Projected Bulk Band Gap Research Articles

Mg-induced Si(111)3 × 1 structure studied by photoelectron spectroscopy

By means of angle resolved photoelectron spectroscopy using synchrotron radiation, we have measured the valence band and surface sensitive Si 2p core-level spectra for the Si(111)3 × 1Mg surface. The dispersion of the valence band shows the fact that this surface has a semiconducting property and two surface states in the projected bulk band gap at the K̄ point. From the fitting results of the Si 2p core-level spectra, we find that the two surface shifted core-level components, S′ 1 and S′ 2 stem from the Si atom with single dangling bond and the Si atom bonding to the Mg atom, respectively. From experimental observations we suggest that the Si(111)3 × 1Mg structure is formed by ordering of Mg atoms on the ideal Si(111)1 × 1 surface, not by reconstruction of Si atoms of the substrate.

Hydrogen-induced reconstruction of a W(110) surface studied by angle-resolved photoemission spectroscopy

We have observed changes in the electronic structure of a W(110) surface caused by the hydrogen-induced reconstruction using angle-resolved photoemission spectroscopy. For the unreconstructed H/W(110) surface, a number of hydrogen-induced states are observed, and some of them are found to be in the projected bulk band gap. As W(110) undergoes the surface reconstruction induced by further hydrogen adsorption, one of those states is rapidly quenched, while others simply shift downward in energy by a fraction of an electron volt. To understand the surface reconstruction mechanism for this system, the surface electronic states before and after the surface reconstruction are calculated using the “simple” tight-binding method. The driving mechanism for the hydrogen-induced surface reconstruction is briefly discussed.

Hydrogen-induced reconstruction of a W(110) surface studied by angle-resolved photoemission spectroscopy

Abstract We have observed changes in the electronic structure of a W(110) surface caused by the hydrogen-induced reconstruction using angle-resolved photoemission spectroscopy. For the unreconstructed H/W(110) surface, a number of hydrogen-induced states are observed, and some of them are found to be in the projected bulk band gap. As W(110) undergoes the surface reconstruction induced by further hydrogen adsorption, one of those states is rapidly quenched, while others simply shift downward in energy by a fraction of an electron volt. To understand the surface reconstruction mechanism for this system, the surface electronic states before and after the surface reconstruction are calculated using the “simple” tight-binding method. The driving mechanism for the hydrogen-induced surface reconstruction is briefly discussed.

Surface electronic structure and dynamical interactions on Ta(011) and H/Ta(011).

A comprehensive study of surface electronic structure of both clean and hydrogen-covered Ta(011) using high-resolution angle-resolved photoemission is presented. A goal of this study was to explore possible implications of the electronic structure for elastic and inelastic dynamical phenomena at these surfaces. A single surface band is observed on the clean surface, lying in a large projected bulk band gap near \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} as a pure surface state of even symmetry. This band makes an unusual continuous transition to become a surface resonance with increasing ${\mathit{k}}_{\mathrm{?}}$. The resonant portions of the band exhibit the expected broadening due to coupling with the underlying bulk continuum, disperse slightly with momentum normal to the surface, and tend to lie near the middle of the associated bulk continuum. Upon hydrogen adsorption, this band is observed to shift between 0.5 and 1.8 eV to higher binding energy, depending upon the value of ${\mathit{k}}_{\mathrm{?}}$. The hydrogen-modified band exhibits qualitatively similar behaviors to the clean-surface band. Hydrogen adsorption also induces a sharp state very close to the Fermi level localized along the \ensuremath{\Sigma}\ifmmode\bar\else\textasciimacron\fi{} symmetry line and having predominantly odd mirror-plane symmetry. The ability of this level to participate in adsorbate vibrational damping is explored. A level associated with the hydrogen 1s atomic state is also observed and characterized.

Empty surface electronic bands on CdTe(110)

Angle-resolved inverse photoemission has for the first time been used to study the surface electronic band structure of a II–VI semiconductor, CdTe. The energy dispersion of an empty electronic surface state on CdTe(110), previously observed in normal emission only, has been mapped \\ ̄ gG-X̄ and \\ ̄ gG-X̄ ' in the surface Brillouin zone. In the course of this study new aspects of the surface electronic structure are observed. Firstly, we find two surface states with similar dispersions approximately 0.4 eV apart, one of which is the previously observed. Along \\ ̄ gG-X̄ ' they are both resonances but move into the projected bulk band gap along \\ ̄ gG-X̄ and become true surface states. Secondly, a rapidly dispersing structure is observed dispersing into the projected bulk band gap close to the X̄-point which we consequently identify as a third true surface state. These findings yield a new value for the surface band gap, located at \\ ̄ gG, of 2.9 eV.

Surface electronic structure of clean and hydrogen chemisorbed Ge(100)2 × 1 studied by angle-resolved photoemission

The electronic structure of the clean Ge(100)2 × 1 surface and the Ge(100)2 × 1-H monohydride surface has been studied in detail by angle-resolved photoemission. The surface state dispersions, E i (k ‖ ) , in the [010] direction are mapped out. For the clean surface three surface states, which correspond to surface states previously reported for the Si(100)2 × 1 surface, are observed in the 1 × 1 projected bulk bandgap. A new, fourth surface state, showing high emission intensity is observed, just above the 1 × 1 projected valence band edge, around the J ′ point of the 2 × 1 surface Brillouin zone. On the 2 × 1-H surface two strong hydrogeninduced surface states/resonances are observed in the energy region 4.4–5.5 eV below E F .

Two-photon photoelectron spectroscopy of Pd(111)

Angle-resolved two-photon photoelectron spectroscopy has been used to study the normally unoccupied band structure of Pd(111). Photoemission features are studied as functions of excitation photon energy hν, polarization state, and emission angle. Below hν=4.9 eV, photoemission is dominated by excitation of bulk states near the Fermi level of Λ3 symmetry through Λ1 excited states between the Fermi and vacuum level. The kinetic energy of the corresponding spectral peak is found to increase according to 2 hν−Φ, where Φ is the Pd(111) work function. At hν=4.9 eV, a surface state with 0.65±0.1 eV binding energy is found. The dispersion of this state with k∥ is well-described by an effective mass m* equal to the free electron mass me. This state is assigned as the n=1 member of the image potential series of states. The relationship between the dispersion of this state and its position in the (111) projected bulk band gap is discussed.

Perturbations on surface electronic structure induced by trace impurities: K on Cu(111)

High-resolution angle-resolved photoemission experiments are reported which describe changes in the surface electronic structure of a Cu(111) crystal induced by the adsorption of trace quantities of potassium. The primary focus is on the zone-center sp intrinsic surface state. In accord with previous studies, a small shift to larger binding energy is observed as the work function decreases. This effect is explained semiquantitatively using a simple phase analysis model. Changing the binding energy of the surface state is equivalent to moving it up and down in its projected bulk band gap. We have observed and quantified the expected changes in bulk penetration length accompanying this movement. The observed lengths allow a fairly precise measure of the copper band structure in the complex plane. The final observations in these experiments concern the coverage-dependent broadening of the surface state peak. This is ascribed to loss of the nearly perfect crystal order with the consequent broadening of the momentum spectrum. A simple analysis allows extraction of the scattering cross section of electrons in the surface band off the impurity site.