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Surface Science 605 (2011) 1807–1811
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Surface Science
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
High energy excitations in ion-induced electron emission from AlF3
G. Ruano a, R.A. Vidal a, b, J. Ferrón a, b, R.A. Baragiola c,⁎
a
b
c
INTEC (CONICET and Universidad Nacional del Litoral) Güemes 3450 CC 91, 3000 Santa Fe, Argentina
Facultad de Ingeniería Química (UNL) Santiago del Estero 2829, 3000 Santa Fe, Argentina
University of Virginia, Laboratory for Atomic and Surface Physics, Charlottesville, VA 22904, USA
a r t i c l e
i n f o
Article history:
Received 25 April 2011
Accepted 11 June 2011
Available online 23 June 2011
Keywords:
Ion induced electron emission
Autoionization
Factor analysis
Insulating films
a b s t r a c t
Measurements of electron emission spectra from surfaces of aluminum fluoride impacted by keV noble gas ions
show a high-energy structure, peaking around 7 eV that increases in intensity with ion energy. The shape of this
structure, identified by Factor Analysis, is independent of the nature and the energy of the impinging ions. We
discuss one electron, two electron and plasmon excitation mechanisms and conclude that the high-energy
structure results from the autoionization of F− 2p4nl n′l′ excited by electron promotion in close atomic collisions.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Electron emission is a primary effect occurring during the irradiation
of a surface by low energy ions. Several mechanisms of electron
production have been identified and divided, according to the energy
source, into potential and kinetic emission [1,2]. Most experimental and
theoretical studies of ion-induced electron emission have been
performed for metal surfaces, which are relatively well understood.
The situation is different with wide band-gap insulators, such as metal
fluorides, where the nature of high-energy excitations such as excitons
and plasmons are a matter of current debate.
Not only is the behavior of insulators under ion and electron
irradiation interesting from the fundamental point of view, but also
because of widespread applications from astrophysics to nanotechnology. Metal fluorides are being used in two prominent areas, coatings for
deep-ultraviolet and vacuum-ultraviolet optical elements, and
nanolithography. Several methods to prepare coatings make use of
ion-surface collisions. Those which use electrical discharges [3–6] need,
for their modeling, information about ion-induced electron emission [7],
which is mostly unavailable. Inorganic metal fluoride resists, such as
AlF3, are particularly appealing in electron and ion lithography [8–12].
Decomposition by electronic excitations results in removal of F2 and
high quality metal deposits that can be tailored to very small dimensions
using nanoscale particle beams. An in-depth understanding of lithog-
⁎ Corresponding author. Tel.: + 1 804 982 2907; fax: + 1 804 924 1353.
E-mail address: [email protected] (R.A. Baragiola).
0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2011.06.015
raphy requires deepening the knowledge of high-energy excited states
and electronic excitation mechanisms.
Measurements of energy distribution of electrons emitted under
particle impact provide direct information on high-energy electronic
states and clues to excitation mechanisms. Potential electron emission
by ion impact, well understood for metals, has not been unambiguously
observed in insulators. The main mechanism for potential emission is
Auger neutralization, where an electron from the solid tunnels the
surface barrier to neutralize the incoming projectile, and the excess
energy is taken by another electron to escape from the solid. In this way,
the maximum allowed energy for this secondary electron is En − 2ϕ,
where En is the neutralization energy and ϕ the metal work function. In
the case of insulators, the work function is replaced by the larger
ionization energy (sum of the band-gap, electron affinity and hole-hole
repulsion energy). Thus, a lower probability for electron emission is
expected for ions of low velocities where potential emission is dominant
for metals. However, an increase of the secondary electron yield for
insulators, as compared to metals, is found [13,14]. Riccardi et al. [15],
based on the energy and bombarding ion type independence of the
electron yield for MgO have proposed the formation and decay of
excitons as the main mechanism in low energy ion induced electron
emission in this insulator. This mechanism requires a negative electron
affinity for the surface, which occurs in MgO, but not in aluminum
fluoride. Since efficient electron emission is observed for AlF3 under low
energy ion impact, it is of interest to know what mechanisms could be
responsible. Thus, we have measured electron energy distributions from
AlF3 under impact with 1–5 keV noble gas ions, and applied Factor
Analysis to the ion energy dependence to discuss the role of possible
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G. Ruano et al. / Surface Science 605 (2011) 1807–1811
0.4
single, double and collective electronic excitations, which has not been
possible in earlier studies of insulators.
