Normal Photoemission Research Articles

UV photoemission study of heteroepitaxial AlGaN films grown on 6H-SiC

This study presents results of UV photoemission measurements of the surface and interface properties of heteroepitaxial AlGaN on 6H-SiC. Previous results have demonstrated a negative electron affinity of AlN on 6H-SiC. In this study Al x Ga 1− x N alloy films were grown by organometallic vapor phase epitaxy (OMVPE) and doped with silicon. The analytical techniques included UPS, Auger electron spectroscopy, and LEED. All analysis took place in an integrated UHV transfer system which included the analysis techniques, a surface processing chamber and a gas source MBE. The OMVPE alloy samples were transported in air to the surface characterization system while the AlN and GaN investigations were prepared in situ. The surface electronic states were characterized by surface normal UV photoemission to determine whether the electron affinity was positive or negative. Two aspects of the photoemission distinguish a surface that exhibits a NEA: (1) the spectrum exhibits a sharp peak in the low kinetic energy region, and (2) the width of the spectrum is hv - E g. The in situ prepared AlN samples exhibited the characteristics of a NEA while the GaN and Al 0.13Ga 0.87N samples did not. The Al 0.55Ga 0.45N sample shows a low positive electron affinity. Annealing of the sample to > 400°C resulted in the disappearance of the sharp emission features, and this effect was related to contaminant effects on the surface. The results suggest the potential of nitride based cold cathode electron emitters.

A new effect in spin-resolved photoemission from Pt(110) in normal emission by linearly polarized VUV-radiation

A new spin polarization effect in normal photoemission from Pt(110) using s-polarized light has been observed. Measurements have been performed with off-normally incoming light of a gas discharge tube which was s-polarized by a 3-mirror reflection type polarizer. The spin polarization vector of the created photoelectrons is perpendicular to the surface of the crystal and occurs only if the E vector of the incident radiation is not aligned with a mirror plane of the Pt(110) crystal. These experimental findings confirm a recent theoretical prediction of Henk and Feder [1].

A new effect in spin-resolved photoemission from Pt(110) in normal emission by linearly polarized VUV-radiation

A new spin polarization effect in normal photoemission from Pt(110) using s-polarized light has been observed. Measurements have been performed with off-normally incoming light of a gas discharge tube which was s-polarized by a 3-mirror reflection type polarizer. The spin polarization vector of the created photoelectrons is perpendicular to the surface of the crystal and occurs only if the E vector of the incident radiation is not aligned with a mirror plane of the Pt(110) crystal. These experimental findings confirm a recent theoretical prediction of Henk and Feder [1].

Angle-resolved photoemission extended fine structure: Multiple layers of emitters and multiple initial states

Recently, angle-resolved photoemission extended fine structure (ARPEFS) has been applied to experimental systems involving multiple layers of emitters and non- s core-level photoemission in an effort to broaden the utility of the technique. Most of the previous systems have been comprised of atomic or molecular overlayers adsorbed onto a single-crystal, metal surface and the photoemission data were taken from an s atomic core-level in the overlayer. For such a system, the acquired ARPEFS data is dominated by the p o final state wave backscattering from the substrate atoms and is well understood. In this study, we investigate ARPEFS as a surface-region structure determination technique when applied to experimental systems comprised of multiple layers of photoemitters and arbitrary initial state core-level photoemission. Understanding the data acquired from multiple layers of photoemitters is useful for studying multilayer interfaces, “buried” surfaces, and clean crystals in ultra-high vacuum. The ability to apply ARPEFS to arbitrary initial state core-level photoemission obviously opens up many systems to analysis. Efforts have been ongoing to understand such data in depth. We present clean Cu(111) 3 s, 3 p, and 3 d core-level, normal photoemission data taken on a high resolution soft x-ray beamline 9.3.2 at the Advanced Light Source in Berkeley, California and clean Ni(111) 3 p normal photoemission data taken at the National Synchrotron Light Source in Upton, New York, USA.

Electronic excitations in Bi2Sr 2CaCu2O8: Fermi surface, dispersion, and absence of bilayer splitting.

