Simulations of germanium epitaxial growth on the silicon (1 0 0) surface incorporating intermixing

We present kinetic lattice Monte Carlo simulations of Ge deposition onto a reconstructed Si (1 0 0) surface. We account for the exchange of Ge with Si atoms in the substrate, considering two different exchange mechanisms: a dimer exchange mechanism whereby Ge–Ge dimers on the surface become intermixed with substrate Si atoms, and the exchange of Ge atoms below the surface to relieve misfit strain. We examine how Si–Ge exchange affects the interface between the materials when the growth simulations are done at different temperatures.     

Defect-induced dimer pinning on the Si(0 0 1) surface

Room-temperature STM images frequently show regions of antisymmetric dimer ordering surrounding certain types of defect on the Si(0 0 1) surface. While it has been generally believed that any defect asymmetric with respect to the dimer row would induce this dimer pinning effect, recent experimental results have shown that this is not the case. We present a model, based on a nearest-neighbour Ising treatment of the surface, which allows the extent of pinning caused by a dimer to be predicted from ab initio calculations. We use this model to predict the pinning extent for three phosphorus-containing structures important in a proposed silicon-based quantum computer fabrication scheme, and identify one of these asymmetric features as causing no appreciable pinning. In addition, we use ab initio calculations to identify the effects governing the interaction between neighbouring dimers.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Towards hybrid silicon-organic molecular electronics: The stability of acetone on the Si(0 0 1) surface

We present the results of a combined study using scanning tunneling microscopy (STM) and density functional theory (DFT) of the interaction of acetone [(CH₃)₂CO] with the Si(0 0 1) surface. Three distinct adsorbate features were observed using atomic-resolution STM. One of the features appears as a bright protrusion located above a Si-Si dimer, while the other two are asymmetric about the dimer row and involve a second neighboring Si-Si dimer. One of the two asymmetric features has a protrusion located between the two dimers, while the other has a protrusion which is located at the site of a single dimer and exhibits a dimer sized depression on the adjacent dimer. DFT calculations have been performed for two structures; the four-membered ring structure and dissociation structure. Our calculations show that the bright single-dimer sized feature observed in the STM images could be attributed to either of these two calculated structures. However, neither of the two calculated structures can explain the appearance of the two-dimer wide asymmetric features observed in the experiment.     

STM study of di-indenoperylene molecules on Cu(1 0 0) surfaces: Mobility, stability and epitaxy

We report a detailed study of the self-assembly process of di-indenoperylene (DIP) on Cu(1 0 0) surfaces, investigated by variable temperature scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. During the initial growth stages a preferred nucleation at the step edges is revealed. Subsequently, large ordered 2D islands grow from the step edges, while smaller 2D islands form on the terrace sites. The equilibrium fluctuation of these 2D clusters has been monitored in real-time, thereby obtaining a direct insight into the temperature dependence of the molecular surface diffusion. The substrate–adsorbate interactions determine the azimuthal molecular orientation, finally lead to the formation of highly ordered chiral domains which are commensurate with the substrate.     

Electron-induced attachment of chlorinated benzenes to Si(1 0 0)2 × 1

Thermal (300 K) and electron-induced reactions of benzene (Bz), chlorobenzene (ClPh), 1,2-dichlorobenzene (1,2-diClPh) and 1,4-dichlorobenzene (1,4-diClPh) with Si(1 0 0)2 × 1 have been examined by scanning tunneling microscopy (STM). Thermal reactions of Bz yielded predominantly the quadruply-σ-bound tight bridge, TB, configuration on top of the Si dimer-rows, For ClPh and 1,2-diClPh, which resembled one another, thermal reaction led with 45-50% yield to the doubly-σ-bound butterfly, BF, configuration, also on top of the dimer-row, and with 20% yield to a novel ‘displaced’, D, configuration to one side of a dimer-row. The adsorbate 1,4-diClPh was alone in favouring a configuration in which neighbouring dimer-rows were ‘linked’ (L) by a bright-feature centrally located between the dimer-rows. By ab initio calculation, we interpret D as due to the rupture of one C-Cl bond per adsorbate molecule, and L to the rupture of two C-Cl’s. The breaking of this weak bond is followed in the former case by attachment of the aromatic ring to one dimer-row, and in the latter to attachment to two adjacent dimer-rows. Application of a −5 V voltage pulse to the STM tip substantially increased the percentage of row-linking structures, L, for 1,4-diClPh, but neither −5 V nor +4-6 V volt pulses resulted in L-type binding of Bz. The postulated L product of 1,4-diClPh, with an aromatic ring linking the two inner Si atoms of adjacent dimer-rows and the two Cl’s on the outer Si atoms of the dimer-rows, is shown to be in accord with ab initio simulation of the observed STM image.     

