Surface states and accumulation nanolayer induced by Ba and Cs adsorption on the n-GaN(0 0 0 1) surface

The Ba and Cs adsorption on the n-GaN(0 0 0 1) surface has been studied in situ by the threshold photoemission spectroscopy using s- and p-polarized light excitation. Two surface bands induced by Ba (Cs) adsorption are revealed in surface photoemission spectra below the Fermi level. The surface-Fermi level position is found to be changed from significantly below the conduction band minimum (CBM) at clean n-GaN surface to high above the CBM at Ba, Cs/n-GaN interfaces, with the transition from depletion to electron accumulation occurring at low coverages. Photoemission from the accumulation nanolayer is found to excite by visible light in the transparency region of GaN. Appearance of an oscillation structure in threshold photoemission spectra of the Ba, Cs/n-GaN interfaces with existing the accumulation layer is found to originate from Fabry–Perot interference in the transparency region of GaN.     

Initial stage of nitridation on Si(1 0 0) surface using low-energy nitrogen ion implantation

The initial stage of the thermal nitridation on Si (1 0 0)-2 × 1 surface with the low-energy nitrogen ion (200 eV) implantation was studied by photoemission spectroscopy (PES). The formation of nitride was shown the different characteristics depending on the annealing temperature. The disordered surface at room temperature was changed to 2 × 1 periodicity with the low-energy electron diffraction (LEED) as increasing the nitridation temperature. By decomposition of Si 2p spectrum, we can identify the three subnitrides (Si¹⁺, Si²⁺, and Si³⁺). By changing the take-off angle of the Si 2p, we can increase surface sensitivity and estimate that Si¹⁺, Si²⁺ and Si³⁺ are the interface states.     

Photoemission study of the Li/Ge(1 1 1)-3 × 1 reconstruction

In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)-3 × 1 reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1 reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the and directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the direction. In the direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)-3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge 3d core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge 3d bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)-3 × 1 case, no double bond between Ge atoms in the top layer was found.     

Resonant photoemission study of Ti interaction with GaN surface

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.     

Photoemission study of cobalt interaction with the oxidized Si(1 0 0)2 × 1 surface

The interaction of cobalt atoms with an oxidized Si(1 0 0)2 × 1 surface was studied by photoelectron spectroscopy with synchrotron radiation at room and elevated temperatures. The SiO_x layer grown in situ on the crystal surface was ∼0.3 nm thick, and the amount of deposited cobalt was varied within eight atomic layers. It was found that Co atoms could penetrate under the SiO_x layer even at room temperature in the initial growth. As the Co amount increased, a ternary Co-O-Si phase was formed at the interface, followed by a Co-Si solid solution. Silicide synthesis associated with the decomposition of these phases started under the SiO_x layer at ∼250 °C, producing cobalt disilicide with a stable CaF₂-type of structure.     

Olefin metathesis reaction on GaN (0 0 0 1) surfaces

Proof-of-concept reactions were performed on GaN (0 0 0 1) surfaces to demonstrate surface termination with desired chemical groups using an olefin cross-metathesis reaction. To prepare the GaN surfaces for olefin metathesis, the surfaces were hydrogen terminated with hydrogen plasma, chlorine terminated with phosphorous pentachloride, and then terminated with an alkene group via a Grignard reaction. The olefin metathesis reaction then bound 7-bromo-1-heptene. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and water contact angle measurements following each step in the reaction scheme. The XPS data was used to qualitatively identify surface chemical species and to quantitatively determine molecular surface coverage. The bromine atom in 7-bromo-1-heptene served as a heteroatom for identification with XPS. The reaction scheme resulted in GaN substrates with a surface coverage of 0.10 monolayers and excellent stability towards oxidation when exposed to oxygen plasma.     

Structural properties of Bi-terminated GaAs(0 0 1) surface

Electronic and structural properties of Bi-terminated reconstructions on GaAs(0 0 1) surface have been studied by scanning tunneling microscopy (STM) and synchrotron radiation core-level spectroscopy. A 2-3 monolayer thick Bi-layer was evaporated on a Ga-terminated GaAs(0 0 1) surface. By heating the surface, the reconstruction changed from (2 × 1) to (2 × 4). The α2 phase with one top Bi dimer and one As or Bi dimer in the third atomic layer per surface unit cell is proposed to explain the STM images of the Bi/GaAs(0 0 1)(2 × 4) surface heated at 400 °C. Bi 5d photoemission from the Bi/GaAs(2 × 4) consisted of two components suggesting two different bonding sites for Bi atoms on the (2 × 4) surface. The variation of the surface sensitivity of the photoemission induced no changes in the intensities of the components indicating that the origins of both components lie in the first surface layer.     

Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces II. Cr on TiO2(0 0 1)

The electronic states of the Cr overlayers on TiO₂(0 0 1) surfaces have been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. At lower coverages, Cr deposition on TiO₂(0 0 1) creates two well separated in-gap emissions due to the formation of surface Ti³⁺ (3d¹) ions and Cr³⁺ (3d³) ions. At higher coverages, the in-gap emission is developed into the 2-peak-structure emission of Cr 3d character. The corresponding state is considered to be of metallic nature from the viewpoint of the high ability of oxygen adsorption, but has no Fermi edge, indicating a possibility of forming small Cr clusters on TiO₂(0 0 1) at this stage.     

Electron surface states in short-period superlattices: (GaAs)2/(AlAs)2(1 0 0)-c(4 × 4)

The electronic structure of (GaAs)₂/(AlAs)₂(1 0 0)-c(4 × 4) superlattice surfaces was studied by means of angular-resolved photoelectron spectroscopy (ARUPS) in the photon energy range 20-38 eV. Four samples with different surface termination layers were grown and As-capped by molecular beam epitaxy (MBE). ARUPS measurements were performed on decapped samples with perfect c(4 × 4) reconstructed surfaces. An intensive surface state was, for the first time, observed below the top of the valence band. This surface state was found to shift with superlattices’ different surface termination in agreement with theoretical predictions.     

Photoemission studies of the interactions of CdTe and Te with Si(100)

The interactions between CdTe, and in particular Te, and the (100) surface of Si have been probed using photoemission and low energy electron diffraction with a view to investigating the mechanisms responsible for (100) and (111) growth orientations for CdTe on Si(100). The interfacial reactions have been studied both on room temperature deposition followed by annealing and on depositions at typical epitaxial growth temperatures. In both cases the same precursor stage of an ordered submonolayer of Te on the Si(100) surface has been identified. Line shape analysis of the Si 2p core level has suggested a structural model in which Te adatoms make up an incomplete monolayer bound in bridge sites. This model is in excellent agreement both with the (1 × 1) LEED pattern and recent SEXAFS studies of this surface. The implications of the cubic symmetry of this surface in terms of the subsequent growth orientation of CdTe are discussed. Termination of the surface by Te was also seen to induce band bending suggestive of Fermi level pinning at around midgap, in contrast to the passivating behaviour of other group VI elements on this surface. The Si 2p core level line shape analysis on termination by Te has also provided evidence to support the “covalent dimer” interpretation of the clean dimerised Si(100) surface.     

Epitaxial growth of Al on Si(1 1 1) with Cu buffer layers

The structure of thin Al films grown on Si(1 1 1) with thin Cu buffer layers has been investigated using synchrotron radiation photoemission spectroscopy. A thin Cu(1 1 1) layer between the Si(1 1 1) substrate and an Al film may enhance quantum well effects in the Al film significantly. The strength of quantum well effects has been investigated qualitatively with respect to the thickness of the Cu buffer layer and to the Al film thickness. Deposition of Cu on Si(1 1 1)7 × 7 leads to formation of a disordered silicide layer in an initial regime before a well-ordered Cu(1 1 1) film is formed after deposition of the equivalent of 6 layers of Cu. In the regime below 6 layers of Cu the disorder is transferred to Al layers subsequently grown on top. The initial growth of up to 8 layers of Al on a well-ordered Si/Cu(1 1 1) layer leads to a disordered film due to the lattice mismatch between the two metals. When the Cu buffer layer and the Al over-layer are above 6 and 8 layers, respectively the Al film shows sharp low energy electron diffraction patterns and very sharp quantum well peaks in the valence band spectra signalling good epitaxial growth.     

Photoemission study of the modification of the electronic structure of transition-metal overlayers on TiO2 surfaces: I. Fe on TiO2(1 1 0)

The electronic states of the Fe overlayers on TiO₂(1 1 0) surfaces have been investigated using normal-emission and resonant photoelectron spectroscopy with synchrotron radiation. It was found that Fe grows in a Stranski-Krastanov mode. At low coverages, Fe deposition on TiO₂(1 1 0) is supposed to create surface Ti³⁺(3d¹) ions leading to the same in-gap emission as that is produced by surface oxygen vacancies of TiO₂. At high coverages, Fe-induced in-gap emission is evolved into a bulk Fe spectrum. However, at the beginning, a Fermi edge is not observed, indicating that the small Fe clusters of non-metallic nature are formed. A sharp Fermi edge is formed at higher coverages, indicating that the cluster becomes metallic as the size increases.     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

RHEED study of the (1 1 0) cleavage surface of CdTe:Cr single crystals

The structure of (1 1 0) plane of Cr-doped CdTe single crystals has been studied by reflection high energy electron diffraction and scanning electron microscopy. Diffraction patterns consist of diffraction spots and Kikuchi lines. However, for very small incident angle, the Debye rings are observed. The constant lattice attributed to these rings is 0.8% less then for pure CdTe. These anomalous properties of the near surface layer are likely to occur due to the concentration of Cr atoms creating compressive surface strains or the effect of crystal cleavage.     

