Dimer Vacancy Lines Research Articles

Toward the III–V/Si co-integration by controlling the biatomic steps on hydrogenated Si(001)

The integration of III-V on silicon is still a hot topic as it will open up a way to co-integrate Si CMOS logic with photonic devices. To reach this aim, several hurdles should be solved, and more particularly the generation of antiphase boundaries (APBs) at the III-V/Si(001) interface. Density functional theory (DFT) has been used to demonstrate the existence of a double-layer steps on nominal Si(001) which is formed during annealing under proper hydrogen chemical potential. This phenomenon could be explained by the formation of dimer vacancy lines which could be responsible for the preferential and selective etching of one type of step leading to the double step surface creation. To check this hypothesis, different experiments have been carried in an industrial 300 mm metalorganic chemical vapor deposition where the total pressure during the annealing step of Si(001) surface has been varied. Under optimized conditions, an APBs-free GaAs layer was grown on a nominal Si(001) surface paving the way for III–V integration on silicon industrial platform.

Evidence of Low Schottky Barrier Effects and the Role of Gap States in the Electronic Transport through Individual CoSi2 Silicide Nanoislands at Low Temperature (9 K)

In this paper, we study the electronic properties of CoSi2 metallic islands grown on a Si(100) surface with a low-temperature (9 K) scanning tunneling microscope (STM). The atomic scale structures of the flat and ridge silicide islands surfaces are described with an ultimate resolution, thanks to the stability of low-temperature STM. A statistical study of the I–V and dI/dV signals acquired along the islands shows their metallic-like properties and a very small residual conduction band gap of ∼30 mV. This reveals that the electronic transport through the individual metallic islands at the silicide–silicon interface is mainly ruled by electronic tunnel processes for positive sample biases and driven by the presence of gap states for negative sample voltage. The role of the gap states is demonstrated by performing conductance measurements along the dimer vacancy lines in which interstitial Co atoms are accessible at the silicon surface. Hence, the electronic transport that occurs from the silicide–silicon interface toward the macroscopic contact of the sample can be explained.

Strain effects and intermixing at the Si surface: Importance of long-range elastic corrections in first-principles calculations

We investigate Ge mixing at the Si(001) surface and characterize the $2\times N$ Si(001) reconstruction by means of hybrid quantum and molecular mechanics calculations (QM/MM). Avoiding fake elastic dampening, this scheme allows to correctly take into account long range deformation induced by reconstruted and defective surfaces. We focus in particular on the dimer vacancy line (DVL) and its interaction with Ge adatoms. We first show that calculated formation energies for these defects are highly dependent on the choice of chemical potential and that the latter must be chosen carefully. Characterizing the effect of the DVL on the deformation field, we also find that the DVL favors Ge segregation in the fourth layer close to the DVL. Using the activation-relaxation technique (ART nouveau) and QM/MM, we show that a complex diffusion path permits the substitution of the Ge atom in the fourth layer, with barriers compatible with mixing observed at intermediate temperature.

Cobalt adsorption on the bare Si(100) 2 × 1 surface at low temperature (12 K)

The adsorption of Co atoms on the bare Si(100)-2×1 surface at low temperature (12K) is performed in the low coverage regime (< 0.1 monolayer). The ensuing surface is studied by means of a low temperature (9K) scanning tunneling microscope (STM). Several adsorption sites are described via STM topographies and differential conductance spectroscopy. Our study reveals that at low temperature (12K), a significant fraction of the Co atoms diffuse into the surface and form the first stage reaction sites that are relevant for the silicidation mechanism of the Si(100). Furthermore, the low temperature conditions allow to describe surface Co adatoms conformations that are stabilized on the Si(100)-c(4×2) surface. Interestingly, we have observed the irreversible transformation of the symmetric pedestal site (Ps) to the under dimer site (UD) via specific STM scanning conditions. Finally, the presence of dimer vacancy lines is discussed and reveals that the activated etching process of the silicon dimer with metals can be induced at very low temperature.

Atomic structure and composition of the2×Nreconstruction of the Ge wetting layer on Si(001) investigated by surface x-ray diffraction

