Narrow Terraces Research Articles

Step structure of arsenic-terminated vicinal Ge (100)

Germanium (100) crystals, 9° off-axis towards the [011] were exposed to 2.0 Torr of tertiarybutylarsine and 99.0 Torr of hydrogen at 650 °C, then heated to between 450 and 600 °C in vacuum or H2. The resulting surfaces consist of narrow dimer-terminated terraces, with (1×2) and (2×1) domains, that are separated by steps between one and eight atomic layers in height. The distribution of (1×2) and (2×1) domains changes with temperature, exhibiting a pronounced maximum in the (1×2) fraction at 510 °C. These results suggest that the arsenic passivation of germanium is a critical step in gallium arsenide heteroepitaxy.

One-dimensional character of the dimer structure of S B terrace on a Si(001) surface

We studied the influence of the finite width of S B terrace on the structure of a Si(001) surface by using Monte Carlo simulations (MCSs) based on the asymmetric dimer model. Coupling constants among the buckling of the dimers are given by the first-principles calculation: We consider a narrow S B terrace and treat the S A and the S B step edges by the free boundary condition for the buckling of the dimers. We find that the system reveals the feature of a one-dimensional Ising spin system at low temperature. As long as the temperature is finite, there is no long-range order and the structure of the system fluctuates. We calculated STM images and show that the images change in accordance with the scale of the observation time. LEED intensity reflects short-range order nearly independently of the scale of the observation time. It is indicated by the present MCSs that information about scanning speed of the STM is very important for interpreting the results of experimental STM images observed on the narrow terrace on the Si(001) surface.

REM Studies of Adsorption-Induced Phase Transitions and Faceting in the Si(111)-Au System

Initial stages of Au adsorption processes on Si(111)-(7 × 7) surfaces at ~780°C, a temperature range where the surface structure undergoes successive phase transitions [(7 × 7)–("1 × 1")–(5×2)–("1 × 1")], were observed in situ by reflection electron microscopy. All of the phase transitions are heterogeneous on the surface and start at surface atomic steps. During Au adsorption on Si(111)-(7 × 7), and subsequently on Si(111)-("1 × 1") surface with wide terraces, steps advance toward the step-down direction. At a Au coverage of ~0.3 ML, the 5 × 2 structure nucleates at step edges, and the nuclei expand both to the lower side terraces and to the higher side terraces. At this stage, an effect of current for heating the specimen was noted. From measurements of such movements of the steps and the domain boundaries, the density of Si atoms in the "1 × 1" phase is estimated to be 1.3–1.7 ML at a Au coverage of ~0.3 ML. Au adsorption on Si(111) surfaces with narrow terraces causes bunching of the steps. After nucleation of the 5 × 2 structure, the bunched [Formula: see text] steps become straight along the [Formula: see text] direction, and are transformed into the (335) facet planes at a Au coverage of 0.50 ML. It was found that the (335) facet planes are stabilized by adsorbed Au atoms. Destruction of the (335) facet is noted at a Au coverage of 0.73 ML.

Atomically flat (0 0 1)GaAs surface prepared by two-step atomic-hydrogen treatment and its application to heteroepitaxy of GaN

Substrate surface treatment techniques and growth conditions are the key of growing high-quality cubic GaN (c-GaN) epitaxial layers on (001) GaAs substrates. An atomically flat (001) GaAs substrate surface can be obtained by a two-step atomic hydrogen (atomic-H) irradiation technique. The typical surface of the substrate treated by the atomic-H consists of narrow terraces of one monolayer step height with acute-angle polygonal structures. The X-ray rocking curve FWHM of GaN (002) grown on the atomic-H treated substrate is less than 90arcsec. It is shown that high-density monolayer steps and kinks at the surface terrace edge would be preferable for the uniform nucleation of the c-GaN buffer layer.

Electronic states of monolayer micrographite on TiC(111)-faceted and TiC(410) surfaces

The monolayer micrographite films, of which the lateral dimensions are in the order of nanometer, have been grown on TiC(111)-faceted and TiC(410) surfaces by chemical vapor deposition. Diffraction spots due to the graphite lattice have been observed for TiC(111)-faceted surfaces, but not for (410). Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) spectra of micrographite on the faceted surface have shown a series of peaks due to the π states of graphite. On the (410) surface, quantized π branches of graphite ribbon with the narrow terrace of 0.886 nm appeared. Any other state characteristics of the micrographite, such as edge state near Fermi level, was not observed for both systems.

