Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 1, pp. R9-R17, 2004

Nuclear and Electronic Sputtering Induced by High Energy Heavy Ions
N. Imanishi* and S. Ninomiya
Department of Nuclear Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan
Received: March 10, 2004; In Final Form: May 24, 2004
This review focuses on recent fruitful findings of dependence of sputtering phenomena on solid-state property andsecondary particle species in the field of interaction of MeV-energy heavy ions with matter. Emission mechanismof singly charged ions from non-insulating targets is explained by the Sigmund-Thompson linear cascade model
combined with increase of ionization probability by transient electronic excitation. However, emission mechanism of multiply charged ions differs from that of singly charged ions, and can be explained by a combined mechanism of simultaneous recoiling and ionization by the projectile and the Coulomb repulsion by a short-lived
ionized track region. Large cluster ions are produced from non-metallic targets. Mostly clusters are emitted
directly from solid surface. However, in the case of Al2O3, they are produced by coagulation of small molecules
in a selvage region near the surface.

When an energetic particle comes into a solid, collisions
induce several processes such as recoil and sputtering of
constituent atoms, defect formation, electron excitation andemission, and photon emission. The sputtering process, especially emission process of secondary ions, has been widely
studied for various target materials under bombardment ofheavy ions. Most of studies are, however, concerned withsecondary ion mass spectrometry (SIMS) at nuclear-collisiondominant low energies.1−7 In a MeV-energy range, an electronic-energy-loss process becomes dominant,8 and the basic
process of secondary ion emission is different from that in the
low energy range because the electronic behavior strongly
depends on the solid state property. How does the highdensity electronic excitation energy deposited by a heavy ion
transfer to the atomic motion? The problem was reviewed
only for frozen gases and organic molecules.9−16 The aim of
this review is to show experimental results of yields and energies of secondary ions emitted from several tightly boundmetallic, semiconductive, and insulating solid targets and tosummarize the dynamic mechanism of secondary ion emission
in the MeV-energy range by relating it to characteristic physical properties of the target materials.
2. Energy Deposition
An energetic ion incident on a solid collides with constituent
atoms. Because of a large mass difference between the nucleus
and the electron, the collision with the nucleus is called anelastic or a nuclear collision, where kinetic energy andmomentum should be conserved during the collision. The collision with the electron is named as an inelastic or an electronic
collision and results in excitation and ionization of constituent
electrons in the atom. Thus, the ion passing through matter
loses its kinetic energy by the electronic excitation and ionization and by the kinetic collision. The energy loss by the electron-related collision is called electronic or inelastic energy loss
and the kinetic-collision induced energy loss is called nuclear or
elastic energy loss. The former and the latter respectively work
mainly at a high and a low velocity range. The term of stop-

ping power is also used instead of the energy loss.
The most familiar stopping power formula is the BetheBloch equation17, 18 which is derived on the basis of the plane
wave Born approximation applicable at a high velocity range.
In a keV energy range, the ion velocity, especially that of aheavy ion, is very low and the nuclear collision could not be
omitted. Lindhard, Scharff, and Schiott (LSS)19 derived theuniversal electronic and nuclear stopping power formulae
based on the Thomas-Fermi atomic model at the low velocity
range. The Bethe-Bloch and the LSS formulae are underlying
stopping power formulae in the high- and the low-energy
range, respectively. However, obtained experimental values of
stopping power depend so much on incident ion and energy,
and target atom. Ziegler et al.8 have developed a versatile
program named SRIM covering stopping power, range, range
straggling, damage distribution, sputtering yield, and so on andcovering any incident ion-target atom combinations over anenergy range between 1 keV and 2 GeV. In this review thenuclear and the electronic stopping powers are calculated with
the SRIM code. Examples of the results are shown in Figure 1
for C, Si, and Ag projectiles passing through a Si target.

10 4

Stopping Power / eV nm-1

1. Introduction

10

3

10 2

10

1

10 0

10 -1 0
10

Se

Sn

Si Si
C Si
Ag Si

Si Si
C Si
Ag Si

10 1

10 2

10 3

104

10 5

10 6

Projectile Energy / keV*Corresponding author. E-mail: imanishi@nucleng.kyoto-u.ac.jp.
FAX: +81-742-45-2356.

Figure 1. The SRIM (Reference 8) calculation of the nuclear andelectronic stopping powers of a Si target for C, Si, and Ag projectiles.

© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/16/2004

R10 J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Imanishiincluding bond-scissions in the molecular backbone andcreating ultimately vibrational excitation and collective motion
at the molecular scale.28

Y=

0.042α (M2 /M1) Sn (E, Z1, Z2)
,
NU0

(1)

where Sn is a nuclear stopping power, Z1 and Z2 are, respectively, the atomic numbers of a projectile and a target atom, E
is a projectile energy, N a density of the target material, U0 an
average surface-binding energy, and α is a function dependingon the mass ratio between the target (M2) and projectile (M1)
atoms.
Particles sputtered from a solid surface during particle
bombardment are mostly neutral atoms emitted with a cosine
like angular distribution and with a broad energy distributionthat peaks at a few eV, about half the surface binding energy.
Experimental data are often compared with the SigmundThompson distribution20−22 as described by
d 3Y ∝KcosΘ,
dKd 2Ω (K+U0) 3−2m(2)where K and Θ are, respectively, a kinetic energy and a polar
angle of the emitted particle with respect to the surface normal,Ω is a solid angle, and m an adjustable parameter close to 0.
Main parts of the sputtering phenomena can be understoodby analytical linear cascade models and by associatedcomputer simulation schemes of binary-collision approximation and Monte Carlo treatments, as far as an energy deposit to
the target is small enough to be treated in the linear cascade
regime and the contribution of the electronic excitation issmall. In the case of high-energy deposition, various
phenomena have been observed as spikes, large cluster emission, chemical effects, and so on. Therefore, the further understanding of these sputtering phenomena needs molecular
dynamic treatments, because these phenomena contain manybody interactions, electron-phonon coupling, and internal excitation of a limited region.
Mass distributions of emitted neutral large clusters havebeen systematically measured by Wucher group using a post
ionization method with a laser induced single ionization technique.23, 24 The obtained cluster yields are very large and show
a power law dependence nδ on the cluster size n. The exponent
δ depends on the total sputtering yield and wave length ofultraviolet.
In sputtering experiments on nonmetals measured yields are
generally different from the expectations by the collisional
theory. Here besides the energy transferred to target atoms byelastic collisions, the energy transferred to electrons producing
electronic excitation and ionization also can contribute toatomic displacements. The sputtering mechanism for thenonmetals is material dependent and very complex. 2 5
Stampfli26 proposed that a dense electronic excitation of electrons from valence band states of insulators or semiconductors
to conduction band states induces an expansion of the bondlengths and results in sputtering. Delcorte et al. carried out amolecular dynamics simulation, which uses the adaptativeintermolecular Brenner potential (AIREBO) including longrange van der Waals forces,27 for organic solids bombarded bylow energy Ar ions, and concluded that the atomic collisiondissipates energy over the first few hundreds of femtoseconds,

4. Sputtering Induced by High Energy Heavy Ions
–Cooperation of Nuclear and Electronic Collisions –4.1. Atomic Ion Emission
4.1.1. Yield. Figure 2 shows typical examples of light mass
regions of time-of-flight (TOF) mass spectra for Al, Si, Al2O3,
and SiO2 targets bombarded by 3 MeV Si2+ ions. Singly andmultiply charged atomic ions of the constituting elements areunambiguously identified in the spectra.29 The corresponding
spectra for GaP, GaAs, and GaSb targets are shown in Figure
3. Other than dominant species of 69,71Ga+ ions, P ions are
6×103
4×10

3

2×10

3

Al

3+

Al2+

(a)

Al

Al +

Si

Si+

Al2O3

Al

×20
0
2×104

Si 4+
1×104

CountsOn the basis of the binary-collision approximation (BCA)
Sigmund proposed a linear collision cascade model, which has
been successfully applied in a low energy-density collision.20
When sputtering yield, Y, is defined as an average number ofatoms removed from a solid surface per incident particle, theyield for backward sputtering at perpendicular incidence isgiven by

Si3+

×200

(b)

2+

Si

×10
0
1×104

(c)

+

2+

Al

5×103
×20
0
3×106

O2+

2×106

(d)

SiO2

Si

20

30

+

3+

Si
4+

×100

Si

1×106

+

O
2+

×2000

Si

0

0

10

m /q / u e-1
Figure 2. Light mass regions of TOF-mass spectra for Al, Si, Al2O3,
and SiO2 targets bombarded by 3 MeV Si2+ ions.

69

Ga+

4×104

P

3+

2×104

P
P 2+

GaP

+

(a)

Ga+

71

× 10

× 50

0
6×104

Counts3. Outline of Nuclear Sputtering–Low Energy Ion Bombardment –

4×10

4

2×10

4

69

Ga+

GaAs (b)
71

+

Ga

As+
× 20

0
69

Ga+

4×104

GaSb (c)
71

+

Sb+

Ga

2×10

4

× 20
0

0

20

40

60

80

100

120

m /q / u e-1
Figure 3. Light mass regions of TOF-mass spectra for GaP, GaAs,
and GaSb targets bombarded by 3 MeV Si2+ ions.

