Si Coverage Research Articles

Atomic models of (√3×√3)R30° reconstruction on hexagonal 6H–SiC(0001) surface

By using reflection high-energy electron diffraction (RHEED) and RHEED multislice dynamical calculations, the atomic structures of the (∛×∛)R30° reconstruction on 6H–SiC(0001) surface were solved. Both the simple adatom structure with a Si coverage of one-third monolayer occupying the threefold-symmetric T4 or H3 sites and a bit complex structure with Si trimers centered on the T4 positions with 1 monolayer coverage are all compatible with our results.

Electronic structure and nature of the bonding at the Cu(110)+c(2×2)-Si surface alloy

The electronic structure of the Cu(110)+c(2×2)-Si surface alloy has been studied experimentally by synchrotron radiation photoemission spectroscopy. We have recorded normal emission photoemission spectra as a function of the photon energy, Si coverage and angle of incidence of the polarized light in order to identify the electronic features induced in the Cu valence band by the presence at the surface of an ordered Si–Cu bidimensional layer. Two main dispersionless bands in the direction perpendicular to the surface and centered around −3 and −1 eV of binding energy are the most characteristic electronic features of the system. Most of the details in the photoemission spectra are well reproduced by a local density of states calculation employing the Extended Hückel Theory. These results suggest that the electronic states from the surface alloy are mainly originated by an in-plane hybridization of the Cu 3d electrons from the outermost layer with the Si 3p electrons.

Thermal Chemistry of Chlorine on Si/Cu(100)

Photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD) studies investigated the chemistry of Cl2 on Si/Cu(100). Si deposition was carried out by exposing Cu(100) to SiH4 at 420 K. PES showed that the exposure of this Si-saturated surface to Cl2 resulted in the formation of SiCl, SiCl2, and SiCl3 species at 120 K, with the latter species becoming more prevalent at higher Cl coverages. Heating of the chlorinated surface between 120 and 500 K increased the concentration of SiCl3 species. TPD studies of Cl2/Si/Cu(100) as a function of Si coverage showed primarily SiCl3 (and probably SiCl2) desorption at the lower Si coverages and SiCl4 at the highest Si coverage. The majority of the Si was not removed from the surface as chlorosilane product, but instead diffused into the Cu bulk at temperatures above 500 K.

Evolution of Si-on-GaAs (001) surface morphology towards self-organized ordered Si structures

Scanning tunneling microscopy studies of the restructuring of GaAs (001) surfaces induced by Si deposition have been performed. With increasing Si coverages, different reconstructions are developing. Their interaction with the surface step structures results in a distinct separation into different surface phases with different Si coverages, revealing unique Si distribution patterns. This phenomenon is explained by considering kinetically accessible thermodynamically determined structures.

Controllable step bunching induced by Si deposition on the vicinal GaAs(001) surface

The evolution of the morphology of the vicinal GaAs (001) surface induced by Si deposition was studied by scanning tunneling microscopy. The observed step bunching is accompanied by a spatial separation of surface phases with different Si coverages on terraces and in step regions. The spacing and height of the bunches depend on the substrate temperature. A model is proposed to account for these effects by considering a kinetic pathway of the surface to an accessible lowest energy state.

Formation and stability of the Cu(110)+c(2×2)-Si surface alloy studied by high resolution XPS

High-resolution synchrotron radiation photoemission has been used to investigate the formation of the Cu(110)+c(2×2)-Si surface alloy. The complex spectra of the Si 2p core-level are analyzed as multiple component spectra for different Si coverages and annealing temperatures of the surface alloy. The results show that c(2×2) islands are formed from the very beginning of the growth and that Si has a high diffusion length on Cu. The thermal stability of the surface alloy has been studied by measuring real-time photoemission spectra at different temperatures. The surface alloy is stable up to 180°C. Above this temperature disruption of the surface alloy and clustering of the Si atoms can be observed.

