Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

In-Si(111)界面上的电荷转移及铟原子的表面电致迁移现象

用反射式高能电子衍射仪首次观察到在Si(111)面上的一部分铟吸附原子在直流电场下,沿电场方向发生迁移的现象——表面电致迁移。根据所观察到的表面电致迁移过程,可以把吸附在Si(111)面上的铟原子的结合状态分成两类:紧靠着硅表面的一个单原子层铟与硅表面结合牢固,几乎不受电场影响,称为紧固层;在紧固层以上的铟层易受电场影响而发生表面电致迁移,称为迁移层。从铟原子的表面电致迁移率与温度的关系,求得表面质量迁移的激活能为0.43eV。用表面电导测量研究了In-Si(111)界面形成过程中的电荷转移现象。结果表明,吸附在硅表面的铟原子形成表面深施主能级。导致表面电致迁移的力是离化了的铟原子在电场中所受到的库仑力。 关键词：     

Shubnikov-de Haas oscillations in p-type inversion layers on n-type silicon

Surface Shubnikov-de Haas oscillations have been measured in p-type channels of (110) silicon field effect transitors between 1.4 and 4.2 K in magnetic fields up to 10 Tesla. Two electric subbands were revealed and the effective masses of the holes in both bands could be determined. For one subband the dependence of the mass on the surface electric field was investigated. At low gate voltages the g-factor of the holes was measured from the spin splitting of the Landau-levels.     

Surface quantum oscillations in (110) and (111) n-type silicon inversion layers

Shubnikov-de Haas oscillations have been studied for (110) and (111) n-type silicon inversion layers. The measured cyclotron masses m_c = (0.38 ± 0.03)m₀ and m_c = (0.40 ± 0.03)m₀ for (110) and (111) planes, respectively, are larger than theoretically predicted values. The experimental valley degeneracy factor g_v = 2 ± 0.2 for both orientations is also at variance with self consistent calculations. The electronic g-factor depends on the surface carrier concentration and is enhanced over its bulk value. There was no evidence for the occupation of other subbands.     

Tight-binding approach to field desorption: N2 ON Fe(111)

We present a tight-binding cluster calculation including interatomic Coulomb repulsion for field-induced adsorption and desorption. For electric field strengths F up to the desorption threshold $\text{F ～ 1.5}\text{V}\text{A}\text{?}$ for N₂ on Fe(111) we calculate total potential energy surfaces. The variation of the Schottky barrier and of the N₂ vibrational frequency is extracted as a function of F.     

Electrical properties of highly (111)-oriented lead zirconate thin films

Antiferroelectric PbZrO₃ thin films were grown on Pt/Ti/SiO₂/Si substrates with predominant (111) orientation using a sol-gel process. The Pt/PbZrO₃/Pt film capacitor showed well-saturated hysteresis loops at an applied voltage of 5 V with remanent polarisation (P_r) and coercive electric field (E_c) values of 8.97 μC/cm² and 162 kV/cm, respectively. The leakage current density of the highly (111)-oriented PbZrO₃ film was less than 1.0×10⁻⁷ A/cm² over electric field ranges from 0 to 105 kV/cm. The conduction current depended on the voltage polarity. The PbZrO₃/Pt interface forms a Schottky barrier at electric fields from 20 to 160 kV/cm. The dielectric relaxation current behaviour of Pt/PbZrO₃/Pt capacitor obeys the well-known Curie-Von Schweidler law at electric field of 20-80 kV/cm, the currents have contributions of both dielectric relaxation current and leakage current.     

Structural determinations of the Rh(100) and Cu(111) surfaces using the reliability factor proposed for LEED by Zanazzi and Jona

The reliability factor (R) proposed for LEED by Zanazzi and Jona has been applied to experimental and calculated intensities for the (100) surface of rhodium and for the (111) surface of copper. The calculations used the dynamical perturbation programs of Van Hove and Tong. For each metal, phase shifts were calculated both from a band structure potential and from a potential calculated with a 13 atom cluster. For Cu(111) the I(E) curves from the two potentials were indistinguishable visually and gave similar minimum R values (0.132 and 0.136); the two potentials used for rhodium showed somewhat greater differences. The approach described by Zanazzi and Jona has been supplemented by a simple statistical analysis of the errors involved in the predicted geometries. This study indicates that the topmost interlayer spacing in Cu(111) is contracted by 4.1 ± 0.6% from the bulk value; in Rh(100) the top spacing equals the bulk value to within 3%.     

