surface science
reports
ELSEVlER
Surface Science Reports 28 (1997) 1777245
Elastic and inelastic processes in the interaction of l-10 eV
ions with solids: ion transport through surface layers
Mustafa
Akbulut,
Norbert
J. Sack, Theodore
E. Madey *
Department of Physics and Astronomy and Laboratoryfor Surface Modification, Rutgers, The State University
of New Jersey, Piscataway, NJ 08855, USA
Manuscript received in final form 5 March 1997
Abstract
We review the escape.depth of secondary ions (or neutrals) desorbing from solid surfaces under the impact of electrons,
photons or ions. We survey ion (or neutral) transport through many materials, but most are wide band gap insulators such
as rare-gas solids and molecular solids. We address the issue of low-energy (< 10eV) ion-solid interactions, and review
experimental and theoretical studies that provide insight into the physical mechanisms of these interactions, such as elastic
scattering,charge transfer and ion-molecule reactions. Although it is usually assumed that most of the secondary ions stem
from the top surface layer, we show that this is not necessarily the case: In certain instances, 1-1OeV ions are able to
transmit solid films which are several monolayers thick. The transport oflow-energy ions through materials has very broad
implications. We point out the importance of these results for electron or photon stimulated desorption (ESD/PSD),
secondary ion mass spectrometry (SIMS), and ion-sputtering of surfaces, and discuss their relevance to other fields, such as
ion beam deposition (IBD), low-energy ion implantation, and electrochemistry.
1. Introduction
Electron, photon or ion bombardment of solid surfaces can lead to the desorption of secondary
particles, including ions or neutrals. Analysis of the secondary particles and their kinetic energy and
angle of desorption has proven useful in the identification of the structure and elemental and
chemical composition of the surface from which they desorb.
* Corresponding
author. Fax: + 1908-445-4991; e-mail: [email protected].
0167-5729/97/$32.00 0 1997 Elsevier Science B.V. All rights reserved
‘PII SOl67-5729(97)00007-l
182
M. Akbulut et al. 1 Surface Science Reports 28 (1997) 177-245
Secondary particles ( < 10 eV) sputtered during ion bombardment of solid samples are utilized in
methods such as secondary ion mass spectrometry (SIMS) or secondary neutral mass spectrometry
(SNMS) [l]. The identification of the mass of secondary ions allows insight into the elemental
composition of the surface layers.
Electron/photon stimulated desorption (ESD/PSD) are based on the principle that energetic
electron/photon beams (a few eV to several thousands of eV) incident on a solid can cause desorption
of ions and neutral species (including metastables) from the surface layers of the solid by inducing
electronic transitions to dissociative states [2]. (Mechanisms of ESD/PSD are discussed in Appendix C in detail.) These beam damage processes are termed desorption induced by electronic transitions
(DIET). The most probable range of kinetic energies of ESD and PSD ions is 1-1OeV. In ESDIAD
(ESD ion angular distribution) one not only measures the ion desorption yield, the ion mass and the
ion kinetic energy distributions but also the angular distribution of the desorbing ions [3,4]. This
provides information about the geometric orientation of the chemical bonds that are broken by the
electrons and therefore about the geometric orientation of the atoms and molecules on the surface.
It is clear that the secondary ions must come from the surface layers. However, the question of the
exact depth of origin of secondary ions has until relatively recently not been addressed in detail. Do
most of the ions stem from the top surface layer as it is often assumed [1,2,5], or can some ions which
are produced below the surface traverse the surface layers and escape from the surface?
The physics behind these questions is the physics of the interaction of ions having kinetic energy in
the range 1-1OeV (low energy ions) with solids or thin films. l-1OeV ions generated in a layer
beneath the surface can interact with the topmost layers through various elastic and/or inelastic
processes. The elastic and inelastic processes that influence ion transport through ultrathin films
depend on the nature of the ion and its kinetic energy, and on the structure and electronic properties
of the surface materials. Although there exists a wealth of information on ion-solid interactions at
collision energies higher than 1 keV [6], the interactions of l-10 eV ions with solids (such as rare gas
and molecular solids) are not known in detail. It is commonly assumed that both kinetic energytransfer and charge transfer neutralization are the dominant types of interactions in the low collision
energy range. Low-energy ion-solid interactions can be rather complex because the cross-sections of
the elastic and inelastic collision processes depend strongly on the details of the interaction potential
between the collision partners. Hence generalizations that can be made for higher-energy collisions
(e.g. keV-MeV) cannot be applied in this energy regime.
The transport of low-energy (l-10eV) ions through atomic and molecular layers is of great
importance in many diverse areas such as electrochemistry, radiation chemistry and physics,
low-energy ion implantation and ion beam deposition [7]. Irrespective of the source of ions, in all
low-energy ion transport processes through ultrathin films, the main physical mechanisms of elastic
and inelastic scattering should be basically the same.
In this review, we survey data on ion and atom transmission through a variety of insulators,
semiconductors and metallic layers. Since most of the available data are for ions passing through
rare-gas solids and molecular films, this review deals mainly with the question of the escape depth of
secondary ions from wide band gap insulators. In order to address the escape depth of secondary ions
from solids, we have developed a novel experimental approach in our laboratory. The essence of our
experimental approach is illustrated in Fig. 1. We generate a source of atomic ions (e.g., O+, H+, F+,
F-, Cl+, Cl-) with k nown kinetic energy and angular distribution by bombarding an appropriate
surface with a focused electron beam, and causing electron stimulated desorption (ESD) (see
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
183
e- beam (-300 eV)
l-10 eV ion (Of I F+I F-, H+)
substrate as an ion source
(e.g., oxidized W(lOO),l-MLPF3/Ru(OOOl))
Fig. 1. Illustration of experimental approach in Section 4.1. An ion is desorbed by ESD from a substrate, and overlayers
are adsorbed onto this substrate and the ion yield is monitored as a function of overlayer thickness.
Appendix C for more detail). We then cover the surface, cooled to m 20 K, with an ultrathin film of
a condensed atomic or molecular solid (Ar, Kr, Xe, NH,, H,O, etc.). During electron bombardment
of the layered surface, we measure the changes in the intensity, kinetic energy distributions, and
angular distributions of the ion beam as a function of film thickness. ESD-produced ions are
detected by means of a high sensitivity digital ESDIAD detector that allows mass, energy and
angle-resolved ion detection. Except for metastable species, escaping neutrals are not detected.
By measuring the changes in total intensity, angular distribution and kinetic energy distribution
of ions that originate at the substrate beneath an overlayer film, as a function of film thickness, we
can gather much information. The change in total ion intensity is expected to be due mainly to
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177-245
backscattering and large angle scattering (> 90” with respect to the surface normal) as well as charge
transfer neutralization and ion-molecule reactions leading to formation of new species (for
molecular overlayers). The changes in ion angular distribution and energy distribution are expected
to arise from elastic and inelastic scattering in a forward direction. In studying ion transport through
surfaces, we address the above-mentioned questions concerning 1-1OeV ion-solid interactions.
A comparison of the velocity of the outer shell electron of a target atom (u,) with a projectile ion
velocity (u,) is very useful in determining the interaction potential during a collision. When u, >>v,
(slow collisions) the collision is called adiabatic and the collision can be described using quasimolecular potentials formed by the colliding partners during the collision. On the other hand, when
u, >>u, (fast collisions) the electronic distributions of the colliding partners are static, except for
abrupt transitions that occur when the particles are at their closest approach.
Hereafter, in this review, we define the energy ranges of ions as follows: (a) ultralow energy ions:
0.0251 eV, (b) low-energy ions: 1 eV to the energy at velocity u, > u, (where u, is the velocity of the
outer shell electron of a target atom of order lo* cm/s, and u, is the projectile ion energy); and (c)
high-energy ions: u, > u,.
This review is organized as follows: we discuss ion attenuation mechanisms in ultrathin films in
Section 2. In Section 3-5, we discuss a selection of experimental and theoretical results related to the
question of the escape depth of neutrals and ions. We present implications for the interpretation of
ESD, PSD and SIMS measurements, and an outlook to neighboring fields in Section 6. Conclusions
are given in Section 7. We discuss basic physics of elastic and inelastic atomic collisions in Appendix
A, and charge transfer processes between ions and solids in Appendix B. We survey ion desorption
from surfaces in Appendix C.
2. Low-energy ( < 10 eV) ion attenuation mechanisms in ultrathin films
Energetic electrons, photons (few eV to several keV) or ions (usually higher than 1 keV) can
penetrate a solid and create secondary particles (such as ions, neutrals) on or below the surface via
electronic excitation or momentum transfer processes (see Appendix C for details of desorption
processes). For DIET of an adsorbed monolayer on a metal surface, all desorbing species (ions and
neutrals) originate from the adsorbed monolayer, because substrate electronic excitations are
quenched too rapidly for desorption of bulk atoms. For compound materials such as oxides and
fluorides, subsurface layers might contribute to the desorption signal. The desorption signal from an
atomic or molecular multilayer film might also include contribution from subsurface layers. This is
the main focus of this review: from what depth do desorbing species originate?
For ions created below the surface, their desorption probability depends strongly on their
interaction with the surface layers on top. As an energetic ion created below the surface enters the
surface layer region, its trajectory and charge state are determined primarily by a series of elastic and
inelastic collisions between the projectile ion and target atoms in the surface layers, and on the
electronic properties of the projectile and the surface layer. The probability that an ion created below
an ultrathin overlayer film survives and exits the film as an ion depends on both the energy loss and
charge transfer reactions between the desorbing ion and target atoms in the film.
Elastic scattering between a projectile ion created below the surface and target atoms in the
overlayer leads to changes in the trajectory and kinetic energy of the ion. In an experimental-
M. Akbulut et al. /&r&ace Science Reports 28 (1997) 177-245
185
measurement ofion escape probability, those ions that are elastically backscattered or scattered with
such a large angle that they cannot escape from the surface are not detected. In the case of scattering
by an angle of 90” or more with respect to the surface normal, an ion can undergo reneutralization by
resonant electron tunneling or Auger neutralization via coupling to the substrate density of states
and finally, either desorb as a neutral or become trapped in the overlayer film. If an ion is scattered in
a forward direction (by an angle of < 90” with respect to the surface normal) through the overlayer, it
may escape from the surface as an ion.
Charge-transfer and ion-molecule reactions are expected to influence low-energy ion transport
through ultrathin films by reducing the ion yield. Note that we use the term ion-molecule reaction to
refer to a chemical reaction leading to formation of new molecular species. By a charge-transfer
reaction we mean a reaction in which an electron is transferred between projectile and film. If
projectile ions, in their passage through a molecular overlayer, interact with the overlayer molecules
via charge transfer or chemical reactions to form new species [S], this results in a decrease in the
projectile ion yield.
Elastic scattering of l-10 eV ions by weakly bonded thin films can be explained in terms of a very
simple model of binary collisions. In this simple model, each of the atoms or molecules in the films is
considered to be free and at rest; the lattice binding energy and vibrational motion of the target
atoms or molecules are neglected. Although this is an oversimplified assumption of collision because
it neglects the influence of the neighboring atoms, it is a very useful model. Since virtually all
measurements described in Section 4 are for condensed films of insulating solids which are weakly
bonded, and the collision time (10-‘5-10-‘4s
for 1-1OeV for F+, O+ ion) is smaller than the
characteristic period for lattice vibrations (- lo- ’ 3 s), the binary collision approximation should be
valid, at least for the initial collision(s).
If the ion kinetic energy is ultralow (< 1 eV), the interaction of a desorbing ion with the lattice
vibrational modes or molecular vibrational modes can become important, because the vibrational
period of the atoms or molecules in the films can be comparable with the collision time. The ion can
lose a small fraction of its energy due to the lattice vibration. In order to take the lattice vibration into
account, the velocity of the target atom is included in the collision kinematics. The influence of the
target velocity on the final energy of the projectile ion can be seen as a shift in the measured ion elastic
energy as a result of a binary collision and cause energy broadening, depending on the temperature
of the solid [9,10]. The effect of lattice vibrations on ion scattering can be estimated from the ratio of
the shift in the final elastic scattering energy (6E) to the final ion energy (E,), 6E/E,. For example, for
ion impact energies larger than N 1 eV, the lattice vibrations have a negligible effect on the scattering
at temperatures between 25 and 300K, i.e. dE/E, z k,T/E, z 2 x 10e3-2 x lop2 for initial ion
kinetic energy E, = 1 eV. Therefore, in ion attenuation processes the effect of lattice vibrations on ion
scattering is important only if the ion kinetic energy is ultralow (< 1 eV).
In a dipolar medium such as water or water ice, it is also possible that a moving ion can lose energy
to dipolar relaxation. If the ion loses sufficient energy due to multiple elastic and inelastic processes
without charge transfer and chemical reaction in the films, the ion may not escape from the overlayer
films; it may become trapped in the overlayer films. This leads to a decrease in the ion yield.
Because most of the processes we consider are based on binary collisions between projectile ions
and target atoms of the wide band gap insulators, we discuss the basic physics of atomic collisions,
binary elastic collisions, one electron charge transfer processes, and ion-molecule reactions
elsewhere in this review (see Appendix A).
M. Akbulut et aLlSurface Science Reports 28 (1997) 177-245
i86
-.....-..
Poisson
layer-by-layer
thickness
(ML)
Fig. 2. Ion attenuation in a film of thickness < 5 ML. Comparison of Poisson statistics (corresponds to a Poisson
distribution of the film thickness) to a layer-by-layer film growth model.
2.1. Statistics of ion attenuation
in thinjilms
In the following we assume that ion transport through a film is determined by a series of binary
collisions during which changes in trajectory and charge transfer can occur. In this case, ion
transport can be described by Poisson statistics:
a@
dx-
-No@,
(2.1)
with CDbeing the ion flux at position x in the film, N the atom/molecule number density (per unit
volume) in the film, and crthe attenuation cross-section. This leads to an exponential attenuation of
the ions in the film, as depicted in Fig. 2 (solid curve):
CD= QOexp( - Nod),
(2.2)
where d is the film thickness. This result assumes that the attenuation cross-section of the ions is
independent of film thickness.
The Poisson statistics are based on a random distribution of the atoms/molecules in the film, i.e.,
the film thickness is not uniform. However, a film may grow in a non-Poisson way, such as
a layer-by-layer growth for which the individual layers can be ordered (crystalline) or disordered.
For layer-by-layer growth, the attenuation of an ion signal in the film is expected to be linear as
a function of atom density within one layer, as shown in Fig. 2, provided that the atomic separation
(radius) is larger than the collision radius measured from the experimental attenuation cross-section.
The ion transmission signals at each completed layer still lie on an exponential curve, but within one
M. Akbulut et aLlSurface Science Reports 28 (1997)
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187
layer the attenuation is linear. Therefore, for very low attenuation, the attenuation cross-section can
be derived using the exponential attenuation mode (Eq. (2.2)).
When the attenuation cross-section is very large (a >>l/N& atom separation less than collision
radius), the effective interaction areas of neighboring overlayer atoms overlap with increasing
coverage, even in a single monolayer. This leads to an exponential attenuation of the ion signal
within one layer for both statistical and layer-by-layer growth modes.
Note that there are similarities between the statistics of ion transmission through thin films and
that of Auger electrons emitted from substrates and transmitted through adsorbates [ll]. If the
adsorbate growth is statistical, then the Auger electron signal from the substrate is attenuated
exponentially by the adsorbate. If the adsorbate grows in a layer-by-layer fashion, then the
attenuation curve shows kinks at each completed layer. Attenuation can be caused entirely by elastic
scattering effects with little or no charge transfer, or charge transfer can dominate. Consider first
what happens in a gas phase charge transfer process.
In a gas phase one-electron charge transfer reaction between a positive ion and an atom or
molecule, the ion captures an electron from the target atom or molecule, and becomes neutral. In the
case of the negative ion-atom/molecule
interaction, the negative ion becomes neutral by transferring an electron to the target atom or molecule. The energy defect, which represents the change in the
total internal energy in a charge transfer reaction, is an important parameter governing the charge
transfer at low energy. Experimental results and theoretical calculations indicate that the smaller the
energy defect, the larger the charge transfer probability [12,13]. The physics of charge transfer in
ion-atom/molecule
collisions is discussed in detail in Appendix A.