The 2 \ifmmode\times\else\texttimes\fi{} $N$ reconstruction of the Ge/Si(001) wetting layer has been investigated by surface x-ray diffraction. At a substrate temperature of $670 {}^{ˆ}\mathrm{C}$, the average $N$ periodicity decreases from $N$ $=$ 11.5 to 8 with an increasing Ge coverage from one to three monolayers (ML). The top layer consists of asymmetric dimers with a bond length in the range of 2.50--2.60 \AA{} and a buckling angle in the range of 9.4\ifmmode^\circ\else\textdegree\fi{}--15.6\ifmmode^\circ\else\textdegree\fi{}, depending on the Ge coverage. The obtained dimer bond lengths are similar to those calculated for alternating asymmetric mixed dimers. Intermixing of Ge with Si is found down to the sixth (eighth) layer for 2 (from 3 to 5) ML coverage. For 2 ML coverage, a quantitative surface x-ray diffraction data set has been measured. It is analyzed using a model describing the atomic structure and Ge occupation probability with a limited set of parameters to bypass the intrinsic lack of appreciable reflections of the 2 \ifmmode\times\else\texttimes\fi{} $N$ ($N$ $=$ 9) reconstruction. The Ge occupation probability varies periodically along the $N$ direction, having its minimum value below the dimer vacancy lines. In addition, a more direct calculation of the Lorentz and detector acceptance corrections is given for rocking and radial scans.

Manganese Nanostructures on Si(100)(2 × 1) Surfaces: Temperature-Driven Transition from Wires to Silicides

The Si(100)(2 × 1) surface serves as a template for the formation of Mn wires at room temperature, which are the starting point for the annealing experiments discussed here. The evolution of the Mn surface structures as a function of annealing temperature was observed with scanning tunneling microscopy for the temperature range between 115 and 600 °C and establishes a surface phase diagram for the Mn−Si(100)(2 × 1) system. The stability of the Mn nanowires is limited; they break up below 115 °C (the lowest annealing temperature studied), and ultrasmall clusters are formed. These clusters are initially positioned on the terraces but migrate to the step edges at around 250 °C, which sets the limit for the Mn surface diffusion length. At around 300 °C the Mn adatoms move into subsurface sites, and the empty and filled states images strongly indicate that Mn acts as an acceptor in this near-surface region. A further increase in temperature leads to the formation of large crystallites (several tens of nanometers), which exhibit the characteristic shape associated with Mn silicides. The modification of the Si surface with temperature is characterized by the dramatic increase in the defect population (defect density and size and step edge roughness). The condensation of dimer vacancies begins around 200 °C and progresses to the formation of long dimer vacancy lines at elevated temperatures. The step edge roughness and the step edge formation energies were calculated for SA and SB steps, and their modulation with annealing temperature illustrates the impact of Mn on the defect and kink stabilities. These data will be used to perform a thermodynamic and kinetic modeling of defect population in the presence of Mn at moderate substrate temperatures. This study presents a surface phase diagram, which includes the evolution of the Mn nanostructures and the modification of the Si surface. The identification of near-surface layers of Mn acceptors is particularly relevant for the design of basic building blocks for future Si-based spintronics.

Study of Ga Adsorption Structure on Ni/Si(100) Surface by Scanning Tunneling Microscopy

The Ga adsorption structures on the Ni-induced Si(100) surface have been studied by scanning tunneling microscopy. The protrusions, which are different from Ga ad-dimer, are observed on the dimer vacancy lines (DVLs) after the Ga ad-dimers cover the Si region. The amount of protrusions increases as Ga coverage increases and these protrusions occupy the DVLs at about 0.50 monolayer. The type of protrusions on the DVLs is similar to the precursor state of the Ga clusters. Islands of different sizes are formed locally on the steps and terraces as coverage increases. Small islands are distributed evenly on the terraces after annealing the surface at 200 °C, whereas large islands are formed again at 400 °C. The changes in the distribution of the islands are associated with the nuclei sites on the DVLs and the transformation of the surface structure.

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S A and rebonded S B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along ( 5 ¯ 0 1 ) direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and [ 5 ¯ 0 1 ] directions.

Surface etching induced by Ce silicide formation on Si(1 0 0)

We investigate the temperature-dependent surface etching process induced by Ce silicide on Si(1 0 0) using scanning tunneling microscopy and X-ray photoelectron spectroscopy. We found that step edges on the Si(1 0 0) surface are gradually roughened due to the formation of Ce silicide as a function of substrate temperature. Unlike the Si(1 1 1) surface, however, terrace etching also occurs in addition to step roughening at 500 °C. Moreover, we found that Si(1 0 0) dimers are released and formed dimer vacancy lines because bulk diffusion of Ce silicide into Si(1 0 0) surface occurs the defect-induced strain at higher temperature (∼600 °C).

Selforganized nanopatterning of Si(0 0 1)

The (2 × n) superstructure of Si(0 0 1) consists of elongated (2 × 1) reconstructed stripes separated by a dimer-vacancy line every few nanometers, thus offering a means to obtain a nanopattered Si(0 0 1) surface. Scanning tunneling microscopy (STM) investigations of Si(0 0 1) substrates that were deoxidized at 880–920 °C reveal that the formation of the (2 × n) depends strongly on the Si coverage of the topmost surface layer. It forms only in a narrow coverage window ranging from 0.6 to 0.8 ML. Systematic Monte Carlo simulations by an algorithm that combines the diffusion of monomers and dimers with the simultaneous deposition of Si onto the Si(0 0 1) surface, corroborate the STM results and suggest Si deposition as a viable alternative for introducing dimer vacancies in a well-defined manner.

