Normal Photoemission Research Articles

Electron band bending and surface sensitivity: X-ray photoelectron spectroscopy of polar GaN surfaces

The role of electron band bending and surface sensitivity in determining the core level binding energies by X-ray photoelectron spectroscopy is investigated. A dominating contribution of surface atomic layers to photoemission intensity is confirmed for normal photoemission. The energy of the photoelectron core level peak does not deviate from core level peak energies of electrons photoemitted from the surface atomic layers of the crystal. The higher surface sensitivity regime, achieved e.g. at off-normal photoelectron detection angle, can be used to study the surface potential barrier in just a few topmost atomic layers. In addition, it is demonstrated that core level binding energy measured by angle-resolved X-ray photoelectron spectroscopy reflect the electron attenuation anisotropy. In particular, core level binding energy changes with emission angle and correlates with the forward focusing directions in a crystal. This effect is demonstrated by measuring the polar angle dependence of Ga 3d core levels on clean GaN(0001) and GaN(0001¯) surfaces with a higher and a lower band bending, respectively. The effect is explained by variation of emission depth in a crystal for normal and off-normal photoelectron emission angles.

Photoemission footprints of extrinsic plasmarons

A prediction how to experimentally distinguish excitations of extrinsic plasmarons from intrinsic plasmarons is presented. In surface systems where excitations of acoustic surface plasmons is possible it is shown that the photo-electron yield in normal photoemission should decay according to an inverse square root dependence with respect to the photon energy. A computational analysis of the system p(2x2)-K/Graphite confirms this prediction.

Photoemission studies of ultrathin Cs, Ba/InN interfaces

Cs/InN and Ba/InN interfaces were studied by UV photoelectron spectroscopy in the submonolayer coverage range for the first time. Normal photoemission spectra from the valence band and spectra from In 4d, Ba 5p, Ba 4d, and Cs 5p core levels were investigated in the excitation energy range of 60–800 eV. It was found that metallization of the interface and narrowing of the valence band is observed upon increasing coverage.

Hard X-ray photoemission spectroscopy: Variable depth analysis of bulk, surface and interface electronic properties

The electronic properties of surfaces and buried interfaces can vary considerably in comparison to the bulk. In turn, analyzing bulk properties, without including those of the surface, is understandably challenging. Hard X-ray photoelectron spectroscopy (HAXPES) allows the well known ability of photoemission to interrogate the electronic structure of material systems with bulk volume sensitivity. This is achieved by tuning the kinetic energy range of the analyzed photoelectrons in the multi-keV regime. This unique ability to probe truly bulk properties strongly compliments normal photoemission, which generally probes surface electronic structure that is different than the bulk selected examples of HAXPES and possible implications towards the study of complex oxide-based interfaces and highly correlated systems are discussed.

Surface electronic structure of fcc Co films: a combined spin-resolved one- and two-photon-photoemission study

The surface electronic structure of face-centred-cubic cobalt films on Cu(0 0 1) was studied by spin-resolved one- and two-photon photoemission. A minority surface state in a Δ1-symmetry gap of the minority band-structure was identified at about 0.45 eV below the Fermi energy. This state causes a resonance-like enhancement in the population of the minority image-potential surface state in the two-photon-photoemission experiment excited by p-polarized light. Additionally, it appears as a surface-sensitive spectral feature in normal photoemission (PE) and its existence is confirmed by calculations within the one-step model of PE. The majority counterpart of the surface state is theoretically expected at a binding energy of about 1.95 eV. Due to lifetime broadening and bulk transitions in this energy range, the majority state does not appear as a pronounced feature in the spectra. Bulk-derived states close to the Fermi level exhibit shifts to higher binding energy with increasing film thickness, while the minority surface state does not change its energy as a function of Co coverage. These results provide a basis for the interpretation of time-resolved measurements concerning ultrafast magnetization dynamics, which rely on a detailed knowledge of the surface electronic structure of ultrathin films.

