Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of ~1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids.http://www.sciencedirect.com/science/article/B6THY-4SHF49N-1S/1/3eb43650299e0466e76cbbbfdaca9fa

Gordo, P. M.

Liszkay, L.

Kajcsos, Zs.

Havancsák, K.

Skuratov, V. A.

Kögel, G.

Sperr, P.

Egger, W.

Lima, A. P. de

Marques, M. F. Ferreira

Estudo Geral

ully
ials.
ncy
the
e of
tion
nes.
eral
tion
r to
few
. In
the
an
ion
on
igh-
igh
nge,
d by
d as
ghly
. For
was
ters
ved.
G Model
APSUSC-17147; No of Pages 3On the defect pattern evolution in sapphire irradiated by swift ions in
a broad ﬂuence range
P.M. Gordo a,*, L. Liszkay b, Zs. Kajcsos b, K. Havancsa´k c, V.A. Skuratov d, G. Ko¨gel e, P. Sperr e,
W. Egger e, A.P. de Lima a, M.F. Ferreira Marques a,f
a ICEMS, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal
bKFKI Research Institute for Particle and Nuclear Physics, P.O. Box 49, H-1525 Budapest 114, Hungary
cDepartment of Materials Physics, Eo¨tvo¨s University, Pa´zma´ny Pe´ter se´ta´ny 1A, H-1117 Budapest, Hungary
d Flerov Laboratory of Nuclear Reactions, Center of Applied Physics, JINR, 141980 Dubna, Russia
e Institut fu¨r Angewandte Physik und Messtechnik LRT2, Fakulta¨t fu¨r Luft- und Raumfahrttechnik Werner-Heisenberg-Weg 39, D-85577 Neubiberg, Germany
f Instituto Superior Engenharia, P-3031-199 Coimbra, Portugal
1. Introduction
Alumina is a widely used material both as a ceramic and as
sapphire single crystal substrate for various industrial applications
where the quality of the structure plays a determining role.
Consequently, methods to investigate and identify speciﬁc
structural changes are highly desirable for routine-like technolo-
gical applications as well. Ion irradiation can induce changes in the
microstructure of materials in a controlled way and this helps to
clarify structure evolution induced by irradiation.
High-energy heavy ions interacting with material lose their
energy through electronic and nuclear processes. Point defects,
defect clusters and/or ion tracks can be induced in the irradiated
materials up to the penetration depth with defect structure
depending on the ion energy, mass and irradiation ﬂuences. In
addition to the elastic damage it has been reported that, depending
on the material and impinging ion, the electronic stopping above a
certain threshold value can also create additional displacement
damage in the vicinity of the ion path [1].
Positron annihilation spectroscopy (PAS) has been successf
used in studying irradiation-induced defects in different mater
As positrons are uniquely sensitive and selective probes of vaca
type defects, their use can provide important information on
defect structure produced by swift ions. Particularly, the us
slow positron beam allows us to study the defect pattern evolu
as a function of ion ﬂuence and/or energy in near surface zo
Despite the numerous experiments reported there are sev
questions to be resolved regarding the details of defect forma
process induced by the impinging swift ions, speciﬁcally nea
the surface region where the electronic stopping power is a
orders of magnitude higher than the nuclear stopping power
this region, far from the end of the penetration depth of
implanted ions, it is expected that each ion produces
independent cylindrical volume full of defects around the
path [1–3]. The aim of this work is to get additional information
the defect production and evolution during and after the h
energy swift heavy ion irradiation process utilizing the h
sensitivity of the positron annihilation methods.
Applied Surface Science xxx (2008) xxx–xxx
A R T I C L E I N F O
Article history:
Available online xxx
Keywords:
Sapphire
Al2O3
Positron lifetimes
Doppler broadening
Defects
Positron trapping
A B S T R A C T
Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad ﬂuence ra
were investigated using a continuous and a pulsed positron beam to study the defect structure create
the passage of the ions in depths of a fewmicrometers. At small doses, monovacancies were identiﬁe
dominant defects and positron trapping centres. These monovacancies are assumed to be hi
concentrated inside a cylindrical volume around the ion path with an estimated radius of 1.5 nm
higher doses a second type of trapping centre emerges. This second class of structural imperfection
associatedwith the overlap of the individual ion tracks leading to the formation of larger vacancy clus
or voids.