Irreversible structural transformation of Si(1 1 4)-2 × 1 induced by subsurface carbon

Using scanning tunneling microscopy (STM), it has been found that the reconstruction of Si(1 1 4) is transformed irreversibly from a 2 × 1 structure composed of dimer (D), rebonded atom (R), and tetramer (T) rows (phase A) to a different kind of 2 × 1 structure composed of D, T, and T rows (phase B) by C incorporation. It has been confirmed by high-resolution synchrotron core-level photoemission spectroscopy (PES) that such an irreversible structural transformation is due to stable subsurface C atoms. They induce anisotropic compressive stress on the surface, which results in insertion of Si dimers to an R row to form a T row.     

Formation of a √3 × √3 surface on Si/Ge(1 1 1) studied by STM and LEED

We have performed a detailed study of the formation and the atomic structure of a √3 × √3 surface on Si/Ge(1 1 1) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a √3 × √3 periodicity but unlike the Sn/Ge(1 1 1) and the Sn/Si(1 1 1) surfaces, the Si/Ge(1 1 1) surface is not well ordered. There is no long range order on the surface and the √3 × √3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.     

Phosphorus and hydrogen atoms on the (0 0 1) surface of silicon: A comparative scanning tunnelling microscopy study of surface species with a single dangling bond

We present a comparative scanning tunnelling microscopy (STM) study of two features on the Si(0 0 1) surface with a single dangling bond. One feature is the Si-P heterodimer—a single surface phosphorus atom substituted for one Si atom of a Si-Si dimer. The other feature is the Si-Si-H hemihydride—a single hydrogen atom adsorbed to one Si atom of a Si-Si dimer. Previous STM studies of both surface species have reported a nearly identical appearance in STM which has hampered an experimental distinction between them to date. Using voltage-dependent STM we are able to distinguish and identify both heterodimer and hemihydride on the Si(0 0 1) surface. This work is particularly relevant for the fabrication of atomic-scale Si:P devices by STM lithography on the hydrogen terminated Si(0 0 1):H surface, where it is important to monitor the distribution of single P dopants in the surface. Based on the experimental identification, we study the lateral P diffusion out of nanoscale reservoirs prepared by STM lithography.     

(6 × 2) Reconstruction of the Ag/Si(1 1 1) surface at 77 K

The ground state of the Ag/Si(1 1 1)-(3 × 1) has been investigated by low temperature scanning tunneling microscopy (STM) and density-functional theory. The Fourier transform of the STM image reveals a (6 × 2) reconstruction, which is theoretically found to yield a reconstruction with lower energy than the (3 × 1). The most stable (6 × 2) structural model leads to excellent correspondence between experimental and simulated STM images, and reveals a dimerization of the silver atoms in the channels formed by neighbouring honeycomb Si chains.     

Point defects along metallic atomic wires on vicinal Si surfaces: Si(5 5 7)–Au and Si(5 5 3)–Au

Point defects on the metallic atomic wires induced by Au adsorbates on vicinal Si surfaces were investigated using scanning tunneling microscopy and spectroscopy (STM and STS). High-resolution STM images revealed that there exist several different types of defects on the Si(5 5 7)–Au surface, which are categorized by their apparent bias-dependent images and compared to the previous report on Si(5 5 3)–Au [Phys. Rev. B (2007) 205325]. The chemical characteristics of these defects were investigated by monitoring them upon the variation of the Au coverage and the adsorption of water molecules. The chemical origins and the tentative atomic structures of the defects are suggested as Si adatoms (and dimers) in different registries, the Au deficiency on terraces, and water molecules adsorbed dissociatively on step edges, respectively. STS measurements disclosed the electronic property of the majority kinds of defects on both Si(5 5 7)–Au and Si(5 5 3)–Au surfaces. In particular, the dominating water-induced defects on both surfaces induce a substantial band gap of about 0.5 eV in clear contrast to Si adatom-type defects. The conduction channels along the metallic step-edge chains thus must be very susceptible to the contamination through the electronic termination by the water adsorption.     