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004 R11

Nuclear and Electronic Sputtering Induced

10

2

10

1

10

0

107

(a) C

106

Si

+

Si

+

2+

Si
3+
Si

3

102
107

4+

Si

4+

Si

106

(b) SiSi(f) SiSiO2(c) GeSi(g) GeSiO2(d) AgSi(h) Ag

SiO2

2

4

105
104
103
102
107
106
105
104
103
102
107
106
105
104
103
102

0

2

4

6

80

Ag

(a)
6

5

-3

10

10
10

(b)

2

1

Intensity / arb. units

Intensity /arb. units

-2

C

10

+

10

10

Ge

Si

Si (SiO2)+
Si (Si)+Al (Al2O3)+
Al (Al)8

Figure 5. Energy dependence of the atomic ion yield for Si+ (circles),Si2+ (rectangles), Si3+ (triangles), and Si4+ (diamonds) from Si and SiO2
samples bombarded by C, Si, Ge, and Ag projectiles.

10

-1

6

Projectile Energy / MeV

(a)

10

SiO2

Si

104
10

(e) C

Si2+
Si3+

105

Intensity / arb. units

unambiguously present as singly to triply charged positive ionsbut 75As and 121,123Sb ions are scantly observed.30 The featureof very low yield of As ions was found for the low-energy
sputtering of GaAs, too,2 and it is caused by the high ionization
potential of As atoms in addition to its heavy mass.
Respective atomic ion yields are plotted in Figure 4 as a function of incident Si2+ ion energy. The yields of ions with q = 1
except for the group V th elements keep constant or slightly
decrease with increasing incident energy, but those for thegroup Vth elements steeply increase with increasing energy.30
In order to understand the energy dependence of the yield,
detailed data for Si and SiO2 samples bombarded by C, Si, Ge,
and Ag projectiles are shown in Figure 5, and the yields of Si+
and Si4+ ions are plotted in Figure 6 as a function of Sn for the
Si and SiO2 targets.31 In the Si+-Si case, the experimental data
are fitted well with a curve of Sn0.78, and the fact implies that
the Si+ ions are still produced through the nuclear collision in
the high-energy range. The same process can be applied forthe emission of Al and Ga ions from the Al and GaVth targets,
respectively. However, the yields of the Si+ ions from the SiO2
target are higher than the corresponding yields from the Sitarget and don’t correlate with the nuclear stopping power.
The result of the SiO2 target will be discussed later. In the caseof Si4+, as shown in Figure 6, the yield doesn’t trace Sn. A
promising mechanism of the emission of multiply charged ionsshould contain any direct ionization caused by the collisions ofprojectiles with target atoms. In fact multiply charged recoil
ions are effectively produced in collision between energetic
ions and rare-gas atoms, and the formation cross section can begiven in the frame of the independent-electron model.32−34 The
model predicts that the multiply charged ions are produced bya transfer- and a pure-ionization process; in the former process
ionization occurs following an exchange of electrons between
the projectile and the atom, and in the latter process the projectile ionizes the atom without accompanying any electronic
exchange. It is then plausible that the atoms on the surface are
simultaneously ionized and recoiled by the same projectiles.

Si
SiO2
4

10

(b)

4

10

Ge

Ag

Si

3

10

0

C

-1

10

(GaP)
(GaAs)(GaSb)

-2

10

Ga+
+
Ga
Ga+

P+
+
As
+
Sb

2

10

-3

10

0

1

2

3

4

5

Projectile Energy / MeV
Figure 4. Singly charged atomic ion yields plotted as a function of Si
projectile energy.

0

10

1

10

2

10

10

3

Nuclear Stopping Power / eV nm-1
Figure 6. The yields of (a) Si+ and (b) Si4+ ions plotted as a functionof Sn for the Si and SiO2 targets.

R12 J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Imanishinσ (q, q'; i) = ∫ i PVi (1– PV) n−i 2π bdb,

(3)

where q and q’ are the charges of projectiles before and after
the collision, respectively, i is the charge of the recoiled ions, n
the number of electrons in the outer shell, PV the ionization
probability of one of the outer-shell electrons, and b the impact
parameter of collisions. For the transfer ionization P V isassumed to be constant inside an impact parameter rV and zero
outside.34 Then, the cross sections for those processes are
expressed as follows:nσ (q, q'; i) = i PVi (1– PV) n−i r V2 π .

