Orientation dependence of oxygen adsorption on a cylindrical GaAs sample: II. Photoemission measurements

The orientation dependence of oxygen adsorption has been investigated by XPS and UPS on the surface of a cylindrically shaped GaAs single crystal with [11̄0] as its axis. This sample exposes the main low-index orientations (001), (111)Ga, (110), (111̄)As, etc. and all intermediate orientations. It was prepared by ion bombardment and annealing [see part I, Surface Sci. 120 (1982) 67). XPS was used to separate the known two forms of adsorbed oxygen quantitatively. The less tightly chemisorbed a-oxygen has an O 1s binding energy 1.50 ± 0.05 eV larger than the β-oxygen for all surface orientations. Comparison with UPS shows that for hv =40.8 eV the excitation probability of the O 2p orbitals for α-oxygen is four times larger than for β-oxygen. The Ga 3d peak shows the known chemical shift of 1.1 ± 0.1 eV induced by β-oxygen whereas α-oxygen causes no Ga 3d shift. Most of the α-oxygen desorbs thermally starting near room temperature, only a portion smaller than 20% is converted to β-oxygen; β-oxygen desorbs starting at 300–350°C slightly depending on orientation. These results confirm the interpretation of β-oxygen as dissociated oxygen. However, the nature of α-oxygen still is not clear. The strong orientation dependence of the total amount of oxygen is consistent with the Auger results published recently (see part I). The nature of α- and β-oxygen is found to be the same for all orientations. Also the ratio of adsorbed α- to β-oxygen depends surprisingly weakly on the orientation and on the exposure varying only between 0.35 and 0.7 as the extreme values. This suggests that the adsorption of α- and β-oxygen is connected. A possible model is that adsorption, probably of β-oxygen, starts from edges or other energetically less favourable sites and that this disturbance creates stresses or defects which serve as adsorption site for the α-oxygen.     

Adsorption states and Ga depletion during oxygen adsorption on clean polar GaAs surfaces

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1̄1̄1̄)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O₂. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga₂O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form.     

Structure analysis of an oxidized nickel (110) surface using low energy ion scattering

Low energy (6 keV) argon and neon ion scattering in the low angle mode (θ = 30°) has been used to investigate changes in the surface structure of a Ni(110) surface caused by the adsorption of oxygen at low exposures (10^?6 Torr s). The experimental energy spectra indicate that due to adsorption of oxygen, the interatomic distance in the 〈1̄10〉 direction increases while in the 〈001̄〉 direction this distance seems to decrease. This represents strong evidence that a reconstruction process is taking place during the early stages of oxidation of the Ni(110) face, in which the interatomic distances in the 〈1̄10〉 direction doubles. The oxygen atoms were found to lie in or close to the nickel 〈001̄〉 rows. These results are not in agreement with recently published dynamical LEED calculations.     

Composition,structure, surface states,and O2 sticking coefficient for differently prepared GaAs(1̄1̄1̄)As surfaces

The polar GaAs(1̄1̄1̄)As surface can be prepared in three stable and ordered states: two by molecular beam epitaxy (MBE), namely the As-stabilized and the Ga-stabilized states and one simply by ion bombardment and annealing at 770 K. The respective LEED structures are $\text{(2 × 2), (}\text{19}\text{×}\text{19}\text{)}\text{R}\text{23.4°}$, and (1 × 1) with a diffuse faint (3 × 3) superstructure. Auger measurements and the comparison with the stoichiometric cleaved (110) surface show that there are different As concentrations in the first atomic layer associated with each of these three surfaces. Whereas about 10 to 15% of the first As layer appears to be missing on the (2 × 2) surface, about 50% is missing on the $\text{19}$ surface. On the (1 × 1) surface the first As layer is removed completely. The intensity of emission from the surface sensitive states between 1 and 4 eV below the valence band edge, as seen by angular resolved UPS, roughly corresponds to the amount of As at the surface thus confirming their interpretation as As-derived surface states. The inital sticking coefficent for oxygen depends strongly on the surface structure: ～10^?8 for the (2 × 2), ～10^?7 for the $\text{19}$, and ～10^?4 for the (1 × 1) surface. The sticking coefficient does not depend on the surface concentration of As but rather on the degree of saturation of dangling bonds on Ga atoms.     

