High-resolution Low-energy Electron Diffraction Research Articles

Dimer Coupling Energies of the Si(001) Surface.

The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ξ_{∥}^{+}/ξ_{⊥}^{+}=5.2 of the fluctuating c(4×2) domains above the critical temperature T_{c}=(190.6±10) K. We obtain effective couplings J_{∥}=(-24.9±1.3) meV along the dimer rows and J_{⊥}=(-0.8±0.1) meV across the dimer rows, i.e., antiferromagneticlike coupling of the dimers with c(4×2) symmetry.

Evolving surface morphology during epitaxy of NiO on Ag(001)

Reactive molecular beam epitaxy of NiO on Ag(001) has been characterized by high-resolution low energy electron diffraction performed during deposition of Ni on Ag(001) in molecular oxygen atmosphere for deposition temperatures of 300–420 K. The initial stage of film growth at high deposition temperature is determined by competing processes of Ni oxidation and Ni interdiffusion into Ag(001) with Ni-Ag alloying. Ni oxidation is favored at low deposition temperatures. Thus, NiO bilayers are immediately formed. At high deposition temperatures, however, alloying with Ag is enhanced and the formation of the initial NiO bilayer is delayed. After formation of a closed NiO bilayer, alloying is prevented and all deposited Ni is oxidized and NiO growth proceeds in the layer-by-layer growth mode. Later stages of epitaxy are governed by relaxation processes induced by the formation of mosaics due to interface dislocations. The tilt angle of the mosaic decreases with increasing film thickness due to elastic deformation of the NiO lattice. Dislocation related mosaics are paired and form grooves. Postdeposition characterization shows periodic arrangement of grooves related to the formation of a regular dislocation network.

Strain measurement of ultrathin epitaxial films using electron diffraction techniques

Many diffraction and imaging techniques have been developed and used to measure the strains in epitaxial films with different degrees of accuracy. In this tutorial, we will focus on measurements of strains in epitaxial films using electron diffraction techniques. These include reflection high-energy electron diffraction (RHEED), azimuthal reflection high-energy electron diffraction, low-energy electron diffraction (LEED), and high-resolution low-energy electron diffraction techniques. These diffraction techniques are surface sensitive and are suitable for in situ measurements of strains in ultrathin epitaxial films quantitatively. In particular, RHEED and LEED are simple to operate and readily available in many laboratories. We discuss the instrument limit of strain measurement in each technique. A nominal definition of the resolving power of an instrument to resolve a strain state is given as the percentage of intrinsic instrumental broadening in the reciprocal unit with respect to the first reciprocal Brillouin zone (2π/lattice constant) of a sample. It is shown that the resolving power of these diffraction techniques ranges from 2% to 0.2%. Experimental strategies without modifying the instrument and to achieve a strain detection capability beyond the definition of the nominal resolving power of these techniques are presented.

Mazes and meso-islands: Impact of Ag preadsorption on Ge growth on Si(111)

The preadsorption of Ag on Si(111) drastically changes the growth of Ge. In a temperature range from $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $650{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, Ag adsorption on Si leads to the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ reconstruction that exhibits a maze-like morphology on the mesoscopic scale, as observed by low-energy electron diffraction (LEED) and low-energy electron microscopy. This maze morphology can be attributed to a surface roughening on an atomic scale, induced by the re-arrangement of top layer atoms during the $7\ifmmode\times\else\texttimes\fi{}7$ to $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ transition. The subsequent deposition of Ge results in the formation of a wetting layer, the evolution of which has been found to be governed by the Ag/Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ template's maze structure, as the latter offers a high density of heterogeneous nucleation sites. Upon further Ge growth, three-dimensional islands with diameters in the micrometer range are formed, which exhibit a large and flat (111) top facet. X-ray photoemission electron microscopy reveals that during Ge growth, Ag is segregating to the surface very efficiently. Grazing-incidence x-ray diffraction and transmission electron microscopy have been used to study the composition, strain state and defect structure of the Ge islands in dependence of the growth temperature. The strain induced by lattice mismatch is found to be largely relaxed (80--90% relaxation) in the investigated growth temperature range from 400 to $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is confirmed by high-resolution LEED measurements. As a main relaxation mechanism, the formation of interfacial misfit dislocations has been identified. Interdiffusion of Si into the Ge islands becomes more and more pronounced for increasing growth temperature, whereas the formation of twinned Ge regions can drastically be suppressed at higher temperature.

In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter

The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε=0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived.

