Swift ion beam analysis (IBA) of materials and their surfaces has been widely applied to many fields over the last half century, constantly evolving to meet new requirements and to take advantage of developments in particle detection and data treatment.
Today, emerging fields in nanosciences introduce extreme demands to analysis methods at the nanoscale. This book addresses how analysis with swift ion beams is rising to meet such needs. Aimed at early stage researchers and established researchers wishing to understand how IBA can contribute to their analytical requirements in nanosciences, the basics of the interactions of charged particles with matter, as well as the operation of the relevant equipment, are first presented. Many recent examples from nanoscience research are then explored in which the specific analytical capabilities of IBA are emphasized, together with the place of IBA alongside the wealth of other analytical methods.
1. Fundamentals of Ion-solid Interactions with a Focus on the Nanoscale.
2. Instruments and Methods.
4. The Place of NanoIBA in the Characterization Forest.
The term "swift ion beam analysis" (IBA) encompasses a selected number of techniques dedicated to analyzing materials with the common basis of sending light ions at high enough energies and with low enough fluxes to be able to obtain information about the sample without damaging it and even less abrading it. Indeed, when a charged particle penetrates the material it interacts with the electrons and nuclei, which causes a slowdown of the particle and a change in its trajectory. The different interactions involved during the passage of the ion in the condensed matter allows for the implementation of different techniques.
The Coulomb repulsion between the ion and the nucleus will allow the implementation of the Rutherford Backscattering Spectrometry (RBS) technique with ions of a few MeV and, for lower energies, the Medium Energy Ion Scattering (MEIS) technique, which employs ions of energies ranging from 50 to 500 keV, is used. The same ion/nucleus repulsion will also allow, in a particular geometry, the ejection of very light ions recoiled from the sample using Elastic Recoil Detection Analysis (ERDA). Ion-electron Coulomb interactions will induce X-ray emissions resulting from the creation of holes in the inner electron shells, allowing the use of the Particle Induced X-ray Emission (PIXE) technique. Finally, if the incident ion has sufficient energy to pass the Coulomb barrier, it can enter the field of strong interactions of the target nucleus and induce a nuclear reaction. The nucleus formed during this collision may be in an excited state and subsequently relaxes by emitting gamma photons or charged particles. Detecting charged particles is the basis of the Nuclear Reaction Analysis (NRA) technique and the gammas at the base of the Particle Induced ?-ray Emission (PIGE) technique. The common phenomenon between these techniques is to perform a spectroscopy of ions scattered by the primary target in the case of RBS and MEIS, of recoil ions in the case of ERDA or of secondary X-rays, ?-rays or charged particles in the case of PIXE, PIGE and NRA. Basically, this spectroscopy allows an elemental analysis of the target and, depending on the technique employed, also provides concentration depth profiles, layer thicknesses, mapping of chemical compositions or even structural information.
These analyzing methods have been successfully and widely employed in bulk materials as well as in layers of various thicknesses. The emergence, since the middle of the 1980s, of nanoscience poses new questions for analysis techniques, including IBA, in terms of spatial resolution and sensitivity.
Although nanoparticles are generally considered as a recent discovery of science, they actually have a very long history. The scientific community discovered the specific optical properties of metallic colloids in the early 20th Century with Gustav Mie's works [MIE 08]. However, the use, probably unwitting, of nanoparticles for their optical properties is even older and dates back several millennia [COL 09, GAR 11]. As an example, red glasses of the late Bronze Age (1200-1000 BC) found at Frattesina di Rovigo (Italy) were colored thanks to the excitation of surface plasmon modes of copper nanoparticles [ANG 04, ART 08].
Nowadays, different types of nano-objects are examined by various scientific disciplines. The feature these objects have in common is that at least one of their dimensions is in the nanoscale. They can be classified into three categories named by their number of nanometric dimensions:
- - Ultrathin layers: the layer's thickness is nanometric and the other two dimensions can be considered infinite (1D objects).
- - Nanotubes and nanowires: here, only two dimensions are in the nanoscale (2D objects).
- - Clusters and quantum dots (QDs): three dimensions of space are in the nanoscale (3D objects).
It should be noted that, in the literature, this nomenclature is often replaced by the number of infinite dimensions. As an example, thin layers are often called 2D layers. The appearance of a mixed classification using both definitions can be seen in some cases. Thus, the transition between a thin layer and a quantum dot plane is called 2D/3D transition. In this case, the nomenclature regarding the thin layer refers to the number of infinite dimensions, while the one concerning the quantum dots refers to the above definition.
