Chemical properties of anion vacancies on zinc oxide

A ZnO(404̄1) surface, which is a stepped [4(101̄0) × (0001)] surface containing a high density of anion vacancies was prepared. When compared with the nonpolar (101̄0) surface, the (404̄1) surface adsorbed O₂ and methanol more strongly, but CO₂ more weakly. The decomposition products of methanol were different on these two surfaces.     

The effect of hydrogen chemisorption on GaAs(100) and GaAs(1̄1̄1̄)

The interaction of hydrogen with the polar (100) and (1̄1̄1̄) surfaces of GaAs has been studied with LEED, angle-resolved photoemission and core level spectroscopy. It was found that the properties of the hydrogen-covered surface were independent of the composition of the initial surface. The core levels also showed an increase in the surface As concentration for initially Ga-rich surfaces. Angle-resolved photoemission results for GaAs(100) and GaAs(100):H are presented and the dispersion of a hydrogen-induced state is shown.     

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 [2̄11] or [21̄1̄]. Steps with edges towards [2̄11] 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 [21̄1̄] 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 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.     

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.     

Angular resolved ups of surface states on GaAs(1&#x0304;1&#x0304;1&#x0304;) prepared by molecular beam epitaxy

The As-rich (2 × 2), a newly found (√3 × √3) and the (√19 × √19) surfaces of GaAs(1&#x0304;1&#x0304;1&#x0304;) are studied by angular resolved UPS (ARUPS). The (2 × 2) surface is prepared by molecular beam epitaxy and the others by mild annealing. For the (2 × 2) surface emission from surface states is observed, which shows dispersion periodic within the (2 × 2) surface Brillouin zone. Using s-polarized light and the known symmetry selection rules the uppermost surface bands between 1 and 2 eV below the valence band maximum are assigned to the As dangling bond orbital. The bands near 4 and 7 eV assigned to the backbonds. From the strong decrease of emission intensity of the As-derived surface states between the (2 × 2) and the annealed surfaces it is concluded that the character of the As dangling bond orbital must have been changed from sp³-hybridic to s-like. This gives further evidence for our recently proposed model for the (√19 × √19) surface, which is particularly applicable for the (√3 × √3) surface.     

Surface core level shift on Be( 112&#x0304;0)

A photoemission study of the Be(112&#x0304;0) surface carried out at a sample temperature of 100 K is reported. A surface shifted Be 1s component, having a shift of - 410 meV, is resolved on this surface. The extracted surface to bulk intensity ratio indicate that this component originates from atoms in the surface layer only. This is opposite to previous observations on both the close-packed Be(0001) surface and the Be(101&#x0304;0) surface where sub-surface shifted Be 1s levels were unambiguously identified. Among these three surfaces a surface layer atom is expected to have the lowest coordination on the (112&#x0304;0) surface but the surface layer shift is found to be smallest on this surface. Compared to findings on other metals this is unusual and reasons contributing to this behaviour are suggested and discussed.     

Atomic relaxation of the BeO (101&#x0304;0) surface

The atomic relaxation of the nonpolar (101&#x0304;0) surface of BeO has been calculated by minimizing the surface energy within the framework of the ab initio Hartree-Fock method. A six-layer two-dimensionally periodic slab model was used, permitting a full symmetry-conserving relaxation of the two outer layers. The BeO surface bonds show a small rotation angle of about 4 accompanied by a large (about 10%) reduction in surface bond length. Significant contraction of backbonds and a small rotation of second layer bonds are also found. The relaxed BeO (101&#x0304;0) surface is thus predicted to be similar to the ZnO (101&#x0304;0) surface but different from the corresponding surfaces of all other II–VI compounds. Various explanations for this difference are discussed, and evidence from a bond population analysis is presented which suggests that this behavior can be described in terms of partial double bond character in the surface bonds. Since multiple bonding is related to small atomic radii, it would follow that the small radius of the oxygen atom is the ultimate cause of the type of surface relaxation we predict.     