For ion transmission through overlayer films near a conducting surface, the valence levels of both
the desorbing ions and the target atoms or molecules of weakly interacting ultrathin film are shifted
by the surface image potential [14]. However, we expect that the energy difSerence between the
relevant levels does not change significantly near the surface. Therefore, we believe that an
understanding of gas phase charge transfer processes is useful to explain a charge transfer reaction
between a desorbing low energy ion and weakly interacting film.
In a thick overlayer film (2 3 ML) a band structure can be well developed [l&16], and one might
consider that this could make the direct application of inelastic binary ion-atom/molecule
collision
concepts to the transport problem inappropriate. In the wide. band gap insulating films, the
electronic excitations are highly localized, because the electronic excitation energies are much larger
than the cohesive energies. Hence, in first approximation, we expect that gas phase ion/atom charge
transfer concepts can still be useful to describe a charge transfer reaction during ion passage through
a thick insulating film.
3. Depth of origin of desorbing neutrals
Although the focus of this paper is the depth of origin of secondary ions from solid surfaces,
we address briefly the depth of origin of desorbing neutrals. It can be expected that the depth
of origin of a neutral, e.g. 0, differs from that of an ion, e.g. O+, because the interaction potentials
are different. This will affect the elastic scattering and energy loss of the different species in the
surface layers; furthermore, inelastic scattering cross-sections, such as charge transfer, will be very
different for the ion and the neutral. Also, the ion escape depth is a lower limit on escape of
M. Akbulut et aLlSurface Science Reports 28 (1997) 177-245
188
0
1
2
3
4
5
6
7
8
9
10 14.5
Ar intemwdiote spacer thickness d I nm
Fig. 3. Attenuation
of Kr,F fluorescence intensity as a function of Ar intermediate spacer layer. From [17].
all particles, because species which begin as ions may still escape from the surface as neutrals after
charge transfer.
Very recently, Bressler and Schwentner [17] have reported a long range migration (mean
penetration depth of 2.8 nm) of photomobilized - 4.2 eV F atoms in an Ar film. As shown in Fig. 3,
they used a sandwich of three films: the top rare gas film Ar was doped with F,, the intermediate film
is pure Ar, and the third film contains Kr that was used for detection. They generated F atoms by F,
photodissociation in the top layer and monitored the migration of the F atoms across the Ar spacer
layer with variable thickness by measuring the intensity of fluorescence from Kr,F formed at an
Ar/Kr interface.
Although the authors did not discuss F atom transport mechanisms (such as elastic and inelastic
collisions) through the Ar film, it is expected that a - 4.2 eV F atom can interact with the Ar layers
through various scattering processes. We compare this result with - 4eV F+ and - 1 eV F- ion
transmission through ultrathin films of Xe, Kr and H,O in Section 4.
In the following we point out some experimental sputtering measurements and theoretical studies
which address the depth of origin of secondary neutrals.
Dumke et al. [18] studied the sputtering of gallium-indium alloys with 15 and 25 keV Ar+ ions.
The principle underlying these experiments is that the surface layer of a gallium-indium alloy
(composition: 16.5% indium, 83.5% gallium) is enriched with indium: 94% indium vs. 6% gallium.
Hence by measuring the indium to gallium ratio of the sputtered atoms one can derive the surface
layer contribution to the total neutral sputter yield (if the sputter yields of pure gallium and indium
are known). They conclude that for 15 keV Ar+, 85% of the sputtered neutrals stem from the surface
layer, while for 25 keV At-+ it is only 70%. The authors do not comment on the difference. We suggest
that this may be due to a difference in beam damage caused by the different energy ions in the surface
layers, which may allow a different amount of neutrals from subsurface layers to escape from the
surface.
Burnett et al. Cl93 determined the depth of origin of neutrals from a Cu overlayer (< 2 ML)
on Ru(000 1) by 3.6 keV Arf ions. By measuring the ratio of sputtered Cu to Ru neutrals
M. Akbulut et al. /Surface
Science Reports 28 (1997)
1777245
189
using the SARISA method (surface analysis by resonance ionization of sputtered atoms) they
could derive the amount of Ru atoms that can desorb through the overlayer. They conclude
that N 66% of the sputtered particles come from the top surface layer. Their experimental data
are in agreement with the results they obtain from the computer simulation program “TRIM”
POIHarrison et al. [21] performed classical computer
simulations
on the ion bombardment
of
Cu(1 00), Cu(1 lo), and Cu(1 1 1). They find differences in the sputter yield for the three crystallo-
graphic orientations. More important for the present paper, they find that the majority of the
sputtered atoms originate in the top layer ( > 99% for the (10 0) face, 81% for the (1 10) face, and
96% for the (1 1 1) face). The differences are explained by the different structures of the three faces:
The (1 10) direction has the most open structure of the top layer and therefore also the largest
percentage of sputtered particles from subsurface layers.
In the experimental studies mentioned above, the attenuation of the neutrals is dominated by
elastic scattering, or “blocking”.
A theoretical study concerning the depth of origin of sputtered atoms has been performed by
Vicanek et al. [22]. The authors use a Monte Carlo simulation, and assume an isotropic initial
angular distribution and an E- 2 energy distribution without an upper or lower cutoff. They
conclude that the most important factor influencing the escape depth is the elastic energy transfer
from the low-energy atoms to the surface atoms. The effect of angular scattering on the escape depth
of atoms is found to be rather small.
A round robin computer simulation published by Sigmund et al. [23] compared the results from
various simulations of the ejection of low-energy copper atoms ( < 50 eV) through a planar copper
surface. In most cases, a Born-Mayer repulsive interaction potential, called Gibson 2 was used:
V(r) = Aexp( - r/a),
(3.1)
where A = 22.5 keV and a = 0.196 A. This potential was augmented where necessary by an attractive
interaction for distances, r, around and above the interatomic distance.
Some of these results are shown in Fig. 4. Although it is shown that some energetic atoms can
traverse a surface layer several A thick (- 5 A), most of the sputtered atoms are found to originate in
the surface layer. As seen in Figs. 4(e)-4(f), the Cu atom ejection probability is very low for Cu atoms
having kinetic energy less than lOeV, and it increases slowly with increasing atom energy up to
50 eV.
Recently, Lill et al. [24,25] have measured abundance distributions of ionic and neutral clusters
sputtered by 4 keV Ar ion bombardment of liquid gallium, liquid gallium-aluminum eutectic alloy
and gallium-indium eutectic alloy by time-of-flight mass spectrometry. They have found that the
depth of origin of sputtered clusters increases with increasing cluster size. Their data indicate that for
small clusters (I containing 3 atoms) the ejection region growth is lateral, whereas for large clusters
(2 containing 3 atoms) the ejection region growth is vertical in the surface.
The escape depths of neutrals are of the same order of magnitude as those obtained for ions
in cases where elastic scattering dominates the attenuation. The sputtering studies described
above reveal that roughly 30% of the desorbing neutrals originate below the top monolayer
(i.e., 70% of the neutrals originate in the surface layer). Similarly, from the results discussed below in
Sections 4.1.2 and 4.1.4 we estimate that for many systems roughly 50% of the ions transmit 1 ML
overlayer.
M. Akbulut et al./ Surface Science Reports 28 (1997) 177-245
190
i
7
+yIi
I
6
x(A)
P
p
I
(4
05
06
01
01
01
x(A)
Fig. 4. (a) -(d) Depth dependence of ejection probability from a random Cu target (from [23]). The straight line is inserted
for orientation (see [23] for detail).(e) -(f) Energy dependence ofejection probability from a random Cu target for(e) x = 0
(surface layer) and(f) x = 1.8 8, (second layer). In (e) the straight line represents P, = 0.5 for E 2 U and 0 otherwise, valid for
a spherical barrier in the absence of collisions (see [23]), and the solid curve represents the expression P, = )[ 1- ( U/E)1’2],
where U is the binding energy of the target atom, E is the kinetic energy of the ion.
4. Depth of origin of desorbing ions: experimental results
There have been a number of experimental studies investigating the depth of origin of secondary
ions from solids that are mainly wide band gap insulators. In each of the following sections we first
introduce the experimental approach and then discuss selected results.
M. Akbulut et al. /Surface Science Reports 28 (1997) 177-245
191
4.1. Ion desorption from substrates through overlayers
4.1 .I. Principles
The approach used in the authors’ laboratory is illustrated in Fig. 1. The basic idea is to desorb
ions from a substrate by ESD, and to condense overlayers (of a different material from the substrate)
on top of the substrate which are nearly transparent to the primary electron beam. The yield, energy
distribution, and angular distribution of the substrate ions are then monitored as a function of
overlayer thickness.
There are a few principal assumptions underlying this approach. First, the substrate and the
overlayer have to be chosen so that the adsorption of the overlayer does not lead to a mixing of the
two components. This is achieved by choosing a “rigid” substrate (such as an oxide) and an overlayer
that is only weakly bound to the substrate. To date, most overlayers used have been physisorbed rare
gases and condensed molecular layers that are weakly bound to the substrate.
Another assumption is that the primary electrons can penetrate the overlayer film without losing
a significant fraction of their energy. This is achieved by using electrons with a kinetic energy
> 100 eV; in most solids, the energy loss is small enough that electrons can penetrate an overlayer
film several monolayers thick without losing a significant portion of their energy [26].
It is also assumed that the primary electrons do not disturb the overlayer significantly in the
vicinity where the secondary ion traverses the films, i.e., that the secondary ions do not traverse
a strongly perturbed film. In order to keep radiation damage in the overlayer (sometimes referred to
as “second order effects” see [l,p. 6721) to a minimum, low electron fluences (- 101”e/cm2) are
used. However, this still does not rule out the possibility that the same electron that initiates one
ESD process may 1ocalIy disturb the overlayer in the vicinity of the desorbing ion (“first-order
effect”). But the total desorption cross-section from a condensed atomic and molecular thin film
for Xe), so it is not expected that
under > 1OOeV electron bombardment is low ( - 3 x lo-“cm2
disturbance of the condensed overlayer films by primary electrons (or secondary electrons) is
a significant problem affecting the measured parameters [27-30-J.
A further assumption is that the adsorption of the overlayer does not affect the ESD process on the
substrate surface significantly (see Appendix C for mechanisms of ESD). This assumption may not
always be valid; however, one can assume that such quenching at the substrate-overlayer interface is
limited to the first overlayer monolayer only, so that there is true ion-thin film transmission for the
second and following monolayers.
When the above conditions are fulfilled, this is a good approach to study the interaction of
1-1OeV ions with thin films, and it can reveal information about the depth of origin of secondary
ions.
4.1.2. Experimental procedures
The experimental measurements of ion transmission through condensed films are carried out in
an ultrahigh vacuum (UHV) chamber equipped with facilities for surface characterization; details of
the apparatus and the methods used have been described previously [31]. The following experiments
can be performed in this UHV system: (a) thermal desorption spectroscopy (TDS), (b) low-energy
electron diffraction (LEED), (c) low-energy ion scattering (LEIS), (d) Auger electron spectroscopy
(AES), and (e) electron stimulated desorption ion angular distribution (ESDIAD) of both positive
and negative ions.
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M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
TDS and ESDIAD are the two main techniques used in these studies. TDS is used to determine the
overlayer coverage, while ESDIAD is used to measure total ion yield, energy and angular
distribution of ESD ions as a function of the overlayer thickness.
The sample, either a W( 100) or a Ru(000 1) single crystal in our studies to date, is mounted on an
XYZ-rotary sample manipulator that is coupled to a closed cycle helium refrigerator to achieve
a sample temperature of N 20 K. The sample can be heated in the range of 20- 1600 K by radiative
heating and electron beam heating from a tungsten filament located behind the sample. The
temperature of the sample is measured by using a Ni/Cr/Si-Ni/Si/Mg,
N-type (generally known as
Nicrosil-Nisil) thermocouple.
The overlayer gases are deposited onto the substrate at 20K using a leak valve through
a directional gas doser capped with a microcapillary array aimed at the surface [29,32]. This
procedure allows precise dosing of various coverages of the gas, from fractional monolayers to
multilayers. The exposure in units of Langmuir (L) is determined by measuring the dosing pressure
with an uncalibrated ionization gauge located in the UHV chamber, and the dosing time.
The coverages of the atomic and molecular overlayers are determined using TDS, which allows us
to resolve the desorption peaks from the first monolayer and subsequent multilayers [29,30].
Monolayer identification is possible if the desorption peaks from the first monolayer and subsequent
layers occur at different temperatures. Since the integrated area of the pressure vs. time trace is
directly proportional to the number of gas atoms or molecules which desorb (coverage), the relation
between coverage and exposure can be measured and calibrated.
The ESDIAD/TOF (time of flight) detector allows mass, energy, and angle-resolved positive or
negative ion detection [29,30]. Positive and negative ions can be generated by bombarding a surface
with a focused electron beam (typically N 300 eV). The ESDIAD/TOF detector (Fig. 5) includes
a set of four high transparency planar grids of which all except the second are grounded, a stack of
five microchannel plates, and a position sensitive resistive anode encoder (RAE). The RAE is
connected to a position analyzing computer to provide direct digital acquisition of two-dimensional
data. By pulsing the primary electron beam, which provides a start pulse coincident with desorption
from the surface, and by gating the retarding potential grid G,, we can perform TOF analysis of the
desorbing ions. The TOF capability allows us to separate easily lighter ions (shorter flight times)
from heavier ions (longer flight times).
For appropriate bias and pulse conditions we can detect both positive and negative ions in
a mass-resolved mode. Energy analysis of desorbing ions is measured in two ways: by use of
a retarding field method in the ESDIAD/TOF detector and by careful analysis of ion signals in TOF
measurements. In the retarding field method, the ESDIAD ion intensity is monitored as a function of
retardation voltage applied to the grid G, to obtain an ion retardation curve. Upon taking the
derivative of the retardation curve with respect to retardation voltage, an ion kinetic energy
distribution is obtained for ions of known mass.
The polar angle of detection (under field free conditions around the crystal) ranges from 0” to 22”.
Application of a positive bias voltage to the substrate compresses the angular distribution of the
desorbing positive ions so that even ions which desorb with angles substantially larger than 22” can
be detected. For example, when a substrate bias of + 100 V is applied, the polar angle of detection is
increased from O”-22” to O”-70” for a positive ion with a kinetic energy N 7 eV [29,30]. Hence, the
application of a positive sample bias allows us to collect nearly the total desorption yield, but it
makes the quantitative measurement of the angular and energy distribution of desorbing ions more
M. Akbulut et al. 1 Surface Science Reports 28 (1997)
G2 grids
\
177-245
193
/c--i
Microchannel Plates (MCP
/
primary
electrons
/
Resistive Anode
Encoder (RAE)
IIII
IIII
J
III I
crystal,
bias: V,
ions
III I
t
+i lo
I
Fig. 5. ESDIAD
>OeV.
detector
electro;;
1
: :
G2 bias voltage
and time of flight (TOF) arrangement.
The ion trajectories
are indicated
for a substrate
bias
difficult. The position sensitive RAE also enables us to measure angle resolved yields over arbitrary
regions. Fig. 5 shows the ESDIAD detector and TOF arrangement.
Typically, the electron pulse length is 0.1 ps, the average electron current is 1 nA and the total
electron fluence for a measurement is - 2 x 1Ol3cmd2 (beam area - 1 mm2) [29,30].
Different samples are used as sources for different ion beams. In our first series of experiments, we
study the transmission of low-energy ESD-produced O+ through ultrathin atomic and molecular
overlayers. In order to generate a well-defined ESD O+ ion beam, we use a W(100) single crystal; it is
oxidized at 860 K in an 0, atmosphere (5 x lop6 Pa) for 10 min to produce a thin oxide film on the
crystal [30]. The oxide surface prepared in this way exhibits a (1 x 3) LEED structure. Based on
earlier studies, the stoichiometry of the surface is assumed to be WO,_, [33,34]. ESD of this surface
produces a well-defined 0 ’ beam having a peak energy of - 7 eV and full width at half maximum
(FWHM) of - 15” desorbing normal to the surface [35]. This is our reference O+ ion source. Fig. 6
shows the profile of the ESD O+ beam obtained under field free condition (zero sample bias) from
the oxidized W( 100) surface measured with the ESDIAD/TOF detector. The inset of Fig. 6 is the O+
ESDIAD pattern from which the angular profile is extracted.