Electrical properties of p-type GaP schottky barrier under stress

The effect of stress on a Schottky barrier height at a metal-semiconductor interface is investigated for metal-p-type GaP contacts. The diodes are fabricated by evaporating metals (Ag, Au) on polar (111)Ga and (1̄1̄1̄)P surfaces. Stress is applied to the diodes by bending the crystal wafers attached to the cantilever. The variation of the barrier height with stress is determined from the measurements of the current-voltage characteristics under stress. The barrier height decreases under compressive stress parallel to the interface and increases under tensile stress. The change in barrier height on the (111)Ga surface is greater than that on the (1̄1̄1̄)P surface. These experimental results are discussed from the point of both the piezoelectricity and the change in band gap caused by stress.     

Adsorption of gold on low index planes of rhenium

On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function $\text{}\text{?}$gf increases at low average gold coverage $\text{}\text{?}$gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a $\text{}\text{?}$gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101&#x0304;1&#x0304;) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101&#x0304;0),but the values taken by φ(clean) are not well defined. Both forms of (101&#x0304;1&#x0304;) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112&#x0304;0) and (112&#x0304;2&#x0304;) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101&#x0304;1&#x0304;) and (101&#x0304;0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures T_s on (112&#x0304;2&#x0304;), (101&#x0304;1&#x0304;) and (112&#x0304;0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ₀(1011) = 0.1 × 10^?30 C-m with α = 10 ų and μ₀(112&#x0304;0) = 0.32 × 10^?30 C-m with α = 22 ų, where μ₀ is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112&#x0304;2&#x0304;) is attributed to its atomically rough structure. No dipole effect is seen on (101&#x0304;0). Energy spectroscopy of electrons field emitted at (202&#x0304;1&#x0304;) and (101&#x0304;1&#x0304;) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of T_s and $\text{}\text{?}$gq gold appears to form states which are well-characterised by a coverage-independent work function. (101&#x0304;0), (101&#x0304;1&#x0304;) and (112&#x0304;0) each form two such states, one in the range 2 < $\text{}\text{?}$gq < 4 (state 1), and the second at $\text{}\text{?}$gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112&#x0304;2&#x0304;) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 $\text{}\text{?}$gq.     

Oxygen adsorption on silver (110): Dispersion,bonding and precursor state

Angle-resolved photoemission, both in the laboratory and with synchrotron radiation, has been used to investigate three states of adsorbed oxygen on Ag(110): chemisorbed atomic oxygen, undissociated chemisorbed dioxygen and physisorbed molecular oxygen. For atomic oxygen, dispersion of adsorbate-induced levels was observed indicating strong oxygen-oxygen interaction in the [001] direction. Polarised light combined with selection rules was used to determine the symmetry of the adsorbate-derived bands. The adsorption geometry and symmetry of the oxygen-induced levels for the chemisorbed dioxygen species were also investigated with the selection rules. The molecule appears to lie parallel to the surface with the OO axis oriented in the [11&#x0304;0] direction. The adsorbate-induced feature of lowest ionisation potential at 1.1 eV relative to E_F is 1 π_g-derived. The very low frequency reported for the OO stretch and the analogy with coordination chemistry also suggest that the chemisorbed dioxygen species is lying down on the surface. At sufficiently low temperature, oxygen was found to physisorb on the bare metal also with its OO axis parallel to the surface. We identify physisorbed oxygen as an intrinsic precursor state for chemisorption.     

