High-resolution Low-energy Electron Research Articles

Oxidized surfaces of Cu films on Ru(0001)

The crystallographic aspects of O-induced structures on strained Cu films grown epitaxially on Ru(0001) are studied by scanning tunneling microscopy and high-resolution low-energy electron diffraction. O adsorption at 520 K removes the thickness-dependent relaxation structures of the O-free Cu film. Instead, structures appear corresponding to a film thickness reduced by one monolayer, accompanied with the formation of a O/Cu surface layer. Wave-like O–Cu–O chains along the [ 1 ̄ 12] direction are formed on the pseudomorphic Cu monolayer. The O/Cu structures on uniaxially reconstructed Cu bilayers depend on both the local stacking sequence and the extension of the unit cell. On moiré-like Cu films (thickness >3 ML), an O/Cu structure of well-established long-range order only occurs if O is preadsorbed on Ru(0001). The results obtained allow one to conclude that Cu–O segments (links) of the O–Cu–O chains should be regarded as surfactants for the O-mediated growth of Cu/Ru(0001). All O/Cu structures can be interpreted on the basis of O–Cu–O strings, in close relation to Cu 2O(111).

Growth and structure of ultrathin FeO films on Pt(111) studied by STM and LEED

The growth of iron-oxide films on Pt(111) prepared by iron deposition and subsequent oxidation was studied by scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction (LEED). Despite a 10% lattice mismatch to the substrate, an epitaxial growth of well-ordered films is observed. The oxide starts to grow layer by layer in a (111) orientation of the metastable cubic FeO structure up to a thickness of about 2.2 monolayers (ML). The completion of the second and third FeO layer depends on the precise oxidation temperature, and at coverages of approximately 2 ML three-dimensional ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ islands start to grow. The FeO(111) layers consist of hexagonal close-packed iron-oxygen bilayers that are laterally expanded when compared to bulk FeO and slightly rotated against the platinum substrate. They all exhibit oxygen-terminated unreconstructed $(1\ifmmode\times\else\texttimes\fi{}1)$ surface structures. With increasing coverage several structural film changes occur, and four coincidence structures with slightly different lateral lattice constants and rotation misfit angles against the platinum substrate are formed. In the submonolayer regime an FeO(111) bilayer with a lattice constant of 3.11 \AA{} and rotated by 1.3\ifmmode^\circ\else\textdegree\fi{} against the platinum substrate is observed. Upon completion of the first layer the film gets compressed leading to a lattice constant of 3.09 \AA{} and a rotation misfit angle of 0.6\ifmmode^\circ\else\textdegree\fi{}. Between 1.5 and 2 ML a coincidence structure rotated by 30\ifmmode^\circ\else\textdegree\fi{} against the platinum substrate forms, and at 2 ML a nonrotated coincidence structure with a lattice constant of 3.15 \AA{} evolves. All these coincidence structures exhibit large periodicities between approximately 22 and 38 \AA{} that are visible in the STM images up to the third FeO layer surface. The LEED patterns exhibit characteristic multiple scattering satellite spots. The different coincidence structures reflect lowest-total-energy arrangements, balancing the contributions of substrate-overlayer interface energies and elastic energies within the strained oxide overlayer for each coverage.

Reconstructed pseudomorphic Co films on the Cr(110) surface

Ultrathin epitaxial Co films on Cr(110) are examined by high-resolution low-energy electron diffraction. Pseudomorphic Co growth occurs over the entire coverage range investigated, i.e., up to 30 ML. A periodic lattice distortion is observed to exist in two equivalent $(3\ifmmode\times\else\texttimes\fi{}1)$ reconstruction domains showing uniaxial superperiodicity along either one of the two in-plane close-packed row directions. The lattice reconstruction is extended over several atomic layers normal to the surface. It is concluded that the overlayer reconstructs due to the constraint to adopt an inherently unstable structure forced by pseudomorphism.

