IBA techniques

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IBA techniques

IBA (Ion Beam Analysis) techniques

When the sample is exposed to the ion beam (of MeV energy range), different atomic and nuclear processes that lead to the emission of products are induced. Each product carries information about the sample composition, or some other material property. If the ion beam is focused in the nuclear microprobe facility, analysis with spatial resolution around 1 mm can be performed. Furthermore, techniques that are depth sensitive can be used for depth profiling of elements with down to 1 nm resolution.

Some of the methods most frequently used in the Laboratory are described below:

PIXE (Particle Induced X-ray Emission)
RBS (Rutherford Backscattering Spectrometry)
ERDA (Elastic Recoil Detection Analysis)
NRA (Nuclear Reaction Analysis)
IBIC (Ion Beam Induced Charge)

o PIXE (Particle Induced X-Ray Emission) spectroscopy

In 1970 Johansson et al. presented PIXE as a novel and powerful analytical method. Characteristic X-rays produced in sample being irradiated by MeV protons, were for the first time detected by semiconductor Si(Li) X-ray detector that had just become available. This analytical method soon became widely accepted in different accelerator laboratories, known under the acronym PIXE - Particle Induced X-ray Emission Spectroscopy.

There are four main physical processes of importance to PIXE: (i) when a charged particle (proton or heavier ion) enters the material it encounters numerous inelastic collisions by sample atoms; (ii) the energy of the ion along its trajectory decreases according to the specific energy loss (stopping power); (iii) from some of the numerous ionised atoms along the particle path, characteristic X-rays are emitted with a probability given by the X-ray production cross section; (iv) finally, X-rays emerging from the sample are attenuated in the material.

Advantages of PIXE spectroscopy can be deduced from these physical processes:

Very high probability of X-ray emission makes PIXE very sensitive analytical technique (for the most of elements and samples limits of detection are of the order of 1 ppm), while simple x-ray spectra obtained by high energy resolution detectors makes PIXE multielemental.
Since the x-ray fluorescence yield and detector efficiency are lower for light elements, PIXE is most often used for the analysis of elements 11 > Z > 92.
Since the sample damage is generally very low, PIXE is considered to be nondestructive technique.

Typical PIXE spectrum is shown on the figure.

o RBS (Rutherford Backscattering) spectroscopy

RBS differs from PIXE in the kind of detected particle. Instead of characteristic X-rays, those incident ions (of M₁ mass and Z₁ atomic numbers) that scatter back elastically from the target nuclei (M₂, Z₂) are detected. Due to the conservation of momentum and energy, the energy of scattered ions, E, is unambiguously determined by M₂, i.e., the energy distribution of scattered ions (energy spectrum) describes the elemental composition of the investigated target specimen:

Here k, the kinematic factor, characterises the target atom; E₀is the energy of the incident ions and Θ is the scattering angle. For RBS the energy and type of the incident ions can be chosen in such a way that their scattering from the target atoms will be predominantly pure Coulomb scattering (e.g., 0.5-2 MeV He , Li ions). In this case the scattering probability (cross section), σ (E₁,M₁,M₂,Z₁,Z₂,Q), can be calculated from basic principles using the Rutherford formula.

Therefore, without applying any calibration standards, the area, A, of the various peaks in the energy spectrum can be simply converted to the elemental areal density. In such cases RBS is inherently standardless quantitative analysis technique. If the proton beam is used instead (PES - Proton Elastic Scattering), cross sections differ from the Rutherford value, which can be favorable in some cases where high cross section increase the sensitivity of analysis. Backscattering techniques can be used for determination of all elements heavier then the projectile used. In the case of protons, these are 2<Z<92.

For thicker samples those ions that scatter back from atoms buried in the sample at various depths, x, loose some energy along both their inward and outward trajectories. Consequently, the peaks characterising the different elements of a buried layer in the sample will shift towards lower energies. Therefore RBS analysis provides also elemental depth profiles. The depth resolution will be determined by the ion energy and type, as well as detection system used. In the most favorable conditions depth resolutions of 1 nm can be obtained.

Example of RBS spectrum is given bellow, for the sample with thin film of elements with masses M3 and M2 at the M2 substrate. Element M3 is also implanted at some depth. (M1<M2<M3).

o ERDA (Elastic Recoil Detection Analysis)

Based on the same physical principles of elastic scattering, ERDA technique use heavier ions in the beam in order to recoil (in forward angles) light nuclei from the sample. ERDA can be performed in transmission mode (for samples thin enough), or which is more frequent, using the sample positioned in the grazing incidence angle (as given below).

Based on the fact that beam of heavier ions has small range as well as high cross section for elastic scattering, ERDA is efficient technique for high resolution depth profiling. It is used for analysis of light elements (all lighter then the projectile 1 < Z < Z (projectile)).

For the analysis of hydrogen depth profiles, the most simplest version of the technique is used. Here, the stopper foil placed in front of the silicon particle detector stops forward scattered particles. Only recoiled light nuclei can reach detector.

More sophisticated detection systems (such as IEE system at IRB) are used for the multielemental ERDA analysis.

o NRA (Nuclear Reaction Analysis) / PIGE (Particle Induced Gamma Emission)

When bombarded by MeV energy beam of protons or especially deuterons, nuclear reactions are becoming very probable for the most of light nuclei. Detection of reaction products is the basis of NRA and PIGE analysis techniques. For NRA, charge particles produced in nuclear reaction are detected by particle detector. For PIGE, reaction products being detected by detector are gamma rays.

o IBIC (Ion Beam Induced Charge)

Contrary to the previously discussed techniques that measure elemental concentrations in samples exposed to the ion beam, IBIC technique is used to measure electronic properties of the sample.

On its way through the material, ion lose its energy in numerous collisions with atomic electrons. Total number of electrons (and holes) depends on the ion energy and material under test. If the sample is semiconductor device with internal electric field (at junction), or externally applied electric field, charge carriers will drift in the electric field, inducing a charge signal at the electrodes of the device. The height of this pulse as well as its time evolution give information about the fundamental electronic properties of material (defects, lifetime of charge carriers, collection length, electric field, etc.)

Examples:

- IBIC analysis of CVD diamond and other detector materials (Nucl. Instr. and Meth. B158 (1999) 458-463)

- IBIC analysis of SiC (Nucl. Instr. and Meth. B188 (2002) 130-134)

- IBIC study of defects in polycristalline silicon (Nucl. Instr. and Meth. B181 (2001) 298-304)

- TRIBIC analysis of CdZnTe (Nucl. Instr. and Meth. B210 (2003) 237-242)

STIM (Scanning Transmission Ion Microscopy)

This is a low current nuclear microbeam technique that uses energy loss of ions that are transmitted through the sample. The amount of energy loss measured by detector is proportional to the thickness of the sample. STIM therefore gives image of microscopic changes in sample density/thickness.

more about STIM - Universitaet of Leipzig link

IL (Ionoluminescence)

The light emitted under ion irradiation originates from electron transitions and recombination processes within the outer electron shells of the sample atoms. The energy levels of these electron shells are affected by the chemical bondings of the atom. Therefore the ionoluminescence method can provide information about the chemical form of elements (speciation), which cannot be obtained by other ion beam analytical methods...

more about IL - Universitaet of Leipzig link