High-resolution Electron Energy Loss Spectroscopy Spectra Research Articles

Surface reactions of trimethylgallium and trimethylarsenic on silicon surfaces

Epitaxy of III–V semiconductor layers from gaseous sources metalorganic chemical vapor depsoition and metalorganic molecular-beam epitaxy is essentially determined by surface reactions of metalorganic components on the semiconductor surface. The present paper deals with the decomposition of trimethylgallium [TMGa=Ga(CH3 )3 ] and trimethylarsenic [TMAs=As(CH3 )3 ] on Si(111) and Si(100) surfaces. High-resolution electron energy-loss spectroscopy (HREELS) is used to identify the adsorbed surface species by means of its vibrational spectrum. Adsorption of TMGa and TMAs is performed at 300 K and the surface reaction, in particular its dependence on temperature is studied by measuring HREELS spectra after each step. A comparison with thermal desorbtion data is given in some cases. TMGa and TMAs exhibit a different decomposition pattern and the reaction of TMAs is dependent on the orientation of the Si surface.

Oxygen induced reconstruction on Cu(100)

The adsorption of oxygen on Cu(100) has been studied by low-energy electron diffraction, Auger spectroscopy, and electron energy-loss spectroscopy (EELS). Using an electron spectrometer for an intensity analysis of the diffracted beams we show that with increasing coverage a first-order phase transition from a disordered (1×1) phase into a long-range ordered ((2)1/2×2(2)1/2)R45 structure takes place. Contrary to most previous studies no evidence is found for a c(2×2) structure. High-resolution EELS spectra (ΔE=16–20 cm−1) change significantly upon the transition, establishing that the ((2)1/2×2(2)1/2)R45 structure involves a reconstruction of the substrate. Both the perpendicular in-phase and out-of-phase vibration of the oxygen atoms are detected. Their observation is consistent with a reconstruction model where second nearest-neighbor rows in the [100] direction show a row pairing. We interpret this reconstruction mechanism as a cooperative Jahn–Teller effect.

HREELS study of epitaxial Si(111)/WSi 2: Correlation with optical properties in the infrared energy range

Electronic properties of epitaxial WSi 2 layers on top of the Si(111)(7 × 7) surface have been studied by means of high resolution electron energy loss spectroscopy (HREELS) in the loss energy range below 1 eV. A characteristic electronic-type excitation is observed and its origin explained, in the framework of the dielectric theory, through consideration of the IR-optical properties of bulk single crystal WSi 2. HREELS spectra measured for different WSi 2 overlayer thicknesses allow the identification of contributions from electron gas excitations at the surface and at the interface. AES and LEED analyses have also been carried out for characterization.

High-resolution electron-energy-loss spectroscopy study of the uv-laser photodissociation of adsorbed Al2(CH3)6 on Si(100)21 and Si(111)77 surfaces.

High-resolution electron-energy-loss spectroscopy (HREELS) has been used to study the adsorption and uv-laser-induced dissociation of ${\mathrm{Al}}_{2}$(${\mathrm{CH}}_{3}$${)}_{6}$ [trimethylaluminum (TMA)] on Si(100)2\ifmmode\times\else\texttimes\fi{}1 and Si(111)7\ifmmode\times\else\texttimes\fi{}7 surfaces. Single-layer adsorption was obtained for substrate temperatures ${T}_{s}$ between 180 and 200 K, while multilayer deposition occurred for ${T}_{s}$180 K. Low-energy electron diffraction patterns showed no evidence of long-range order in either the single-layer or multilayer deposits. HREELS spectra from TMA single layers adsorbed on Si(100)2\ifmmode\times\else\texttimes\fi{}1 and Si(111)7\ifmmode\times\else\texttimes\fi{}7 were consistent with those expected for nondissociatively adsorbed TMA dimers whose Al-Al axes are orthogonal to the substrate surface. No changes in the adlayer spectra were observed under KrF-laser irradiation (248 nm, 5.0 eV, 20-ns pulses) even after ${10}^{4}$ pulses with intensities up to 200 mJ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ per pulse. Low-intensity (20mJ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) ArF-laser irradiation (193 nm, 6.4 eV, 20-ns pulses), however, led to monomerization of the adsorbate. Higher-intensity (200 mJ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) ArF-laser irradiation caused dissociation of the TMA monomer and desorption of some methyl ligands. Both KrF- and ArF-laser irradiation of multilayer samples gave rise to immediate thermally induced desorption of loosely bound upper layers, while the remaining adlayer behaved in a manner similar to that of the single-layer samples.