Addimer chain structures: Metastable precursors to island formation on Ge-Si(0 0 1)-(2 × n) alloyed surface

We have identified addimer chain structures as metastable precursors to compact epitaxial islands on the (2 × n) reconstructed SiGe wetting layer, using polarity-switching scanning tunneling microscopy (STM). These chain structures are comprised of 2-12 addimers residing in the troughs of neighboring substrate dimer rows. The chain structures extend along equivalent 〈1 3 0〉 directions across the substrate dimer rows in a zigzag fashion, giving rise to kinked and straight segments. We measure a kink-to-straight ratio of nearly 2:1. This ratio corresponds to a free energy difference of 17 ± 4 meV, favoring the formation of kinked segments. The chain structures convert to compact epitaxial islands at elevated temperatures (?90 °C). This conversion suggests that the chain structures are a precursor for compact island formation on the SiGe wetting layer. We digitally process filled- and empty-state STM images to distinguish chain structures from compact islands. By monitoring the populations of both species over time, the chain-to-island conversion rates are measured at substrate temperatures ranging from 90 to 150 °C. The activation energy for the conversion process is measured to be 0.7 ± 0.2 eV with a corresponding pre-exponential factor of 5 × 10^4±2 s⁻¹.     

GaAs(0 0 1) (2 × 4) to c(4 × 4) transformation observed in situ by STM during As flux irradiation

Atomic resolution scanning tunnelling microscopy (STM) has been used to study in situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As₄ flux. The relationship between the As-rich (2 × 4) and c(4 × 4) surfaces is observed throughout the gradual evolution of the reconstruction transformation. The results suggest that during the initial stage of the transformation, Ga-rich As-terminated variations of the c(4 × 4) form in order to accommodate excess mobile Ga produced by pit formation. These transient structures later planarize, as excess Ga is incorporated at step/island edges. Successive imaging of the same sample area during As₄ irradiation allows point-by-point adatom binding to be analysed in a way inaccessible to MBE–STM systems relying on sample quenching and transfer.     

Tetra-σ bonding adsorption of pyridine on Si(1 0 0)-2 × 1

We studied adsorption of pyridine on Si(1 0 0) at room temperature using high resolution photoemission spectroscopy (PES) and near edge X-ray adsorption fine structure (NEXAFS) in the partial electron yield (PEY) mode. The Si 2p, C 1s, N 1s spectra of pyridine on Si(1 0 0) showed that pyridine is chemisorbed on Si(1 0 0)-2 × 1 through the formation of the tetra-σ-bonded structure with the N atom and three C atoms. NEXAFS was conducted to characterize the adsorption geometry of pyridine on Si(1 0 0). The π* orbital of CC bond showed a good angle dependence in C K-edge NEXAFS spectra, and we were able to estimate the adsorption angle between chemisorbed pyridine of CC bond and the Si(1 0 0) surface using an analytical solution of NEXAFS intensity. We find the coexistence of two different tight bridges with the adsorption angles 42 ± 2° and 45 ± 2° with almost equal abundance.     

Electron-induced H atom desorption patterns created with a scanning tunneling microscope: Implications for controlled atomic-scale patterning on H-Si(1 0 0)

We have used scanning tunneling microscopy (STM) to explore the details of single and multiple H atom desorption from the H-Si(1 0 0)-2 × 1 surface induced by the inelastic scattering of electrons from an STM tip. The desorption of pairs of H atoms from individual Si dimers is rarely observed. Two-H atom desorption most often involves pairs of dimers, in the same or adjacent rows. This suggests that recombinative H₂ desorption via an interdimer reaction pathway, like that observed recently under nanosecond laser heating, may also be operative for electron-induced excitation using STM. Repeatable fabrication of desired size-selected dangling bond (DB) clusters is also achieved. The single atomic precision of the fabrication is a result of the intrinsically unfavorable paired H atom desorption from a single dimer, but does not result from the spatial localization of excitation energy of the Si-H bond under the STM tip as suggested in previous studies.     