Structure and width of Au-adsorbed narrow Si(111) terraces

The structure and width of Au-adsorbed narrow terraces on a vicinal Si(111) surface misoriented toward the $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2]$ direction are studied by scanning tunneling microscopy. We find that, at a coverage of 0.15 ML, narrow Au-adsorbed bunched steps are separated by wide $7\ifmmode\times\else\texttimes\fi{}7$ terraces. The Au-adsorbed terraces are classified into three types according to the structures on the neighboring terraces. Where the Au-adsorbed terrace is next to the $7\ifmmode\times\else\texttimes\fi{}7$ terrace, the boundary facing the $7\ifmmode\times\else\texttimes\fi{}7$ terrace has a characteristic structure depending on whether it faces the faulted or the unfaulted halves. On the other hand, where the Au-adsorbed structure is sandwiched by the other Au-adsorbed terraces, the width of the Au-adsorbed terrace along the $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2]$ direction can be expressed by $(5N+5+2/3)\sqrt{3}/2$, where the length is represented in units of the lattice constant of the $1\ifmmode\times\else\texttimes\fi{}1$ surface, and the $N$ is the number of the $5\ifmmode\times\else\texttimes\fi{}2$ unit cells along the $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2]$ direction. For $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$, the terrace width is equal to that of a (755) facet. On this terrace, $5\ifmmode\times\else\texttimes\fi{}2$ reconstruction cannot be observed although the size of the terrace is sufficient to form $5\ifmmode\times\else\texttimes\fi{}2$ reconstruction. We propose a model for the boundaries between the $7\ifmmode\times\else\texttimes\fi{}7$ terrace and the Au adsorbed structure on the narrow (111) terraces.

Comparison of bromine etching of polycrystalline and single crystal Cu surfaces

The etching of polycrystalline Cu surfaces by Br2 was studied using temperature programmed desorption and scanning tunneling microscopy, and the results were compared with a recent study of the interaction of Br2 with Cu(100). For both Cu surfaces, the etching mechanism could be described as a two step process: adsorption rate limited CuBr formation followed by CuBr desorption. The adsorption rate was found to be structure sensitive. At 325 K, the sticking coefficient for CuBr formation was higher on Cu(100) than on polycrystalline Cu. For both surfaces, increasing the temperature during dosing decreased the CuBr formation rate. The decrease was larger for polycrystalline Cu and so the structure sensitivity increased with increasing temperature. The removal of CuBr by sublimation was found to depend more strongly on surface structure. For low CuBr coverages on Cu(100), a CuBr desorption peak was observed at 450 K, a temperature lower than anticipated from CuBr vapor pressure data. When the CuBr coverage was increased, the peak at 450 K shifted to higher temperatures and eventually disappeared; the remaining CuBr desorption peaks were consistent with sublimation of bulk CuBr. A CuBr desorption peak at 450 K was also observed on polycrystalline Cu. On the polycrystalline surface, however, this peak saturated at high coverages but did not shift to higher temperatures. The intensity of the 450 K desorption peak depended on the treatment of the polycrystalline surface. Annealing was found to increase the peak intensity while sputtering and Br2 etching were found to decrease the intensity. Scanning tunneling microscopy images following each of these treatments suggested that the 450 K desorption peak is associated with narrow terraces on the surface. The images also revealed that etching greatly increases terrace widths suggesting that steps act as Cu atom sources for CuBr formation, and thus that Br2 etching of Cu can be considered the reverse of step flow growth.

