MeV-cluster impacts and related phenomena

Nuclear Instruments and Methods in Physics Research B 116 (1996) 13-17

__ !iB ELSEVIER

NOM B Beam Interactions

with Materials&Atoms

MeV-cluster impacts and related phenomena

A. Perez a, * , M. DSbeli b, H.A. Synal b

a Dipartement de Physique des Mat&iaux. Uniuersiti Claude Bernard Lyon 1, 69622 Villeurbanne, France b Pout Scherrer Institute c/o IPP HPK, ETH Honggerberg, CH-8093 Zrich. Switzerland

Abstract Point defect creation in 8.7 MeV C,-cluster irradiated LiF crystals have been studied in a dose range from 2 X 10” up to

1 X 1013 CT/cm* and compared to the defect creation in similar conditions with single energetic carbon ions. An enhancement of the primary defect creation (F-centres) and aggregation (Fa-centres) has been measured in a near surface region as deep as about 0.3 pm corresponding to an energy per carbon atom of the incident cluster decreasing from 1.74

MeV at the impact down to 1.3 MeV. In this region, the carbon atoms of a cluster are confined in a track with a radius (N 100 nm) comparable to that of a single carbon ion track. This leads to a very high locally deposited energy density which is directly responsible of the enhanced defect production measured.

1. Introduction

The field of clusters which concerns the edifices of few

atoms to few thousands of atoms, appears specially fasci- nating, since these systems are intermediate between small molecules and bulk solids. One of the first challenges for researchers in cluster science was to produce clusters of any kind of materials in a wide range of size. This was achieved with the development of supersonic molecular beams [l-4] and particularly with the laser vaporization source [5]. ‘Ihis last technique which combines the advan- tages of laser ablation and supersonic expansion allows the production of clusters of any kind of materials, even the most refractory or complex systems (bimetallic clusters, oxides . . . ). However, since the discovery of new species

in molecular beams is interesting 161, a second challenge consists of depositing these clusters on substrates to syn- thetize new materials or to use these clusters as heavy projectiles to bombard targets. Different regimes can be distinguished depending on the kinetic energy per atom stored in the cluster (E,) compared to the intra-bond energy (E,,). For Ek -=c E,, the corresponding regime is the low energy cluster beam deposition (LECBD) 171. In this technique, clusters do not fragment at the impact upon the substrate leading to the formation of nanostructured materials by random stacking of clusters. The high energy ionized cluster beam deposition (ICBD) consists of ionizing and accelerating clusters in the keV energy range (Ek > E,) before deposition on the substrate. In this case, the energetic clusters fragment upon impact on the surface

* Corresponding author.

of the substrate enhancing the formation of nucleation sites and adatom diffusion. Using this technique, Yamada [8] obtained epitaxial coating of Al/Si (111) at room temperature despite a 25% misfit. Using clusters with a large energy per atom (E, > keV to MeV), we enter into the sputtering and the ion implantation domain (HECBB: high energy cluster beam bombardment). In classical ion implantation the target undergoes physical or chemical changes due to the damaging and doping effects. In the case of cluster impact, it is known that fragmentation occurs at the target surface [9]. However, the individual ions originating from the cluster dissipate their energy in the same interaction volume (electronic excitation track or nuclear collision cascade volume depending on the energy of ions) during the same time (- 1014 to lo-” s). This confinement effect leads to very high local densities of

energy unattainable by any other energy deposition process. The results are high and nonlinear desorption and sputtering yields [lo- 131 and damage rates [W-17]. Spe- cific non-equilibrium phase formation could be also considered.

In a previous paper [ 151, we reported some preliminary results on the defect production by carbon cluster (C, to C,) bombardments in LiF crystals. From the comparison of the damage rates for clusters and for high energy heavy ions (Ne, Ar, Kr, Xe) we showed a specific and significant cluster effect from C,. Thus point defect creation and energy confinement effect in the track studied using ionic crystal (LiF) bombarded with carbon clusters CC,) having energies per atom in the MeV-range is the subject of this paper. Light atoms such as carbon in the MeV-energy range dissipate their energy mainly by electronic pro- cesses. In this case, alkali halides are appropriate targets

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14 A. Perez et al.JNucl. Instr. and Meth. in Phys. Res. B 116 (1996) 13-17

since they are very sensitive to electronic excitations for the creation of point defects in the anionic sublattice (colour centres) [ 181. Using an optical absorption technique it is possible to study the point defect creation in the near surface region of the crystals in which individual ions originating from the incident cluster are confined in the same track. Comparison of these results with those of single carbon ion bombardments is able to discuss the specific high energy density effects characteristic of cluster bombardments.

2. Sample preparation, irradiations and optical absorption measurement techniques

Thin platelets of LiF (15 X 15 X 0.5 mm3) were cleaved from an ultra-high purity single-crystal block prehardened by exposure to 60Co-gamma rays (N lo7 rad) and then annealed for 1 h at 500°C to remove the induced damage. These platelets were irradiated at room temperature with carbon ions C+ and clusters Cl from the tandem accelera- tor of the Paul Scherrer Institute. The energies varied from 1.3 to 1.74 MeV/carbon atom and the total doses from 10” to 1014 carbon atoms/cm*. The current densities on the targets during irradiations were maintained in the range 109-10” carbon atoms/cm* s depending on the total dose attained. The beam was scanned in order to obtain a homogeneously irradiated surface of 1 cm*.

