
Project
A4:
Kinetic Energy Transfer Processes at Surfaces
This
project deals with energy dissipation processes at solid surfaces that are
induced by the impact of atoms or molecules with velocities ranging from
about 10 m/s to 10 km/s. The imposed kinetic energy dissipates by means
of elastic collisions (“nuclear stopping”) and electronic excitation
processes (“electronic stopping”). For impact energies in the
keV range, it is well known that nuclear stopping largely dominates the
energy loss experienced by the projectile, thus generating a cascade of
atomic subsurface collisions which can lead to the emission of surface atoms
into the gas phase (sputtering). However, part of the kinetic energy is
converted into electronic excitation, which manifests, for instance, as
the presence of hot electrons in excited states above the Fermi level.
Both processes are investigated by i) mass spectrometric characterization
of the sputtered particle fluxes emitted from the surface and ii) spectrometric
detection of hot electrons with excitation energies below the work function
of the sample. Particular emphasis is put on the comparison between processes
induced by atomic and polyatomic projectile species, since in the latter
case the collision cascades induced by the constituent atoms overlap in
space and time, generating a nonlinear superposition effects which are to
be studied here.
Mass Spectrometry

An example for the type of experiments performed to characterize the
flux of material emitted from the surface is shown in Fig. 1, which
depicts the measured velocity distribution of neutral atoms and clusters
emitted from a polycrystalline indium surface under bombardment with
Au- and Au2- projectiles of the same impact velocity. These distributions
carry a clear signature of the nature of the sub-surface collision
processes induced by the projectile impact. For the monoatomic projectile,
the data coincides with what is expected theoretically for a so-called
linear collision cascade (solid line). Note that the distributions
measured for atoms and dimers are largely different.
Switching to diatomic projectiles leads to a drastic change of the distributions
towards low emission velocities. Moreover, atoms and clusters are now ejected
with practically identical velocity distributions. Both observations indicate
a drastic change the nature of the emission process, which is now dominated
by a phase explosion from a dense collisional spike.
Electron Spectroscopy
To detect
hot electrons in low-energy excited states located between the Fermi level
and the vacuum niveau, we utilize a Metal-Insulator-Metal (MIM) tunnel junction.
The working principle of such a device is illustrated in . The top electrode
represents the actual surface bombarded by the projectile beam. The central
idea is that weakly excited electrons (~ 1 eV above the Fermi level) generated
in this layer can travel to the insulator interface, tunnel through the
thin oxide barrier and are detected as an ion bombardment induced tunneling
current in the underlying aluminum base electrode. In order to allow ballistic
transport of the hot electrons to the interface and – at the same
time - prevent damage to the oxide interface due to penetration of projectiles,
the top electrode thickness is chosen as about 20 nm.
As is seen in Fig. 2 a tunneling
current into the base electrode is only observed if the impact zone
of the pulsed projectile beam is located inside the junction area
(defined by the overlap of top and base electrodes). Kinetic excitation
mechanisms like electronic friction or electron promotion in close
collisions may lead to direct transfer of kinetic energy to the electronic
system. These processes must strongly depend on the impact energy
of the projectile. Furthermore, kinetic excitation can be superimposed
by effects induced by the potential energy of the projectile charge.
In order to differentiate between both excitation mechanisms, Fig.
3 shows the measured tunneling yield (i.e., the tunneling current
normalized to the projectile flux) as a function of the projectile
energy for singly charged Ar+ ions as well as for neutral Ar atoms
impinging onto a polycrystalline silver surface.
The results show a clear dependence on the kinetic energy, thus indicating
that the excitation is due to kinetic processes. It can be seen that the
signals for Ar+- and Ar0-bombardment are practically the same for low projectile
energies, whereas for higher impact energies electronic excitation induced
by the ions exceeds that induced by the neutrals. This finding is understandable
in terms of the interaction time of the projectile with the surface before
the actual impact. When the ion approaches the surface slowly, electron
transfer processes like Auger neutralization are fast enough to fully neutralize
the ion before it penetrates into the solid.
In this case, there is obviously no difference between the signals measured
for ion and neutral projectile bombardment. At higher velocities, the neutralization
process is not fully completed, leading to a growing probability to reach
the surface in the charged state. Since the electronic stopping power for
the charged particle is larger than that for the corresponding neutral species,
a larger tunneling current is observed for the initially charged projectile.
The data displayed in Fig. 3 have been acquired with zero bias voltage
across the tunnel junction. By applying a negative bias voltage to
the aluminum base electrode, it is possible to generate an energy
dispersive element permitting energy spectroscopy of the excited electrons.
As an example, Fig. 4 shows the measured tunneling yield as a function
of the bias voltage. At first sight, the observed signal decrease
with increasing negative bias voltage appears to be qualitatively
expected from the increasing energy discrimination. The same is true
for the signal increase at positive bias voltages. A very interesting
finding is the change in the sign of the tunneling yield for voltages
below -1V. Analysis shows that this observation is due to the detailed
shape of the tunneling barrier, leading to contributions of both excited
electrons and holes to the measured current, the relative weight of
both contributions being controlled by the bias voltage. A detailed
interpretation of this experiment is currently under way, simulating
the effect of ion induced excitation by means of a temporally increased
electron temperature in the silver layer during the lifetime of the
collisional spike induced by the projectile impact. Preliminary results
reveal electron temperatures of the order of 1000 K to be reached
within the collision cascade initiated by the projectile impact.
Theory
The experimental
results are compared to a new computer simulation model which allows to
incorporate low-energy electronic excitation into the molecular dynamics
(MD) simulation of atomic collision cascades in solids. The transfer of
kinetic into electronic excitation energy is described in the frame of a
quasi-free electron gas model. It is assumed that the friction-like electronic
energy loss experienced by all moving atoms leads to a space and time dependent
electronic excitation that spreads around the original point of generation
with a diffusivity D. The resulting excitation energy density E(r,t) is
parametrized in terms of a space and time dependent electron temperature
Te. Although the underlying assumption of quasi-instantaneous energy equipartition
among the electron system is not generally justified, corresponding ab-initio
DFT-calculations carried out in Project B5 reveal that the original energy
spectrum generated by electronic friction is dominated by low-energy excitations
and therefore closely resembles a Fermi distribution at all times.
First pioneering calculations of electronic excitations in an atomic collision
cascade initiated by the 5-keV impact of an atomic silver particle onto
a monocrystalline Ag(111) surface have not only demonstrated that electron
temperatures of several thousand Kelvin may be reached, but also clearly
indicated the necessity to account for the influence of space- and time-dependent
reduction of crystallographic order caused by the evolution of the collision
cascade on the diffusivity D. In our most recent work, we have therefore
extended the model towards a full three dimensional treatment based on a
finite differences approach which allows for a spatial and temporal variation
of D, which can then be coupled both with local temperature and lattice
disorder. The latter is quantified in terms of a local order parameter calculated
from the time dependent atom positions delivered by the MD calculation.
As an example of such a calculation, Fig. 1 depicts temporal snapshots of
the resulting lateral distribution of (a) the electron temperature and (b)
the diffusivity D evaluated immediately at the surface, i.e., in the uppermost
cell layer of the model crystallite. In order to emphasize the role of lattice
dynamics, Fig.1 (c) shows a three-dimensional animation of the corresponding
MD trajectory.
A more complete set of animations can be viewed electronically at our web
address
http://www.exp.physik.uni-due.de/wucher/.

