Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Electronic structure of the (111) and (111) surfaces of GaAs

Inward relaxation effects of the outermost Ga layer on the electronic structure of GaAs (111) Ga and outward expansion effects of the outermost As layer on that of GaAs ($\text{111}$) As are studied by extended Hückel theory. Three different surface geometries are examined for the respective surfaces. It is shown that upon relaxation on GaAs (111) or upon expansion on GaAs ($\text{111}$) new surface states associated with dangling- and back-bonds are revealed. The character and dispersion behaviour of strongly localized surface states are described.     

Field evaporation of silicon (111) surfaces in the presence of hydrogen

The field evaporation rate of Si(111) face in H₂ imaging gas is measured by counting the removal rates on individual net planes in a field ion microscope. The rate is found to decrease with increasing temperature and to be proportional to H₂ gas pressure. The evaporation voltage increases with temperature (T<300). A model explaining this temperature dependence is based on the rate-determining formation of surface hydrides due to field-induced chemisorption of hydrogen.     

Polytypism in GaAs nanowires: determination of the interplanar spacing of wurtzite GaAs by X‐ray diffraction

In GaAs nanowires grown along the cubic [111]_c direction, zinc blende and wurtzite arrangements have been observed in their stacking sequence, since the energetic barriers for nucleation are typically of similar order of magnitude. It is known that the interplanar spacing of the (111)_c Ga (or As) planes in the zinc blende polytype varies slightly from the wurtzite polytype. However, different values have been reported in the literature. Here, the ratio of the interplanar spacing of these polytypes is extracted based on X‐ray diffraction measurements for thin GaAs nanowires with a mean diameter of 18–25 nm. The measurements are performed with a nano‐focused beam which facilitates the separation of the scattering of nanowires and of parasitic growth. The interplanar spacing of the (111)_c Ga (or As) planes in the wurtzite arrangement in GaAs nanowires is observed to be 0.66% ± 0.02% larger than in the zinc blende arrangement.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

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.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

Superconducting transition temperatures of amorphous binary alloy systems based on Bi,Pb, Ga and Be

Quench condensed binary alloy films are produced by evaporation from two separated furnaces. The films contain the whole composition range of the respective alloy system in well defined arrangement.T _c is measured as a function of concentration. Eight predominantly amorphous alloy systems are studied: Bi—Ga, Pb—Ga, Pb—Bi, Be—Bi, Be—Pb, Be—Ga, Be—Al, Be—Li. In Bi—Ga and Pb—GaT _c is a linear function of concentration in the amorphous composition range. In Pb—BiT _c has a maximum. All Be-alloys show lower transition temperatures than pure quench condensed Be. Except for Be—Li all systems have aT _c minimum. The experiments are compared to aT _c calculation using tunelling spectroscopy data. Except for the Be-alloys the agreement is satisfying.     

Effect of electron-beam irradiation on the magnetic properties of Ga1−xMnxAs thin films grown on GaAs (100) substrates

The effect of electron-beam irradiation on the magnetic properties of (Ga_1−xMn_x)As thin films grown on GaAs (100) substrates by using molecular beam epitaxy was investigated. The ferromagnetic transition temperature (T_c) of the annealed (Ga_0.933Mn_0.067)As thin films was 160 K. The T_c value for the as-grown (Ga_0.933Mn_0.067)As thin films drastically decreased with increasing electron-beam current. This significant decrease in the T_c value due to electron-beam irradiation originated from the transformation of Mn substituted atoms, which contributed to the ferromagnetism, into Mn interstitials or Mn-related clusters. These results indicate that the magnetic properties of (Ga_1−xMn_x)As thin films grown on GaAs (100) substrates are significantly affected by electron-beam irradiation.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Hyperfine interactions in intermetallic rare earth-gallium compounds studied by 111 Cd PAC

Magnetic and electric hyperfine interaction of the nuclear probe ¹¹¹In/¹¹¹Cd in intermetallic compounds of the rare earth-gallium system have been investigated by perturbed angular correlation (PAC) spectroscopy. The PAC measurements, supported by X-ray diffraction, provide evidence for a marked phase preference of ¹¹¹In for hexagonal RGa₂ over orthorhombic RGa and of RGa₃ with the L12 structure over RGa₂. In the case of SmGa₂, the magnetic hyperfine field B_hf, the electric quadrupole interaction and the angle β between B_hf and the symmetry axis of the electric field gradient have been determined as a function of temperature. The angle β?=?0 is consistent with the results of previous magnetization studies. Up to T?≤?17 K the magnetic hyperfine field has a constant value of B_hf?=?3.0(2) T. The rapid decrease at higher T gives the impression of a first-order transition with an order temperature of T_N?=?19.5 K. In the RKKY model of indirect 4f interaction the ratio T_C/B_hf(0) is a measure of the coupling constant. For ¹¹¹Cd:SmGa₂ (T_C/B_hf(0)~6.5 K/T) this ratio is significantly smaller than for the same probe in other R intermetallics (SmAl₂ ~9.5 K/T, Sm₂In ~13.5 K/T).     

