Orientation dependence of oxygen adsorption on a cylindrical GaAs sample: I. Auger measurements

The orientation dependence of oxygen adsorption has been investigated by AES on the surface of a cylindrically shaped GaAs single crystal with [111̄0] being its axis. It thus exposes the main low index orientations (001), (111)Ga, (110), and (111̄)As, as well as all their vicinal surfaces and intermediate orientations on its surface. It is shown that it is possible to prepare all these orientations simultaneously and with reasonable quality by ion bombardment and annealing (IBA). The orientation dependence of the amount of adsorbed oxygen in the range (001)(111)Ga(110)(111̄)As can be understood in terms of different sticking coefficients on the different types of terrace site and of enhanced adsorption on edge-adjacent sites. These edge-adjacent sites show saturation at about 4 × 10⁵ L. Starting from (110) towards (111)Ga, at first, steps one atomic layer high are found, changing to a height of two layers when approaching (331). This behaviour can be understood in terms of the known relaxation on (110). A deep minimum in the amount of adsorbed oxygen between (111̄)As and (001̄) is interpreted to be due to an As stabilized low sticking coefficient phase between (112̄) and (113̄). Early saturation (at～10⁵ L) on (001) and (111̄)As is consistent with the fact that these surfaces usually do not reach their room temperature equilibrium phase upon preparation by IBA. Sudden and accidental oxygen induced composition changes towards As-richer substrate compositions further confirm this.     

Inelastic scattering of Ne atoms from a LiF(001) surface

Scattering experiments with a ²⁰Ne nozzle beam from a LiF(001) surface in the 〈100〉 azimuth are reported. The (11) and (1̄1̄) Bragg reflections show broad tails due to inelastic scattering. These tails can be attributed by time-of-flight measurements to single phonon scattering on acoustic modes. The inelastic contribution decreases rapidly with increasing energy of the phonons involved.     

The adsorption of Xe and CO on a Cu (311) single crystal surface

LEED, electron energy loss spectroscopy and surface potential measurements have been used to study the adsorption of Xe and CO on Cu (311). Xe is adsorbed with a heat of 19 ± 2 kJ mol^/t-1. The complete monolayer has a surface potential of 0.58 V and a hexagonal close-packed structure with an interatomic distance of 4.45 ± 0.05 Å. CO gives a positive surface potential increasing with coverage to a maximum of 0.34 V and then falling to 0.22 V at saturation. The heat of adsorption is initially 61 ± 2 kJ mol^?1, falling as the surface potential maximum is approached to about 45 kJ mol^?1. At this coverage streaks appear in the LEED pattern corresponding to an overlayer which is one-dimensionally ordered in the [011̄] direction. Additional CO adsorption causes the heat of adsorption to decrease further and the overlayer structure to be compressed in the [011̄] direction. At saturation the LEED pattern shows extra spots which are tentatively attributed to domains of a new overlayer structure coexisting with the first. Electron energy loss spectra (EELS) of adsorbed CO show two characteristic peaks at 4.5 and 13.5 eV probably arising from transitions between the electronic levels of chemisorbed CO.     

Ex situ STM imaging with atomic resolution of Ni(111) electrodes passivated in sulfuric acid

Scanning tunneling microscopy has been used to study in air Ni(111) electrodes passivated in 0.05M H₂SO₄ at + 650 mV/SHE. Before passivation, the Ni(111) single crystal surface has steps along the 〈1̄10〉 directions with terraces having a width of a few hundred nanometers. After passivation, a mosaic structure is formed with crystallites of 2 to 3 nm in size and strip-like features extended mainly along the [12̄1] direction whose width ranges from 2 to 3 nm and which appear to be locally tilted from 5 to 13°. Atomic resolution imaging has been achieved on both of these features. It reveals a corrugated lattice whose parameters are 0.30 ± 0.02 nm and 117 ± 3° in agreement with those of NiO(111). The crystallites have smooth step edges along [1̄01] and [011̄]. The STM imaging of the passive film as well as its mosaic structure are discussed.     

An investigation of the kinetics of structural changes during the early oxidation stages of a Ni(100) surface using low energy ion scattering (LEIS)

Low energy ion scattering has been used to investigate the early stages of the oxidation of a Ni(100) surface. This technique allows simultaneous study of the oxygen uptake in the surface and the development of surface structures. Bombardment induced surface damages was minimised by performing the experiments with low ion doses, while keeping the target at 200–300°C. The measured kinetics of the oxygen uptake are in good agreement with recent work, using different techniques. It is concluded that during the early chemisorption, a two stage process takes place: an initial oxygen adsorption during which the O atoms probably reside within the fourfold surface hollows, followed by a reconstruction process, caused by the combined action of at least two nearest neighbour O atoms, trapping mobile Ni adatoms, after which the O atoms stabilise at a site in or close to the reconstructed 〈001̄〉 row. Observed structural changes at higher exposures are compatible with a transition into a (3 × 1) structure and subsequently NiO, but cannot, as yet be positively identified.     

