Band bending variation of the Si(111) surface during its thermal oxidation

During the thermal oxidation of Si(111) surfaces performed under ultra high vacuum conditions and investigated with various surface techniques such as Auger electron spectroscopy, low energy electron diffraction, ultra violet and X-ray induced photoemission spectroscopy, the Si 2p core level binding energy was measured and provided directly the band bending variation of the Si surface. A net interface charge then could be deduced. This is an elegant in situ method to have access to the electrical characteristics of the SiSiO₂ interface during its formation.     

Angle-resolved photoemission study of CoSi2 nanofilms grown on Si(111) substrates

We have carried out an angle-resolved photoemission study for CoSi₂ nanofilms grown on the Si(111)-7×7 substrates. The surface of CoSi₂(111) nanofilm changes from the bulk-truncated surface to the surface with additional Si-bilayer by annealing at higher temperature above 825 K. The angle-resolved photoemission spectra of the CoSi₂ nanofilm annealed at 853 K show the spectral features originated from the surface resonance state on the CoSi₂ surface terminated by Si-bilayer. From the detailed photoemission study, we discuss the surface electronic structure in CoSi₂(111) nanofilms grown on Si(111) substrates.     

Characterization of alkyl monolayer covalently bonded to Si(111) and soft-landing of vanadium-benzene sandwich clusters onto the alkyl monolayer substrate

A hexadecyl monolayer covalently attached to Si(111) surfaces (C₁₆–Si(111)) was prepared at 200 ^○C from 1-hexadecene. Formation of the monolayer was characterized by water contact angle measurement, attenuated total reflection infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Gas phase synthesized vanadium (V)-benzene (Bz) 1:2 (VBz₂) sandwich clusters were size-selectively deposited onto the C₁₆–Si(111) substrate thus prepared and an oxidized Si substrate. Investigation of the resultant clusters was implemented by thermal desorption spectroscopy (TDS). About 30 K increase in threshold desorption temperature of the landed clusters was observed on going from the oxidized Si to the C₁₆–Si(111) substrate, a result indicating that the clusters are more strongly bound to the C₁₆–Si(111) than to the oxidized Si. This result was explained by the penetration of the landed clusters into the hexadecyl monolayer.     

Photoemission study of the growth of the NdF3/Si(111) interface

Photoemission spectroscopy with synchrotron radiation was used to study the NdF₃/Si(111) interface as a function of annealing temperature for NdF₃ films. These films range in thickness from 1–20 monolayers and were deposited at room temperature. Without annealing, both F-Si and Nd-Si bonding is observed, indicating that the planar triangular NdF₃ molecules lie flat on the Si(111) substrate. At annealing temperatures between 400 and 500° C, the NdF₃/Si(111) interface is dominated by Nd-Si bonding as evidenced from a line-shape analysis of the Si 2 p and Nd 4 f core levels. By resonant excitation of the giant 4 d-4 f absorption resonance, the photoemission signal from the partially occupied 4 f orbitals is enhanced and can be distinguished from the photoemission signal of the overlapping F 2 p valence band. At higher temperatures F is completely lost due to the decomposition of NdF₃.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

Investigation of the initial stages of oxidation of microcrystalline silicon by means of X-ray photoelectron spectroscopy

XPS Si2p spectra of microcrystalline silicon (μc-Si), prepared by plasma chemical transport are reported and the initial stages of oxidation are studied: In comparison with single crystal Si(111) surfaces, μc-Si samples are remarkable resistant to surface oxidation. A short exposure to air results in negligible oxygen adsorption (intensity ratio of 01s to Si2p peaks is less than 0.018 after more than3 × 10¹¹ Langmuirs exposure to air). Intensive oxidation treatment is required to produce an oxide layer and evidence supporting a preferential grain boundary oxidation mechanism is presented. The surface plasmon structure observed in the Si2p spectrum provides complementary information on the remarkably low oxidation rate of the crystallite surfaces as compared to the grain boundaries.     

Synchrotron-radiation photoemission study of the ultrathin Ba/3C–SiC(111) interface

Electronic structure of the Ba/3C–SiC(111) interface has been detailed studied in situ in an ultrahigh vacuum using synchrotron radiation photoemission spectroscopy with photon energies in the range of 100–450 eV. The 3C–SiC(111) samples were grown by a new method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. Valence band photoemission and both the Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage. Under Ba adsorption two induced surface bands are found at binding energies of 2 eV and 6 eV. It is obtained that Ba/3C–SiC(111) interface can be characterized as metallic-like. Modification of both the Si 2p and C 1s surface-related components were ascertained and shown to be provided by redistribution effect of electron density between Ba adatoms and both the Si surface and C interface atoms.     

