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Ti/GaN interface formation on GaN(0 0 0 1 )-(1 × 1) surface has been investigated by means of resonant photoelectron spectroscopy (for photon energies near to Ti 3p → 3d excitation). The sets of photoelectron energy distribution curves were recorded for in situ prepared clean GaN surface and as a function of Ti coverage followed by post-deposition annealing. Manifestations of chemical reactions at the Ti/GaN interface were revealed in the valence band spectra as well as in the Ga 3d core level peak—the discerned contribution of Ti 3d states to the valence band turned out to be similar to that reported in the literature for titanium nitride. The interaction between Ti and N was further enhanced by post-deposition annealing. The study was complemented with SIMS and AFM measurements.

Mn/GaN interface formation on a GaN(0 0 0 1)-(1 × 1) surface has been investigated by means of resonant photoelectron spectroscopy, as a function of Mn coverage. A contribution of Mn 3d states to the emission from the valence band of the system was discerned in photoemission spectra taken for photon energies near to Mn 3p → 3d excitation. Interaction between Mn and GaN was also monitored by spectroscopy of the Ga 3d core level. It was shown that Mn on GaN formed a reactive interface with contribution of a compound containing Mn and Ga. Annealing of Mn/GaN at 400 °C changed the surroundings of large part of Mn ions. The Mn 3d related emission spectrum was found to be similar to that characteristic of Mn built into tetrahedrally coordinated semimagnetic semiconductors.

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.

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.

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.

A CdTe(110) surface, prepared in situ by argon ion bombardment and annealing, was oxidised at a pressure of 1×10 −5 –5×10 −5 Torr of molecular oxygen excited by the hot filament of the ion gauge. The adsorption on the surface was monitored by recording the energy position and shape of Te 3d, Cd 3d, O 1s and C 1s core level peaks by X-ray photoemission spectroscopy. The increase of oxygen and carbon bound on the surface was observed as a function of the gas exposure. Simultaneously, the development of additional features connected with the Te 3d peaks was revealed. The extremely slow formation of the oxide layer with strongly enhanced uptake of gaseous carbon compounds (probably CO) has been demonstrated.

Surface differential reflectivity experiments with polarised light have been carried out for cleaved and then oxidised CdTe(110) surfaces. It was shown that the optical anisotropy of the surface investigated is small and almost independent from photon energy. The interpretation of the spectra in terms of optical transitions between surface states is discussed.

We present data on photoluminescence (PL) and the Franz-Keldish (FK) effect on ultra-high-vacuum-cleaved CdTe(110)1 × 1 surfaces by surface differential reflectivity (SDR). PL and the FK effect on a clean surface have been studied as a function of argon laser power with the sample at room temperature. PL spectra have been deconvoluted into two contributions: a free excition peak, which determines the PL peak position, and an interband recombination structure, which dominates most of the PL spectra. The linear dependence of the area of the PL spectra on laser power confirms that the PL spectrum is mainly due to the contribution from free-carrier recombination. The PL peak position shifts towards lower energies (18 meV maximum), linearly with increasing laser power, presumably due to local heating of the sample. The FK effect has an exponential dependence upon laser power. Upon switching off the laser, the FK effect exponentially disappears with a time constant of about 1300 s. Angle-resolved photoemission has been used to evaluate the shift of the Fermi level upon laser irradiation. FK and photoemission results are compared to evaluate the magnitude of the FK effect. Moreover, the photovoltage induced by the photoemission lamp has been measured through the FK effect and resulted to be 80 meV.

The surface band structure of non-cleavable CdTe(111)-2 × 2 reconstructed surfaces is determined by means of angle-resolved photoemission and constant-final-state (CFS) spectroscopy. The experiments were performed with He I radiation and synchrotron radiation from the DORIS II storage ring at HASYLAB. High-quality (111)-2 × 2 surfaces were prepared by sputtering and annealing controlled by electron diffraction (LEED and RHEED). In order to distinguish between surface and bulk related emissions in the spectra we utilized, besides the criteria that the k ∥ dispersion of surface states should reveal the 2 × 2 periodicity of the surfac mesh, also photon energy dependent CFS series at several critical points of the surface Brillouin zone. The data on CdTe(111) will be compared with experimental and theoretical results which are available for the electronically similar GaAs(111) surface.

Angle-resolved photoemission and constant-final-state (CFS) spectroscopy are performed with synchrotron radiation to determine the experimental surface band structure E ( k ∥ ) along the high symmetry lines ГX′MXГgG of in situ cleaved CdTe(110). In order t distinguish between surface and bulk related features we utilized the k ⊥ dispersion behaviour of surface states by photon energy dependent normal emission spectra and off-normal CFS spectra at several critical points of the surface Brillouin zone (BZ) as well as their particular k ∥ dispersion and polarization behaviour. Three surface states and one surface resonance are clearly identified, whereas the resonance is only visible in the second surface BZ. In particular, the energy position of the anion derived dangling bond state with respect to the bulk valence band maximum Γ 8 is determined. The surface band structure is compared with previous experimental and theoretical results.

Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.

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.

This paper examines whether the Schottky formula ϕ ss = E T − W is fulfilled during barrier formation for a CdTe(p) crystal. The parameters ϕ ss , the surface position of the Fermi level, E T , the photoemission threshold and W , the work function, and their changes were determined during oxygen adsorption and In deposition on a CdTe(110) surface freshly cleaved in UHV. The following techniques were used: yield and internal photoemission spectroscopy (YPS, IPS) and contact potential difference (CPD). During oxidation of the CdTe crystal crystal surface ϕ ss did not change and was equal to 0.52 eV. The first stage of In deposition leads to an E T change of about 80 meV and ϕ ss changes to 0.98 eV in good agreement with 0.96 eV determined by IPS for the formed diode. The Schottky formula is well fulfilled with a small deviation which corresponds well to the change in E T (80 meV) obtained after the first stages of In deposition and can be correlated to the reactivity of the In with the crystal surface. The long decay time of the surface potential after illumination disappears after the first stages of metal deposition.