A comparative study of clean and Bi-stabilized InP(1 0 0)(2 × 4) surfaces by the core-level photoelectron spectroscopy

The bismuth-stabilized (2 × 4)-reconstructed InP(1 0 0) surface [Bi/InP(1 0 0)(2 × 4)] has been studied by synchrotron-radiation core-level photoelectron spectroscopy. The spectra are compared with previous core-level data obtained on a clean InP(1 0 0)(2 × 4) surface. The findings support that the P 2p surface-core-level shift (SCLS) of the clean InP(1 0 0)(2 × 4), which has higher kinetic energy than the bulk emission, arises from the third-layer P atoms and that the second P 2p SCLS, which has lower kinetic energy than the bulk, arises from the top-layer P atoms. Similar In 4d SCLSs are found on the clean and Bi-stabilized InP(1 0 0)(2 × 4) surfaces, indicating that these shifts contain contributions of the In atoms that lie in the second and/or fourth layers. In addition to this, the results improve our understanding of the atomic structure of the Bi/InP(1 0 0)(2 × 4) surface and lead to refined surface models which include Bi-Bi and Bi-P dimers.     

Core-level shifts of the c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) surfaces

We have studied In-stabilized c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III–V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the ζ and ζa models, which have been previously established to describe the atomic structures of the III–V(1 0 0)c(8 × 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III–V(1 0 0)c(8 × 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated ζ and ζa SCLS values shows that the InSb spectra are reproduced by the ζ SCLSs better than by the ζa SCLSs. Interestingly, the ζa fits agree better with the InAs spectra than the ζ fits do, indicating that the ζa model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III–V(1 0 0)c(8 × 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.     

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.     

Atomic geometry and electronic properties of the Ge(1 1 1)2 × 1-Sb surface studied by scanning tunneling microscopy/spectroscopy and core-level photoemission

By scanning tunneling microscopy and spectroscopy (STM/S) and high-resolution core-level photoemission using synchrotron radiation, we have investigated the atomic structure and electronic properties of Sb-induced 2 × 1 reconstruction on Ge(1 1 1). Our results support well the zigzag-chain model proposed for this phase in the literature; in particular, the STM images visualize the Sb zigzag (Seiwatz) chain in a real space, and the STS I-V spectrum suggests this surface to be semiconducting, in good agreement with the atomic configuration proposed. However, a closer inspection of the STM results does not support the buckling of Sb chains reported in earlier studies. Moreover, the analysis of the Sb 4d core-level line shape of the (2 × 1) reconstruction shows that the bonding state of the Sb atoms is very similar, suggesting an unbuckled Seiwatz chain. In addition, the Ge 3d core-level emission reveals only one component, giving evidence for the ideal bulk-terminated structure of the Ge substrate.     

Non-contact atomic force microscopy studies of (2 × 4) InP(0 0 1) surface

A sputter-cleaned indium-rich (2 × 4) InP(0 0 1) surface was investigated by non-contact scanning atomic force microscopy (NCAFM). Atomically-resolved images of the surface exhibit two different patterns. The patterns can be interpreted within the mixed dimer model of (2 × 4) reconstructed InP(0 0 1) surface. It is shown that due to contrast formation mechanism in NCAFM the features resolved are in close correspondence to scanning tunnelling microscopy (STM) data. Due to chemical interaction a P-terminated tip gives the image similar to an empty-state STM image, whereas an In-terminated tip gives the image resembling a filled-state STM one. Moreover, it is shown that due to dipole-dipole interaction, NCAFM can be sensitive to orientation of In-P dimers.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

SEM and XPS studies of nanohole arrays on InP(1 0 0) surfaces created by coupling AAO templates and low energy Ar ion sputtering

The aim of the present study is to demonstrate the feasibility to form well-ordered nanoholes on InP(1 0 0) surfaces by low Ar⁺ ion sputtering process in UHV conditions from anodized aluminum oxide (AAO) templates. This process is a promising approach in creating ordered arrays of surface nanostructures with controllable size and morphology. To follow the Ar⁺ ion sputtering effects on the AAO/InP surfaces, X-ray photoelectron spectroscopy (XPS) was used to determine the different surface species. In_4d and P_2p core level spectra were recorded on different InP(1 0 0) surfaces after ions bombardment. XPS results showed the presence of metallic indium on both smooth InP(1 0 0) and AAO/InP(1 0 0) surfaces. Finally, we showed that this experiment led to the formation of metallic In dropplets about 10 nm in diameter on nanoholes patterned InP surface while the as-received InP(1 0 0) surface generated metallic In about 60 nm in diameter.     

