Secondary Electron Spectra Research Articles

Angular distribution of K Auger electrons ejected by highly charged ions interacting with an Al(111) surface.

Secondary electron spectra of the H-like Ne[sup 9+] ion incident with impact energies of 135 eV up to 90 keV on a solid Al(111) surface were measured. The dependence of the [ital K] Auger electron yield on the angle of observation is studied in detail. It is found to be cosine like in case of the 90-keV Ne[sup 9+] ions and to be more and more isotropic at lower ion energies although a clear anisotropy remains. Information about the rates of the filling of the [ital L] and [ital K] shells inside the solid is obtained from a comparison of the measured angular distributions with the calculation of a two-step model for the successive filling of the [ital L] and [ital K] shells. The data show clear evidence for Auger electron emission from below the surface for ion energies as low as 135 eV.

Quantitative Auger analysis of A 3B 5 pseudobinary compounds by means of spectrum synthesis based on a non-linear least-squares fitting technique

Quantitative procedures for AES analysis and composition depth profiling of semiconductor heterostructures based on III-V pseudobinary compounds are demonstrated. The procedure involves the extraction of pre-filtered elastic Auger currents by linear digital filtering of the secondary electron spectra; this is followed by spectrum synthesis. Digitally filtered spectra of binary, as well as ternary compounds, can serve as external standards. The non-linear least-squares fitting technique is used to compensate some effects of sample charging and misalignment of the Auger spectrometer.

Voltage measurements using the time-of-flight electron spectrometer

The time-of-flight (TOF) electron spectrometer measures the time taken for a short pulse of secondary electrons to travel from the specimen surface to a fast microchannel plate detector placed some way up the electron-optical column. The shape of the resulting pulse is directly related to the complete energy spectrum of the emitted secondary electrons. Results are presented for an experimental set-up based on a standard scanning electron microscope column with an additional “single-pole” final lens which allows a relatively large unobstructed working distance above the specimen. Methods for the rapid extraction of quantitative voltage data are described.

Virtual excitation of a core-level hole as the origin of extended fine structure of Auger spectra: Theory and experiment

It has been shown that the contribution from the intermediate excited state of the system at the moment of the Auger transition causes the extended fine structure of secondary electron spectra, which extends several hundred electron volts above the main Auger line. Two types of oscillations are observed in the spectrum, due to the electron scattering on the atomic environment in the intermediate and final states. The comparison between Cu and Ni theoretical and experimental spectra was carried out.

Damage correlations in semiconductors exposed to gamma, electron and proton radiations

The use of nonionizing energy loss (NIEL) in predicting the effect of gamma, electron, and proton irradiations on Si, GaAs, and InP devices is discussed. The NIEL for electrons and protons has been calculated from the displacement threshold to 200 MeV. Convoluting the electron NIEL with the slowed down Compton secondary electron spectrum gives an effective NIEL for CO/sup 60/ gammas, enabling gamma-induced displacement damage to be correlated with particle results. The fluences of 1 MeV electrons equivalent to irradiation with 1 Mrad(Si) for Si, GaAs, and InP are given. Analytic proton NIEL calculations and results derived from the Monte Carlo TRIM agree exactly, as long as straggling is not significant. The NIEL calculations are compared with experimental proton and electron damage coefficients using solar cells as examples. A linear relationship is found between the NIEL and proton damage coefficients for Si, GaAs, and InP devices. For electrons, there appears to be a linear dependence for n-Si and n-GaAs, but for p-Si there is a quadratic relationship which decreases the damage coefficient at 1 MeV by a factor of approximately 10 below the value for n-Si.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Scattering in electron spectrometers, diagnosis and avoidance. I. Concentric hemispherical analysers

AbstractThe unwanted scattering of electrons in electron spectrometers leads to a loss of sensitivity, distortion of the spectral background and problems in calibrating spectrometer intensity/energy transmission functions. Here we develop three methods to analyse the extent of the contribution and how to reduce it to an insignificant level. The three diagnostic methods involve either the measurement of the region of the M4,5VV Auger electron peaks from Ag in AES, or the reflection of an electron beam from a biased sample, or the 3p1/2 photoelectron peak from Ag irradiated with Mg Kα x‐rays. In this work analyses are made for concentric hemispherical analysers. In Part II, cylindrical mirror analysers are studied. It appears that in our spectrometers, a modified VG scientific Escalab II with either the original analyser or a 210 analyser, the major scattering contributions arise from high‐energy electrons that strike the outer hemisphere giving rise to a spectrum of secondary electrons. These cause the scattered electron contribution to vary approximately as E, where Ep is the pass energy, to increae approximately in proportion to the spectrometer input slit area for AES studies, to fall approximately inversely with an increase in the lens magnification from 1 to 3 and, in the constant ΔE mode, to increase with the analysed electron energy. Thus, for each lens magnification, there exists a range of slits and pass energies for which the scattering may be maintained at &lt;1%. In the constant ΔE mode, pass energies of ≥50 eV are generally safe, whereas in the constant ΔE/E mode, retardation ratios of ≤4 are required.

