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The energy range of primary electrons, for which the direct model is
valid, is approximately 200eV to 50 keV.
For energies below 200eV solid state effects are increasingly pronounced.
These are not accounted for in the partial wave approach for elastic
scattering. Furthermore the dielectric approach is based on an approximation
done for electron energies above 200eV.
The main difference between the implementation of the hybrid model and
the direct simulation
model is the description of inelastic scattering
processes.
The continuous slowing down approximation (CSDA) (used in both the
hybrid as well as the single scattering model) does not localize inelastic
scattering events. It only accounts for the mean energy loss
for a given flight path.
In contrary to this, the direct simulation approach simulates
discrete scattering events.
This approach leads to following advantages :
-
Using CSDA there is a rigid relationship between flight path and energy loss.
A given electron will always lose the same amount of energy for a certain
flight path. In the direct simulation approach the energy losses are
subject to random deviations, thus leading to a distribution of energy losses
for a given flight path with more realistic results for some applications.
-
In the CSDA a flight path of an electron is always associated with an energy
loss. This approach thus cannot be applied in simulating the escape process
of Auger (AES) or photo electrons (XPS) as well as the elastic peak
area of backscattered electrons. Due to the stochastic element in the description
of inelastic scattering, this problem does not arise in the direct simulation
model. Auger and photo electrons have a chance to leave the sample
without being subject to a loss of energy. primary electrons can
be backscattered conserving their initial energy.
-
As further advantage, the direct simulation model allows to simulate secondary
electrons. Assuming the energy losses to be caused by electron-electron
scattering events, energy is transferred from the primary electron
to a second electron. The trajectory of this secondary electron
can be simulated and its possible emission be detected.
The results of the simulation of secondary electrons are not reliable, as
the energy of the secondaries usually is in the range of 20-30eV where
there is only a limited validity of the physical models used in SESAME.
Quantative results cannot be expected.
The main restriction in the application of the direct simulation model is
given by the limited availability of dielectric data. The dielectric
behaviour of a material is a solid state property and can not be determined
using the appropriate elemental dielectric data.
The presently available elements and compounds for the direct simulation
model is shown in Table A.1.
Table A.1: Available elements for the DIR model
Furthermore, the direct simulation approach leads to slightly higher
computational effort in both the initialization of the simulation as well
as during the simulation itself.
Next: Applications
Up: Direct Simulation Model
Previous: Implementation
Horst Wagner
Tue Mar 19 10:24:55 MET 1996