Solid phase epitaxy amorphous silicon re-growth: some insight from empirical molecular dynamics simulation

The modelling of interface migration and the associated diffusion mechanisms at the nanoscale level is a challenging issue. For many technological applications ranging from nanoelectronic devices to solar cells, more knowledge of the mechanisms governing the migration of the silicon amorphous/crystalline interface and dopant diffusion during solid phase epitaxy is needed. In this work, silicon recrystallisation in the framework of solid phase epitaxy and the influence on orientation effects have been investigated at the atomic level using empirical molecular dynamics simulations. The morphology and the migration process of the interface has been observed to be highly dependent on the original inter-facial atomic structure. The [100] interface migration is a quasi-planar ideal process whereas the cases [110] and [111] are much more complex with a more diffuse interface. For [110], the interface migration corresponds to the formation and dissolution of nanofacets whereas for [111] a defective based bilayer reordering is the dominant re-growth process. The study of the interface velocity migration in the ideal case of defect free re-growth reveals no difference between [100] and [110] and a decrease by a mean factor of 1.43 for the case [111]. Finally, the influence of boron atoms in the amorphous part on the interface migration velocity is also investigated in the case of [100] orientation.     

Element specific surface reconstructions of islands during surfactant-mediated growth on Si (111)

The early stages of surfactant- (As, Sb) mediated homoepitaxial growth on Si (111) are examined by scanning tunneling microscopy and extensive ab initio calculations of Si(n) clusters (n相似文献   

Structural transformations on a platinum surface under mechanical tension

The effect of uniaxial tension on the structure of a recrystallized platinum surface is investigated using low-energy electron diffraction (LEED). The initial LEED patterns indicate that the (111) facet emerges on the surface of a platinum foil after a series of heating cycles under vacuum and in oxygen. After loading at ～80 MPa, the clean platinum surface is characterized by systems of regular and irregular atomic terraces. The regular terraces have a (9(111) × 100) structure. As the load increases to 90–100 MPa, the ordered arrangement of terraces transforms into a disordered arrangement. After the samples are held under these loads for ～2 h, the surface structure undergoes a transformation into the diffraction-disordered state. Under tensile deformation, the island structure of graphite molecules on the recrystallized platinum surface containing ～10 at. % C also undergoes a transformation.     

Orientation dependence of the surface segregation kinetics in Cu-6 at % Ge alloy

The surface segregation kinetics of germanium on the (110), (111), and (100) faces of Cu-6 at % Ge alloy has been investigated in the temperature range of 525–700 K by electron spectroscopy. In the initial annealing stage, Ge is accumulated on the surface, forming a phase similar to a supersaturated solid solution. When the Ge concentration reaches 25–40 at % (depending on the face), it forms well-ordered 2D phases, whose structure is determined by the face orientation. In the surface region the bulk diffusion parameters exhibit anisotropy; the Ge diffusion activation energy is minimum near the (110) face.     

Diffusion of silver over atomically clean silicon surfaces

The diffusion of silver the (111), (100), and (110) silicon surfaces is studied by Auger electron spectroscopy and low-energy electron diffraction. The mechanisms of diffusion over the (111) and (110) surfaces are revealed, and the temperature dependences of diffusion coefficients are measured. An anisotropy of silver diffusion over the (110) surface is detected.     

The adsorption of hydrogen on face centered tetragonal FePd surfaces

Adsorption of hydrogen on the ordered FePd face centered tetragonal alloy is investigated using a tight binding theoretical method. The changes in the electronic structure and bonding in the (100), (110) and (111) surfaces are analyzed. A simplified model for H diffusion to the bulk is also developed. The H-surface bond is achieved at the expense of the metal-metal bond being the Pd-Pd the more affected in the (100) and (110) cases.     

Direct observation of the phase transition between the (7 × 7) and (1 × 1) structures of clean (111) silicon surfaces

The phase transition process of clean (111) silicon surfaces between the (7 × 7) and (1 × 1) structures at about 830°C was directly observed by reflection electron microscopy, which had been briefly reported in a previous short communication (Osakabe et al., Japan. J. Appl. Phys, 19 (1980) L309). Smooth atomic steps, whose shapes change spontaneously and continually in a microscopic scale at high temperature of the (1 × 1) structure, transform into zig-zag steps at low temperature of the (7 × 7) structure, where the changes of the step shape stop. On cooling, domains of the (7 × 7) structure nucleate preferentially on upper terraces along the steps and expand on the terraces to the neighbouring steps. Out of phase boundaries with phase differences of 2πn/7 are seen to be formed. On heating the reversed process takes place. The out of phase boundaries are easy places to transform to the (1 × 1) structure. The observations clearly suggest the phase transition of the first order and the models of the (7 × 7) structure of ordered vacancies or adatoms rather than of static displacements of surface atoms.     

Electron confinement in surface states on a stepped gold surface revealed by angle-resolved photoemission

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width ( approximately 3.8 nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM.     

