R. S. Bauer

Reconstructions of GaAs and AlAs surfaces as a function of metal to As ratio

The room temperature surface reconstructions of GaAs and AlAs have been investigated with angle integrated photoemission as a function of the metal to arsenic ratio. Detailed information on both the (110) and (100) surface as a function of reconstruction has been obtained from changes in the surface core level intensities, binding energy shifts, and changes in the surface valence band density of states. The GaAs(100) surface shows ordered reconstructions over a wide composition range. The ordering proceeds through a series of centered structures formed from As and vacancies in the outer layer in the As rich range to noncentered structures derived from As, Ga antisite defects, and vacancies on the Ga rich end. The AlAs(100) surface is predominantly disordered and only a 3×2 reconstruction was found in a narrow composition range. Aspects of the Al–Ga exchange reaction on GaAs(110) are discussed.

Ultraviolet photoemission studies of chemisorption and point defect formation on ZnO nonpolar surfaces

Abstract Ultraviolet photoemission (He I and He II) studies are reported on point defect formation as well as CO2 and O2 interaction with ZnO(1010) under thermodynamically well defined conditions. Physisorption spectra can be correlated with UPS spectra in the gas phase if a uniform relaxation/polarization shift is taken into account. CO2 chemisorption may be characterized by formation of carbonate like surface complexes. Small concentrations of intrinsic point defects lead to significant changes in the surface electronic structure and provide specific CO2 chemisorption sites which are intermediates during heterogenous oxidation of CO on ZnO.

Surface phases of GaAs(100) and AlAs(100)

The surface phases of GaAs(100) and AlAs(100) have been investigated with angle integrated photoemission. Detailed information about the surface reconstructions as function of Ga/As ratio has been obtained from changes in the surface core level intensities as a function of reconstruction. Large changes are also shown to occur in the surface valence band density of states and surface core level binding energy shifts as a function of reconstruction. We find that whereas the GaAs(100) surface shows ordered reconstructions over a wide composition range, the AlAs(100) surface is predominately disordered and only a 3×2 reconstruction was found in a narrow composition range.

Ge—GaAs(110) interface formation

The heterojunction chemistry for Ge grown by molecular beam epitaxy (MBE) on in situ cleaved GaAs exhibits significant interdiffusion in short times at growth temperatures TG of 430°C (significantly lower critical TG than that reported for moderate‐vacuum physical vapor deposition). This results in profound changes in the electronic properties of the interface as probed by synchrotron‐radiation‐excited 3d core electron photoemission. Even when there is significant alloying of the two lattice‐matched semiconductors, there is nearly equal probability for Ge to bond to either a Ga or an As atom at the initial stage. As Ge becomes the dominant species, we find As preferentially diffusing toward the Ge side of the junction. This As is distributed throughout the overlayer in contrast to metal–semiconductor interface formation where the diffusing constituent resides only on the free, growing surface. We show that these behaviors are consistent with the kinetic and thermodynamic properties of the atomic species. ...

Evidence of SiO at the Si—oxide interface by surface soft x-ray absorption near edge spectroscopy

Abstract Using synchrotron radiation a new surface sensitive spectroscopy has been applied to determine the local structure of the first surface oxide layer formed on the Si(111) surface. The Surface Soft X-ray Absorption (SSXA) spectra have been measured. From the analysis of the X-ray Absorption Near Edge Structures (XANES) we have extracted structural information. We have first determined that bulk amorphous SiO has a characteristic microsopic structure, which cannot be described by the random alloy or microcrystalline (Si + SiO 2 ) mixture models. The oxide layer formed on the Si(111) surface by ground-state molecular excitation in ultra high vacuum at temperatures (∼700°C) approaching the oxide dissociation point has this unique SiO local structure. Such SiO layer not formed at room temperature is expected to be present in the SiSiO 2 interface grown at high temperature. An electronic transition to empty states at the SiSiO 2 interface has been observed.