Formation and Equilibration of Submonolayer Island Distributions in Ag/Ag(100) Homoepitaxy

We present an analysis both of the nucleation and growth of two-dimensional (2D) islands or clusters during deposition of Ag on Ag(100) at 295 K and of the subsequent postdeposition equilibration of such island distributions at coverages below about 0.25 monolayer. Island formation during deposition is shown to be effectively irreversible, and the island density and size and separation distributions are characterized using a combination of scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction. Postdeposition coarsening of the adlayer is monitored via STM and is shown to be dominated typically by diffusion and subsequent coalescence of large 2D clusters rather than by Ostwald ripening. Tailored studies of such coarsening elucidate both the kinetics and the underlying cluster diffusion process.

New information on the sublimating CdTe(001) surface from high resolution LEED

The sublimation of the CdTe(001) surface (≥ 300°C) was investigated by high-resolution low energy electron diffraction (HRLEED). From intensity oscillations of the specular diffraction spot as a function of time and energy we deduce a periodic variation of the step density, in agreement with the interpretation of earlier reflection high energy electron diffraction (RHEED) data within the step density model. However, our quantitative profile analysis shows that the relative change of the step density is only very small (≈ 15%). Thus the dominant contribution to the total desorption flux is due to step-flow, but not due to layer-by-layer sublimation. An important and general consequence is that absolute values of sublimation rates cannot be determined from intensity oscillation periods alone.

COMBINED HRLEED AND STM INVESTIGATIONS ON THE INITIAL STAGES OF Cu HETEROEPITAXY Ru(0001)

Recent scanning tunneling microscopy (STM) observations revealed different layer structures in the heteroepitaxial Cu/Ru(0001) system with increasing film thickness attributed to various stages of strain relaxation. High-resolution low-energy electron diffraction (HRLEED) analysis permits one to derive more exactly both lattice periodicities and lattice rotations. Furthermore, the representative character of local STM results can be proved. However, STM measurements are needed to identify and to assign the satellite spots to coexistent different superstructures which are superposed incoherently in the diffraction pattern. Generally, the integral LEED results confirm the crystallographic data obtained by STM in a local scale.

Reconstruction, morphology, and stoichiometry of CdTe(001) and Cd 0.96Zn 0.04Te(001) surfaces

The (001) surfaces of commercial, “MBE-ready” CdTe and Cd 0.96Zn 0.04Te substrates were studied by high resolution low energy electron diffraction (SPA-LEED) and Auger electron spectroscopy (AES) with respect to reconstructions, morphologies and stoichiometries. Contrary to the commonly used surface preparation by growth of additional buffer layers, contamination-free and structurally long-range ordered surfaces were achieved by Ar +-sputtering and subsequent annealing at 260–350°C. For CdTe, a mixed c(2 × 2)+(2 × 1) reconstruction with average terraces of up to at least 1000 Å width, separated by double layer steps of (3.3±0.1) A ̊ height, are deduced. The c(2 × 2) domains have a size of ∼65 A ̊ × 300 A ̊ . No facetting was detected. For the Cd 0.96Zn 0.04Te surface, Zn enrichment occurs at temperatures > 300°C, leading to various different surface reconstructions. Depending on annealing conditions and on the number of sputter-annealing cycles performed, (3 × 1), (2 × 1) and c(2 × 2) reconstructions, an increase of the step density and a formation of (011) facets were observed. Different mechanisms for the variation of the surface Zn concentration and of the surface reconstructions are discussed.

Vertical layer distribution and atomic step height of ultrathin Co films on Cu(001) determined by lattice G-factor analysis

The molecular beam epitaxial growth of ultrathin Co films on the Cu(001) surface at room temperature has been monitored in situ through the angular profile measurement of integer order beams using the high resolution low energy electron diffraction (HRLEED) technique. A simple kinematic lattice G-factor analysis of energy dependent angular profiles at a fixed coverage gives the vertical layer distribution of Co films and atomic step height. The vertical layer distribution is consistent with the nearly layer-by-layer growth mode. The atomic step height ∼ 1.77 Å is consistent with the structure determined from dynamical LEED calculations. A precaution in the application of the lattice G-factor analysis in heteroepitaxial systems is discussed.

Growth and melting of a Pb monolayer on Cu(111)

The growth and thermal behavior of thin Pb films on Cu(111) up to 1 monolayer (ML) coverage was investigated by means of high resolution low energy electron diffraction (SPA-LEED). The lattice constant of the Pb monolayer depends on the coverage varying from 0.497 nm (0.5 ML) to 0.480 nm (1 ML) and is incommensurate. At a coverage of 0.7 ML the thermal expansion coefficient of this film is twice the Pb bulk value. The melting temperature of the film depends on the coverage, too, varying from 550 K (0.5 ML) to more than 700 K (>1ML) compared to the Pb bulk value of 601 K. The melting process was studied in detail by fitting the profiles with different models. Although not fully applicable a comparison has been tried with the predictions of the KTHNY theory (developed by Kosterlitz, Thouless, Halperin, Nelson and Young) of two-dimensional melting. Below the melting temperature the film can be described as a “floating solid” with a “quasi” long-range order.

Growth instabilities of CaF 2 adlayers deposited at high temperature on Si(111)

The growth of ultrathin CaF 2 films on Si(111) at high deposition temperature has been studied by UHV-atomic force microscopy (AFM) and by high-resolution low energy electron diffraction (SPA-LEED) during growth. The CaF 2 film starts to grow in the step flow mode reproducing the Si substrate steps. The atomic force microscopy investigations show that after deposition of 2 TL CaF 2 instabilities of the growing film lead to the formation of triangular islands on top of the initial CaF interlayer at these steps. These instabilities are enhanced with increasing CaF 2 coverage so that the CaF 2 film forms wedges with extremely large flat terraces separated by steps that are some nm high. Additionally, small three-dimensional islands nucleate at these steps acting as preferential nucleation centers.

Critical scattering at the order-disorder phase transition of Si(111)-3×3R30°-Au surface: A phase transition with particle exchange

The Si(111)-$\sqrt{3}$\ifmmode\times\else\texttimes\fi{}$\sqrt{3}$-Au phase, which coexists with three-dimensional islands of excess Au at coverages above 1 monolayer and at temperatures above 700 K, is shown to undergo a temperature driven order-disorder phase transition at 1057 K. The islands act as a reservoir of Au atoms and adjust the chemical potential for the two-dimensional layer during the transition. Due to this particle exchange, the phase transition within the $\sqrt{3}$\ifmmode\times\else\texttimes\fi{}$\sqrt{3}$R30\ifmmode^\circ\else\textdegree\fi{}-Au has been observed for a chemisorbed layer which is not under the usual constraint of constant coverage, but controlled by the chemical potential. We quantitatively analyzed the critical scattering and experimentally determined its critical exponents \ensuremath{\mathrm{B}}, \ensuremath{\gamma}, and \ensuremath{\nu} by high-resolution low-energy electron diffraction. Their values are in good agreement with the expected values of the three-state Potts model, which shows that the transition is continuous without any finite-size effects detectable.

Oxygen-induced restructuring of strained Cu layers on Ru(0001)

The interaction of O with strained Cu layers on Ru(0001) has been studied by means of high-resolution low-energy electron diffraction and scanning tunneling microscopy. An O-induced (3 × 2√3) structure with glide-plane symmetry is observed after exposure in the range 50–200 Langmuir at temperatures between 500 and 700 K on thin Cu films on Ru(0001). At Cu coverages > 2 monolayers the structure coexists with different heteroepitaxial Cu Ru(0001) strain structures. Since O adsorption is accompanied by a drastic mass transport of Cu atoms, we suggest that a two-dimensional O Cu surface structure is formed containing Cu in more than one layer.

High-resolution LEED analysis of strained Cu layers on Ru(0001)

Scanning tunnelling microscopic observations have revealed that the relaxation of the lattice strain in the Cu film growing on Ru(0001) occurs in four different stages that are connected with different superstructures depending on the film thickness. Using high-resolution low-energy electron diffraction (HRLEED) the satellite spots of the different superstructures of Cu films with a thickness up to 7 ML (monolayers) grown at 520 K could be identified and quantitatively analysed. However, for Cu films thicker than 2 ML the diffraction patterns are very complex because satellite spots of several superstructures are incoherently superposed. Surprisingly, the structural data derived in a local scale by scanning tunnelling microscopy (STM) are highly representative for the entire surface, analysing by low-energy electron diffraction (LEED). This demonstrates the stability of the relaxation process. Corrugated Cu(111) layers formed after a deposition of 4 ML are rotated with respect to the Ru lattice within a small range of angles of only ±0.7°.