Temperature-dependent formation and evolution of the interfacial dislocation network ofAg/Pt(111)

We have investigated the formation of a dislocation network that develops at the interface of a Ag film on $\mathrm{Pt}(111)$ and its evolution with film thickness at 600 K and above. This system is commonly considered to be representative of a surface-confined alloy, and we have studied the structure and topography of the films' surfaces up to thicknesses of 35 ML using high-resolution low-energy electron diffraction. The network is formed during the growth of the second layer and is fully developed already for films only a few layers thick. The spacing of the network varies between 5.8 and 6.6 nm when grown at 600 and 800 K, respectively. This dependence on the growth temperature is attributed to a temperature-dependent material mixing in the first (few) layers of the film. This mixing is irreversibly set by the deposition temperature and even persists during prolonged annealing at a temperature close to the desorption of Ag from $\mathrm{Pt}(111)$.

Hysteresis proves that the In/Si(111)(8×2)to(4×1)phase transition is first-order

Indium on silicon (111) exhibits a Peierls-like phase transition from a $(4\ifmmode\times\else\texttimes\fi{}1)$ reconstructed high-temperature phase to a $(8\ifmmode\times\else\texttimes\fi{}2)$ reconstructed ground state. A controversial debate is going on what kind of phase transition it is: first- or second-order. We employed high-resolution low-energy electron diffraction under slow thermal heating and cooling cycles to follow the phase transition. A robust hysteresis of diffraction spot intensities with a width of 8.6 K has been observed, which is independent of the heating and cooling rate. This hysteresis directly proves the existence of an energy barrier at the phase transition temperature and thus the first-order behavior of this phase transition.

Surface morphology of ultrathin hex-Pr2O3 films on Si(1 1 1)

In this work, the morphology of the surface of hexagonal Pr2O3(0 0 0 1) films grown on Si(1 1 1) is studied by high-resolution low-energy electron diffraction combined with spot profile analysis. For this purpose, praseodymia films prepared by molecular beam epitaxy were capped with protecting amorphous germanium films. After removal of the capping layers due to heating in diluted oxygen atmosphere the surface properties of the oxide film were investigated in situ with Auger electron spectroscopy and spot profile analysis low energy electron diffraction. The removal of the capping layer has no impact on the hexagonal Pr2O3(0 0 0 1) film structure which is shown by x-ray diffraction. Surface sensitive electron diffraction confirms that the surface of the oxide film has hexagonal structure. Diffraction spot profile analysis shows that the film surface has grain structure without any mosaic spread due to the negligible lateral lattice mismatch between hexagonal Pr2O3(0 0 0 1) and Si(1 1 1). In addition, single atomic steps with complete bulk unit cell height are present at the surface. The density of the atomic steps is small pointing again to the high quality of the surface of hexagonal Pr2O3 films compared to cubic Pr2O3 films grown on Si(1 1 1).

Stabilization of the ceria ι-phase (Ce7O12) surface on Si(111)

In this work a 250 nm CeO2(111) film grown on a hex-Pr2O3(0001)/Si(111) system is annealed at 660 °C for 30 min to form the ι bulk phase of Ce7O12 as controlled by x-ray photoelectron spectroscopy. The (111) surface of the stabilized ι phase is characterized via high-resolution low-energy electron diffraction. The ι-phase surface exhibits a (7×7)R19.1° superstructure with two mirror domains. This structure is attributed to a periodic ordering of oxygen vacancies compared to the fluorite structure of CeO2.

Tailor-made ultrathin manganese oxide nanostripes: ‘magic widths’ on Pd(1 1 N) terraces

The growth of ultrathin two-dimensional manganese oxide nanostripes on vicinal Pd(1 1 N) surfaces leads to particular stable configurations for certain combinations of oxide stripe and substrate terrace widths. Scanning tunneling microscopy and high-resolution low-energy electron diffraction measurements reveal highly ordered nanostructured surfaces with excellent local and long-range order. Density functional theory calculations provide the physical origin of the stabilization mechanism of ‘magic width’ stripes in terms of a finite-size effect, caused by the significant relaxations observed at the stripe boundaries.