In order to study transmission of F+ and F- ions through condensed layers, a chemisorbed PF,
layer on a Ru(0001) surface is used to produce F+ and F- ion beams with well-defined energy and
angular distributions [36,37]. The clean Ru(0001) surface exhibits a (1 x 1) LEED pattern.
A saturation coverage of PF, (opF3 = 0.33 monolayer (ML)) is dosed onto the Ru(000 1) surface at
lOOK; subsequently, the crystal is annealed at 270 K for a few seconds in order to produce
a hexagonal array of off-normal ion emission under electron bombardment, for both F- and F+
194
M. Akbulut et al. / Surface Science Reports 28 (1997)
100
200
300
400
177-245
500
position channel
Fig. 6. Profile of the O+ desorption beam from oxidized W(10 0) surface measured with the ESDIAD detector. Primary
electron energy: 300 eV. The inset shows the O+ ESDIAD pattern from which the profile is obtained. The periodic
structure in the O+ yield is due to ion transmission through a metal grid.
ions. The F+ ions desorb with a peak energy of - 4eV, while the F- ion kinetic energy distribution
from adsorbed PF, has a peak at - 1 eV [38]. This surface exhibits a (fi x ,_/?)R30” LEED
pattern. Afterwards the PF,/Ru(OOO 1) surface is slightly damaged under electron bombardment;
ESD of this surface gives rise to a strong F+ emission (due to PF fragments) [36,37] normal to the
surface, in addition to the hexagonal off-normal beams. Fig. 7 shows F+ and F- ESDIAD patterns
from the slightly damaged PF,/Ru(OOOl) surface obtained with a bias voltage of + 160 and
- 120 V, respectively. The F- ESDIAD pattern is unchanged by beam damage [36,37]. After these
initial surface treatments, the slightly damaged surface remains stable throughout the measurements.
A well-defined ESD H+ (D+) ion beam can be produced either from a bilayer H,O adsorbed on
a Ru(000 1) surface [39] or a thick H,O film (> 7 ML) on Ru(000 1) [40]. ESD of the bilayer H,O
adsorbed on Ru(000 1) produces an H+ (D+) beam having a peak energy of - 4 eV and a broad
angular distribution (FWHM - 40”) centered on the normal to the surface. Surprisingly, H+ (D+)
ions generated from the thick H,O film on Ru(0001) desorb in directions closer to the surface
normal with a FWHM - 22” under field free conditions [40].
4.1.3. 0 ‘transmission through rare gas$lms
Sack et al. [29,32] have investigated the transmission of 7 eV O+ ions from an oxidized W(100)
substrate through ultrathin overlayer films of Ar, Kr and Xe. They find that the ions can penetrate
several monolayers of Kr or Xe, while < 2 ML Ar suppress the 0’ signal to -C 1% (Fig. 8). They also
M. Akbulut et al. / Surface Science Reports 28 (1997)
177-245
195
(4
(b)
F’lg. 7. (a) F’ ESDIAD pattern from slightly damaged 1 ML PF,/Ru(O 00 1) beam surface obtained with a sample bias of
+ 160 V (incident electron energy: 360 eV) and (b) F- ESDIAD pattern from a slightly damaged 1 ML PFJRu(000
1)
beam surface obtained with a sample bias of - 120 V (incident electron energy: 200 eV). Note that the differences in the
angular distribution of F+ and F- hexagonal beams obtained from the PF,/Ru(OOO 1) surface are due to field distortion
and the differences in the sample mounting.
find a change in the kinetic energy distribution towards lower energies for Kr or Xe films thicker than
2 ML (Fig. 9), and they present indirect evidence for a slight broadening (change) in the angular
distribution of the desorbing Of ions for Xe films thicker than 2 ML.
M. Akbulut et al./ Surface Science Reports 28 (1997) 177-245
196
i
I
I
4
5
rare gas coverage
(ML)
V
m
.“”
I
0
h\
m
2
1
I
3
6
Fig. 8. Total O+ yield from an oxidized W( 10 0) substrate as a function of rare gas overlayer thickness. The lines are guides
to the eye. From [32].
1 .I
?
=r
0.t
4
a
z
0.E
‘&
g
‘PI
c;
0.4
&
%
0.2
1,
2
II,,,I,,,,,,,,,I,,,,,,,,,,,,,,,,,
4
kinetic
at
erg)
(eV)
I
10
I
I
1
1
12
Fig. 9. Energy distribution of the O+ ions from clean oxidized W( 10 0) surface and of O+ after passage through Xe films of
various thickness (in monolayers), measured by a retarding-field method. Yields are normalized to unity at their maximum.
From [32].
M. Akbulut et al. / Surface Science Reports 28 (1997)
/
3. Iaye?
177-245
1.
197
layer
2. layer
Fig. 10. Schematic structure of a fcc( 1 1 1) single crystal film. It can be seen that there exist channels perpendicular to the
surface which are being closed by the third layer.
They attribute the attenuation of the O+ ions in the Kr and Xe films predominantly to elastic
scattering (see Appendix A.1 for the atomic collision discussion). The attenuation cross-sections for
Kr and Xe (of order lo- ’ 5 cm’) agree well with a molecular dynamics simulation [41] (see Section 5);
this simulation assumes only elastic scattering between the 0’ ions and the rare gas atoms, but no
charge transfer. However, the simulations do not produce the strong attenuation observed for the Ar
films, which is so far unexplained. Furthermore, the change in the attenuation of Of in Kr and Xe
around 2 ML is correlated with the suggested rare gas structure: Bulk rare gas solids are known to be
fee which means that in the (111) direction they show an A-B-C-A _. . layer structure [42] (Fig. 10).
The authors conclude that the existence of channels allows the O+ ions to traverse the rare gas films
with relatively small attenuation and energy loss up to 2 ML [29]. The third monolayer closes the
channels and hence leads to increased attenuation and energy loss. Channeling leads to a lower
attenuation cross-section of an ion in a crystalline solid than in an amorphous solid if the ion
traverses the solid along one of the channeling directions. Steering means that an ion changes its
trajectory from a non-channeling direction to a channeling direction.
Hence, this study shows that the structure of the surface layers of a solid can influence the escape
depth of ions from the solid significantly. If the surface layers are well ordered (crystalline) and if
there are channels through which ions can escape from the surface, the ion escape depth may be
larger than if the surface layers were amorphous. The angular distributions of the desorbing ions are
expected to be affected by the structure of the surface layers.
An alternative interpretation of Of attenuation by rare gas films has been presented by Ageev
[43], who bases the attenuation on elastic backscattering and local Coulomb surface field
relaxation. According to this model, an 0 + ion backscattered by an overlayer atom decelerates
toward the surface and then accelerates from the surface by means of the same local repulsive
Coulomb surface field that produces the ESD of Of from the surface. If the backscattered O+ ion
.has sufficient time to attain kinetic energy exceeding the image surface potential it may desorb,
M. Akbulut et al. /Surface Science Reports 28 (1997) 177-245
198
0.0
0.5
1.0
1.5
Coverage
2.0
2.5
3.0
(ML)
Fig. 11. Total O+ yield from an oxidized W( 100) substrate as a function of H,O and NH, overlayer thickness. The lines
are exponential curve fits. From [30].
otherwise it readsorbs. This model predicts quite well the experimental attenuation
and kinetic energy distributions of Of ions in rare gas films.
cross-sections
4.1.4. 0’ transmission through H,O, NH,Jilms
Akbulut et al. [30] have studied the transmission of 7 eV O+, generated from an oxidized W(10 0)
substrate, through ultrathin films of H,O and NH,. They find strong suppression of O+ by H,O
(Fig. 11): Less than 1 ML H,O suppresses the O+ signal to < 0.1%. The attenuation cross-section
for Hz’*0 is 9 f 2 x lo- l5 cm2; for NH, the cross-section is 3 k 1 x lo- l5 cm2. The authors
suggest that the observed attenuation cross-section cannot be explained primarily by backscattering
and large angle elastic scattering alone, for several reasons. First, NH, and H,‘*O have nearly the
same mass and similarly large dipole moments, so that very similar Of attenuation cross-sections
for both NH, and H,‘*O would be expected due to elastic scattering; this is not observed. Second,
the measured cross-sections are too large to be caused only by elastic scattering (see below). They
attribute the strong attenuation mainly to charge transfer to the O+ ions in the overlayer:
O++H20-,0+H20+,
(4.1)
O++NH,-tO+NH;
(4.2)
(see Appendix A.3 for more details about charge transfer reactions in the gas phase). In thermal gas
phase reactions [44] the cross-sections of reactions (4.1) and (4.2) differ by a factor 3, similar to the
ratio of the cross-sections found in these studies. The absolute cross-sections at thermal energies are
significantly larger than those at 7 eV collision energy as expected on the basis of the orbiting theory
(see Appendix A.3).
M. Akbulut et al. / Surface Science Reports 28 (1997)
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199
The exponential attenuation of the ions even in the fractional monolayer range is explained by
statistical growth of the overlayer films at 25 K, at which temperature the mobility of the overlayer
molecules is strongly suppressed. Because the effective interaction area is much larger than the
molecular size (based on intermolecular spacing), with increasing overlayer coverage the effective
interaction areas of neighboring molecules overlap. This leads to Poisson statistics of ion attenuation in the overlayer film.
Earlier studies by Diebold and Madey [45] revealed an attenuation cross-section for 5eV 0’
transmission through NH, similar to that for 7 eV O+ through NH, [30]. In that experiment [45],
the oxygen ions are desorbed from a TiO, substrate. The fact that the O+ attenuation cross-section
by NH, is nearly the same in these two experiments suggests that the experimental approach allows
insights into ion-thin film transmission, since the results are independent of the substrate used as
a source of 0 +.
4.1.5. Of transmission through alkali metal overlayers
Very recently, Ageev et al. [46] have investigated ESD produced O+ transmission through
fractional monolayer films of Li, Na, K and Cs on an oxidized tungsten substrate. Fig. 12 shows
a semilogarithmic plot of the normalized ESD O+ signal as a function of alkali metal concentration.
As seen in Fig. 9, the ESD Of signal is attenuated by the alkali overlayer: For alkali metal
coverage < 0.3 ML the attenuation rate of the O+ signal is lower than the attenuation rate of O+ for
alkali coverage > 0.3 ML. For alkali coverage > 0.3 ML, the attenuation cross-sections of O+ are
(18&2)x 10-15cm2 for Cs, (13+1)x 10-15cm2 for K, (5+1)x 10-‘5cm2 for Na and
(3.3 + 0.3) x lo- l5 cm2 for Li.
In an earlier study, Yu [47] has investigated ESD O+ and 0 - yields from an oxygen chemisorbed
Mo(100) surface as a function of Cs overlayer coverage, for thickness less than a monolayer. Yu has
found that at low Cs coverages the 0 - yield increases with coverage, whereas the 0 ’ yield decreases
slightly, and at high Cs coverages both ESD Of and O- yields are strongly attenuated. He suggested
that shielding of the oxygen atoms from incident 200 eV electrons by elastic electron scatte?ing from
o-
-1 -
;”
Z-2J-
I
0
4
2
I
I
e
a
N.10"an"
Fig. 12. Total O+ yield from an oxidized W( 10 0) substrate
.C461.
as a function
of Li, Na, K and Cs overlayer
thickness.
From
200
M. Akbulut et al. /Surface
Science Reports 28 (1997)
1777245
the Cs overlayer may be the reason for the high coverage attenuation. However, as discussed in
Section 4.1.1, attenuation of > 1OOeV electrons by elastic backscattering in an overlayer film is
expected to be very low [29,30]. Based on the above argument, Ageev et al. [46] do not believe that
primary electron backscattering in the alkali overlayer has strong influence on the measured
attenuation of O+ in their measurements. These authors also rule out the possibility of elastic
scattering of ions as a dominant process in transmission of O+ ions through alkali metal films,
because the measured O+ attenuation cross-sections are much too large to be realistic for elastic
backscattering and large angle scattering, for which smaller impact parameters are necessary.
However, there is a correlation between the ionization potential of the alkali metal and the O+ for
attenuation cross-sections for > 0.3 ML, which is consistent with the resonance one-electron charge
transfer model proposed by Rapp and Francis [48]. Hence, Ageev et al. attribute the attenuation of
O+ in alkali metal films for > 0.3 ML mainly to resonance one-electron charge transfer from
adsorbed alkali metal atoms to 0* 3S, and O*?S, excited states of oxygen with excitation energies of
- 4.09 and 4.4 eV, respectively.
4.1.6. F+ , F- transmission through rare gas$lms
Sack et al. [35,36,49] studied the transmission of F+ and F- ions from a monolayer of
PF,/Ru(OOOl) through Kr and Xe overlayers. The kinetic energy of F+ is - 4 eV and that of
F- - 1 eV [38]. The F+ ions are attenuated to < 10% by 1 ML of Kr or Xe (Fig. 13). The F+ angular
distributions do not change significantly as a function of overlayer coverages. The measured
attenuation cross-sections are - 1.4 x lo- l5 cm2 for Kr and - 2.6 x lo-r5 cm2 for Xe. The authors
discuss the results in terms of elastic scattering, and perform a molecular-dynamics simulation which
indicates that the attenuation may be dominated by elastic scattering. However, since the F+
angular distributions do not change significantly as a function of overlayer coverages, the authors
also suggest that charge transfer reactions may contribute to the observed attenuation crosssections.
rare gas coverage
Fig. 13. Total F+ yield from a monolayer
of PF,/Ru(OOO
1) as a function
(ML)
of rare gas overlayer
thickness.
From [49].
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
rare gas coverage
Fig. 14. Total F- yield from a monolayer of PFJRu(000
201
(ML)
1) as a function of rare gas overlayer thickness. From [49].
1.50
f
Y
Fig. 15. F- ESDIAD surface plots from 1 ML PF,/Ru(OOO 1) and from 0.15-ML-Xc/l-ML-PF,/Ru(OOOl).
From [36].
Surprisingly, the total F- yield increases upon adsorption of 1 ML of Kr or Xe (Fig. 14), and
decreases exponentially for films thicker than 1.5 ML. This increase is accompanied by a dramatic
change in the F- angular distribution (Fig. 15): While F- ions desorb from 1 ML PF,/Ru(OOO 1) on
a hexagonal array of trajectories (cf. Fig. 7(b)), the distribution changes to one which is broad and
centered around the surface normal upon passage through the Kr film.
The authors explain the enhancement in F- by a decrease in the neutralization rate of F- with the
substrate during desorption (see Section 4.1.7 for details). The change in the F- angular distribution
is caused by elastic scattering: As was concluded in Section 4.1.3 (see also [29]), there are channels in
the rare gas films (up to 2 ML) through which ions can escape with little attenuation. These channels
are normal to the surface for fcc(ll1) films; hence the F- ions which desorb from the clean
PF,/Ru(OOO 1) surface with polar angles of - 60” have their trajectories changed to smaller polar
angles (“steering”). It is also suggested that the attenuation of the F- ions in Kr and Xe films thicker
than 1 ML is mainly caused by elastic scattering, because inelastic processes such as charge transfer
reactions are unlikely to occur.
202
M. Akbulut et al. /Surface Science Reports 28 (1997)
177-245
As discussed in Section 3, Bressler and Schwentner [17] have reported that the mean penetration
depth (l/e) of - 4.2 eV F atoms in the Ar film is - 2.8 nm; this is an interesting result as compared to
the short escape depths (-0.4-0.8 nm) for F+ and F- ions through Kr and Xe layers. Since the
diameter of an F atom is smaller than an F- ion, we expect that the mean penetration depth of
F atoms in a rare gas film should be greater than that of F- ions. However, considering that the Ar
film grows in fee fashion, and the energetic F atoms undergo a series of (mainly) elastic collisions in
the Ar film, it is somewhat surprising that a - 4.2eV F atom can penetrate so many Ar layers.
4.1.7. F ‘, F- transmission through H,Ojlms
Akbulut et al. [37,50] studied the transmission of Ff and F- from 1 ML PF,/Ru(OOOl) through
H,O overlayers. They also compared the behavior of F+ ions which desorb from the surface with
different polar angles, 8, - 0” and - 60”. The F+ ESDIAD patterns for various H,O thicknesses are
shown in Fig. 16. Fig. 17 shows the total angle integrated F+ ESDIAD intensity from the l-ML
PF,/Ru(OOO 1) surface as a function of H,O thickness. As seen in Fig. 17, 1% of the F+ ions survive
transmission through several monolayers of H,O. They find that the attenuation of the off-normal
(60”) F’ ions is about twice as strong as that of the normal (0’) ions, which is consistent with a simple
path length argument (l/cosQ).
