High-resolution Core-level Photoemission Spectroscopy Research Articles

Thermal decomposition of NH 3 on the Si(1 0 0) surface

The adsorption and thermal reaction of NH 3 on the Si(1 0 0) surface are investigated by high-resolution core-level photoemission spectroscopy using synchrotron radiation. The existence of different reaction products and their chemical bonding configurations at different substrate temperatures are revealed from N 1s and Si 2p core-level spectra. We clearly identified a series of Si–NH 2, Si 2NH and Si 3N species in N 1s spectra indicating a successive N–H bond dissociation during thermal decomposition. The depth distribution and the population changes of each N species with annealing suggest that (i) the intermediate Si 2NH species include insertion into the back-bond site between the first and the second Si layers as well as bridging Si dimer site and (ii) the fully dissociated N atoms are incorporated into the Si subsurface layers first. At a high temperature above 900 K, the incorporated N atoms partly segregate back to the surface to form stoichiometric silicon nitride patches. The Si 2p core levels consistently show progressive changes in subnitride formation and the liberation of H atom upon increase of annealing temperature. The implication of the present result on the proposed reaction mechanism is discussed.

Pb/Si(1 0 0)2 × 1 surface studied by high resolution core-level photoemission spectroscopy

A Pb/Si(1 0 0)2 × 1 surface has been studied at T=120 K by high resolution core-level spectroscopy using a third generation synchrotron light source. A strong component (S), could be identified at a binding energy (BE) of −0.20 ± 0.01 eV with respect to the bulk peak. This is indicative of some reorganization of the topmost silicon atoms after Pb adsorption with the formation of Si–Pb bonds and Pb–Pb symmetric dimers. However, this shift is in the opposite site respect to the bulk peak, as compared with Sb/Si(1 0 0)2 × 1, thus suggesting the interplay of final states screening effects and charge transfer in the core-level position. Another component (smaller than S) is present at +0.15 eV BE, and might be due to contribution from subsurface silicon atoms, as on the clean surface, and/or to surface or interface defects. On the Si 2p core-level taken in bulk sensitive mode, we found a very narrow bulk component with a total full width half maximum (FWHM) of 160 meV at T=120 K indicative of an unreacted Si–Pb interface on top of an ideal Si bulk termination. The Pb 5d core-level spectrum is well represented by one doublet, thus suggesting that each Pb atom is adsorbed in a unique environment, i.e., there is no multisite adsorption.

Core-level photoemission study of the Pb overlayers on Si(001)

High-resolution core-level photoemission spectroscopy using synchrotron radiation and low-energy electron diffraction have been applied to study the intriguing structures of the Pb overlayers on Si(001) up to a coverage of 1 monolayer (ML). At a coverage of less than 0.6 ML, two distinct components are observed in Pb $5d$ core levels. This suggests that the Pb chains formed at this coverage range, as observed in the previous scanning tunneling microscopy studies, are composed of buckled Pb dimers consistent with the recent structure model. The Pb $5d$ core-level shift due to this buckling is measured to be 0.35 eV. The spectral line shape of Pb $5d$ at an intermediate coverage around 0.75 ML, exhibiting the $c(8\ifmmode\times\else\texttimes\fi{}4)$ phase in the low-energy electron diffraction, differs significantly from that of the lower coverages in contradiction to the available structure models with asymmetric buckled dimers. In contrast, the $2\ifmmode\times\else\texttimes\fi{}1\ensuremath{-}\mathrm{Pb}$ surface developed at a full monolayer coverage exhibits only single component in Pb $5d.$ This result is compatible only with the close-packed symmetric dimer model. The spectral line shape of Pb $5d$ suggests a gradual change from a semiconducting to a metallic surface at 0.75--1.0 ML, which agrees with the previous valence-band photoemission study indicating the metallic nature of the $2\ifmmode\times\else\texttimes\fi{}1$ phase. Detailed analyses of the Si $2p$ core levels for the well-ordered $2\ifmmode\times\else\texttimes\fi{}2,$ $c(8\ifmmode\times\else\texttimes\fi{}4),$ and $2\ifmmode\times\else\texttimes\fi{}1$ phases at 0.5, 0.75, and 1.0 ML, respectively, are given for the discussion of the Pb-induced Si surface reconstructions.