Orientation Dependence of Strained-Ge Surface Energies near (001): Role of Dimer-Vacancy Lines and Their Interactions with Steps

Recent experiments and calculations have highlighted the important role of surface-energy (gamma) anisotropy in governing island formation in the Ge/Si(001) system. To further elucidate the factors determining this anisotropy, we perform atomistic and continuum calculations of the orientation dependence of gamma for strained-Ge surfaces near (001), accounting for the presence of dimer-vacancy lines (DVLs). The net effect of DVLs is found to be a substantial reduction in the magnitude of the slope of gamma vs orientation angle, relative to the highly negative value derived for non-DVL, dimer-reconstructed, strained-Ge(001) surfaces. The present results thus point to an important role of DVLs in stabilizing the (001) surface orientation of a strained-Ge wetting layer.

Dynamic Ad-Dimer Twisting Assisted Nanowire Self-Assembly on Si(001)

Based on ab initio total energy calculation, we show that a dynamic ad-dimer twisting assisted (DATA) process plays a crucial role in facilitating a novel structural reconstruction involving surface and subsurface atoms on Si(001). It leads to self-assembly of long nanowires of group-V elements (Bi, Sb) in the trenches of surface dimer vacancy lines (DVLs) with a characteristic double-dimer configuration. The key to this is the lowering of the kinetic barrier by the DATA process in conjunction with a favorable interaction between ad-dimers and step edges in DVLs. The present results provide an excellent account for experimental observations and reveal the atomistic origin and the dynamic transformation path for nanowire self-assembly on Si(001).

Crossover energetics for halogenated Si(100): Vacancy line defects, dimer vacancy lines, and atom vacancy lines

Crossover energetics for halogenated Si(100): Vacancy line defects, dimer vacancy lines, and atom vacancy lines

Surface energetics and structure of the Ge wetting layer on Si(100)

Ge deposited on Si(100) initially forms heteroepitaxial layers, which grow to a critical thickness of $\ensuremath{\sim}3$ MLs before the appearance of three-dimensional strain relieving structures. Experimental observations reveal that the surface structure of this Ge wetting layer is a dimer vacancy line (DVL) superstructure of the unstrained Ge(100) dimer reconstruction. In the following, the results of first-principles calculations of the thickness dependence of the wetting layer surface excess energy for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $4\ifmmode\times\else\texttimes\fi{}6$ DVL surface reconstructions are reported. These results predict a wetting layer critical thickness of $\ensuremath{\sim}3$ MLs, which is largely unaffected by the presence of dimer vacancy lines. The $4\ifmmode\times\else\texttimes\fi{}6$ DVL reconstruction is found to be thermodynamically stable with respect to the $c(4\ifmmode\times\else\texttimes\fi{}2)$ structure for wetting layers at least 2 ML thick. A strong correlation between the fraction of total surface induced deformation present in the substrate and the thickness dependence of wetting layer surface energy is also shown.

Confinement of one-dimensional electrons in dimer row segments at Ge/Si(001) surfaces

During the initial growth, Si(001) surfaces were covered by thin Ge wetting layers. At the wetting layer surfaces, Ge atoms were reconstructed into dimers, which were aligned along the [110] direction to form one-dimensional (1D) ‘dimer rows’. In addition, the dimer rows were partitioned by dimer vacancy lines at the Ge/Si(001) surfaces. As a result, the surfaces were covered spontaneously by an array of 1D dimer row segments. Scanning tunneling microscope images demonstrated that the 1D free-electrons in the dimer rows are confined in each segment and form an ultra-high density array of 1D quantum wells at the surfaces.

Initial surface roughening in Ge/Si(001) heteroepitaxy driven by step-vacancy line interaction.

The initial surface roughening during Ge epitaxy on Si(001) is shown to arise from an effective repulsion between S(A) surface steps and dimer vacancy lines (VLs). This step-VL interaction gradually inactivates a substantial fraction of adatom attachment sites at the growth front, causing a rapid increase in the rate of two-dimensional island nucleation. The mutual repulsion hinders the crossing of S(A) surface steps over VLs in the second layer, thus organizing the developing surface roughness into a periodic array of anisotropic 2D terraces. Isolated (105) facets forming at specific sites on this ordered template mediate the assembly of first 3D Ge islands.