Optical characterization of the evolution of ion-induced anisotropic nanopatterns on Ag(001)

Reflection anisotropy spectroscopy is used as an in situ probe for the emergence and evolution of surface patterns on Ag(001) during oblique incidence ion bombardment. The information is extracted from plasmon resonances induced by the nanoscale patterns, utilizing the fact that smooth Ag(001) is optically isotropic. The Rayleigh-Rice perturbation approach delivers the temporal development of the average periodicity and amplitude of the surface patterns. For ion bombardment at a polar angle of incidence of ${70}^{\ensuremath{\circ}}$ along a $\ensuremath{\langle}110\ensuremath{\rangle}$ azimuth, strongly anisotropic surface features develop, giving rise to a single plasmon resonance, which is described well with a one-dimensional power spectral density function. For a smaller polar angle of incidence of $61.{5}^{\ensuremath{\circ}}$ multiple plasmon resonances are observed, which demand a two-dimensional power spectral density function for a perfect description. These result compare well with high-resolution low-energy electron diffraction data, taken after ion bombardment at both angles of incidence. The optical data, obtained at $61.{5}^{\ensuremath{\circ}}$, show coarsening and seem to suggest scaling of the periodicity and roughness, with critical exponents $0.27$ and $0.40,$ respectively.

Anomalous anisotropy in athermal Bradley-Harper roughening of Cu(001)

Ion bombardment induced surface structures on Cu(001) have been studied under conditions obeying the previously “coined athermal Bradley-Harper (BH) region.” Off-normal ion impact along the ⟨110⟩ and the ⟨100⟩ azimuth at 200 K gives rise to different high-resolution low-energy electron-diffraction patterns. Unanticipated and marked deviations from the inherent fourfold symmetry are obtained already at a polar angle of incidence as low as 10°. Experiments with 800 eV Ar+ ions (flux 6×10 12 ions cm−2 s−1 and fluence 4.3×10 16 ions cm−2) clearly show BH behavior for bombardment along [100] in contract to bombardment along [110]. This observation is attributed to the higher probability for surface penetration of ions when incident along ⟨100⟩ . This remarkable finding is further corroborated by measurement at various energies between 0.2 and 2 keV along [100].

Submonolayer growth of copper-phthalocyanine on Ag(111)

The growth of high-quality thin films is a key issue in the ability to design electronic devices based on organic materials and to tune their properties. In this context, the interfaces between metals and organic films play a decisive role. Here, we report on the interface formation between copper-phthalocyanine (CuPc) and an Ag(111) surface using various complementary methods. High-resolution low-energy electron diffraction revealed a rich phase diagram for this system with disordered (two-dimensional (2D)-gas-like) and ordered structures (commensurate and point-on-line). In particular, a continuous change in lattice parameters with increasing coverage was found for long-range ordered structures, indicating a substrate-mediated repulsive intermolecular interaction similar to the case of tin-phthalocyanine/Ag(111). Chemisorptive bonding to the substrate was found by x-ray standing waves and ultraviolet photoelectron spectroscopy, and this weakened with increasing coverage at low temperature. This remarkable effect is correlated to a shift in the highest occupied molecular orbital (HOMO) and a HOMO-1 split off band to higher binding energies. Based on our experimental results, we present a comprehensive study of the adsorption behavior of CuPc/Ag(111), including the mechanisms for phase formation and molecular interaction.

Oblique incidence deposition of Cu/Cu(001): Enhanced roughness and ripple formation

Polar angle of incidence, substrate temperature, and atom flux are of key importance to the self-organisation process on a substrate surface during oblique incidence deposition. The substantial influence of these parameters in homoepitaxial growth on Cu(001) is studied using scanning tunneling microscopy, high-resolution low-energy electron diffraction, and extensive growth simulation. The quantitative simulations reproduce the experimentally observed phenomena in great detail provided the steering effect is incorporated and accurate barriers for interlayer transport are used. Both experimental techniques reveal the formation of rectangular mounds for sufficiently large angles of incidence that are initially elongated in the direction perpendicular to the deposition plane of incidence. The simulations show a transition from perpendicular to parallel orientation, which occurs through pyramid formation at a coverage that depends on the angle of incidence and the substrate temperature. The elongation of the islands can be quite strong, leading to ripple formation and a very strong roughening of the growth front. The enhanced roughening of the growth front, compared to the already rough growth observed at normal incidence, is identified as super Poisson growth.

Stable tungsten disilicide contacts for surface and thin film resistivity measurements

High-temperature stable electric contacts of tungsten disilicide (WSi2) on Si(001) are fabricated by a simple two-step process: vacuum deposition of W on the native Si dioxide and subsequent annealing under ultrahigh-vacuum conditions. Silicidation starts at 1000K, as, it is believed to occur, the Si diffuses to the surface through the defects in the oxide. Flash annealing to 1500K removes the oxide, resulting in stable WSi2 contacts on the surface. Contamination due to migrating W is confined to within a micrometer of the edge of the WSi2 contacts. Beyond this micrometer-sized zone, the surface is free of contamination as confirmed by low-energy electron microscopy and high-resolution low-energy electron diffraction. Reproducible resistance curves during annealing and cooling of the Si(001) sample confirm the reliability of the contacts, which can withstand many flash-annealing cycles without degradation.