The experiments were performed in a commercial surface analysis
system (PHI SAM 590A) with a base pressure in the low 10 − 10 Torr.
Thin (4–15 nm) aluminum fluoride films were deposited from a
Knudsen cell onto a Cu substrate. Analysis of the films was performed
with Auger electron spectroscopy (AES) and electron energy loss
spectroscopy (EELS). The film thickness was sufficient to suppress any
electron-induced Auger signal coming from the substrate, but small
enough to prevent electrostatic charging of the sample. The AlF3 films
grow stoichiometrically, with no preferred surface orientation [16].
The AlF3 films were irradiated with singly charged He, Ne and Ar
ions incident at 54° with respect to the surface normal. Ion beam
fluences were low enough to prevent any noticeable damage to the
films, as tested by AES, using low enough doses so as to avoid
decomposition, signaled by the appearance of metallic Al [17]. The
substrate was biased (− 6 V) to avoid space charge effects near the
sample, and to ensure the electron collection by the analyzer. Electron
energy spectra were acquired with a single-pass cylindrical mirror
analyzer with a resolution of 0.6% and a transmission proportional to
the electron energy. Spectra acquisition times were kept low enough
to prevent changes due to preferential sputtering, verified by the
reproducibility of the spectra with time.
To analyze the electron energy spectra we applied the Factor
Analysis (FA) method, which serves to identify the presence of different
electron generation mechanisms. This requires that the intensity, but
not the shape, of components of the electron spectra have distinct
energy dependence. The FA method has been extensively discussed in
our previous publications [18], including applications to electron
emission [19], therefore we will limit ourselves to a brief description.
The first step in FA is the determination of the minimum number of
linearly independent factors required to describe the complete series of
spectra corresponding to the evolution under study. In doing that, we
compare the experimental error with the error performed in reproducing the experimental data with a minimum set of factors. This
procedure is performed as electron emission spectra are added
sequentially [18], and each time this error surpasses the experimental
error a new factor appears in the process. Once the number of
independent factors is known, the shape of the contribution coming
from each mechanism (base) is determined through a least square fit
procedure called Target Transformation (TT) [18,20].
N(E) (arb. units)
2. Experimental methods and data treatment
Cu
0.2
AlF3/Cu
0.0
0
10
20
30
40
Energy Loss (eV)
Fig. 1. Energy loss spectra of 500 eV electrons reflected from an AlF3 thin film on Cu and
from pure Cu(100). The width of the no loss peak is due to the energy spread of the
electron source and the analyzer resolution.
the high-energy peak is not resolved but its presence is apparent from
the width of the electron spectra. As we will show later, for Ar, the
energy dependence of the normalized spectra is also compatible with
the presence of such a peak. The presence of this high-energy
structure is clearly differentiated from the low-energy electrons that
result from the electron collision cascade in the solid. In the inset of
Fig. 2 we included the secondary electron spectra coming from a clean
copper surface bombarded by He + and Ne +, where the emission is
due to the Auger neutralization mechanism [23]. We included data for
copper to exemplify the difference between insulators and metals.
3.3. Analysis of the electron spectra for He-AlF3 collisions
Fig. 3 depicts the results of the FA treatment for both AlF3 and Cu,
which illustrate the ability of FA to extract distinct component spectra
that can be assigned to different mechanisms. The center column
shows the bases and the right column their relative weights along the
spectral series corresponding to different ion energies. In the first
column we show the quality of the reproduction of the experimental
spectra using the obtained bases. The results show that all the
experimental spectra can actually be represented by a linear
combination of the only two different bases shown in the figure.
3. Results
10
3.1. Electron energy loss spectroscopy
+
Ne
+
He
Cu(001)
+
Cu(001)
5 keV
4 keV
3 keV
2 keV
8
N(E) (arb. units)
In Fig. 1 we compare the EELS spectra for a 15 nm thick AlF3 film and
for copper, taken with 500 eV electrons. Since the electron bombardment of AlF3 produces radiolysis of the sample, the ELS measurements
were continuously controlled by AES against the appearance of metallic
Al. The absence of excitations below ~10 eV for AlF3 is due to the
excitonic band gap of the insulator. From the results depicted in Fig. 1 we
estimate this band gap to be 10.0 ± 0.2 eV, close to 10.8 eV of a previous
report [21] and shows an overestimation of a cluster model calculation
that gave a value around 14 eV [22].
