NUM B Nuclear Instruments and Methods in Physics Research B 91 (1994) 187-191 North-Holland Beam Interactions

with Materials A Atoms

Defect creation in tracks produced by high energy carbon clusters in LiF

A. Perez @*, M. Dijbeli b and H.A. Synal b a LXpartement de Physique des Matiriaux, Universite’ Claude Bernard Lyon 4 F-49622 Weurbanne, France b Paul Scherrer Institute, c/o IMP HPK ETH Hiinggerberg, CH-8093 Ziirich, Switzerland

LiF single crystals have been bombarded at room temperature with C3 and C, clusters with energies of 1.74 MeV/carbon atom and fluences ranging from 101’ to 1014 carbon atoms/cm’. Point defects (color centers: F, F, etc.) resulting from electronic excitation processes were measured by optical absorption spectroscopy. The defect concentrations were compared to those produced by irradiations with single “C ions with the respective energy and dose. Also using a differential optical absorption technique, it was possible to determine the defect concentrations close to the sample surface, when the tracks associated to each carbon atom of the cluster overlap. In this zone where the “cluster effect” is maximum due to the very high density of electronic excitations, enhanced defect production is observed. Defect concentrations as large as those obtained previously by Kr and Xe irradiations at GANIL have been measured. In addition to the high production rates of defects observed, aggregation laws (i.e.

F --f F2) characteristic of cluster irradiations are also deduced.

1. Introduction

Energetic ions penetrating matter are slowed down by momentum transfer to target atoms (nuclear stop- ping), and by excitation of the electronic system of the target (electronic stopping). These interaction mecha- nisms are rather well known at the present time, and appropriate theoretical models exist to calculate the energy depositions and ranges of ions in solids [1,2]. Concerning the damage resulting from the energy dissi- pation we have to consider the specificity of the energy deposition by ions in matter: a very high density of energy dissipated in a very short time (- lo-l4 s) in a small volume surrounding the ion trajectory. For low- energy ions (- keV/amul when the interaction by nuclear elastic collision dominates, this energy localiza- tion volume is the collision-cascade volume [3,4]. For higher-energy ions ( - MeV/amu), the electronic stop- ping is preponderant, and in this case the deposited energy will be localized in a track containing the &rays emitted from the ion trajectory that can be considered as a linear source of electrons [5,6]. In the case of energetic ion-clusters, it is known that fragmentation occurs on the impact on the surface of the target. However, in a limited depth from the surface, the individual ions originating from the cluster are suffi- ciently close to propagate in the same track [7]. In this so called “cluster zone” the superposition of the en-

* Corresponding author.

ergy depositions of each ion, in the same track-volume, during the same time, leads to very high local densities of energy unattainable by any other energy deposition process. After the cluster zone, due to the repulsion between ions and because of straggling, they are suffi- ciently separated to be considered as individual ions propagating in individual tracks.

The works presented in this paper are especially concerned with the defect creation in the cluster zone near the surface of the target in which a very high density of energy is deposited via electronic processes. As for the appropriate targets that can be used for these studies, a special mention must be given to ionic crystals, and especially alkali halides. Such materials are very sensitive to electronic excitations for the cre- ation of point-defects in the anionic sublattice (color centers). These result from the rather large amount of ionic relaxation that follows any electronic change in- ducing a motion of a halide atom [8]. Also, the primary defects (Frenkel pairs) as well as the aggregate-centers are well known and easily revealed using optical ab- sorption measurements, which are very sensitive, non- destructive, and which give a quantitative evaluation of the local concentrations of defects in the tracks [6]. It is also interesting to remark that pure ionic crystals are not amorphized by heavy-ion bombardments. In this case, the point-defect structure which exists up to a very high level of energy deposition allows one to study some interesting effects, such as the departure of lin- earity in the defect production, saturation effects, and aggregation mechanisms. Among all the alkali halide

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III. HALIDES

188 A. Perez et al. /Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 187-191

crystals, we have chosen for our studies lithium fluo- ride (LiF), which is one of the less hygroscopic and which can be easily cleaved into thin platelets very convenient for the optical absorption measurements. These crystals have been bombarded with carbon ions and carbon clusters (C, and C,) using the tandem accelerator of the Paul Scherrer institute. Energies per carbon atom ranging from 1.3 up to 1.8 MeV were sufficiently large to consider only the electronic energy loss process for the energy deposition. Fluences in the range from 5 X 1011 to 1 x 1014 carbon atoms/cm’ were used in order to investigate a system with individ- ual tracks (- 5 X 1011 to 1 X 101’ ions/cm’) and to study the overlapping effects between tracks (- 1 x 101’ to 1 X 1Ol4 ions/cm’). A differential optical ab- sorption technique has been developed to study the cluster zone near the surface of the samples. This technique consisted in subtracting the optical absorp- tion spectrum of a crystal irradiated with individual carbon ions from the spectrum of the crystal irradiated with clusters. The energy of individual carbon ions was chosen in order to correspond to the energy of carbon ions of the cluster at the end of the cluster zone, when they can be considered as completely separated.

