High-resolution Low-energy Electron Diffraction Research Articles

Instability in Low-Temperature Molecular-Beam Epitaxy Growth of Si/Si(111)

Using high-resolution low-energy electron diffraction we studied in situ low-temperature molecular-beam epitaxial growth of Si/Si(111). At $\ensuremath{\sim}275 \ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and a deposition rate of $\ensuremath{\sim}7$ bilayers/min, a roughening evolution occurs after an initial transient layer-by-layer growth. It shows dynamic scaling characteristics with a roughness exponent $\ensuremath{\alpha}\ensuremath{\approx}1$ and an interface growth exponent $\ensuremath{\beta}=\frac{1}{4}$. More importantly, the average local slope is found to increase with time as $\ensuremath{\sim}\sqrt{\mathrm{ln}(t)}$. However, such a roughening behavior does not exist either at temperatures higher than 350 \ifmmode^\circ\else\textdegree\fi{}C or under a slower deposition rate of $\ensuremath{\sim}1$ bilayer/min. The phenomena are consistent with a statistical model of linear diffusion dynamics.

Dynamic scaling of island-size distribution in submonolayer growth of 1 x 1 films.

The scaling of island-size distribution of an epitaxially grown 1\ifmmode\times\else\texttimes\fi{}1 Fe film on a Au (001) surface as a function of coverage was observed using high-resolution low-energy electron diffraction. The scaling function extracted from the intensity distribution of integer-diffracted beams is best described by a \ensuremath{\gamma} function. The island density is almost a constant in the aggregation growth regime. The forms of the scaling function and the island density are in qualitative agreement with recent simulations of molecular-beam-epitaxy growth.

Anomalous dynamic scaling on the ion-sputtered Si(111) surface.

The time evolution of an ion-sputtered Si(111) surface was investigated between 300 and 650 \ifmmode^\circ\else\textdegree\fi{}C using the high-resolution low-energy electron diffraction technique. Below 450 \ifmmode^\circ\else\textdegree\fi{}C, a (1\ifmmode\times\else\texttimes\fi{}1) rough phase shows an anomalous dynamic-scaling behavior on the short-range scale where the measured height-height correlation has a form of \ensuremath{\sim}ln(t)${\mathit{r}}^{2\mathrm{\ensuremath{\alpha}}}$, which grows in time with a roughness exponent \ensuremath{\alpha}=1.15\ifmmode\pm\else\textpm\fi{}0.08. Such a behavior is consistent with the prediction by a recent dynamic-scaling theory. Above 530 \ifmmode^\circ\else\textdegree\fi{}C, we found that the (7\ifmmode\times\else\texttimes\fi{}7) phase does not undergo roughening evolution. The dramatic morphology change with temperature indicates a dynamic phase transition.

Energetics of diffusion processes during nucleation and growth for the Cu/Cu(100) system

Various diffusion processes during sub-monolayer epitaxy of Cu/Cu(100) were studied using the embedded atom method. These processes include (1) single atom diffusion on a flat Cu(100), (2) adatom diffusion around steps running both in 〈110〉 and 〈100〉 directions, and (3) migration of small clusters of up to 5 atoms on a flat terrace. The results show that the energy barrier for adatom diffusion along the step ledge of 〈110〉 directions is actually lower than that for adatom diffusion on a flat terrace and that diffusion across the descending steps depends on step orientation. The exchange mechanism is favored energetically over direct jumps across 〈110〉 steps, while direct jumps are favored across 〈100〉 steps. Dissociation energies and diffusion barriers for small clusters are oscillatory among clusters of odd and even number atoms. Comparison of dissociation energies and diffusion barriers indicates that (1) tetramers are relatively stable compared to dimers, trimers, and pentamers, and (2) small clusters of up to 5 atoms are quite mobile near room temperature. The calculated results support the conclusions from a recent high-resolution low-energy electron diffraction investigation on sub-monolayer epitaxy for Cu/Cu(100).

Growth and coalescence in submonolayer homoepitaxy on Cu(100) studied with high-resolution low-energy electron diffraction.

By measuring the dependence of the island separation [ital L] on the flux [ital F] during submonolayer epitaxy on Cu(100), the scaling exponent [ital p] in [ital L][similar to][ital F][sup [minus][ital p]/2] is determined in the steady-state and island coalescence regimes. In both regimes at low temperature (223 K), a crossover of [ital p] is observed from a low-flux value of 1/3 to a high-flux value of 1/2. At elevated temperatures (263--305 K), [ital p][similar to]3/5 is obtained. These results agree with classic nucleation theories and recent Monte Carlo simulations, and imply that the smallest stable island changes directly to a tetramer from a low-temperature dimer with increasing temperature. Dissociation energy calculations using the embedded-atom method support these results.