Ar
6
0
10
20
0
10
20
4
2
3.2. Ion-induced electron emission
0
In Fig. 2 we present a compilation of the electron emission data for
He + and Ne + impact. The electron spectra are normalized to their
area and compared with results for Cu(100), shown in the inset. The
appearance of a high-energy secondary electron peak around 15 eV
for He and Ne is one of the most interesting results. In the case of Ne,
0
10
20
0
10
20
0
10
20
Electron energy (eV)
Fig. 2. Ion induced electron emission from an AlF3 thin film (~5 nm) on Cu for different
ions and primary energies. Inset: electron emission from Cu(100). The spectra are
normalized to the same area.
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G. Ruano et al. / Surface Science 605 (2011) 1807–1811
Experiments, FA Fits
Bases from FA
1809
Weights from FA
Cu
100
Experiment
He 5 keV
He 2 keV
FA Fits
He 5 keV
He 2 keV
l-base
h-base
FA weights (%)
N(E) (arb. units)
50
l-base
h-base
100
50
AlF
3
0
10
20
0
10
20
0
1
30
Electron energy (eV)
2
3
4
5
Ion energy (keV)
Fig. 3. FA treatment results for He+ on Cu and AlF3. Left panel: experimental electron spectra and FA fit. Center panel: bases obtained from FA, low energy component (l-base) and
high-energy component (h-base). Right panel: weights of the components obtained after FA vs. primary ion energy.
The dependence of the weights on He + energy, shown in the third
column, clearly shows the different origins of the high-energy
electrons coming from both samples. While for He + on Cu the high-
energy component (the h-base) decreases with ion energy, a clear
manifestation of potential emission mechanisms [1]; for AlF3 the
behavior is opposite: the h-base increases with energy. This result
Bases from FA
100
FA weights (%)
N(E) (arb. units)
l-base
h-base
Experiment
Ar 2 keV
Ar 5 keV
FA Fit
Ar 2 keV
Ar 5 keV
0
5
10
15
20
Electron energy (eV)
25
30
50
0
1
2
3
4
5
Ion energy (keV)
Fig. 4. FA results for Ar+ on AlF3. Bottom left panel: experimental Ar+ induced electron spectra and FA fit. Top left panel: bases obtained from FA, low energy component (l-base) and
high-energy component (h-base). Right panel: weights of the components obtained after FA vs. primary ion energy.
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G. Ruano et al. / Surface Science 605 (2011) 1807–1811
points out to a kinetic mechanism whose possible origin we discuss
below.
3.4. Analysis of the electron spectra for Ne- and Ar-AlF3 collisions
Since, for Ne, the spectral shape is nearly independent on impact
energy, as we previously mentioned, FA is useless for extracting possible
mechanisms in this case, i.e. only one base (any spectrum) is enough to
reproduce all experimental spectra within our experimental error.
For Ar, on the other hand, the appearance of a crossing point in the
electron energy spectra for different impact energies (Fig. 2) suggests
again the presence of two different mechanisms [19]. The results of FA
(Fig. 4) are similar to the He case, showing two bases with different
energy dependence. The importance of the kinetic mechanism (weight
of h-base) is clearly lower for Ar than for He.
Finally, in Fig. 5 we summarize what we think is one of the
noteworthy results from this group of measurements, i.e. the similarity
among the high-energy components for all impinging ions. In the
following we will center our discussion on the possible mechanisms that
could explain such behavior.
4. Discussion
In this section we analyze kinetic electron emission mechanisms that
can produce a high-energy structure in the electron energy spectra
(Fig. 5) that is independent of the type of projectile, within
uncertainties. Kinetic electron emission in slow ion-surface collisions
is known to result from electron–electron interactions that lead to
promotion of some electronic orbitals of the transient molecule formed
in the collision. Excitation by direct transfer from the nucleus of the
projectile to a single target electron occurs at impact velocities larger
than those used in this work. Electron promotion can be analyzed using
the Barat–Lichten rules that correlate molecular orbitals (MOs) to the
atomic orbitals of the separated atoms (large internuclear separation)
and those of the united atom in the limit of zero separation [24]. At the
crossings of MOs, electrons can be transferred if vacancies exist. A single
orbital promoted to the continuum leads to ionization with electron
N(E) (arb. units)
He FA h basis
Ar
Ne (average
spectrum)
0
10
20
Electron energy (eV)
Fig. 5. The high-energy spectral base (h-base) obtained by applying FA to the case of
He+ and Ar+ on AlF3 (line) compared with the Ne+ results.
energies that decrease approximately as f(ε) = exp(−cε/v) [25] with ε
the electron kinetic energy and v the collision velocity. Such electrons
appear together with the cascade electrons in the low-energy part of the
energy distribution, e.g. the l-base in the FA results for Ar and He on AlF3.
