Misao Murayama

Reflectance Difference Spectra Calculations of GaAs(001) As- and Ga-rich Reconstruction Surface Structures

The reflectance difference (RD) spectra of nine GaAs(001) surface reconstructions, (2×4)β2, (2×4)α, (2×4)γ2, (2×4)β1, (2×4)γ1 and c(4×4) reconstructions on As-rich surface and (4×2)β2, (4×2)α and (4×2)β1 reconstructions on Ga-rich surface, were studied by using the nearest-neighbor sp3s* tight-binding method. The surface atomic positions and the tight-binding interaction parameters were obtained by the ab inito pseudopotential calculations. We found that the RD spectra have considerably different features between As- and Ga-rich surface reconstructions. The RD spectra of As-rich surfaces are mainly understood by transitions between top As-dimer states, while the RD spectra of Ga-rich surfaces are explained by the surface electronic states resulting from the sinkage of surface Ga atoms into bulk layers. These calculations are compared with the results of recent experiments.

Electronic Structures of Hetero-Crystalline Semiconductor Superlattices

Crystal structures of hetero-crystalline superlattices made of materials with different crystalline structures are proposed and band structures of (hexagonal-diamond Si)/(cubic-diamond Si) and (wurzite ZnS)/(zinc-blend ZnS) superlattices are calculated using the first-principles pseudopotential method within the local-density approximation. It is shown that Si superlattices have indirect band gaps of energy around 1.1 eV, while ZnS superlattices have direct band gaps of energy around 3.8 eV. In both superlattices, the states around the fundamental gap show charge densities localized in one constituent layers, indicating that these superlattices give quantum well systems of good quality. Band offsets are also evaluated.

Atom-scale optical determination of Si-oxide layer thickness during layer-by-layer oxidation: Theoretical study

Optical anisotropy spectra of SiO2/Si(001) interfaces were theoretically investigated based on the sp3s* tight-binding calculation. In the spectra, we found three types of optical transitions originating from the E1 and E2 transitions of bulk Si, the interface Si–Si bonds, and the dangling-bond states at the interface. It was shown that the sign of these transitions oscillates during the layer-by-layer oxidation, which indicates that by counting the oscillation one can determine the layer thickness of oxidized Si layers in an atomic scale.

Tight-Binding-Calculation Method and Physical Origin of Reflectance Difference Spectra

The accurate tight-binding-calculation method of reflectance difference (RD) spectra is developed for surfaces and interfaces with various orientations, taking into consideration the crystalline nature and the finite size of the system adopted in the calculation. It is shown that the bond-polarization picture, i.e., that every bond becomes a unit of polarization, is applicable to the optical response of surfaces and interfaces, and the RD spectra are calculated by averaging the optical responses of even and odd finite-layer systems. The physical origins of the RD spectra are classified into two groups; the RD spectra have either the peak shape or the energy-derivative-of-peak shape depending on whether the electronic states are localized or extended around the surface/interface.

Decay process of a crater/hillock and step structure transformation

Both the decay process of a crater or a hillock on Si(111) and the step bunching formation accompanying the decay of a periodic array of craters or hillocks on vicinal Si(111) are investigated by using the lattice gas model. We considered two elementary atomic processes, hopping with the Schwoebel effect and evaporation, and carried out the time dependent Monte Carlo simulation. The decay process of a crater or a hillock is caused by the difference of the equilibrium adatom concentrations between at the crater or the hillock edge and at the nearest step edge. The size of a crater or a hillock decreases approximately linearly with time for both kinetics of the diffusion limited and the capture limited. For a periodic array of craters or hillocks on a vicinal surface, the step bunching is formed at the upper side of craters or the lower side of hillocks with the decay of craters or hillocks, in the diffusion-limited kinetics. The evaporation of adatoms accelerates the growth rate of the step bunching and extends the bottom width of craters, reducing the step bunching width.

Au–Si Bonding on Si(111) Surfaces

To investigate the Au–Si bonding feature on Si(111) surfaces, surface energy calculations have been performed for 1 ×1- and √3 ×√3-Au/Si(111) systems by the first-principles method in a local density approximation. It was found that the Au overlayer considerably stabilizes the Si(111) surface. By analyzing the changes in the surface energy and the charge density at various Au positions on the surface, it was shown that the large electronegativity of Au produces covalent-like Au–Si bonds by either terminating dangling bonds of surface Si or inducing the charge transfer from Si–Si to Au–Si bonds, both of which promote surface stabilization and determine the Au overlayer height on Si(111) surfaces.

Effect of the Surface-Atomic Positions on Reflectance Difference Spectra of [001]-GaAs β2 Structure

The role of surface atomic positions in reflectance difference (RD) spectra of the GaAs(001)-(2 ×4)β2 structure is studied using the sp3s* tight-binding method. Here we assume that the surface atomic positions are effectively changed by variation of two kinds of transfer energies. It was found that the As-As distance change in surface As dimer causes an energy shift of the RD spectral peak, while the distance change between surface As and substrate Ga affects not the peak position but the peak width. These calculation results are compared with the results of recent experiments.

Band Offsets at Wurtzite / Znic-blende Interfaces and Their Applications

Analyzing the calculated band offsets at wurtzite(WZ)/zinc-blende(ZB) interface by the first-principles pseudopotential method, general features of electronic structures of WZ/ZB systems are presented. Using the calculated offsets, band gap energies of polytypes such as ZnS and SiC and of stacking-fault in Si are calculated. It is found that a general idea of band offset can be applied to explain the electronic properties of both polytypes and stacking-fault.

Electronic structures and the charge transfer of Au overlayer on Si(111) surfaces

Abstract Electronic structures of 1×1-Au/Si(111) systems have been calculated by the ab initio pseudopotential method in a local density approximation. Au and top-layer Si produce the bonding and a large charge transfer is seen from Si to Au. Comparing the results with and without on-top Si layer, it is shown that which Si layers release the charge depends on the number of surface Si dangling bonds. The relation between the charge transfer and the change of bond strength has also been clarified.

Atomic structures of Ag on √3 X √3 Ag/Si(111) and on 7 X 7 Si(111)

Time evolution of atomic structures during Ag deposition and in subsequent annealing on both substrates, √3 X √3 Ag/Si(111) and 7 X 7 Si(111), have been studied with use of the lattice gas model. The interrelation between the calculated atomic structures and the observed surface conductance is investigated. On √3 X √3 Ag/Si(111), Ag grows in an island mode and Ag adatoms coexist with 3-dimensional Ag islands. It was found that the time dependence of adatom density shows peculiar behavior and is strongly correlated to the observed variation of the surface conductance. On 7 X 7 Si(l 11) substrate, Ag grows in a layer mode. The observed variation of surface conductance indicates that the conductance from 3 lower overlayers is suppressed.