(4)Another feature shown in Figure 4 is that the yields of thegroup Vth ions, whose ionization potentials are higher than thework functions of the samples, depend strongly on the incident
energy (and the electronic stopping power). This characteristic
dependence different from those of the other secondary ionscan be accounted by the following process 30: In the case of
semiconductive and insulating materials, the fast projectile
excites electrons from the valence band to the conduction bandand the produced electrons excite further other electrons to the
conduction band. As a result, lots of electron-hole pairs areproduced along the path of the projectile, forming a very shortlived electronically high-excited spot. The holes increase theionization probability compared to the static valence band byacting as accepters of electrons in the emitting atom. Šroubek
has deduced the ionization probability P e + as a function ofeffective electron temperature parameter Teff characterizing theelectronic excitation,

On the other hand for the pure ionization it is better to assume
that the parameter PV depends exponentially on the impact
parameter,34
PV (b) = PV (0)exp(–b/rV ),(5)

where PV(0) represents the ionization probability of the outershell electrons at the impact parameter b = 0. The ionization
cross sections calculated for the Si target (where n = 4) using
the parameter values in Table 1 are plotted in Figure 7 alongwith the experimental data for the Si-Si system.31 Those parameter values are derived from the results for a Ne q+ + Ne
system at an energy of 1.05 MeV/u. The qualitative agreement
supports the process caused by simultaneous occurrence ofionization and recoil induced by the projectiles.
TABLE 1: Values of the Parameters P V and r V (in nm)
Used for (a) the Transfer Ionization and Those of PV (0) andrV for (b) the Pure Ionization (Reference 31)
q

q’

PV

rV

(a) transfer ionization

2
2
4

3
4
3

0.19
0.28
0.34

0.049
0.036
0.007

(b) pure ionization

2
4

2
4

0.35
0.40

0.033
0.055

Pe+ = exp –10-1

10 -16

10 -17

10 -18

1

2

3

4

(6)

10

-2

10

-3

+

P (GaP)
As+ (GaAs)+
Sb (GaSb)

Calculated result
Experimental result

Intensity / arb. units

Cross Section / cm24v⊥t0,
kBTeffwhere ϕ and t 0 are respectively the work function and therelaxation time of the excited spot, I and v⊥ are respectively theionization potential and the normal velocity of the emitting
atom, k B is the Boltzmann constant and e is the electronic
charge.35, 36 The effective electron temperature parameter Teff isconnected with the electronic stopping power Se. Then, in theframework of the above transient electronic excitation model,
it is expected that yields of secondary ions with high ionization
potentials depend exponentially on 1/Se. This expectation is
confirmed by the clear exponential dependence shown in
Figure 8. The transient electronic excitation can explain thefollowing particulars of observed secondary ion yields which
are beyond the scope of the ordinary surface ionization
process. The high yield of Si + compared to that of Al+ , asshown in Figure 4, can be understood by the reason that thehigh-excited spot sustains longer in Si than in Al.30 In spite of
the comparable ionization potential of P and As, P+ ions are
more frequently produced than As ions. 30 The fact comespartly by the low electron mobility in GaP of 300 cm 2V−1s−1that is 0.03 times as high as that for GaAs,37 and the resultant
high transient value of Teff is expected. Thus, the increase of
the yields of the secondary ions whose ionization potentials aremuch higher than the work functions of the samples is caused

10 -15

10 -192
I–ϕ– e143

According to the independent-electron model,
the crosssections for multiple ionization of recoiled-atoms are given by
the sum of cross sections for the pure and transfer ionization,
and are expressed as

648

32−34

Electric Charge q of Secondary Ion / e
Figure 7. The ionization cross sections calculated (open circles) for
the Si target (where n = 4) applying parameters derived from theresults for a Neq+ + Ne system at an energy of 1.05 MeV/u along withthe experimental data (closed rectangles) for the Si-Si system. Theline is drawn to guide the eye.

0

0.5

1

1.5
-1

2

1/S e / keV nm
Figure 8. Secondary ion yields plotted as a function of 1/Se. Thelines are drawn to guide the eye.

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004 R13

Nuclear and Electronic Sputtering Induced

bution of multiply charged ions does not differ from that ofsingly charged ions. However, Figure 9 shows that the emission energy is higher than the value of the singly charged ions.
The most probable and the mean energies of the axial emission
energy distributions are overall proportional to the electriccharge of the secondary ion, as shown in Figure 10 for the 3
MeV Si impact. These facts support the formation mechanismby the simultaneous process of ionization and kinetic recoil.
The microscopic charging up in the track region stands for avery short period of about 10−15–10−14 s. Then, the period is too
short for a stationary atom to get a kinetic energy by the repul-

10

0

10

0

Al

+

Al2O3
Al

Intensity / arb. units

10

Si+

(a)

10

Mean Axial Emission Energy /eV

20

10

0

(b)

40

30

20

10

0

1

2

Si

2+

3

Electric Charge q of Secondary Ion / e

(d)

10

10

0

+

0

69

Ga+

(h)

P

10

(e)

(k)
GaP
GaAs
GaSb

GaP
-1

+

5.5keV, Ar
GaAs

-1

100

Al2+ (b)10

P 2+ (i)

-2

-1

0

0

4

Figure 10. The most probable and the mean energies of the axial
emission energy distributions for the 3 MeV Si2+ impact.