Irreversible structural transformation of Si(1 1 4)-2 × 1 induced by subsurface carbon

Using scanning tunneling microscopy (STM), it has been found that the reconstruction of Si(1 1 4) is transformed irreversibly from a 2 × 1 structure composed of dimer (D), rebonded atom (R), and tetramer (T) rows (phase A) to a different kind of 2 × 1 structure composed of D, T, and T rows (phase B) by C incorporation. It has been confirmed by high-resolution synchrotron core-level photoemission spectroscopy (PES) that such an irreversible structural transformation is due to stable subsurface C atoms. They induce anisotropic compressive stress on the surface, which results in insertion of Si dimers to an R row to form a T row.     

Phosphine and tertiarybutylphosphine adsorption on the indium-rich InP (0 0 1)-(2 × 4) surface

Phosphine and tertiarybutylphosphine adsorption on the indium-rich InP (0 0 1)-(2 × 4) surface at 25 °C have been studied by internal reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and low energy electron diffraction. Both molecules form a dative bond to the empty dangling bonds on the In-P heterodimers and the second-layer In-In dimers and vibrate symmetrically at 2319 (2315) and 2285 (2281) cm⁻¹ and asymmetrically at 2339 (2339) and 2327 (2323) cm⁻¹. A fraction of these species dissociate into adsorbed PH₂ with the hydrogen and tertiarybutyl ligands transferring to nearby phosphorus sites. The calculated energy barriers for desorption (<11 kcal/mol) of these molecules is less than that for dissociation (>17 kcal/mol) and explains their low sticking probabilities at elevated temperatures under InP growth conditions.     

Valence band and surface core-level shift studies of the oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

Using synchrotron radiation, W 4f_7/2 and valence-band (VB) photoelectron spectroscopy was used to study the oxidation states of the p(3 × 1) reconstructed surface of W(1 0 0) produced by oxygen adsorption at 1500 K. The W 4f_7/2 study showed two features at 0.40 and 1.50 eV higher binding energies relative to the bulk feature which represent shifts to lower binding energies compared to the W 4f_7/2 features of the as-grown oxygen films on W(1 0 0). Co-existence of these features is a characteristic of the formation of the p(3 × 1) structure. The VB studies of this system and as-grown oxygen films at room temperature (RT) showed an oxygen-induced feature for the p(3 × 1) structure for which the peak maximum had shifted by 0.9 eV to higher binding energies compared to the as-grown films. The direction of shifts in the W 4f_7/2 and VB spectra for the p(3 × 1) structure confirmed the reduction of the charge transfer from W to oxygen compared to the as-grown films. A study of the VB changes with photon energy shows that the main component of the oxygen-induced feature in the p(3 × 1) VB at 6.6 ± 0.2 eV is related to a hybridized state between W 5d and O 2p electrons.     

The effect of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface

We have studied the effects of controlled ion bombardment on the electronic structure of the Si(0 0 1) surface. The surface was exposed to various doses of Ar⁺ ions accelerated towards the surface at 500 eV. X-ray photoelectron spectroscopy (XPS) spectra of the irradiated H-terminated Si(0 0 1) surface reveal the appearance of peaks that are associated with the presence of cleaved Si bonds. Ultraviolet photoelectron spectroscopy (UPS) spectra of the irradiated Si(0 0 1)2 × 1 surface show that the dimer dangling-bond surface state decays monotonically with increasing dose. These results, coupled with previous scanning tunneling microscopy (STM) studies, indicate that the breaking of dimers, and possibly the creation of adatom-like defects, during ion irradiation are responsible for the changes in the electronic structure of the valence band for this surface.     

A new structural model for the SiC(0 0 0 1)(3 × 3) surface derived from first principles studies

A new structural model with fluctuant Si-trimers and missing Si-adatom is proposed for Si-terminated 6H-SiC(0 0 0 1)(3 × 3) reconstruction. The atomic and electronic structures of the model are studied using first principles pseudopotential density-functional approach. The calculated surface electronic density of states coincides quantitatively with the experimental results of photoemission and electron energy loss spectroscopy. Based on the calculations, the Patterson map and scanning tunneling microscopic (STM) images simulated for the new model agree more satisfactorily with the experimental X-ray diffraction and STM observations than that for previously proposed models. The calculations of formation energies suggest that the new structure would be formed under the environment of dilute Si vapor around the surface in the preparation process.     