Identification of surface hydronium: Coadsorption of hydrogen fluoride and water on platinum (111)

The adsorption of HF and its coadsorption with water were studied on Pt(111) by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) as a step in the UHV modeling of the acidic aqueous electrolyte/electrode interface. Anhydrous HF adsorbs without dissociation. HF coadsorbed with water reacts to form several phases distinguishable by TPD. HREELS spectra show that the reaction forms the H 3O + ion. The stoichiometries of thermal desorption identify an acid monohydrate phase ([H 3O +][F −]) and fully hydrated phases with stoichiometries of HF · 5H 2O in the monolayer and HF · 8H 2O in the multilayer. To the as-yet-unknown extent that low-temperature measurements are relevant to normal aqueous electrochemistry, these results indicate that even such classically “non-specifically” adsorbed ions as H + and F − interact sufficiently strongly with Pt surfaces to displace some water from their inner solvation shells. These data also show that Brönsted acid-base chemistry can be carried out and spectroscopically observed in low temperature monolayers in UHV, and point the way towards UHV studies of such pH-dependent phenomena as corrosion and electrocatalysis.

A comparison between water adsorbed on Rh(111) and Pt(111), with and without predosed oxygen

The adsorption of water and its interactions with oxygen on Rh(111) were studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS), and low energy electron diffraction (LEED); and comparison was made with similar data for Pt(111). On Rh(111) water absorbs molecularly in hydrogen-bonded clusters; no evidence for dissociation was seen on the clean surface. Reaction of water with adsorbed oxygen on Rh(111) produces hydrated surface hydroxyls. While the gross features of adsorption and hydroxyl formation are similar to those previously reported on Pt(111), significant differences in detail were found. In particular, the complex librational and OH-stretching regions of the HREELS spectra for H 2 O/Rh(111), more closely resemble those for other noble metal surfaces than the sharp, single feature observed for Pt(111). HREELS peaks at 970, 1020 and 1950 cm −1 seen for H 2 O/Pt(111) were absent on Rh(111). The middle (3a 1 ) molecular orbital for molecular water on Rh(111) is shifted towards the Fermi level, while on Pt(111) the spacing between the three orbitals is the same as in water vapor. Comparison with spectral data for bulk phases suggests that water on Pt(111) exists primarily in a state with O-O nearest neighbor distances closer to those of liquid water than of ice, allowing better match with the Pt(111) surface mesh. Additional minority species account for the additional EELS peaks specific to Pt(111). Water on Rh(111) is a mixture of ice-like water and water similar to the majority species on Pt(111). The structural differences lead to different chemistry. On both surfaces adsorbed oxygen and water react to yield a surface phase which evolves water upon heating to 210 K. On Pt(111) this phase contains OH but no H 2 O. On Rh (111) this phase contains both OH and H 2 O in association. The differences in the interactions between water and the (111) surfaces of these two catalytically and electrochemically similar metals may help explain electrochemical effects peculiar to the (111) face of Pt.

A new mechanism for K promotion of surface reactions: N2 on K-precovered Fe(111)

The adsorption of N2 on K-precovered Fe(111) at 74 K has been studied with low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and high resolution electron energy loss spectroscopy (HREELS). The presence of low precoverages of K (<1.4×1014 cm−2) dramatically enhances the sticking coefficient of α1-N2, the π-bonded precursor to dissociation, and causes an increase in the maximum population of this species. We conclude that the effects of K on this system are primarily mediated by long range interactions; we have modeled the nonlocal K-induced changes of the adsorption and desorption of N2 for temperatures ≤430 K and found that by slightly decreasing the γ-N2 adsorption energy and increasing the α1-N2 adsorption energy we can quantitatively account for both the increase of the α1-N2 sticking coefficient at 74 K and the increase of the dissociative sticking coefficient at 430 K previously reported by Ertl, Lee, and Weiss [Surf. Sci. 114, 527 (1982)]. The promoted α1 state has an N–N stretch frequency less than 20 cm−1 lower than that of unpromoted α-N2, as expected for a weak long range interaction with K, indicating that the N–N bond is not appreciably perturbed. This is significant, as the catalytic effects of K have been previously attributed to bond weakening in the dissociation precursor. At higher K precoverages, local N2–K interactions dominate, characterized by broad N–N vibrations at 1600–1800 cm−1. The HREELS spectra of H2 and H2O, common vacuum contaminants, are also reported for adsorption on a K-precovered surface at 83 K.

