Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

Surface electromigration of metals on Si(001): In/Si(001)

The possibility of surface electromigration (SE) of metals of In, Ga, Sb and Ag on a very flat Si(001)2×1 substrate (single domain 2×1) was examined by SEM, μ-RHEED and μ-AES under UHV conditions. It was found that Ga, Sb and Ag show no SE on Si(001) surface even at DC annealing temperatures for the desorption of these metals. For In on Si(001), a very fast SE (8000 μm/min) towards the cathode side was found that suddenly sets in at 450°C DC annealing, which was related to a surface phase transition. μ-RHEED and μ-AES observation showed that the SE is related to an ordered 4×3-In phase together with two-dimensional In gas phase over the 4×3-In phase and an In-disordered phase at the front end of SE. Single domain 4×3-In phases were found to occur under sequences of In deposition and DC annealing which involve the In SE on Si(001).     

NiO-thickness dependent magnetic anisotropies in Fe/NiO/Au(001) and Fe/NiO/MgO(001) systems

The in-plane magnetic anisotropy of Fe/NiO bilayers was studied quantitatively as a function of NiO thickness using the magneto-optical Kerr effect with a rotating field. For NiO thicker than the ordering transition thickness, the total in-plane fourfold anisotropy of the Fe layer decreases with NiO thickness in Fe/NiO/Au(001), but increases in Fe/NiO/MgO(001). Our result indicates that the exchange coupling in an Fe/NiO bilayer might induce an additional in-plane fourfold anisotropy, and the opposite thickness dependent behaviors may be attributed to the different Ni²⁺ antiferromagnetic spin orientations for NiO films grown on Au(001) and MgO(001) surfaces.     

Scanning tunneling microscopy investigation of CoO/Fe(001) and Fe/CoO/Fe(001) layered structures

We report on the nanometer scale morphology of CoO thin films grown on top of Fe(001) substrates from the early stages of interface formation (few atomic layers), and on the surface topography of Fe/CoO/Fe(001) layered structures. The growth of the CoO films is dominated by formation of islands up to about 5 nominal atomic layers, then it proceeds in the layer-plus-island regime. The surface topography of thin Fe films grown on top of the CoO/Fe systems is strongly influenced by the morphology of the latter. Moreover, we observe a strong relationship between the growth mode and the chemical interactions at the CoO/Fe interface, since thick layers of iron oxides develop only below the CoO islands, as an effect of the proximity between Fe and Co atoms. We finally discuss possible implications of our observations on the magnetic properties of these layered magnetic structures.     

Surface Reconstruction of Pt/Si(001)

Platinum-induced surface reconstructions on Si(001) were investigated by scanning tunneling microscopy. The Si(001) surface shows c(4×6)+c(4×2)-type reconstruction after Pt hot deposition at 750 °C. The c(4×2) reconstruction is formed by regular arrangement of Pt ad-atoms at the hollow sites between two Si dimers on Si(001). The c(4×6) structure is formed by long-range ordering of Si dimers superimposed on the c(4×2) reconstruction. The surface reconstruction changes with the local Pt coverage and missing dimer density. An atomic model for c(4×6)+c(4×2) is proposed based on the observed c(4×4) surface reconstruction, a new surface reconstruction, which is observed following a high-temperature anneal at 900 °C. The c(4×4) surface reconstruction is regular and ordered in both the dimer-row direction and the direction perpendicular to the dimer rows. The reconstruction is formed by the regular arrangement of Si dimers and Pt ad-atoms sitting on top of the hollow sites surrounded by two dimers. An atomic model for this new reconstruction is proposed. PACS 68.35.Bs; 68.37.Ef; 68.43.Fg     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

Vibrational spectroscopy of ordered oxygen adiayers on Ni/Cu(001) and Co/Cu(001) thin film systems

We report surface vibrations in c(2 × 2) oxygen adlayers on Ni and Co thin films on a Cu(001) substrate measured at gG by high resolution EELS. For the Ni thin film surface, one phonon peak is measured for varying film thicknesses from 1.3 ML (monolayer) to 6 ML with a constant energy of 221 cm⁻¹. For the Co thin film surface, three loss peaks are found, whose relative intensities change as the film thicknesses are varied. One loss peak at ˜520 cm⁻¹ is tentatively assigned to the Fuchs-Kliewer mode of cobalt oxide (CoO). The other two peaks at 317 and 376 cm⁻¹ are likely related to different bonding sites. Surface phonons on the p(2 × 2) Co thin film (389 cm⁻¹) and a bulk resonance mode (115 cm⁻¹) are also reported.     

Hydrogen on WO3(001)

Ultraviolet photoemission measurements of the (001) surface of WO₃ show that after exposure to atomic hydrogen new states appear in the energy gap region at 2.3 and 0.9 eV below the Fermi energy. A peak at 2.2 eV was also produced by hydrogen ion bombardment. To investigate whether these surface effects were due to the formation of hydrogen tungsten bronze H_xWO₃, comparison was made with spectra from the electronically similar sodium bronze. The form of the spectra were different both in the valence band and energy gap regions indicating that hydrogen adsorption or loss of oxygen from the surface rather than formation of H_xWO₃ is the major effect.     

Magnetism of the Fe/ZnSe(001) interface

The magnetism of epitaxial ultrathin films of Fe on ZnSe(001) has been investigated by x-ray magnetic circular dichroism down to the submonolayer regime. In contrast to other metallic ferromagnet/semiconductor interfaces, no reduction of the Fe magnetic moment was found at the Fe/ZnSe(001) interface. Furthermore, a significant enhancement of the Fe magnetic moment compared to the bulk value was observed for coverages up to one monolayer in agreement with theoretical predictions. We also demonstrate that the magnetic properties of the Fe/ZnSe(001) interface remain stable against thermal annealing up to 300 degrees C, a prerequisite for the future development of efficient spintronics devices.     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.     

Electronic structure of strained Sin/Gen(001) superlattices

Using the empirical tight binding method we have investigated the electronic properties of the Si_n/Ge_n(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit.     

Surface charge-density wave phase transitions on Mo(001) and W(001)

The recently reported phase transitions of Mo(001) and W(001) are discussed in terms of surface charge-density waves. Band structure, surface lattice distortion and possible accompanying surface antiferromagnetism are considered. The expected two-dimensional critical behaviour is also investigated.