The surface structure of BaO on Pt(1 1 1): (2 × 2)-reconstructed BaO(1 1 1)

The surface structure of BaO(1 1 1) has been determined using STM and computer modelling. The BaO(1 1 1) surface was prepared in thin film form on Pt(1 1 1) and presents a surface with twice the lattice parameter expected for that of the bulk termination, i.e. a (2 × 2) reconstruction. Computer modelling indicates that the bulk termination is unstable, but that the (2 × 2) reconstructed BaO(1 1 1) surface has a low surface energy and is hence a stable surface reconstruction. The (2 × 2) reconstruction consists of small, three-sided pyramids with (1 0 0) oriented sides and either oxygen or barium ions at the apices. Less regular surface reconstructions containing the same pyramids are almost equally stable, indicating that we may also expect less regular regions to appear with a fairly random distribution of these surface species. The simulations further suggest that a regular (4 × 4) reconstruction built up of bigger pyramids is even more energetically favourable, and some evidence is found for such a structure in the STM.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.     

Simulations of iodine adsorbed Ge(0 0 1) surface and its STM images

A layer of iodine at Ge(0 0 1) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(0 0 1) surface covered by 0.25 monolayer of iodine. The p(2×4), p(2×2), c(2×4) and p(1×4) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I₂ molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data.     

Spectroscopic interpretation for Na-induced phase transitions on Na/Si(1 1 1)3 × 1

Na adsorption at room temperature causes the Na/Si(1 1 1)3 × 1 surface with Na coverage of 1/3 monolayer (ML) to transit into the Na/Si(1 1 1)6 × 1 surface at 1/2 ML and sequentially into the Na/Si(1 1 1)3 × 1 surface at 2/3 ML. The phase transition was studied by Si 2p core-level photoemission spectroscopy. The detailed line shape analysis of the Si 2p core-level spectrum of the Na/Si(1 1 1)3 × 1 surface (2/3 ML) is presented and compared to the Na/Si(1 1 1)3 × 1 surface (1/3 ML) which is composed of Si honeycomb chain-channel structures. This suggests that as additional Na atoms form atomic chains resulting in the Na/Si(1 1 1)3 × 1 surface (2/3 ML), the inner atoms of the Si honeycomb chain-channel structure is buckled due to the additional Na atoms.     

Formation of single layer h-BN on Pd(1 1 1)

The structure of hexagonal boron nitride (h-BN) on Pd(1 1 1) was studied with low energy electron diffraction (LEED), photoelectron spectroscopy and scanning tunnelling microscopy (STM). h-BN forms flat monolayers on the Pd(1 1 1) surface in contrast to Rh(1 1 1) where a complex self-assembled double layer structure, the nanomesh [M. Corso, W. Auwärter, M. Muntwiler, A. Tamai, T. Greber, J. Osterwalder, Science 303 (2004) 217], appears. The LEED patterns reveal a dominating 10 × 10 h-BN superstructure, with a second, distinct structure rotated by 30° and further azimuthally randomly oriented h-BN overlayers. This is consistent with STM images which show several different Moiré patterns associated with different rotation angles of the overlayer. Additionally the use of thin Pd(1 1 1) films instead of single crystal substrates was studied. No significant differences in the h-BN film quality were found.     

Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Structural and electrical analysis of S ion bombarded p-InP(1 0 0)

The chemical state of sulfur and surface structure on low-energy S⁺ ion-treated p-InP(1 0 0) surface have been investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). S⁺ ion energy over the range of 10-100 eV was used to study the effect of ion energy on surface damage and the process of sulfur passivation on p-InP(1 0 0) by S⁺ ion beam bombardment. It was found that sulfur species formed on the S⁺ ion-treated surface. The S⁺ ions with energy above 50 eV were more effective in formation of In-S species, which assisted the InP surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form In-S species in the sulfur passivation of p-InP(1 0 0). The subsequent annealing process removed donor states that were introduced during the ion bombardment of p-InP(1 0 0). Results of theoretical simulations by Transport of Ions in Materials (TRIM) are in accordance with those of experiments.     

Scanning tunnelling microscopy and spectroscopy of the reduced TiO2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO₂(1 0 0) surface. The STM results showed that the TiO₂(1 0 0) surface is capable to form (1 × 7) reconstruction which can transform to (1 × 3) reconstruction due to reoxidation of the surface. The CITS results showed that the (1 × 7) reconstruction is much more metallic in compared to the (1 × 3) reconstruction showing pronounced surface states at energy 1.3 eV and 0.8 eV below the Fermi level and at energy 1.0-1.2 eV above the Fermi level.