Search

Changes of the valence band electronic structure of 6H polytype SiC crystal were studied. The changes were caused by deposition of Mn atoms on SiC(0 0 0 1) surface cleaned by annealing at 773 K in ultra high vacuum (UHV) conditions. The Mn atoms were deposited in situ sequentially 1 and 2 ML, and the sample was annealed at 773 K. Synchrotron radiation in the energy range from 47 to 51 eV was applied to study photoemission spectra of the valence band. The photoemission spectra corresponding to the Fano resonance for Mn 3p–3d transition were measured and the contributions of Mn3d electrons to the valence band were determined. The results showed appearance of the contribution of Mn3d band corresponding to Mn islands on the SiC. The annealing of the sample leads to the Mn atoms diffusion into the crystal and appearance of the new structure.

The electronic structure of polycrystalline Ge 0.86 Mn 0.14 Te—ferromagnetic semiconductor with T C =110 K, has been studied by means of resonant photoemission spectroscopy for photon energies close to the Mn 3p→3d excitation. The contribution of Mn 3d states to the electronic structure of the system was revealed and position of the Mn ions in the crystal, characteristic of GeTe-based diluted magnetic semiconductors was proved.

The Fano resonance photoemission studies of Gd/(Pb,Gd)Te layers using synchrotron radiation were carried out and the electronic structure parameters like binding energies of Gd 4f and 5p shells, resonance and antiresonance energies for Gd were determined. The presence of Eu ions was observed in the (Pb,Eu)Te and (Eu,Gd)Te layers grown by MBE technique. The comparison of data for (Pb,Gd)Te compound with corresponding data for (Eu,Gd)Te and (Pb,Eu)Te layers indicates that we are not able to distinct the Eu4f and Gd4f electrons contribution to the valence band photoemission spectra because of small content od Gd and similar binding energy values. The key parameters allowing to prove exactly the presence of either Eu or Gd are the resonance and antiresonance energies which are significantly different for these ions and equal to 143 eV/137 eV and 150 eV/142 eV, respectively.

The substitution of Gd 3+ for Eu 2+ ions introduces conducting electrons and qualitatively changes the model of description of magnetic properties of these semiconductor alloys: antiferromagnetic order observed in insulating layers of EuTe is replaced by ferromagnetic (RKKY interaction driven) state in metallic n-type (Eu, Gd)Te. The high-energy photoemission spectroscopy ( hν = 3000 eV) was used to study the electronic structure and valence of magnetic ions in about one micron thick monocrystalline, ferromagnetic n-type (Eu, Gd)Te layers grown by molecular beam epitaxy on BaF 2 (1 1 1) substrate with EuTe buffer layer. The photoemission spectra of Eu (3d, 4d, 4f), Gd (3d, 4d, 4f), Te (3d, 4d) electrons as well as valence band electrons were measured after sequential Ar ion etching. For surface of the sample exposed to the air the oxidation of surface takes place and measured spectra showed the dominant Eu 3+ contribution in comparison to Eu 2+ . The chemical shift of Te 3d and Te 4d peaks was observed for Te atoms oxidized on the surface. Surface sputtering by Ar ions removes the top layer with oxygen adsorbed at the surface and oxygen diffused in the surface region and clean (Eu, Gd)Te surface of the layer was measured. For clean surface the Eu 2+ dominate in the spectra and the peaks of oxidized Te atoms disappear. The contribution of Eu 2+ 4f electrons to the electronic structure of the (Eu, Gd)Te was found at the top of the valence band.

