Determination of the thickness and density of the ion bombardment induced altered layer in SiC by means of reflection electron energy loss study

The steady state surfaces of ion bombarded 3C-, 4H- and 6H-SiC samples were studied by means of reflected electron energy loss spectroscopy (REELS). The REELS exhibit a well-defined loss peak in the region of about 20 eV. The position of the maximum of the loss peak depends on the bombarding ion energy (decreasing with increasing ion energy), and on the primary electron beam energy (increasing with increasing primary energy). This behavior can be explained if we suppose that the plasmon energy in the altered layer (produced by ion bombardment) is different from that of the unaltered bulk. In this case the measured loss peak is the sum of two overlapping plasmon peaks. With modeling the system as a homogeneous altered layer and a homogeneous unaltered substrate the plasmon energy in the altered layer was derived to be 19.8 eV. The large change of the plasmon energy with respect to the bulk value of 23 eV is explained by a thin low density overlayer on the surface of the sample produced by the ion bombardment.     

Surface plasmon excitation at a Au surface by 150-40,000 eV electrons

Spectra of electrons with energies between 5 and 40 keV reflected from a homogeneous Au surface have been measured and analyzed to give the normalized distribution of energy losses in a single surface and volume excitation, as well as the total probability for excitation of surface plasmons. The resulting single scattering loss distributions compare excellently in (absolute units) with data from previous work taken at lower energies (150-3400 eV). An empirical relationship is derived for the total surface excitation probability as a function of the energy. For high energies the surface scattering zone represents only a small fraction of a typical electron trajectory and hence interference effects should be small at these energies. Since we find that both the energy dependence of the surface plasmon excitation probability and the shape of the single scattering loss distributions are the same at high and low electron energies, we conclude that there is no evidence for interference effects in the entire energy range studied.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Angular dependence of the electron energy loss spectra from a clean and adsorbate covered W(001) surface

A dispersion analysis of the EELS from a W(001) surface in the range 1 < ΔE < 35 eV has been performed and compared with recent and complete optical data for tungsten. The non-dispersive (k ～ 0) EELS correlated well with a combination of the surface and bulk loss functions calculated from the optical data. Losses at 1–5 eV and a pair at 32 and 34.5 eV were assigned to interband and N_6,7 core ionization excitations respectively. The principal bulk and surface plasmon losses were identified at 24.0 and 20.3 eV respectively. Two further losses at 14.0 and 9.6 eV were also observed and assigned to subsidiary plasmon losses. All four plasmon losses showed only minimal energy dispersion, never exceeding 1.5 eV. A momentum selectivity for separating bulk and surface interband losses was demonstrated with the non-dispersive losses arising from excitations within the bulk even with incident energies as low as 88 eV, whereas their dispersive counterparts were extremely sensitive to the chemical state of the surface. New adsorbate derived losses which develop during adsorption were associated with excitations from the new deep lying adsorbate levels to final state levels at or near the Fermi level. It was concluded that this final state was also responsible for the N_6,7 ionization losses.     

New interpretations of XPS spectra of nickel metal and oxides

A current interpretation of XPS spectra of Ni metal assumes that the main 6 eV satellite is due to a two hole c3d⁹4s² (c is a core hole) final state effect. We report REELS observation in AES at low voltages of losses (plasmons and inter-band transitions) corresponding to the satellite structures in Ni metal 2p spectra. The satellite near 6 eV is attributed to a predominant surface plasmon loss. A current interpretation of Ni 2p spectra of oxides and other compounds is based on charge transfer assignments of the main peak at 854.6 eV and the broad satellite centred at around 861 eV to the cd⁹L and the unscreened cd⁸ final-state configurations, respectively (L is a ligand hole). Multiplet splittings have been shown to be necessary for assignment of Fe 2p and Cr 2p spectral profiles and chemical states. The assignments of Ni 2p states are re-examined with intra-atomic multiplet envelopes applied to Ni(OH)₂, NiOOH and NiO spectra. It is shown that the free ion multiplet envelopes for Ni²⁺ and Ni³⁺ simulate the main line and satellite structures for Ni(OH)₂ and NiOOH. Fitting the NiO Ni 2p spectral profile is not as straightforward as the hydroxide and oxyhydroxide. It may involve contributions from inter-atomic, non-local electronic coupling and screening effects with multiplet structures significantly different from the free ions as found for MnO. A scheme for fitting these spectra using multiplet envelopes is proposed.     