Leed studies of vicinal surfaces of silicon

Clean silicon surfaces inclined at small angles to (111), (100) and (110) planes were investigated by LEED. Surfaces oriented at low angles to the (111) plane contain steps with edges towards [2&#x0304;11] or [21&#x0304;1&#x0304;]. Steps with edges towards [2&#x0304;11] have a height of two interplanar distances d₁₁₁ at low temperatures. At 800°C the reversible reconstruction of this step array into the steps of monolayer height takes place. Steps with edges towards [21&#x0304;1&#x0304;] can be seen at low temperatures only. They are of monolayer height and disappear at annealing in vacuum. Surfaces oriented at low angles to the (100) plane contain steps with (100) terraces and have a height of about two interplanar distances d₁₀₀. Surfaces at low angles to (110) planes are facetted and contain facets of the (47 35 7) type. The information about surface self-diffusion of silicon may be obtained using the kinetic data of structural reconstructions on surfaces close to (111) at different temperatures.     

Orientation of adsorbed ethylene oxide on Ni(110) by X-ray photoelectron diffraction

The adsorption of ethylene oxide on Ni(110) was studied at 95 K and monolayer coverage by angle-resolved X-ray photoelectron spectroscopy. A slow radiation-induced decomposition at hv = 1486.7 eV to most likely methoxy was noted. The orientation of the adsorbed ethylene oxide was determined by measuring forward scattering enhancements in the O 1s intensity distribution. Peaks in polar (θ) as well as azimuthal (φ) scans occurred at four angular positions in 2π above the surface: (θ = 54°, φ = 36°, 144°, 216°, 324°). These positions were evaluated to yield the tilt angle of the molecule at 48°_relative to normal, and the COC bond angle of adsorbed C₂H₄O of about 57°. The molecule is tilted towards the [001] and [001&#x0304;] directions (two domains), with a mirror plane in the [001] azimuth.     

Surface structures formed upon oxidation of the (311) face of copper

The oxidation of the copper (311) surface at temperatures from 25 to 900° C, and at oxygen pressures from 1 atm to 10⁻⁴ torr has been investigated by reflection high energy electron diffraction (RHEED). At room temperature, a poorly organized three-demensional epitaxial layer of Cu²O initially covers the surface, but it disappears when heated in vacuum to 200 °C. Between 300 and 600 °C, two symmetry-equivalent versions of a (4 × 1) two-dimensional surface structure form. Above 500 °C, this structure transforms into another having a hexagonal primitive cell with one axis coinciding with the Cu [011&#x0304;] direction and an axial length of 5.212 Å. This is the same cell which has been observed previously with oxidation of copper (100), (111), and (110) faces above 600 °C. Upon oxidation above 600 °C, the surface decomposes into (111) and (100) facets having the copper [010&#x0304;] direction in common.     

Electron stimulated oxidation of GaAs,studied by quantitative auger electron spectroscopy

The oxidation properties of the clean polar (111) faces of GaAs, prepared by Ar⁺ bombardment and proper annealing, are investigated. Considering the adsorbed layer as a continuum and using empirical values for the escape depth of the Auger electrons from literature, the coverage of oxygen on these faces is quantitatively determined. For a coverage of up to 10% of a monolayer the sticking coefficients are about 10^?3 for the ($\text{111}$) As face and about 10^?4 for the (111) Ga face, respectively. They decrease rapidly with increasing coverage. The oxidation is strongly stimulated by electron irradiation causing dissociation of the oxygen which is originally adsorbed in molecular form. In this way a compact oxide layer is formed which shows As depletion as a result of sublimation of As₄O₆ and a chemical shift of the Ga Auger peaks is observed. The cross section for the O₂ dissociation is calculated to be 1.8–2.5 Ų depending on electron energy.     