Oxygen adsorption on Cu(110): Determination of atom positions with low energy ion scattering

The position of adsorbed oxygen on Cu($\text{1}$10) surfaces was determined with Low Energy Ion Scattering (LEIS). The experiments were performed by bombarding the copper surface at small angles of incidence with low energy Ne⁺ ions (3–5 keV). Measurements of the Ne⁺ ions scattered by adsorbed oxygen showed regular peaks in the azimuthal distribution of the scattered ions due to a shadowing effect. From the symmetry of the azimuthal distributions it follows that the centre of an adsorbed oxygen atom on the Cu(1̄10) surface lies about 0.6 Å below the midpoint between two neighbouring Cu atoms in a 〈001〉 row. A comparison of the azimuthal distributions of Ne⁺ ions scattered from clean Cu surfaces and oxygen-covered Cu surfaces showed that hardly any surface reconstruction had occurred in the oxygen-covered surfaces. The applied method seems to be an appropriate one for locating adsorbed atoms because it uses only simple qualitative considerations about azimuthal distributions of scattered ions.     

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 [011̄] 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 [010̄] direction in common.     

Structure determination of Ni(110)-(2 × 3)-N by use of SEXAFS measurements: on-surface and sub-surface sites on a pseudo-(100) reconstructed surface

Surface-extended X-ray-absorption fine-structure measurements have been performed on the Ni(110)-(2 × 3)-N system. The data are consistent with a model in which nitrogen chemisorbs on a pseudo-(100) reconstructed and largely corrugated surface with a nearest-neighbour N-Ni bond length of 1.86 ± 0.03 Å. The corrugation results in two raised and two lowered [11̄0] Ni rows which are not uniformly distributed in the [001] direction. Nitrogen chemisorbs in four-fold hollow sites slightly (0.19Å) above the lowered Ni rows and in pseudo-four-fold hollow sites slightly (0.38 Å) below the plane defined by a raised and a lowered Ni row. In both sites N is equidistantly bonded to Ni atoms in the second layer. Structural models with long-bridge adsorption sites can be safely excluded.     

Nucleation and orientation of graphite{0001} layers on Pt{110}

Investigations by field electron microscopy revealed that graphite{0;0001} layers grow preferentially on Pt{110}. Layer nucleation and initial growth occurs in the vicinal area of {110} along the 〈112〉 and the 〈100〉 zones and not on the unstepped {110} plane itself. The growth rate in the vicinal area is determined by the epitaxial misfit parallel to the step edges. An alignment of the graphite layer with the underlying {110} is observed. It corresponds to graphite〈101̄0〉 ∥ Pt〈100〉, which coincides with the calculated orientation of smallest epitaxial misfit. The change of relative emission from {110} upon adsorption of graphite is discussed.     

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 (511̄), (711̄), (911̄) 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.     

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 [11̄0] 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.     

An experimental investigation of the elastic scattering of He and H2 from Ag(111)

Experimental diffraction probabilities for 63 meV He and 66 meV H₂ scattering from Ag(111) along the 〈112̄〉 direction are reported. Debye-Waller experiments for the He/Ag(111) system yield a mean well depth of 9.3 meV and an effective surface Debye temperature of 253 K.     

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.     

The reactivity of the clean and contaminated (0001̄) polar and the (101̄0) prism ZnO surfaces to chlorine

The reactivities of the (0001̄) and (101̄O) surfaces of zinc oxide to chlorine gas have been studied by a range of techniques. In the case of the (0001̄) oxygen polar surface investigations were made with the surface both atomically clean and with a known level of carbon and calcium contamination. Comparison is made with our earlier results on the (0001) surface which showed a high level of reactivity due to the increased electrostatic stability on adsorption of the electronegative gas. Both the oxygen polar and the prism surface showed a much lower reactivity to chlorine than the zinc face: contamination by carbon and calcium on the former surface reduced the reactivities still further. This result conflicts with comparable data for oxygen adsorption where previous work has shown a greater take-up of oxygen on the oxygen face than the zinc face. Unlike the zinc face, no LEED superstructures were observed on any of, the three surfaces, but in common with the (0001) there were significant electron beam desorption effects. Two states could be identified: one was rapidly removed in ~10 μA min exposure to the beam, the other in much longer periods. Work function and ELS data were consistent with atomic adsorption of chlorine on all surfaces. An exception was the (101̄O) at high exposures where a work function decrease took place following the initial increase: this may indicate a second molecular state.     