For F-, Akbulut et al. find a similar increase in F- yield around 1 ML H,O as discussed in
Section 4.1.6 for F- through Kr and Xe. Fig. 18 shows the total angle-integrated F- ESDIAD
intensity as a function of H,O thickness. It can be seen in Fig. 18 that the F- ESDIAD intensity
increases in the H,O coverage range O-l ML, and this increase is accompanied by a strong change
in the F- angular distribution [37]. However, it is interesting to note that there is no increase in the
F- yield upon adsorption of - 1 ML H,O when the F- ions are desorbed from a multilayer of PF,
(> 10 ML) instead of 1 ML, as shown in the inset of Fig. 18. This points out that the increased Fyield is a phenomenon associated with the substrate-overlayer interface, and not a charge transfer
process in the film. Based on this observation, the authors suggest that dielectric screening of the
H,O overlayer leads to a reduction in the neutralization probability of F- from 1 ML
PF,/Ru(OOO 1) with the surface [37,50].
Nordlander and colleagues [SO] have carried out quantum mechanical calculations in order to
estimate the influence of the dielectric water overlayer on the resonance electron tunneling rates
between an F- ion and the Ru(0001) surface. The water overlayer was modeled as a uniform
dielectric film with dielectric constant E and thickness d. They have shown that the presence of
a dielectric film introduces a potential barrier between a desorbing F- ion and the metal surface.
This potential barrier reduces the electron tunneling rates near the surface, so that the F- survival
probability increases. The calculations have also revealed that the increase in survival probability of
the F- saturates after a water bilayer is formed, which is in excellent agreement with the experiment.
It is noteworthy that condensed H,O attenuates the Ff signal (the attenuation cross-section is
- 1 x lo- l5 cm”) much more effectively than the F- signal; - 1% of the F- ions can penetrate
10 ML of H,O (the attenuation cross-section is only 6 x lo- I6 cm”). The long penetration depth for
F- is explained by a low probability for electron detachment and dissociative attachment reactions
(all of the known reactions between F- and H,O are endothermic), and the open structure of the ice
H,O films, through which the ions can channel without significant energy loss and attenuation. In
contrast, the strong attenuation of F+ without substantial changes in angular distribution suggests
that charge transfer processes are important in this case. Although some ion-molecule reactions
M. Akbulut et al. / Surface Science Reports 28 (1997)
177-245
203
(a)
(b)
Fig. 16. F+ ESDIAD pat
tern obtai ined wi.th a sample bias of + 160 V (incident electron 1 energy r: 360 eV) from (a) a sligl 7t1y
beal m-d amaged 1 ML PF JR u(OO( 3l)sur .face (no H,O present). (b) 0.5 ML of H,O an Id(c) 1.61ML H,O adsorbed on the
sligl 1t1y beam-damage :d 1 ML 1 PF,,iRu(O( 3 0 1) surface at 20 K. From [37].
204
M. Akbulut et aLlSurface Science Reports 28 (1997)
Water
Coverage
177-245
(ML)
Fig. 17. Total F+ yield from a monolayer of PF,/Ru(OOO 1) as a function of H,O overlayer thickness. Shown are the
attenuation curves for an F+ beam centered around the surface normal, and for one off-normal beam (polar angle: - 60 “).
From [37].
J
L,
0.5 :-
8
0
0
3
#
Fig. 18. Total F- yield from a monolayer of PF,/Ru(OOO 1) as a function of H,O overlayer thickness. The inset shows
semilogarithmic plots of the total angle-integrated F- ESDIAD intensity from 1 ML PFJRu(000
1) (solid circles) and
10 ML PFJRu(000
1) surfaces (open circles) as a function of H,O overlayer thickness. From [37].
(such as F+ + H,O +(HF)+ + OH, F+ + H,O + HF + (OH)+) are energetically possible and may
occur in the film, we were not able to detect evidence for ion-molecule reactions.
4.1.8. Cl- from CH,Br/CCI,/Ag(lll)
There is another report on the enhancement of an ion yield upon condensation of an overlayer on
top of the substrate. Dixon-Warren et al. [Sl] investigated the effect of an overlayer of CH,Br on the
M. Akbulut et al. /Surface Science Reports 28 (1997)
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205
Cl- yield from a layer of CCl,/Ag(l 11). They observe an increase in the Cl- yield for small CH,Br
coverages and an exponential decrease with higher CH,Br coverages. Based on their finding that the
photoelectron yield also increases with CH,Br coverage for small coverages, they conclude that the
production of Cl- by photoelectrons is increased. The increase in photoelectrons is caused by
a decrease in the work function of the surface by the CH,Br overlayer.
4.1.9. Evidence for ion-molecule chemical reactions in overlayers
Sanche and Parenteau [52,53] studied the transmission of O- through hydrocarbon overlayers,
1 ML n-C,H,,+, (such as C,H,, and C,H,,) and 1 ML 1 ML n-C,H,, (such as C,H, and C,H,). In
these experiments, a Pt substrate was covered by a 4 ML 0, film, on top of which the hydrocarbon
layers were deposited. Under bombardment by electrons with energies w 13.5, they find OH- to
desorb from the surface. They also observe the desorption of OH- ions from films composed of
a mixture of 0, at 25% vol in n-CnH2n+2 and n-C,H,,. Fig. 19 shows the thickness dependence of
the O- and OH- yields and O-/OHintensity ratio for a film containing 25% 0, (by vol) in
n-C,H,, deposited on the Pt surface. Since n-C,H2n+2 and n-C,H,, contain only hydrogen and
carbon, the OH- signal bears the signature of dissociative electron attachment (DEA) from
adsorbed 0,. Sanche and Parenteau suggest that the OH- ions are formed via the reaction between
ground state molecules (C,H,, +2 and n-C,H,,) and O- produced directly from adsorbed 0, by the
I
1
I
2
COVERAGE
1
3
I
4
I
5
(MONOLAYERS)
Fig. 19. Thickness dependence of the 0 - and OH - yields and OH -/O - intensity ratio for a film containing
in n-C,H,, deposited on Pt. The incident electron energy is 13.5 eV. From [52,53].
0, at 25% vol
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
206
electron beam:
0-
+C,H 2n+2+OH-
0- +C,H,,+OH-
+ CnH~n+l,
+C,H,,_;
(4.3)
(4.4)
This is one of the few direct measurements of ion-molecule reactions in ion transmission experiments in this low-energy regime.
In another experiment, Sanche [54] investigated the ESD from thin films of 0, and N,, and
observed NO+ as one of the desorption products. By measuring the dependence of the NO+ yield on
the primary electron energy, Sanche concluded that the formation of NO’ is initiated by the
ionization of O,, and that NO+ must be formed in an ion-molecule reaction
O++N,+N,O++NO++N.
(4.5)
4.1.10. Cr on SiO, and SiO, on Cr
To conclude this section we mention a study of the escape depth of the secondary ions resulting
from electronic sputtering by 9 MeV Ar ions in inorganic thin films [55]. Two cases have been
investigated: sputtering of Si(OH), from SiO, with Cr as overlayer, and sputtering of CrO,H- from
Cr with SiO, as overlayer. A SIMS operating in TOF mode was used in order to determine the depth
of secondary ions. Based on the experimental observations, the authors derive escape depths of
0.5 nm for the Cr overlayer and 1 nm for the SiO, overlayer. This result indicates that the depth of
origin of the secondary ions resulting for electronic sputtering is larger than that for knock-on
(nuclear) sputtering (- 0.5 nm). However, it is possible that beam damage of the overlayers and
surface roughness affect the apparent depth of origin of ions in this experiment, because it is expected
that the primary ion beam can cause much more damage to the inorganic materials than the metals
[6]. This beam damage of surface layers may affect the depth of origin of ions, because the ions
traverse strongly disturbed overlayers, instead of nearly undisturbed overlayers.
In Table 1, we summarize the available experimental data on low-energy ion and neutral
transmission through overlayer films.
4.2.
Ion desorption from multilayer adsorbates and compounds
4.2.1. Principles
In this final part of Section 4 we summarize a number of experimental results from ESD yields
from multilayer adsorbates on metal surfaces, and their dependence on the adsorbate film thickness.
We try to extract some information from these data concerning the origin of the detected ions: do
they originate from the surface or from subsurface layers? We illustrate this approach in Fig. 20: The
total ESD ion yield from a multilayer adsorbate can contain contributions from ions originating on
the surface and from ions in subsurface layers. As the first example, we discuss Ccl, multilayers,
which are interesting because they produce both positive and negative ESD ions.
4.2.2.
Cl+, Cl- desorptionfrom Ccl,
The ESD Cl+ and Cl- yields from CCl,/Ru(O 0 0 1) were investigated as a function of Ccl, film
thickness from monolayer to multilayer thicknesses by using a - 300eV electron beam (Fig. 21)
ESD
ESD
ESD
ESD
- 5eVO+
-7eVO+
-7eVO+
-4eVF+
-leVF-
MD (theory)
-7eVO+
Technique/method
W (10 0)
Oxidized
1)
1)
1 ML PF,/Ru(OOO
tungsten
Oxidized
1 ML PF,/Ru(OOO
W ( 10 0)
Oxidized
10)
W ( 10 0)
Oxidized
TiO,(l
or ion source
ion and neutral
Neutral
data on very low-energy
ESD
(experiment)
or ions
of the available
-7eVO+
Neutral
Table 1
Summary
overlayer
90
30
33
50
130
180
14
26
10
- 0.5 ML H, I80 (0.18 nm)
- 3 ML NH, (0.7 nm)
<lML(<O.l6nm)
< 1 ML( <0.19nm)
< 1 ML( <0.24nm)
< 1 ML( <0.27nm)
- 1 ML Kr (0.4 nm)
- 1 ML Xe (0.44 nm)
- 4 ML H,O (1.5 nm)
H “0
N;I,e
Li’
Na’
Kf
cs’
Krp
Xeg
H,Oh
Kra
Xeg
H,Oh
transfer
Charge transfer
transfer
Charge
Charge transfer
scattering
and
transfer
Elastic
and charge
Charge transfer
(O-l ML)
(O-l ML)
(0- 1
Charge
Charge transfer
- 5 ML Ar (1.9nm)
-4MLKr(1.6nm)
- 6 ML Xe (2.6 nm)
Arc
Kr’
Xe’
28
Elastic scattering
scattering
Elastic
13 (1-5 ML range)
17(224 ML range)
ll(226 ML range)
- 6 ML Xe( 2.6 nm)
Xeb
NHxd
Elastic
Elastic scattering
scattering
Elastic
- 4 ML Kr (1.6 nm)
Krb
Dominant
mechanisms
range)
range)
range)
range)
range)
range)
ML
ML
ML
ML
ML
ML
Attenuation
cross-section
( x lo-r6 cm’)
5(0-l
60(1-2
5 (O-2
22 (2 4
5 (O-2
15 (226
films
- 2 ML Ar (0.75 nm)
Escape-depth”
through
Arb
Overlayer
transmission
Cu(ll1)
Cu(ll0)
Cu( 100)”
-2ML
-2ML
-3ML
Ion-molecule reaction
(OH - formation)
Enhancement (O-l ML)
Elastic scattering
Elastic scattering
Elastic scattering
Dominant
mechanisms
neutral or ion yield to
- 70% of the Ru yield
suppressed
11
15
6
- 2 ML Kr (0.8 nm)
- 2 ML Xe (0.88 nm)
- 10 ML H,O (3.7 nm)
-15nm
Krg
Xeg
H,Oh
Ar’
n-C, H,,’
n-C, H,, + 2
CH,Brk
Cr’
SiO,
cum
- 0.5 nm
- 1.0 nm
>lMLCu
Attenuation
cross-section
( x lo-l6 cm2)
Escape-depth”
Overlayer
a In order to compare the available data from different sources, we define escape-depth as an overlayer thickness that attenuates
approximately 1% of the original yield.
b Refs. [29,32].
’ Ref. [41].
d Ref. [45].
e Ref. [30].
’ Ref. [46].
g Refs. [36,49].
h Refs. [37,50-J.
i Ref. [17].
J Refs. [52,53].
k Ref. [Sl].
’ Ref. [55].
m Ref. [19].
” Ref. [21].
l-20 eV Cu
CCl,/Ag( 11 1)
SiO,
Cr
Ru(000 1)
PSD
Sputtering
clSi(OH) ;
CrO,HRu and Cu
cu
3.6 keV Ar
sputtering
and TRIM
Theory
F,/Ar/Kr
WPt
Photodissociation
ESD
- 4.2 eV F
- 1.5 eV O-
1 ML PF,/Ru(OOO 1)
ESD
-leVF-
Neutral or ion source
Technique/method
Neutral or ions
Table 1 (continued)
M. Akbulut et al. /Surface Science Reports 28 (1997)
209
177-245
eV
V ions
‘\
overlayer
4
substrate
Fig. 20. Illustration of experimental approach in Section 4.2. An ion is desorbed by ESD from a multilayer on a substrate,
and the ion yield is measured as a function of coverage.
!
1.0 -
7
0.8 -
0.6 -
0.4 -
O.*0.
3
(
I
d
8
1
2
3
CC14 coverage
4
5
6
(ML)
Fig. 21. Cl- and Cl+ ion desorption yield as a function of Ccl, coverage for CCl,/Ru( 0 0 0 1). 1 ML corresponds to room
temperature saturation coverage. From [57].
210
M. Akbulut et al./ Surface Science Reports 28 (1997)
177-245
[56,57]. As seen in Fig. 21, the Cl+ yield continues to increase up to 5 ML, while the Cl- signal
increases up to 2 ML, and then decreases to a saturation value between 3 and 6 ML.
If the Cl+ ESD cross-section were independent of film thickness, and if all the detected ions
originated in the surface layer, one would expect a constant ion yield for films thicker than 1 ML.
This is obviously not the case. One factor that could lead to an increase in ion yield beyond 1 ML is
that quenching of the primary excitation with the surface (which leads to reduced ion emission) is not
as strong for the second and subsequent layers as for the first layer which is directly in contact with
the metal substrate. This would lead to higher ESD cross-section for the second and subsequent
layers than for the first layer. We also cannot exclude the possibility that not all ions detected
originate from the surface layer; some ions may originate in subsurface layers and traverse the
surface layers and contribute to the ion signal. This may explain why the ion yield increases so
strongly for adsorbate thicknesses > 1 ML. As discussed in Section 4.1, it is clear that ions produced
in subsurface layers can traverse overlayer films.
As seen in Fig. 21, the Cl- yield rises very steeply between 1 and 2 ML. This may be due to
a decrease in the neutralization probability of the desorbing Cl- with the surface. This effect is
expected to enchance the Cl- yield due to dielectric screening caused by the second Ccl, layer,
similar to the observation discussed in Sections 4.1.6 and 4.1.7.
There is another factor which can influence the thickness dependence of negative ion yields.
Sanche [58] and Akbulut et al. [38] have reported that the ESD O- and F- yields produced via
both DEA and dipolar dissocation (DD) processes from 0, and PF, on Pt vary strongly with the
thickness of the 0, and PF, layer, respectively. The DEA O- (F-) yield increases to a saturation
value with increasing 0, (PF,) thickness, whereas the DD O- (F-) yield first increases as a function
of O2 coverage up to l-2 ML and then decreases with increasing 0, coverage. Sambe et al. [59]
have also reported a similar observation, when 0, is condensed on the Pt substrate with a rare
gas spacer layer between the 0, and Pt. They have argued that the image potential induced at
the Pt surface affects the O- yield via DEA and DD processes differently. Since the Cl- yield
from CCl,/Ru(O 0 0 1) generated under - 300eV electron bombardment, it is expected that the
Cl- ions from CCl,/Ru(O 0 0 1) are produced mainly via DD process. Therefore, it is also possible
that the change in the Cl- yield as a function of Ccl, is influenced by the image potential induced in
the Pt surface.
4.2.3. H+ desorption from H,O, NH,
Madey and Netzer [60] investigated the H+ ESD yield from H,O on Ni( 1 1 1). They observe that
the H+ yield increases with H,O film thickness far beyond 1 ML. Benndorf and Madey [61] and
Netzer and Madey [62] also studied the H+ ESD yield from NH,/Ru(O 0 0 1) and NH,/Ni(l 1 1) and
find that the H+ signal increases strongly with adsorbate film thickness beyond 1 ML. In all these
cases, it seems likely that the ion signal contains contributions from ions produced in subsurface
layers.
Recently, Ma et al. [40] have investigated ESD of H+ ions from amorphous ice films (l-300 ML
thick) condensed onto a Ru (0 0 0 1) surface at - 30 K. H + ions generated by using a focused electron
beam (- 300 eV) desorb normal to the surface with a peak energy of - 4.5 eV. The FWHMs of H+
become narrow with increasing film thickness, from - 40” at 1 ML to - 20” at 10 ML. Annealing the
samples leads to a phase transformation from an amorphous to crystalline ice which decreases the
H+ signal and increases the FWHM of H+. This observation indicates that for amorphous ice, the
M. Akbulut et al. /Surface
Science Reports 28 (1997) 177-245
211
H+ are not only created at the top layer; they also originate from deeper layers (3-5 ML), mainly
from interstitial water molecules, and contribute significantly to the observed H+ yield. In contrast, the ESD Hf yield from crystalline ice originates mainly from the top layer. Measurements
of H+ transmission through ultrathin Xe overlayers condensed onto ice layers support this idea
[39,40].
4.2.4. Oxygen depletion in TiO, by electron bombardment
It has been reported [63,64] that electron beam damage can lead to a significant oxygen depletion
in the surface layers of a TiO, crystal. McCartney and Smith [63] used a high-resolution electron
microscope for both sample analysis and for electron irradiation of rutile TiO,. A layer of rocksalt
TiO is produced which is several nanometers thick. Apparently, oxygen species produced by ESD in
subsurface layers traverse the surface layers, desorb, and thereby contribute to the reduction of TiO,.
5.
Depth of origin of desorbing ions: computer simulations
Computer simulations have served as helpful tools in addressing the question of the depth of
origin of desorbing ions. In particular, molecular dynamics (MD) methods are suited because they
enable the study of trajectories of low-energy (- 7 eV) ions as they penetrate a solid film. One can
compare computed parameters such as the total transmission yield, the energy distribution and the
angular distribution of ions after they traverse the film to the corresponding experimental values.
Moreover, one can study the energy losses and trajectories of the ions in the thin film, which are
usually not accessible experimentally.
In order for the results of the MD simulation to be accurate and meaningful, one has to know the
interaction potentials of the ions and neutrals involved; this is often the main limitation on the
accuracy of the MD simulation. In contrast to high-energy collisions, where the exact shape of the
interaction potential surface does not matter as much and where good approximations are available,
the results of the simulation at these low collision energies depend strongly on the details of the
interaction potential.
In the following, we discuss one example of a MD simulation: The transmission of 7 eV 0 ’
ions through ultrathin rare gas films of Ar, Kr, and Xe [29,32] by Klein et al. [41]. This MD
simulation was performed in connection with the experiments [29] which we describe in Section
4.1.3. The authors use O+-rare gas interaction potentials from the literature [65] and calculate the
transmission of O+ ions through fcc(1 1 1) rare gas films. The initial 0 ’ energy and angular
distributions are chosen to agree with experiment. No inelastic effects, such as charge transfer, are
considered.
As can be seen from Fig. 22 the simulation of the total transmission yield agrees very well with the
experimental data for Kr and Xe. For Ar, there are deviations for film thicknesses > 1.5 ML.
The excellent agreement for Kr and Xe strongly supports the explanations given in Section 4.1.3,
and demonstrates that MD can predict the ion transmission yield through a thin film in this low
collision energy range. One of the reasons for the excellent agreement in this case is that the
interaction potentials for O+-Ar, O+-Kr and O+-Xe are available [65] (based on experimental
studies and on calculations). The authors confirmed that the transmission yield depends strongly on
the details of the shape of the interaction potentials [41]: If the onset of the repulsive part of the
212
M. Akbulut et al. / Surface Science Reports 28 (1997)
f simulation
!
IO
t
.
I
0.1
0
177-245
,
f
i
.
.
7
i
(clAf
12
1 (mo3nolayA)
5
6
Fig. 22. Transmission yield Yas a function of film thickness for three rare gases. From [41], with experimental data taken
from [29].
potential energy curve is increased to slightly larger internuclear distances, the ion transmission rate
drops significantly.
Klein et al. [41] also investigated the sputtering of (the weakly bound) xenon atoms from the
overlayer film by the 7 eV oxygen ions (a “kick-off” process, as suggested in [27,28-J).They find that the
sputter yields are of order 1, and that the sputter yield exhibits a maximum for Xe films of thickness
3 ML. The decrease in sputter yield for thicker Xe films is attributed to the fact that the O+ ions have
lost too much kinetic energy in subsurface collisions to be able to cause Xe desorption; for thinner
M. Akbulut et al. / Surface Science Reports 28 (1997)
177-245
213
films, the probability for a collision of O+ with Xe becomes small. The sputtering of Xe atoms is
correlated with the back scattering of Of ions, i.e., only small impact parameter collisions provide
enough momentum transfer from O+ to a Xe atom to sputter the Xe.
We discuss in Section 4.1.3 that the rare gases are expected to grow in a layer structure
A-B-C-A-B-C-A...,
corresponding to a (1 1 1)-oriented fee structure. Klein et al. [41] also
investigated the transmission through a hypothetical hcp structure with a layer structure A - B -A B-A... The transmission yield is found to be much higher for thicker rare gas films with the hcp
structure than for the fee structure. This is in agreement with a “channeling”mode1: In contrast to the
fee case, where the layer C closes the last channels, the channels persist beyond the third layer for the
fee structure, and ion transmission yield is therefore higher (Fig. 10). This is a nice example where
a MD simulation can provide insights into physics where the experiment cannot.
6.
Applications and implications for surface analysis
In this section we briefly present some conclusions from the discussions so far concerning the
interpretation of ESD, PSD and SIMS measurements. One obvious conclusion is that these
techniques are not always sensitive to the top surface layer only, contrary to what is often assumed
[1,2,5]. The secondary ion signal from an unknown sample to be analyzed may well contain
contributions from subsurface layers. The percentage of subsurface contributions depends on
a variety of physical surface properties, such as chemical composition, structure and electronic
properties, but total subsurface contributions of order 50% would not be surprising. Since these
properties are usually not known for a sample to be analyzed, it may be difficult to attribute
a secondary ion signal to a specific surface layer.
Besides the total yield, the energy distribution and angular distribution of the secondary ions can
contain information about the chemical bonds and the geometry of the bonds on the surface (see
Appendix C). However, if ions that originate below the surface layer can escape through the surface
layers into vacuum and be detected, their kinetic energy may be reduced upon transmission through
the surface layers and their trajectory changed, both due to elastic scattering. Hence in certain cases
it can be difficult to draw conclusions from ion energy and angular distributions concerning the
properties of an unknown surface.
6.1. Ion beam deposition
Low-energy ion beam deposition has been shown to be a promising technique for deposition of
smooth, homogeneous and ordered films on substrates. For instance, Lifshitz et al. [7] demonstrated that smooth, diamond-like films can be produced by low-energy C+ deposition on a carbon
matrix. Rabalais [66] deposited 15 eV Si+ ions onto a Si substrate and observed homoepitaxial film
growth.
The physical processes involved in low-energy ion beam deposition are very similar to the ones
determining the escape of low-energy ions from surfaces. Instead of the ion being generated in the
solid and traversing the surface layers in order to escape from the surface, the solid is bombarded
with low-energy ions which may penetrate the solid a few monolayers or be adsorbed on the
.surface. The interaction mechanisms of the ions with the surface are reflection of the ions (elastic
214
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
backscattering), neutralization (charge transfer), or penetration
These processes are discussed throughout this paper.
of the surface (e.g. channeling).
6.2. Low-energy ion implantation
Ion implantation is the most common process to place dopant (impurity) ions (such as B+, I’+,
As+) in semiconductor crystals [67,68]. With ion implantation, the penetration depth of the ions and
dopant ion concentration are controlled precisely. The ion penetration depth depends mainly on the
energy and mass of the ions, and the atomic mass of the solid. For example, the average penetration
range of B+ ions incident on Si in the energy range from a few keV to 3 MeV is about 100-4000 nm,
while that of 10 keV P+ ions in silicon is 14 nm [67,68].
Continued reduction or scaling of the semiconductor device dimensions is demanding a need for
ultrashallow source/drain junctions only 60 nm deep for the 0.25 ,um device generation [67,69]. The
ion penetration depth can be, in principle, reduced by decreasing the ion beam energy to form an
ultrashallow junction. For p-channel devices where the source/drains are heavily doped with boron,
low-energy (< 1 keV) ion implantation is required. This is an extremely challenging problem in the
semiconductor industry, because at low energies, it is difficult to produce high beam currents to
perform industrial processes [67,69]. (Since the n-type source/drains are heavily doped with
phosphorus or arsenic, the formation of ultrashallow junctions with > 1 keV ions is not a major
problem for n-channel source/drains.) There has been significant progress to develop low-energyhigh current ion implanters [69,70].
It is expected that continued transistor scaling will require even lower-energy (< 500 eV) ion
implantation [69]. However, only little is known about the processes and properties that determine
low-energy ion implantation, and hence, there are considerable research efforts to understand
l-5 keV or even lower-energy (< 500 eV) processes [69,70]. A future study of low-energy B+ ion
transport through ultrathin Si films will be very important to understand the limitation of very
low-energy ion implantation processes.
6.3. Electrochemistry
Low-energy ion transport through liquids or ultrathin films plays a central role in the field of
electrochemistry [71]. In particular, ion transport through liquid water is of fundamental interest in
electrochemistry, because water is the most important electrolyte in electrochemistry [72]. Since
amorphous ice can serve as a useful model of liquid water [72], ion transport through amorphous ice
water layers can have important implications.
On the other hand, very slow ions ( < 1 eV) can be solvated in water. For example, as discussed by
Akbulut et al., the attenuation of - 1 eV F- ions in amorphous ice films is mainly due to energy loss
processes (multiple elastic and inelastic scattering) [37,50]. (Note that - 1 eV F- ions can penetrate
- 10 ML of condensed water.) After an F- loses sufficient energy via collisions in ice films, it may
become trapped in the ice films as a result of ion-solvent interactions.
6.4. Electronic aging
In radiation physics and chemistry, transport and relaxation of excess low-energy charge carriers
(such as electrons and ions) is a topic of major importance for engineering materials (such as
M. AkbulutlSurface Science Report 234 (1997) 177-245
215
dielectrics) subjected to high electric fields [73-761. Hot electrons produced in dielectric materials
subjected to high voltages can induce electronic excitations and ionization events. These can lead to
physical and/or chemical changes which contribute to the aging process in the dielectric [76].
As discussed earlier, low-energy ions can interact with target atoms/molecules via various elastic
and inelastic processes. If ions, in their passage through an atomic or molecular film, interact with
the films via charge transfer and/or ion-molecule reactions these processes can produce reactive
chemical species which contribute to the electronic aging. Therefore, it is expected that the ion
transmission through ultrathin dielectric films is also relevant to the field of electronic aging.
7. Conclusions
In this paper we have addressed the fundamental question of the depth of origin of secondary ions
from surfaces. We have provided basic background knowledge and experimental and theoretical
results from investigations which can shed light on the interaction of ultralow energy ions with thin
solid films. We have correlated the different physical attenuation mechanisms with material
properties of solid surfaces. Finally, we pointed to a few neighboring fields of research where similar
physical processes occur as in the area discussed here.
Contrary to “common belief” secondary ions from solid surfaces desorbing under the impact of
electrons, photons or ions can originate in subsurface layers, sometimes several layers below the
surface. We have shown that elastic and charge transfer processes are the dominant processes in
determining the depth of origin of ions created below the condensed overlayers. Our experimental
results on ion transmission through condensed atomic and molecular films have revealed that these
processes depend strongly on the overlayer electronic and chemical properties as well as the nature
of the ion and its kinetic energy.
We believe that ion transmission through ultrathin film measurements can have an impact in
areas as diverse as ion transport in electrochemistry, astrophysics, radiation physics and chemistry,
ion beam deposition, the physics of atomic and molecular collisions at low energies and surface
analysis using electron, photon and ion beams.
Acknowledgements
The authors acknowledge, with pleasure, valuable discussions with Prof. R.E. Johnson. This work
was partially supported by the National Science Foundation, Grant CHE-9408367.
Appendix A. Ion-atom/molecule
collisions
Since most of the processes leading to attenuation of l-10 eV (low energy) ions in condensed films
having wide band gaps are believed to involve binary collisions between projectile ions and target
atoms in the surface layers, we now focus on the basic physics of atomic collisions, binary elastic
collisions, charge transfer processes, and ion-molecule reactions.
Collisions between ions and atoms can be classified according to the collision energy. If the
relative collision velocity of the two particles is fast relative to the motion of the electrons (u, >>0,; v,:
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M. Akbulut et al. /Surface Science Reports 28 (1997)
177-245
collision velocity; u,: electron velocity), then the electron motion during the collision can be
neglected, and the electron distribution is static, except for transitions which occur when the
particles are at their closest approach [77,78]. For low-energy collisions (0, <<u,), on the other hand,
the electrons adjust constantly to the changing potentials during the collision; this is an adiabatic
process. This means that the electrons of the two collision partners may be shared by both centers
during the collision. This results in a complex potential diagram with many possible transitions
between potential curves, leading to high cross-sections for inelastic processes such as charge
transfer. It is useful to remember that the speed of an electron of a ground state H atom is
2.2 x lo6 m/s, which corresponds to a collision energy of about 25 keV/amu. Hence all collisions that
we have been discussing in this paper fall in the limit of low-energy collisions.
A.1. Binary collisions: collision kinematics and elastic collisions
We begin with a two-body binary collision model [79]. If the collision time during the ion-solid
collision is much shorter than the period of a lattice vibration, the collision is called fast (or
impulsive). The collision time is defined as the time during which the projectile ion experiences the
repulsive interaction with a surface atom. (Assuming that the range of a repulsive interaction is
d - 1 A, one can estimate roughly the collision time z for a given ion velocity v, as z N d/v.) Assuming
that the collision is fast and elastic, any bonding forces acting on the target solid atoms during the
collision can be neglected and the target atoms are treated as initially at rest; the projectile has
already left the vicinity of the surface before the interaction between the target surface atom and its
neighbors becomes significant.
In elastic collisions, both momentum and total kinetic energy of the collision partners are
conserved during the collision [80-821. Fig. 23 shows a schematic of a two-body collision. Particle
A (ion) with mass M, and kinetic energy E,i scatters off particle B (target atom) (mass MB) which is
initially at rest. The scattering energy is 0,, and the final energy of A is E,,. Particle B has a final
energy of EBP,and the recoil angle is 8,. If there is no change in the internal energy of the system AB,
MA
Fig. 23. Schematic of an elastic two-body collision. Particle A with mass M, and initial kinetic energy EAi scatters off
particle B (mass B) and changes its energy to E,,. Scattering angles: 0, and Oa. Energy of particle B after collisions: E,,.
M. Akbulut et al. /Surface Science Reports 28 (1997)
177-245
217
which could, for instance, be the result of a charge transfer process, then we can write the equations of
conservation of energy and momentum as
(A.11
and
M,U,i = M,u,,
COS 8, + M,u,,
COS 8,,
64.2)
where uAi and uAf are the initial and final velocities of particle A, respectively, and uBf is the final
velocity of particle B. Through elimination of r+$fand 8Bwe can derive the formula for energy loss in
an elastic collision in the laboratory system:
(A-3)
This ratio is referred to as the kinematic factor and gives the energy transfer in a binary collision in
terms of the mass ratio p (p = MB/MA) and the scattering angle eA [81,82-J. For MB > M, only the
expression with the plus sign in Eq. (A.3) is valid. When the projectile ion is heavier than the target
atom (MB < M$ both plus and minus expressions in Eq. (A.3) hold, indicating that two final energies
are possible for each scattering angle. For MB < M,, the heavier projectile ion cannot be backscattered by the lighter target atom; there is only a limited regime of scattering angle. The maximum
scattering angle is given by sin 8, = MB/M,. Fig. 24 shows energy loss (E,,/E*,) as a function of
laboratory scattering angle for various values of p = MB/M, as given by Eq. (A.3).
For 8, = 90 ’ and MB > M,, Eq. (A.3) becomes particularly simple:
EAr/EAi= (MB - MA)I(MB + MA) = CL- I/P + 1.
Note that for MB < M, there is no solution possible for Eq. (A.4).
Fig. 24. The ratio E,,/E,,
as a function of scattering angles for various mass ratios (MA/MB). From [82].
(A.4)
218
M. Akbulut et al. / Surface Science Reports 28 (1997)
177-245
For a scattering angle of QA= 180 ‘, Eq. (A.3) reduces to
EAf/EAi
=
Ct”B - M*)/(“B + M*)12*
(A.3
For primary ion energies above lOOeV, it is well established that the scattering of ions from
solid surfaces can be easily explained in terms of elastic binary collisions with single surface
atoms, because the collision time is very much shorter than the period of a lattice vibration
[81,82].
For ion energies below lOOeV, the validity of the binary collision model has been addressed by
several authors [83-851. Hulpke [84] has reported that even for low-energy (2-20eV) ion-solid
scattering the vibrational period of the surface atoms is larger than the collision time. For example,
for a 10 eV Li f -W(1 10) surface collision the vibrational period of the W atoms at 190 K is 40 times
larger than the collision time, while at 2 eV the vibrational period of the W atoms is 18 times larger
than the collision time. These results indicate that the binary collision approach for ion-solid
interactions is still valid even at such low ion energies. However, a comparison of the experimental
results with the calculated energy loss from Eq. (A.3) shows that the calculated results deviate slightly
from the experimental results. Hulpke [84] has shown that in this low-energy range (2-20 eV) the
attractive image potential plays a very important role in determining the energy loss processes;
a better agreement between the experiment and the calculation has been found by modifying Eq.
(A.3) by including a potential step of height E (E, -+ E, + E,E, + E, + E).
Low-energy ion transmission through weakly bonded thin films (such as rare gas solids) can be
viewed as proceeding via a single binary collision or a series of binary collisions between a desorbing
ion and atoms in the films, because the vibrational period (- l/phonon frequency) of the
atoms/molecules in the film is much larger than the collision time of low-energy ions; for example,
the vibrational period of atoms in solid rare gas is - 1 x lo- l2 s, the collision time for 7 eV O+ is
- 1 x lo-i4 s (time for - 7 eV 0+ ion to travel N 1 A). However, the ion collision time may be
comparable with the lattice vibration time after the ion loses sufficient energy due to a series of
binary collisions in the film, and ion-phonon scattering in the film becomes important. Ion-phonon
scattering may be a very important energy loss process in the transmission of < 1 eV ions through
a molecular solid such as ice (sublimation energy of ice is - 0.5 eV/molecule).
A.2. Interaction
potentials
So far we have discussed the situation for a hard sphere elastic collision. Note that Eq. (A.3)
does not depend on the actual interaction potential. The details of the collision, such as the exact
trajectories of the particles, the differential cross-section, and the possible transitions, depend on
the nature of the interaction potential between the two colliding particles [SS]. In principle,
the primary force determining this potential is the Coulomb force. For high-energy collisions
(u, >>u,), nuclear repulsion dominates the collision, and the interaction potential can usually be
approximated by a screened Coulomb potential [20]. However, for the low collision energy
regime (a, <<UJ this approximation is not appropriate. In this energy range, both repulsive
(short range) and attraction (long range) interactions contribute to the interaction potential;
the interaction of the two collision partners at large distance, R (R > rA, rB; the atomic radii of
A, B), becomes important. The long-range interaction can sometimes be approximated by a power
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
2
L
6
219
a
Fig. 25. Adiabatic potential energy curves vs. internuclear separation, R, for H + -Xe system. From [87].
law potential:
V(R) =
1
CJR"
(A-6)
n
with C, being a constant. For ion-dipole interactions the leading term is n = 2; for interactions of an
ion with a neutral without a dipole moment, the ion-quadrufiole interaction dominates and the
leading term is n = 3. For the collisions between an ion and a polarizable atom (such as Kr, Xe), the
ion-induced dipole interaction dominates (n = 4). A reasonable interaction potential describing the
interaction of an ion with a polarizable atom/molecule (or a molecule having a permanent dipole
moment) can be obtained by superposition of the Lennard-Jones interaction and the ion-induced
dipole interaction (or the ion-dipole interaction).
At low energies (v, <<u,) an inelastic ion-atom/molecule
collision can be best viewed as involving
transitions between a set of quasi-molecular electronic states formed in the collision by the colliding
patterns. As an example, we depict in Fig. 25 the calculated potential energy curves for the H+-Xe
system [87]. It can be seen in Fig. 25 that there are a number of possible transitions at small
intermolecular distances. Some of these transitions can lead to charge transfer, which is discussed in
the next section. Therefore, for an exact description of scattering a knowledge of the potential energy
curves is necessary. These can be obtained from theoretical molecular orbital calculations and from
scattering experiments.
220
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
A.3. Charge transfer reactions in the gas phase
A charge transfer reaction between an ion and weakly interacting ultrathin film can be explained
by utilizing the knowledge of gas phase charge transfer processes, provided the film is very thin
( < 3 ML) so that the band structure is not well developed. The following is a discussion about charge
transfer processes in the gas phase and near solid surfaces.
Electron transfer in collisions between ions and atoms or molecules is important from a fundamental as well as a practical standpoint [12,88,89]. During collisions of multiply charged or singly
charged ions with atoms or molecules, several reaction pathways are possible. These include electron
transfer (single or multiple), electron transfer accompanied by target excitation or ionization, etc. In
this review, we are concerned with the one-electron charge transfer processes between a singly
charged ion and an atom or molecule, since this case is most appropriate to our experiments. The
reader interested in multiply charged ion-neutral charge transfer reactions is referred to reviews by
Kamber and Cocke [90], and Appell [91].
In a charge transfer reaction between a singly charged positive ion and atom/molecule, the
projectile ion picks up an electron from the target atom/molecule and becomes neutral. This process
can be described by the relation:
A++B-+A+B++AE,
(A.7)
where AE is the energy defect of the reaction, and A and B can be atomic or molecular species
[77,78,89]. AE is the total change in internal energy; when reactants are in their ground states, AE is
just the difference between ionization energies of A and B at infinite separation. For asymmetric
systems (A # B) AE can be either positive (exoergic) or negative (endoergic); these processes are
known as non-resonant charge transfer [12,88,89]. For exoergic reactions (AE > 0), no external
energy is needed for the reaction to proceed. For endoergic reactions (BE < 0), the needed energy
usually can be obtained from the kinetic energy of the projectile ion; there is a limitation on the
primary ion energy range to which the charge transfer reaction can proceed.
The one-electron charge transfer reaction between two identical particles (symmetric system) is
described as
A++A-,A+A+.
(A.8)
For symmetric systems, AE = 0 when reactants and products have the same internal energy state.
These reactions are classified as symmetric resonant charge transfer reactions [12,88,89].
One-electron charge transfer between an unlike ion and an atom can also be a resonant or nearly
resonant process if appropriate electronic states are available so that AE is nearly zero, and these
processes are called accidental resonant charge transfer [88,89].
In the following we discuss each charge transfer process separately. We first begin with a symmetric resonant charge transfer process.
A.3.1. Symmetric resonant charge transfer
The cross-section for a symmetric resonant charge transfer collision increases with decreasing
collision velocity. The variation of symmetric resonant charge transfer cross-section, c, with impact
velocity, V,is given by
a=(a-blnv)‘,
(A.9)
M. Akbulut et aLlSurface Science Reports 28 (1997) 177-245
221
impact velocity (cm/set)
Fig. 26. Resonant charge transfer cross-section for hydrogen (H+ + H -+ H + H+) as a function of impact velocity. From
~921.
where a and b are constants [89]. As an example, we depict in Fig. 26 the symmetric resonant charge
transfer collision velocity dependence of the process
H++H+H+H+.
(A.lO)
Sakabe and Izawa [92] present calculated and measured cross-section values for symmetric
resonant charge transfer for all non-transition elements. The data cover mainly the high collision
energy regime, but go down to as far as a few eV; note that for hydrogen a collision energy of 5 eV
corresponds to a velocity of 3 x lo6 cm/s.
Rapp and Francis [48] have shown that symmetric resonant charge transfer cross-sections
depend strongly on the ionization potential. Fig. 27 shows the calculated symmetric resonant charge
transfer & as a function of ion velocity for several systems [48]. It is apparent from Fig. 27 that the
lower the ionization potential the higher the charge transfer cross-section.
A.3.2. Non-resonant charge transfer
Experimental and theoretical gas phase studies have shown that cross-sections for non-resonant
charge transfer are small at low energies (u, < uJ, rise to a maximum, and fall off in the same way as
symmetric resonant processes for u, > u, [48,89,93]. However, at energies from thermal energy to
10 eV, the non-resonant charge transfer cross-sections may increase if the reactions are exoergic.
This is mainly due to the long-range polarization potential between the ion and neutral. In the
222
M. Akbulut et al./ Surface Science Reports 28 (1997)
10’5 2
2
s
10'
2
177-245
s
v. cm SK-’ -
Fig. 27. Variation of symmetrical resonant charge transfer cross-section
calculated by Rapp and Francis. From [48].
(X’ + X+X + X’) with impact velocity,
following, we first discuss the application of the Massey adiabatic criterion to non-resonant charge
transfer in order to explain the energy dependence of the non-resonant charge transfer crosssections.
The Massey criterion [13,89] (adiabatic maximum rule) states that if the time of collision is
comparable to the time of transition, the charge transfer cross-section c is maximum. The velocity at
which c is expected to peak is
(A.1 1)
Here a is known as the “adiabatic parameter”, the range of the interaction between the colliding
partners, u is the relative impact velocity, and AE is the energy defect. The collision time is defined as
a/v. If v C-C
aAE/h, v is small compared to the electronic velocity of the target (v,). The target
atom/molecule can have enough time to adjust the perturbation imposed by the projectile ion
without a charge transfer occurring; this type of collision is called adiabatic. Hence, at low energies
(E <<u2 1BE1 2m/2h2, but not smaller than 100 eV), the cross-section is small unless AE is very small.
Fig. 28 shows clearly that the smaller the energy defect, the larger the charge transfer probability at
least for energies down to N 100 eV. At very high velocities (v >>uAE/h), the cross-section decreases
with increasing ion energy, because the interaction time becomes too short for the transition to
occur; the collision is said to be sudden.
Experimental data analysis shows that the Massey criterion is very useful [94]. For example,
Hasted [ 13,891 has derived values of a using the experimental cross-section data for non-resonant
charge transfer. For a large number of data, Hasted has shown that a is quite large, a -N 7Aln, where
n is the number of electrons transferred in the collision.
M. Akbulut et aLlSurface Science Reports 28 (1997) 177-245
102
1oJ
10’
105
Energy
(eV)
Fig. 28. Charge transfer cross-sections
for protons
10’
223
10’
in rare gases. From [13].
The determination of a and A E can be rather difficult for certain collisions [13,89]. For example,
the determination of AE is complicated by the final states of the ion and neutral. Another factor
which can influence the energy defect during collision is polarization forces; the energy defect during
collision may be rather different from the energy defect at infinite nuclear separation due to
polarization forces. Since the determination of a and AE during collision is problematic, by using the
Massey criterion one may not accurately estimate the projectile velocity (or energy) for the
maximum. However, the Massey criterion can provide qualitative understanding of the energy
dependence of the cross-sections for non-resonant charge transfer.
Rapp and Francis [48] derived an approximation for non-resonant charge transfer processes at
low collision energies ( > 100 eV) which has proved to be very useful. For low energies ( > 100 eV),
the Rapp - Francis non-resonant charge transfer cross-section is reduced to the following form:
(A.12)
with y = JE,/13.6eV, E, is the ionization potential of B, U,is the collision velocity and a is the Bohr
radius. Eq. (A.12) indicates that the smaller the energy defect AE and the larger the collision velocity,
the larger the cross-section [48,93]. Fig. 29 shows the energy defect (AE) dependence of the
non-resonant cross-sections calculated by the approximation of Rapp and Francis. Fig. 30 compares the experimental cross-section curves for charge transfer in collisions of Hf with Xe and Kr
atoms with the calculated cross-section curves for A E = O.l,OS, 1,3,5 eV. Since the rare gas ions
may be formed in either 2P1,2states or 2P3,2states [48,93], the energy defects for H+-Kr and H f -Xe
collisions are:
H++K~+H+KI-+(~P,,J,
AE= -0.4eV,
(A.13)
H+ + Kr-+H + Kr+(2P,,J,
AE = - l.OeV,
(A.14)
H++Xe+H+Xe+(2P,,,),
AE= +lSeV,
(A.15)
H+ + Xe--+H + Xe+(2P,,J,
AE = +0.2eV.
(A.16)
224
M. Akbulut et al. JSurface Science Reports 28 (1997) 177-245
i-
.
i-
/4
/ -
1
Impact velocity (cm s-l)
Fig. 29. Cross-section for charge transfer calculated by the approximation of Rapp and Francis; (a)-(c) refer to cases in
which ionization potentials are chosen to correspond, when AE = 0, to the symmetrical charge transfer reactions He+-He,
H+-H, and Cs+-Cs, respectively; (-) symmetrical resonant case (AE = 0); (-----) non-symmetrical resonant case with
IAEl as indicated in eV. From [48].
As seen in Fig. 30, for H ‘( D+)-Xe collisions, although the observed cross-sections for both sets of
data are different in absolute magnitude, the data show maxima located at nearly the same impact
velocity. This corresponds to BE = 1 eV. This is suggestive that Xe+(‘P,,,) is the predominant
product. For H+-Kr collisions the observed data disagree both in absolute magnitude and location
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
4
6
810’
Impact vdociry
01
IOQ
24
68
(cm 3-l)
I
I
I
f
1
t
2
4
I
10’
2
+
velocity
km 9)
Impact
225
I
tw
I
1
2
Fig. 30. Cross-sections for charge transfer collisions between H+ ions and (a) He, Ne and Ar atoms, (b) Kr and Xe atoms.
Numbers refer to authors as indicated in the references below; (-) calculated by semi-empirical method by Rapp and
Francis for different assumed energy defects as indicated; (-0 -o-) calculated for H+-He by Green [139]. Actual energy
defects are 11.0,8.0,2.2 eV for H+ collisions with He, Ne and Ar, respectively (From [48,93]). 1. J.B.H. Stedeford and J.B.
Hasted, Proc. R. Sot. A 227 (1955) 466; 2. J.B. Hasted, Proc. R. Sot. A 212 (1952) 235; 3. M. Becher and A.Z. Scharmann, Z.
Naturf. 24 (1969) 854.
[93]. Therefore, based on these experimental data it is difficult to draw a conclusion about the Kr
ion state.
Using Eq. (A.1 1) (the Massey criterion) and taking a - 7 A, one can estimate the peak velocity for
the H ‘-Xe collision to be - 2.6 x lO’cm/s ( - 360eV) for AE = 1.5 eV and - 3.4 x lo6 cm/s
226
M. Akbulut et al. /Surface
Science Reports 28 (1997) 177-245
(A+e++B
Nuclear Separatron
Fig. 31. Effective potential energy curves for the trajectories leading to a transition. The curves cross at an internuclear
distance R = R,. From [90].
( 1~6 eV) for AE = 0.2 eV. A comparison of these results with the experimental result shown in Fig. 30
indicates that (a) reaction (A.15) is dominant and (b) the Massey criterion and the Rapp-Francis
model are very useful to predict the final state of the collision products.
For low-energy collisions (0, CU,), a charge transfer reaction can be described as a transition
between electronic states of a temporary molecule formed as an intermediate state by the colliding
partners during the collision. A typical charge transfer process at low energies is
A+ +B-QAB)++A+B+.
(A.17)
A charge transfer is assumed to be induced at curve crossings in the temporary molecular potentials
or between temporary states which are energetically close at finite internuclear distance [ 13,88,95].