The challenge posed by the analysis of these objects is different in each case. This will be discussed in the following chapters and illustrated by a selected number of application examples. Before addressing these questions, a brief description of the analytical techniques of secondary radiation will be presented followed by a summary of the fundamentals of ion-material interactions. Some of the techniques described here have not yet found applications in nanosciences; however, they are included in the interests of completeness.
I.1. Interactions with electrons
- - Ion electronic energy loss: in a process that is common to all of the IBA analytical techniques discussed here, ions lose energy through interactions with electrons as they traverse the material being analyzed. The energy spectrum of an elastically scattered primary beam or of charged particle nuclear reaction products thus contains information about the depth from which the particle originated, with the depth resolution being directly related to the energy resolution of the detection system. On the other hand, photons (gamma rays, X-rays and optical photons) do not lose energy as they traverse the material. Thus, an ion-beam induced photon energy spectrum does not contain depth information on its own accord. Ion energy loss after transmission through thin self-supporting films has been exploited as an analytical technique for film thickness determination (Scanning Transmission Ion Microscopy (STIM)) and in normalization procedures for other IBA techniques applied to such films. The electronic excitations induced by the passage of the ion beam may sometimes relax due to the emission of optical photons giving rise to Ion Beam Induced Luminescence (IBIL) and, in semiconductor devices, these may induce charge movements that are measurable by external circuits. This is a technique termed Ion Beam Induced Charge (IBIC) and it is in fact the effect at the heart of semiconductor-charged particle detectors.
- - PIXE (Particle Induced X-ray Emission): This analyzing method is based on the ionization of the inner electronic shells (K, L, M, .) of the target atom by the incident ion and on the detection of characteristic X-rays emitted subsequent to the electronic rearrangement. This radiation is characteristic of the chemical element and the elemental composition of the sample can be inferred from the X-ray spectrum, similar to the use of energy dispersive X-ray analysis (EDX) available on the Scanning Electron Microscope (SEM), except that sensitivity is better since the primary ion beam produces a much less bremsstrahlung background than that due to the electron beam of the SEM. Optimum sensitivity is obtained with the use of proton beams of energies of a few MeVs. A thin film placed in front of the detector to prevent the elastically scattered primary beam from entering absorbs the fluorescence of elements lighter than sodium. For elements of higher atomic numbers, the technique has sensitivity in the order of a few ppms, as illustrated in Figure I.1 [JOH 76].
Figure I.1. Sensitivity of PIXE as a function of atomic number and proton energy for organic samples [JOH 76]
The cross-section for X-ray production is sufficiently high that PIXE is often used on accelerators equipped with an ion microprobe, where incident beam currents are very low, for elemental mapping of the sample with a lateral resolution that can be in the submicron scale [WAT 98].
I.2. Elastic scattering from nuclei
- - RBS (Rutherford Backscattering Spectrometry): the principle here is to measure the kinetic energy and the scattering angle of charged particles, usually protons or alpha particles, elastically scattered by the nuclei of the sample. In favorable cases, one can separate the elements according to their atomic mass and determine their depth distribution. RBS is practiced with beam energies ranging from 0.5 to 5 MeV and a surface barrier detector of an energy resolution of 12-15 keV is used for probing the sample over a few microns with a depth resolution of about 10-20 nm. In fact, at energies near the Coulomb barrier the interaction may remain elastic but it has a cross-section that varies from that of Rutherford, including resonant structures similar to those described for NRP (below). In this case, the term Elastic Backscattering Spectrometry (EBS) is often employed and the resonant structures may be exploited to obtain enhanced depth information from samples.
- - MEIS (Medium Energy Ion Scattering): The MEIS technique is similar to the RBS in that elastic scattering is exploited but it uses particles with energies between 50 and 500 keV. An electrostatic detector analyzes the energy of scattered particles with a resolution of 0.3% and the scattering angle with an accuracy of the order of 0.1°. The analysis focuses on a few tens of nanometers below the sample surface and, in favorable cases, the depth resolution can reach 0.25 nm. Since the electrostatic detector determines angular distributions and energy distributions simultaneously, ion blocking effects (described in section 1.3.3) in single crystal materials are...