The surface and electronic structure of Ga0.92In0.08N layers grown by metal organic chemical vapor deposition (MOCVD) have been investigated by means of photoemission. Stability of chemical composition of the surface subjected to Ar+ ion sputtering was proven by means of X-ray photoemission spectroscopy. The analysis of the relative intensities of In 3d, Ga 3p, and N 1s peaks showed that argon ion bombardment does not change significantly the relative contents of the layer constituents. Simultaneous efficient removal of the main contaminants (O and C) was observed during the sputtering procedure, proving that argon sputtering can be used as a method for preparation of clean Ga1−xInxN surfaces. For a clean (0001)–(1×1) surface prepared by repeated cycles of Ar+ ion sputtering and annealing, electronic structure was investigated. The band structure was explored along the Γ–A direction of the Brillouin zone, measuring angle-resolved photoemission spectra along the surface normal. A similar set of data was also acquired for the same surface of GaN layer. Comparison of the collected data revealed an additional feature at the valence band edge, which can be ascribed to the presence of In in the layer.

The electronic structure of monocrystalline ZnTe was investigated for a clean, relaxed ( 110)-(1 x 1) surface and for the same surface after sequential deposition of Mn atoms. The resonant (Fano-type) photoemission technique was applied. The spectra were measured as energy distribution curves and in the constant initial state mode, for the valence band and Zn 3d core level. At very low Mn coverage (θ < 0.15 nm), manganese interaction with the crystal surface results in creation of ternary alloy Zn 1-x Mn x Te(110) surface. Further increasing of Mn deposition leads to formation of metallic manganese islands on the surface. The successive annealing leads to Mn atoms diffusion and to formation of Zn 1-x Mn x Te at the surface region.

High-energy photoemission spectroscopy (hν=3510 eV) was used to study the electronic structure of EuS buried layers deposited on a thick PbS layer and covered by about 30 A of PbS layer. The study of Eu 3d, 4d and valence band photoemission spectra showed the existence of only Eu2+ ions in the as-grown buried EuS layer. The angular behavior of the Eu 3d spectra revealed that Ar ion bombardment of the sample surface, i.e. initially the PbS cap layer, leads to a change of stoichiometry in the near surface region of the EuS layer leading to depletion of S and concomittant accumulation of Eu3+ at the surface. In addition, prolonged X-ray exposure is found to gradually increase the Eu3+ concentration in the surface region. The contribution of Eu2+ 4f electrons to the electronic structure of the PbS/EuS/PbS multilayer is found at the top of the valence band.

We report on a comprehensive study of the Sm/GaN(0 0 0 1 ) interface using synchrotron radiation photoemission. Spectra were taken after stepwise Sm deposition onto a clean GaN(0 0 0 1 ) surface at room temperature and after annealing. Based on the analysis of resonant photoemission spectra taken for hν=136 eV (divalent samarium resonance) and hν=141 eV (trivalent samarium resonance) we find that both divalent and trivalent samarium states are present in the GaN–Sm interface layer after each step of deposition. The average valence of the system increases from 2.57 for a coverage of 0.6 A to 2.73 for 3.2 A. Above this coverage the average valence drops and reaches a final value of 2.63 for a coverage of 10 A. The Ga3d core level evolution confirms that two valence Sm states are present in the interface region. A large chemical shift of the Ga3d core level after samarium deposition and a moderate decrease in the Ga3d attenuation curve indicate that a reactive interface is created when Sm atoms are deposited on the GaN(0 0 0 1 ) surface. Annealing of the Sm/GaN system to 500 °C induces a valence change from Sm2+ to Sm3+ and promotes diffusion of Sm from the interface into the bulk.

The ultraviolet photoemission and resonant photoemission spectroscopy were used to study the electronic structure of EuS/PbS multilayers with the contribution of europium’s 4f orbitals to the density of states of the valence band. The valence band photoemission spectra of galena and europium monosulfide layers show good agreement with theoretical density of state computations. The relative ratio of Eu 3+ and Eu 2+ ions located at the surface of EuS layer strongly depends on conditions of preparation procedures of the sample surface. For the buried EuS layer only Eu 2+ ions were observed.