Theoretical study of Ni adsorption on the GaN(0 0 0 1) surface

First-principles pseudo-potential calculations within density-functional theory framework are performed in order to study the structural and electronic properties of nickel adsorption and diffusion on a GaN(0 0 0 1)-2×2 surface. The adsorption energies and potential energy surfaces are investigated for a Ni adatom on the Ga-terminated (0 0 0 1) surface of GaN. This surface is also used to study the effect of the nickel surface coverage. The results show that the most stable positions of a Ni adatom on GaN(0 0 0 1) are at the H₃ sites and T₄ sites, for low and high Ni coverage respectively. In addition, confirming previous experimental results, we have found that the growth of Ni monolayers on the GaN(0 0 0 1) surface is possible.     

Photoemission study of glycine adsorption on Cu/Au(1 1 1) interfaces

The adsorption of glycine on Au(1 1 1) pre-deposited with different amounts of Cu was investigated with both conventional X-ray photoelectron spectroscopy (XPS) and synchrotron-based photoemission. In the Cu submonolayer range, glycine physically adsorbs on the Cu/Au(1 1 1) surfaces in its zwitterionic form and completely desorbs at 350 K. The C 1s, O 1s and N 1s core level binding energies monotonically increase with Cu coverage. This indicates that, in the Cu submonolayer range, the admetal is alloyed with Au rather than forming overlayers on the Au(1 1 1) substrate, consistent with our recent experimental and theoretical results [X. Zhao, P. Liu, J. Hrbek, J.A. Rodriguez, M. Pérez, Surf. Sci. 592 (2005) 25]. Upon increasing the amount of deposited Cu over 1 ML, part of the glycine overlayer transforms from the zwitterionic form to the anionic form (NH₂CH₂COO⁻) and adsorbs chemically on the Cu/Au(1 1 1) surface with the N 1s binding energy shifted by −2.3 eV. When the amount of deposited Cu is at 3.0 or 6.0 ML, the intensity of the N 1s chemisorption peak increases with aging time at 300 K. It indicates that glycine adsorption induces Cu segregation from the subsurface region onto the top layer of the substrate. Judging from the initial N 1s peak intensities, it is concluded that 64% and 36% of the top layer are still occupied by Au atoms before glycine adsorption even when the amounts of deposited Cu are 3.0 and 6.0 ML, respectively. On the Au(1 1 1) surface pre-dosed with 6.0 ML of Cu, part of the chemisorbed glycine will desorb and part will decompose upon heating to 450-500 K. In addition, about 20% of the glycine exists in the neutral form when the glycine overlayer was dosed on Cu/Au(1 1 1) held at 100 K.     

Mn/PbTe(111)界面行为的光电子能谱研究

利用X光电子能谱(XPS)对Mn在PbTe(111)表面上沉积生长的界面性质进行了研究.研究表明Mn的沉积使衬底发生了原子尺度上的突变及金属/半导体界面的形成.从X光电子能谱的芯态能级峰来看,随着Mn膜的沉积Pb 4f峰的低结合能端出现了金属Pb的特征新峰,而Te 3d峰的高结合能端却出现了MnTe特征新峰.且随着Mn膜厚度的增加这些新峰变得越来越明显,当Mn膜厚度超过7 ML(monolayer)(即超过Pb,Te的探测深度)时,衬底信号峰完全消失,只剩下金属Pb和MnTe的芯态能级峰.Mn膜厚度继续增 关键词： PbTe半导体 界面形成 光电子能谱 偏析     

Conformational isomers of stilbene on Si(1 0 0)

Stilbene (1,2-diphenylethylene) has shown an intriguing isomerisation behavior and may serve as a model system for “molecular switches” incorporating a CC double bond. To evaluate the possible use of such molecules as molecular switches on semiconductor surfaces, the adsorption of cis- and trans-stilbene on Si(1 0 0) has been investigated. Identification of both isomers is achieved by differences in adsorption geometry as revealed by NEXAFS, and differences in electronic structure in the occupied and unoccupied molecular orbitals. For both isomers, bonding takes place via the CC double bond to the Si dimer atoms allowing for free movement of the aromatic rings, a necessary prerequisite for photoinduced isomerisation on the surface. Our experimental results agree well with theoretical calculations.     

Coverage dependent organic–metal interaction studied by high-resolution core level spectroscopy: SnPc (sub)monolayers on Ag(1 1 1)

We study the electronic structure of tin-phthalocyanine (SnPc) molecules adsorbed on a Ag(1 1 1) surface by high-resolution photoelectron spectroscopy. We particularly address the effect of different SnPc coverages on the interaction and charge transfer at the interface. The results give evidence for a covalent molecule–substrate interaction, which is temperature and coverage dependent. The valence and core level spectra as well as the work function measurements allow us monitoring subtle differences in the strength of the interface interaction, thus demonstrating the sensitivity of the methods. The results consistently show the effect of charge exchange between substrate and molecules which obviously leads to a net charge transfer into the SnPc molecules, and which is increased with decreasing coverage. Surprisingly, the Sn3d core levels are neither effected by variations of charge transfer and interaction strength, nor by a possible “Sn-up” or “Sn-down” orientation, which have been observed for sub-monolayers.