Low-energy ion scattering


Low-energy ion scattering spectroscopy , sometimes referred to simply as ion scattering spectroscopy, is a surface-sensitive analytical technique used to characterize the chemical and structural makeup of materials. LEIS involves directing a stream of charged particles known as ions at a surface and making observations of the positions, velocities, and energies of the ions that have interacted with the surface. Data that is thus collected can be used to deduce information about the material such as the relative positions of atoms in a surface lattice and the elemental identity of those atoms. LEIS is closely related to both medium-energy ion scattering and high-energy ion scattering, differing primarily in the energy range of the ion beam used to probe the surface. While much of the information collected using LEIS can be obtained using other surface science techniques, LEIS is unique in its sensitivity to both structure and composition of surfaces. Additionally, LEIS is one of a very few surface-sensitive techniques capable of directly observing hydrogen atoms, an aspect that may make it an increasingly more important technique as the hydrogen economy is being explored.

Experimental setup

LEIS systems consist of the following:
  1. Ion Gun, used to direct a beam of ions at a target sample. An electron ionization ion source is typically used to ionize noble gas atoms such as He, Ne or Ar, while heating of wafers containing alkali atoms is used to create an alkali ion beam. The ions thus created hold a positive charge, typically +1, due to the ejection of electrons from the atoms. The range of energies used most often in LEIS is 500 eV to 20 keV. In order to attain good experimental resolution it is important to have a narrow energy spread in the outgoing ion beam.
  2. Ion beam manipulator, includes the electrostatic lenses of the ion gun for focusing and beam-chopping. Lenses consist of a series of either plate or cylinder geometries and serve to collimate the beam as well as to selectively filter the beam based on mass and velocity. Beam chopping is performed using a pulsed-wave generator when time-of-flight experiments are performed. Ions only pass through the chopper when there is no applied voltage.
  3. Sample manipulator, allows an operator to change the position and/or angle of the target in order to perform experiments with varying geometries. Using directional controls, azimuthal and incident angle adjustments may be made.
  4. Drift tube/drift region, used in TOF setup. TOF measurements are used when analysis of particle velocity is required. By pulsing ions towards the sample with a regular frequency, and observing the time to travel a certain distance after surface impact to a detector, it is possible to calculate the velocity of ions and neutrals coming from the surface. An accelerator may also be used in this setup, prior to the drift tube, in order to achieve separation of ions from neutrals when desired.
  5. Detector/electrostatic analyzer, used to detect the velocities and/or energies of scattered particles including ions and, in some cases, neutral species. As opposed to TOF analyzers, electrostatic analyzers achieve ion energy resolution using electrostatic deflectors to direct only ions of a particular energy range into a collector, while all other ions are redirected. This type of analyzer can give good energy resolution but typically suffers from poor sensitivity due to the fact that it only detects ions of a certain energy range and ignores neutral species altogether. Two types of detectors are used: channel electron multiplier and microchannel plate detectors. CEMs operate in a similar manner to photomultipliers, displaying a cascade of secondary electron emission processes initiated by ion or fast neutral impact to give a gain in signal current. In this way it is possible to efficiently detect even small ion or neutral particle fluxes. MCP detectors are essentially 2-dimensional arrays of CEMs, and they allow additional information about particle position to be obtained at the cost of sensitivity at any given position.
  6. Vacuum pumps; Studies are performed in ultra-high vacuum conditions in order to prevent unwanted interference with the ion beam and/or sample. Common UHV pumps include turbomolecular and ion pumps, with roughing pumping typically performed using a rotary vane pump. Due to the extreme surface sensitivity of LEIS, samples also need to be rigorously cleaned prior to analysis. Some common processes used to clean samples include sputtering and annealing. Appropriate equipment for cleaning must be contained within the vacuum chamber.
  7. Other analysis tools; in many cases it is desirable to perform multiple types of analysis on a sample within the same UHV system, or even at the same time. Some additional tools may include Auger electron spectroscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy. Use of these tools typically requires the presence of additional detectors as well as electron and/or x-ray sources where applicable.

    Physics of ion-surface interactions

Several different types of events may take place as a result of the ion beam impinging on a target surface. Some of these events include electron or photon emission, electron transfer, scattering, adsorption, and sputtering. For each system and each interaction there exists an interaction cross-section, and the study of these cross-sections is a field in its own right. As the name suggests, LEIS is primarily concerned with scattering phenomena.