Figure
5: Snapshots of (a) the electron temperature Te and (b) the diffusivity
D at the surface; (c) shows the corresponding trajectory pictures in a perspective
view.
At 150 fs after the projectile impact, the temporal and spatial evolution
of the collision cascade has already led to sputter emission of two surface
atoms as well as to a significant damage of the crystallographic order within
a circular near-surface volume of approximately 1 nm diameter around the
original impact point. By inspecting the diffusivity distribution at that
time, the correlation between lattice disorder and the magnitude of D becomes
clearly visible. The surface electron temperature exhibits a much broader
distribution in connection with much smaller absolute values, mainly caused
by the rapid diffusion of the original excitation. This distribution is
superimposed by local excitation sources originating from fast recoil atoms
moving within the surface layer.
The next snapshots captured at t=350 fs reveal (i) the onset of massive
sputtering and (ii) the spatial spread of the collision cascade predominantely
propagating in direction towards the front-right crystal edge. Looking
at the diffusivity plot, one finds a nearly homogenous distribution
with rather low values of D?0.5-5 cm2/s . The corresponding electron
temperature distribution peaks at values around 1000 K and still shows
prominent local structure reflecting the ongoing kinetic heating of
the electronic sub-system by fast recoils. Finally, at t=750 fs an
electron temperature distribution is obtained which exhibits a gaussian-like
shape with a maximum of approximately 2000 K. The location of the
maximum excitation correlates with the core of enhanced collision
dynamics and crystallographic disorder. To allow a better visualization
of the time dependence of surface excitation, the temporal evolution
of Te has been calculated for different radial distances r from the
impact point.
To allow a better visualization of the time dependence of surface excitation,
the temporal evolution of Te has been calculated for different radial distances
r from the impact point.

There are several interesting observations. Shortly after the impact, the
predicted excitation exhibits a sharp peak of about 10 fs duration. The
maximum electron temperature reached in this time interval is calculated
as 6500 K, decreasing with increasing distance from the impact point. After
the projectile has passed the surface layer, the electron temperature is
found to rapidly decay due to the onset of diffusion without any notable
electronic energy source term, until at times of about 50 fs values close
to room temperature are reached.
Probably the most striking observation in Figure 6 is the fact that, after
this initial decay, the surface excitation is found to rise again, leading
to a second maximum of Te at times around 500 fs. This finding is of utmost
importance, since it reflects the trapping of electronic excitation in a
collision cascade by means of local heating and, more importantly, atomic
disorder. The electron temperature reached in the later stage of the cascade
is clearly sufficient to influence the ionization and excitation processes
of sputtered particles leaving the surface. Moreover, the time scale at
which these temperatures are reached almost perfectly coincides with that
of maximum particle emission . A shortcoming of the present model is the
fact that electronic excitations via electron promotion in violent binary
collisions have not yet been taken into account. Our ongoing present work
focuses on the proper inclusion of those processes into our nonlinear diffusion
model in order to study the relative role of both (electronic friction and
electron promotion) excitation.


Poster:

Internationaler
Workshop 2008
A4 Duvenbeck et al.
PDF (1.0 MB) |

Internationaler
Workshop 2008
A4 Heuser et al.
PDF (1.8 MB) |

IISC-17
Hernstein-2006
A4 Duvenbeck et al.
PDF (0.8 MB) |

Publications: