Strain-stabilized highly concentrated pseudomorphic Si1-xCx layers in Si

Two-dimensional carbon incorporation into Si(001): C amount and structure of Si(001)-c(4 x 4)

The C amount and the structure of the Si(001)-c(4 x 4) surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The c(4 x 4) phase is found to contain 1/8 monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The c(4 x 4) phase constitutes the subsurface Si(0.875)C(0.125) delta layer with two-dimensionally ordered C atoms.

Highly concentrated Si1-xCx alloy with an ordered superstructure

An ordered Si-C alloy was observed in the products resulting from thermal reduction of molybdenum disilicide heating rods. High-resolution transmission electron microscopy measurements indicate that the Si1-xCx alloy with x approximately 25% is pure, single crystalline, and possesses a superlattice structure. The superlattice periodicity occurs along the diamond [001] direction and corresponds to the quintupling of the primary (002) periodicity. The possible growth mechanism is discussed. The Mo element is likely to play a crucial role in the growth process of the Si1-xCx alloy, since it can both reduce the energy required for breaking up C clusters and contribute to surface modification, which are of great benefit to enhance the carbon concentration and induce an ordered structure.

Self-organized superlattice formation during crystal growth from continuous beam fluxes

Alloy superlattice structures consisting of alternating Si-rich and C-rich layers form spontaneously during gas-source molecular beam epitaxy of Si(1-y)C(y) on Si(001) from constant Si2H6 and CH3SiH3 precursor fluxes at T(s)=725-750 degrees C. The self-organized patterning is due to a complex interaction among competing surface reactions. During growth of the initial Si-rich layer, strain-driven C segregation to the subsurface results in charge transfer from surface Si atom dangling bonds to C backbonds. This decreases the Si2H6 sticking probability, and, hence, the instantaneous deposition rate, thereby enhancing C segregation. The Si-rich layer continues until a critical C coverage is reached allowing nucleation of a C-rich layer which grows until the excess subsurface C is depleted. The process then repeats with periods tunable through the choice of T(s) and y(avg).

Initial stage of carbon incorporation into si(001) and one-dimensional ordering of embedded carbon

We investigate the initial stage of the C incorporation into Si(001) using thermal dissociation of C2H2. The scanning tunneling microscopy shows that C-induced dimer vacancies (DVs) with depressed adjacent dimers are generated on the surface and aligned in the dimer direction, forming the 2xn structure. The ab initio pseudopotential calculations reveal that, with the presence of a DV in the surface, the alpha site in the fourth subsurface layer directly below the DV is the most favorable for the incorporated C atoms. The embedded C atoms align one dimensionally due to the interaction which is attractive in neighboring dimer rows but repulsive in the same dimer row.

Thermodynamics of C incorporation on Si(100) from ab initio calculations

We study the thermodynamics of C incorporation on Si(100), a system where strain and chemical effects are both important. Our analysis is based on first-principles atomistic calculations to obtain the important lowest-energy structures, and a classical effective Hamiltonian which is employed to represent the long-range strain effects and incorporate the thermodynamic aspects. We determine the equilibrium phase diagram in temperature and C chemical potential, which allows us to predict the mesoscopic structure of the system that should be observed under experimentally relevant conditions.

Computational design of direct-bandgap semiconductors that lattice-match silicon

Crystalline silicon is an indirect-bandgap semiconductor, making it an inefficient emitter of light. The successful integration of silicon-based electronics with optical components will therefore require optically active (for example, direct-bandgap) materials that can be grown on silicon with high-quality interfaces. For well ordered materials, this effectively translates into the requirement that such materials lattice-match silicon: lattice mismatch generally causes cracks and poor interface properties once the mismatched overlayer exceeds a very thin critical thickness. But no direct-bandgap semiconductor has yet been produced that can lattice-match silicon, and previously suggested structures pose formidable challenges for synthesis. Much recent work has therefore focused on introducing compliant transition layers between the mismatched components. Here we propose a more direct solution to integrating silicon electronics with optical components. We have computationally designed two hypothetical direct-bandgap semiconductor alloys, the synthesis of which should be possible through the deposition of specific group-IV precursor molecules and which lattice-match silicon to 0.5-1% along lattice planes with low Miller indices. The calculated bandgaps (and hence the frequency of emitted light) lie in the window of minimal absorption in current optical fibres.