Ion-induced electron emission from a cold (40 K) nickel target

The energy spectra of secondary electrons emitted in the wide region of exit angles δ' (from δ' = 0.8° up to δ' = 90°) were examined for the cold (40 K) (100) nickel target bombarded with 5 keV Ne + ions. The electron energy spectrum shows some structure which strongly shifts to the side of low energy while δ increases. This shift can exceed the work function for the nickel. Such result suggests that the angular distribution of generated electrons should be anisotropic, and it is possible under assumption that the electron creation takes place on the target surface.

Anomalous behavior in the spin polarization of low-energy secondary electrons from Gd(0001).

We have measured the spin-polarization spectrum of low-energy secondary electrons from the surface of thick single-crystalline Gd(0001) films grown on W(110). Below 1.5 eV an anomalous decrease in the polarization is observed and is attributed to the presence of the 4${\mathit{f}}^{8}$ states just above the vacuum level, which leads to increased emission of spin-down electrons. Between 1.5 and 7 eV the spin polarization remains nearly constant at about 33% (at 150 K), demonstrating that the phenomenon of polarization enhancement for low-energy secondary electrons is not universal. The observed large polarizations indicate that the completely polarized 4f electrons contribute significantly to the secondary-electron yield.

SEMICLASSICAL STUDY OF δ-ELECTRON SPECTRA

This short communication contains an analytical expression for secondary-electron spectra. The obtained solution is confirmed by comparison with experimental data and analytical calculation of the Livingstone-Bethe asymptotics.

X-ray photoemission spectroscopy studies of cesium and oxygen on GaAs(100)

We have studied the interface formation of Cs on an As rich GaAs(100) surface and the subsequent oxidation of the cesiated surface using x-ray photoemission spectroscopy at room temperature. Several aspects of this interface are in sharp contrast to the nonreacting Cs/GaAs(110) interface. We found that Cs disrupted the GaAs(100) surface and caused As to segregate. Unlike the one monolayer (ML) saturation coverage for Cs/GaAs(110) at room temperature, no saturation coverage for Cs/GaAs(100) could be found. Further exposure to oxygen seemed to resurface the Cs atoms originally buried under the segregated As. The work function reduction was found to be 3.1 eV at Cs coverage of 3.0 ML, and the work function increased by 0.45 eV after 100 L oxygen, unlike the work function decrease found on cesiated GaAs(110). A feature of the conduction band electronic states of GaAs, which is below the vacuum level for a clean surface, was observed to appear in the secondary electron spectrum during the process of cesiation.

Effect of specimen bias on secondary electron images in the STEM

Modifications to a Vacuum Generators HB501 STEM to enable electrical contacts to be made to a sample during the operation of the microscope are described. An application of this system is demonstrated by the investigation of secondary electron image contrast as a function of specimen voltage bias. It is shown that the contrast of some heterogeneous specimens can be enhanced by the application of small positive voltage biases. This is attributed to differences in the secondary electron spectra from the different materials.

Surface electron emission induced by ion bombardment: methods of observation

The energy spectra of Ne + ions reflected from the cold (100) nickel surface covered by an adsorbed layer of Ar and the energy spectra of secondary electrons emitted under ion bombardment were measured simultaneously. The dependenes of the single-scattering nickel peak and ion-induced electron yield on the incidence angle were compared. It was found that in the range of incidence angles when the violent collisions are impossible due to shadowing of the substrate atoms by the adsorbed layer (the scattering angle was fixed at θ s = 150°) secondary electrons are emitted and their energy spectrum shows some structure. This structure disappears when violent collisions with substrate atoms become possible. It is assumed that the additional structure of the energy spectrum is connected with weak surface collisions and creation of (Ne + + Ni) quasimolecules. The decay of the inner-shell excited states of such quasimolecules can lead to escape of the electrons in the direction θ c = 90°. In the case of violent collisions the production of electrons increases, but they leave the quasimolecules mainly in the directions θ c > 90° and cannot be detected (the detection angle of electrons was fixed at θ c = 90°).

Evidence for subsurface Auger processes during interactions of N7+ ions with a Ni(110) target.

We present secondary-electron spectra arising from collisions of ${\mathrm{N}}^{7+}$ ions with a Ni(110) target for kinetic energies ranging between 150 eV and 20 keV and also for different observation angles. The filling of the two K-shell holes in ${\mathrm{N}}^{7+}$ requires two consecutive KLL Auger processes, giving rise to a double structure in the KLL peak. The ratio of the intensities of the two KLL peaks depends on the collision energy as well as on the angle of observation. We show that this dependence can be explained by assuming that the KLL electrons are emitted after the projectile has penetrated the surface.