Acetylene chemisorption and decomposition on the Co( [formula omitted]) single crystal surface

Acetylene chemisorption and dissociation on the Co( 1 1 2 ̄ 0 ) surface has been studied using high-resolution core level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). The adsorbed acetylene molecules are found to dissociate at about 200 K, which is significantly lower than the dissociation onset reported for the system C 2H 2/Co(0 0 0 1). NEXAFS measurements show that acetylene hybridises strongly with the Co( 1 1 2 ̄ 0 ) surface, forming antibonding states below the ionisation limit, which are not present in the gas-phase. In the temperature region from 200 to 300 K a dehydrogenated fragment, possibly of the form C 2H or C 2, is found to co-exist with molecular acetylene. Further heating to 450 K leads to decomposition of this fragment to graphitic carbon, while an ordered (5×2) carbon overlayer starts to form at the expensive of molecular acetylene. At 570 K this ordered overlayer is fully developed. By combining results from photoemission spectroscopy measurements, LEED and STM, a hard sphere model for the carbon overlayer relative to the Co substrate is proposed. Above ∼600 K, a substantial decrease in the amount of ordered carbon atoms is seen, leaving mainly graphitic carbon on the Co( 1 1 2 ̄ 0 ) surface.

CO adsorption on Pd(1 1 1): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss spectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various CO–Pd(1 1 1) bonding configurations have been identified from the decomposition analysis of the adsorbate C 1s and the substrate Pd 3d 5/2 core-level photoemission spectra obtained after CO adsorption at 120 K. The c(4×2) structure at 0.5 monolayer (ML) and the (2×2)-3CO structure at 0.75 ML formed at low temperature have been used for calibration purposes. The core-level results are consistent with CO adsorbing in a mixture of fcc and hcp threefold hollow sites in the c(4×2) structure and of hollow and on-top sites in the (2×2) structure, as reported in the literature. For CO adsorption at 300 K, a different site occupation is evidenced by the presence of two components in the C 1s and Pd 3d 5/2 core-level and C–O stretching vibration lineshapes. At coverages up to 0.1 ML only fcc threefold hollow sites in a ( 3 × 3 )R30° structure are occupied. The population of these sites continues with increasing the CO coverage up to 0.32 ML, but in addition adsorption of CO in bridge sites takes place. The latter sites become predominant at the 300 K saturation coverage of 0.5 ML, and are considered to form domain boundaries in a poorly ordered ( 3 × 3 )R30° phase with split low energy electron diffraction reflexes. Between 0.32 and 0.5 ML the domain boundary regions grow at the expense of the ( 3 × 3 )R30° domains.

Surface modification and cleaning enhancement of TaSi(N) films with dilute hydrofluoric acid

Amorphous TaSi(N) alloy films are of great interest for application in the fabrication of reticles for next generation lithography and for incorporation into semiconductor devices as barrier layers for Cu processing. The stress characteristics of TaSi(N) films are critical when used as an absorber on x-ray and extreme ultraviolet lithography masks, or as a scatterer for electron projection lithography masks. One little understood, but critical characteristic of these films, is that they undergo a stress change towards a less tensile or more compressive state upon interaction with a buffered oxide etch (BOE). We have investigated the cause of this behavior using synchrotron radiation based high-resolution core level (Ta 4f and Si 2p) photoemission spectroscopy. Our results indicate that, upon interaction with a BOE, the surface oxide undergoes a major reorganization and the Ta gets heavily oxidized, resulting in the formation of Ta2O5. Such a reaction would lead to a buildup of strain in the oxidized region, which we interpret as being the contributing factor for the observed stress changes in the TaSi(N) thin films. Furthermore, annealing the surface results in a significant reduction of the oxide desorbtion temperature from 650 to 450 °C due to the HF interaction. This can be attributed to a change in the bonding configuration by HF, wherein the Si–O bonds are broken and weak Ta–O bonds are formed, causing the loosely bonded oxide to desorb at lower temperatures.

Thermal reactions of phosphine with Si(100): a combined photoemission and scanning-tunneling-microscopy study

This study investigates the adsorption and thermal decomposition of phosphine (PH 3) on the Si(100)-(2×1) surface. The adsorption species, dissociation reactions, atomic ordering, and surface morphology of the phosphine/Si(100) surface at temperatures between 300 and 1060 K are examined by scanning tunneling microscopy (STM) and high-resolution core-level photoemission spectroscopy employing synchrotron radiation. The P 2p core level spectra clearly indicate that phosphine molecularly adsorbs at room temperature and partially dissociates into PH 2 and H on a time scale of minutes at low (<0.2 ML) coverages. An exposure of >15 Langmuirs (L, 1 Langmuir=10 −6 Torr s −1) of phosphine on the Si(100)-(2×1) surface at room temperature produces a saturated and disordered surface. The total amount of P on the saturated surface is ∼0.37 ML as calibrated by the P 2p photoemission intensity. Successive annealing of the saturated surface at higher temperatures converts PH 3 into PH 2, converts PH 2 to P–P dimers, and causes the desorption of PH 3. These processes become complete at ∼700 K, and the resulting surface is a H/Si(100)-(2×1) surface interspersed with one-dimensional P–P islands. Desorption of hydrogen from that surface occurs at ∼800 K, and is accompanied by partial displacement of P with Si atoms on the substrate. At 850 K, the Si(100) surface, interspersed with 0.22 ML of two-dimensional islands, is a random alloy of nominal 0.5 ML Si–P heterodimers and 0.5 ML Si–Si dimers.