Surface electron accumulation in indium nitride layers grown by high pressure chemical vapor deposition

Surface termination and electronic properties of InN layers grown by high pressure chemical vapor deposition have been studied by high resolution electron energy loss spectroscopy (HREELS). HREEL spectra from InN after atomic hydrogen cleaning show N-H termination with no indium overlayer or droplets and indicate that the layer is N-polar. Broad conduction band plasmon excitations are observed centered at 3400 cm⁻¹ in HREEL spectra with 7 eV incident electron energy which shift to 3100 cm⁻¹ when the incident electron energies are 25 eV or greater. The shift of the plasmon excitations to lower energy when electrons with larger penetration depths are used is due to a higher charge density on the surface compared with the bulk, that is, a surface electron accumulation. These results indicate that surface electron accumulation on InN does not require excess indium or In-In bonds.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

Characteristic energy loss structure of solids from x-ray photoemission spectra

Energy loss peaks in x-ray photoemission spectra of nine metals are presented. No strong evidence for intrinsic plasmon structure was observed. Spectra from the free electron-like metals, Al, Li, and Na under ultra-high vacuum (10^?11 torr) conditions show intense bulk plasmons and surface plasmons. Systematic variations in the characteristic energy losses are reported for the series Ag to Te. In addition to losses that may be attributed to plasma oscillations of the 5s5p bands, with N = 1 and 2 electrons, respectively, both Ag and Cd show additional high-energy losses that may arise through 4d-shell participation, with N = 11 and 12.     

Assignment of quantum number for plasmon energies in carbon layer systems

Electron-emission distribution curves of carbon layer surfaces excited by primary electrons of energies in the 118-534 eV range have been measured. The first four peaks in the plasmon spectrum are observed. It is concluded that the oscillator energies are presented to explain the assignment of the quantum number (n = 0,1,2,3) for internal plasmons in carbon layer systems. The preliminary assignment is in good agreement with the experimental results. It is also shown that the existence of limit between internal and surface plasmons. It is pointed out that the plasmon energy does not depend on both the external electrostatic voltage and the sample temperature. Moreover, the quantum number was adopted to the names of internal plasmons in the observed spectra.     

Characteristic energy loss spectra of copper crystals with surfaces described by LEED

Energy distributions of electrons back-scattered from copper (100) and (110) surfaces have been obtained for incident electron energies in the range 30 to 350 eV. The relations between optical measurements and the characteristic energy losses, as well as the effect of interband transitions on the bulk and surface plasmon frequencies in metals which do not have ideally free electron plasmas are discussed. By chemisorbing increasing amounts of oxygen on the clean surface, the surface plasmon loss peak was identified in the copper energy loss spectrum from its intensity dependence on the dielectric constant at the surface. This peak has been identified by previous authors as the bulk plasmon loss of a single s-electron plasma oscillation. Our identification of the surface plasmon loss peak implies that the d-electrons in copper do participate in the plasma oscillation and that the bulk plasmon frequency is shifted from its free electron value because of interband transitions.     

Surface valence band and plasmon features on clean cleaved silicon

The L₁L_2,3V Auger transition from vacuum cleaved surfaces of silicon has been studied in detail. Features in the distribution which are not due to plasmon losses are shown to correspond with primary features in the valance band density of states. However, effects of transition probability energy dependence apparently smear the structure, which may also be due in part to particular surface region properties. Bulk plasmon losses up to 9 in number are clearly resolved and the relative intensities compared with simple theory. A mean free path for bulk plasmon emission is deduced which is approximately twice the m.f.p. for other loss processes. Background contamination produces a new peak at approximately 4.5 eV.     

Probability and shape of the spectral line of a single bulk characteristic energy loss of a fast electron in a medium with electron absorption and strong spatial dispersion

The probability of single characteristic energy loss of a fast electron in a reflection experiment has been calculated. Unlike many works concerning this subject, the bremsstrahlung of bulk plasmons in the non- Cherenkov ranges of frequencies and wavevectors of a plasmon has been taken into account. The contributions to the probability of single loss and to the shape of the spectral line from a quantum correction that is due to the interference of elastic and inelastic electron scattering events have been determined. The probability has been calculated in the kinetic approximation for the relative permittivity, where the short-wavelength range of the plasmon spectrum is correctly taken into account. In view of these circumstances, the expression for the mean free path of the electron with respect to the emission of a bulk plasmon that was obtained by Pines [D. Pines, Elementary Excitations in Solids (Benjamin, New York, 1963)] has been refined. The coherence length of the fast electron in the medium-energy range under consideration has been estimated. The shape of the spectral line of energy losses in the non-Cherenkov frequency range has been determined. It has been shown that the probability of the single emission of the bulk plasmon incompletely corresponds to the Poisson statistics.     

Enhanced photoluminescence of conjugated polymer thin films on nanostructured silver

Photoluminescence of a conjugated polymer in the presence of surface plasmons on metallic nanoparticles is studied. A layered device structure was constructed that enabled control over nanoparticle diameter and separation between the polymer and nanoparticles layers. The dependence of the surface plasmon evanescent field and energy transfer has been investigated with the largest enhancement in photoluminescence observed at a 40 nm distance separation between the fluorophore and the surface plasmon. A spectrum of surface plasmon resonances ranging from the emission to the absorption energies of the conjugated polymer revealed largest enhancements when the resonance was tuned to the conjugated polymer emission energy. At peak photoluminescence the maximum photoluminescent enhancement was found to be 5.6 times of the photoluminescence of the control structure and the total integrated enhancement was 5.9 times.     