0

10
10

30

10

0

10

Si (SiO 2)
q+
Si (Si)q+
Al (Al2O3)q+
Al (Al)q+
P (GaP)

-1

10

(a)

q+

40

SiO2
Si

-1

10

Most Probable Axial Emission Energy /eVby the transient electronic excitation. That is, the electronic
process plays an important role in the production of singly
charged ions with high ionization potentials as well as multiply
charged ions.
4.1.2. Energy Distribution. Particles sputtered from a solid
surface have a broad energy distribution. An axial energy,
which is defined as a kinetic energy along the surface normal,
is usually measured and compared with theoretical results.
The axial emission energy distributions for Alq+, Siq+, Pq+ (qe:electric charge), and Ga+ secondary ions are shown in Figure 9
for a 3 MeV Si2+-ion bombardment, for example, and the result
obtained for Ga+ ions emitted from GaAs bombarded by 5.5
keV Ar + ions is also included. 2, 30, 38, 39 The distributions
scarcely depend on the incident energy of Si ions and arestrongly asymmetric extending to a high-energy side with apower law dependence on the emission energy. The SigmundThompson calculation of the linear cascade model, which has
been developed for sputtering in the nuclear-collision-dominant low incident energy region, 2 0 − 2 2 can reproduce themeasured distributions using a reasonable surface potential
energy of about 3 eV in the case of the singly charged ionseven at the high energies, as far as noninsulating targets areconcerned.30, 38 The result agrees with the conclusion deduced
from the yield measurement; that is, in the case of non-insulating targets, singly charged atomic ions are produced throughthe nuclear collision even in the high-energy range. In the caseof insulating SiO2 target, the energy distribution of Si+ ionsemitted from the target is symmetric and is narrower than thevalue expected from the Sigmund-Thompson distribution. 38
The result combined with the yield increase compared to the Sitarget leads to the conclusion that a large part of the secondary
atomic ions are formed not by the linear collision cascade but
mainly by a bond breaking through a repulsive energy levelproduced by the electronic process, as discussed earlier onlaser-induced atomic desorption in some insulating materials
such as alkali halides and SiO2.40, 41
Regarding to the multiply charged ions,30, 38 as shown in
Figure 5, the observed yields are very high compared to data of
the low-energy incidence. At a keV-energy region, a yieldratio of Si 2 + to Si 1 + , for example, is about 10 − 3 and it isexplained by inner-shell excitations accompanied by Auger
electron emission.1 In this process, the emission energy distri-

100

200

3+ (f)

Si

-1

10-1

10

0

(l)

121
10

0

Al

10

3+ (c)

-1

10

Sb+

0

3+

P

0

10

( j)

GaSb

(g)
10

10

-1

10

0

40

80

120

4+

Si

-1

0

40

80

10

120

Axial Emission Energy /eV Axial Emission Energy /eV

-1

-1

0

40

80

120

0

40

80

120

Axial Emission Energy /eV Axial Emission Energy /eV

Figure 9. Axial emission energy distributions for Alq+, Siq+, Pq+ (qe: electric charge), Ga+, and Sb+ secondary ions for the 3 MeV Si2+ impact alongwith the result obtained for Ga+ ions emitted from GaAs bombarded by 5.5 keV Ar+ ions.