Depth distribution function calculated by quantum scattering theory

The depth distribution function (DDF) $\ensuremath{\phi}(z)$ in normal photoemission from solids is studied by use of quantum full multiple scattering theory and non-Hermitian optical potential $\ensuremath{\Sigma}$. The damping of the photoelectron waves is taken into account without use of ad hoc assumption: $\mathrm{Re}\phantom{\rule{0.2em}{0ex}}\ensuremath{\Sigma}$ has influence on the elastic scatterings from surrounding atoms and the $\mathrm{Im}\phantom{\rule{0.2em}{0ex}}\ensuremath{\Sigma}$ on the damping of the photoelectron wave. The present theoretical approach explicitly takes the details of atomic arrangement in solids into account, which is in contrast to the currently used classical approaches applied to jellium models. The latter approach cannot study interference effects caused by elastic scatterings from different atomic sites. To properly describe the DDF, full multiple scattering renormalization is inevitable even at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which needs large scale computations. The elastic scatterings give rise to a peak at $z\ensuremath{\sim}4.1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ in the DDF both for one- and three-dimensional models. Temperature effects on the DDF are also discussed, which smear the DDF because the quantum interference effects are destroyed because of the thermal motions. The asymptotic behavior of $\ensuremath{\phi}(z)$ for large $z$ shows simple exponential decay from which we can estimate ``mean free path.'' It is, however, different from the original one, since elastic scatterings are renormalized to it.

Surface electronic structure of NiMnSb(001)

We report on surface band structure measurements for the (001) surface of single crystal NiMnSb by angle-resolved photoelectron spectroscopy. Through comparisons of our data with previous angle-integrated experiments on various samples, polycrystals, and single crystals, we show that the binding energies of the observed photoemission peaks are sensitive to the particular surface condition and/or preparation. Additionally, our $\mathbf{k}$-resolved measurements reveal a clear energy dispersion of the observed spectral features and a surface state candidate. We compare our measurements to fully-relativistic theoretical calculations and find that while they do not agree with the experimentally observed band dispersion they do explain the elemental origin and energy position of the normal emission photoemission results.

Normal state spectral lineshapes of nodal quasiparticles in single layer Bi2201 superconductor

Abstract A detailed study of the normal state photoemission lineshapes and quasiparticle dispersion for the single layer Bi 2 Sr 2− x La x CuO 6+ δ (Bi2201) superconductor is presented. We report the first experimental evidence of a double peak structure and a dip of spectral intensity in the energy distribution curves (EDCs) along the nodal direction. The double peak structure is well identified in the normal state, up to 10 times the critical temperature. As a result of the same self-energy effect, a strong mass renormalization of the quasiparticle dispersion, i.e. kink, and an increase of the quasiparticle lifetime in the normal state are also observed. Our results provide unambiguous evidence on the existence of bosonic excitation in the normal state, and support a picture where nodal quasiparticles are strongly coupled to the lattice.

Electron mean free path for GaAs(100)-c(4×4) at very low energies

Electron mean free path (MFP) was determined by the angular resolved photoemission in 10-40 eV energy range for GaAs(100)-c(4 x 4) by the overlayer method. The investigation was based on the attenuation of the normal photoemission intensity of the Al 2p line from the molecular-beam-epitaxy grown GaAlAs layer buried four monolayers of GaAs below the surface. The energy dependence of the MFP shows a pronounced maximum at about 30 eV which is related to the corresponding section of the electron band structure of GaAs(100). (C) 2004 Elsevier B.V. All rights reserved.

Electronic structure of GaN(0 0 0 [formula omitted])-(1 × 1) surface

The (0 0 0 1 ̄ ) (N-polar) (1 × 1) surface of bulk gallium nitride crystals has been investigated by means of angle-resolved photoemission spectroscopy. The surface was prepared by repeated cycles of Ar + ion sputtering and annealing. The electronic band structure was explored along the Γ– A and Γ– K– M directions of the Brillouin zone, by measuring normal and off-normal photoemission spectra. A similar set of data was also collected for the same surface subjected to Ga deposition under MBE conditions. The identification of bulk and surface related spectral features was based on analysis of the acquired experimental data and supported by comparison with the results of available band structure calculations. The features characteristic of both (1 × 1) N-terminated and (1 × 1) Ga-adlayer terminated surface configurations were revealed for the surface under investigation.