For example, note that some potential energy curves of the H+-Xe system cross at particular
Hf-Xe internuclear distances, as shown in Fig. 25. Hence, the exact value of the cross-section
depends strongly on the details of the potential energy curves.
One of the most useful formulae for predicting low-energy charge transfer cross-sections was
developed independently by Landau, Zener and Stuckelberg [ 13,881. The Landau-ZenerStuckelberg (LZS) model is based on the crossing of two potential-energy curves at a finite
internuclear separation R,. According to the LZS model, the charge transfer takes place only at the
crossing point. As shown in Fig. 31[903, for impact parameter less than R, the system passes over the
crossing point twice, on the incoming and outgoing passages. p is the probability that a transition
from one potential curve to another occurs in traversing the crossing point and (1 -p) is the
probability that the system remains on the same potential curve. If a transition occurs on the first
passage but not the second then the total transition probability is [p(l - p)] and vice versa
[( 1 - p)p], this leads to a net change in the state of the system (charge transfer). Therefore, the
M. Akbulut et al. / Surface Science Reports 28 (1997) 177-245
221
averaged charge transfer transition probability P,,, at the crossing point R, is given by
PLZS
= 2P(l - PI.
(A.18)
The crossing probability
p
is given by
-2)
p=l-exp(
(A.19)
with
Al/
Of
_
dvf dvo
I
dR
I
dR R=R,’
(A.20)
Here, the quantity H,, is the transition matrix element, u is the relative velocity, V, and I’, are the
initial and final potential energies and R is the internuclear separation. Then, using the LZS model,
the charge transfer cross-section can be written in the form [95]
fsLZS
=
(A.21)
f’,,z(&).
Note that LZS model breaks down in the threshold region where the distance of closest approach is
approximately equal to R,.
Demkov developed a model considering non-crossing interaction potentials [13,39]. For a review
of the theoretical low-energy charge transfer models, readers are urged to refer to books by Johnson
[95], McDanielet al. [13] and Bransden and McDowell [96], and review papers by Hasted [89] and
Johnson [SS].
Although non-resonant charge transfer collisions in the collision energy regime higher than keV
have been studied intensively, there exist only limited reports in the low collision energy range
l-100eV. Based on the available experimental studies, there does not appear to be a trend in the
non-resonant charge transfer cross-sections in the low collision energy range l-100eV [l&13]. At
these low energies, charge transfer reactions can take place at relatively large impact parameters and
the deflection of the projectile by the target may be negligible (glancing collision).
At ultralow energies (0.025-l eV), charge transfer between an ion and a neutral occurs during
orbiting due to long-range polarization potential [12,13,95]. The orbiting effect increases the
probability for a charge transfer reaction if the reaction is exoergic [88,95]. There are several
classical theoretical treatments of ion-neutral atom/molecule collisions. At ion energies much below
1 eV, the charge transfer cross-sections can be estimated according to the Langevin ion-induced
dipole theory or according to the average dipole orientation (ADO) theory [97]. The Langevin pure
polarization theory considers the interaction between an ion and a non-polar atom/molecule,
assuming that both ion and neutral are point particles with no internal energy. If the reaction is
exoergic, the Langevin capture cross-section and rate constant are given by
CC=
(A.22)
,
(A.23)
228
M. Akbulut et al./Surface Science Reports 28 (1997)
177-245
where aP is the polarizability of the neutral, e is the charge on the ion, E, the relative energy of the
system and M, is the reduced mass. The Langevin ion-induced dipole theory predicts that the
microscopic capture cross-section is inversely proportional to Ef’* and the rate constant is
independent of velocity v. The Langevin theory only agrees well with a few simple low-energy
ion-non-polar molecule reaction rate constants but underestimates the rate constants of most of
ion-polar molecule collisions.
Various theoretical models of ion-molecule collisions in which the neutral molecule has a permanent dipole moment have been developed. For example, the ADO theory, which includes the
contribution of the ion-dipole potential to the collision cross-section, predicts quite accurately both
the magnitude of the rate constants and their dependence on dipole moment for some selective
ultralow energy charge transfer reactions [97]. However, both ADO and Langevin theories estimate
cross-sections for ion-molecule reactions very poorly at energies higher than 1 eV. The high-energy
limit of the collision cross-section approaches zero, because the collision partners are assumed to be
point particles in these theories.
Low-energy ion-molecule non-resonant charge transfer collisions are expected to be significantly different from ion-atom charge transfer collisions, provided that the molecule has a
permanent dipole moment [12,97]. In addition, in ion-molecule charge transfer reactions, at
least one of the collision partners is a molecule, the vibrational and rotational excitations may play
an important role during charge transfer reactions. Fig. 32 shows measured cross-sections for
1.0
0.8
0.6
.,
_
4
Impact
Fig. 32. Observed cross-section for charge transfer reactions
et al. [140], (-) observed by Koopman [141]. From [93].
Ii
8
energy (
I2
IO'eV)
ofH+ with O,, N,, CO, and H,O; (---) observed by Stebbings
M. Akbulut et al. /Surface Science Reports 28 (1997) 177-245
229
H+-0,
(AE = + lS3eV), H+-N2 (AE = 1.98eV), H+-CO2 (AE = O.l7eV), and H+-HZ0
(AE = + 0.98 eV) charge transfer reactions [93]. It is clear from the shape of the curves shown that
the factors determining the ion-molecule charge transfer cross-sections are considerably more
complex than for atoms.
A.3.3. Accidental resonant charge transfer
Accidental resonant charge transfer processes (charge transfer processes between an unlike ion
and an atom/molecule when the energy defect is zero or very small) exhibit characteristics similar to
either the symmetrical or the non-symmetrical charge transfer process [93].
The O’(4S) + H(ls) +O(3P,) + H+ collision presents a very good example of an accidental
resonance charge transfer process [S9,93]. The energy defect of the 0+-H charge transfer collision
depends on the total angular momentum J of the oxygen:
0+(4S)+H(1~)+0(3PJ)+H+
AE= -O.OleV
(J=O),
A E = 0.00 eV
(J = 11,
i AE= +O.O2eV
(A.24)
(J=2).
The measured charge transfer cross-sections as a function of ion energy for the 0+-H charge
transfer collision and the H+-H symmetric charge transfer collision are shown Fig. 33. A comparison of the Of-H charge transfer cross-section with the H+-H charge transfer cross-section
indicates that both exhibit cross-sections which decrease monotonically with increasing impact
energy [89,93]. However, note that the 0+-H charge transfer cross-section is smaller than the
H +-H charge transfer cross-section. This can be understood by the fact that interaction energies of
initial and final states are not identical at a finite nuclear separation. Although the energy defect is
zero at infinite separation, it is non-zero as the projectile approaches the target atom; the difference
in polarizabilities induces an effective energy defect. In the symmetrical resonant process, the energy
defect is always zero, independent of the nuclear separation.
A.4. Ion-molecule chemical reactions
In this paper, we refer to ion-molecule
of an ion-molecule collision, such as
A+‘-‘+ BC+AB+‘-‘+
C.
chemical reactions when new species are formed as a result
(A.25)
At low energies (l-lOeV), the ion-molecule chemical reaction cross-section depends on whether
a reaction is exoergic or endoergic. For the endoergic reactions, the reaction cross-sections are
generally zero up to a threshold energy and increase rapidly as the translational energy increases
above the threshold energy (Fig. 33) [8,98-1001. At low energies, the typical ion-molecule reaction
cross-section is - lo-i6cm2. Many gas phase ion-molecule reaction constants are listed in the
literature (e.g., [44,101]).
Several different ion-molecule chemical reaction channels between a projectile ion and a target
molecule can be possible [S,SO]. For example, in the case of - 7 eV O+ and N 4 eV F + transmission
through H,O overlayers, the ion-molecule reactions:
230
M. Akbulut et al./Surface
Science Reports 28 (1997) 177-245
L------
2.0
cl
ICI
E/E.
n;(v
I
I
0)
l
Hr-HeH*
1
l
I
n
I
0.15 -
>
it 0.10 3
0.05 -
0
0
t
Em
2
4
(I
8
Et/N
Fig. 33. (a) Translational energy E, dependence of the reaction cross-section for reactions with and without energy
threshold (from [98]). (b) Translational
energy E, dependence of the reaction cross-section for reaction
H: + He -+ HeH+ + H (from [99]).
M. Akbulut et al. / Surface Science Reports 28 (1997)
177-245
231
0+ + H,O -QOH)+ + OH,
(A.26)
F+ + H,O +(OH)+ + HF,
(A.27)
F+ + H,O+(HF)+
(A.28)
+ OH
are energetically possible; reaction (A.26) is endoergic by w 0.4 eV, reactions (A.27) and (A.28) are
exoergic by N 1.0 and 4.8 eV, respectively.
Appendix B. Charge transfer processes between ions and solids
In the following, we present a very brief discussion of ion-solid charge transfer processes.
A detailed and complete account of the charge transfer processes near surfaces can be found in
papers by Nordlander [14], Yu and Lang [102] and in a forthcoming review paper by Nordlander
[103].
Charge transfer is the dominant inelastic process during low-energy ion or atom scattering from
surfaces as well as during the desorption of energetic particles from solids [14,104-1071. Charge
transfer between an ion and a solid can occur via two mechanisms: (a) Auger neutralization and (b)
resonance neutralization. These processes depend strongly on the ionization potential and the
electron affinity of the ion (or atom) as well as the work function of the solid. The energy levels of the
energetic ion near a surface broaden and shift (the ionization level is shifted up, while the electron
affinity level shifted down) due to the screening by the electrons in the metal.
In Auger neutralization, the Coulomb field of the ion induces an electronic transition in the solid
involving two electrons, if unoccupied electronic states (hole states) of the ion are located well below
the conduction band states of the solid (Ei > 2e& Ei is the ionization energy of the ion, and 4 is the
work function of the solid). The neutralization of positive ions (except alkali metal ions) by a surface
is mainly due to Auger neutralization processes, because unoccupied positive ion states are typically
located below the bottom of the metal valence band. Positive alkali metal ions and negative ions near
a metal surface usually undergo charge transfer by resonance tunneling from the occupied level of
the metal or into the unoccupied level of the metal, respectively.
A change in the work function of the substrate could affect the resonant neutralization ion
probability significantly. There have been several studies demonstrating the influence of the change
in the work function on the ion yields [47,102,108,109]. Since adsorption of alkali metal atoms on
a metal surface decreases the work function of the surface significantly, in some of these studies the
effect of submonolayers of alkali metal on the secondary ion yields were investigated. For example,
Joyce et al. [lOS] studied the coadsorption of potassium, K, with PF,/Ru(O 0 0 1) and its influence on
F- and F* (metastable F neutrals) yields. Fig. 34 shows the influence of K on the desorption yields
from PF, chemisorbed ( < 10% of saturation coverage) on Ru(0 0 0 1). As seen in Fig. 34 the F + yield
decreases with increasing K coverage, while the F- and F* yields increase. The authors argue that an
explanation of the decrease in Ff is not straightforward due to a complex chemical interaction of
K with PF,, while the increase in the F- and F* yields can be explained due to a decrease in the work
function. Since K decreases the work function of the substrate by several eV, this increases the Fand F* desorption yields by decreasing the neutralization probability of ESD-produced F- ions, as
illustrated schematically in Fig. 35.
232
Akbulut et aLlSurface Science Reports 28 (1997) 177-245
PF3 Coverage
it
0.00
0.02
= 0.02 ML
0.04
Pota88hn
0.06
Coverage
0.08
0.10
0.12
(ML)
Fig. 34. Electron stimulated desorption yields of F+, F- and F’ from 0.02 ML of PF, coadsorbed with K on Ru(000 1).
From [lOS].
Appendix C. Ion desorption from surfaces
Since the main focus of this paper is the depth of origin of secondary ions from the surfaces of solids,
we briefly summarize in the following section the principles of ion desorption from surfaces induced by
various electronic and momentum transfer processes. We discuss desorption induced by electronic
transitions (DIET) (electron and photon stimulated desorption (ESD/PSD)) and ion sputtering.
C.1. Electron/photon
stimulated desorption
In ESD/PSD, beams of energetic electrons or photons (usually I 500 eV) impact on solid surfaces
containing either adsorbed atoms or molecules, or terminal bulk atoms. These can induce electronic
excitations to states that are repulsive in character, and the subsequent conversion of potential
energy into motion of the excited species can result in the emission of energetic particles (e.g., positive
and negative ions, and neutral species, including metastables) from the surface layers of the solid.
Depending on the quenching rate of the excitations, the desorption of energetic species from the
surface may occur (see Appendix C.2) [ 1lo]. Extensive reviews on ESD and PSD phenomena can be
found in the literature [2-4,ll l-l 161.
Since the penetration depths of electron or photon beams (energies of hundreds to several
thousands of eV) incident on solids are of hundreds to thousands of &rgstrbms, electronic excitations
can be produced far below the surface. Hence, for DIET of atomic or molecular multilayer films, or
compound materials, subsurface layers might contribute to the desorption signal.
C.1 .I. Mechanisms of desorption induced by electronic transitions (DIET)
Direct momentum transfer between an incident low-energy (I 500 eV) electron and an adsorbed
species (even the lightest adsorbed species) is generally insufficient to lead to anything but
233
M. Akbulut et al. J Surface Science Reports 28 (1997) 177-245
b
b
4
= 3.5
E
0 = 5.5
4
B
E vat
eV
e* = 4.7 eV
%
B/B
E vat
E vat
b
2
Fig. 35. Schematic of the energy levels of F relative to the substrate
surface region, for three different work functions. From [lOS].
Fermi level and the shifting of these levels in the near
vibrational excitation, so that desorption must result from electronic excitation of the adsorbate, the
adsorbate-substrate
bond, or the bulk atoms. In this energy range, the electronic excitations leading
to ESD/PSD ion or neutral desorption can involve either valence or core level excitations.
Several models have been developed to explain DIET from surfaces. The Menzel-GomerRedhead (MGR) model and Knotek-Feibelman
(KF) models have been very successful in explaining desorption from covalent adsorbates and desorption from ionic adsorbates, respectively. In the
following we summarize the fundamentals of ESD/PSD.
C.l.l.l.
Menzel-Gomer-Redhead
(MGR)
model. The MGR model provides a good starting
point for a discussion of desorption induced by electronic transition processes. The MGR model is
234
M. Akbulut et al. /Surface Science Reports 28 (1997) 177-245
Fig. 36. Schematic potential energy diagram illustrating stimulated desorption of surface species for an adsorbate
(A)-substrate(M) system. Electronic excitation from the attractive ground-state potential curve (M + A) to a repulsive
excited-state potential curve (such as MA’, MA+, MA +*)can lead to desorption of energetic species.
very general [2,3], and is based on a description of electronic excitation and dissociation of gaseous
molecules. According to this model, the initial excitation is sudden (Franck-Condon-like)
occurring
on a timescale that is short in comparison to nuclear motion. Fig. 36 shows schematic potential
energy curves for an adsorbate(substrate(M)
system to illustrate the MGR model. The system is
assumed initially in a ground-state configuration (MA), and a sufficiently large excitation energy can
make a Franck-Condon
(FC) transition from the (MA) state to a repulsive excited-state (such as
MA*, MA+, MA+*) possible, as shown in Fig. 36. After transition to a repulsive excited-state
potential, the adsorbed species acquires nuclear motion (Fig. 36). If this excited state has a sufficiently long lifetime, the excited species can gain enough kinetic energy to desorb from the surface [4].
(The time required to break the surface bond is typically - lo- r4 s, corresponding to the time
necessary for a typical ESD ion to travel - 1 A.)
In the gas phase, if a molecule is excited to an antibonding state, it will typically dissociate.
However, for a molecule adsorbed on a metal surface, there are final state effects (such as
reneutralization and image force) that can influence the desorption probability. At the surface, the
electronic excitations can be quenched rapidly; the de-excitation can proceed via reneutralization or
by “a bond-healing” transition due to charge transfer to (or from) the desorbing species, e.g., by
resonant tunneling or Auger neutralization [ 1lo]. Quenching of the excitation (de-excitation) can
lead to recapture of the excited species, unless it has already gained an amount of kinetic energy
M. Akbulut et al. /Surface Science Reports 28 (1997) 177-245
235
sufficient to result in desorption along the ground-state potential curve. Desorption along the
excited-state curve occurs if no quenching transitions occur. Since the quenching processes at
surfaces can be very efficient, the overall desorption cross-sections are smaller than those for
gas-phase dissociation, even when the primary excitation cross-sections are comparable. The typical
cross-sections for gas phase dissociative ionization are N lo-i6 cm2, whereas the maximum crosssections are in the range of 10-20- 10-23cm2 for desorption of ions and in the range of lo-‘*10-20cm2 for desorption of neutrals from surfaces [4]. Thus the ESD cross-section may be
expressed in the form
0 = o,P,
(C.1)
where ce is a primary excitation cross-section and P is an escape probability [3] (see Appendix C.1.3).