R14 J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Imanishisive Coulomb force enough to override the potential barrier.
However, when a target atom on the surface is recoiled with akinetic energy of a few eV to a direction of about 90º withrespect to the incident beam axis, and is ionized at the sametime, the atom is accelerated further to the backward direction
by the repulsive Coulomb force from neighboring ionizedtarget atoms in the track region and can get out the surface
within 10−15–10−14 s. The Coulomb repulsive energy is estimated to be about (10 to 15) × q eV on an average when thenearest lying atoms are assumed to be doubly ionized.30 This
simple mechanism initiated by the simultaneous recoiling andionization is supported by the independence of the yield ratios
on incident energy and target species.31
4.2. Cluster Ions
4.2.1. Yield. Figure 11 shows mass distributions of secondary ions emitted from Al, Al2O3, Si, SiO2, GaP, GaAs, and
GaSb under the bombardment of 3 MeV Si2+ ions. No cluster
ion is emitted from the Al and Si targets. On the other hand,large cluster ions were observed for the Al2O3, and SiO2 targets; (Al2O3)p-1AlO+, (Al2O3)p-1AlO2+, (Al2O3)p-1Al2O2+, and(Al 2 O 3 ) p-1 Al 2 O 3 + (p is a positive integer) from Al 2 O 3 , and(SiO2)p-1Si+, (SiO2)p-1SiO+, (SiO2)p-1SiO2+, and (SiO2)p-1Si2O+
from SiO2.42, 43 In the cases of the GaP and GaAs targets nolarge cluster ion was detected.25 On the other hand, large cluster ions composed of Sb were found for the GaSb target.25 The
yields of the observed clusters emitted from SiO2 are shown in
Figure 12 as a function of the Si and Ag incident energy.43
Figure 13 shows the case for the Si-GaSb system.25

We treat the mechanism of the sputtering from the SiO2
sample. 43 The experimental result that the cluster yields
steeply increase with increasing incident energy indicates thatthe electronic energy transfer governs the mechanism ofcluster-ion formation even in the competing region of the electronic and nuclear energy depositions. Figures 14 and 15 show
power law dependences of the cluster ion yields on the electronic stopping power and on the cluster size p, respectively.43
From the results, it is obvious that the yields are represented
well by a simple power function of Y ∝ pASeB, where A and B
are fitting parameters, A taking a negative value and B being
positive. That is, the intensity of the cluster ions depends
inversely on the size of the clusters and the intensity of thelarge cluster ions increases more steeply with increasing electronic stopping power than that of the small clusters. The
unambiguous correlation indicates that the cluster ions areproduced through the process initiated by the electronic-energy
deposition. Mechanisms based on the electronic energy deposition have been proposed for the explanation of secondaryparticle emission under high-energy ion bombardment.10, 13, 14, 44
However, most of the models try to explain the total yield of
the secondary particles and do not step into the explanation of
the yields of individual components. Hedin et al.12 proposed
an ion-track model. In this model, it is assumed that secondary
electrons produced by a projectile hit bonding electrons in asolid and a cluster with a size of p is emitted by multiple hits.
The probability emitting the cluster depends both on the electronic stopping power and size. The larger cluster has thesteeper increase with the stopping power. The prediction qualitatively explains the feature of the experimental dependence.
That is, the cluster ions are not formed by the coagulation of
SiO2 molecules in a vacuum nor formed by decomposition ofvery large metastable clusters after sputtering, but are formedin the near-surface region due to the multiple breaking ofbonds directly by the incident ions and indirectly by theshower of secondary electrons. For the Ag projectile 60% of
the deposit energy, that is, 500 – 1000 eV/nm, is consumed in
the excitation process of bonding electrons and sufficient tobreak a large number of covalent bonds (4.7 – 8.2 eV/bond) at
the same time.45
On the other hand, the ion-track model cannot be applied tothe semiconductive materials because of the fast recovery of

200

400

600

4

10

2

10

1

(c)

Al

10 3

Counts

(b)

SiO 2

0
10

(a)

Si

CountsCounts

6

10
5
10
4
10
3
10
2
10
1
10
0
10
10 6
5
10
4
10
3
10
2
10
1
10
0
10

10

4

10

3

10

2

10

1

10

0

10

4

10

3

10

2

10

1

(e)

GaP

(f )

GaAs

100
10

4

10

3

10

2

1

(g)

0

10 4
10

Al2O3

(d)

10

3

10

2

10

1

10

10 0

100

0

200

400

600

800

1000

GaSb

0

200

m/q /u e -1

400

600

m/q /u e-1

Si

Figure 11. Mass distributions of secondary ions emitted from Al,
Al2O3, Si, SiO2, GaP, GaAs, and GaSb under the bombardment of 3
MeV Si ions.

10

GaSb
GaSb+

-1

+

Ga2 Sb+10(a) Si

10

4

p =2

10

3

10

2

10

1

4

p =2

10

3

3

4
5

10

2

4

10

+

(SiO2)p-1Si

7

10

3

10

2

10

1

5

10

4

p =2

10

3
4
6

10

10

3

10

2

10

1

10

0

3
4
5

2

10

+

(SiO2)p-1SiO

7

10
5
10
4

p =2

3

0

10

(SiO2)p-1Si+

5

1

10

0

4

(b) Ag

3

10
105
10

5

10

Intensity / arb. units

Intensity / arb. units

GaSb 2
5

1(SiO2)p-1SiO+

+

Sb
10

-2

10

-3

7

10

5

10

104

p =2
3
4
5
0

1

p =2
3
4
6

3

10

2

10

(SiO2)p-1SiO2+

7

101

2

3

4

Projectile Energy / MeV

5

6

0

1

+

Sb 4
(SiO2)p-1SiO+2

7
2

3

4

5

10
6

Projectile Energy / MeV

Figure 12. The yields of the observed clusters emitted from SiO2 as a function of the (a) Si and (b) Ag incident energy.

