Low-energy Electron Microscopy Study Research Articles

Kinetics-driven anisotropic growth of pentacene thin films

The growth of nonepitaxial as well as epitaxial structures of (001)-oriented pentacene (${\mathrm{C}}_{22}{\mathrm{H}}_{14}$, Pn) thin films on silicon surfaces has been extensively studied in order to elucidate the intrinsic thin-film growth mechanism. The kinetically driven growth processes in pentacene films were found to be modified significantly by the anisotropy of the crystal structure. In situ real-time low-energy electron microscopy studies of diffusion-limited growth of Pn islands on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ and $\ensuremath{\alpha}\ensuremath{\surd}3$-Bi-Si(111) surfaces reveal a definite anisotropy in their shapes. Although this anisotropy is associated with organic film crystal structure, it cannot be predicted directly from the equilibrium crystal shape. It has been found that under kinetic growth conditions the Pn islands are always elongated along the $\mathbit{b}$ axis of the in-plane unit cell, even though the step along the $\mathbit{a}$ axis has the lowest energy, regardless of crystalline polymorph or epitaxial relation, indicating that Pn thin-film growth has an intrinsic kinetic preference along the $\mathbit{b}$-axis direction. Utilizing this kinetic preference and constraining the direction of available flux on inert surfaces during Pn deposition enabled us to control the crystal orientation of Pn domains.

Diffusion Dynamics during the Nucleation and Growth ofGe/SiNanostructures on Si(111)

We report a low energy electron microscopy study of the relation between self-organized Ge/Si(111)nanostructures and their local environment. By comparison with Monte Carlo simulations, three-dimensional islands are shown to display a substantial tendency towards self-ordering. This tendency may result from the diffusive nature of the nucleation processes. The size of individual nanostructures does not significantly correlate with the distance between neighboring islands. Thus energetic factors are thought to govern the competition among coexisting nanostructures to capture the deposited mass.

Temperature dependent low energy electron microscopy study of Ge growth on Si(113)

Abstract We investigated the initial Ge nucleation and Ge island growth on a Si(1 1 3) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 °C and 590 °C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the [ 3 3 2 ¯ ] direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced.

Synthesis of Si-Ge nanoscale structures via deposition of single-source (GeH3)4−nSiHn hydrides

Growth of nanoscale islands with distinct Si0.33Ge0.67, Si0.25Ge0.75, and Si0.20Ge0.80 compositions and uniform sizes is conducted on Si(100) via dehydrogenation of the single-source hydrides (H3Ge)2SiH2, (H3Ge)3SiH, and (H3Ge)4Si, respectively. High-spatial-resolution electron energy loss spectroscopy and Raman spectroscopy indicate homogeneous elemental concentrations within and among islands and confirm that their Si-Ge content is predetermined by the stoichiometry of the corresponding precursors. Z-contrast electron microscopy reveals distinct and perfectly epitaxial islands with atomically sharp interfaces grown via a smooth and continuous wetting layer ∼10Å thick. Cross-sectional electron microscopy shows monomodal distributions of islands with defect-free microstructures. Low-energy electron microscopy studies of the film formation reveal that the growth proceeds via the Stranski-Krastanov mode. Assemblies of coherent quantum dots with highly controlled Ge-rich concentrations produced by this method are desirable for their potentially useful optical properties.

Selective growth of Co nanoislands on an oxygen-patterned Ru(0001) surface

We present a combined low energy electron microscopy and photoemission electron microscopy study of Co nanoislands grown on pure and partially oxidized Ru(0001) surfaces at 650--850 K. When Co is deposited on pure Ru(0001) at the temperature above $\ensuremath{\approx}700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it first forms a wetting monolayer and then Co nanoislands abruptly nucleate. Partial oxidization of the clean Ru(0001) surface gives rise to large rhombus-shaped Ru--O patches (of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ scale). On this surface at $\ensuremath{\approx}730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Co islands form with different density in the Ru and Ru--O regions, and at $\ensuremath{\approx}800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Co islands only grow in the pure Ru regions. We attribute this selectivity to differences in the temperature dependence of the sticking coefficients for Co on the pure Ru and Ru--O regions.

