Chemistry Reference
In-Depth Information
Surface
Hydrogen ion beam (1 keV)
0
Graphite bulk
50
100
150
Hydrogen trajectory
200
Recoils
250
300
350
400
-250
-200
-150
-100
-50
0
50
100
150
0
)
Y
(
A
FIGURE 9.18
Collisional cascade created in graphite by 1 keV hydrogen ion beam. The
simulation was done using the TRIM code.
or it penetrates into the target and, after a collision cascade, comes to rest in thermal
equilibrium with the target atoms (implantation) (Figure 9.18) [96]. The target atoms
that reach the surface and have more energy than the surface binding energy of the
target material can be released from the surface (physical sputtering). The physical
sputtering and backscattering depends on the atomic masses of the incident ion and
the target as well as the energy and the angle of incidence of the incident ion. The
erosion of the surface by physical sputtering and the implantation of atoms can lead
to changes in the composition and structure of the material.
Incident hydrogen atoms that have equilibrated with the surface diffuse within the
target and can undergo chemical reactions with the target atoms to form hydrocarbons
and hydrogen molecules. Some of these molecules diffuse to the target surface and
can either be desorbed by incident atoms or be released thermally from the surface,
depending on the surface temperature (chemical erosion) [97]. In some cases, the
incoming energetic hydrogen atom or ion breaks a covalent C-C bond on the graphite
surface by pushing apart the carbon atoms due to its short range repulsive potential.
This can result in the ejection of either a carbon atom or a hydrocarbon and is called
swift chemical sputtering [98].
All sputtering processes are characterized by the
sputtering yield Y
, which is the
number of atoms or molecules ejected per impacting particle [99-102]. For physical
sputtering to occur, enough momentum must be transferred to the lattice atom to
overcome the surface-binding energy; thus, a
threshold energy E
th
exists for physical
sputtering below which no sputtering occurs. The value of
E
th
can be estimated
from the surface-binding energy of the target. The physical sputtering yield can be