Dispersion of quantum well states in Cu/Co/Cu(001)

Quantum well states (QWS's) in Cu thin films grown on Co/Cu(001) were studied using angle-resolved photoemission spectroscopy. For the normal photoemission, QWS's from both lower and higher energy bands relative to the vacuum level were measured, and explained by phase accumulation method. QWS's from the lower band were studied in detail as a function of the in-plane momentum k(parallel to). We found that the QWS dispersion depends on the Cu film thickness. From the experimental data, we deduced the quantized perpendicular momentum k(perpendicular to) and the energy as a function of k(parallel to). Our results show that the in-plane effective mass can not be obtained by a simple parabolic fitting of the E vs k(parallel to) due to the dependence of k(perpendicular to) on k(parallel to).

Electronic Structure of Sr2Cu2ZnO2S2 Layered Oxysulfide with CuS Layers

The electronic structure of p-type semiconducting Sr2Cu2ZnO2S2 (SCZOS) was examined experimentally and theoretically. The band gap of SCZOS estimated by the optical absorption of its thin film was 2.7 eV, which is relatively large in chalcogenide semiconductors. Normal and inverse photoemission spectra were measured to observe the valence band or conduction band structure of this material. The observed band structure revealed that the Fermi level of SCZOS was located at the top of the valence band, and the bottom of the conduction band was approximately 2.5 eV above the Fermi level. These results demonstrate the wide-gap and p-type semiconducting nature of SCZOS. Comparison of the photoemission spectra with the density of states calculated by the tight-binding method led to the conclusion that the top of the valence band is mainly composed of a hybridized (Cu 3d)−(S 3p) band, and the bottom of the conduction band primarily consists of a dispersed Zn 4s band.

One-step photoemission calculations for ideal GaAs(001) and AlAs(001) surfaces and(GaAs)m(AlAs)nsuperlattices

Normal photoemission spectra for the (001) surface of $(\mathrm{AlAs}{)}_{2}(\mathrm{GaAs}{)}_{2}$ and $(\mathrm{AlAs}{)}_{3}(\mathrm{GaAs}{)}_{3}$ superlattices are calculated. They represent the first calculations within the one-step model for layered superlattices. The aim is to develop tools to extract general features from measured spectra specific to superlattices. The spectra show the opening of band gaps at the Brillouin zone edge, which are characteristic for the modified periodicity of the superlattice. Furthermore, the layer-resolved photocurrent allows to identify emissions from AlAs layers hidden below the first two or three GaAs layers. As a reference, theoretical normal emission spectra from the pure bulk GaAs and AlAs(001) surfaces are shown.

Electronic structure and nature of the bonding at the Cu(110)+c(2×2)-Si surface alloy

The electronic structure of the Cu(110)+c(2×2)-Si surface alloy has been studied experimentally by synchrotron radiation photoemission spectroscopy. We have recorded normal emission photoemission spectra as a function of the photon energy, Si coverage and angle of incidence of the polarized light in order to identify the electronic features induced in the Cu valence band by the presence at the surface of an ordered Si–Cu bidimensional layer. Two main dispersionless bands in the direction perpendicular to the surface and centered around −3 and −1 eV of binding energy are the most characteristic electronic features of the system. Most of the details in the photoemission spectra are well reproduced by a local density of states calculation employing the Extended Hückel Theory. These results suggest that the electronic states from the surface alloy are mainly originated by an in-plane hybridization of the Cu 3d electrons from the outermost layer with the Si 3p electrons.

