxygen adsorption on the α-Mo₂C(0 0 0 1) surface has been investigated with X-ray photoelectron spectroscopy and valence photoelectron spectroscopy utilizing synchrotron radiation. It is found that oxygen adsorbs dissociatively at room temperature, and the adsorbed oxygen atoms interact with both Mo and C atoms to form an oxycarbide layer. As the O-adsorbed surface is heated at ≧800 K, the C-O bonds are broken and the adsorbed oxygen atoms are bound only to Mo atoms. Valence PES study shows that the oxygen adsorption induces a peculiar state around the Fermi level, which enhances the emission intensity at the Fermi edge in PES spectra.