-4

0

1

2

3

4

5

Projectile Energy / MeV
Figure 13. The yields of the observed clusters emitted fromGaSb as a function of the Si incident energy.

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004 R15

Nuclear and Electronic Sputtering Induced

(a)

10 0

10

Intensity / arb. units

10

+

Ga

-2

Intensity / arb. units

10

0

-1

+

SiO (3.7)+
SiO2 (2.7)+
Si 2O (2.7)+
Si 2O2 (3.1)

10-3

10-4

(b)

10 0

+

(SiO 2)SiO (3.9)+(SiO 2) 2(4.5)+(SiO2) 2SiO (3.9)+(SiO2) 6
(8.5)

10-1

10

-1

10

-2

10

-3

10

-4

GaSb+
+
Ga2Sb
+
GaSb2

+

Sb

10-2

Sb+4
0

10-3

1

2

3

Electronic Stopping Power / keV nm-1

10-4

0.5

1

2

5

Electronic Stopping Power / keV nm-1

Figure 16. The yields of cluster ions from GaSb as a function of theelectronic stopping power. The yield of Ga+ is shown for comparison.

Figure 14. Power law dependences of the cluster ion yields on theelectronic stopping power. The exponents of the power law function
are shown in the parentheses. The lines are drawn to guide the eye.

2

+

10-2
-1

10

-2

Intensity / arb. units

10

-3

10

(exponent)+
1.0MeV, Si (6.4)
2+
3.0MeV, Si (4.9)
4+
5.0MeV, Si (3.4)

-4

10

-5

10
10

GaP
GaSb+

(a)

Intensity / arb. units

10

0

5

2

10-3

0

(b)

-1

10

5

-2

10

0.5

1

1.5

2

-1

1/S e / keV nm
Figure 17. Secondary cluster ion yields plotted as a function of 1/Se
for the GaP and GaSb targets. The lines are drawn to guide the eye.

-310

(exponent)
2+

3.0MeV, Si (4.9)
4+
5.0MeV, Cu (4.2)
5+
6.0MeV, Ag (4.1)

-4

10

-5

10

0

1

2

5

10

Cluster Size p
Figure 15. Power law dependences of the cluster ion yields on thecluster size p. The exponents of the power law function are shown in
the parentheses. The lines are drawn to guide the eye.

the bond breaking. An experimental study of the sputtering
phenomena of GaAs was done by Schenkel et al.46, 47 using
multiply charged slow ions. They plotted the total sputtering
yield for GaAs as a function of potential energy, and foundthat the total yield is very low up to a value of the potential
energy and then increases by two or three orders of magnitude
beyond the value. Meanwhile, a theoretical work predictedthat structural changes are induced in covalent solids when
10% of valence electrons are promoted from bonding states inthe valence band to anti-bonding states in the conduction
band.48 Therefore, Schenkel et al.46, 47 concluded that the steep
increase of the sputtering yields is caused by the destabilization of atomic bonds resulted from high-density electronic

excitations. A critical laser fluence49 to induce such a phasetransition in GaAs is accepted to be 5 keV/nm2 where a characteristic absorption depth50 is 1 µm. Then a similar phase transition can happen for GaP, GaAs, and GaSb at the electronic
stopping power of the MeV-energy region. The ionization
potentials of a di-atomic cluster Ga2 are low and they are easily
ionized. Then, the yield dependence of Ga2 ions on the incident energy is expected to represent the sputtering-yielddependence on the incident energy in the case of the GaV thtargets.51 As a result, the sums of the yields of neutral andionized clusters containing Sb from GaSb should have asimilar energy dependence.51 However, as shown in Figure 16,
the yields of the ionized clusters containing Sb increase withincrease in the electronic stopping power; this behavior contrasts
strongly with a scarce dependence of the Ga2 ion yield on theelectronic stopping power. The fact reflects the increase ofionization efficiency by the transient electronic excitation
described above, as shown in Figure 17. Figure 18 shows thecluster size dependence of the secondary cluster ion yields for the
GaSb target. The result shows the clear power-law dependence.