 2008 Elsevier B.V. All rights reser
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier .com/locate/apsuscs of
ure
* Corresponding author. Tel.: +351 239 410 629; fax: +351 239 829 158.
E-mail address: pgordo@ci.uc.pt (P.M. Gordo).
0169-4332/$ – see front matter  2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2008.05.185
Please cite this article in press as: P.M. Gordo et al., On the defect patt
range, Appl. Surf. Sci. (2008), doi:10.1016/j.apsusc.2008.05.1852. Experimental
Al2O3 (0 0 0 1) single crystal samples with a typical thicknes
few hundred of micrometers were irradiated at room temperatern evolution in sapphire irradiated by swift ions in a broad ﬂuence
with
Dub
The
avoi
mai
I
men
tem
Coim
wer
at 1
char
dept
3. R
I
posi
sam
para
the r
sam
are c
that
hom
ﬁtte
norm
(free
In F
ﬂue
pres
I
valu
effec
high
anni
S va
ﬂue
expl
vaca
state
diffu
39 
tivel
sign
Fig.
refer
2
G Model
APSUSC-17147; No of Pages 3
Ple
ran245 MeV Kripton ions at the U-400 FLNR JINR cyclotron in
na, Russia. The ﬂuence range was 5  109–1.6  1014 ion/cm2.
incident ion beamwas perpendicular to the sample surface. To
d overheating of the irradiated targets the ion ﬂux density was
ntained below 2  108 cm2 s1.
n situ positron annihilation Doppler broadening measure-
ts as a function of energy were carried out at room
perature with a magnetically guided slow positron beam in
bra [4], whereas the in situ positron lifetime measurements
e performed at the pulsed positron beam facility in Munich [5]
6 keV positron beam energy. These two set-ups were used to
acterize the damaged region from the surface up to 3mm
h.
esults and discussion
n Fig. 1, the Doppler S parameter values – as a function of
tron implantation energy E – are shown for the reference
ple and for some irradiated a-Al2O3 samples. The S(E)
meter values have all been normalized to the bulk value of
eference sapphire sample. As can be observed for all irradiated
ples, the S(E) values above10 keV of incident positron energy
onstant. This behaviour is in agreement with our expectation
near the surface, in the ﬁrst few mm depth of the material, a
ogeneous distribution of defects exists. Each S(E) curve was
d using the VEPFIT [6] computer code to determine the
alized Ssample value in this region of the irradiated sample
of surface contribution) and the positron diffusion length L+.
ig. 2, the normalized Ssample values as a function of the ion
nce in the range from 1011 up to 1.6  1014 ion/cm2 are
ented.
n the ﬂuence range of 1011–1013 ion/cm2, the normalized S
es do not change indicating that there is a saturation trapping
t. The corresponding Doppler S value is approximately 2.5%
er than the defect-free bulk and this value can be related to the
hilation of positron in a monovacancy. For higher ﬂuences, the
lues increase drastically and a value of 1.15 is achieved for a
nce of 1.6  1014 ion/cm2. This high S value can only be
ained due to the appearance of a new trapping site, i.e., large
ncy clusters or voids, where a longer living trapped positron
can be formed. The increase observed in the positron
sion length for ﬂuences above 1013 ion/cm2 (L+ = 15  1,
1 and 44  1 nm for 1013, 5  1013 and 1014 ion/cm2, respec-
y) combined with a reduction of the F+ center ionoluminescence
al produced above the same ﬂuence values [7], indeed strongly
suggest the clustering of these ﬁrst kind of traps, i.e., the clustering of
monovacancies sets in.
In a previous work [8] the lifetimes related to these two kinds
(with differing sizes) of trapping sites induced by the swift ions in
sapphire were identiﬁed as 186 ps and 500 ps, respectively. The
creation of the extended positron traps yielding in longer lifetimes
was associated with the overlap of individual ion effects. An
estimation of the effective region radius can be obtained through
the mean distance between the near-neighbour regions. In the
simplest geometrical description (one-dimension) we can write
the relation between the region radius r, the ﬂuence F, and the
mean distance d, as:
dðnmÞ  10
7ﬃﬃﬃ
F
p
 2r : (1)
From Fig. 2 it can be seen that the second trapping site appears
for ﬂuences above 1013 ion/cm2 and assuming d = 0 at this ﬂuence
the use of Eq. (1) provides an upper limit for the region radius rlimit
1.6 nm.
In order to understand the nature of structural imperfections
and their pattern induced by swift ions, a new set of Kr irradiated
sapphire samples in the ﬂuence range 5  109–5  1010 was used
