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.     

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.     

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.     

Fabrication of quantum wires and dots on GaAs (111)A patterned substrates by molecular beam epitaxy

Flat GaAs/AlGaAs multilayers without any extra facets were successfully grown on ridge-type triangles with (021)-related sidewalls having an inverted mesa on GaAs (111)A substrates by molecular beam epitaxy. The result was obtained on the basis of the large difference in the growth rate between the [111]A and [01̄1] directions. Fabrication of quantum dots on ridge-type triangles and hexagons with smaller sizes and quantum wires on stripes running in the [1̄1̄2]A direction was proposed.     

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.     

Observation of Si(111) and gold-deposited Si(111) surfaces using micro-probe reflection high-energy electron diffraction

Observations of clean Si(111) and gold-deposited Si(111) surfaces have been performed using micro-probe reflection high-energy electron diffraction. It was found that many atomic steps on a Si(111) surface run in nearly the same direction, about 9° off the [1̄1̄2] direction. When gold was deposited on this surface at a substrate temperature of about 800°C, 5 × 1, diffuse √3 × √3R30°, sharp √3 × √3 R30° structures and Au clusters appeared on the surface with continuation of the deposition. During the deposition process, it was found that one kind of Si(111) 5 × 1 Au domain grew selectively along these atomic steps and nearly covered the entire surface. A phenomenon of gold clusters moving during the deposition was also observed. These clusters all moved in nearly the same direction so as to climb the atomic steps.     

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.     

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 (1&#x0304;1&#x0304;1&#x0304;)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 (1&#x0304;1&#x0304;1&#x0304;)P surface. These experimental results are discussed from the point of both the piezoelectricity and the change in band gap caused by stress.     

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.     

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 〈112&#x0304;〉 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.     

Chemical properties of anion vacancies on zinc oxide

A ZnO(404&#x0304;1) surface, which is a stepped [4(101&#x0304;0) × (0001)] surface containing a high density of anion vacancies was prepared. When compared with the nonpolar (101&#x0304;0) surface, the (404&#x0304;1) surface adsorbed O₂ and methanol more strongly, but CO₂ more weakly. The decomposition products of methanol were different on these two surfaces.     

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.     

Molecular beam epitaxy of monotype CrSi2 on Si(111)

Low energy electron diffraction (LEED), angle-resolved ultraviolet (ARUPS), and X-ray (XPS) photoemission spectroscopy and work function measurements were used to investigate the growth of epitaxial CrSi₂ on a Si(111) surface. The CrSi₂layers ) (~ 100 Å) are formed by the MBE technique, in which Cr and Si are coevaporated in their stoichiometric ratio on the Si(111) substrate maintained at ~450°C. In comparison with the CrSi₂ epitaxy previously obtained by the SPE technique, where two kinds of CrSi₂ domains with equal formation probability are always observed, the epitaxial CrSi₂ layers obtained by the MBE technique essentially present one definite orientation characterized by CrSi₂(0001)∥Si(111) and CrSi₂[112&#x0304;0] ∥[112&#x0304;].     

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.     

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 (1&#x0304;1&#x0304;) 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.     

Neutralization of low energy helium ions scattered from Au adatoms on Si(111)-√3 × √3-Au

The neutralization of 5 keV He⁺ ions scattered from Au adatoms on the Si(111)-√3 × √3-Au surface was studied by impact-collision ion-scattering spectroscopy (ICISS). The He⁺ ICISS data contained false shadowing features that were actually the result of local neutralization effects. The radially dependent ion-atom neutralization theory of Woodruff, when used in our simulations of the ion scattering results, was reasonably successful in describing the neutralization of the He⁺ ions by the Au atoms. Good agreement for both the [112&#x0304;] and [1&#x0304;10] azimuths was obtained for a neutralization rate R = A exp(− ar), where A and a are 15.5 fs⁻¹ and 1.94 Å⁻¹ , respectively. An Auger neutralization model assuming a planar ion-solid interaction surface was also tested, yielding much poorer agreement.     

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.     

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.     

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 [011&#x0304;] direction. Additional CO adsorption causes the heat of adsorption to decrease further and the overlayer structure to be compressed in the [011&#x0304;] 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.     

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.