From a detailed study, including polarization dependence, of the normal state angle-resolved photoemission spectra for Bi$_2$Sr$_2$CaCu$_2$O$_8$, we find only one CuO$_2$ band related feature. All other spectral features can be ascribed either to umklapps from the superlattice or to ``shadow bands''. Even though the dispersion of the peaks looks like band theory, the lineshape is anomalously broad and no evidence is found for bilayer splitting. We argue that the ``dip feature'' in the spectrum below $T_c$ arises not from bilayer splitting, but rather from many body effects.

Temperature-dependent surface Fermi contours: Origin of unusual surface geometries on Ag(1 1 1)?

The well-known Shockley-type surface state observed in normal photoemission from Ag(1 1 1) shifts linearly with temperature T and is located above the Fermi energy at T > 450 K. In consequence it gets partially empty at increasing T. We raise the question whether this is the reason for a surface reconstruction observed under particular electrochemical conditions [Y. Tang et al., Phys. Rev. Lett. 67, 2814 (1991)] and for an unusual surface relaxation reported recently [P. Statiris et al., Phys. Rev. Lett. 72, 3574 (1994)].

Surface states and photoemission of magnetic multilayer systems.

Key features of the surface electronic structure of magnetic multilayer systems are derived by analytical tight-binding Green function calculations for the simple model of a semi-infinite chain. In the case of antiferromagnetic (AFM) coupling, for which the bulk is macroscopically nonmagnetic, spin-up and spin-down surface states emerge in a bulk energy gap associated with the reduction of the Brillouin zone with respect to the nonmagnetic case. Explicit expressions for the surface state (ss) characteristics are obtained in terms of the main band parameters, displaying a strong sensitivity of the ss energies to deviations of the surface magnetic moments from the bulk ones. The simple model findings are confirmed and put on a quantitative footing by means of relativistic layer--Korringa-Kohn-Rostoker (KKR) calculations for multilayer systems ${\mathrm{FePt}}_{\mathit{n}}$(001) consisting of ferromagnetically ordered Fe monolayers coupled ferromagnetically (FM) or antiferromagnetically through n=0,1,2 Pt layers. In particular, we retrieve for AFM coupling magnetic surface states in symmetry-specific bulk energy gaps. For magnetization normal to the surface, normal valence-band photoemission spectra have been calculated by a relativistic one-step-model layer-KKR approach. Spin-polarized features in these spectra reflect the magnetic surface states. Due to spin-orbit coupling, they further exhibit an intensity asymmetry upon magnetization reversal known as magnetic circular dichroism (MC). For AFM coupling, the surface-induced MCD can in fact be stronger than the MCD found for the corresponding FM system.

Photoemission Calculations Including Both Band Structure Effects and Photon Field Variation

AbstractPhotoemission cross‐sections are calculated, using band wave functions and a spatially varying photon field. The solid is considered to be composed of a stack of identical layers of atoms parallel to the surface with a potential of muffin‐tin form. The photoemission matrix element is calculated by appropriate expansion of the initial and final state wave function and the vector potential of the photon field in the muffin‐tin, interstitial, and vacuum regions. The case of normal photoemission from the (100) face of aluminium is considered as an application.

Studies of the electronic structure of the ferrimagnetic alloy Pt3Cr using normal take-off angle-resolved photoemission

The ordered alloy, Pt2Cr has the L12 structure and is ferrimagnetic with Tc=170 degrees C. Angle-resolved photoemission experiments using synchrotron radiation (18-45 eV) have been made in normal take-off geometry from the (110) surface in order to elucidate the band structure. The valence band energy-momentum relationship in the Gamma Sigma M direction has been determined assuming a free-electron final state. The symmetries of these initial states have been determined using the polarization of the synchrotron radiation.

Magnetic linear dichroism and spin polarization in 3d-band photoemission

Spin-orbit effects in normal photoemission from ferromagnetic cubic (001) surfaces with in-plane magnetization M\ensuremath{\rightarrow} have been studied by symmetry considerations and by relativistic one-step-model Green-function calculations for d-band emission from Ni(001). Spin-averaged spectra due to s-polarized light in two orthogonal states show an intensity asymmetry, i.e., magnetic linear dichroism (MLD). For off-normally-incident p-polarized light, there is another type of MLD, which is maximal for M\ensuremath{\rightarrow} normal, and zero for M\ensuremath{\rightarrow} parallel to the plane of incidence. In the latter case, the electron spin polarization P\ensuremath{\rightarrow} has components exclusively due either to spin-orbit coupling or exchange. Individual spectral features are explained by the relativistic bulk band structure and by analytical considerations.

