Structural study of Co/Pd(1 1 1) and CO/Co/Pd(1 1 1) by surface X-ray absorption fine structure spectroscopy

The structure of the Co thin films on Pd(1 1 1) and the effect of the CO adsorption on Co thin films were studied by Co K-edge surface X-ray absorption fine structure (XAFS). The polarization dependences of the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicate that Co thin films grow in the fcc stacking mode on Pd(1 1 1) up to 12 ML. The analysis of the nearest neighbor shell shows little mechanical strain at the interface, indicating that Co atom does not grow pseudomorphically on Pd(1 1 1). There is no alloy-like structure at the interface. CO adsorption causes no structural change of the Co thin films but modifies the Co surface electronic state. These structural studies provide deep insight in the magnetic property of the Co thin films on Pd(1 1 1).     

The dependence of I–V and C–V characteristics on temperature in the H-terminated Pb/p-Si(1 0 0) Schottky barrier diodes

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of H-terminated Pb/p-Si/Al contacts fabricated by us have been measured in the temperature range of 77–300 K. The experimental values of the barrier height (BH) Φ_bo and the ideality factor n for the device range from 0.674 and 1.072 eV (at 300 K) to 0.352 and 2.452 eV (at 77 K), respectively. The ideality factors become larger with lowering temperature while the barrier height decreases. The Φ_bo(n) plot shows a linear dependence in the temperature range of 77–300 K that can be explained by the barrier inhomogeneity at the metal/semiconductor interface. The extrapolation of the linear Φ_bo(n) plot to n = 1 has given a homogeneous barrier height of approximately 0.713 eV for the Pb/p-Si(1 0 0) contact. A Φ_bo versus 1/T plot was drawn to obtain evidence of a Gaussian distribution of the BHs, and values of and σ_s = 80.5 mV for the mean BH and zero-bias standard deviation have been obtained from this plot, respectively. Then, a modified versus 1/T plot gives and A^* as 0.828 eV and 54.89 A/cm² K², respectively. Furthermore, an average value of −0.687 meV/K for the temperature coefficient has been obtained, the value of −0.687 meV/K for hydrogen terminated p-type Si differs from those given for p-type Si without hydrogen termination in the literature.     

Heteroepitaxial thin film of iron phthalocyanine on Ag(1 1 1)

Ordering of submonolayer iron phthalocyanine (FePc) molecules deposited on Ag(1 1 1) was investigated using scanning tunneling microscopy. The room temperature deposition of FePc alone, without any annealing, results in no ordered overlayers. However, posterior annealing the substrate to 475 K leads to the formation of a two-dimensional oblique lattice with the lattice constants of 16.2 ± 0.3 Å and the angle of 78 ± 1° between them. The resulting FePc lattice is commensurate to the substrate lattice. In addition, the nearest neighbor distance in the lattice is significantly increased through a distinctive molecular orientation of the FePc molecules within the unit cell. The commensurate lattice with a large intermolecular distance is in sharp contrast to that observed from a close-packed square lattice that many other metallo-phthalocyanine molecules often self-assemble into. A possible reasoning behind this intriguing structure is discussed.     

STM studies on the growth of monolayers: Co on Cu(110) with one half monolayer of preadsorbed oxygen

The growth of the first cobalt monolayer (ML) on the Cu(110)-(2×1)O surface was studied by scanning tunneling microscopy. Extensive exchange of Cu and Co atoms takes place in the first stages of the deposition. The displaced Cu atoms form new Cu---O---Co mixed islands, with the same structure as those of the terrace surface. At 0.25 ML Co, a new structure nucleates, which contains three Cu atoms, four Co atoms and two O atoms per 2×2 cell. The structure consists of rows in the [ 10] direction with an internal periodicity of two lattice units. The rows are separated from one another by two lattice units along the [001] direction, and are found both in-phase and out-of-phase relative to one another. The result is a mixed p(2×2) and c(2×4) surface. The fraction of the surface covered by the new structure increases with Co coverage, and completely covers the surface at 1 ML Co.     