Observation of Si(001) surface topography at temperatures below 1140t°C using a reflection electron microscope

The changes in the surface topography of a Si(001) vicinal sample caused by heating it to temperatures below 1140t\ifmmode^\circ\else\textdegree\fi{}C by direct current are investigated by reflection electron microscopy: the surface normal of the sample is misoriented by less than 2\ensuremath{'}. At temperatures below 1000t\ifmmode^\circ\else\textdegree\fi{}C, the surface is mostly covered by preferential terraces (for example, 2\ifmmode\times\else\texttimes\fi{}1 terraces) determined by the direction of the heating current. At temperatures below about 850t\ifmmode^\circ\else\textdegree\fi{}C, pairs of steps are uniformly distributed on the surface, while step bands are formed by the aggregation of steps at temperatures near 950t\ifmmode^\circ\else\textdegree\fi{}C. A few wide terraces remain between the step bands: in our sample, they are about 5 \ensuremath{\mu}m wide in the direction parallel to the current. At temperatures between 1000t\ifmmode^\circ\else\textdegree\fi{}C and 1100t\ifmmode^\circ\else\textdegree\fi{}C, the step bands relax and the narrow terraces (1\ifmmode\times\else\texttimes\fi{}2 terraces) widen due to the evaporation of Si atoms. At temperatures between 1070t\ifmmode^\circ\else\textdegree\fi{}C and 1100t\ifmmode^\circ\else\textdegree\fi{}C, atomic steps appear due to the relaxation in pair steps; both the 2\ifmmode\times\else\texttimes\fi{}1 and 1\ifmmode\times\else\texttimes\fi{}2 terraces coexist with approximately equal widths (about 1 \ensuremath{\mu}m). At temperatures above 1100t\ifmmode^\circ\else\textdegree\fi{}C, the surface topography changes to one composed of mosaic domains due to the evaporation of the atoms. For the Si(001) vicinal surface, the evaporation temperature of Si atoms is about 1000t\ifmmode^\circ\else\textdegree\fi{}C for Si atoms released from the ${\mathrm{S}}_{\mathrm{b}}$ step and about 1100t\ifmmode^\circ\else\textdegree\fi{}C for the atoms released from the ${\mathrm{S}}_{\mathrm{a}}$ step, where the evaporation temperature is determined by the activation energy, which consists of the release energy and the evaporation energy.

Influence of SiH4 deposition on the Si(111) 1 x 1-->7 x 7 phase transition.

The kinetics of the transition from the high-temperature Si(111) 1\ifmmode\times\else\texttimes\fi{}1 phase to the 7\ifmmode\times\else\texttimes\fi{}7 reconstruction is affected by an external Si source, which retards the transition velocity significantly. The phase transition can be frozen at much slower cooling rates and on narrower terraces than without Si supply. Areas of a high-density 1\ifmmode\times\else\texttimes\fi{}1 and of a 7\ifmmode\times\else\texttimes\fi{}7 phase are observed by scanning tunneling microscopy after quenching across the transition on terraces of less than 2000 \AA{} in width. The 7\ifmmode\times\else\texttimes\fi{}7 domain size distribution follows a $\ensuremath{\Gamma}$ distribution, pointing to random nucleation and uniform growth rate of the domains on the terraces.

Probing electrical transport, electron interference, and quantum size effects at surfaces with STM/STS

We use scanning tunneling microscopy (STM) and spectroscopy (STS) to probe electrical transport through the dangling-bond surface states of semiconductors and electron scattering and electron confinement effects in metal surface states. Specifically, we use point contacts between the STM tip and the sample to show the existence of surface electrical transport in Si(111)-7 × 7. Point contacts to silicon islands provide further support for the existence of the surface transport channel and illustrate the role played by carrier scattering at the boundaries of the nanostructure in electrical transport. In contrast to the silicon case, electrons in Shockley-type metal surface states act like a quasi-two-dimensional free-electron gas (2DFEG). This 2DFEG is scattered by steps, adsorbates, and defects, and the interference between incident and reflected electron waves leads to an oscillatory local density of states (LDOS). This LDOS is imaged in STM spectroscopic maps, and analysis of the oscillations provides novel information regarding electron scattering by individual surface features. Steps are found to act as barriers for surface electrons, and this property is utilized to confine them and form structures of lower dimensionality. Quasi-1D structures (quantum wires) are generated at narrow terraces of stepped surfaces, while small metal islands behave as 0D structures (quantum dots). Confined states with discrete spectra are observed even at 300 K, and their probability distributions are imaged.