Chemical bonding and electronic properties of SeS2-treated GaAs(100)

SeS 2 -passivated n-type GaAs (100) surfaces, formed by treatment of GaAs in SeS2:CS2 solution at room temperature, were studied with high-resolution core-level photoemission spectroscopy excited with synchrotron radiation source. The SeS2-treated surface consists of a chemically stratified structure of several atomic layers thickness. Arsenic-based sulfides and selenides reside in the outermost surface layer while gallium-based selenides are adjacent to the bulk GaAs substrate. The shift of the surface Fermi level within the band gap was monitored during controlled thermal annealing, allowing for the identification of the specific chemical entities responsible for the reduction in surface band bending. Arsenic-based species are removed at low annealing temperature with little shift of the Fermi level. Gallium-based selenides are shown to be associated with the unpinning of the surface Fermi level.

Core-level and valence band photoemission study of perovskite oxide powders synthesized by mechanically and thermally activated solid-state reaction

High-resolution core-level and valence band x-ray photoemission spectroscopy measurements were performed on perovskite oxide powders synthesized for applications in solid-oxide fuel cells by high-temperature solid-state reaction (x = 0.3 and 0.19) and by room-temperature mechanical activation of the precursors (x = 0.3). A structure in the valence band at about 1 eV below the Fermi level was clearly observed and assigned to the emission from the Mn 3d-derived states, thereby allowing the extraction of information about correlation effects in this type of material. Both the core-level and valence band spectral features were found to be independent of the choice of synthesis route. This finding indicates that mechanical activation, due to its lower synthesis temperature, can represent a valid alternative method of synthesis allowing a better control of the microstructure.

High-resolution Si 2p core-level photoemission spectroscopy on Sb-terminated Si(100) surfaces: evidence for a strong interfacial component

High-resolution core level spectroscopy using a third-generation synchrotron light source has been used to characterize the Sb-terminated Si(100) surface. On the Sb-terminated surfaces, all the components of the Si 2p core level related to the clean surface have disappeared, and one new Sb-induced component could be identified at a binding energy +0.20±0.01 eV with respect to the bulk peak. On the Sb-1×1 reconstruction, we found a very narrow bulk component [Gaussian full-width half maximum (FWHM) of 150 meV and Lorentzian FWHM of 85 meV], indicative of an ideal bulk termination. On the Sb-2×1 reconstruction, a broader bulk component is needed to fit the Si 2p line (Gaussian FWHM of 180 meV), indicative of some reorganization of the topmost silicon atoms following Sb–Sb dimers formation. A small component, probably due to local disorder, is present on the high-kinetic-energy side for both surfaces at −0.2 eV. The Sb 4d core-level spectrum is well represented by one doublet, thus suggesting that each Sb atom is adsorbed in a unique environment, i.e. there is no multisite adsorption.

Growth process of Ge on Si(100)-(2×1)in atomic-layer epitaxy fromGe2H6

This study investigates the growth process of Ge on Si(100) during atomic-layer epitaxy (ALE) utilizing digermane. The surface ordering, morphology, and stoichiometry of the digermane saturated Si(100) surface at temperatures between 300 and 900 K, as well as the grown films are examined both by scanning tunneling microscopy (STM) and high-resolution core-level photoemission spectroscopy employing synchrotron radiation. An exposure of more than 15 L (1 L=${10}^{\ensuremath{-}6}$ T s) of digermane on the Si(100)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surface at room-temperature results in a saturated and disordered surface. When the digermane saturated surface is heated to 725 K for 60 s, diluted Ge dimer chains are created and surrounded by SiH species. Further annealing to 810 K completely desorbs hydrogen from the surface, leaving large two-dimensional islands covering $\ensuremath{\sim}41%$ of its area. The surface recovers its smooth $2\ifmmode\times\else\texttimes\fi{}1$ structure, but is interspersed with poorly ordered short dimer vacancy lines at 900 K with no observable contrast on the atomic terraces, indicating displacive adsorption of Ge on the terraces. Significant differences of the surface diffusion phenomena between the gas-phase and solid-phase molecular-beam epitaxy (MBE) are observed. Multilayer Si deposition is performed by ALE, i.e., cyclic digermane adsorption at near room temperature, followed by thermal annealing at 900 K. STM images reveal the formation of $2\ifmmode\times\else\texttimes\fi{}n$ structures and increasing roughening of the surface as the growth cycle increases, similar to what occurs during MBE. Issues related to the atomic origins of surface core-level shifts and the chemical composition of the surface layer resulting from the formation of mixed Ge-Si or Ge-Ge during the submonolayer adsorption of Ge on Si(100) are also discussed.