Reaction of chlorinated benzenes with Si(1 0 0)2 × 1: a theoretical study

Theoretical (HF + DFT) investigations of the adsorption of chlorobenzene (ClPh), 1,2- and 1,4-dichlorobenzene (1,2-diClPh and 1,4-diClPh) on a silicon (1 0 0) surface are reported for the first time, and are compared with one another and with benzene. Binding energies for various structures with the molecules attached on-top and in-between the surface dimer rows are correlated with the STM experimental data. Novel structures with the molecules linking two dimer rows, stabilised by detachment of Cl (or H)-atoms forming Cl-Si (or H-Si) bonds, are described. For 1,4 and 1,2 binding, these linking structures are predicted to attach the phenyl ring parallel or perpendicular to the Si surface, respectively, while preserving its aromaticity. The potential-energy barriers between several different structures are evaluated, and compared with available experimental evidence. For 1,4-diClPh it is shown that the potential-energy barrier for the second Cl transfer is significantly lower than for the first one in contrast to the gas-phase, and comparable to the barrier for lifting the Bz-ring into a vertical position and forming a singly bonded ‘displaced’ structure. The predicted barrier-heights are consistent with the experimentally observed relative occurrence of the on-top, linking, and displaced structures.     

Mixed PbSi dimer chains on Si(1 0 0): a first-principles study

It has been a common belief that the one-dimensional structures observed by STM at low coverage of Pb on Si(1 0 0) are buckled Pb-Pb dimer chains. However, using first-principles density functional calculations, we found that it is energetically favorable for Pb adatoms to intermix with Si atoms to form mixed dimer chains on Si(1 0 0), instead of Pb-Pb dimer chains as assumed in previous studies. Up to a Pb coverage of 0.125 ML, mixed PbSi dimer chain is 0.19 eV per Pb atom lower in energy than Pb dimer chain.     

The effect of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface

We have studied the effects of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface. The surface was exposed to various doses of Ar⁺ ions accelerated towards the surface at 500 eV. X-ray photoelectron spectroscopy (XPS) spectra of the irradiated H-terminated Si(0 0 1) surface reveal the appearance of peaks that are associated with the presence of cleaved Si bonds. Ultraviolet photoelectron spectroscopy (UPS) spectra of the irradiated Si(0 0 1)2 × 1 surface show that the dimer dangling-bond surface state decays monotonically with increasing dose. These results, coupled with previous scanning tunneling microscopy (STM) studies, indicate that the breaking of dimers, and possibly the creation of adatom-like defects, during ion irradiation are responsible for the changes in the electronic structure of the valence band for this surface.     

Atomic surface structure of Si(1 0 0) substrates prepared in a chemical vapor environment

Subsequent III-V integration by metal-organic vapor phase epitaxy (MOVPE) or chemical vapor deposition (CVD) necessitates elaborate preparation of Si(1 0 0) substrates in chemical vapor environments characterized by the presence of hydrogen used as process gas and of various precursor molecules. The atomic structure of Si(1 0 0) surfaces prepared in a MOVPE reactor was investigated by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) available through a dedicated, contamination-free sample transfer to ultra high vacuum (UHV). Since the substrate misorientation has a fundamental impact on the atomic surface structure, we selected a representative set consisting of Si(1 0 0) with 0.1°, 2° and 6° off-cut in [0 1 1] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(1 0 0) surfaces indicated two-domain (2 × 1)/(1 × 2) reconstructions for lower misorientations implying a predominance of single-layer steps undesirable for subsequent III-V layers. However, double-layer steps developed on 6° misoriented Si(1 0 0) substrates, but STM also showed odd-numbered step heights and LEED confirmed the presence of minority surface reconstruction domains. Strongly depending on misorientation, the STM images revealed complex step structures correlated to the relative dimer orientation on the terraces.