Continuous order-disorder phase transitions of the p(2 x 2) and ( sqrt 3 x sqrt 3 )R30( degrees) superstructures of sulfur on Ru(001): Effective critical exponents and finite-size effects.

Critical properties of the two-dimensional order-disorder phase transitions of the p(2\ifmmode\times\else\texttimes\fi{}2) and the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} superstructures of sulfur chemisorbed on Ru(001) were determined by spot-profile analysis using high-resolution low-energy electron diffraction. Both transitions are continuous, as evident from the power-law behavior observed for 0.01\ensuremath{\le}\ensuremath{\Vert}t\ensuremath{\Vert}\ensuremath{\le}0.1 (t=T/${\mathit{T}}_{\mathit{c}}$-1) and from the values obtained for the critical exponents. For smaller \ensuremath{\Vert}t\ensuremath{\Vert} the phase transitions are finite-size rounded by an interaction of the superstructure domains with steps (average distance between steps \ensuremath{\sim}275 \AA{}). The values of the effective exponents \ensuremath{\beta} of the order parameter, \ensuremath{\nu} of the correlation length, \ensuremath{\gamma} of the susceptibility, and the exponent \ensuremath{\eta} [only determined for p(2\ifmmode\times\else\texttimes\fi{}2)] fall close to the values theoretically predicted for the four-state and three-state Potts universality classes, respectively. Deviations of experimental values from Potts values, found for \ensuremath{\beta} and \ensuremath{\gamma}, are attributed to corrections to scaling which in part might be specific of the lattice gas. Additional experiments on vicinal surfaces with higher step densities show that pinning of the superstructure domains at monoatomic steps limits the correlation length to values below the average terrace width. The finite-size-induced effects are quantitatively compatible with predictions from finite-size scaling theory.

Roughening and faceting in a Pb thin film growing on the Pb(110) surface.

Using high-resolution low-energy electron diffraction, we have observed nonconventional dynamic scaling during the molecular-beam-epitaxy growth of Pb/Pb(110). The growth front becomes increasingly rough as more Pb atoms are deposited. At a low growth rate, the interface width w changes with deposition time t in a scaling form w\ensuremath{\sim}${\mathit{t}}^{\mathrm{\ensuremath{\beta}}}$ with \ensuremath{\beta}=0.77\ifmmode\pm\else\textpm\fi{}0.05. The other scaling hypothesis involving the lateral correlation length \ensuremath{\xi}\ensuremath{\sim}${\mathit{t}}^{\mathrm{\ensuremath{\beta}}/\mathrm{\ensuremath{\alpha}}}$ is no longer valid and the local roughness increases dramatically. However, the short-range height-height correlation function H(r) still scales with r, the horizontal distance between two points in the surface, in the form of H(r,t)\ensuremath{\sim}f(t)${\mathit{r}}^{2\mathrm{\ensuremath{\alpha}}}$ with \ensuremath{\alpha}=1.33\ifmmode\pm\else\textpm\fi{}0.05. At the later stage of growth, the rough surface eventually induces an anistropic {111} faceting. Faceting is found to occur earlier when the deposition rate is high.

The rotational disorder of Pb/W(112)

Using high-resolution low-energy electron diffraction, we have observed the rotational disorder of Pb/W(112) during MBE growth. Pb atoms grow on W(112) substrate in a typical 3D fashion, forming Pb(111) islands as the thickness of the Pb layers increases. While the surface normal of the Pb(111) islands aligns with that of the W(112), the lateral lattice orientation within the Pb(111) plane is not unique with respect to the W(112) substrate. Depending on the growth rate, the Pb overlayer may have different morphologies. At the lower deposition rate, random orientations as well as six discrete orientations are observed. At the medium deposition rate, four discrete orientations are observed. At the higher deposition rate, only one orientation is observed but with certain lateral deviations. These orientations can be attributed to the substrate geometry, lattice matching at the interface and the symmetry principle for epitaxial rotation.

Step-Edge Barriers, Re-Entrant Oscillations, and Unstable Epitaxial Growth

AbstractThe influence of step-edge barriers that inhibit the interlayer transport of atoms is discussed in the context of several material systems by using simulations of a solid-on-solid model. We show how the combined effect of this step-edge barrier and a non-thermal short-range incorporation process of freshly-deposited atoms can be used to explain several epitaxial phenomena on metal and semiconductor surfaces: (i) re-entrant oscillations seen with He-atom scattering and high-resolution low-energy electron diffraction during Pt(111) homoepitaxy, (ii) instabilities during growth on GaAs(001) and several metal (001) and (111) surfaces in the form of multilayer features that dynamically coarsen, while maintining their shape, and (iii) re-entrant oscillations during etching of GaAs(001). In (iii), the incorporation is replaced by a short-range search for the atom to be removed.

