S.J. Jenkins

Theoretical evidence concerning mixed dimer growth on the surface

The local atomic geometry, chemical bonding and energetics for half- and full- monolayer Ge coverages on the Si(001)(2 1) surface have been studied by applying the ab initio pseudopotential method. For one monolayer coverage we find that the adatoms form an asymmetric dimer with length equal to the bond length in bulk Ge. At half-monolayer coverage we find that the formation of mixed Si-Ge dimers is comparable to growth of Ge-Ge dimers in islands, confirming recent experimental observations. Our work also shows that Ge diffusion into the substrate will only become important at unusually high growth temperatures, in the region of 1000 K or above.

Bonding and structure of the Si(001)(2 × 1)-Sb surface

Abstract A thin layer of Sb adsorbed on to the Si(001) surface is believed to show surfactant behaviour for epitaxial growth of Ge, important for device applications. In this work we describe results of ab initio density functional calculations on the Si(001)(2 × 1)-Sb surface. The surface reconstruction is due to dimerisation of the Sb atoms in the direction perpendicular to the dimer rows of the clean Si(001) surface. Our relaxed geometry is found to be in accord with recent theoretical work and SEXAFS measurement in predicting a structure with symmetric dimers. Furthermore, we investigate the nature of the bonding and band structure of the relaxed Sb-capped surface. We find that the normal energy ordering of ungerade and gerade π orbitals localised on the Sb dimer is reversed.

Energetic evidence for mixed dimer growth on the Si(001)Ge(2 × 1) surface

Abstract Recent experimental evidence has indicated that the predominant growth mode of Ge on the Si(001)(2 × 1) surface at submonolayer coverage is by the formation of mixed SiGe dimers. We present results of ab initio pseudopotential calculations for the half-monolayer and full-monolayer Ge covered Si(001)(2 × 1) surface. Comparison of the total energy results for these surfaces supports the picture of mixed dimer formation for the half-monolayer coverage, and limited interdiffusion at higher coverages.

Comparative ab initio pseudopotential studies of group V overlayers on Si(001)

We present a comparative theoretical study of a single-monolayer coverage of the group V elements P, As, Sb and Bi on the Si(001) surface. The generic surface reconstruction, due to formation of symmetric overlayer dimers, is considered and a first-principles pseudopotential method is used. In addition to presenting well-relaxed structural data, we identify chemical trends in geometry, bonding and electronic structure in moving down the group V column of the periodic table. Overlayer-induced distortion of the substrate is found to decrease with increasing overlayer atomic number. In general, P and As overlayer systems share many similarities, while Sb and Bi form another such natural pair. A novel feature of group V overlayers is that the bonding and occupied antibonding states formed by the dangling bonds lie very close in energy.

An ab initio pseudopotential calculation of ground-state and excited-state properties of gallium nitride

In this work, the electronic, ground-state and vibrational properties of both alpha -GaN (i.e. wurtzite structure) and the recently fabricated beta -GaN (i.e. zincblende structure) have been studied using the ab initio pseudopotential method within the local density approximation and a simple quasi-particle scheme. The calculated equilibrium lattice constants, bulk moduli, the pressure derivatives of the bulk moduli, and the Al TO( Gamma ) phonon frequency are in good agreement with available experimental and other recent ab initio theoretical results. The self-energy band gap corrections are found to be highly k-dependent. The calculated fundamental band gap is direct in both cases and for the experimental lattice constant is calculated to be 3.36 eV in beta -GaN and 3.48 eV in alpha -GaN, in excellent agreement with experiment.

Thermodynamic evidence for surfactant behaviour of Sb in the growth of Ge on Si(001)

Abstract Thermodynamic evidence of the Stranski—Krastanov growth of Ge on the Si(001) surface is provided, based upon ab initio density functional theory calculations. Island formation is predicted to begin at a Ge coverage of around three monolayers, in good agreement with recent experimental findings. The presence of a terminating Sb layer is shown to promote epitaxial growth. The analysis of energetic data in terms of surface free energy as a function of chemical potential provides a general theoretical signature for both Stranski—Krastanov and epitaxial growth modes.

A study of atomic vibrations on Si(001)/Sb(2×1)

Abstract We have investigated vibrational properties of the Si(001)/Sb(2×1) surface by applying the adiabatic bond-charge model using the recently determined relaxed atomic geometry. We compare the phonon spectrum of this surface in detail with that of the Si(001)(2×1) surface presented in a recent bond-charge model study. We observe that the surface acoustic phonon modes are mainly localized on the adsorbate layer atoms due to the large mass difference between Sb and Si atoms. Adsorption of Sb is shown to result in two new peaks in the phonon density of states; one below the bulk continuum, and the other near the boundary between bulk acoustic and optical states.

Ab initio calculation of geometry and bonding for overlaid and inlaid models of Si(001)/Sb(0.25 ML)-c(4×4)

Abstract Recent experimental work has revealed the existence of a c(4×4) reconstruction of the Si(001)–Sb surface in the Sb coverage range between 0.2 and 0.3 monolayers. The best fit to the experimental data was found with a structural model in which Sb dimers are adsorbed above the Si(001) dimer rows (overlaid above row structure), although a structure in which Sb dimers are incorporated into the top Si layer (inlaid structure) also fitted reasonably well. In a previous theoretical work we have shown the overlaid geometry to have the lowest surface free energy of a number of structural models considered, with the inlaid structure coming a close second. In this work we present a detailed discussion of bonding information for both of these models.

Atomic structure and bonding on GaAs(001)Sb(2 × 4)

Abstract We present results of ab initio pseudopotential calculations for the (2 × 4) reconstruction of the Sb-terminated GaAs(001) surface. Using the structural model recently proposed by Moriarty et al. we provide a detailed analysis of the atomic geometry and the nature of chemical bonding at the surface. The geometrical results are compared with the X-ray standing wave analysis of Sugiyama et al.

Structural studies of Si(001)/Sb(0.25 ML)-c(4 × 4)

Abstract Recent experimental work reveals a c(4 × 4) reconstruction of the Si(001)/Sb surface in the coverage range between 0.2 and 0.3 monolayers. In this paper we present the results of ab initio pseudopotential density functional calculations for the 0.25 monolayer covered surface. We consider three possible structural models for the c(4 × 4) reconstruction: in the first, Sb dimers sit directly above the Si(001) dimer rows, in the second they bridge the gap between neighbouring Si(001) dimer rows, and in the third they are embedded into the existing Si(001) dimer rows. We determine that the configuration in which Sb dimers sit directly above the dimer rows is the most energetically favourable of the three geometries. The structural parameters of this reconstruction are discussed in some detail.