Dangling bond surface state of Si(111): New results

By angle resolved photoemission on Si(111) 2 × 1 surfaces, the main surface structure at ?0.85 eV below E_F is shown to exhibit a strong s?p_z character in contradiction with previous measurements. The similarity of the results on the 2 × 1 and 7 × 7 surfaces, together with theoretical calculations lead us to favor a buckled model for the 2 × 1 surface and a model based on ring like arrangements of lowered and raised atoms for the 7 × 7 surface.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Adsorption of potassium on iron

Adsorption of K on Fe(110), (100) and (111) surfaces was studied by means of LEED, AES, thermal desorption and work function measurements. The monolayer capacity is about 5.5 × 10¹⁴ K-atoms/cm² in all three cases. With Fe(111) an ordered 3 × 3 overlayer was found at fairly low coverages. The work function decreases to a minimum and the initial dipole moments were determined to μ₀ = 7.0 Debye for Fe(110), μ₀ = 4.4 Debye for K/Fe(100) and μ₀ = 3.9 Debye for K/Fe(111). The heat of adsorption decreases from its initial value (Fe(110): 57; Fe(100): 54; Fe(111): 52 kcal/mole) continuously with increasing coverage which parallels the continuous decrease of the dipole moment of the adsorbate complex.     

Time-evolution of thermal oxidation on high-index silicon surfaces: Real-time photoemission spectroscopic study with synchrotron radiation

The initial oxidation on high-index silicon surfaces with (113) and (120) orientations at 820 K has been investigated by real-time X-ray photoemission spectroscopy (Si 2p and O 1s) using 687 eV photons. The time evolutions of the Siⁿ⁺ (n = 1–4) components in the Si 2p spectrum indicate that the Si^2 + state is suppressed on high-index surfaces compared with Si(001). The O 1s state consists of two components, a low-binding-energy component (LBC) and a high-binding-energy component (HBC). It is suggested that the O atom in strained SiOSi contributes to the LBC component. The reaction rates are slower on high-index surfaces compared with that on Si(001).     

Application of synchrotron radiation to investigation of the mechanism of increase in the yield of alkali metal ions in electron-stimulated desorption

Core-level photoelectron spectroscopy with synchrotron radiation (hv = 140 eV) has been applied to study the variation in the Si⁺ charge state in silicon films deposited on the W(100) surface after thermal annealing of the substrate. The purpose of this study is to check the mechanism responsible for the sharp increase in the yield of Na⁺ ions in electron-stimulated desorption from a sodium layer adsorbed on the Si/W(100) surface after high-temperature annealing. The evolution of the W 4f _7/2 and Si 2p photoelectron spectra and the valence band photoemission spectra is investigated for two silicon coverages (1 and 3 ML) on the W(100) surface in the temperature range 300<T<2200 K. It is shown that annealing of 1 ML Si on the W(100) surface results in the formation of a W-Si covalent bond, which can weaken the Si-Na bond and lead to an increase in the equilibrium distance X ₀ and, hence, to an increase in the yield of Na⁺ ions in electron-stimulated desorption. The variation in the photoelectron spectra under annealing of 3 ML Si differs from that observed after annealing of 1 ML Si in the direction of charge transfer, thus correlating with the opposite effect of annealing of 3 ML Si/W on the Na⁺ yield in electron-stimulated desorption.     

A photoemission study of the interaction of Ni(100), (110) and (111) surfaces with oxygen

Photoelectron spectroscopic studies of the oxidation of Ni(111), Ni(100) and Ni(110) surfaces show that the oxidation process proceeds at 295 and 485 K in two distinct steps: a fast dissociative chemisorption of oxygen followed by oxide nucleation and lateral oxide growth to a limiting coverage of 3 NiO layers. The oxygen concentration in the 295 K saturated oxygen layer on Ni(111) was confirmed by ¹⁶O(d,p) ¹⁷O nuclear microanalysis. At 295 and 485 K the oxide growth rates are in the order Ni(110) > Ni(111) > Ni(100). At 77 K the oxygen uptake proceeds at the same rate on all three surfaces and shows a continually decreasing sticking coefficient to saturation at ~2.1 layers (based upon NiO). An O 1sb.e. = 529.7 eV is associated with NiO, and O ls b.e.'s of ~531.5 and 531.3 eV can be associated, respectively, with defect oxide (Ni₂O₃) or (in the presence of H₂O) with an NiO(H) species. The binding energies (Ni 2p, O 1s) of this NiO(H) species are similar to those for Ni(OH)₂. Defect oxides are produced by oxidation at 485 K, or by oxidation of damaged films (e.g. from Ar⁺ sputtering) and evaporated films. Wet oxidation (or exposure to air) of clean nickel surfaces and oxides, and exposure of thick oxide to hydrogen at high temperature results in an O 1s b.e. ~531.3 eV species. Nuclear microanalysis ²H(³He,p) ⁴He indicates the presence of protonated species in the latter samples. Oxidation at 77 K yields O 1s b.e.'s of 529.7 and ~531 eV; the nature of the high b.e. species is not known. Both clean and oxidised nickel surfaces show a low reactivity towards H₂O; clean nickel surfaces are ~10³ times less reactive to H₂O than to oxygen.     