Ge δ-Layers on Si(111) and Si(001) Grown by MBE and SPE

AbstractGe δ-layers on Si(111) and Si(001), grown by molecular beam epitaxy (MBE) and solid phase epitaxy (SPE) were characterized in-situ by high-resolution low-energy electrondiffraction and post-growth by x-ray standing waves. LEED intensity oscillations are used to determine the growth mode of Ge on Si which is found to proceed in a double bilayer fashion for Ge on Si(111). X-ray standing waves are employed to investigate crystal quality of the Ge layer. SPE on Si(111) requires high annealing temperatures (600°C) for sufficient recrystallization of defects in the Ge δ-layer. On Si(001), Ge δ-layers of surprisingly high crystalline quality are grown by solid phase epitaxy at room temperature.

Interaction of Pt Films With ZnO(0001) and ZnO(0001)

AbstractThe growth, structure, and electronic properties of Pt films supported on the (0001) and (0001) surfaces of ZnO were investigated using high-resolution electron energy-loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). Vapor-deposited Pt films were found to grow in a layer-by-layer fashion on both surfaces and exhibited hexagonal LEED patterns. HREELS results indicate that there are only weak electronic interactions at the Pt/ZnO(0001) interface, while charge transfer and Schottky barrier formation occurs at the Pt/ZnO(0001) interface.

Observation of atomic place exchange in submonolayer heteroepitaxial Fe/Au(001) films.

Submonolayer Fe films on a Au(001) surface were found to exhibit atomic place exchanges with Au atoms during room temperature growth by using the angular profile measurement of high-resolution low-energy electron diffraction. The place exchanged atoms result in 2D Au surface islands and Fe subsurface structures. These inverted 2D islands form a structure with a roughly equal island size and spacing in \ensuremath{\sim}0.5 monolayer FeAu(001) surfaces. The observed mechanism of the atomic place exchange is consistent with recent first-principles calculations for a heteroepitaxial system.

Ordering and phase diagrams of xenon adsorbed on thin epitaxial NaCl(100) films and on Ge(100).

An adsorption study of Xe on both the bare Ge(100) surface, as well as on thin NaCl films grown pseudomorphically on Ge(100), was carried out using high-resolution low-energy electron diffraction. Ordered phases and phase diagrams were investigated in quasiequilibrium with room-temperature gas. Starting from low pressures, a transition from a disordered lattice gas to a distorted hexagonal phase was found for Xe on NaCl(100) in addition to the hexagonal bulk phase, which grows as a multilayer phase. The distorted hexagonal phase was found to be commensurate with the substrate in only one direction. At lower temperatures and/or higher pressures, the onset of three-dimensional growth is observed in the form of a hexagonal nanocrystalline phase (average grain size 80 \AA{}) with Xe bulk lattice constant which is still oriented along the [110] direction with one of its axes. Both phase transitions are of first order, with pronounced hysteresis effects for the second. On Ge(100), no additional superstructure spots are induced by Xe before the bulk phase grows. The (2\ifmmode\times\else\texttimes\fi{}1) reconstruction is maintained, but characteristic jumps in intensity of integral order as well as of half-order beams are observed as a function of temperature and pressure so that Xe seems to lock on defined sites in (2\ifmmode\times\else\texttimes\fi{}1) registry. Phase diagrams for Xe adsorbed on both surfaces were derived and heats of transition at the phase boundaries were determined.

Thermal stability and intermixing of ultrathin Fe films on a Au(001) surface

Thermal stabilities and atomic intermixing of ultrathin ferromagnetic Fe films grown on a Au(001) surface have been studied using high-resolution low-energy electron diffraction (HRLEED). Both the average terrace size and the average inhomogeneity length can be quantitatively determined as a function of temperature by properly decomposing the energy dependent angular profiles of the (00) beam measured from Fe/Au(001) films at various temperatures. Atomic place exchange between Au and Fe is processed in Fe films up to ~ 2.5 ML thick at room temperature. For higher coverage Fe films (3.5–5.0 ML), the Fe adatoms bury the intermixed interface (⩽ 2.5 ML thick) formed at room temperature. For all the films studied (1.5–5.0 ML), the occurrence of intermixing — i.e., intermixed Fe and Au patches due to interdiffusion of Fe adatoms into Au bulk and exchanges of Au atoms to the surface — is observed clearly in different elevated temperature ranges. This conclusion is supported by the results that dominant surface steps for the thicker Fe films change from Fe/Fe steps to Au/Fe steps, and eventually, to Au/Au steps as the annealing temperature is increased. The average inhomogeneity length as a result of intermixing is extracted and is in the order of 20 Å over a wide range of temperatures.