The MGR model has been successful in estimating the magnitude of P, but it cannot be used to
predict specific excitations that will lead to desorption.
C.1.1.2. Knotek-Feibelman
(KF) model. The model proposed by Knotek and Feibelman describes a mechanism for positive ion desorption from ionic solids [2,3,115]. The KF model is
applicable to maximal valency materials, such as TiO, and WO,. In these oxides (maximal valency
materials), the cation (metal ion) is ionized down to the noble gas configuration, such as
Ti+4[...3p6] in TiO,, and the highest occupied electronic level of the cation is a core level. This
model explains both the observed thresholds and the large charge transfer involved in the ESD of
positive ions (such as O+ and OH+) from maximal valency oxides.
In the KF model, the desorption of ions from surfaces is initiated by the creation of a core-hole on
a cation site at the surface [115-J.This core-hole can be filled by an Auger decay process, resulting in
a multihole state. Since in the maximal valency configuration there are no valence electrons on the
nearest neighbor cations, the core-hole decay can be described as an interatomic Auger process, and
two additional electrons are ejected, as shown in Fig. 37 for an Of desorption from a TiO, surface.
The interatomic Auger decay of the core-hole creates a two-hole (2h) positive anion. In this way
a positive ion is formed at an initially negative ion site leading to a repulsive Coulomb interaction
(reversal of Madelung potential), which provides the driving force for expulsion of a positive ion
from the surface.
C.1 .I .3. Auger stimulated desorption model. An extension of the KF description of ion desorption
is the Auger stimulated desorption (ASD) model, which is applicable to both covalent and
non-maximal valency materials, such as adsorbed gases (CO, NO) as well as Cr,O, and WO,. The
ASD model gives a generalized picture of the desorption of ions from surfaces following creation of
a core-hole and Auger decay [117-1221. This model for ion desorption is based on the ionization of
a core level as the primary process and the production of an electronic state with multiple holes that
are created by an Auger process. If the holes are localized for a sufficiently long time in a bonding
orbital, a hole-hole repulsive state can arise resulting in the desorption of a positive ion. The
localization of the holes is possible if the effective hole-hole repulsion U’ is greater than the valence
bandwidth I/ (U’ > V) [2,123].
Recent experimental and theoretical studies on ESD from adsorbed molecules (such as N,, CO,
H,O, C6H,IF, C,H,F, and PF,) have shown that multi-hole final states (such as 2hle and 2h) may
initiate the desorption of positive ions from surfaces [3&l 17-1201. The presence of multi-holes in
236
M. Akbulut et al. /Surface Science Reports 28 (1997)
177-245
Auger Electrons
-_Fermi Level
\
IY
f Valence Band
02s
Ti 3p
Ti*+
02-
Fig. 37. Schematic diagram of the Knotek-Feibelman
(KF) model.
a covalent system can result in a repulsive interaction between the unscreened nuclei and subsequent
formation of ion fragments. ESD and PSD measurements from adsorbed CO, NO and N,O at
excitation energies near the 0 1s core level have shown dramatic increases of O+ production (by up
to a factor of 50-100) [113,122,124]. Although the core excitation cross-sections are much smaller
than the valence excitation cross-sections, the ion desorption probability for excitations caused by
core ionization is higher than that from valence excitations. An initial core excitation in a covalent
system may lead to a two or three valence-hole Auger final state that is highly repulsive, and result in
the desorption of ions.
C.1 .I .4. Negative ionformation. There are important differences between desorption mechanisms
for negative and positive ions from surfaces under photon and electron bombardment. ESD positive
ion emission is generally initiated by valence or core ionization processes, as discussed above, and
the threshold energies for ESD positive ion emission are typically > 15 eV [4,123]. However, the
threshold energies to cause ESD of negative ions can be considerably smaller than for positive ions
(O-15eV) [125,126] and the mechanisms of ESD of negative ions are substantially different.
Negative ion production by electron impact can proceed via dissociative electron attachment (DA
or DEA) and/or DD [125,126]. For electron energies from 0 to N 15 eV, negative ion (anion)
formation proceeds via the temporary capture of the incident electron to form a short-lived negative
ion resonance, which is dissociative in the FC region. Dissociative electron attachment (DEA)
processes usually proceed via one-hole two-electron states. For a diatomic molecule AB this process
may be represented by the relation
e+AB+[AB]-+A+B-.
(C.2)
At the surface of a solid, an anion must have sufficient kinetic energy to overcome the induced
electronic polarization potential to produce an ESD signal. The most detailed studies of DA
processes have been performed by Sanche and co-workers on condensed and physisorbed atomic
and molecular layers [75,125-1271.
M. Akbulut et al. /Surface Science Reports 28 (1997) 177-245
237
I i-
DEA
DD
.e
4-
5
_
d
2"
z
a
._
2-
t
G
l-
0
2
4
6
0
lo
12
14
16
18
20
Electron Energy (eV)
Fig. 38. Typical negative fluorine ion yield as a function of electron energy from a 10 ML thick PF, layer on a Pt surface.
At electron energies above about 15 eV, negative ion formation may proceed via the formation of
an electronically excited neutral intermediate state which decays by DD. For a diatomic molecule
this process may be represented by
e+AB-+[AB]*+A++B-+e.
(C.3)
This occurs at energies which lie above the dissociation energy of the fragments (e.g., A+ + B-). The
DD process produces a signal, which, beyond threshold, increases monotonically with electron
energy. Fig. 38 shows a typical negative ion yield as a function of electron energy from a thick PF,
layer on a Pt substrate [38].
C.l.2. ESDIPSD ion angular distribution and kinetic energy distribution
In ESD/PSD, ions do not desorb isotropically from a surface; ions are often observed to desorb in
well-defined cones determined by the bond direction which is ruptured by electron or photon
excitation. Measurements of the ion angular distribution (IAD) can provide direct structural
information about adsorbed species on the surface.
The ESDIAD experimental technique has been successfully used to obtain direct information
about the structure of the molecules oriented on surfaces, and to study dynamical aspects of surface
species [3,4,128].
The initial direction of ion desorption in ESD occurs along the chemical bond direction which is
ruptured by electron impact excitation, because the excitation to a localized repulsive state is short
on a timescale compared to characteristic vibration times of molecules. ESD ions often desorb in
M. Akbulut et al/Surface
238
Science Reports 28 (1997) 177-245
electron
beam
\
F,
Fig. 39. Illustration
F+
of the relationship
between
surface bond angle and ion desorption
angle in ESDIAD.
specific directions with respect to the substrate symmetry axis. That is, ground state bond angles are
directly related to ion desorption angles in measurements of ESDIAD. Therefore, the measurement
of the angular distribution of the desorbing species can provide information about the bonding
geometry of species adsorbed on surfaces. As illustrated in Fig. 39, if the primary excitation leads to
breaking of the C-O bond of CO bonded in a “standing up” configuration on a surface, then the
oxygen ion (one of the possible desorption products) desorbs on trajectory from the surface that
is perpendicular to the terrace plane. ESD of H+ from H,O bonded via the 0 atom of “inclined”
OH, and ESD of F+ (6r F-) from PF, bonded through the P atom to the surface are expected to
occur away from the surface normal, as shown in Fig. 39. The width of the angular distribution is
determined largely by the amplitude of bending vibrations of the surface molecule [3,4].
The energy distributions of ESD/PSD ions generated at the topmost surface layer display peaked
structures, and the ion energies usually range from 1-15 eV. The ion energy distribution can provide
information about both the chemical state of the surface and the ESD process [129,130]. However,
care must also be taken in interpretation of the ion energy distribution; the final state effects (such as
image force) can influence the ion energy distribution (see Appendix C.1.3).
C.l.3. Final state efects in ion desorption
As ions desorb from the topmost layer of a solid surface containing either adsorbed atoms or
molecules, or terminal bulk atoms, they interact with the surface in two ways which can change its
state of charge and its trajectory: (a) surface neutralization and (b) image force interaction [131,132].
Surface neutralization means that there is an electron transfer from the desorbing ion to the
surface o,r from the surface to the ion. This charge transfer takes place at ion-surface distances of up
to a few A, and depends strongly on the electronic states of the ion and the surface. Usually, the ion
M. Akbulut et al. / Surface Science Reports 28 (1997)
177-245
239
survival probability, P, is modeled as
P = exp
[
7R(z(t))dt
1
Z0
with z(t) being the time dependent ion-surface separation, 2, the initial ionsurface
R the reneutralization rate:
R(z)=Ae-a2
(C.4)
separation, and
(C.5)
(A and a are constants). In Appendix B, we present a more general discussion about charge transfer
processes between ions and solids.
The image force interaction between a desorbing ion and the surface leads to a change in the
trajectory of the ion. Since the interaction is attractive, the image force causes an increase in the polar
desorption angle of an ion leaving a planar surface: the ion trajectory is bent towards the surface.
Since the larger the desorption angle with respect to the surface, the more time the ion spends near
the surface, the image force results in an increase in the ion neutralization probability. Thus, there is
preferential reneutralization of ions desorbing off-normal, which can affect the measured ion angular
distribution [131,132].
Miskovic et al. [ 1321 have developed a classical model describing the influence of the image force
interaction on ion desorption processes. The screened image potential in which the ion moves is
approximated by
v,(z) = V,l(z
+ z0)
with the image potential at the initial ion-surface
V, = - e2/4(s, + k-i).
(C.6)
separation s,,
(C.7)
Here k-’ is the Fermi-Thomas screening length, and s0 is the distance of the ion from the surface
image plane at the instant the ion is formed with initial kinetic energy E, and z,, = s0 + k-i. The
z-axis and the x-axis are chosen along the surface normal and parallel to the surface, respectively.
Fig. 40 shows the calculated trajectories of desorbing and trapped ions in the image potentials [132].
These results show clearly that the image potential causes an increase in the polar desorption angle
of an ion leaving a planar metal surface.
In conclusion, the ion escape probability from a metal surface increases with increasing ion kinetic
energy and with decreasing polar angle of desorption (towards the surface normal). The trajectories
of the ions are most strongly bent towards the surface by the image force for larger polar angles of
desorption and for small ion kinetic energies.
C.l.4. Substrates
There are three types of substrates which are commonly studied in ESD/PSD experiments. The
first type is a metal or semiconductor surface covered by a monolayer of adsorbed gas. Another
common substrate for ESD/PSD studies includes homogeneous compounds, such as oxides,
fluorides, etc. The stoichiometry in the surface layers may not differ significantly from that in the
bulk. Since the primary radiation in ESD or PSD experiments can penetrate well beyond the first
monolayer into the solid and cause electronic excitations or ionization in subsurface layers, the
desorbing particles could, in principle, originate below the surface. The question (which is addressed
M. Akbulut et aLlSurface Science Reports 28 (1997) 177-245
240
I-
b
3
.
N
4
2
E
20
IO
30
-X*
Fig. 40. Trajectories of desorbing and trapped ions in the image potential x* = x/z,y lateral coordinate, z* = z/zO:
ion-surface distance (both dimensionless). BO:initial desorption polar angle, BC:critical initial desorption polar angle (for
B0> 8,, the image potential will bend the ion back to the surface. From [132].
in Sections 3 to 5) is whether they are able to traverse the surface layers and escape from the surface as
charged particles.
A third type of substtate is multilayer adsorbates (condensed gases, etc.). Similar to the compound
surfaces, a fraction of the ions may be generated below the surface, and their probability for
transmission through the surface layers determines whether they can desorb from the surface.
C.2. Sputtering
Besides electrons and photons, ion bombardment (sputtering) of a surface can also lead to the
desorption of ions or neutrals (sputtering) [95,133-1351. An ion with a kinetic energy of a few eV up
to several MeV impacts a solid; depending on the energy, the ion can penetrate from a few
monolayers up to several pm. On its way into the solid, the ion loses energy by collisions, which leads
to motion of atoms in the solid. If the target atoms are close to the surface, they can desorb (either as
ions or as neutrals); in some instances, these atoms can collide with and eject other atoms from the
surface layer [27,28-J. These processes are indicated in Fig. 41.
The two basic physical processes that lead to the sputtering of solids by energetic ion bombardment are knock-on (nuclear elastic) collisions and electronic excitations. When an energetic ion
collides with a target solid atom it can lose a substantial fraction of its energy to the target atom; the
M. Akbulut et al. 1 Surface Science Reports 28 (1997) 177-245
241
ion -
1_---
secondary
secondary
_--.
ions,
neutrals
solid
Fig. 41. Illustration of sputtering. A primary ion impacts on a surface and penetrates into the solid. Secondary ions and
neutrals are emitted from the surface through interaction of the primary ion with the solid and through secondary
processes.
target atom recoils into the solid. If the energy transferred to the target atom is larger than the local
binding energy of atoms within the solid, the target particle overcomes the binding forces. It can
either escape from the solid or collide with other target atoms and transfer energy and momentum to
the surface, leading to substantial atom displacement in the solid and particle ejection. This process
is the so-called “knock-on sputtering” or “nuclear sputtering”. The sputtering yield depends strongly
on the amount of energy deposited at the surface and the sublimation energy of the materials. The
number density of atoms set in motion is a very important parameter determining the momentum
transport in the solid.
In electronic sputtering, the incident ion causes excitations of the solid. Secondary ions or neutrals
can be created as a result of these processes. Electronic excitations and ionizations in the solid can be
produced two ways by ion impact. First, the fast ion can transfer sufficient energy to cause excitation
or ionization during a close collision with a target electron in a binary encounter. Second, a fast
charged particle produces a time varying field. If the frequency components of this field can be
absorbed by a target atom, this can initiate dipole excitations and ionizations.
Sputtering in metals is mainly due to the energy deposited in elastic nuclear collisions, whereas
sputtering in inorganic insulators (such as rare gas solids, water ice, carbon dioxide ice) is determined
by the electronic energy deposition processes. Since in condensed atomic and molecular films (large
band gap insulators) the electronic excitations are localized, and the electronic excitation energies
are larger than the cohesive energies, the electronic energy deposition by ions can result in the
motion of lattice atoms or molecules [136-1381. This is not the case for metals; because the
M. Akbulut et al. 1 Surface Science Reports 28 (1997) 177-245
242
electronic excitations in metals diffuse rapidly from their origin around the incident particle track,
the energy density is not sufficient to eject an atom or molecule from the solid.
The ability of a solid to slow down and stop an incident high-energy ion at an incident energy E, at
some characteristic depth is expressed by the stopping power. The total stopping power can be
expressed to a first approximation as a sum of the nuclear (elastic) and electronic (inelastic)
contributions,
dE,/dx
= (dE,/dx),
+ (dE,/dx),.
(C.8)
The first term is the elastic-nuclear stopping power, while the second term is the electronic stopping.
The total stopping power can also be expressed in terms of the nuclear and electronic cross-sections
W,) and UE,) as
dE,W = N{W,) + W,))>
(C-9)
where N is the material number density. The reader interested in more details about sputtering and
ion stopping is referred to books by Johnson [95] and Nastasi et al. [6].
One of the main differences between sputtering and ESD/PSD processes is that in ESD/PSD only
a few bonds are broken in the solid due to the (low-energy) electron/photon
impact, while
a high-energy ion can lead to significant beam damage (atom displacement) in the vicinity of its
impact on the solid. Hence the question of the depth of origin of ions becomes more complicated:
Instead of a situation where the ions generated below the surface have to traverse nearly unperturbed surface layers (in ESD/PSD) [29], the surface layers can be disturbed significantly
in the case of sputtering. This can affect the depth of origin of sputtered ions. For instance,
the atoms/molecules from the surface layers may be displaced by the primary ion so that the
secondary ion from a subsurface layer can escape without attenuation through a “crater” in the
surface layers.
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