Resonant photoemission spectroscopy, with application of synchrotron radiation, was used to study of the valence band electronic structure of clean surface of the Si crystal doped with Gd and of these surface after deposition of 1 2 ML of Gd atoms. Fano-type resonant photoemission spectra corresponding to the Gd 4d–4f transition were measured to determine the contribution of Gd 4f electrons to the valence band of Gd/Si crystal surface. The resonant and antiresonant photon energies of Gd atoms deposited on Si surface were found as equal 151.8 and 146.8 eV, respectively. Position of Gd 4f electrons contribution was found at 9.8 eV below the valence band edge of Gd/Si valence band.

Angle-resolved photoemission measurements on gallium nitride single crystals and epitaxial thin films with wurtzite structure were performed using synchrotron radiation. Calculated theoretical final state bands were used to determine the corresponding k vectors in reciprocal space using the direct transition model. We were able to identify several previously unobserved features including several surface states and transitions to non-free-electron final states. Significant differences in the surface electronic band structure between thin film and single crystal samples were observed. 2002 Elsevier Science B.V. All rights reserved.

X-ray photoelectron and ultraviolet resonant photoemission spectroscopy was used to study the Sn0:96Mn0:04Te MBE layers. The surface chemical composition of the samples after different treatment conditions was studied by means of XPS. Resonant photoemission spectroscopy with application of the synchrotron radiation was applied to investigate the electronic structure and the contribution of Mn 3d electrons to the valence band of Sn0:96Mn0:04Te layers. The contribution of the Mn 3d electrons to the valence band electronic structure appears at the valence band with the maximum located at 4.0 eV below the valence band edge. 2002 Published by Elsevier Science B.V.

Experimental photoemission results on GaN (0 0 0 1 ) and CdTe(1 0 0) surfaces on a function of samarium adsorbate coverage are presented. Energy distribution curves were recorded at photon energies around the Sm 4 d →4 f threshold (135–136 eV) to reveal the Sm contribution to the electronic structure and enhance the sensitivity. The evolution of core-level Ga 3d and Cd 4d spectra was also studied. The interaction of Sm with both GaN and CdTe leads to reactive interface formation. In the case of CdTe, it results in creation of an interfacial Sm–CdTe compound, which inhibits further diffusion of Sm into the bulk. The interaction of Sm with GaN leads to the release of Ga atoms on the surface and segregation of Ga clusters.

Photoemission spectroscopy was applied to determine the valence band density-of-states in zinc blende MnTe grown by MBE on a CdTe(100) substrate. The Mn 3d derived contribution to the electronic structure was revealed by resonant photoemission experiments. Our experimental results are compared to the available configuration-interaction cluster models. An important role of the core hole screening (through charge transfer from ligand orbitals) on the whole Mn 3d derived photoemission spectrum (including the satellite structure observed below the valence band) was clearly confirmed.

The paper presents the experimental results of the electronic band structure study of the semiconductor crystal clean surface and this surface doped by transition metal or rare earth metal atoms. For clean surfaces of the CdTe crystal the two-dimensional electronic band structure E( k ) dependence was determined for surface states located in the valence band energy region. The doping of the clean surface of CdTe was performed by the controlled, sequential deposition of metal atoms (Fe, Gd or Yb) on the surface. After each deposition the synchrotron radiation was used to measure the resonant photoemission spectra (energy distribution curve – EDC, constant initial states – CIS and constant finale states – CFS). The results of the study showed that in the first stages of the metal atoms' deposition in the range of thickness equal to about 0.5 ML the effect of the crystal clean surface doping occurs. For bigger metal layer deposition the metallic islands electronic structure gave the contribution to the measured spectra. Heating of the sample surface covered by metal atoms leads to the diffusion of the atoms into the sample and results in an increase of the crystal doping and decrease of the metallic islands contribution to the measured spectra.