If energetic electrons in the conduction band inside the solid have a
kinetic energy lower than the band gap, they cannot excite other
electrons and will lose energy only through phonon excitations with
negligible effects on electron emission. Thus, those electrons observed in
vacuum with energies lower than the band gap minus the electron
affinity χ (~0.7 eV for AlF3) [21], or ~10 eV, will be observed at their
original excitation energy. This means that the cascade distribution in
the spectra is weak compared to the case of metals since only the
relatively few high-energy electrons are allowed to transfer energy to
other electrons in the valence band of the solid.
We now turn to the mechanism responsible for the high-energy
electrons in the h-base, seen for all noble gas ions. The shape of the
h-base, since it is independent of the type of ion, suggests that it is
dominated by the electronic states of the solid. For low-energy ion
impact of MgO, where the energy spectra are independent of the type of
ion, Riccardi et al. [15] suggested that the electrons originate from the
decay of an exciton excited by MO promotion of an O-2p electron. The
exciton decay is allowed by the location of the exciton above the vacuum
level due to the negative electron affinity of MgO. In the case of AlF3, a
promoted F-2p electron may be transferred also to an exciton. However,
the positive electron affinity of AlF3 blocks an exciton located below the
conduction band to be tunneled into vacuum. A state with energy above
the vacuum level would be immersed in the continuum and thus not
qualify strictly as an exciton. Although such states can contribute with
structure in the density of states of the electron–hole complex, the
structure is not seen in conditions of well-defined photon excitation,
such as the ultraviolet photoelectron spectra measured with 21.2 eV
photons [21].
We note the contrast to the case of electron emission from graphite,
where a similar peak in the electron energy distribution was interpreted
by us to be due to the Auger decay of an energetic exciton [26]. The
exciton in graphite is enabled by a many-body interaction of nearly free
electrons [27], which does not occur in an insulator with tightly bound
electrons, such as AlF3.
At this time, we cannot rule out that an excited electron in the
continuum of the AlF3 lattice may become somewhat stabilized by the
presence of the nearby projectile. It is conceivable that its lifetime is
prolonged by the proximity of the projectile ion to allow the state to
decay by a 2-electron process, the simultaneous excitation of a valence
band electron. However, the independence of the h-base on projectile
type and energy suggest that such final electron-projectile interaction is
not important in our case.
Another possibility to explain the h-base is two-electron excitations
in fluorine. A doubly excited F− 2p4nl n′l′ state would decay by
autoionization to F 2p5 emitting an energetic electron. This process has
been observed in the gas phase in collisions of F− with He atoms [28]
through a peak in the electron energy distribution at 14.85 eV,
attributed to autoionization of the 2p4(1D) nl nl excited state. The
same doubly-core excited stated has been inferred to explain collisions
of protons with LiF [29] and may explain the h-base in our experiments.
Since fluorine has an electron affinity of 3.4 eV, the 2p 4(1D) nl nl state
has an energy 18.25 eV above F − 2p 6. In solid AlF3, the kinetic energy of
the emitted electrons will be equal to the excitation energy minus the
ionization energy of the solid (band gap+ affinity), i.e. 18.25 eV −
10.8 eV − ~ 0.7 eV ≈ 6–7 eV, that agrees quite well with the peak
position in the h-base.
Double 2p excitations have been observed in Ne, which is
isoelectronic to F −, in ion surface collisions with second row elements
Na to Si, [30,31] and also in F + collisions with Al and Si surfaces [32].