Vibrational study of the SiO2/Si interface by high resolution electron energy loss spectroscopy

By using high resolution electron energy loss spectroscopy (HREELS) the surface phonon spectra of SiO2(0001) and of thick amorphous SiO2 layers have been measured. The spectra recorded on the crystalline surface are characterized by three first order peaks at 498, 798, and 1176 cm−1. The HREELS data could be interpreted by the dielectric theory applied to anisotropic material, and the optical constants were compared to infrared spectroscopy values. HREELS spectra as a function of oxide thickness could be obtained from oxidized Si(100) wafers by sputter erosion. Peak positions shifted to lower frequencies and intensities decreased regularly with decreasing oxide thickness. At the interface a transition layer of about 25 Å was observed and carbon contamination was identified. The dielectric theory applied to a thin homogeneous supported film cannot account for the observed frequency shift.

HREELS and Auger studies of conducting polymers

High Resolution Electron Energy Loss Spectroscopy (HREELS) and Auger Spectroscopy have been used to examine the surface of conducting polymer samples. The samples were 100 nm thick films of polypyrrole doped with either BF 4 or ClO 4 negative ions. The films were made in Sweden and shipped to the USA for study. HREELS spectra could be obtaineed without difficulty in the samples' “as-is” condition. These spectra showed broad bands corresponding to known polypyrrole vibrational modes. Exposure of the polymer to oxygen had no effect, but water could be adsorbed on the samples at 130 K. In addition to vibrational excitations, broader electronic transitions were also identified. Auger studies were performed in an effort to characterize the surface. These studies indicate that considerable non-stoichiometry exists in the surface region. There were, as expected, problems due to questionable sample cleanliness, sample charging, and electron beam damage.

HREELS and Auger studies of conducting polymers

High Resolution Electron Energy Loss Spectroscopy (HREELS) and Auger Spectroscopy have been used to examine the surface of conducting polymer samples. The samples were 100 nm thick films of polypyrrole doped with either BF 4 or ClO 4 negative ions. The films were made in Sweden and shipped to the USA for study. HREELS spectra could be obtaineed without difficulty in the samples' “as-is” condition. These spectra showed broad bands corresponding to known polypyrrole vibrational modes. Exposure of the polymer to oxygen had no effect, but water could be adsorbed on the samples at 130 K. In addition to vibrational excitations, broader electronic transitions were also identified. Auger studies were performed in an effort to characterize the surface. These studies indicate that considerable non-stoichiometry exists in the surface region. There were, as expected, problems due to questionable sample cleanliness, sample charging, and electron beam damage.

Multiple vibrational excitations of H 2O and D 2O on Si(100)(2 × 1): A HREELS study

The HREELS (high-resolution electron energy-loss spectroscopy) spectra from H 2O and D 2O chemisorbed on the clean Si(100)(2 × 1) surface have been investigated in the energy region between about 500 and 1400 meV. For the first time, second overtones have been observed for a chemisorbed species. This observation is believed to be due to the enhancement of the O-H stretching fundamental and overtone modes because of the temporary formation of a negative ion that occurs when the incident electron is briefly trapped in a shape resonance. Additional multiple and combination modes are observed involving the resonance enhanced overtones as well as the fundamental modes. Furthermore, the dissociation energy of the hydroxyl group has been determined and found to be much larger than the actual energy needed to dissociate hydroxyl groups on the surface, indicating that the driving force for the observed atomic rearrangements is probably Si-O bond formation rather than O-H dissociation.