Photoemission determination of dipole layer and VB‐discontinuity formation during the MBE growth of GaAs on Ge(110)

Synchrotron radiation‐excited soft x‐ray photoemission is used to study the heterojunction formation of GaAs on Ge(110) by examining 3d‐core levels, the valence band (VB), and the secondary electron distribution cutoff in a photoemission spectrum. Arsenic saturation of the Ge(110) surface prior to MBE growth of GaAs causes a work function increase of 0.26±0.045 eV and a decrease in Ge valence‐band maximum (VBM) from the Fermi level by 0.96±0.09 eV. The increase in work function can be explained by the formation of a GeAsx layer. The large VBM decrease is partly due to a band bending of 0.61±0.03 eV and a chemisorption‐induced charge transfer reaction. Using two different photoemission methods, the VB discontinuity at the interface is determined to be 0.23±0.08 and 0.26±0.07 eV. This value is different from the one determined for the inverse heterojunction where Ge is deposited on top of GaAs(110) at the same epitaxial growth temperature of 320 °C. The difference in electron affinity between the clean Ge(1...

Electronic and atomic structure of Si(111) Al, Ag, and Ni metal overlayer induced surface reconstructions

The electronic structure of the ordered metal overlayers Si(111) √3× √3: Al and √3×√3: Ag and Si(111) √19×√19: Ni prepared from Si(111)7×7 surfaces has been studied with angle‐resolved photoelectron spectroscopy. On all these surfaces we find dangling bond derived bands which for Al and Ag overlayers have a dispersion of 0.4 eV. We also report a new Si(111) √7×√7: Al structure at ?0.5 monolayer coverage of Al. A new structural model for the √19×√19: Ni surface is proposed.

Investigation of plasmon sidebands by synchrotron radiation tuning of electron escape depths

Photoelectron spectra of Al and Si have been excited by monochromatized synchrotron radiation at different photon energies. The plasmon loss structures produced by the elastic electrons from the Si 2p and Al 2p core levels have been investigated for different kinetic energies at the elastic electrons. Variation of the plasmon loss intensities for electron kinetic energies of 30–330 eV explains earlier reported absence of plasmon structure in uv photoelectron spectroscopy. The results also indicate that a model of extrinsic bulk plasmon excitation developed by Mahan and modified to account for surface effects could explain the observed threshold behavior of bulk plasmon intensity at low electron kinetic energies.

Surface resonances and the oxidation of single‐crystal aluminum

Valence‐band photoemission spectra have been studied on single‐crystal aluminum faces using polarized sychrotron radiation. New intrinsic and oxygen‐related extrinsic surface resonance features are observed within 5 eV of the Fermi level. These surface resonances are correlated with oxygen uptake on the (100), (110), and (111) faces as observed in both valence and core‐level spectroscopy. Considerable face dependence to the oxidation is shown. Although aluminum is a free‐electron‐like metal, its surface behavior on single‐crystal faces shows effects due to localized charge.

ON THE ADJUSTABILITY OF THE “ABRUPT” HETEROJUNCTION BAND-GAP DISCONTINUITY

Abstract There is growing evidence that the offset in the band edge between two different semiconductors is intimately linked to the chemical and geometric structure at the interface. By conducting experiments sensitive on an atomic scale, we focus on the relation of the nanostructure changes to the electronic barrier formation for heterojunctions between Ge, GaAs, and AlAs. We demonstrate that there is not a single value for the distribution of band gap differences between valence band and conduction band offsets. Attempting to apply the commonly used Anderson electron affinity rule (i.e., ΔEc = Δχ), shows no systematic behavior of band-gap discontinuities as measured by photoelectron spectroscopy. The discontinuity depends not on a simple scaling parameter such as electron affinity or LCAO bond strength, but rather depends on the precise processing and substrate conditions prior to and during the heterostructure growth. Examples are presented of dependences on crystallographic orientation, semiconductor crystallinity, and growth parameters (such as deposition sequence and rate, substrate temperature and preparation). The link between the chemistry and structure at the heterojunction interface is demonstrated by measurements of the evolution of GaAs/Ge(100) “band” offsets as the nanostructure changes during growth for a variety of starting GaAs surface stoichiometries and reconstructions. Such chemical rearrangements are controlled by local, interatomic interactions at the free surfaces, neither bulk, equilibrium thermodynamic parameters (such as enthalpy of formation) nor free surface parameters (reconstruction, or surface stoichiometry) are important. Rather, the free surface and interface phase diagrams must be understood to give proper consideration to the stable or even metastable compounds which form. In order to accommodate such energy considerations as surface-induced strain and electrostatic dipole forces, the “abrupt” epitaxial heteroj unction will have an interface whose extent may be greater than a single atomic layer even for nonpolar orientations of compound semiconductors. With this understanding, we consider the possibilities for experimentally adjusting the band edge discontinuity. Atomic site selection during growth, creation of stabilizing phases and other growth and processing possibilities are also explored.