Interaction of carbon dioxide with Fe(110), stepped Fe(100) and Fe(111)

The adsorption of CO₂ on single crystal surfaces of Fe(110), regularly stepped Fe(110) and Fe(111) in the temperature range between 77 and 340 K was studied by means of He(I) UPS and measurements of the change in work function. The smooth Fe(110) face proved to be completely inactive with respect to CO₂ adsorption. On a stepped Fe(110) and an Fe(111) face CO₂ is adsorbed at 77 K in the form of a linear molecule and in the form of a species the nature of which is not yet clarified. This latter form is predominant at 140 K. With increasing temperature decomposition into CO and O and finally into C and O takes place.     

Exchange Interactions and Curie Temperatures in Dilute Magnetic Semiconductors

We discuss the origin of ferromagnetism in dilute magnetic semiconductors based on ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb. We use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. Curie temperatures (T _C) are calculated from first-principles by using a mapping on a Heisenberg model in a mean field approximation. It is found that if impurity bands are formed in the gap, as it is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic √c dependence of T _C as a function of the Mn concentration c. On the other hand, if the d-states are localized, as in (Ga, Mn)Sb, Zener's p–d exchange prevails resulting in a linear c-dependence of T _C. In order to have more precise estimations of T _C, effective exchange coupling constants J _ij 's are calculated by using the formula of Liechtenstein et al. It is found that the range of the exchange interaction in (Ga, Mn)N is very short. On the other hand, in (Ga, Mn)As the interaction is weaker but long ranged. Monte Carlo simulations show that the T _C values of (Ga, Mn)N are very low since percolation is difficult to achieve for small concentrations and the mean field approximation strongly overestimates T _C. Even in (Ga, Mn)As the percolation effect is still important.     

Adsorption isotherms of oxygen on tungsten (100), (110) and (111) surfaces in the pressure range 10−9 to 10−4 torr and temperature range 1500 to 2600 k,as measured by AES

This paper is a continuation of a previous investigation of oxygen adsorption on tungsten at high temperature using Auger electron spectroscopy. In this paper the adsorption isotherms of oxygen on (100), (110) and (111) faces of tungsten are reported. It is shown that these isotherms can be described by an equation of the form pO₂ = AF(θ) exp [?q(θ)/ kT]. The coverage depended functions F(θ) and q(θ) evaluated from the isotherms are different for all three investigated faces. The isosteric adsorption energy q has following initial values at very low oxygen coverage: q₁₀₀ = 6.1 eV, q₁₁₀ = 6.8 eV and q₁₁₁ = 6.5 eV. Increasing the oxygen coverage has only small influence on q₁₁₁; it changes from the initial value to q₁₁₁ ≈ 6.0 eV at θ ≈ 0.3 and remains constant at this value up to θ ≈ 1. q₁₁₀ shows the strongest dependence on oxygen coverage. It decreases rapidly at low coverages, slowly at moderate coverages and reaches the value q₁₁₀ = 5.0 atθ ≈ 1. The variation of q₁₁₀ with increasing oxygen coverage is monotonie from the initial value to q₁₁₁ = 4.9 eV at θ ≈ 1. Assuming that the atomic oxygen is the dominant species leaving the tungsten surface at high temperatures the functions F(θ) are used to calculate the oxygen equilibration probability ζO₂ (high temperature sticking probability) as a function of oxygen coverage θ. The main characteristic of ζO₂(θ) for all three faces is that it shows a maximum for (100) and (111) faces at θ = 0.3 and for (110) face at θ = 0.55.     