Formula omitted] a surface phase with a variable composition

Using scanning tunneling microscopy, the changes in the morphology and atomic structure of the Si(111)5 3 ×5 3 -Sb surface during Sb desorption have been studied. It has been found that the Si(111)5 3 ×5 3 -Sb surface phase does not have a definite composition. In attempting to explain the 5 3 ×5 3 long-range order stability in a wide Si and Sb coverage range it has been suggested that Sb atoms partially substitute for the boundary Si dimers in the basic 5×5 dimer–adatom–stacking-fault structure. A possible model of the Si(111)5 3 ×5 3 -Sb reconstruction satisfying this requirement has been proposed.

Silicon-induced nanostructure evolution of the GaAs(001) surface

By using scanning tunneling microscopy and reflection high-energy electron diffraction it is demonstrated that self-organized ordered Si structures develop during submonolayer Si deposition on vicinal GaAs(001) surfaces, provided the preferred adsorption sites in the trenches of the $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction are filled with Ga. The evolution of different reconstructions with increasing Si coverages is accompanied by step bunching and de-bunching processes. This unexpected behavior is explained by the interaction between reconstructions and steps from a thermodynamic equilibrium view. For particular coverages the complex process leads to a separation of different surface phases and Si coverages on terraces and in step regions.

Thermal Chemistry of CH3 on Si/Cu(100)

Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH{sub 3} (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal decomposition of SiH{sub 4} at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu{sub 2}Si structure. These Cu{sub 2}Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH{sub 3}){sub 3}SiH desorbed near 200 K from CH{sub 3}/Si/Cu(100) prepared with a low Si concentration. With increasing Si concentration a (CH{sub 3}){sub 3}SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH{sub 3}){sub 3}SiH at 200 and 420 K were believed to be due to the thermal reaction of CH{sub 3} with the low Si density and high Si density (i.e., Cu{sub 2}Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu{sub 2}Si phase covered the surface, only the 420 K peak was present during CH{sub 3}/Si/Cu(100) TPD. Results also suggested that (CH{sub 3})Simore » and possibly some (CH{sub 3}){sub 2}Si intermediates predominated on the surface below room temperature, and (CH{sub 3}){sub 3}Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH{sub 3}){sub 3}SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).« less

Formation of the Si/Cu interface

The growth of evaporated Si on polycrystalline Cu has been investigated by means of ultra-violet photoemission spectroscopy, Auger electron spectroscopy (AES) and atomic force microscopy techniques. Thus, by analyzing the AES intensity evolution with Si coverage, AES lineshape and energy position of the main valence band features, we have found that room-temperature Si deposition leads to the formation of a reacted phase ∼20 A thick, most likely corresponding to the eutectic Cu 3 Si. On top of this phase grows an amorphous Si layer. In this paper we study the difference between the height of the Schottky barrier formed on the reacted phase and that obtained on an atomically sharp interface, and some reported values.

Si overgrowth of self-assembled Ge clusters on Si(001) — a scanning tunnelling microscopy study

The first stages of the Si overgrowth of self-assembled Ge quantum dots on Si(001) have been studied by scanning tunnelling microscopy. Both the smaller ‘hut’ clusters (width 30–60 nm, {105} side facets) and the larger ‘domes’ (width 60–l00 nm, mostly {113} side facets) have been investigated. We obtain direct evidence of a distinct change of the morphology of the clusters even for low Si coverages. In case of the hut clusters this leads to a transition towards the shape of truncated pyramids. Even for the large dome clusters an abrupt decrease in the aspect ratio z/ x can be observed at Si coverages as low as a few monolayers. We discuss the mechanisms, which can lead to this behaviour and compare with the observations of other studies.

