John P. LaFemina

Total-energy calculations of semiconductor surface reconstructions

Abstract In this article, the current understanding of semiconductor surface reconstructions and the role of total-energy calculations in achieving this understanding is reviewed. The methods used to compute semiconductor surface reconstructions are also surveyed. Central unifying themes such as the role of topology and surface stress in determining surface reconstructions, and the existence of universal reconstructions for the zincblende- and wurtzite-structure compound semiconductor cleavage surfaces are identified and discussed.

Electronic structure and relative stability of the MgO (001) and (111) surfaces

The relative stability and electronic structure of the MgO (001), (111)‐Mg, and (111)‐O surfaces have been studied with the Harris–Foulkes total energy functional. The computed energies of the (001), (111)‐O, and (111)‐Mg surfaces of MgO are 2.64, 12.80, and 13.02 J m−2, respectively. Our calculations exhibit a qualitative distinction between the Mg‐terminated polar (111) surface, which has a nonzero density of one‐electron states at the Fermi level, and the other (001) and (111)‐O surfaces which retain the band gap properties of the bulk material. The projected local density of states for atoms in the bulk and at the various surfaces are presented, and used to explain the relative stability of different surfaces in terms of the properties of surface bonds.

Dependence of oxide surface structure on surface topology and local chemical bonding

The atomic geometries of the charge neutral surfaces of several oxides exhibiting different crystal structures and varying participation of O(2p) electrons in the chemical bonding have been calculated using tight‐binding total energy models. Surface structures have been computed for exemplary cubic (MgO), wurtzite (ZnO), β‐tridymite, and ideal β‐cristobalite (SiO2) oxides. The cubic oxide exhibits a minimum energy structure involving small outward relaxations of the oxygens and inward relaxations of the cations. For the cleavage faces of wurtzite ZnO, large bond‐length‐conserving relaxations occur because the surface atoms can relax without appreciable distortion of the local bond lengths. The charge neutral faces of β‐tridymite and ideal β‐cristobalite SiO2 also undergo bond‐length‐conserving relaxations. Thus the mechanism for the surface relaxation of tetrahedrally coordinated oxides is significantly different from that of the cubic oxides as is the role of the oxygen p electrons in the surface chemica...

New surface atomic structures for column V overlayers on the (110) surfaces of III–V compound semiconductors

Two new minimum‐energy surface structures have been identified for p(1×1) overlayers of Sb on the (110) surface of III–V compound semiconductors using a tight‐binding total‐energy formalism previously developed for these systems. The first is the ‘‘epitaxical on top structure’’ (EOTS) in which Sb zig–zag chains are commensurate with, and on top of, the Ga–As unreconstructed surface zig–zag chains. This structure differs from the previously found ‘‘epitaxical continued layer structure’’ (ECLS) by virtue of the registry of the Sb chains ‘‘on top of ’’ rather than ‘‘in between’’ the substrate Ga–As chains. Like the ECLS, the EOTS is compatible with both scanning tunneling microscopy (STM) and photoemission data. The second new structure is the ‘‘epitaxical overlapping chain structure’’ (EOCS) in which the Sb chains are 180° out‐of‐phase with, and on top of, the Ga–As substrate chains. This structure is, however, incompatible with both low‐energy electron diffraction and STM data for GaAs(110)–p(1×1)‐Sb. Comp...

Dynamical strain at semiconductor interfaces: Structure and surface‐atom vibrations of GaAs(110) and GaAs(110)–p(1×1)–Sb

The dynamical force fields of the clean GaAs(110) surface, an isolated Sb chain, and the GaAs(110)–p(1×1)–Sb(1‐ML) interface have been computed utilizing tight‐binding total energy models that have been used successfully to describe the atomic and electronic structure of the clean and adsorbed cleavage faces of the tetrahedrally coordinated compound semiconductors. Since the main consequences of the different chemical bonding in these two cases are manifested in changes in the force field associated with the dynamics of the top‐layer atoms, we explore these consequences using a restricted dynamical model in which only the top‐layer atoms are allowed to vibrate. The resulting vibrational energies are in remarkably good agreement with experimental measurements, and hence afford a vehicle to obtain quantitative relationships between the nature of the surface chemical bonds and the vibrational energies of the surface atomic species.