The relativistic layer-resolved electronic structure and photoemission of the adsorbate system Xe/Pt(111)

For the ( square root 3* square root 3)R30 degrees commensurate physisorption system Xe/Pt(111), the K/sub //-/, layer- and double-group-symmetry-resolved density of states (LDOS) as well as spin-resolved normal photoemission spectra for circularly and linearly polarized light have been calculated by means of a fully relativistic Green function formalism. Comparison with the LDOS obtained for an isolated Xe monolayer shows that the mj splitting of the 5ps2/-derived states of the Xe adsorbate is almost exclusively due to the lateral interaction within the Xe layer. In contrast. Pt derived features are strongly determined by the Xe overlayer via 'surface umklapp': firstly, electronic states localized in the two topmost Pt monolayers; and secondly, photoemission features originating from umklapped Pt bulk initial states. For circularly polarized light, we obtain very good agreement with experiment with regard to peak positions and sign of the spin polarization. Substantial discrepancies in absolute polarization values are ascribed to Auger-like electron-hole processes and call for an extension of our photoemission theory to incorporate these additional excitation channels. For linearly s-polarized light, the calculated spin polarization spectra depend very strongly on the lateral position of the Xe layer relative to the substrate.

Angle-resolved photoemission from Au deposited on Ag(111): evidence for surface alloy formation

We have performed an extensive study of angle-resolved normal photoemission from Au deposited on Ag(111) at temperatures T ⩽ 330 K. In contrast to other authors we do not observe a layer-by-layer growth of Au(111) with an abrupt interface to the substrate. Our results present clear evidence that in the first deposition steps an ordered alloy interface develops, with a silver-rich outermost layer and an estimated thickness of at least 4–5 monolayers, until after more than 5 nominal Au-monolayers bulk-like gold starts to grow. Besides information on the particular Au/Ag(111) system our results demonstrate the capabilities of angle-resolved photoemission to explore extended interfaces.

The surface electronic structure of Ag8Au4 superlattice

We present the calculations of electronic structure and photoemission spectra for a lattice-matched Ag-Au(111) superlattice. The selfconsistent band structure exhibits a superlattice gap at about 1 eV below the Fermi level. A surface state is found in this gap and its dispersion properties are investigated. Its energy location is varied with location of surface terminating plane within the superlattice period. The calculated normal photoemission spectra explain well available experimental data.

Electronic structure and spin-resolved photoemission of Cu3Au (001)

A fully relativistic Green function formalism is applied to calculate the layer-, k/sub ///- and double group symmetry-resolved densities of states (DOS) and the spin-resolved normal photoemission intensities produced by normally incident circularly polarized light for the (001) surface of Cu3Au in its ordered phase. In the calculated photoemission spectra, a number of features can be ascribed to interband transitions in the authors simultaneously calculated bulk band structure, with a real self-energy correction of -0.45 eV for the lower and of 2.5 eV for the upper states. In addition, the authors identify seven features as due to surface states or surface resonances. A comparison with spin-resolved experimental data shows reasonable agreement with regard to existence, energy and preferential spin polarization (i.e. symmetry type of initial states) for most features.

Spin polarization and magnetic dichroism in photoemission from core and valence states in localized magnetic systems. II. Emission from open shells.

With the use of many-particle theory general equations are derived for the six fundamental spectra, which are special linear combinations of the spin-polarized photoemission spectra measured with left-, Z-, and right-circularly polarized radiation. These fundamental spectra, which are the isotropic spectrum, spin spectrum, magnetic circular dichroism, spin-orbit spectrum, anisotropic spectrum, and anisotropic spin spectrum give the correlation between the spin, orbital, and quadrupole momenta in the ground state and spin and orbit of the hole created after photoemission. For emission from an incompletely filled shell the integrated intensities of the fundamental spectra are proportional to the expectation values of the number of electrons, spin magnetic moment, orbital magnetic moment, the alignment between orbital and spin magnetic moment, the quadrupole moment, and the correlation between quadrupole and spin magnetic moment, respectively. This can be used to study the magnetic anisotropy of localized magnetic systems. We calculated the fundamental spectra for the 4f photoemission of the rare-earth ions and show that the multiplet structure displays a strong polarization dependence in agreement with our analysis in terms of correlations of moments and the rules for the integrated intensities. Finally we present the relation between the polarization effects in inverse photoemission and normal photoemission and we discuss the magnetic dichroism in metallic iron, cobalt, and nickel.