Dynamics of surface roughening of Cl-terminated Si(100)-(2×1) at 700 K

The dynamics of surface roughening and healing induced by Cl on Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) were studied with variable-temperature scanning tunneling microscopy. Clean samples were exposed to ${\mathrm{Cl}}_{2}$ at room temperature, heated to as high as 700 K, and imaged for periods that exceeded 20 h. Chlorine caused surface roughening via a Cl recycling pathway that created pits and regrowth islands with minimal desorption of ${\mathrm{SiCl}}_{2}.$ This reaction pathway is accessible at $\ensuremath{\sim}675\mathrm{K}$ if the surface is not saturated with Cl. Images showed dimer vacancy creation and pit growth, together with vacancy capture by pits. They also showed dimer vacancy escape from pits and pit annihilation, which had not been reported previously. When the terrace coverage was 0.93 ML, the diffusivity of dimer vacancies along the dimer rows was $\ensuremath{\sim}0.7{\mathrm{\AA{}}}^{2}/\mathrm{sec},$ about three orders-of-magnitude lower than that on clean Si(100). Single Si adatoms were created on terraces and bonded to Si dimers that were Cl-free. They could form new regrowth dimers or they could be accommodated at the ends of existing islands. Adatoms could also be released from regrowth islands. Pairs of atom vacancy lines separated by one or two Si dimers were observed and the transition between those lines and dimer vacancy lines was recorded. Local areas with (3\ifmmode\times\else\texttimes\fi{}2) symmetry were created from the latter. These dynamic changes all require Cl-free Si dimers, and the rate of change of the surface morphology reflects the surface concentration. The rate of change increases with time as the density of bare dimers increases due to the creation of new sites for Cl on an increasingly rough surface and limited desorption.

Dimers at Ge/Si(001) surfaces: Ge coverage dependent quenching, reactivation of flip-flop motion, and interaction with dimer vacancy lines

We studied Ge coverage $({\ensuremath{\theta}}_{\mathrm{Ge}})$ dependent quenching, reactivation of the flip-flop motion, and interaction with dimer vacancy lines (DVLs) of dimers on Ge/Si(001) surfaces using a scanning tunneling microscope (STM) combined with a molecular beam epitaxy apparatus. Deposition of $\ensuremath{\sim}0.3\mathrm{ML}$ (monolayer) Ge quenched the flip-flop motion, making all dimers asymmetric. Further deposition introduced DVLs at ${\ensuremath{\theta}}_{\mathrm{Ge}}&gt;~\ensuremath{\sim}0.5\mathrm{ML},$ and symmetric dimer domains appeared again locally at $\ensuremath{\theta}&gt;~1.5\mathrm{ML}.$ High-resolution STM images indicated that asymmetric dimer rows always invert their phase in alternation with buckled dimer's up-end at the DVLs. Low-temperature STM images indicated that the symmetric dimer domains were due to flip-flopping of asymmetric dimers activated by large ${\ensuremath{\theta}}_{\mathrm{Ge}}$ at room temperature. The symmetric dimer domains extended along the dimer rows over the DVLs due to the phase correlation.

Distribution of dimer vacancy lines on Si(001)

We report an extensive energetics study of dimer vacancy lines (DVLs) on Si(001), using a highly accurate tight-binding total-energy approach. An orientational transition of DVLs from perpendicular to parallel to surface dimer rows occurs over a range of surface dimer vacancy density N DV=0.2–0.3. Below N DV=0.25, perpendicular DVLs with maximum widths are preferred due to the rebonding of excessive dangling bonds at the nonbonded edges of neighboring vacancy lines. At higher N DV, single-dimer wide parallel DVLs are favored but are nearly degenerate with multiple wider parallel DVLs. These results are in quantitative agreement with experiment and provide a comprehensive understanding for the distribution of DVLs on Si(001).

Acetylene molecules on the Si( [formula omitted]) surface: room-temperature adsorption and structural modification upon annealing

Acetylene molecules adsorbed on the Si(0 0 1) surface are studied by scanning tunneling microscope and ab initio pseudopotential calculations. In case of room-temperature adsorption, there are basically two different kinds of di-σ bonding geometries at low coverage: (i) on-top and (ii) end-bridge di-σ configurations. The tetra-σ configurations are not found. Theoretical calculations show that the paired end-bridge configuration is dominant at high coverage. Based on these findings, we discuss the results at saturation coverage in the existing literature consistently. Subsequent annealing of the C 2H 2/Si(0 0 1) surface induces different C-induced surface structures depending on the acetylene coverage. At low coverage, parallel lines of dimer vacancies are observed. As the amount of the adsorbed acetylene increases, the c(4×4) ordered phase replaces the dimer vacancy line phase. Near saturation, clusters are formed and the surface becomes rough.