Synchrotron radiation photoemission has been used to study the formation of Sm overlayer on the surface of CdTe(1 0 0) crystal. The data were recorded at room temperature for Sm coverage ranging from 0 to 9 ML. The analysis of constant initial state spectra and energy distribution curves taken in the valence band region showed that at very low Sm coverage (0.1 and 0.2 ML) the trivalent Sm state dominates. For higher coverages the average valence of the system drops down from the value of 2.7 to 2.2 for 1.8 ML. The analyses of Sm4f multiplet peaks binding energy lead to the conclusion that the average valence of Sm in the Sm/CdTe(1 0 0) system is heterogeneous (the valence has site-dependent but integer value). The photoemission results indicate strong Sm–CdTe interaction and confirm the assumption about the creation of a surface compound with spinel CdSm 2 Te 4 structure.

We present the results of photoemission studies on (0 0 0 1 ) (N-polar) surfaces of bulk gallium nitride crystals. The procedures that yield clean and well-ordered (0 0 0 1 ) (1×1) surfaces were developed and tested. The surface crystallinity was assessed by low-energy electron diffraction. Angle-resolved photoemission spectroscopy with synchrotron radiation was used to determine the valence-band electronic structure of GaN along the Γ – Δ – A direction of the Brillouin zone. The band structure derived from our experimental data is compared with the results of recent band structure calculations.

The aim of the work is to present the XPS and UPS studies of a new kind of material MBE grown -CdYbTe thin layers. The analysis of the valence band electron states was performed for three samples (with different Yb contents and different crystallographic quality) by means of resonant photoemission based on the electronic transition between Yb 4d–4f shells. Analysis of Cd 3d, Te 3d and Yb 4d core levels made by XPS gave the qualitative information about the samples before and after Argon ion sputtering. The photoemission spectra for the CdYbTe sample containing 5% Yb, which corresponds to the limit of Yb solubility, were compared with the spectra of the CdYbTe sample with 1% Yb. By means of resonant photoemission Ytterbium was found in two charge states Yb(+2) and Yb(+3).

Resonant photoemission spectroscopy has been applied to study the group of Pb 1− x RE x B VI (RE=Eu, Gd; B=Te, Se, S) diluted magnetic semiconductors. Energy distribution curves, as well as constant-initial-state or constant-final-state photoemission spectra have been taken for photon energies near to the intra-ion Eu 4d→4f or Gd 4d→4f transitions. The Eu 4f and Gd 4f contributions to the electronic structures of the crystals have been determined by comparison of photoemission spectra taken on and off resonance. The localized character of the Eu(Gd) 4f 7 shell has been confirmed by the analysis of the shape of the corresponding spectral feature. The influence of the host crystal ionicity on the position of the RE 4f shells with respect to the valence band is discussed.

The electronic structure of Cd 0.985 Fe 0.015 Te has been studied by resonant photoemission in the synchrotron radiation photon energy range from 48 to 67 eV. Photoemission spectra revealed that Fe contributions to the valence band density of states are situated at 1.2 eV, 4.1 eV and 5.9 eV below the valence band maximum. The contribution of Fe 3d states to the valence band electronic structure was also observed in situ during evaporation of Fe atoms on the clean CdTe(110) surface. Photoemission measurements show that after Fe evaporation and heating to 265°C the CdTe(110)/Fe crystal, a ternary compound Cd 1− x Fe x Te is created in the surface region. The results of photoemission experiments are consistent with the theoretical calculations made by the CPA (coherent potential approximation) method.

Resonant photoemission spectroscopy was applied to the study contribution of Gd 4f states to the electronic structure ofSn 1− x Gd x Te. The energy distribution curves (EDC) were analyzed and the binding energy of Gd 4f shell was determined. The correspondence between the charge state of Gd ions and the binding energy of Gd 4f electrons was discussed.