Iodine etching of the GaAs(1&#x0304;1&#x0304;1&#x0304;)As surface studied by LEED,AES, and mass spectroscopy

The iodine interaction with the GaAs(1&#x0304;1&#x0304;1&#x0304;)As surface prepared by molecular beam epitaxy has been studied by LEED, LEED intensity measurements, Auger electron spectroscopy (AES) and computer controlled mass spectroscopic study of the whole desorption spectrum. It is shown that an iodine beam hitting the GaAs(1&#x0304;1&#x0304;1&#x0304;)As face at 300 K under UHV conditions etches the surface continuously. After this etching there remains an adsorbate of GaI_x where x is a number between 0 and 3. By thermal desorption of this GaI_x adsorbate an As stabilized GaAs(1&#x0304;1&#x0304;1&#x0304;)As surface showing a (2 × 2) structure can be prepared, which up to the present could be done only by molecular beam epitaxy.     

A new type of reconstruction on the InSb(1&#x0304;1&#x0304;1&#x0304;) surface determined by grazing incidence X-ray diffraction

The (3×3) reconstruction of the InSb(1&#x0304;1&#x0304;1&#x0304;) surface has been investigated by grazing incidence X-ray diffraction and scanning tunneling microscopy. The structure is characterized by 6-atom rings on top of a slightly buckled InSb top double layer. Two types of rings have been found, an elliptic ring consisting of 4 In and 2 Sb atoms and a trigonal ring with 3 In and 3 Sb atoms. The bond angles and lengths are consistent with the concept of rehybridization and depolarization which explains the reconstructions of the (111) and (110) surfaces.     

LEED,AES and work function measurements on clean and cesium covered polar faces of GaP

After argon bombardment and annealing both the (111) and (1&#x0304;1&#x0304;1&#x0304;) faces of GaP show a (1 × 1) LEED pattern. The stabilization of the polar termination is probably obtained by charging of surface states. Measurements of the work function, the Auger spectrum and the LEED pattern during cesium deposition at room temperature suggest disordered cesium adsorption limited to a monolayer.     

Chemisorption,surface structural chemistry,and electron impact properties of carbon monoxide on rhodium (110)

The adsorption, desorption, surface structural chemistry, and electron impact properties of CO on Rh(110) have been studied by LEED, Auger spectroscopy, thermal desorption, and surface potential measurements. At 300 K, CO adsorbs into a single chemisorbed state whose desorption energy (E_d) is ~130kJmol^-1. The initial sticking probability is unity, and at saturation coverage a (2 × 1)plgl ordered phase reaches its maximum degree of perfection, thus demonstrating that this CO structure is common to the (110) faces of all the cubic platinum group metals. The saturated adlayer corresponds to θ = 1 and shows a surface potential of Δ? = +0.97 V. Under electron impact, desorption and dissociation of CO occur with about equal probability, the relevant cross sections being ~10^-22 m² in each case. Slow thermal dissociation of CO occurs at high temperature and pressure, leaving a deposit of C and O atoms on the surface. The thermal, electron impact, and Δ? properties of Rh(110)CO resemble those of Ni(110)CO rather closely, and are very different from those of Pt(110)CO. Surface carbon is shown to inhibit CO chemisorption, whereas surface oxygen appears to lead to the formation of a new more tightly bound form of CO with a considerably enhanced desorption energy (E_d ~ 183 kJmol^-1). Similar oxygen-induced high temperature CO states have been reported recently on Co(0001) and Ru(101&#x0304;1).     

The vibration of atoms at high miller index surfaces: Face centred cubic metals

Using a nearest neighbour model of interatomic forces we have obtained surface mode requencies for the face centred cubic (100), (110), (111), (210), …, (332) surfaces. In addition mode frequencies for the (511&#x0304;), (711&#x0304;), (911&#x0304;) and (553) surfaces were obtained so effects of increasing terrace size could be further studied. Frequencies and surface eigenvectors are presented for nickel. Since a nearest neighbour model is used a simple conversion factor allows a determination of the frequencies for other metals for which the nearest neighbour model is appropriate.     