Symmetry of reaction sites and anisotropy in angular and velocity distributions of desorbing product carbon dioxide on platinum (110)(1 × 2)

The angular and velocity distributions of reactive CO 2 desorption were studied along the narrow terrace of Pt(110)(1 × 2) over a wide range of CO coverage by means of angle-resolved thermal desorption combined with the cross-correlation time-of-flight technique. The angular distribution of CO 2 appearing in four peaks in the range 170–400 K shows a maximum in common at the bulk surface normal. It is broader than that perpendicular to the terrace. The translational temperature of CO 2 in each peak is much higher than the surface temperature and is maximized at the bulk surface normal. The translational temperature indicates less dependence on the desorption angle than that perpendicular to the terrace. The anisotropy in both angular and velocity distributions is related to the symmetry of reaction sites.

Periodic step-bunching on a miscut Pt(111) surface

The temperature dependence of the equilibrium surface morphology of a stepped Pt(111) surface, miscut by 1.8° towards [1̄1̄2], has been studied by X-ray scattering techniques between 300 and 1900 K. Below T c = 1389 K, the surface is found to be uniformly stepped and unreconstructed. Above T c, the surface transforms into a periodic array of wide terraces and narrow terraces. Unlike macroscopic facets, the wide terraces are mesoscopic in extent, ranging from 230 Å at 1389 K to 94 Å at 1900 K. Reflectivity measurements suggest that multiple-height steps are not prevalent. The wave vector characterizing the array (δ) follows a power law versus reduced temperature [t = (T - T c ) T c ] : δ = δ 0t \\ ̄ gb with \\ ̄ gb = 0.66 ± 0.05 .

Observation of Quantum-Size Effects at Room Temperature on Metal Surfaces With STM

Surface steps act as confining barriers for electrons in metal-surface states. Thus, narrow terraces and small single-atom-high metal islands act as low-dimensional, electron-confining structures. In sufficiently small structures, quantum-size effects are observable even at room temperature. Scanning tunneling spectroscopy is used to image the probability amplitude distributions and discrete spectra of the confined states. Examination of the electronic structure of the steps provides evidence for electron-density smoothing and the formation of step-edge states. Estimates of the electron-confining barriers are obtained.

Archaeological Research at the Rowland Clark Site (41RR77), Red River County, Texas

The Rowland Clark site is located in Red River County, Texas, on a prehistoric river channel of the Red River that has been intersected by a more recent channel of the river. It is approximately 16 km south of the Arnold Roitsch or Sam Kaufman (41RR16) site. The site had small Coles Creek and Early Caddoan period occupations, but the major occupation was of a McCurtain phase group of the Late Caddoan period. I The site has been in the Clark family for more than a century. The land the site is on is the property of Mr. Rowland Clark. The site lies on a narrow alluvial terrace at the edge of the floodplain, and it is bounded on the north by a comparatively recent river bank with springs at the base. An older river bank is to the east, and to the west is a small range of sandy pimple mounds, some 5-6 feet tall. These pimple mounds may be ancient sand dunes. A shallow swampy depression, part of an older watercourse, lies between the sandy mounds and the site.

The interaction of C 60 with noble metal surfaces

The interaction of C 60 with Ag(111) was studied using UHV-STM, LEED, AES, and TOF-SIMS and compared with previous results for Au(111). On both surfaces, c 60 grows in a layer-by-layer manner with nucleation occurring at steps separating narrow terraces followed by growth from the steps onto larger terraces. However, differences in C 60 growth on the two surfaces due to the Au(111) surface reconstruction were observed. Also, on Ag(111), C 60, was found to form a 2√3 × 2√3R30° structure while on Au(111) a 38 × 38 in-phase structure was also observed indicating a stronger driving force for adsorption into specific sites on Ag(111). Adsorbed c 60 molecules were found to rotate faster on Au(111) than on Ag(111) also indicating a stronger interaction between C 60 and Ag. Annealing studies, however, showed that C 60 desorbs from both Au(111) and Ag(111) at approximately 500°C indicating that differences in interaction strength between the two surfaces cannot be large. The mobility of first and second layer molecules was also studied using STM. The results indicated that individual adsorbed molecules are sufficiently mobile for thermodynamic considerations to be important in determining nucleation and growth on both surfaces. It was also found that small second layer islands rearrange over the time-scale of minutes, but that concerted motion and rearrangement of large first layer domains is not likely at room temperature on both surfaces.