The growth and electrical properties of silicon on GaAs(110) studied by scanning tunneling microscopy and scanning tunneling spectroscopy

A scanning tunneling microscope (STM) has been used to study the structural formation of silicon overlayers deposited at room temperature on GaAs(110). In addition spectroscopic measurements were obtained simultaneously to reveal electrical properties associated with the interface and overlayer formation. The Si coverage varied in thickness from submonolayer growth up to ∼16 monolayers. The height variations of the STM images indicate that silicon islands did not exceed ∼3 Å at either submonolayer growth or thicker coverages. The lack of atomic resolution of the adsorbed silicon suggests that the silicon overlayers are amorphous in nature, confirming soft x-ray photoelectron spectroscopy (SXPS) measurements. The scanning tunneling spectroscopy measurements clearly indicate a Fermi shift of 0.8 eV when silicon was deposited on the clean cleaved surface of p+ GaAs(110). This shift is also observed on regions of the substrate which remained exposed after the silicon deposition. The Fermi shifts and resultant band bending are attributed to the formation of the Si–GaAs heterojunction and correlation with earlier SXPS measurements prove favorable. The possibilities of measuring the band offsets are also considered.

Application of UV-VIS and FTIR Spectroscopic Ellipsometry to the Characterization of Wet-Chemically Treated Si Surfaces

In this paper, investigations of roughness, oxide and hydrogen coverage of Si(111) surfaces after various wet-chemical treatments using spectroscopic ellipsometry (SE) both in the ultraviolet/visible (UV-VIS) and in the infrared (IR) spectroscopic region are reported. FTIR SE measurements yielded detailed information on the Si–H bonds on the surface of H-terminated Si(111) wafers. The characteristic structure in the IR ellipsometric spectra at 2083 cm—1 can be modeled by an anisotropic oscillator. A correlation is established between the morphological structure of the cleaned Si surface caused by chemical preparation and the resulting electronic surface properties monitored by additional surface photovoltage (SPV) measurements. The results of these systematic investigations have been successfully used for optimization of wet-chemical preparation methods aimed at atomically flat H-terminated Si(111) surfaces with very low surface state densities. Additionally, native oxide growth on initially H-terminated Si(111) was correlated to the evolution of rechargeable interface states determined by SPV measurements.

Theoretical study of the(3×2)reconstruction ofβ-SiC(001)

By means of ab initio molecular dynamics and band structure calculations, as well as using calculated STM images, we have singled out one structural model for the (3x2) reconstruction of the Si-terminated (001) surface of cubic SiC, amongst several proposed in the literature. This is an alternate dimer-row model, with an excess Si coverage of 1/3, yielding STM images in good accord with recent measurements [F.Semond et al. Phys. Rev. Lett. 77, 2013 (1996)].

Si-adsorption induced phase transition on the 3C–SiC(001) surface

The formation of Si-rich surfaces on the 3C–SiC(001) crystal using Si molecular beam epitaxy has been investigated using scanning tunneling microscopy. We observed a surface phase transition from the c(4×2) reconstruction to the (3×2) reconstruction by the adsorption of Si atoms. The adsorbed atoms formed one-dimensional (1D) strings, followed by narrowing the width of the adjacent strings with increasing adsorption. We find that the adsorption by narrowing of the 1D strings is in Langmuir-type adsorption. The Si adsorption saturated at the surface with (3×2) reconstruction having the Si coverage of 5/3 monolayer. No Si island was observed on the saturated (3×2) surface. We discuss the Si adsorption mechanism in terms of the structural phase transition.

Work function measurements of Gd/W(111) with and without silicon interface layers: field emission study

The adsorption of gadolinium on the (111) plane of tungsten field emitters with and without silicon precovered layers has been investigated. Gd and Si depositions and work function measurements were made at the room substrate temperature. The “thickness” of silicon interface layers has ranged from a fraction of a monolayer (ML) to 1ML, whereas Gd was condensed up to the stabilization of the adsystem work function. With the silicon precoverage increasing (from 0 to 1ML), the dependence of work function on the Gd concentration was modified from the pronounced non-monotonic dependence to a monotonic one, the adsystem final work function was increased from 2.6eV to 3.05eV, and the Gd film growth mode was changed from the layer-by-layer mode (the Frank–van der Merwe mechanism) to the three-dimensional (3D) cluster formation (the Volmer–Weber one). In addition, it was observed that the monolayer coverage of Si effectively passivates the W(111) surface. The mixing and the chemical reaction between the coadsorbed Si and Gd atoms were not found. In all cases of Gd and Si atom adsorption, as well as of their coadsorption, the changes in Fowler–Nordheim pre-exponential with coverage were calculated and their behaviour was discussed.