The off-axis pyroelectric effect observed for lithium tetraborate

We find a pyroelectric current along the 〈110〉 direction of stoichiometric Li₂B₄O₇ so that the pyroelectric coefficient is nonzero but roughly 10⁻³ smaller than along the 〈001〉 direction of spontaneous polarization. Abrupt decreases in the pyroelectric coefficient along the 〈110〉 direction can be correlated with anomalies in the elastic stiffness contributing to concept that the pyroelectric coefficient is not simply a vector but has qualities of a tensor, as expected. The time dependent surface photovoltaic charging suggests that an inverse piezoelectric effect occurs at the (110) surface but not the (100) surface. Both effects along the 〈110〉 direction or at the (110) surface are distinct the conventional as a bulk pyroelectric effect.     

Surface phonons in GaAs(110)

Surface phonon dispersion curves have been measured along the <001>, <11&#x0304;2>, and <11&#x0304;0> azimuths of GaAs(110). Features of note include a very low frequency (5.5 meV at zone boundary) surface acoustic mode in the first two directions; this may arise through the known (1 × 1) reconstruction of this surface. A higher frequency surface mode (7.3–8.8 meV, depending on azimuth) is seen in all directions. The helium scattering intensities are greatly influenced by bound state resonances. A careful survey of the selective adsorption signatures in extremely high resolution scans of polar angle, azimuthal angle, and incident beam wavevector allows the bound state energies to be determined with some confidence. Initial results indicate energies of roughly 1, 2, and 4 meV.     

Initial stages of hydrogen reduction of NiO(100)

The reduction of single crystal NiO(100) under hydrogen has been followed by AES, XPS and LEED for the pressure range of 1.0 × 10^?7 to 1.3 × 10^?6 Torr and for substrate temperatures of 150–350°C. The kinetics of reduction are controlled both by the rate of removal of lattice oxide at the surface and by the diffusion of subsurface oxygen to the oxygen-depleted surface. The rate of oxygen removal is first-order in surface oxide concentration and in hydrogen pressure. An induction period precedes the reduction reaction and its length is postulated to be controlled by surface defect concentration. The stoichiometric and reduced lattice oxygen species appear to be chemically identical and give a single symmetric XPS peak at 529.4 eV. Nickel spectra indicate a shift in XPS binding energies from those expected of the oxide to those of nickel metal early in the reduction process, although LEED indicates the NiO(100) surface lattice to remain the stable structure for surface reduced to approximately 20% of the stoichiometric oxygen concentration. Ni(100) island formation is observed, with Ni 〈010〉 and 〈001〉 directions along the NiO 〈010〉 and 〈001〉, respectively, but only after the NiO surface is severely depleted in oxygen.     

Anisotropic photocorrosion of n-type MoS2 MoSe2, and WSe2 single crystal surfaces: the role 0f cleavage steps,line and screw dislocations

The photocorrosive behavior of the semiconducting transition metal dichalcogenides in contact with aqueous electrolytes is extremely anisotropic. Enhanced reactivity was observed at cleavage steps and on either line or screw dislocation cores emerging at the surface exposing mainly (101&#x0304;1) faces lying in 〈112&#x0304;0〉 directions. Corrosion patterns, however, could never be observed on smooth (0001) faces. The corrosion products of the molybdenum compounds are soluble, whereas WSe₂ formed WO₃ precipitates on the surface.     

Oxidation of W(110): I. LEED study of the oxide formation at 1000 K

Oxidation of the (110) surface of a tungsten crystal in 2 × 10⁻³ torr of oxygen at 1000 K for 80 sec has revealed a LEED pattern which exhibits a complex variation with incident electron energy. A kinematic analysis has shown that the LEED patterns result from three pairs of twin related WO₃(111&#x0304;) nuclei, each facetted to pyramids exposing three well developed (100) type surfaces. Detailed analysis of the pattern has shown that the nuclei exhibit the same monoclinic symmetry at the surface as that existing in bulk WO₃. A pseudo-cubic lattice constant of 7.58 ± 0.1 Å was estimated from the electron energies at which Laue back reflexion beams were excited.