Evidence for two-electron excitations comes from the strong peak in the
energy spectra of emitted electrons due to the autoionization of Ne
2p4nl n′l′ to Ne+ 2p 5. In these cases, the excitation of the 2p electron of
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G. Ruano et al. / Surface Science 605 (2011) 1807–1811
the lighter collision partner (Ne or F) occurs through the promotion of a
molecular orbital (4fσ) and electron transfer to unoccupied MO that
correlate to the levels nl n′l′ that are empty before the collision [24]. The
radial coupling of the 4fσ with the unoccupied MO occurs at a diabatic
crossing distance of the order of the sum of the radii of the interacting
orbitals. The cross sections are characterized by a sharp increase from a
projectile energy threshold (required to achieve the crossing distance)
to a very broad plateau over a wide energy range. The different
projectile-energy behavior of cross sections due to radial vs. rotational
coupling of MO is very distinctive and useful to separate the different
processes [33]. Thus, the radial coupling probability in the F-2p
excitation in Ne–F collisions is expected to be high and have a weak
energy dependence, as observed.
The Barat–Lichten rules [24] show that the F-2p orbital is promoted
along the 3dσ MO in He-F and Ar–F collisions. The case He–F is similar to
that for He–Ne studied by Brenot et al. [34]; F-2p excitation occurs
through the 3dδ MO, with double excitation requiring small internuclear
distances, b0.5 Å at which the 3dδ MO can couple rotationally to the 3dπ
MO which, in the outgoing path of the collision, evolves into the empty
He-2p atomic orbital. Since rotational coupling requires small internuclear distances (and, hence, impact parameters), the cross section for
this process must be small and strongly energy dependent, consistent
with our observations.
From the analysis of the analogous Ar–Ne collision [35], the F-2p
level is promoted through the 3dπ MO and a rotational coupling to 3dδ
MO at the turning point due to the rapid rotation of the internuclear axis.
The electrons in the 3dδ can then be transferred by radial coupling to
higher lying MOs on the second half of the collision and the 3dδ MO
evolves into the empty Ar-3d atomic orbital. However, in the case of Ar,
F-2p can also be promoted through the 4fσ MO [36] that can couple
radially with empty higher σ levels. Whether this channel is more
important than the 3dδ–3dδ will depend on details of the diabatic
crossing, which have not been calculated. At any rate, the low velocities
of the Ar projectile make any of those promotions unlikely.
A more likely F-2p excitation mechanism for Ar projectiles occurs
in F–F collisions initiated by a fast F recoil energized in a violent Ar–F
encounter. Such target-target collisions occur for a variety of simple
solid elements in our energy range and are responsible for most of
target 2p excitations in collisions of Ar with second row elements at
keV energies [37,38].
Although the type of excitation mechanism varies with the type of
the projectile, they all lead to the production of doubly excited F¯ 2p 4nl
n′l′. The close agreement of the spectra of Fig. 5 indicates that the
outer shell configuration (nl n′l′) is similar for the three projectiles.
Finally, we consider the possibility that the observed h-base is due
to electrons from the decay of a collective (plasmon) excitation of the
F-2p valence electrons in the solid. The only known mechanism for
plasmon excitation by slow ions is the shake-up in the surface
neutralization of the projectiles [39], which is energetically impossible
in AlF3 due to the large binding energy of the F-2p electrons.
5. Conclusions
We measured electron energy spectra induce by keV He, Ar and Ne
ion impact at keV energies on AlF3 thin films. The application of Factor
Analysis to the impact energy dependence of the spectra shows that
only two components (energy distributions) are sufficient to reproduce
the data, indicating two excitation mechanisms. For Ne there is only one
component that is the same as that produced in high-energy He and Ar
impacts. We discuss the results by considering possible 1-electron,
2-electron and plasmon processes. We find that the best candidate to
explain the structure in the electron energy distribution is the
autoionization of a 2-electron excited F − 2p4nl n′l′ state populated
through electron promotion in close collisions of the projectile with a
lattice F (case of He and Ne) or F–F collisions involving an energetic
recoil fluorine.
1811
The importance of electron promotion in electron emission shows
that analysis of molecular orbital correlations is necessary before one
can extrapolate these results to ion interactions with other insulators.
However, one can predict that the autoionization mechanism will occur
in other fluorides. For those cases where F–F collisions are responsible
for 2p promotion, there will be an increase in the probability of
excitation with the concentration of F. In particular, the autoionization
peak should be prominent in condensed fluorine or SF6.
Acknowledgments
This work was supported by the ANPCyT (PICT 1138/2006 Raíces)
and the U.N.L. (CAI+D 6-6-62). RAB acknowledges additional support
from the Alice and Guy Wilson professorship.
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