Secondary-electron and energy-loss spectra of copper

The spectra of secondary electrons from copper show structure up to a kinetic energy of 300 eV, analogous to features observed in electron energy loss spectroscopy. We report EELS and SE data, up to 300 eV, obtained for a Cu(100) surface in order to show that they both are closely related to the unoccupied DOS. Features in SE spectra originate mostly from the emission channels out of the 3d and 3p levels, and those of EELS up to the threshold of 3p excitation energy stem from the transitions out of the 3d bands. This is well-reproduced by real-space multiple-scattering calculations of unoccupied density of states for different angular momentum components. The success of the interpretation confirms that the density of states dictates the observed features.

Evidence for a surface resonance feature in the very low-energy electron spectrum of ZrN

Previous investigations of the low-energy (0-40 eV) electron spectra of Ti and V nitrides have been extended to ZrN. The lowenergy spectra of ZrN films prepared by nitrogen ion implantation contain the corresponding features to those previously observed for TiN and VN. However, the ZrN spectrum is dominated by an extremely large feature at about 3 eV, which has not been previously reported. The amplitude of this peak varies directly with the nitrogen concentration. The peak shape and intensity are sensitive to primary electron beam energy in the range 0.5 to 2.0 keV, but the peak position does not change. The strong dependence of the amplitude of this feature on physisorption of gases and crystallographic orientation suggest that this peak arises from structure in the secondary electron spectrum induced by a surface resonance.

Bulk and surface electron emission induced by ion bombardment: methods of observation

It was found that the energy spectrum of secondary electrons emitted under ion bombardment of the cold (below 100 K) metal targets contains an additional maximum, the position of which on the energy scale depends on the ion incidence angle and the angle of electron detection. This maximum is also observed at room temperature but only at grazing exist angles. The additional maximum is supposed to be a result of the creation of electrons on the surface. Such electrons can be distinguished from the bulk electron emission at the special experimental arrangement (low target temperature and small exist angle of electrons).

Relativistic model of secondary-electron energy spectra in electron-impact ionization.

A relativistic model for differential electron-impact-ionization cross sections that allows the energy spectrum of secondary electrons to be calculated over a wide range of primary-electron energies is presented. The semiempirical method requires only experimental total ionization cross sections and optical oscillator strengths for the target species of interest, but other information, if available, can be incorporated to make the formulation still more accurate. Results for ionization of helium indicate that the lower limit on primary-electron energy for application of the model is about 100 eV. The simple analytic form of the model facilitates investigation of the regions of the secondary-electron energy spectrum where relativistic effects are important.

Core excitations and secondary electron spectra of Cu(100)

Abstract Electronic transitions near the 2s, 2p, 3s and 3p edges of Cu(100) are investigated with electron energy loss spectroscopy and compared with real-space multiple-scattering calculations that take into account the momentum transfer q in the electron scattering process. The interplay between q and the different angular momentum channels involved in the transitions show that while for the 2p-edge with a large energy loss (and with a large q ) dipole transitions dominate the spectrum, the non-dipole channels are very strong for the 3s-edge even with small q . The methods are extended to scattering states up to 300 eV above the Fermi level in order to identify structures in the secondary electron emission spectra. The strength of electron energy loss spectra in reflecting the electronic structure of solid surfaces is emphasized along with the necessity of a complete theoretical analysis before the dipole approximation may be applied.

Core excitations and secondary electron spectra of Cu(100)

Electronic transitions near the 2s, 2p, 3s and 3p edges of Cu(100) are investigated with electron energy loss spectroscopy and compared with real-space multiple-scattering calculations that take into account the momentum transfer q in the electron scattering process. The interplay between q and the different angular momentum channels involved in the transitions show that while for the 2p-edge with a large energy loss (and with a large q) dipole transitions dominate the spectrum, the non-dipole channels are very strong for the 3s-edge even with small q. The methods are extended to scattering states up to 300 eV above the Fermi level in order to identify structures in the secondary electron emission spectra. The strength of electron energy loss spectra in reflecting the electronic structure of solid surfaces is emphasized along with the necessity of a complete theoretical analysis before the dipole approximation may be applied.

Secondary electrons induced by fast ions under channeling conditions. I. Production and emission of secondary electrons.

Energy spectra of ion-induced secondary electrons emitted from Si and GaAs single crystals have been measured for various ions with a velocity of 3.75 MeV/amu. A simple {ital Z}{sub 1}{sup 2}-scaling behavior of the electron yields indicates that the observed keV electrons stem from simple binary collisions between the ions and the target electrons. Measurements for GaAs crystals preamorphized with 2-keV Ar{sup +} ions provide evidence for the localization of the binary collisions near the surface region under channeling incidence conditions. Furthermore, these measurements are used to determine the effective target thicknesses both for channeling and random incidence conditions.