Optical anisotropy induced by ion bombardment of Ag(001)

Grazing incidence ion bombardment results in the formation of nanoripples that induce an anisotropic optical reflection The evolution of the reflectance anisotropy has been monitored in situ with reflectance anisotropy spectroscopy. The Rayleigh-Rice theory (RRT) has been used to analyze the optical spectra quantitatively and provides the evolution of the average ripple period and root-mean-squared surface roughness. After an incipient phase, both the increase in the periodicity and the roughness vary roughly with the square root of the sputter time. Additional high-resolution low-energy electron diffraction (HR-LEED) measurements have been performed to characterize details of the average structure created by ion bombardment.

Adsorbate induced refacetting: Pb chains on Si(557)

The structure on the atomic and mesoscopic scale of Pb adsorbed on Si(557) has been investigated by high-resolution low energy electron diffraction (SPA-LEED). Depending on Pb coverage in the range between 1.2 and 1.6 monolayers (ML), formation of various facets [(112), (335), (223), and a meta-stable (557) orientation] is induced by the Pb layers. The facet orientation in general does not coincide with the macroscopic orientation of the (557) surface. After an initial annealing step to 600 K, starting with 1.2 ML of Pb, this new vicinality can be tuned gradually and reversibly even at temperatures below 180 K by further adsorption, but also by desorption of Pb. Superstructures of the Pb layers on the terraces were identified on the most stable (223) facets. Here parts of the devil's staircase and the stripe-incommensurate (SIC) phases known from Si(111) surfaces (Yakes et al 2004 Phys. Rev. B 69 224103) develop. A new mechanism for facet formation with different orientations, based on avoidance of step decoration by adsorbed Pb, is proposed.

Strain relief during Ge hut cluster formation on Si(001) studied by high-resolution LEED and surface-stress-induced optical deflection

The kinetics of Ge hut cluster formation and the evolution of film stress have been studied during Ge deposition at 400 and $500\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ by high-resolution low-energy electron diffraction and surface-stress-induced optical deflection. The hut clusters grow coherent to the Si substrate but show an increased layer distance of 4% due to strain-induced tetragonal distortion. The distortion of the Ge unit cell adds up to more than 8%. The sudden onset of hut formation is observed at 3.5 monolayers of Ge. Individual hut clusters instantly grow to a width of $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The strain relief of the order of 15%--20% is maximized by complete dissociation of the Ge wetting layer. Together with this Ge only two additional monolayers of Ge are necessary to cover the entire surface with fully evolved hut clusters.

Adsorption of Benzene on a Mo(112)−c(2 × 2)-[SiO4] Surface

The orientation and growth of benzene and pyridine on a well-ordered Mo(112)−c(2 × 2)-[SiO4] have been investigated with high-resolution electron energy loss spectroscopy (HREELS), Auger spectroscopy (AES) and low-energy electron diffraction (LEED). Benzene on the c(2 × 2)-[SiO4] surface is bound with its molecular plane parallel to the surface plane at submonolayer coverages. At intermediate coverages (1−3 ML), in contrast to benzene adsorption on metal surfaces, a layer-by-layer growth mode is observed with the benzene molecules bonded parallel to the surface. Pyridine on the c(2 × 2)−[SiO4] surface, on the other hand, undergoes a phase transition from a parallel to a tilted configuration. This is the first reported observation of benzene multilayer growth in which the benzene molecules bond exclusively parallel to the surface plane. The origin for this unusual adsorption behavior of benzene is discussed.

Novel Local Free Energy Minimum on the Cu(001) Surface

High-resolution LEED (low-energy electron diffraction) data of Cu(001) reveal an uniaxial in-plane lattice reconstruction by 1%. One-dimensional nanogrooves induced by ion bombardment involve the creation of steps that enable this reconstruction. This is the first verification of van der Merwe's prediction of step facilitated reconstruction. We confirm the predicted dependence on step orientation: <100> steps allow stress-relief and <110> steps do not, consistent with the known elastic anisotropy. Similar behavior is predicted for other nonreconstructed (001) surfaces of 3d and 4d metals.