Kinetic length and step permeability on the Si(1 1 1) (1 × 1) surface

Information on the kinetic regime of step motion and step permeability on the Si(1 1 1) (1 × 1) surface has been obtained from observations of island decay that were made with low energy electron microscopy. Island area during decay exhibits the expected power law dependence on time, with exponent, α, that is a qualitative indicator of the kinetic regime. A new method is presented for determining the kinetic length quantitatively from measurements of the decay exponent in the symmetric island decay geometry on top of a larger concentric circular island. Using this approach, we determine the kinetic length on the Si(1 1 1) (1 × 1) surface at 1163 K to be d ∼ 75a, where a is the lattice constant. It is shown that this result locates step motion firmly in the diffusion limited regime. Mass conservation of decaying island stacks is also observed at this temperature, which indicates that steps are effectively impermeable in the context of diffusion limited step kinetics.     

Magnetic pole pinning at rectangular defects on MnAs/GaAs(0 0 1)

Strong magnetic poles at characteristic rectangular defects have been observed using a magnetic force microscope on a MnAs(  1 0 0) thin film with the thickness of 30 nm. The MnAs thin film was epitaxially grown on a GaAs(0 0 1) substrate. The magnetic poles were in one-arranging direction, being independent of the magnetization direction of the film. The poles were pinned at the edges of the rectangular defects until just below the Curie temperature, and formed a stable magnetic-field loop on the MnAs surface. The stability of the magnetic pole pinning shows the distinctive feature of the magnetic domain structure on the surface with a strong anisotropy, which was built in the heterostructure of MnAs and GaAs.     

Combined effects of substrate compliance and film compositional grading on strain relaxation in layer-by-layer semiconductor heteroepitaxy: the case of InAs/In0.50Ga0.50As/GaAs(1 1 1)A

A systematic theoretical analysis is presented of the combined effects of substrate compliance and film compositional grading on the relaxation of strain due to lattice mismatch in layer-by-layer semiconductor heteroepitaxy. The analysis is based on a combination of continuum elasticity theory and a novel atomistic simulation approach for modeling structural and compositional relaxation in layer-by-layer heteroepitaxial systems. Results are presented for InAs epitaxy on GaAs(1 1 1)A compliant substrates with some marginal film compositional grading that consists of one monolayer of In_0.50Ga_0.50As grown on the substrate surface prior to InAs growth. A parametric study is carried out over a wide range of substrate thicknesses. Interfacial stability with respect to misfit dislocation formation, the dependence on substrate thickness of a thermodynamic critical film thickness, and the completion of the coherent-to-semicoherent interfacial transition are examined in detail. In addition, the structural characteristics and compositional distribution of the corresponding semicoherent interfaces, the associated strain fields, as well as the film surface morphological characteristics are analyzed. Most importantly, the role of segregation at defects of a semicoherent interface in the thermodynamics of layer-by-layer heteroepitaxial growth is demonstrated. Our study shows that systematic combination of the mechanical behavior of thin compliant substrates with grading of the epitaxial film composition provides a very promising engineering strategy for strain relaxation in heteroepitaxy.     

Mechanism and energetics of dimerization of SiH2 radicals on H-terminated Si(0 0 1)-(2×1) surfaces

The mechanism and energetics are presented of the dimerization of two adsorbed surface SiH₂ groups on the H-terminated Si(0 0 1)-(2 × 1) surface to form Si₂H₄ species during the initial stages of growth in plasma deposition of hydrogenated amorphous silicon (a-Si:H) films. The reactions are observed during classical molecular-dynamics (MD) simulations of a-Si:H film deposition from SiH₂ radical precursors impinging on an initially H-terminated Si(0 0 1)-(2 × 1) surface and substrate temperature, T, over the range 500T700 K. The Si₂H₄ species resulting from the surface SiH₂ dimerization reactions undergo surface conformational changes resulting in either a non-rotated (NRD) or a rotated dimer (RD) configuration. The RD configuration is found to be the energetically favorable one. The MD simulation results for the structure of the NRD and RD surface Si₂H₄ configurations corroborate with ab initio calculations of optimized adsorption configurations of SiH₂ radicals on crystalline Si surfaces, as well as results of STM imaging of the thermal decomposition of disilane on Si(0 0 1).     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

On cobalt magnetization suppression at bimetallic interfaces

We study theoretically the Co magnetization suppression at the Co–M (M=Ti, Nb, Mo, Re, Os, Ir and Pt) interface. We consider (1) M(1×1) overlayer on the FCC(1 1 1) or HCP(0 0 0 1) slab, (2) c(2×2) Co–M alloy above the same surfaces. In the latter case, the Co magnetization is reduced to about 0.5 μ_B by Ti, Nb, Mo and Re, but the effect is probably an overestimation because of compression of M–Co bonds. At Co atoms below the M(1×1) overlayer, the Co magnetization does not drop below 1 μ_B. We discuss also the Co–M antiferromagnetic coupling.     