2. Experimental procedure

Thin platelets of LiF were cleaved from an ultra- high purity single-crystal block purchased from Quartz et Silice. The dimensions were 15 mm X 15 mm and the thickness was 0.5 mm. These platelets were irradi- ated at room temperature with carbon ions C+ and clusters C; and Cl from the tandem accelerator of the Paul Scherrer Institute. The energies varied from 1.3 to 1.74 MeV/carbon atom and the total doses from 5 x 1011 up to 1 X 1014 carbon atoms/cm*. The cur- rent densities on the targets during the irradiations were in the range 1Og-1O11 carbon atoms/cm’s, de- pending on the total dose attained. The beam was scanned in order to obtain a homogeneously bom- barded surface of 1 cm’.

Optical absorption measurements were performed after irradiations using a Cary 17 double beam spec- trophotometer. A differential optical absorption tech- nique was used to measure the defect production in the cluster zone near the surface (Fig. 1). A simple model based on the Coulomb repulsion between the atoms of the cluster from the target surface allowed one to determine the energies and depths where the carbon atoms originating from the clusters can be considered as completely separated. As shown in Fig. 1, for clusters having an energy of 1.74 MeV/carbon atom, the total penetration depth determined using the TRIM calculation code [9] is 2.06 ym. At a depth of 0.33 pm, the carbon atoms of the incident cluster are

Cluster i Single carbon i zone : ion zone

i 2.06 km * *

I_ 1.73 w +i

Fig. 1. Schematic view of a cluster-irradiated target showing the cluster zone and the individual ion zone. The depths indicated have been calculated using the TRIM code [9] for carbon atoms originating from the cluster having the energy of 1.74 MeV/C-atom and single carbon ions of 1.3 MeV to

simulate the individual ion zone.

separated by a mean distance of about 3-4 nm which corresponds nearly to the estimated radius of the tracks associated to individual carbon-ions. Consequently, af- ter this depth of 0.33 km, we can consider that the crystal is irradiated by individual carbon ions having an energy of 1.3 MeV. In such a case, the defect wncen- tration in the cluster zone was obtained by subtracting the optical spectrum of a crystal irradiated with 1.3 MeV single carbon-ions from the spectrum of a crystal irradiated with the same dose of carbon atoms in the form of clusters having an energy of 1.74 MeV/carbon atom.

3. Results

3.1. Cluster effect on the primary defect creation

In Fig. 2 the F (optical absorption band at 250 nm) and F,-centers (optical absorption band at 450 nm) growth curves obtained with LiF crystals bombarded with C, C, and C, particles at an energy of 1.74 MeV/carbon atom are presented. These curves pre- sent a classical behavior with a linear part in the log-log scale at low doses (< 1 X 1013 C/cm2) and a saturation effect for higher doses, as observed with various heavy ions [6,10]. However, one has to remark that no significant difference is observed in the defect

A. Perez et al. /Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 187-191 189

production by C, C, and C, when the measurements

integrate the complete colored depth (2.06 pm). On the contrary, in the cluster zone near the surface (see section 2) an increase of the defect production is observed as a function of the size of the incident clusters. This increase is not greatly important for C, compared to C-irradiations but becomes significant for C,-irradiations. This observation is in a good agree- ment with the non-linearity effect [ll] that can be expected in the cluster zone from the superposition of the energy depositions in the same track by all the ions originating from the incident cluster.

3.2. Compakon of the primary defect creation for clus- ters and heavy ions

In a previous study [lo], using high energy (GeV- range) heavy ions produced in the Grand National accelerator GANIL in Caen, we studied the defect creation in LiF crystals in a wide range of energy deposition via electronic processes (lo-’ to 20 MeV/p,m). These results are interesting to compare with our present results with carbon clusters. In Fig. 3 the F-center creation as a function of the electronic energy loss for alpha particles (14 MeV/amu), Ne (40 MeV/amu), Ar (60 MeV/amu), Kr (42 MeV/amu) and Xe (27 MeV/amu) irradiations are reported. The F-center productions measured in the cluster zone with C, C, and C, having an energy of 1.74 MeV/carbon atom are shown on the same diagram. If the defect

. c, b 1.74 MeVlC AC 51 10'8

101s lOI2 10'3 1oL4

FLUENCE (C/cd)

Fig. 2. F (a) and F2 center (b) growth curves obtained with LiF crystal bombarded with C, C, and C, particles at an energy of 1.74 MeV/carbon atom. Curves c, d and e repre- sent the F-center production and curves f, g, h the F,-center production in the cluster zone near the crystal surface for C,

C, and C,, respectively.