Energetically Driven Reorganization of a Modified Catalytic Surface under Reaction Conditions

The compositional and structural rearrangements at the catalyst surface during chemical reactions are issues of great importance for understanding and modeling the catalytic processes. Low-energy electron microscopy and photoelectron spectromicroscopy studies of the real-space structure and composition of a Au-modified Rh(110) surface during water formation reveal reorganization processes due to Au mass transport triggered by the propagating reaction fronts. The temporal evolution of the surface reaction results in a 'patterned' surface consisting of separated Au-rich and Au-poor phases with different oxygen coverage, Rh surface structure, and reactivity. The experimental results are complemented by ab initio electronic-structure calculations of the O and Au adsorption phases, which demonstrate that the reorganization of the Au adlayer by the propagating reaction fronts is an energetically driven process. Our findings suggest that reaction-induced spatial inhomogeneity in the surface composition and structure is a common feature of metal catalysts modified with adatoms which become mobile under reaction conditions.

Real-time low-energy electron microscopy study of Ga adsorption and facet array formation on Si(113)

Real-time low-energy electron microscopy study of Ga adsorption and facet array formation on Si(113)

A LEEM study of bamboo-like growth of Ag crystals on Si(0 0 1) surfaces

We report a low-energy electron microscopy study of novel bamboo-like (quasi-one-dimensional) growth during deposition of Ag on Si(0 0 1) surfaces at elevated temperatures. The bamboo crystals, with typical dimensions of 10 μm in length and varied height and width (tens to hundred of nanometers), align primarily along Si[1 1 0] or Si[1 −1 0] orientation. Low-energy electron microscopy imaging further demonstrates that the Ag bamboo crystals are initially stable against annealing, but break into segments upon prolonged annealing at 843 K. Possible growth mechanisms of the bamboo-like crystals are discussed.

Low energy electron microscopy studies of steps on single crystal thin films of refractory metals

We describe low energy electron microscopy (LEEM) studies of the behavior at high temperatures of step edges on the close-packed surfaces of refractory metal thin films grown by molecular beam epitaxy. The research includes bcc Mo (011), V (011), Nb (011), and Ta (011), and comparison with bulk single crystals of Nb (011) and fcc Pt (111). Properties observed include faceting of individual steps, step–step interactions, and the interactions of steps with defects such as threading dislocations and with surface reconstructions. Step fluctuations on these surfaces are quantified. For Mo (011) observed by LEEM, the Fourier amplitudes are fit with an attachment model to obtain the step stiffness and its entropy, the step free energy, and its orientation dependence.

Low energy electron microscopy study of In on Si(111)

The structural phase transitions in the multiphase system indium (In) on Si(111) are studied as a function of coverage at different temperatures between −100 and 550 °C by low energy electron microscopy (LEEM) and low energy electron diffraction (LEED). All phase transitions observed with increasing In coverage are first order. Nucleation of a new phase starts only after the previous phase is completed. At high temperatures only a disordered monolayer with high atomic density forms. When In is deposited at room temperature on a (√3×√3)-R30°–In surface four new phases are observed: a (2×1), a near coincidence (1.5√3×1.5√3), a “(1×1)-R30°” and a (6×6) phase. At room temperature and up at least to 400 °C a double layer forms. Below 120 °C this layer is compressed and ordered in two coexisting structures, (√7×√3) and “(1×1)-R30°”. Above 120 °C it shows a (1×1) LEED pattern, which is attributed to a disordered layer on the Si(111)-(1×1) surface. A third monolayer with the packing density of a slightly compressed In(111) plane is unstable at room temperature against formation of three-dimensional crystals but becomes stable during growth at temperatures below about −80 °C. Three-dimensional crystals grow at room temperature on the double layer in the Stranski–Krastanov mode. These crystals are primarily (100)-oriented and are bounded by the equilibrium planes known from bulk crystals, including the reconstructed (100) surface, which is no longer reconstructed above 120 °C.

Surface stress-induced island shape transition in Si(001) homoepitaxy

A low-energy electron microscopy study of two-dimensional Si(001) island shapes near thermal equilibrium on $10\ifmmode\times\else\texttimes\fi{}15 \ensuremath{\mu}{\mathrm{m}}^{2}$ large single-domain terraces reveals a continuous increase of island aspect ratio and a shape transition from elliptical to ``American-football''-like with increasing island size. The size-dependent island shapes are driven by elastic relaxation caused by the intrinsic surface stress anisotropy present on Si(001). Analysis of the measured elliptical island shapes based on an elastic-model calculation allows a quantitative determination of step energies and of the surface stress anisotropy as a function of temperature.