Structural effects in the growth of giant magnetoresistance (GMR) spin valves

An investigation has been made of the thin-film structure and interface morphology of giant magnetoresistance (GMR) spin valves of the cobalt/copper/cobalt (Co/Cu/Co) type that were grown on polycrystalline NiO substrates at three different temperatures (150, 300 and 450 K). Sputter-depth-profile analyses indicate that the quality of the layering in the Co/Cu/Co structure was only slightly better for the 150 K sample than for the 300 K sample. For the 450 K sample, however, the Co/Cu/Co structure showed extensive disruption. The similarity in the depth-profiles for the 150 and 300 K samples indicates the sensitivity of the GMR to subtle structural differences.     

On the microscopic origin of the kinetic step bunching instability on vicinal Si(0 0 1)

A scanning tunneling microscopy/atomic force microscopy study is presented of a kinetically driven growth instability, which leads to the formation of ripples during Si homoepitaxy on slightly vicinal Si(0 0 1) surfaces miscut in [1 1 0] direction. The instability is identified as step bunching, that occurs under step-flow growth conditions and vanishes both during low-temperature island growth and at high temperatures. We demonstrate, that the growth instability with the same characteristics is observed in two dimensional kinetic Monte Carlo simulation with included Si(0 0 1)-like diffusion anisotropy. The instability is mainly caused by the interplay between diffusion anisotropy and the attachment/detachment kinetics at the different step types on Si(0 0 1) surface. This new instability mechanism does not require any additional step edge barriers to diffusion of adatoms. In addition, the evolution of ripple height and periodicity was analyzed experimentally as a function of layer thickness. A lateral “ripple-zipper” mechanism is proposed for the coarsening of the ripples.     

Subsurface miscibility of metal overlayers with V, Nb and Ta substrates

The high solubility and diffusivity of oxygen in Nb, Ta and V are responsible for the difficulty in the preparation of a clean, well-ordered (1 0 0) surfaces of these early transition metals. The deposition and subsequent annealing of a metal overlayer on Nb(1 0 0) are a convenient route for the preparation of flat surfaces with the Nb lattice constant and a metallic character. Such well-ordered, stable, inert and easily reproducible Nb(1 0 0)-like surfaces can be produced due to the suppression of oxygen surface segregation by a suitable layer blocking sub-surface oxygen diffusion. For example, a Nb(1 0 0)-like surface can be obtained by annealing thin Au or Pd films deposited on Nb(1 0 0), since this results in a Au–Nb or Pd–Nb alloy situated just below the surface which preserves the structure of the Nb-lattice and effectively suppresses the oxygen segregation toward the surface. In contrast, Ag and Cu layers do not show this property as these metals do not form a bulk alloy with Nb. The presence or absence of bulk alloying and its relation to surface oxygen contamination is a general phenomena observed for many metallic layers deposited on V, Nb and Ta substrates. A discussion of results reported in the literature is given for these adsorbate systems.     

Time-evolution of the GaAs(0 0 1) pre-roughening process

The GaAs(0 0 1) surface is observed to evolve from being perfectly flat to a surface half covered with one-monolayer high spontaneously formed GaAs islands. The dynamics of this process are monitored with atomic-scale resolution using scanning tunneling microscopy. Surprisingly, pit formation dominates the early stages of island formation. Insight into the nucleation process is reported.     