R16 J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Imanishi
1

Si

10 -1

10

-2

10

-3

10 -4

GaSb

0.5 MeV (6.0)

0
1

0
1

5

(f) (Al2O3)p-1 Al2O3+

p =2
p =3
p =4

p= 3
p= 4
p= 5

10

Cluster Size p0Figure 18. The cluster size dependence of the secondary cluster ion
yields for the GaSb target bombarded by 0.5, 3, and 5 MeV Si ions.
The exponents of the power law function are shown in the parentheses. The lines are drawn to guide the eye.
10

p= 3
p= 4
p= 5

(c) (SiO2)p-1SiO+2

0.52(e) (Al2O3)p-1 Al2O2+

p =2
p =3
p =4

0.5

5.0 MeV (5.6)
1

p= 3
p= 4
p= 5

(b) (SiO2)p-1SiO+

3.0 MeV (5.9)
10 -5(d) (Al2O3)p-1 AlO+

p =2
p =3
p =4

0.5

Intensity / arb. units

Intensity / arb. units

10(a) (SiO2)p-1Si+

0

0

10

20

30 0

10

20

30

Axial Emission Energy / eV
Figure 20. Axial emission energy distributions of oxide cluster ionsfrom SiO2 and Al2O3.

0

6

(a)

+

(Al2O3 ) p -1 Al2O2 (0.59)
10

10

10

+

(Al2O3 ) p -1 Al2O3 (0.69)

-1

-2

-3

0

2

4

6

8

10

12

Cluster Size p
Figure 19. The cluster size dependence of the secondary cluster ion
yield for Al2O3. The exponents of the power law function are shownin the parentheses. The lines are drawn to guide the eye.

Most Probable Axial Emission Energy /eVIntensity / arb. units(exponent)(Al2O3 ) p -1 AlO+ (0.48)
4(SiO2 ) p-1 Si

+

SiO2

(SiO2) p-1 SiO+(SiO2) p-1 SiO 2

0
6

(b)

4

+

2

(Al2O3) p-1 AlO

+

Al2O3

(Al2O3) p-1 Al 2 O 2+(Al2O3) p-1 Al 2 O 30

So far quantitative explanation of the power-law dependence isstill an open problem even in the nuclear-collision-dominant low
energy sputtering.
Most experimental results of the yield dependence on size ofcluster have shown power-law dependences irrespective ofprojectile energy. However, the result for Al2O3 shows exceptionally an exponential dependence as shown in Figure 19.
The exponential dependence is expected in the process ofcoagulation of atoms and small molecules sputtered from thesurface in a selvage region just outside the surface.51
4.2.2. Energy Distribution. As shown in Figure 20, axial
emission energy distributions of oxide cluster ions from SiO2
and Al2O3 are symmetric, and are very narrow compared tothose of the atomic ions.38 The peak energies are plotted in
Figure 21 as a function of mass for the three series of the oxide
clusters emitted from SiO2 and Al2O3 in the 3-MeV Si incidences. In the case of SiO2 the peak energy decreases very
slowly with the size of clusters. 38 On the other hand, theenergy of clusters from Al2O3 increases first with increasing
size and then levels off.51 The feature supports again that theclusters observed in Al2O3 are not directly emitted from thesurface of the target but are probably produced indirectly bycoagulation of molecules in the selvage region near the surface.

+

2

0

2

4

6

8

Cluster Size p
Figure 21. The peak energies plotted as a function of mass for thethree series of the oxide clusters emitted from SiO2 and Al2O3 in the 3
MeV Si2+ incidences.

5. Summary
The experimental results and the mechanism of secondary
ion formation for the Al, Si, Al 2 O 3 , SiO 2 , GaP, GaAs, and
GaSb targets under MeV-energy heavy ion bombardment arepresented. The secondary ion emission depends characteristically both on the secondary ion species and on the target
species.
The yield and the emission energy distribution for the singly
charged ions from the non-insulating targets are explained
qualitatively by the Sigmund-Thompson linear cascade model
combined with the increase of ionization probability by thetransient electronic excitation.
However, the results for the multiply charged ions differ
from those of the low-energy incidence in the high yield andthe proportionality of the most probable and mean energies tothe electric charge. The facts can be explained by the simple

Nuclear and Electronic Sputtering Induced

combined mechanism of the simultaneous recoiling and ionization by the projectile and the Coulomb repulsion by the shortlived ionized track region.
Large cluster ions are produced from the Al2O3, SiO2, and
GaSb targets. The clusters from SiO2 and GaSb are emitted
directly from the surface. On the other hand, the clusters
observed for Al2O3 are probably produced indirectly by coagulation of molecules in a selvage region near the surface.
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