The interaction of water with SiMo interface. A core and valence photoemission investigation

Core level photoemission with MgKα radiation (hv = 1253.6 eV) and valence photoemission (hv = 21.2 eV) are presented for Si(1 1 1)-Mo interfaces at various Mo coverages (up to 3.2·10¹⁵ at cm⁻²) exposed to 6000 L of H₂O. The metal deposition strongly enhances the oxidation of Si with the complete H₂O dissociation and the formation of a SiO₂-like compound. The results are consistent with an interface growth with the formation of silicide islands.     

Growth mode and novel structure of ultra-thin KCl layers on the Si(1 0 0)-2 × 1 surface

This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results.     

Potassium and potassium oxide monolayers on the platinum (111) and stepped (755) crystal surfaces: A LEED,AES, and TDS study

The adsorption of potassium and the coadsorption of potassium and oxygen on the Pt(111) and stepped Pt(755) crystal surfaces were studied by AES, LEED, and TDS. Pure potassium adlayers were found by LEED to be hexagonally ordered on Pt(111) at coverages of θ = _K0.9–;1. The monolayer coverage was 5.4 × 10¹⁴K atoms/cm² (0.36 times the atomic density of the Pt(111) surface). Orientational reordering of the adlayers, similar to the behavior of noble gas phase transitions on metals, was observed. The heat of desorption of K decreased, due to depolarization effects, from 60 kcal/mole at θ_K <0.1, to 25 kcal/mole at θ_K = 1 on both Pt(111) and Pt(755). Exposure to oxygen thermally stabilizes a potassium monolayer, increasing the heat of desorption from 25 to 50 kcal/mole. Both potassium and oxygen were found to desorb simultaneously indicating strong interactions in the adsorbed overlayer. LEED results on Pt(111) further indicate that a planar K₂O layer may be formed by annealing coadsorbed potassium and oxygen to 750 K.     

On the structure of chemisorbed acetylene and ethylene on Ni,Pd and Pt surfaces

Filtered He II (hv = 40.8 eV) photoemission spectra for acetylene and ethylene molecularly chemisorbed at T ～ 100 K on Ni(111), Ni(110), Pd(111) and Pt(111) have been obtained. The resulting vertical ionization potentials are presented and used within the framework of an approximate model to obtain information of the geometric structure of these molecules. Two initial state effects are discussed which are found to be important in deducing the molecular structures. These include an initial state shift of the lowest lying carbon-2s derived orbital and a metal atom induced shift of the σ_CC valence orbital for strongly distorted species. The magnitudes of both effects are estimated — the latter using Hartree — Fock LCAO calculations of Be interacting with acetylene or ethylene. The deduced geometries of chemisorbed ethylene are found to differ only slightly from those determined without considering these effects, but for acetylene two classes of structures are found. One class of structures is weakly distorted while the other is strongly distorted (～sp^2.5 hybridization). The latter structure is consistent with recent vibrational loss studies of chemisorbed acetylene on Ni(111) and Pt(111). In contrast to chemisorbed acetylene, chemisorbed ethylene on Ni shows relatively weak distortions. More subtle crystallographic and structural effects for acetylene and ethylene on (111), (100) and (110) Ni surfaces are also discussed.     

Adsorbate band structure of bromine on Pd(111) studied by angle resolved ultraviolet photoemission

The photoemission spectra from Pd(111) and from the Pd(111)-Br(√3 × √3)R30° system are reported; some new features for the clean surface are detected and assigned. The principal effect of the Br overlayer on the direct transitions is a general intensity reduction. Three adsorbate derived features are detected; one at 4 eV with no dispersion is probably an adsorbate-induced feature of the metal, and the other two which disperse are assigned as Br 4p_x, (4.5 eV) and Br 4p_z (6 eV) at $\text{}\text{?}$gG.     

TiO2 chemical vapor deposition on Si(111) in ultrahigh vacuum: Transition from interfacial phase to crystalline phase in the reaction limited regime

The interaction between the metal organic precursor molecule titanium(IV) isopropoxide (TTIP) and three different surfaces has been studied: Si(111)-(7 × 7), SiO_x/Si(111) and TiO₂. These surfaces represent the different surface compositions encountered during TTIP mediated TiO₂ chemical vapor deposition on Si(111). The surface chemistry of the titanium(IV) isopropoxide precursor and the film growth have been explored by core level photoelectron spectroscopy and x-ray absorption spectroscopy using synchrotron radiation. The resulting film morphology has been imaged with scanning tunneling microscopy. The growth rate depends on both surface temperature and surface composition. The behavior can be rationalized in terms of the surface stability of isopropoxy and isopropyl groups, confirming that growth at 573 K is a reaction limited process.