and analysed utilizing the pulsed positron beam at Munich. The
sensitivity of Doppler broadening measurements for this ﬂuence
range is weak as compared to the lifetime measurement. For each
irradiated sample the observed mean lifetime was below 186 ps,
the value given above for saturation monovacancy trapping. The
analysis of each spectrum indicates that the positron annihilates in
two different places, a defect-free sapphire bulk state (tb = 145 ps)
and in a trapped state related to the monovacancy defects
(td = 186 ps). For example the intensity I2 takes values of
37  1%, 64  1% and 85  1% for ﬂuences of 5  109, 2  1010 and
5  1010 ion/cm2, respectively.
For the ﬂuence range of this sample set, if it is assumed that the
positron trapping sites (monovacancies) are inside a cylindrical
volume around the ion path, the radius of the effective ion region
Fig. 2. The S values of irradiated sapphire samples normalized to the defect-free
reference sample in the ﬂuence range of 1011–1.6  1014 ion cm2. The line is only
to guide the eyes.
P.M. Gordo et al. / Applied Surface Science xxx (2008) xxx–xxx1. Doppler S parameter vs. incident positron energy E for the defect-free
ence and for the Kr-irradiated sapphire samples.
ase cite this article in press as: P.M. Gordo et al., On the defect pat
ge, Appl. Surf. Sci. (2008), doi:10.1016/j.apsusc.2008.05.185can be estimated. In fact, in the vicinity of the surface, far from the
projected range of the implanted ions, it is expected that each ion
produce an independent cylindrical volume full of defects around
the ion track [1–3]. AFM pictures of the damage induced by swift Bi
ion irradiation in sapphire support this assumption. Consequently,
the positron implanted into the irradiated sapphire sampletern evolution in sapphire irradiated by swift ions in a broad ﬂuence
s of
and
een
to
our
1,
ion,
no
cies
ce a
nm,
va-
ion
in
ose
hes
hly
ped
sed
in
tion
ted
are
ion
ant
ath.
ius,
cks
of
an-
ting
eral
ed.
the
nts.
os, F.
Phys.
n Ion
21.
262–
. 218
ater.
3
G Model
APSUSC-17147; No of Pages 3diffuses until the fully defected cylindrical volume around the ion
path is reached and than occur the trapping and the annihilation.
Assuming this picture, the trapping of positrons is limited by
diffusion and the mean distance, d, between the border of these
cylinders around the two near-neighbour paths, and those
parameters should deﬁne the trapping rate k, of positrons through
Eq. (2) as [6]:
k ¼ 1
tb
Lþ
d=2
 2
 1
" #
(2)
where tb = 145 ps and L+ represent the positron lifetime and the
positron diffusion length, respectively, in the defect-free bulk
sapphire sample. The positron diffusion length value,
L+ = 62  1 nm was obtained by the VEPFIT [6] program applied to
the S(E) curve of the unirradiated sample shown in Fig. 1.
Fig. 3 shows the experimental values of trapping rate as a
function of the ion ﬂuence obtained from the mean lifetime values
as [9]:
k ¼ 1
tb
t¯  tb
td  t¯
; (3)
where t¯ is the mean lifetime of positron in the irradiated sample,
td = 186 ps is the lifetime of positrons trapped in a monovacancy
and tb = 145 ps the positron lifetime in the substrate.
Combining Eqs. (1) and (2) and adjusting the values to the
experimental data in Fig. 3, the effective radius of the cylindrical
volume around the ion track can be estimated. The best agreement
is obtained for an effective radius value of 1.5 nm. This value agrees
well with the upper limit value of rlimit  1.6 nm obtained above
when the overlap of tracks was taken into account to explain the
creation of extended defects revealed by the second, longer
lifetime component.
TRIM code [10] simulations, however, suggest that vacancies
created by nuclear collision processes extend over several tens of
nanometers around the ion path in the near surface region of Kr
implanted sapphire samples. Thus the small extension (1–2 nm) of
the damaged region around the Kr ion paths estimated by positron
sapphire irradiated with swift 238U ions, and the mean radiu
these damage zones was calculated to be between 1.1 nm
2.6 nm, depending on the incident ion energy. It has also b
shown in ref. [1] that the extended defect formation due
inelastic collisions has a threshold value of 21 keV nm1. In
case the (dE/dx)e value for our 245 MeV Kr ionswas 15.8 keV nm
well below the threshold value. This is an apparent contradict
which might be resolved if we assume that although there is