The results of valence band electronic structure study of GaN ( 0 0 0 1 _ ) 1 × 1 (N-side) clean surface covered by Fe atoms and annealed are presented. The very first stage of Fe deposition (below 1 ML) leads to the Fe doping contribution to the GaN valence band electronic structure. With sequential Fe depositions (above 1 ML) the islands of Fe are created and sharp Fermi edge is appearing over the edge of the GaN valence band. Annealing (500 °C) of the sample in ultra high vacuum conditions, leads to the diffusion of Fe atoms to the surface region of GaN. It leads to the decrease of the contribution of Fe islands and increase of Fe 3d electrons contribution to the valence band of Ga1−xFexN. Spectra obtained after annealing possess elements of structure well corresponding to the calculated electronic structure of FeN.

Resonant photoemission spectroscopy was applied to investigate the electronic structure of semimagnetic semiconductor (SMSC) Pb0.2Sn0.72Mn0.08Te and the contribution of Mn 3d electrons to the valence band. The set of energy distribution curves (EDC) was taken for energies in the region close to the Mn 3p-Mn 3d transition (50eV). The spectra contain extended valence bands (with core-like electrons of Pb 6s, Sn 5s and Te 5s), and core electrons of Pb 5d, Sn 4d, Te 4d and Mn 3p. The electrons Mn 3d hybridize and contribute to the valence band electrons of the crystal; their main contribution to the density of states appears in the energy 3.7 ± 0.3eV below the valence band edge.

The electronic structure of epitaxial SrRuO 3 thin films annealed at 300 °C in ultra high vacuum is studied by resonant photoemission spectroscopy using synchrotron radiation. The valence band photoemission spectra show a good agreement with theoretical band structure calculations indicating a good quality of the film surface. A method was proposed, which allows extracting the partial density of states in the case of weak and broad resonance. The Ru 4d spectral weights obtained experimentally are in a very good agreement with theoretical predictions, confirming a strong mixing between Ru 4d and O 2p states, as well as the signature of electron correlation near the Fermi level in SrRuO 3 .

In this work we present a comparison of the experimental results, which have been obtained in the resonant photoemission study of electronic structure of epitaxial layers and bulk crystals of GeTe doped with Mn and/or Eu. Narrow-gap IV–VI semiconductors are known to form solid solutions not only with magnetic ions with partially filled 3d shell (e.g. Mn, Fe), but also with the elements with partially filled 4f shell (e.g. Eu, Gd). Such diluted magnetic semiconductors (DMS) exhibit transport, optical and magnetic properties attracting large interest from the point of view of basic research as well as of applications in infrared radiation sources, detectors, thermoelectric generators and, recently, spintronic devices. The ferromagnetic ordering in IV-VI –based diluted magnetic semiconductors (DMS) is governed by the Ruderman–Kittel–Yoshida–Kasuya interaction mediated by free holes related to native defects. Ge

Unreconstructed CdTe(100) surface prepared by ion bombardment and annealing is investigated by angle-resolved photoemission. The experimental band structure E(k || ) is determined along high-symmetry lines of the surface Brillouin zone by measuring energy-distribution curves of photoelectrons. Different criteria were applied to separate surface and bulk related spectral features, e.g. calculating the position of bulk-derived emissions in the frame of the free-electron final state approximation assuming k conservation. In this way, most dispersing features could be explained. All remaining features were compared with theoretical surface band structures for different polar surface terminations which were calculated within a layer doubling procedure on the basis of an EHT-flt to the bulk band structure. The investigated CdTe(100)-(1x1) surface could be identified as Cd terminated. Two surface bands were observed, one located above the valence-band edge and the second in the open pocket of the projected bulk band structure along the Γ K direction. At 4.6 eV binding energy an additional weakly dispersing band was found, which contains mixed surface and bulk character. The high density of bulk states associated with this edge of the heteropolar gap is also expected to contribute to this feature.