Gain-assisted propagation of electromagnetic energy in subwavelength surface plasmon polariton gap waveguides

The propagation of electromagnetic energy via coupled surface plasmon polariton modes in a metal-insulator-metal heterostructure is analyzed analytically for a core material exhibiting optical gain. It is shown that a sufficiently large gain can completely compensate for the absorption losses due to energy dissipation in the metallic boundaries, enabling long-range transport with a confinement below the diffraction limit for on-chip switching and sensing applications. For a free-space wavelength of 1500 nm, lossless propagation in a gold-semiconductor-gold waveguide with a core size of 50 nm is predicted for a gain coefficient γ = 4830 cm⁻¹, comparable to that of semiconductor gain media. The gain requirements decrease with the use of low-index nanocrystal-doped glasses or polymers as core materials.     

Joint experimental and computational study of aluminum electron energy loss spectra

Experimental reflection electron energy loss (REEL) spectra are measured from aluminum for primary energies ranging from 130 eV to 2 keV. A Monte Carlo simulation is shortly described and used to calculate the same spectra. The focus is on reproducing the variable weight of surface and bulk losses as the surface sensitivity of spectra changes by changing the primary electron energy. The intensity of surface losses in the simulations is modulated by the thickness of the region where surface excitations occur. Simulations based either on a constant or an energy-dependent thickness for this layer are considered. In both cases, simulated spectra reproduce the experimental trend as a function of energy, though the correct surface-to-bulk intensity ratio for each energy is either underestimated or overestimated.     

Characteristic energy losses on carbon contaminated layers correlated with secondary electron,auger electron emission and contrasts in scanning microscope

The secondary electron (SE) spectrum (0 < E < 50 eV) has been analysed by means of a CMA. Samples were clean aluminum, aluminum becoming carbon contaminated, sintered graphite powder, electro chemically deposited polymer on platinum and monocrystals of silicon carbon contaminated. When the clean Al surface is becoming carbon contaminated a quick decrease of surface plasmon and bulk plasmon losses is observed whereas a main characteristic energy loss peak (ELS) at 20 eV and a secondary electron peak at 20 eV appear simultaneously. Both peaks are very sensitive general features of carbon contaminated surfaces. The main loss peak is attributed to the excitation of the carbon-carbon bounds (σ → σ¹) as already proposed in the transmission ELS. The few eV change of the loss peak energy of various carbon compounds may correspond to slightly different carbon-carbon distances. The 20 eV secondary electrons could be produced by the relaxation of the excited state (σ¹ → σ transition) via an Auger process. The cross section for molecular electronic excitation is higher than that of atomic ionization for inner level. The loss peak is as intense as the SE peak and higher by more than two orders of magnitude than the C KLL Auger peak. The modification of secondary emission under carbon contamination has been observed on a silicon sample by Scanning Electron Microscopy (SEM) in the Secondary Electron Image (SEI) mode.     

Analysis of the plasmon structure in XPS experiments of simple metals

The plasmon structure of core lines in Be, Na, Mg, and Al metal has been analysed. It is shown that in each case extrinsic and intrinsic plasmons are present and that the probability of intrinsic plasmon creation is proportional to bⁿ, where b is the creation rate for a single plasmon.     

Messung der Energieverlustspektren von Aluminium- und Silberfolien mit hoher Auflösung

The energy loss spectra of polycrystalline aluminium and silver foils have been measured with high resolution. For the plasmon energy of aluminium 14.97 eV is obtained. The half width of the plasmon peak is 0.60 eV corresponding to a relaxation time τ=1.1×10^?15 sec in fair agreement with optical data. In silver films of certain thicknesses the surface plasmon peak and the plasmon peak can be separated in the energy loss spectrum. The values of the energy losses are 3.64 and 3.78 eV respectively.     

Energy dependence of electron energy loss processes in Ge 2s photoemission

Deep core Ge 2s photoelectron spectra from polycrystalline Ge films induced by monochromatic synchrotron radiation, of 4, 6 and 8 keV were measured and analysed using two different methods, the partial intensity analysis and the extended Hüfner method to determine the spectral contributions from different electron energy loss processes due to bulk extrinsic, intrinsic and surface excitations. The obtained photon energy dependence of the ratio of these contributions was compared as a function of the photoelectron kinetic energy. It was found that the relative contribution of intrinsic excitations increase with the photon energy.     

Electron Energy Loss Spectroscopy study of the near surface region of the ternary Co-Cr-Mo alloy

Nature of the characteristic electron energy losses in the second electron emission spectra from the ternary Co-Cr-Mo alloy surface are studied in the low energy range of the primary electron energy E₀. The main types of losses were found: surface and bulk plasmons and their hybrid modes, interband transitions and ionization losses. For Co, Cr, Mo and Co-Cr-Mo alloy the experimental values of the plasmon energy were established to be less than it was predicted by free-electron gas model. Excess of conductive electrons in the surface layers for Co, Cr and Mo was observed by dependence of the surface plasmon dispersion from E₀, while for Co-Cr-Mo alloy the situation is quit opposite. Such behavior is explained by the complex phase structure of the ternary alloy. The analysis of intensity lines of plasmons from E₀ showed deeply changed alloy profile. Ionization Spectroscopy was used for studying the alloy elements distributing on the depth. Mo atoms preferred segregation in the outermost layers of Co-Cr-Mo alloy and enrichment with Cr competitive atoms in underlayers is displayed.