Effect of interface roughness on hysteresis loops of ultrathin Co films from 2 to 30 ML on Cu(001) surfaces

Ultrathin Co films have been grown on a smooth Cu(001) surface and a rough Cu(001) surface by molecular beam epitaxy (MBE). Using high-resolution low-energy electron diffraction (HRLEED) angular profile measurement and a kinematic LEED model, we quantitatively extracted the interface width w, the roughness exponent α film and the lateral correlation length ξ for Co films grown on a smooth Cu surface from 2 up to 30 ML thick. Corresponding magnetic properties, including the Kerr intensity I k, coercivity H c and d〈M〉 dH H = H c (the slope of the hysteresis loop at H = H c) were obtained from hysteresis loops measured by the surface magneto-optic Kerr effect (SMOKE). For 2 to 7 ML thick Co films grown on the rough Cu surface the hysteresis loop shape is much less square-like compared with that of Co films grown on teh smooth Cu surface; the Kerr intensity I k is a factor of 3–4 smaller than that of the smooth film; the H c value is ≈3 times larger than that of the smooth film. H c is almost thickness independent in the 2–7 ML regime, in contrast to that of the smooth film in the same thickness region. The difference in magnetic properties can be qualtitatively explained in terms of the quantitative roughness parameters associated with the Co films.

Diffraction spot profile analysis for heteroepitaxial surfaces applied to the initial growth stages of CaF 2 adlayers on Si(111)

We have studied the heteroepitaxial growth of CaF 2 on Si(111) by high resolution low energy electron diffraction. Analysing spot profiles recorded during CaF 2 deposition (SPA-LEED) we obtained information about the film morphology. The spots show a central spike and diffuse shoulder caused by atomic steps of the CaF 2 film. In order to evaluate quantitatively the layer distribution from the intensity oscillations of the central spike at the out-of-phase condition we included the interference between substrate and adlayer in our analysis. Depositing CaF 2 at 300°C we conclude from the central spike intensity that the nucleation of the second CaF 2 layer starts immediately. We attribute this behaviour to the poor binding of the CaF 2 molecules to the 7 × 7 substrate so that obviously the nucleation at surface defects (atomic steps, anti-phase boundaries) is favoured. The analysis of the central spike oscillations also reveals that the film grows in the multi layer growth mode at higher coverages.

Entropic Step Doubling on W(430).

We present high resolution low energy electron diffraction results for the W(430) surface. Unlike every other system studied to date, single atomic height steps on this surface become thermodynamically unfavorable at elevated temperatures. Above 940 K a new phase consisting of bound single and double height steps forms. The transition to double height steps is predicted on the basis of a simple model provided that kinks on the double height steps have a lower formation energy than kinks on single height steps. The new phase is stabilized by a step edge interaction between single-double pairs.

Electronic and geometric structure of NH3 on Ge(001) under equilibrium adsorption conditions.

The adsorption of ${\mathrm{NH}}_{3}$ on the Ge(001) surface is studied by angle-resolved UV-photoelectron spectroscopy using synchrotron radiation and by high-resolution low-energy electron diffraction (LEED) measurements. On the clean surface, the phase transition from 2\ifmmode\times\else\texttimes\fi{}1 to c(4\ifmmode\times\else\texttimes\fi{}2) is observed over a temperature range from 400 to 220 K. Measurements during ${\mathrm{NH}}_{3}$ admission at varying temperature in adsorption-desorption equilibrium yield a sequential occupation of several states. Up to 1/2 ML (one molecule per reconstruction dimer), ${\mathrm{NH}}_{3}$ is bound strongly on the dimer down atoms with the Ge-N axis perpendicular to the surface (\ensuremath{\alpha} state). The adsorption occurs via a mobile precursor state. The \ensuremath{\alpha} state is connected with a (2\ifmmode\times\else\texttimes\fi{}2) superstructure. Yet, the transition c(4\ifmmode\times\else\texttimes\fi{}2)\ensuremath{\rightarrow}(2\ifmmode\times\else\texttimes\fi{}2) starts already at very low coverages (0.01 ML) and is completed for about 0.04 ML. We propose that a long-range attractive interaction causes \ensuremath{\alpha}-${\mathrm{NH}}_{3}$-island formation and that a short-range repulsive interaction is responsible for a change to 2\ifmmode\times\else\texttimes\fi{}2 within the islands by a dimer flip of every second dimer row. At the island edges, this dimer flip continues over the clean part of the surface thus changing its structure to 2\ifmmode\times\else\texttimes\fi{}2 long before 1/2 ML saturation. Beyond 1/2 ML, the structure changes again. A good-quality 2\ifmmode\times\else\texttimes\fi{}1 structure is seen up to 4 ML indicating pseudomorphic growth up to this coverage. Beyond 4 ML, the adsorbate structure changes irreversibly to ${\mathrm{NH}}_{3}$-ice without any LEED pattern. \textcopyright{} 1996 The American Physical Society.