Segregation of tin on (111) and (100) surfaces of copper

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (√3 × √3)R30° structure for the (111) surface and a p(2 × 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to ΔH = ?(53 ± 5) kJ/g-atom.     

Ga- and As-Adatom phases on the Ge(111) and Si(111) surfaces: analogies and differences

The (1 × 1) and (√3 × √3)R30° (T4) structures of Ga and As adatoms on the Ge(111) and Si(111) surfaces are studied using first-principles calculations. The surface energetics predicts, in some cases, a transformation of the T4 structure (surface covered with 1/3 monolayer (ML) of adatoms) into domains of the 1-ML covered (1 × 1) structure and areas of clean reconstructed suface. For As adatoms, such phase separation is favored on both substrates, while for Ga adatoms, it is only preferred on the Ge(111) surfaces. These results compare well with experimental observations.     

Electric field gradient at111In on Cu (100) surfaces and its utilization for indium surface diffusion

The electric field gradient at¹¹¹In probe atoms on Cu (100) surfaces was studied. At clean surfaces all probes are exposed to a well-defined surface field gradient. This is used to investigate indium surface diffusion, where the applied PAC method allows to observe diffusion steps on an atomistic scale. The jump rate for indium on Cu (100) was found to be in the order of 10^–3 Hz at 200 K.     

Angular dependent negative magnetoresistance in Si-MOS (111) inversion layers

The negative transverse magnetoresistance effect was observed in n-inversion layers in Si-MOS (111) surfaces at temperatures between 1.5 and 8.3 K. The negative magnetoresistance depends only on the normal component of the magnetic field to the surface and has a saturation value at high fields. The difference between the resistivity at zero field and that at saturation field increases logarithmically with decreasing temperature such as the effect due to the s-d scattering (Kondo effect).     

Light-emitting structures based on nanocrystalline (Si/CaF2) multiquantum wells

Periodic nanocrystalline (Si/CaF₂) multilayers, deposited on (111) silicon by Molecular Beam Epitaxy (MBE) at room temperature, were used to fabricate simple light emitting structures and to study their electrical and optoelectronic properties. Photoluminescence (PL) and electroluminescence (EL) spectra from the same area of the devices are approximately the same, indicative of the same emission mechanism. Current–voltage characteristics reveal important phenomena in vertical carrier transport. Regions of negative differential resistance and current oscillations were observed and were tentatively attributed to resonant tunneling at high electric fields under field domain formation.     

Adsorption sites of111In and111mCd on Pd(111) surfaces characterized by the electric field gradient

PAC measurements of the electric field gradient at the nucleus of isolated probe atoms on Pd(111) surfaces lead to the identification of five adsorption sites successively occupied by the parents¹¹¹In and^111mCd during annealing between 80K and 600K. The data are consistent with an estimated activation energy for Pd surface self diffusion of E_a=0.76(8)eV.     

Electroreflectance spectra of Ag(111) electrodes

Electroreflectance (ER) spectra of Ag(111) evaporated electrodes on mica in aqueous electrolytes were measured within the wavelength range of 500 to 200 nm for p- and s-polarized light as a function of the angle of incidence, bias potential and surface roughness. These spectra show structures which were not seen in previously obtained ER spectra on slightly rough, polycrystalline Ag electrodes. The effect of volume and surface plasma excitation on the spectra is clearly indicated. The experimental results cannot be reproduced satisfactorily by any currently employed theoretical model which takes only free electron effects into account. The influence of the electric field on the bound electrons in the surface layer obviously cannot be neglected. It is demonstrated that the surface plasma resonance is shifted in energy by the applied electric field.     