An unoccupied Shockley surface state on a carbon deficient TiC (1 1 0) surface

Pendry's one-step model is used to calculate angle-resolved normal incidence inverse photoemission spectra for a carbon deficient unrelaxed TiC(100) p(1 × 1) surface, as found experimentally. Both a bulk and a surface related peak with Γ 5(Δ 5) symmetry are present. The latter is identified as a Shockley surface state which is strongly attenuated by carbon vacancies, and exists only for vacancy concentrations well below 20%. Vacancies destroy the hybridization gap between the almost completely occupied lower and unoccupied upper Δ 5 bulk band, necessary for the existence of a surface state of Shockley type. Experimental evidence forthis particular behaviour is discussed.

Electronic band structure of Cu3Au: An angle‐resolved photoemission study along the [111]‐direction

AbstractHigh‐resolution normal photoemission (ARPE) spectra have been recorded for Cu3Au(111) with the use of polarized synchrotron and rare‐gas resonance radiation in the photon energy range from 9 to 27 eV. It is for the first time that dispersions of the gold‐like bands have been found experimentally. Using a fully relativistic layer‐KKR photoemission formalism, occupied and unoccupied bands as well as one‐step‐model photoemission spectra have been calculated. The comparison of calculated spectra with experimental ones and the observation of direct‐transition resonances upon photon energy near the Brillouin zone‐center reveal a shift of the unoccupied ground‐state bands by about +2.5 eV (self‐energy shift). The direct‐transition structures in the experimental spectra have been exploited to determine the dispersions of the occupied bands along the [111] direction (A line in k space). In order to determine the wave vector of the experimental direct transitions we used as final state that calculated unoccupied band along [111], which also exists in pure copper and gold up to about 20 eV above the Fermi energy (“unfolded” band structure), shifted by + 2.5 eV. The experimental occupied bands with Cu character are in very good agreement with theory after shifting the latter by about 0.3 eV to lower energy, whereas somewhat bigger discrepancies exist for the gold‐like bands.

Angle resolved resonant photoemission spectra of GaAs(110)

Off normal resonant photoemission originating from the decay of a surface core exciton on GaAs(110) studied with high angle resolution shows spectra of complex fine structure, concerning the relative contributions of surface or bulk states, resonance or anti-resonance of the particular states or the strength of their resonant enhancement. In conjunction with theory this leads to a more refined view of several aspects of resonant photoemission and its usual interpretation in oversimplified terms. The k-resolved resonant current at critical points of the surface Brillouin zone was calculated, starting from first principle scattering theory, and good correspondence with the experimental spectra was found, thereby confirming the theoretical model.

The surface electronic structure of (100) centered tetragonal copper

We present a theoretical investigation of the surface electronic structure for the (100) surface of centered tetragonal copper. The calculated band structure and k ∥- and layer-resolved densities of states for the surface and bulk are compared with corresponding quantities obtained for the face centered cubic (fcc) structure. We have found a surface state at about 5 eV below the Fermi level which is splitted off the bulk s-d band for both the tetragonal and the fcc structure. Furthermore, we have calculated the normal photoemission spectra within the one-step approach. The direct transition model explains all features of available experimental data. Particularly, we can conclude that the invisibility of the surface state for the fcc structure in most of the photoemission experiments is driven by the final states.

Spin-polarized normal photoemission from non-magnetic (111)-surfaces by p-polarized light

In normal photoemission from (1 1 1) surfaces of non-magnetic centro- symmetric cubic crystals by off-normally incident linearly and unpolarized light, spin-orbit coupling produces two different spin polarization effects. One, which already occurs for normal incidence, arises from half-space initial states of double group symmetry Λ 4+5. The other comes from Λ 6 states and reflects the hybridization of basis functions of spatial symmetries Λ 1 and Λ 3. For a special azimuthal angle of photon incidence the two surface-parallel spin polarization vectors are perpendicular to each other. Relativistic dynamical calculations for Pt(1 1 1) reveal large polarization features, which are sensitive to the surface barrier.