101 I 10-Z 10-l loo lo1

(dE'dx)c,nct. (MeV/ym)

Fig. 3. F-center production as a function of electronic energy loss in tracks of heavy ions (alpha-particles - 14 MeV/amu, Ne - 40 MeV/amu, PJ - 60 MeV/amu, Kr - 42 MeV/amu and Xe - 27 MeV/amu), carbon ions (1.74 MeV) and carbon-clusters (C, and C, - 1.74 MeV/carbon atom) in the

cluster zone near the crystal surface.

creations with C and C, are roughly aligned with those of other heavy ions, it is clear that the F-center cre- ation with C, starts to depart from the general behav- ior. The F-center production with C, is as high as those of Kr or Xe ions. On the other hand, it is remarkable that the slope of the F-center growth curve, observable by joining C, and C, data in Fig. 3, is larger than those characteristic of heavy ions from alpha-par- ticles up to xenon.

3.3. Point defect aggregation

Another evidence of the cluster effect concerns the aggregation of primary defects in the tracks. In fact the F,-center production is enhanced in the case of C, and C, irradiations compared to heavy ions (Ne, Ar, Kr, Xe) (Fig. 4). If we assume that the separate F-center creation is followed by pairing of randomly close de- fects to form F,-centers we must observe an F,-center concentration proportional to the square of the F- center concentration ([F,] =K[F]‘). In fact, the law experimentally observed with heavy ions is [Fz] = K[F]1.85 [lo]. Thus, the enhancement of the F,-center production for C, and C, is not surprising since for these low velocity ions the effective track radius (e.g. range of &electrons) is smaller and therefore the vol- ume concentration of F-centers for a given dE/dx is larger. In addition it has been shown that the propor- tionality factor K depends on the rate of energy depo- sition in the track [10,12]. In the case of clusters we

III. HALIDES

190 A. Perez et al. /Nucl. Ins@. and Meth. in Phys. Res. B 91 (1994) 187-191

I 1 C

5

*

Xe C

3

I :;;I Kr

Ar - Ne C

101' I

0.1 1 10 <dE/dx),_,. (MeV/ pm)

Fig. 4. F,-center production as a function of electronic energy loss in tracks of heavy ions (alpha-particles - 14 MeV/amu, Ne - 40 MeV/amu, Ar - 60 MeV/amu, Kr - 42 MeV/amu and Xe - 27 MeV/amu), carbon ions (1.74 MeV) and carbon-clusters (C, and Cs - 1.74 MeV/carbon atom) in the

cluster zone near the crystal surface.

also observe a decrease of the K-factor from C, and C, parallel to the curve obtained with heavy ions but shifted towards higher K-values by a factor of 4-5 as shown in Fig. 5. In order to be able to compare the cluster results with the heavy ion irradiation it has been assumed that the atomic number of the clusters is

I I.. I., 0,. (,, ‘, 1 ‘, ‘1

0 10 20 30 40 50 60

Atomic number of the incident ion

Fig. 5. Evolution of the proportionality factor K in the aggre- [7] M. Dobeli, U.S. Fischer, M. Suter and H.A. Synal, Int.

gation law for Fz-centers ([F,] = K[F]1s5) as a function of the Conf. on Ion Beam Modification of Materials, Heidel- atomic number of the incident ions and clusters. berg, September 7-11, 1992, Poster Session PS4.

equal to the sum of the atomic numbers of their constituents.

4. Conclusion

The study of the point defect creation in ionic crystals bombarded with high energy cluster-ions is an attractive method to demonstrate the high energy den- sity effects in the tracks near the target surface when the atoms originating from the clusters are concen- trated. In the case of C!, and C, clusters in LiF, a departure from linearity in the defect production be- comes significant for C, leading to higher color center concentrations than observed with single heavy ions. The aggregation kinetic of F-centers into F,-centers is also enhanced in the case of cluster bombardments. Finally the nanostructures of defects in the tracks in the cluster zone are different for clusters compared to single heavy ions which must induce different proper- ties for the cluster irradiated targets as well as differ- ent annealing behaviors. Experiments in this field are in progress to complete the preliminary results re- ported in this paper.

Acknowledgements

We are indebted to M. Fallavier and J.P. Thomas of the Institut de Physique Nucleaire, Univ. Claude Bernard, Lyon I, for fruitful discussions and calcula- tions of carbon atom repulsion following the explosion of the cluster at the crystal surface. These calculations allowed the determination of the convenient energies of clusters and single carbon ions to apply the differen- tial optical absorption technique.

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