Biquadratic exchange in ferromagnetic/nonferromagnetic sandwiches: A spin-polarized low-energy electron microscopy study

The magnetic domain structure of Co/Au/Co sandwich layers grown on W(110) is studied in situ by spin-polarized low energy electron microscopy as a function of Au spacer layer and top Co layer thickness with the goal to better understand the causes and consequences of biquadratic coupling for the resulting domain structure. It is found that biquadratic coupling not only strongly influences the coupling between the layers near the zero of the bilinear coupling but also at spacer thicknesses at which strong ferromagnetic coupling occurs. Biquadratic coupling appears in a spin reorientation transition between 4 and 5 monolayers. The existence of bilinear and biquadratic coupling produces a wrinkled in-plane magnetization.

Low-energy electron microscopy study of step mobilities on Si(001).

We have analyzed low-energy electron microscopy observations of the equilibrium fluctuations of steps on Si(001) in the temperature range 640--1170 \ifmmode^\circ\else\textdegree\fi{}C. By examining the wavelength dependence of the time constants of the fluctuations, we find that the step motion is limited by the rate of random attachment and detachment of adatoms at the step edges. From the values of the time constants, we determine the step mobility which in principle governs how fast a step responds to being out of equilibrium. This mobility is the same, within experimental uncertainty, for ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ steps. By studying the decay of nonequilibrium rough step profiles, we explicitly show that the step motion is curvature driven, and that the mobility deduced from the thermal fluctuations quantitatively accounts for step smoothing rates. From the amplitude of the equilibrium fluctuations, we determine the stiffnesses of the ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ steps as a function of temperature. \textcopyright{} 1996 The American Physical Society.

Low energy electron microscopy studies of Ge and Ag growth on Si(111)

In this paper we demonstrate how low energy electron microscopy can be used to study in situ, and in real time, the growth of thin films on single-crystal surfaces. By studying the growth of Ge and Ag on both the clean and the Sb-terminated Si(111) surface we show how nucleation, growth, island formation, island shapes, thermal stability and the formation of dislocations in the grown films may be observed. Pre-adsorption of Sb strongly modifies the growth of Ge and Ag films on Si(111), leading to more uniform coverage and smoother films in both cases.

Shape transition in growth of strained islands: Spontaneous formation of quantum wires.

Strained epitaxial layers tend initially to grow as dislocation-free islands. Here we show that such islands, as they increase in size, may undergo a shape transition. Below a critical size, islands have a compact, symmetric shape. But at larger sizes, they adopt a long thin shape, which allows better elastic relaxation of the island's stress. We have observed such elongated islands, with aspect ratios greater than 50:1, in low energy electron microscopy studies of growth of Ag on Si (001). These islands represent a novel approach to the fabrication of «quantum wires»

Spin-polarized low-energy electron microscopy study of the magnetic microstructure of ultra-thin epitaxial cobalt films on W(110)

Magnetic domain formation in ultra-thin Co films on W(110) was studied with spin-polarized low-energy electron microscopy (SPLEEM). Films with the expected hexagonal structure were grown on the clean tungsten surface, but also fcc structured films with a (100) plane parallel to the surface could be prepared on a stabilizing tungsten carbide layer. Generally, fcc Co films showed very irregular in-plane magnetization between 3 and 11 ML thickness. We were able to record subtle changes in the magnetic domain structure with increasing film thickness. For films with hexagonal structure, first in-plane contrast was observed at 3 ML. Rotation of the substrate crystal established the presence of a uniaxial in-plane anisotropy with the easy axis along the W[1 1 0] direction for a 6 ML film.

A low-energy electron microscopy study of the system Si(111)Au

The system Si(111)Au is studied in the submonolayer range between about 550 and 1100 K by low-energy electron microscopy (LEEM) in situ in real time with the goal to understand the microscopic aspects of the film formation and of the phase transitions of the various phases ((5 × 1), (√3 × √3) R30° and (6 × 6)) found in this system at elevated temperatures.

A low-energy electron microscopy study of the system Si(111)Au

A low-energy electron microscopy study of the system Si(111)Au