Correlation between dynamic magnetic hysteresis loops and nanoscale roughness of ultrathin Co films

Ultrathin Co films were epitaxially grown on a Cu(001) surface with different initial roughness created by Ar ion sputtering. The roughness of the Cu substrate and the Co film were characterized by high resolution low energy electron diffraction. The measured angular profiles were compared with a diffraction theory for rough surfaces and the roughness parameters were extracted quantitatively. Magnetic hysteresis loops of these characterized films were measured by surface magneto-optic Kerr effect. The hysteresis loop shape and loop area can be related to the nanoscale roughness in the Co films. For the roughest film with interface width ≊1.2t, where t is the single atomic step height, the magnetization is reduced several fold compared with that of smooth films with interface width ≊0.5t. Also, the coercivity in the roughest film is the highest and there exists a wide range of nucleation centers and coercive fields for magnetic domain reversals. These are related to the high step density in the rough substrate as the pinning centers. The hysteresis loop changes its shape and area under a sinusoidal external magnetic field as functions of frequency and field amplitude for all films. For the smooth films in the low frequency and low field regimes the loop area shows a 2/3 power law scaling behavior. The 2/3 value of scaling exponents are consistent with the prediction of a dynamic mean field theory with a double-well energy barrier. For a film with the same interface width but different step density and lateral correlation length the scaling exponents deviate from 2/3 value drastically.

HRLEED and STM Study of Misoriented Si (100) with and without a Te Overlayer

AbstractThe growth of high quality Te on misoriented Si(100) is important as an intermediate phase for epitaxial growth of CdTe. The misorientation angle plays a key role in the growth quality of CdTe/Si(100); this incited our curiosity to investigate the effect of the misorientation angle on the topography of the surface structure of Si(100). Our main goal is to show the relation between the misorientation angle, the terrace width and the step height distributions. HRLEED ( High Resolution Low Energy Electron Diffraction ) provides information in reciprocal space while STM gives real space topographic images of the surface structure. STM and HRLEED measurements were performed on Si(100) with misorientation angle υ= 0.5°, 1.5° and 8° towards the ]110] direction and υ= 4° towards the ]130] direction. Except for the 8° misorientation in which case a regular step array with diatomic step height was observed, for the other misorientations the terrace width was variable. The average terrace width decreased with increasing misorientation angle. A mixture of diatomic and monatomic step heights was observed on the 0.5° and 1.5° misoriented Si(100) samples. It proves that one can not assume purely monatomic step height for low misorientation angles. Our results do not agree with the belief that at low miscut angle A and B terraces are equal and that as the misorientation angle increases the B terrace tends to be wider than the A terrace. In fact, pairing of terraces was not observed at all. Te was deposited at a substrate temperature of 200 C. We observed a significant reduction in the terrace widths for all miscut angles.

Influence of H on low temperature Si(111) homoepitaxy

Molecular beam homoepitaxy on H terminated Si(111)-(1 × 1) surfaces has been studied by high resolution low energy electron diffraction (LEED). Exponentially decaying LEED intensity oscillations reflect the formation of an increasingly rough growth front during Si deposition. For temperatures below 480°C bulk defects are generated which finally lead to non crystalline films. The strong influence of H (even submonolayer coverages) on the growth behaviour is attributed to the high binding energy of 3.1 eV of the HSi bond, which hinders the place exchange process during growth.