Twisted intramolecular charge transfer and the torsional potential function of dimethylaminobenzonitrile

Recent experimental and theoretical studies of the twisted intramolecular charge transfer process and its interdependence with the torsional potential for dimethylaminobenzonitrile (DMABN) are discussed. It is shown that while the available experimental and theoretical evidence does not effectively discriminate between an untwisted or twisted ground state for DMABN in either the gas phase or solution, strong indirect evidence exists for a twisted DMABN ground state in the presence of a polar solvent.

Surface structure, bonding, and dynamics: Universality of zinc blende (110) potential energy surfaces

Using a tight‐binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated zinc blende structure compound semiconductors exhibit a common ‘‘universal’’ form if expressed in terms of suitably scaled parameters. TBTE calculations on both III–V and II–VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low‐energy, electron‐diffraction surface‐structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, InSb, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the ...

Semiconductor surface and interface dynamics from tight‐binding molecular dynamics simulations

Tight‐binding molecular dynamics simulations have been performed to compute the bulk, (110) surface, and (110)‐p(1×1)‐Sb(1 ML) interfacial atomic vibrational spectra for GaAs and InP. The same tight‐binding total energy model that successfully described the static surface and interfacial atomic and electronic structure for these systems is utilized in the molecular dynamics simulations. The results for the bulk vibrational energies are in semiquantitative agreement with experimental results, displaying approximately the same level of variance as other model computations. Moreover, these simulations are used to examine the effects of anharmonicity in the system by investigating the temperature dependence of the vibrational spectra. The (110) surface vibrational energies are in quantitative agreement with the scattering data, and a comparison of the results for GaAs(110) and InP(110) supports the existance of a surface vibrational mode that is characteristic of the relaxed (110) surface, and whose energy is...

Surface atomic structure and bonding of GaAs(110)‐p(1×1)–Bi (1 ML)

The surface atomic and electronic structure of GaAs(110)‐p(1×1)–Bi (1 ML) has been examined using a tight‐binding total‐energy (TBTE) model. Two structural candidates were considered; the epitaxial continued‐layer structure (ECLS) and the epitaxial on‐top structure (EOTS). The TBTE computations indicate that the ECLS is more stable than the EOTS by ∼0.06 eV/surface atom/unit cell in agreement with a recent low‐energy electron diffraction (LEED) intensity analysis which selected the ECLS as the best‐fit structure. The predicted TBTE structural parameters for the ECLS are in good agreement with the best‐fit LEED results. The energy difference between the ECLS and EOTS is much smaller for the Bi/GaAs interface than for the Sb/GaAs interface (∼0.11 eV/surface atom/unit cell) in agreement with the proposition that the relative stability of the ECLS and EOTS is a function of the overlayer‐substrate mismatch; with the EOTS being favored for larger overlayer‐substrate mismatch. The primary difference between the ...

Atomic structure of the cassiterite SnO2(111) surface

Abstract We predict the relaxed (111) growth surface structure of cassiterite (rutile-structure SnO2) by applying simple physical and chemical constraints. Experiments have shown that the surface symmetry is (1 × 1). We show that the simplest surface which is (1 × 1) and autocompensated has a single oxygen adatom per surface unit cell, located on the cells mirror symmetry plane. The predicted relaxations of other surface atoms from their bulk positions are large enough to be detected by LEED analysis. This result demonstrates the power of the autocompensation principle, which straightforwardly shows that none of 63 possible truncated bulk structures should be stable, and points to the single-adatom stoichiometry.