Coverage-dependent thermal reactions of digermane on Si(100)-(2 x 1).

In this study, we examine the adsorption and thermal reactions of digermane (${\mathrm{Ge}}_{2}$${\mathrm{H}}_{6}$) on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface with high-resolution core-level photoemission spectroscopy using synchrotron radiation. At 325 K, the digermane dissociatively chemisorbed to produce ${\mathrm{GeH}}_{3}$, ${\mathrm{GeH}}_{2}$, GeH, and SiH species. The sticking coefficient at zero coverage deduced from the photoemission intensity is \ensuremath{\sim}0.5. Successive annealing of the digermane-saturated surface to higher temperatures causes further decomposition of ${\mathrm{GeH}}_{3}$ and ${\mathrm{GeH}}_{2}$ and the desorption H from GeH and SiH, leaving atomic Ge on the surface. In light of the sufficiently large chemical and surface shifts in their core-level binding energies for different surface species, those processes were identified by examining the evolution of Ge 3d and Si 2p line shapes. Experimental results indicate that the reaction for H release from GeH not only occurred in a large temperature range but also depended heavily on the ${\mathrm{Ge}}_{2}$${\mathrm{H}}_{6}$ adsorption coverages. Two reaction routes for H release from Ge sites were used to describe the large reaction temperature spreads accurately. GeH decomposition by transferring the H atom to a surface Si dangling bond took place for low coverages at \ensuremath{\le}590 K, and ${\mathrm{H}}_{2}$ thermal desorption occurred for higher coverages in the range of 590--770 K. The former process of atom transfer of H from Ge to Si sites was directly observed in the Ge 3d and Si 2p photoemission spectra. \textcopyright{} 1996 The American Physical Society.

Rapid thermal N2O oxynitride on Si(100)

High-resolution x-ray photoelectron spectroscopy (XPS) in conjunction with secondary-ion-mass spectrometry was used to study the chemical nature and distribution of N in oxynitride films formed by rapid thermal N2O processes (RTPs) or conventional furnace methods. The kinetics of furnace oxide growth in N2O are slower than that in O2. During reoxidation the oxidation rate increased to that in pure O2 and the N in the SiO2–Si interface region is displaced into the bulk of the oxide. High-resolution synchrotron Si 2p core-level photoemission spectroscopy (PES) was used to study the oxide–Si(100) interface suboxide structures produced by RTP with and without the presence of N. XPS N 1s studies indicated that there are two types of N in the RTP oxynitride films. The chemical bond configuration of the first type of N is similar to that of N in Si3N4 and is mainly distributed within the first 1 nm from the interface. The second type of N is distributed mainly outside of the first 1 nm region, and the N is likely bonded to two Si and one oxygen atom. PES studies showed that Si formed suboxides with oxygen at the interface for all oxynitride films. It is found that there is no change in the Si+1 structure while there is a dramatic decrease in the Si+2 and Si+3 states with the inclusion of N in the oxide. Both the XPS and PES results are explained in terms of a strain reduction as N is incorporated in the film near the interface region, where Si3N4 functions as a buffer layer which reduces the stress caused by the large Si lattice mismatch between the bulk Si and the oxide overlayer. About 1/5 of the Si+2 and 1/3 of Si+3 atoms at the SiO2–Si interface have been replaced by the Si3N4 buffer layer at the oxynitride–Si interface.

The effect of rapid thermal N2O nitridation on the oxide/Si(100) interface structure

High-resolution x-ray photoelectron spectroscopy (XPS) was used to study the chemical nature and physical distribution of N in oxynitride films formed by rapid thermal N2O processes (RTPs). High-resolution synchrotron Si 2p core level photoemission spectroscopy (PES) was used to study the oxide/Si(100) interface suboxide structures with and without the presence of N. XPS N 1s studies indicated that there are two types of N in the RTP oxynitride films. The chemical bond configuration of the first type of N is similar to that N in Si3N4 and is mainly distributed within the first 1 nm from the interface. The second type of N is distributed mainly outside of the first 1 nm region, and the N is likely bonded to two Si and one oxygen atom. PES studies showed that Si formed suboxides with oxygen at the interface for all oxynitride films. It is found that there is no change in the Si+1 structure while there is a dramatic intensity decrease in the Si+2 and Si+3 peaks with the inclusion of N in the oxide. Both the XPS and PES results are explained in terms of a strain reduction as N is incorporated in the film near the interface region, where Si3N4 functions as a buffer layer which reduces the stress caused by the large Si ‘‘lattice’’ mismatch between the bulk Si and the oxide overlayer. About 1/5 of the Si+2 and 1/3 of Si+3 atoms at the SiO2/Si interface has been replaced by the Si3N4 buffer layer at the oxynitride/Si interface.

