Low-energy Electron Energy Loss Spectroscopy Research Articles

Structures near the L2,3 core edges of clean and: Oxygen covered Si(111) studied with low-energy electrons

The L2,3 core excitation spectra of clean and oxygen covered surfaces of Si(111), measured in low-energy electron energy loss Spectroscopy reproduce previous results obtained with highenergy electrons or synchrotron radiation. It is inferred that among the different transition channels, the matrix elements for the dipole term dominate. Spectra on oxidized surfaces indicate that oxygen adsorption on Si(111) induces the formation of SiO4 structural units.

The room-temperature oxidation of Cu/Si(100) and Cu/Si(111) interfaces studied by Auger electron spectroscopy, electron energy-loss spectroscopy, and high-resolution electron energy-loss spectroscopy

A study by integrated Auger electron spectroscopy, electron energy-loss spectroscopy (EELS), and high-resolution low-energy electron energy loss spectroscopy (HREELS) has been performed on the oxidation of copper silicides, which had been formed by electron beam vapor deposition of Cu onto Si(100) and Si(111) surfaces at 293 K. The splitting of the 92-eV (L2,3VV) silicon Auger intensity peak during increasing copper coverage and with oxidation at 295 K is analyzed in detail. The oxidation process is furthermore investigated by EELS and HREELS through an analysis of the changes in the fine-structure electron energy-loss spectra. An enhanced oxidation due to a catalytic interaction of Cu upon the surface electronic structure is found apparently to be due to an increased production of surface SiO species at the initial stages. No Cu–O bond was detected.

Oxygen adsorption on Pd(110) at 300 K-low-energy electron diffraction and electron energy loss spectroscopy studies

The adsorption of oxygen on a Pd(110) surface has been studied using in situ combined techniques of low-energy electron diffraction (LEED) and vibrational electron energy loss spectroscopy (EELS). Changes in the adsorbed structure of oxygen with increasing exposure are discussed.

High density molecule-like species formed in surface layers of C 8K

The surface of C 8K is very active at room temperature and it is easily deteriorated even under ultrahigh vacuum atmosphere. Freshly cleaved surfaces of C 8K were exposed to H 2, H 2O or O 2 gas systematically, and investigated with low-energy electron energy loss spectroscopy and Auger electron spectroscopy. It is proved that at room temperature the C 8K surface reacts strongly to oxygen, and a high density of molecular species, presumably K 2O or its related compound molecules, are formed on the surface. This results in the very easy deterioration of the C 8K surface. The surface of C 8K at 140 K, on the other hand, is much less active when it is exposed to gasses.

Experimental Study of Inelastic Process at the Crystal Surface in Angle Resolved Low-Energy Electron Energy Loss Spectroscopy

Angle-intensity profiles of inelastically scattered electrons were measured around several diffraction spots for the energy loss less than 20 eV. Peak structures were observed near diffraction spots. The energy loss dependence and the incident energy dependence of the peak position are discussed on the basis of the two step inelastic diffraction model, which suggests three characteristics in electron energy loss processes at a surface: (1) maximum probability of surface electronic excitation occurs at the momentum transfer parallel to the surface, (2) peaks due to loss before diffraction process dominate for grazing incidence and (3) a part of inelastic electrons due to surface electronic excitations have coherency as well as the incident electron according to the spot-like distribution of inelastic electrons.

Excitonic transitions of ordered layers of Xe on ag(111) and Al(111): A low-energy EELS study

We present the results of low-energy electron energy loss spectroscopy (EELS) studies of the lower excitonic transitions of Xe thin films on single crystal Ag(111) and Al(111) surfaces. We find that the spectra of cold (≈20 K) deposited, disordered Xe films can be interpreted in terms of broadened atomic-like transitions. However, the spectra of ordered Xe layers. obtained upon annealing, show extra transitions with no optical counterparts. These observations are explained in terms of significant dispersion and lifting of the degeneracy of levels in ordered layers of Xe. Low-energy EELS allows transitions to the split excitonic levels involving large momentum transfer.