Atom-probe fim studies of β-SiC whiskers

The 〈111〉-oriented whiskers of β-SiC, which is a semiconductor of the zinc blende structure, were studied by an atom-probe field ion microscope. Spectra of the number of field-evaporated ions versus their mass to charge ratio for A [111]-and B [1&#x0304;1&#x0304;1&#x0304;]-oriented tips were different from each other. Both the atomic ions and the molecular ions of SiC were observed for the two oriented tips. The mechanism of the field-evaporation of β-SiC has been discussed from the spectra in connection with the fact that the distinct difference between the two orientations was found in the field emission patterns of the field evaporated tips.     

A LEED-AES study of the oxidation of Fe (110) and Fe (100)

Simultaneous LEED and AES observations have been used to study the initial stages of oxidation of the Fe(110) and Fe(100) single crystal surfaces at 300 K and 400 K and of a clean Fe polycrystal at 300 K. Accurate surface lattice spacings of the precursory oxide structures have been measured and attempts have been made to quantitatively evaluate the corresponding surface oxygen density.On the (110) single crystal surface the final structure is FeO-like with a lattice spacing 4% larger than that of bulk FeO. The transition to the FeO-like structure starts with a surface lattice expansion in the [11&#x0304;0] direction followed by an expansion in the [001] direction in order to accommodate the (111) face of the FeO-like structure. On the (100) single crystal face the oxygen and iron form an fcc (100) face which initially contracts and then expands with increasing oxygen doses. The structure formed at 300 K is spinel-like but heat treatment causes a transition to FeO(100).The changes of the surface unit cell dimensions are interpreted as the result of an interaction between adsorbate and substrate. This interaction is strongest in a direction parallel to the close packed rows of the substrate, making the corresponding periodicity of the adsorbate more resistant to lattice changes.In the case of the polycrystal a hexagonal structure was observed after oxygen adsorption with no simple relation to the oxide structures observed on the single crystals. The initial sticking coefficients in the interval 0–10^?5 torr sec ranged from 0.07 to 0.36 depending on temperature and crystal face observed. The latter dependence is explained in terms of the surface structure.     

Surface-melting induced faceting of aluminium

Medium-energy ion scattering measurements have been used to study the temperature dependent behaviour of aluminium surfaces with surface orientations in the (11&#x0304;0) zone. The ion-scattering measurements show that surface-melting induced faceting occurs between the melting (110) and the non-melting (111) orientations. The dry and melted facet orientations Θ_d and Θ_d have been determined up to 0.3 K from the bulk melting point. Close to the melting point these orientations are almost independent of temperature, and amount to Θ_d = 7 ± 2° and Θ_m = 29 ± 2° with respect to the (111) plane.     

Surface chemistry of the non-basal planes of cobalt: The structure,stability, and reactivity of Co(101&#x0304;2)-CO

The structural and chemisorptive properties of the stepped, non-unique, (101&#x0304;2) surface of cobalt have been investigated by standard LEED/Auger/Δφ/thermal desorption methods. The clean surface is well-ordered, unreconstructed, and reversibly undergoes the predicted structural changes on cycling through the phase transition. CO chemisorption is rapid and non-dissociative at 300 K, leading ultimately to a (3 × 1) structure with a COCO spacing of 3.8 Å. Heating of the adlayer can, depending on the conditions, lead to competitive desorption and dissociation reactions. The data suggest that the transition state to desorption is mobile whereas that for dissociation is localised. Dissociation is accompanied by diffusion of oxygen into the bulk and formation of a very well-ordered (2 × 3) carbon structure. This structure is interpreted in terms of epitaxial growth of the (001) plane of Co₃C. The carbide surface is still capable of chemisorbing a substantial amount of CO, but cannot dissociate it. Some other ordered phases of the CoCCO system are also observed, and an attempt is made to interpret them in a consistent way. The CO chemistry of the (101&#x0304;2) surface is very different from that of the basal plane.