HIGH-RESOLUTION CORE-LEVEL PHOTOEMISSION INVESTIGATIONS OF ALKALI ON ALUMINUM ADSORPTION SYSTEMS

The use of high-resolution core-level photoemission spectroscopy for investigating the geometrical aspects of alkali adsorption on aluminum surfaces is reviewed. Examples are given of the use of the technique for studies of the submonolayer growth modes of alkali overlayers, of the geometry of the strongly reconstructed ordered overlayer structures, and of temperature-induced phase transformations between metastable and stable alkali-induced structures.

NH3 and NO interaction with Si(100)-(2 x 1) surfaces.

The adsorption of ${\mathrm{NH}}_{3}$ and NO on Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surfaces at room temperature and the stepwise nitridation have been studied by high-resolution Si 2p core-level photoemission spectroscopy. Both molecules adsorb dissociatively, but only for NO does a reaction occur beyond the first monolayer. For the growth of stoichiometric silicon nitride, annealing to 1200 K is necessary.

Adsorbate-to-Si(100) bonding coordination numbers and structural determinations from synchrotron photoemission studies

The initial stages of interface formation between In, Sb, Sn, and Ag adsorbates and the Si(100)-(2 × 1) surface have been examined with high resolution core-level photoemission spectroscopy. In each case, the Si 2p surface-shifted core-level component is seen to be converted into a component which is indistinguishable from the bulk component through adsorption. The average number of Si surface dimer atoms modified in the presence of an adatom, which is referred to as the adsorbate-to-Si bonding coordination number (BCN), is obtained for various coverages. The relative homogeneity of the adsorbate site bonding is evaluated by examining the line shapes of the adsorbate core-level spectra. The In-to-Si BCN is 3 for very low coverages and decreases to 2 for 1/2-monolayer coverage. The results are consistent with an In/Si(100)-(2 × 2) structural model which involves sp2 hybrid bonding of In dimers. The Sn 4d core levels reveal the existence of two different Sn bonding sites which form sp3 and s2p2 hybrid bonding configurations; a structural model for the Sn/Si(100)-c(4 × 4) is presented. The Ag-to-Si BCN measurement indicates that adsorbed Ag forms linear sp hybrid bonds between two neighboring Si dimers along the direction of dimerization at low coverages. Sb chemisorption is found to yield a BCN behavior similar to that of In.

High-resolution photoemission study of Co/Si(111) interface formation.

We have examined the formation of the Co/Si(111) interface at room temperature using high-resolution core-level photoemission spectroscopy. Two chemically shifted Si 2p core-level components have been identified. The evolution of these components with Co coverage makes it possible to model the development of this interface. Heterogeneous ${\mathrm{CoSi}}_{2}$-like cluster formation is observed for nominal Co coverages of less than \ensuremath{\approxeq}4 A\r{}. Continued reaction to form ${\mathrm{CoSi}}_{2}$ becomes diffusion limited when the clusters coalesce, and a solid solution forms with Si atoms in a Co matrix. For Co coverages of more than 8--10 A\r{}, the interfacial region is buried by a metallic Co film.

Coordination determination of In on Si(100) from synchrotron photoemission studies.

High-resolution core-level photoemission spectroscopy was used to study the initial growth and interaction of In on Si(100). The In---Si bonding coordination number, determined by quantification of the number of Si surface atoms selectively modified in the presence of an In adatom, is 3 for very low In coverages, and decreases to 2 for (1/2) -monolayer coverage. The results are consistent with a structural model deduced from electron diffraction, Auger, and scanning electron microscopy studies.

Evidence for germanium segregation on thin films of Ag on Ge(111)

Ag films prepared on Ge(111) at nearly room temperature were studied with high-resolution core-level photoemission spectroscopy. By selectively modifying the sample structure to label surface sites on the Ag film, we unambiguously identified the presence of a small amount of Ge segregating on top on the growing Ag overlayer. The origin and behavior of these segregated atoms and the structure of the overlayer are discussed.