Interatomic Resonant Photoemission from Quantum-Well States in Ultrathin Films of Ag on V(100)

Normal emission photoemission spectra from a single ordered monolayer of Ag on V(100) show a sharp and intense peak associated with a quantum-well state. The photon energy dependence of the intensity of this peak shows an identical enhancement profile to the V $3d$ photoemission in the vicinity of the V $3p\ensuremath{\rightarrow}3d$ excitation threshold, characteristic of resonant photoemission. The significance of this result for resonant photoemission characterization of materials is discussed.

COLLECTIVE MODES, PSEUDOGAPS, AND PAIRING POLARON EFFECTS IN HIGH-TEMPERATURE SUPERCONDUCTING MATERIALS

The interpretation of the pseudogap observed in the normal phase photoemission spectra of Bi2212 is discussed in terms of a boson scattering model. Also, the nature of quasi-particle excitations in short coherence length superconductors is presented, illustrating the importance of chain-like structures.

Surface valence bands of GaSb(110) investigated by UV photoemission

Normal and off-normal photoemission experiments are performed with He I radiation on a GaSb(110) surface using a fast photoelectron analyzer. The results are compared with volume and surface electronic structures calculated by the tight-binding method. Care is taken to distinguish true surface emissions from diffracted secondary emissions, also called “umklapp” emissions. The umklapp bands can be followed in off-normal data with continuous variations which have not previously been observed. Surface states at the top of the valence band and surface resonances have been recognized. The tight-binding calculation of the energy dispersion of the umklapp bands confirms the allocation of these emissions (called S 1 and S 2). They seem very similar to those already reported for other group III–V compounds.

Band Dispersions in Photoluminescent Porous Si

The same band dispersions as those obtained from crystalline Si along the $\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Delta}\ensuremath{-}X$ symmetry line are first observed from photoluminescent porous Si independent of its microstructure and porosity using an angle-resolved photoemission technique. The electronic structures of porous Si observed in the normal photoemission, valence photoemission, and $L\ensuremath{-}\mathrm{edge}$ x-ray absorption spectra are interpreted consistently to result from the overlap of that of bulk crystalline Si and that relating to the receding valence structure, which is suggested experimentally to be linked to the luminescence origin.

A model for photoemission from prepoled ferroelectric ceramics

The emission of electrons from a prepoled ferroelectric material when shined with a laser light pulse cannot be interpreted as a normal photoemission process, nor due to polarization switching. Another picture is proposed for the emission: in a prepoled ferroelectric disk a very narrow layer of electrons is set at the surface. The high-power laser pulse is absorbed by the electrons and an Auger process occurs. Hence, part of the hot electrons acquire enough energy to tunnel and/or to overcome the potential barrier and exit the crystal.

Angle-resolved photoemission extended fine structure of the Ni3p,Cu3s,and Cu3pcore levels of the respective clean (111) surfaces

We report a non-s initial-state angle-resolved photoemission extended fine-structure (ARPEFS) study of clean surfaces for the purpose of further understanding the technique. The surface structure sensitivity of ARPEFS applied to clean surfaces and to arbitrary initial states is studied using normal photoemission data taken from the Ni 3p core levels of a Ni(111) single crystal and the Cu 3s and the Cu 3p core levels of a Cu(111) single crystal. The Fourier transforms of these clean surface data are dominated by backscattering. Unlike the s initial-state data, the p initial-state data show a peak in the Fourier transform corresponding to in-plane scattering from the six nearest neighbors to the emitter. Evidence was seen for single-scattering events from the same plane as the emitters and double-scattering events. Using a recently developed, multiple-scattering calculation program, ARPEFS data from clean surfaces and from p initial states can be modeled to high precision. Although there are many layers of emitters when measuring photoemission from a clean surface, test calculations show that the ARPEFS signal is dominated by photoemission from atoms in the first two crystal layers. Thus ARPEFS applied to clean surfaces is sensitive to surface reconstruction. The best-fit calculation for clean Ni(111) indicatesmore » an expansion of the first two layers. {copyright} {ital 1997} {ital The American Physical Society}« less