Nuclear Instruments and Methods in Physics Research B 88 (1994) 25-28 North-Holland

NOM B Beam Interactions

withMaterials&Atoms

Measurements of point defect creation related of electronic excitations produced by energetic cluster bombardments

A. Perez a~*, M. DGbeli b and H.A. Synal b

to high densities carbon

a Dbpartement de Physique des Matkriaux, Universite’ Claude Bernard Lyon I, F-69622 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 C, and C, clusters with energies of 1.74 MeV/carbon atom and fluences ranging from 10” to 1Or4 carbon atoms/cm’. Point defects (color centers: F, Fa, . . ) 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 same respective energy and dose. 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 defect production rates observed, aggregation laws (i.e. F + F,) 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 deposition 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/amu) 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), electronic stopping is preponderant, and in this case the deposited energy will be localized in a track containing the b-rays emit- ted 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 sufficiently close to propagate in the same track [7]. In this so called

* Corresponding author.

“cluster zone” the superposition of the energy deposi- tions 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. Alter the cluster zone, due to the repulsion between ions and because of straggling, they are sufficiently separated to be considered as individual ions propagat- ing in individual tracks.

The work presented in this paper is especially con- cerned 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, special mention must be made of ionic crystals, and especially alkali halides. Such materials are very sensi- tive to electronic excitations for the creation of point defects in the anionic sublattice (color centers). These result from the rather large amount of ionic relaxation that follows any electronic change inducing directed ionic motion of a halide ion [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 subsists up to a very high level of energy deposition allows one to study

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26 A. Perez et al. / Nucl. Znstr. and Meth. in Phys. Res. B 88 (1994) 25-28

some interesting effects, such as the departure of lin- earity in the defect production, saturation effects, and aggregation mechanisms. Among all the alkali halide 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 101’ to 1 X 1014 carbon atoms/cm’ were used in order to investigate a system with individ- ual tracks (- 5 x 1O1l to 1 X 101’ ions/cm’) and to study the overlapping effects between tracks (- 1 x 1012 to 1 X 1014 ions/cm2). 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 to 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 Slice. 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 Cf and clusters C; and C: 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 lOI carbon atoms/cm2. 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 to reach. The beam was scanned in order to obtain a homogeneously bom- barded surface of 1 cm2.

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 to determine the orders of magnitude of 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

I Cluster i zone i

Single carbon i ion zone

1 2.06 pm 1 c:

0.33 pm 1 t i

c _?)

1.3 MeV

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 an energy of 1.74 MeV/C atom and single carbon ions of 1.3 MeV to

simulate the individual ion zone.

MeV/carbon atom, the total penetration depth deter- mined using the TRIM calculation code [9] is of the order of 2.06 p.m. At a depth of 0.33 pm, the carbon atoms of the incident cluster are separated by a mean distance of about 3 to 4 nm, which nearly corresponds to the estimated radius of the tracks associated with individual carbon-ions. Consequently, after this depth of 0.33 pm, we can consider the crystal to be irradiated by individual carbon ions having an energy of 1.3 MeV. In such a case, the defect concentration in the cluster zone was obtained by subtracting the optical spectrum of a crystal irradiated with 1.3 MeV single carbon ions to 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 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 on the log-log scale at low doses (< 1 X 1013 C/cm2) and a

A. Perez et al. /Nucl. I&r. and Meth. in Phys. Res. B 88 (1994) 25-28 27

saturation effect for higher doses, as observed with various heavy ions [6,10]. However, no significant dif- ference is observed in defect 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 in- crease of defect production is observed as a function of the incident clusters size. This increase is not too important for C, compared to C irradiations, but be- comes significant for C, irradiations. Such observation is in good agreement with the nonlinearity effect [ll] that can be expected in the cluster zone from the superposition of the energy depositions in the same track by all ions originating from the incident cluster.

3.2. Comparison 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 GANIL accelerator in Caen, we studied defect creation in LiF crystals in a wide range of energy deposition via electronic pro- cesses (lo-’ to 20 MeV/pm). These results are inter- esting 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

b

10'2 10'3 10'4

FLUENCE (Chd)

Fig. 2. F (a) and F, 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 in the cluster zone near the

crystal surface for C, C, and C,, respectively.

1 ,,““I ““‘1

Xe

;/

10' 1 ,,l,d ,“,d ,,u/ 1O-2 10-l 100 10'

(dE’dx)e,ece. (Mel’/ 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, Ar 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.

1.74 MeV/carbon atom are shown in the same dia- gram. If the defect creations with C and C, are roughly aligned with those of other heavy ions, it is clear that the F-center creation with C, starts to depart from the general behavior. 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-particles up to xenon.

Another evidence for the cluster effect concerns the aggregation of primary defects in the tracks. In fact the Fz-center production is enhanced in the case of C, and C, irradiations compared to heavy ions (Ne, Ar, Kr, Xe> [lo]. If we assume that the separate F-center creation is followed by pairing of randomly close de- fects to form F, centers we should observe an F,-center concentration proportional to the square of the F- center concentration ([FJ = K[F]‘). In addition it has been shown that the proportionality factor K depends on the rate of energy deposition in the track [10,12]. In the case of clusters we also observe a decrease of the K factor from C, and C, parallel to the curve obtained with heavy ions [lo] but shifted towards higher K

values by a factor of 4 to 5.

4. Conclusion

The study of point defect creation in ionic crystals bombarded with high energy cluster ions is an attrac-

28 A. Perez et al. /Nucl. Instr. and Meth. in Phys. Res. B 88 (1994) 25-28

tive method to demonstrate the high energy density effects in the tracks near the target surface when the atoms originating from the clusters are concentrated. In the case of C!, and C, clusters in LiF, a departure from linearity in the defect production becomes signifi- cant for C, leading to higher color centre concentra- tions than observed with single heavy ions. The aggre- gation kinetic of F centers into Fa centers is also enhanced in the case of cluster bombardments. Finally the nanostructures of defects in the tracks in the clus- ter zone are different for clusters compared to single heavy ions which must induce different properties for the cluster irradiated targets as well as different an- nealing behaviors. Experiments in this field are in progress to complete the preliminary results reported in this paper.

We are indebted to M. Fallavier and J.P. Thomas of the Institut de Physique Nu&aire, 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 us to determine the convenient energies of clusters and single carbon ions to apply the differential optical absorption technique.

References

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