Reactive molecular dynamic simulations of hydrocarbon dissociations on Ni(111) surfaces

Empirical potential parameters for H, C and Ni elements have been developed for the ReaxFF force field in order to study the decomposition of small hydrocarbon molecules on nickel using molecular dynamics simulations. These parameters were optimized using the geometrical and energetic information obtained from density functional (DFT) calculations on a subset of hydrogen and methane reactions with nickel (111) surfaces. The resulting force field was then used to obtain a molecular perspective of the dynamics of the methane dissociative adsorption on Ni(111) as well as two other small alkane molecules, ethane and n-butane. NVT simulations of dissociative adsorption of methane over a range of temperatures enabled the estimation of the sticking coefficient for the adsorption as well as the activation energy of the first C–H bond breaking. The rate constants of each elementary step (both forward and reverse) of CH_x dissociation on Ni(111) were obtained by monitoring the surface species and a microkinetic model was constructed as a result. Qualitative analyses of the simulations of ethane and n-butane decompositions on Ni(111) demonstrate that such reactive MD technique can also be used to obtain useful information on complex reaction networks.     

de Haas van Alphen (dHvA) effect in two-dimensional (2D) conductors: susceptibility oscillations

A new scheme for analyzing the de Haas van Alphen (dHvA) effect in nearly two dimensional (2D) metals (i.e. with nearly cylindrical Fermi surface) is presented. The envelope of the magnetic susceptibility oscillations is calculated in the entire range of magnetic fields and temperatures. The resulting envelope function is found to be proportional to a universal function of the dimensionless parameter Q=hω_c/k _B T. The upper (i.e. paramagnetic) branch of the susceptibility envelope has a maximum at a certain Q = 5.45. This universal value may be useful for determining the effective cyclotron mass and the Fermi energy of nearly 2D metals. A simple relation between magnetization oscillations amplitude and calculated susceptibility amplitudes is derived. The corresponding limiting formulae for the magnetization oscillations envelope are found to match smoothly around the value X = 2π²/Q?2 of the Lifshitz-Kosevich (LK) smearing parameter. The influence of Fermi surface sheets with open orbits on magneto-quantum oscillations is considered. Triangle-like rather than saw-tooth-like oscillations at ultralow temperatures are obtained and substantially diminished magnetization and susceptibility amplitudes are calculated. This suggests the possibility of estimating the band structure parameters of Fermi surface sheets from magneto-quantum oscillations measurements.     

The Tamm surface state on Cu(111) and Ag(111)

It is shown, that the well-known d-electron Tamm surface-state emission observed in photoelectron spectra from Cu(111) at the M̄ point in the surface Brillouin zone, is indeed due to such a surface state and not a bulk band transition as recently suggested L. Wallden, Solid State Commun. 59, 205 (1986). A similar surface state on Ag(111) is reported.     

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.     

Reversible Vertical Manipulations of Single Pt Adatom on Pt（111） Surface with a Triple-Apex Tip

With a triple-apex tip, we investigate theoretically the vertical manipulation of single Pt adatom on the Pt（111） surface. The adatom adsorbed on the f cc site of the flat Pt（111） surface can be transferred vertically to the tip by adjusting the tip height properly. Moreover, based on the strong vertical trapping ability and the relatively weak lateral trapping ability of the tip, we propose a simple method to realize a reversible vertical manipulation of the Pt adatom from the highly coordinated sites, the kink and the step sites, of the stepped Pt（111） surface. All the vertical manipulations are completed using only the atomic force between the tip and the adatom, without the electric field.     

Hyperfine interactions at R and In sites in RNiIn (R = Gd, Tb, Dy, Ho) compounds measured by perturbed angular correlation spectroscopy

Perturbed gamma–gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (mhf) in RNiIn (R = Gd, Dy, Tb, Ho) intermetallic compounds using the ¹¹¹In→¹¹¹Cd and ¹⁴⁰La→¹⁴⁰Ce probe nuclei. The PAC spectra for ¹¹¹Cd measured above magnetic transition temperature show a major fraction with a well defined quadrupole interaction for all compounds except GdNiIn where a single frequency was observed. PAC measurements below T _C showed a combined electric quadrupole plus magnetic dipole interaction for ¹¹¹Cd probe at In sites, and a pure magnetic interaction for ¹⁴⁰Ce at R sites. The temperature dependence of mhf measured with ¹⁴⁰Ce at R sites shows that the values of fields drop to zero at temperatures around the expected T _C for each compound. However, in the measurements with ¹¹¹Cd at In sites, the mhf values become zero at temperatures which are smaller than T _C . The difference between the temperatures at which mhf is zero for ¹⁴⁰Ce and ¹¹¹Cd probes correlates with T _C . For each compound this difference decreases with T _C . The results are discussed in terms of the RKKY model for magnetic interactions and the existence of two magnetic systems, with distinct exchange interaction energies due to different types of atomic layers in these compounds.