Oxygen and hydrogen adsorption on GaAs(110)

The interaction of molecular, ‘‘unexcited’’ oxygen and of atomic hydrogen with clean, cleaved GaAs surfaces was studied by Auger-electron and low-energy electron energy-loss spectroscopy as well as by a Kelvin probe. Up to ≊3×105 L of O2 the coverage slightly increases only up to θ=0.05–0.1, depending on the ‘‘quality’’ of the cleave. This chemisorption, which is attributed to cleavage-induced imperfections, is further correlated with the annihilation of the 20 eV energy-loss, usually characterized as an excitonic Ga(3d) core level-to-dangling bond states transition. The main chemisorption is observed after exposures above 3×105 L and yields θ=0.8 after 1010 L of O2. Two successive stages can be distinguished. The first one saturates at θ=0.4–0.5 after 3×106 L of O2. Larger exposures further increase the coverage and cause two new losses to grow, an O(2p) related one at 7 eV and an As(3d) related one shifted by 2.5 eV to larger binding energies. Illumination of the surface with the light of a Xe-arc lamp dramatically stimulated the oxygen uptake. Under the present experimental conditions, the initial sticking coefficient at RT is increased by a factor of 100, and the coverage reaches θ=1.5 after 1010 L of O2. The chemisorption of hydrogen causes the work function to increase on n-type and to decrease on p-type substrates, by this indicating the formation of depletion layers on both substrates. After exposures of up to 5 L the chemisorption-induced surface states of acceptor- and donor-type, respectively, pin the Fermi level at approximately Evs+0.85 eV and Evs+0.63 eV for n- and p-doped samples, respectively. Above exposures of 3 L the 20 eV energy-loss peak is observed to decay, and it has vanished after 3×104 L of hydrogen. This result clearly demonstrates that hydrogen atoms do not initially bond to the surface Ga atoms. In the exposure range from 50 to 3×104 L the intensity ratio of the surface sensitive As(31 eV) and Ga(55 eV) AES peaks decreases by approximately 20% while the Ga(55 eV) signal itself remains unchanged with respect to the primary peak. This result corresponds to a chemisorption-induced loss of approximately 75% of the As atoms in the topmost layer.

Segregation of As on GaAs(110) surfaces observed immediately after cleavage

A local excess of pure As was observed on surfaces cleaved from Te-, Si-, and Zn-doped boat-grown GaAs and from a sample fabricated by the travelling heater method. The surface segregation was studied by locally resolved Auger-electron and low-energy electron energy-loss spectroscopy. The diameter of the arsenic spots which varies up to 70 μm immediately after cleavage and the number of spots increase during storage at room temperature and by anneals at up to 170°C. With the larger spots a fine-structure is distinguishable.

Study of OCS by electron energy loss spectroscopy

A high resolution study of the OCS molecule is presented using low-energy electron energy loss spectroscopy. Measurements have been carried out at various incident electron energies and scattering angles, covering the whole range from 5 to about 17 eV. The high energy resolution (∼30 meV) has allowed the detailed analysis of the vibrational progressions and Rydberg series, and some identifications are supported by the plots of their differential cross sections. For the first time, the two overlapping progressions of the (10σ) 1Π state, around 7.4 eV, are resolved by electron impact. A new triplet state is observed at 7.59 eV, assigned to the (11σ) 3Π known state, whose 1Π term would correspond to a known progression lying under the strong 1Σ+ band. Above the first ionization limit, the spectrum presents a striking similarity with the photoionization efficiency curves, with the strong autoionizing features which form Rydberg series converging to the ? 2Σ+ state of OCS+. Even the weak series VIII is present, appearing as a small dip. Further measurements carried out at low residual electron energy (?5 eV) reveal that such features then become peaks of strength comparable with the other features. Typical triplet behavior are pointed out for some of them. Two new series (X and XI) are thus proposed, while another feature is assigned to the n = 3 term of series VII converging to the ? 2Σ+ state. For all of the series observed in this region, the quantum defects are determined and orbital assignments are proposed.

High resolution studies of dipole-forbidden states of N2using low-energy electron energy-loss spectroscopy

Energy-loss spectra for molecular nitrogen have been measured with impact energies between 0.5 and 100 eV above the threshold for excitation and at scattering angles between 2 and 60 degrees . The energy-loss range between 5 and 25 eV was studied, with a typical resolution of approximately 0.025 eV. At energy losses below 11 eV the observed spectra can be fully assigned, with the exception of one progression of very weak peaks, using dipole-forbidden states previously observed in emission studies. The angular distribution of one of these states, w1 Delta u was measured. Between 14.30 and 15.90 eV, six energy-loss peaks were found to have angular distributions typical of dipole-forbidden transitions which cannot be related to previously observed states of N2. Above the ionisation limit, three new Rydberg series of states converging to the nu =0 level of the B2 Sigma u+ state of N2+ were detected at high scattering angles and low impact energies.