Elemental composition and two-body collision model

Due to the energy range typically used in ion scattering experiments, effects of thermal vibrations, phonon oscillations, and interatomic binding are ignored since they are far below this range, and the interaction of particle and surface may be thought of as a classical two-body elastic collision problem. Measuring the energy of ions scattered in this type of interaction can be used to determine the elemental composition of a surface, as is shown in the following:
Two-body elastic collisions are governed by the concepts of energy and momentum conservation. Consider a particle with mass mx, velocity v0, and energy given as impacting another particle at rest with mass my. The energies of the particles after collision are and where and thus. Additionally, we know. Using trigonometry we are able to determine
Similarly, we know
In a well-controlled experiment the energy and mass of the primary ions and the scattering or recoiling geometries are all known, so determination of surface elemental composition is given by the correlation between E1 or E2 and my. Higher energy scattering peaks correspond to heavier atoms and lower energy peaks correspond to lighter atoms.

Getting quantitative

While obtaining qualitative information about the elemental composition of a surface is relatively straightforward, it is necessary to understand the statistical cross-section of interaction between ion and surface atoms in order to obtain quantitative information. Stated another way, it is easy to find out if a particular species is present, but much more difficult to determine how much of this species is there.
The two-body collision model fails to give quantitative results as it ignores the contributions of coulomb repulsion as well as the more complicated effects of charge screening by electrons. This is generally less of a problem in MEIS and RBS experiments but presents issues in LEIS. Coulomb repulsion occurs between positively charged primary ions and the nuclei of surface atoms. The interaction potential is given as:
Where and are the atomic numbers of the primary ion and surface atom, respectively, is the elementary charge, is the interatomic distance, and is the screening function. accounts for the interference of the electrons orbiting each nucleus. In the case of MEIS and RBS, this potential can be used to calculate the Rutherford scattering cross section :
As shown at right, represents a finite region for an incoming particle, while represents the solid scattering angle after the scattering event. However, for LEIS is typically unknown which prevents such a clean analysis. Additionally, when using noble gas ion beams there is a high probability of neutralization on impact due to the strong desire of these ions to be in a neutral, closed shell state. This results in poor secondary ion flux. See AISS and TOF-SARS below for approaches to avoiding this problem.

Shadowing and blocking

and blocking are important concepts in almost all types of ion-surface interactions and result from the repulsive nature of the ion-nucleus interaction. As shown at right, when a flux of ions flows in parallel towards a scattering center, they are each scattered according to the force of the Coulomb repulsion. This effect is known as shadowing. In a simple Coulomb repulsion model, the resulting region of “forbidden” space behind the scattering center takes the form of a paraboloid with radius at a distance L from the scattering center. The flux density is increased near the edge of the paraboloid.
Blocking is closely related to shadowing, and involves the interaction between scattered ions and a neighboring scattering center. As shown, ions scattered from the first nucleus are now on diverging paths as they undergo interaction with the second nucleus. This interaction results in another “shadowing cone” now called a blocking cone where ions scattered from the first nucleus are blocked from exiting at angles below. Focusing effects again result in an increased flux density near .
In both shadowing and blocking, the "forbidden" regions are actually accessible to trajectories when the mass of incoming ions is greater than that of the surface atoms. In this case the region will have a finite but depleted flux density.
For higher energy ions such as those used in MEIS and RBS the concepts of shadowing and blocking are relatively straightforward since ion-nucleus interactions dominate and electron screening effects are insignificant. However, in the case of LEIS these screening effects do interfere with ion-nucleus interactions and the repulsive potential becomes more complicated. Also, multiple scattering events are very likely which complicates analysis. Importantly, due to the lower energy ions used LEIS is typically characterized by large interaction cross-sections and shadow cone radii. For this reason penetration depth is low and the method has much higher first-layer sensitivity than MEIS or RBS. Overall, these concepts are essential for data analysis in impact collision LEIS experiments.

Diffraction does not play a major role

The de Broglie wavelength of ions used in LEIS experiments is given as. Using a worst-case value of 500 eV for an 4He+ ion, we see λ is still only 0.006 Å, still well below the typical interatomic spacing of 2-3 Å. Because of this, the effects of diffraction are not significant in a normal LEIS experiment.

Variations of technique

Depending on the particular experimental setup, LEIS may be used to obtain a variety of information about a sample. The following includes several of these methods.