The electronic structure of semimagnetic semiconductor ternary alloys Cd1-xMxSe (M = Mn, Fe, Co) crystals was investigated by photoemission. The ground state of impurity level of Fe 3d6 and Co 3d7 were found to determine the position of the edge of the valence band of the ternary alloys crystals Cd1-xFexSe and Cd1-xCoxSe. The core level Se 3d spectra measured for Cd1-xFexSe crystals with different composition x correspond to the appearance of local structure in the crystal. The valence band density of states distribution was estimated for MnSe, FeSe and CoSe with CdSe structure as a difference between valence band density of states distribution for Cd1-xMnxSe, Cd1-xFexSe or Cd1-xCoxSe respectively and CdSe.

Angle Resolved Photoemission Spectroscopy (ARPES) with high resolution (energy 60 meV, angle ±0.5°) and with the HASYLAB Synchrotron Radiation was applied to determine the electronic structure of volume as well as the surface states of clean CdTe (110) surface. The dangling bond surface states were found rather over the edge of the valence band and other surface states were found at the valence band region.

Photoemission yield spectroscopy (PYS) was used to investigate HgSe and Hg0.88Fe0.12Se crystal surface freshly cleaved in a UHV chamber at a pressure of about 10−9 Torr. The spectral photoemission yield curves were measured in the vacuum-ultraviolet range (4 < hv < 11 eV). The experimental results show that the contribution of the Fe3d6 electrons to the electronic structure of the valence band leads to a shift of the work function from 4.30 to 4.15 eV. This contribution appears as an additional shoulder on the edge of the spectral photoemission yield curve. The interaction of the ambient atmosphere with the surfaces of freshly cleaved Hg1−xFexSe (x = 0.0, 0.12) crystals causes a decrease of the ionization energy Ei and the work function W.

Ultraviolet photoemission spectroscopy (UPS) as an investigative method for surface states is presented. The measured energy distribution curve (EDC) furnishes information on the electronic density of states distribution in the valence band, nearest to the core levels and surface states. The measured angle resolved EDC's provide the possibility to determine the energy-momentum E(k) dependence of electrons in the bulk of the crystal and on the surface. Application of the synchrotron-storage ring system as a source of ultraviolet radiation in the energy range from 10 to 300 eV opens new avenues to investigate structure of electronic states. It features the following possibilities: (1) To distinguish the contribution of the surface and bulk states to the obtained EDC by measuring the change of the EDC when varying the exciting energy hv around the minimum of the escape depth (E = 80 eV) and outside of this region. (2) To discriminate in the valence band the contribution of the d-electrons from that of the s-p electrons due to a different change of the photoemission cross section of the d and s-p electrons with a change of hv. (3) To recognize the localized and delocalized contribution to the density of states in the valence band and to determine E(k) for these electrons by measurements of the angle resolved EDC. (4) To obtain information on the initial and final state distribution using the constant initial states (CIS) and/or constant final states (CFS) techniques. The potential of the photoemission technique will be illustrated by the results of the electronic structure investigation of some metals and semiconductors.

Publisher Summary The UPS /Ultraviolet Photoemission Spectroscopy/ and AR UPS/Angle Resolved UPS/ methods are presented as powerful tools in investigations of the electronic structure of the crystals. The electronic structure investigations of the three-component crystals are illustrated in the chapter by the results obtained for two sets of crystals with different ionicities. The AR EDCs obtained for Cd 1-x Mn x Se crystals have shown the localized and delocalized contributions of the Mn3d states to the valence band. The maximum obtained at the distance of 3.35eV below the valence band edge of Cd .7 Mn .3 Se and Cd .6 Mn .4 Se crystals corresponds to the localized part of the Mn3d states. The delocalized part of the Mn3d states contribution is obtained in the upper part of the valence band corresponding to the region of 0 - 2,5 eV below the valence band edge.

The energy distribution of the valence band states of cubic β-HgS was investigated by means of photoemission spectroscopy. The energy positions of the spectral features observed were used to determine band energies at high symmetry points of the Brillouin zone. Changes in the binding energy of Hg 5d states in cubic mercury chalcogenides were analyzed and the role of the interaction between Hg 5d and S 3p states in β-HgS was estimated.