Identification of Ge/Si intermixing processes at the Bi/Ge/Si(111) surface

The chemical contrast between Si and Ge obtained by scanning tunneling microscopy on Bi-covered Si(111) surfaces is used as a tool to identify two vertical Ge/Si intermixing processes. During annealing of an initially pure Ge monolayer on Si, the intermixing is confined to the first two layers approaching a 50% Ge concentration in each layer. During epitaxial growth, a growth front induced intermixing process acting at step edges is observed. Because of the open atomic structure at the step edges, relative to the terraces, a lower activation barrier for intermixing at the step edge, compared to the terrace, is observed.     

Molecular dynamics study of anisotropic growth of silicon

Based on the Tersoff potential, molecular dynamics simulations have been performed to investigate the kinetic coefficients and growth velocities of Si(100),(110),(111), and(112) planes. The sequences of the kinetic coefficients and growth velocities are μ_((100)) μ_((110)) μ_((112)) μ_((111))and v_((100)) v_((110)) v_((112)) v_((111)), respectively, which are not consistent with the sequences of the interface energies, interplanar spacings, and melting points of the four planes. However,they agree well with the sequences of the distributions and diffusion coefficients of the melting atoms near the solid–liquid interfaces. It indicates that the atomic distributions and diffusion coefficients affected by the crystal orientations determine the anisotropic growth of silicon. The formation of stacking fault structure will further decrease the growth velocity of the Si(111) plane.     

STM study of SiC crystallite growth on vicinal Si(111) surfaces

The initial stage of epitaxial growth of cubic β-SiC on vicinal Si(111) misoriented towards the [112̄] direction is studied by scanning tunneling microscopy in ultra-high vacuum. The clean Si(111) surface contains terraces separated by groups of atomic steps. The separation between the atomic steps within a group is observed to be approximately equal to the length of the long axis of the Si(111)7 × 7 unit cell. We postulate that the SiC forms three-sided pyramids with surfaces of (110) orientation. The pyramids are located mostly at the step edges and are sharper than the end of the tip. This results in a series of identically shaped tip images located at the step edges, which display the structure of the tip.     

Surface diffusion of Pd and Au on W single crystal planes: II. Anisotropy of Pd surface diffusion due to the influence of substrate structure

Large anisotropy effects have been observed for the spreading of Pd on W single crystal planes exhibiting regular step structures. The spreading rate was studied as function of step density and step direction at temperatures around 1000 K. The terraces had always (110) orientation. Step densities as low as 3.3 × 10⁴cm^?1 corresponding to an average terrace width of 3000 Å already cause enhanced spreading rates parallel to the step direction. Steps parallel to the [001] direction showed the largest promoting action. The rate perpendicular to steps does not depend markedly on step density. The diffusion behaviour on the “step free” (110) plane turned out to be isotropic. A model is proposed which assumes the mass transport to occur in the second Pd layer. The increase of the diffusion rate along steps is correlated with the much larger density of diffusing atoms in step sites as compared to terrace sites due to the difference in binding energy.     

Chemisorption and ordering of naphthalene and azulene on Pt(s)[7(111) × (100)]: The effect of periodic defects on long range order

Naphthalene and azulene are molecularly adsorbed on the stepped platinum surface Pt(s)[7(111)×(100)] at room temperature. The (111) terraces of this surface are wide enough to accommodate the unit cell of ordered naphthalene on Pt(111). The periodic defects of this surface disrupt the long range ordering seen for both naphthalene and azulene on Pt(111). There appears to be no correlation between ordered islands on neighboring terraces. In the case of naphthalene, a LEED intensity anomaly allows us to place restrictions on the relative orientations of neighboring naphthalenes and the orientation of the naphthalene islands on each terrace. The naphthalene molecules adjacent to the steps have the long axis of the molecule close to perpendicular to the step.     

The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.     

LEED studies of clean high Miller index surfaces of germanium

Clean high Miller index Ge surfaces the poles of which lie on the sides of a unit stereographic triangle were studied by LEED. Clean surfaces of germanium may coincide with the atomic planes of the same indices or consist of the regular steps with (111) or (100) terraces, or contain the “hill and valley” configuration depending on the orientation. Surface atomic planes are reconstructed. On clean Ge surfaces at certain temperatures the reversible order-disorder and order-order transitions take place.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.     

Surface structures of W(110) and W(100) faces by the dynamical LEED approach

By means of an efficient dynamical LEED method, the surface structures of W(110) and W(100) faces are examined. It is found that the W(110) surface maintains the bulk structure, despite the possibility for the top tungsten atoms to settle into sites of higher coordination number. The W(100) face exhibits a possible contraction of the top atomic layer by 0.10 ± 0.10 Å. These results fit into a trend correlating a top-layer contraction with the absence of close-packing in the top layer, i.e. with the roughness of the surface.     

Surface diffusion of Pb on clean Si surfaces

Pb diffusion on clean Si(111), (100), and (110) surfaces was studied by Auger electron spectroscopy and low energy electron diffraction in the temperature range from 100 to 300°C. It is shown that lead transport along sillicon surfaces takes place via the mechanism of solid-phase spreading with a sharp moving boundary. The temperature dependence of the Pb diffusion coefficients on Si(111), (100) and (110) surfaces have been obtained. A Si(110)-4×2-Pb surface structure has been observed for the first time.