Anisotropy in the spatial distribution of reactive carbon dioxide desorption from narrow terraces on a reconstructed iridium (110)(1 × 2) surface

The spatial distribution of reactive CO 2 desorption on a reconstructed Ir(110)( l × 2) surface was measured by means of angle-resolved thermal desorption. Two-directional desorption collimated along the terrace-surface normal was analyzed in detail. The desorption component shows a sharp angular distribution parallel to the surface trough and a much sharper distribution perpendicular to it. This anisotropy is correlated with the structure around the reaction sites.

Anisotropy in the spatial distribution of reactive carbon dioxide desorption from narrow terraces on a reconstructed iridium (110)( [formula omitted]) surface

Anisotropy in the spatial distribution of reactive carbon dioxide desorption from narrow terraces on a reconstructed iridium (110)( [formula omitted]) surface

Electrochemical study of step reconstruction on platinum surfaces belonging to the [0 [formula omitted]1] zone between Pt(311) and Pt(111)

In this paper we present experimental results concerning the effect of thermal and electrochemical oxygen adsorption on the surface structure of platinum faces vicinal to Pt(111) in the [0 1 1] zone whose ordered equilibrium surface structure, when clean, is denoted by Pt(S)[ n (111)×(100)]. Two different types of electrochemical behaviour have been observed depending on terrace width. Under the effect of oxygen adsorption, surfaces with n ⩾ 4 develop a well-marked adsorption state at 0.115 V which reflects the formation of (110) sites to the detriment of the (100) step sites which would normally correspond to the orientation of the sample. This phenomenon can be avoided by cooling the sample under an H 2 + Ar atmosphere, or the step order can be almost totally restored by applying a fast cycling treatment under the same conditions as those used to restore the two-dimensional long-range order in Pt(100) samples. A hard-sphere model of the perturbed surfaces based on a (100) step reconstruction is used to analyse quantitatively the corresponding voltammograms leading to results in agreement with the geometry of the (110) kink sites formed after oxygen desorption. The electrochemical behaviour of orientations with n ⩽ 4 has also been studied. The absence of the adsorption state related to (110) sites after oxygen adsorption-desorption indicates the stabilization of (100) sites when narrow terraces are intrinsic to the orientation of the sample. For Pt(311), representing the turning point of the [0 1 1] zone, analysis of the voltammograms obtained after thermal or electrochemical oxygen adsorption reveals the existence of a microfaceting process leading to the formation of multi-atomic (100) and (111) microfacets no wider than three or four atomic rows.

Nucleation, growth, and structure of fullerene films on Au(111)

The growth of fullerene films by vapor deposition on Au(111) was studied using UHV-STM, LEED, AES, and TOF-SIMS. The fullerenes were found to grow in a layer-by-layer manner. The molecules initially adsorb at intersections of multiple steps and edges of monatomic steps on narrow terraces. As the coverage is increased, periodic arrays of short chains form at steps separating wider terraces with the periodicity determined by the (23 × √3)− Au(111) reconstruction. At higher coverages, hexagonal layers with a lattice constant of 1.0 nm grow out from the steps on both the upper and lower terraces. The adsorption of the fullerenes lifts the reconstruction indicating a strong interaction between the fullerenes and Au(111). STM indicates that two ordered commensurate structures predominate on the Au(111) surface: (1) a layer with a periodicity of approximately 38 Au spacings; and (2) a layer with a (2√3 × 2√3)R30° unit cell. Second layer molecules were found to display no affinity for step edges. On Au(111), the second layer desorbs at 300°C, while the first layer is strongly bound and does not begin to desorb until 500°C. After annealing to 500°C the remaining C 60 is largely disordered. Intramolecular structure was observed within molecules at the step edges and in the hexagonal arrays.

Adsorption of xenon on stepped nickel (hk0) surfaces by LEED and photoemission

Xenon adsorption on stepped Ni( hk0) faces has been studied by LEED, photoemission and work function measurements. The dipole moment per step atom, measured on the clean surfaces, has been found equal to 0.2 D. The step sites have been evidenced on Ni(810) by xenon adsorption: an increase of the binding energy of the Xe 5 p 1 2 level of about 700 meV is reported between the terrace and the step adsorption sites. On Ni(410) and Ni(310) no terrace sites have been observed. The adsorption is dominated by the steps and two types of xenon adsorption sites have been evidenced. It is assumed that the potential associated with the step edges in the case of narrow terraces presents a lateral extension of the same order of magnitude as the xenon probe diameter.