Island ordering on clean Pd(110)

High resolution low-energy electron diffraction measurements are reported that demonstrate the existence of semiordered islands on the clean Pd(110) surface. The islands are stable up to 1000 \ifmmode^\circ\else\textdegree\fi{}C and are approximately 90 atoms long in the 〈001〉 direction. A simple model is presented that makes use of step-step interactions to generate the periodic island structure. This model predicts that ordered islands form below the roughening temperature if the step creation energy is small compared to the step-step interaction. A justification for this condition is given for the Pd(110) surface.

Step-induced deconstruction and step-height evolution of the Au(110) surface.

We use temperature-dependent high-resolution low-energy electron diffraction and spot-profile analysis low-energy electron diffraction to study the Au(110) surface at room temperature up to 786 K. The experimental data were analyzed within the framework of the kinematic theory. Oscillations were determined of the positions of half order and fundamental Bragg peaks as well as of the full width at half maximum of the specular peak as a function of perpendicular momentum transfer. Evidence of mono- atomic steps occurring in the [001] direction was found below and around the (2\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\rightarrow}(1\ifmmode\times\else\texttimes\fi{}1) transition at ${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$, the surface gets smoother in the [001] direction; at the roughening temperature, ${\mathit{T}}_{\mathit{R}}$, the evolution of multiple-height steps starts in both symmetry directions.

Periodic faceting on TaC(110): Observations using high-resolution low-energy electron diffraction and scanning tunneling microscopy.

Unidirectional alternating (100)-(010) faceting has been observed for the clean TaC(110) surface using both high-resolution low-energy electron diffraction and scanning tunneling microscopy. Facets propagate periodically along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction and form a ridge-and-valley grating, characterized by an average periodicity of \ensuremath{\sim}6 lattice spacings in the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction, where (110) terraces are completely absent. This faceted surface is shown to be an energetically favored structure.

Modification of surface structures and roughening phase transition in Pb(110) by impurities

Using high-resolution low-energy electron diffraction, we show that the thermal roughening transition of a clean unreconstructed Pb(110) surface can be altered by impurities in the surface. The flat surface becomes a two-level regular terrace structure (1D) along the close-packed direction above 350 K with terraces modeled by a cut-off geometric distribution. The original Kosterlitz-Thouless roughening transition at 415 K for the clean surface was delayed until 480 K and the transition was irreversible due to the change in the amount of impurities in the surface.

Modification of surface structures and roughening phase transition in Pb(110) by impurities

Using high-resolution low-energy electron diffraction, we show that the thermal roughening transition of a clean unreconstructed Pb(110) surface can be altered by impurities in the surface. The flat surface becomes a two-level regular terrace structure (1D) along the close-packed direction above 350 K with terraces modeled by a cut-off geometric distribution. The original Kosteriitz-Thouless roughening transition at 415 K for the clean surface was delayed until 480 K and the transition was irreversible due to the change in the amount of impurities in the surface.

Chemisorption of H, H2O and C2H4 on Si(113): implications for the structure

Abstract The chemisorption of H, H 2 O and C 2 H 4 on Si(113) is studied by means of high-resolution electron energy loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). By saturating the dangling bonds of the clean Si(113)3 × 2 surface, H forms a monohydride first. At higher exposures a dihydride phase is formed, clearly distinguishable in HREELS through the appearance of the H-Si-H scissor mode. H 2 O dissociates into H and OH. Its sticking coefficient is of the order of one, indicating that Si dimers may constitute the Si(113)3 × 2 surface to a large part. H, OH and C transfer the surface structure from 3 × 2 into 3 × 1 adsorbate. At least in the case of H chemisorption the change of structure occurs very likely without lateral Si transport. This has strong implications for the structure of the clean Si(113)3 × 2 surface. To account for these findings together with our photoemission results and two scanning tunneling microscopy (STM) studies from the literature we propose a buckling model for the Si(113)3 × 2 surface. Our investigation underlines that only the absence of H, OH and C — the main residues from contamination through background gas — guarantees the occurrence of the 3 × 2 reconstruction for the clean Si(113) surface.