Adsorption of pentamethylcyclopentasiloxane on the Pd(100) surface studied by high resolution X-ray photoemission spectroscopy

Adsorption and reaction of pentamethylcyclopentasiloxane (PMPS) on a clean Pd(100) surface were investigated with the goal to determine whether anchoring of siloxane compounds on this metal occurs via strong dissociative chemisorption and whether siloxane monolayers can be formed. The room temperature exposure resulted in disordered, partially dissociated PMPS while retaining the Si–O backbone bonds, as indicated by high resolution X-ray photoemission spectroscopy (HRXPS). The quantitative HRXPS analysis showed that the surface Si coverage corresponded to 0.74 ML and that the PMPS adsorbed “flat” with partially broken Si–CH 3 bonds serving as anchoring points. Subsequent exposure of the PMPS-covered Pd surface to CO and O 2 showed no adsorption of these molecules, demonstrating that the adsorbed PMPS completely blocked the metal surface. However, exposure to Cl 2 gave rise to two distinct Cl 2p peaks in the HRXPS spectrum: a main component at 199.73 eV and a much smaller peak shifted from the main peak by ΔBE=−2.11 eV, corresponding to Cl atoms covalently bonded to PMPS and atomic Cl bonded to the Pd metal respectively. Thus, Cl 2 reacts with PMPS and the Cl atoms gain access to the Pd surface by rearranging the Cl/PMPS overlayer.

Band discontinuity fluctuations and local chemistry at the GaSe–Si interface

Photoemission spectromicroscopy data on Si–GaSe with 0.2 μm lateral resolution revealed laterally inhomogeneous interface Se–Si chemical reactions. The local Si 2p fits agrees with interface Si selenide parameters; the Se 3d peaks indicate a Si–selenide component; the Ga 3d peaks exhibit a metallic component whose intensity increases with Si coverage. This local interface reaction is related to lateral changes in the band lineup.

Submonolayer Si deposition at low temperatures on the GaAs(001)-(2×4) surface studied by scanning tunneling microscopy

Si deposition at low substrate temperatures (240 and 360°C) on the GaAs(001)-(2×4) surface has been studied by scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). The Si was deposited in the absence of a direct As 4 flux up to a coverage of 0.8 monolayer (ML). The STM images reveal that the preferential Si incorporation site lies in the trenches between the As dimer rows of the (2×4) reconstruction. Here, it substitutes the missing Ga and is subsequently covered with As from the residual As 4 background to result in fully As-terminated areas. Above 0.4 ML, Si dimer rows form on top of this As. The RHEED patterns continuously change with increasing Si coverage from the initial (2×4) to an asymmetric (3×1) pattern. Annealing at 450°C results in a (2×4) reconstructed surface with disorder and kinks in the As dimer rows.

Si adatom surface migration biasing by elastic strain gradients during capping of Ge or Si1−xGex hut islands

Hut cluster formation during Ge or Si1−xGex solid source molecular beam epitaxial growth on Si(001) is a well-known kinetic pathway for partial strain relief. It results in undulated morphologies with {105} facets allowing a∥ lattice parameter relaxation on the island apexes. Here, we show how subsequent Si coverages, grown at 500 °C, avoid being tensile strained and impede further increase of stored elastic strain energy. Dominant inhomogeneous Si surface diffusions take place as proven by a Ge marker technique able to provide transmission electron microscopy or high-resolution transmission electron microscopy images of the initial Si morphology stages and by reflection high-energy electron diffraction examinations. This mechanism prevails for high enough Si growth rates, able to quench lateral Ge diffusion and limit chemical strain relief. Mediated by stress variations on the noncapped island curvatures, Si is depleted from the top of the islands and accumulates in the troughs of the ripples where it accommodates mostly unstrained. By this selective Si coverage, the surface undergoes a rapid smoothing and a∥ recovery toward the Si bulk value. When the Ge containing islands are completely buried, their strain, dictated by the Si buffer and cap layers, ends by being mainly along the growth direction or tetragonal (Δa∥=0).