Growth of metallic nanowires on anisotropic Si substrates: Pb on vicinal Si(0 0 1), Si(7 5 5), Si(5 3 3), and Si(1 1 0)

The spontaneous formation of mesoscopic Pb-wires, on 4° off-cut Si(0 0 1) vicinal surface, Si(7 5 5), Si(5 3 3), and Si(1 1 0) substrates was studied by low-energy electron microscopy. Before the deposition of Pb the substrates were modified by predeposition of a submonolayer amount of Au followed by annealing. The Au-induced reconstruction creates quasi-one-dimensional facets and superstructures. Their width ranged from several hundred nm in the case of the vicinal Si(0 0 1) down to atomic scale size, for the Si(1 1 0) surface. The best-developed arrays of parallel aligned mesoscopic wires were obtained during the deposition of Pb on substrates cooled slightly below room temperature. Wires with length to width ratio reaching 130 were produced on the Si(7 5 5) and the Si(5 3 3) substrates. The width of these nanowires was uniform over the whole substrate and was about 60 nm. The driving forces for the formation of the mesoscopic wires are the anisotropic strain due to the large misfit between the Pb and the Si lattice and one-dimensional diffusion of Pb.     

STM and LEED observations of erbium silicide nanostructures grown on Si(1 0 0) surface: atomic-scale understandings

In this work, erbium silicide is grown on the Si(1 0 0) surface by depositing Er onto the substrate and annealing at 600–700 °C. Many nanowires of Er silicide are formed with lengths in the range 10–100 nm. The formation and evolution of this nanostructure are investigated at atomic scale directly with scanning tunneling microscopy and low-energy electron diffraction. The direction of these nanowires is found perpendicular to that of Si dimer rows. It is shown that Er coverage and annealing temperature have an effect on the formation of nanowires. On the surface between nanowires, new (5×2) and c(5×4) reconstructions are observed, giving an implication to understand the growth behaviors of Er silicide on Si(1 0 0) surface.     

Surface segregation in bimetallic Pt3M (M = Fe, Co, Ni) alloys with adsorbed oxygen

Surface segregation of Pt₃M (M = Fe, Co, and Ni) alloys under oxygen environment has been examined using periodic density functional theory. The segregation trend at a (1 1 1) surface is found to be substantially modified by the adsorbed oxygen. Our calculations indicate that under 1/4 monolayer O coverage both the Pt-segregated and M-segregated surfaces are less stable than the non-segregated one. Further analysis reveals that segregation energy under adsorption environments can be expressed as the sum of the segregation energy under vacuum conditions and the adsorption energy difference of the segregated and non-segregated alloy systems. Therefore, the surface segregation trend under adsorption conditions is directly correlated to the surface-adsorbate binding strength.     

A scanning tunneling microscopy study on the effect of post-deposition annealing of copper phthalocyanine thin films

Using scanning tunneling microscopy, overlayer lattice structures and thin film morphology of copper phthalocyanine (CuPc) have been investigated while varying posterior, annealing temperatures. The room temperature deposition of CuPc on Ag(1 1 1) resulted in ordered domains, which were three molecular layers high and separated with disordered regions. The ordered surface also exhibited an oblique lattice with the lattice constants of 13.3 Å and 16.4 Å and the angle of 74° between the two. By annealing the thin films systematically to 580 K, the symmetry of overlayer lattice increased from the oblique to the square lattice, commonly observed on various substrates. Further annealing to 780 K desorbed much of the CuPc molecules. However, the remaining molecules reacted and formed dendrite-like chains, in which each CuPc molecule was linked through its isoindole rings. A possible model for the formation of dendritic chains through polymerization is discussed.     

Coalescence kinetics of two-dimensional TiN islands on atomically smooth TiN(0 0 1) and TiN(1 1 1) terraces

In situ high-temperature scanning tunneling microscopy (STM) was employed to follow the coalescence kinetics of two-dimensional TiN islands on atomically smooth TiN(0 0 1) and TiN(1 1 1) terraces. The results are analyzed using a numerical approach based upon Mullin’s theory for surface relaxation and experimentally determined absolute orientation-dependent step stiffnesses. From the calculated time-dependent island shapes during coalescence, in combination with the STM data, we obtain quantitative values for edge mobilities.     

Stabilization against initial oxidation using surface segregation of sulfur on Fe(1 0 0)

Surface segregation process of dissolved elements on a Fe(1 0 0) surface and the initial oxidation processes of the sulfur segregated surfaces were studied in situ by AES and RHEED. The surface enrichment of sulfur has formed a c(2×2) superstructure at elevated temperatures. The Fe(1 0 0)c(2×2)-S surface was found to have stabilization effect against initial oxidation. The retardation of oxidation was most enhanced when the sulfur surface concentration reached at the saturation coverage (1/2 ML). The possible physical origins of the stabilization effect are proposed.