Secondary knock-on atoms

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

In addition to recoil mixing, other ballistic phenomena are possible during ion irradiation and implantation. For example, enhanced atomic mixing can occur when multiple displacements of target atoms result from a single incident ion. In the multiple displacement process, an initially displaced target atom (primary recoil) continues the knock-on-atom processes, producing secondary recoil atom displacements which in turn displace additional atoms. The multiple displacement sequence of collision events is commonly referred to as a collision cascade. 

Figure 1.3 Highly simplified pictorial illustration of the knock-on sputtering process (this is the most common form of sputtering as outlined in Section 3.2.1.1). The solid black ball represents an incoming primary ion, the hollow circles refer to the atoms making up the solid surface, the solid arrows refer to incoming ion trajectories, and the gray arrows refer to substrate atom trajectories. Also illustrated are approximate time scales over which the respective processes take place. As outlined in Section 3.3, some percentage of the sputtered population will exist in the ionized state. It is the ionized population (the secondary ion population) that is recorded in SIMS.

Figure 3.1 Highly simplified illustration of a two-step process in which atomic secondary ion emission proceeds via knock-on sputtering resulting from keV primary ion impact (left box) followed by secondary ion formation/survival (right box) from the sputtered population. M in the right box can represent any element with the superscript referring to its associated charge and e" referring to an electron. The ejection of molecular ions appears to follow similar albeit more complicated routes.

When an incident beam of electrons is focused on a specimen at low pressures then many things can happen. Some electrons hit the nucleus of atoms and are bounced and scattered back towards the source. The heavier the atoms, the more back-scattering occurs. Some of the electrons cause electrons in the sample to be ejected. Some of the incident electrons knock out secondary electrons from the sample and the energy of these is characteristic of the atoms concerned. Some electrons cause X-rays to be emitted from the sample and some electrons pass straight through the sample. 

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