visible track formation below the threshold value, monovacan
can still be created in the vicinity of the ion path, i.e., ions indu
highly disordered columnar volume, with a radius of 1.5
surrounded by a larger volume with the presence of mono
cancies. The positron lifetime value in this highly disordered reg
of the bulk material should be quite similar to that
monovacancies, i.e., it is not possible to distinguish between th
highly similar defectswith positrons.When the ion ﬂuence reac
a critical value that corresponds to the overlap of these hig
disordered regions, a second type with a longer living trap
positron state emerges. To better clarify the mechanism discus
here and better understand the defect pattern evolution
sapphire irradiated by swift ions, further positron annihila
measurements are in progress on high energy Bi ion irradia
sapphire samples, where the electronic stopping power values
well above the threshold.
4. Conclusions
Positron annihilation measurements on swift heavy
irradiated samples have identiﬁed monovacancies as domin
defects inside the damaged cylindrical volume around the ion p
A value of1.5 nmwas calculated for the effective cylinder rad
a value that is in good agreement with the estimation of tra
overlapping above 1013 ion/cm2 ﬂuence value. The overlap
cylinders/tracks seems to lead to the aggregation of monovac
cies forming large vacancy clusters or voids, revealed by the set
in of a second, longer living trapped positron state.
Acknowledgements
The support provided by the Hungarian-Portuguese Bilat
Scientiﬁc agreement TET No. P23/03 is gratefully acknowledg
The irradiation in Dubna was supported by a special fund of
Hungarian Academy of Sciences.
The authors are thankful to Dr. G. Huhn for AFMmeasureme
References
[1] B. Canut, A. Benyagoub, G. Marest, A. Meftah, N. Moncoffre, S.M.M. Ram
Studer, P. Thevenard, M. Toulemonde, Phys. Rev. B 51 (1995) 12194.
[2] V.A. Skuratov, S.J. Zinkle, A.E. Eﬁmov, K. Havancsak, Nucl. Instrum. Methods
Res. B 203 (2003) 136.
[3] V.A. Skuratov, A.E. Eﬁmov, D.L. Zagorskii, Phys. Solid State 44 (2002) 165.
[4] A.P. de Lima, C. Lopes Gil, P.M. Gordo, M. Duarte Naia, in: Proc. Workshop o
and Slow Positron Beam Utilisation, Costa da Caparica, Portugal, 1998, p. 1
[5] W. Bauer-Kugelmann, P. Sperr, G. Ko¨gel, W. Triftshauser, Mater. Sci. Forum
365 (2001) 529.
[6] A. van Veen, H. Schut, J. Haakvoort, R.A. Vries, M.R. Ijpma, AIP Conf. Proc
(1990), p. 171.
[7] V.A. Skuratov, private communication.
[8] L. Liszkay, P.M. Gordo, K. Havancsa´k, V.A. Skuratov, A. de Lima, Zs. Kajcsos, M
Sci. Forum 445–446 (2004) 138.
Fig. 3. Overall positron trapping rates in monovacancies as a function of ﬂuence
inside the cylindrical volume around the ion track. Fluence range: 5  109–
5  1010 ion cm2. The line serves only to guide the eyes.
P.M. Gordo et al. / Applied Surface Science xxx (2008) xxx–xxx79, p.
olids,annihilation results indicate that these defects might be also
formed by inelastic collisions with electrons. It was also observed
[1] that collective electronic excitations can create defects inPlease cite this article in press as: P.M. Gordo et al., On the defect patt
range, Appl. Surf. Sci. (2008), doi:10.1016/j.apsusc.2008.05.185[9] R.N. West, in: P. Hautoja¨rvi (Ed.), Positrons in Solids, Springer, New York, 19
89.
[10] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in S
Pergamon, New York, 1985.ern evolution in sapphire irradiated by swift ions in a broad ﬂuence


On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range

Council on Academic Affairs

Eastern Illinois University

Eastern Illinois UniversityThe KeepMinutes Council on Academic Affairs1991January 24, 1991Council on Academic AffairsFollow this and additional works at: http://thekeep.eiu.edu/caa_minutesThis Article is brought to you for free and open access by the Council on Academic Affairs at The Keep. It has been accepted for inclusion in Minutes byan authorized administrator of The Keep. For more information, please contact tabruns@eiu.edu.Recommended CitationCouncil on Academic Affairs, "January 24, 1991" (1991). Minutes. 886.http://thekeep.eiu.edu/caa_minutes/886

English

January 24, 1991

The Keep

http://estudogeral.sib.uc.pt/jspui/bitstream/10316/4333/1/file4101581100924b3c9290a89972da6f4d.pdf

On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range

Abstract

Similar works

Full text

Available Versions

Estudo Geral

Eastern Illinois University

The Keep