On the order-disorder transition of chemisorbed H on Ni (111)

The order-disorder transition of chemisorbed H on Ni (111) has been analyzed by Kittler and Bennemann in terms of the lattice gas model. The main purpose of this paper is to refine the above calculation. The expression with regard to adlayer domains is given for the half-order LEED spot intensity, from which the dependence of the transition temperature and of the LEED spot intensity on H-coverages is calculated. In particular, the H-coverage dependence of the transition-temperature T_c and the LEED intensity I for θ? 0.25 is restudied. We present results obtained by using only pair interactions between the H-atoms and no 3H-interactions. It is shown that the observed asymmetric phase diagram with respect to H-coverages can result from the two kinds of pair interactions between chemisorbed H atoms.     

A study of the (00) LEED beam intensity at normal incidence from CdS(0001), Cu(001), Cu(111), and Ni(111)

A Faraday cage apparatus is used for the measurement of the (00) LEED beam intensity, I₍₀₀₎, and the total secondary emission coefficient, δ(E_k), for angles of incidence from 0° ± 2° to 8° ± 2°, with an energy resolution of ± 0.037 of the incident beam energy, in the energy range 1 to 200 eV. The data are normalized and expressed as a fraction of the incident beam intensity. The basic principle of operation is the separation of the incident and specularly diffracted beams in a uniform magnetic field. Monolayer, or in-plane, resonances associated with the emergence of nonspecular beams, as well as beam threshold minima, are observed in I₍₀₀₎ at normal incidence from clean CdS(0001), Cu(111), and Ni(111). Some major differences are observed in the I₍₀₀₎ profiles for the clean (111) surfaces of nickel and copper. All secondary Bragg peaks, except the 2$\text{2}\text{3}$ order, have greater intensities for Ni(111) in the energy range 50–150 eV, thus indicating that the atomic scattering cross-section for electrons in this energy range is larger for nickel than for copper. For the (111) surface of nickel, the (11) resonance is missing, but the (10) resonance and all $\text{1}\text{3}$ order secondary Bragg peaks between the second and fifth orders are observed. For Cu(111) both the (10) and (11) resonances are observed, but the $\text{1}\text{3}$, $\text{2}\text{3}$, 1$\text{2}\text{3}$, and 3$\text{1}\text{3}$ order secondary Bragg peaks are missing in this energy range. These data indicate that multiple scattering with evanescent intermediate waves, or “shadowing”, is predominate on the (111) surfaces on nickel and copper for energies above 30 eV, and that below 30 eV multiple scattering with propagating intermediate waves is predominate on Cu(111). Correlation of the (00) beam intensity profiles from clean Ni(111) at 0°, 2°, and 6° with the intensity profiles of the (10). (1&#x0304;0), and (11) non-specular beams is nearly one-to-one from 30 eV to 100 eV, thus supporting the dynamical theories of LEED in which peaks in the (00) beam are expected to occur at nearly the same energies as peaks in the non-specular beams.     

Annealing of gold nanoparticles on GaP(111)B: initial stage of GaP nanowire growth

GaP(111)B substrate was strewn with 30 nm colloidal Au nanoparticles. Organic residues were removed by: A) boiling in acetone and isopropylalcohol followed by a DI water rinse, B) treatment A + HF:H₂O, C) treatment A + O₂ plasma for 10 min, 20 min, and 40 min, and D) treatment A combined with O₂ plasma (10 min) and HF:H₂O. The substrate thus had original ‘epi‐ready' oxides (A), or fresh native oxides (B and D), or new added oxides (C). The samples were annealed at T_a = 650 °C for 10 min under PH₃ and H₂ in an MOVPE chamber. This resulted in the growth of GaP stumps along [111]B on each sample. Their length was <3 nm (B and D), ～20 nm (A), and ～220 nm (C 40 min). Elemental Ga is left as P₂O₅/Ga₂O₃ oxides form on etched GaP(111)B at room temperature. We believe that as the oxides disintegrated during annealing, they released the elemental Ga that combined with phosphorus from PH₃, and this led to the growth of the GaP stumps. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

AFM morphological characterization and Raman study of germanium grown on (111)GaAs

In this communication we report on the growth of Ge heterolayers on (100), (111)Ga and (111)As surfaces of GaAs substrates. Arsenic can play a role as both dopant and surfactant, changing the growth mechanism for the Ge/GaAs growth on (001) oriented substrates. The use of substrates oriented in different directions can change the growth mode and produce different results, since surface polarity can induce different growth modes: we have compared the results on (111) substrates with respect to the non-polar surface case.The growth behavior on (001), (111)Ga and (111)As substrates is discussed. The layer morphology was investigated by Atomic Force Microscopy and Raman spectroscopy has been carried out as a function of the sample thickness.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.