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.     

The reactivity of the clean and contaminated (0001&#x0304;) polar and the (101&#x0304;0) prism ZnO surfaces to chlorine

The reactivities of the (0001&#x0304;) and (101&#x0304;O) surfaces of zinc oxide to chlorine gas have been studied by a range of techniques. In the case of the (0001&#x0304;) 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 (101&#x0304;O) at high exposures where a work function decrease took place following the initial increase: this may indicate a second molecular state.     

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.     

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.     

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, (1&#x0304;1&#x0304;1&#x0304;)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.     

Site-specific interaction of H2O with ZnO single-crystal surfaces studied by thermal desorption and UV photoelectron spectroscopy

The adsorption and condensation of H₂O(D₂O) on ZnO(101&#x0304;0), (0001)Zn and (0001&#x0304;)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001&#x0304;)O face defect-induced features were identified. The water lone-pair orbital 1b₁, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at E_B^V(1b₁) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001&#x0304;)O face and are compared with the multilayer ice emission at 1&#x0304;1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101&#x0304;0) face the 1b₁ emission is very broad, indicating some overlap between different states.     

Structure of Si(111)−7 × 7

A model of the Si(111)?7 × 7 surface atomic arrangement is put forward on the basis of results already established for Si(111) and Si(100) surfaces. The unit mesh contains a triangular double-layer island with 21 first-layer atoms. The island is laterally expanded and is bounded by [1&#x0304;1&#x0304;2] steps with second-layer edge atoms forming asymmetric dimers. It is shown that salient features of low energy electron diffraction (LEED) patterns for Si(111)?7 × 7 can be explained by the model. The LEED patterns are interpreted qualitatively by a double-diffraction mechanism involving forward diffraction in the selvedge. It is shown that the patterns contain characteristic formations of fractional-order spots attributable to the dimers at the island boundaries. The best agreement with observed patterns is obtained with the following parameter values: dimer bond length 2.5 ± 0.2 Å, island lateral expansion 3 ± 2%. Some of the implications of the model for the chemical reactivity and electronic properties of the Si(111)?7 × 7 surface are discussed.     

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 〈1&#x0304;10〉 direction increases while in the 〈001&#x0304;〉 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 〈1&#x0304;10〉 direction doubles. The oxygen atoms were found to lie in or close to the nickel 〈001&#x0304;〉 rows. These results are not in agreement with recently published dynamical LEED calculations.     

Evaluation of the electrostatic potential at the surface of an ionic solid

A method to evaluate the electrostatic potential at and near the surface of a semi-infinite ionic solid is presented. It utilizes the Ewald equation for an infinite crystal and Parry's extension of the Ewald equation for an infinite slab. The method was applied to calculate the potentials at the (100), (110), (021&#x0304;), and the (111) surfaces of NaCl. Results for the nonpolar (100) and (110) surfaces agree with results by other methods. Results for the polar (111) surface differ from results of slab calculations. The difference is thought to be due to the presence of a second surface in the slab calculations. Its application to a relaxed surface is also demonstrated.     

Observation of extrinsic surface states on (112&#x0304;0) CdS

Surface states have been detected by surface photovoltage spectroscopy on (112&#x0304;0) CdS surfaces subjected to various treatments in UHV and studied by Auger electron spectroscopy and LEED. All surface electronic features can be related to chemical contamination or lattice nonstoichiometry. Energy level spectra of air-exposed CdS exhibit a set of discrete states due to adsorption of C, O, and Cl. Ion bombardment generates a pair of states 2.35 eV and ~0.8 eV above the valence band edge due to S interstitials and vacancies, respectively. Oxygen adsorption produces a broad continuum of states. Changes in surface atomic order show no direct effect on these electronic features. No intrinsic surface states, filled or empty, are observed by surface photovoltage spectroscopy on clean, stoichiometric (112&#x0304;0) faces of CdS.     

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 〈001&#x0304;〉 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.