Ion-solid interactions

(solid-state physics) An atomic process that occurs as a result of the collision of energetic ions, atoms, or molecules with condensed matter.

Physical processes resulting from the collision of energetic ions, atoms, or molecules with condensed matter. These include elastic and inelastic backscattering of the projectile, penetration of the solid by the projectile, emission of electrons and photons from the surface, sputtering of neutral atoms and ions, production of defects in crystals, creation of nuclear tracks in insulating solids, and electrical, chemical, and physical changes to the irradiated matter resulting from the passage or implantation of the projectile.

When an energetic ion impinges upon the surface of condensed matter, it experiences a series of elastic and inelastic collisions with the atoms which lie in its path. These collisions occur because of the electrical forces between the nucleus and electrons of the projectile and those of the atoms which constitute the solid target. They result in the transformation of the kinetic energy of the projectile into internal excitation of the solid.

One of the most simple interactions occurs when the projectile collides with a surface atom and bounces back in generally the opposite direction from which it came. This process is known as backscattering. Its observation in 1911 led Ernest Rutherford to conclude that most of the matter in atoms is concentrated in a small nucleus. Now it is used as an analytical technique to measure the masses and locations of atoms on and near a surface. This technique for surface characterization is appropriately named Rutherford backscattering analysis, and is most commonly performed with alpha particles of about 2 MeV. Another backscattering technique, known as ion-scattering spectrometry, uses projectiles with energies of perhaps 2 keV.

Although backscattering events are well enough understood to be used as analytical tools, they are relatively rare because they represent nearly head-on collisions between two nuclei. Far more commonly, a collision simply deflects the projectile a few degrees from its original direction and slows it somewhat, transferring some of its kinetic energy to the atom that is struck. Thus, the projectile does not rebound from the surface but penetrates deep within the solid, dissipating its kinetic energy in a series of grazing collisions.

The capacity of a solid to slow a projectile is called the stopping power, and is defined as the amount of energy lost by the projectile per unit length of trajectory in the solid. Stopping power is of central importance for many phenomena because it measures the capacity of a projectile to deposit energy within a thin layer of the solid and this energy drives secondary processes associated with penetration. In many insulating solids (including mica, glasses, and some plastics) the passage of an ion with a large electronic stopping power creates a unique form of radiation damage known as a nuclear track. When the substance is chemically etched, conical pits visible under an ordinary microscope are produced where ionizing particles have penetrated. The passage of single projectiles may thereby be observed.

In the nuclear stopping region it is relatively likely that the projectile will transfer significant amounts of energy to individual target atoms. These atoms will subsequently strike others, and eventually a large number of atoms within the solid will be set in motion. This disturbance is known as a collision cascade. Collision cascades may cause permanent damage to materials, induce mixing of layers in the vicinity of interfaces, or cause sputtering if they occur near surfaces.

Ion implantation is used in the manufacture of integrated circuits and in the improvement of surface properties of metals. Ion-solid processes permit highly sensitive analyses for trace elements, the characterization of materials and surfaces, and the detection of ionizing radiation. Techniques employing them include secondary ion mass spectrometry (SIMS) for elemental analysis and imaging of surfaces, proton-induced x-ray emission (PIXE), ion-scattering spectrometry (ISS), and Rutherford backscattering analysis (RBS). They are also fundamental to the operation of silicon surface-barrier detectors which are used for the measurement of particle radiation, and of nuclear track detectors which are used in research as diverse as the dating of meteorites and the search for magnetic monopoles. See also Activation analysis; Proton-induced x-ray emission (PIXE); Secondary ion mass spectrometry (SIMS).