Optical absorption measurements were performed after irradiations using a cary 17 double beam spectrophotome- ter. A differential optical absorption technique has been specially developed to measure the defect production in the near surface region where the cluster effect is maximum. This technique was described in details elsewhere [ 161. On the other hand, for low dose irradiated crystals (< 10’2 ions/cm*), the absorption band intensities are weak. In this case, several platelets were irradiated separately with the same dose and then stacked for the optical measurements.

As mentioned previously, in the HECBB regime clusters fragment upon impact on the surface. In this case, a simple calculation based on the Coulomb repulsion between the atoms of the cluster from the target surface is able to determine the energies and depths where the carbon atoms originating from the cluster can be considered as completely separated. For example, for clusters having an energy of 1.74 MeV/carbon atom, the total penetration depth in LiF estimated using the TRIM calculation code [19] is 2.06 p.m. However, at a depth of 0.33 p,m, the carbon atoms of the incident cluster having an energy of 1.3 MeV are separated by a mean distance of about 3-4 nm which corresponds roughly to the estimated radius of the core of the tracks associated to single carbon-ions. Consequently, after this depth of 0.33 l.Lrn, 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

concentration in the near surface region (0.33 p,m) the so-called “cluster zone” was obtained by subtracting the optical absorption spectrum of a crystal irradiated with 1.3 MeV single carbon-ions from the spectrum of a crystal irradiated with the same fluence of carbon atoms in the form of clusters having an energy of 1.74 MeV/carbon atom.

3. Point defect creation in LiF bombarded with single carbon ions

Typical optical absorption spectra obtained with LiF crystals irradiated at room temperature with 1.3 MeV C+-ions in the dose range from 2X 10” up to 1014 ions/cm* are reported in Fig. 1. Absorption bands corresponding to well known point defects are observed as a function of dose: F-centres (band at 250 nm>, F,-centres (band at 445 nm), Fs-centres (bands at 316 and 374 nm), F,-centres (band at 520 nm), and Fz-centres (band at 435 nm). Assuming that these bands are Gaussian in shape as a function of the photon energy and using their characteristic parameters (position and FWHM), it is possible to decom- pose the spectra into individual absorption bands in order to determine their intensities and then the corresponding

i i

0.5 e

0.4 h

.f %

4 03 z y

e 0.2

0.1

Ot. 1 200 300 400 500

wavelength (nm) 600

Fig. 1. Typical optical absorption spectra obtained with LiF crystals irradiated at room temperature with 1.3 MeV carbon ions

at doses of (a> 2X1O’2,(b) 5X 10’*,(c) 1 X 1013, (d) 2X lOI and

(e> 5 X lOI ions/cm2. The positions of the main bands due to F and F-aggregate centres (F2 to F4) are marked by the arrows.

A. Perez et al./Nucl. Insir. and Meth. in Phys. Res. B 116 (1996) 13-17 15

10 F 10” lo’* 10’3 1oE4 I

fluence (ions/cm2)

Fig. 2. F and F-aggregate cenmes @2 to F4) growth curves deduced from the Rls of the optical aborption spectra of 1.3 MeV carbon ion irradiated LiF cryStak

defect concentrations which are directly prapminnal to the band areas. In Fig. 2 are reponed the F and F-aggregate centre growth curves deduced from the fits of the optical absorption spectm of 1.3 MeV C+-ion irradiated crystals. The absorption coefficient at the maximum of the band (a,,) reported on the venical axis in Fig. 2 has been calculated using the thickness of the coloured zone which corresponds to the penetration depth of 1.3 MeV carbon ions in LiF: 1.73 pm [19]. For the particular case of F and F,-centres, since the oscillator strengths of the corresponding optical transitions are known, it is possible to estimate

the defect concentrations (C (cm-3)> from the absorption coefficients (amax (cm -I)) using the relations: C, = 9.48

x 10’5 x (Y F,,ax and CF* = 5 X 1015 X +&ax. In a previous paper 1201 concerned with the point defect

depth profiles measured in LiF bombarded with various high energy (GeV range) heavy ions (Ne, Ar, Kr, Xe), we deduced the radius of the mcks containing the point defects in a wide range of ian mass and energy. From the extrapolation of these results towards lower ion masses, we can estimate a radius of the Uacks for single carbon ions in the MeV-energy range of rhe order of 8 to 9 nm. In the particular case of 1.74 MeV C +-ions, taking into account the electronic energy loss ((d E/dx)e = 1.37 keV/mn) calculated using the TR[M code [19], we deduce a total energy density deposited inside af each individual single

carbon ion track in the near surface region of the LiF

target of the order of 5.4 eV/nm’. The average F-centre

concentration measured in the corresponding region, using the differential optical absorption technique described in Section 2 [OD, (near surface region) = OD, ( I.74 MeV C’-ions) - OD, (1.3 MeV Cc-ions), OD, being the apti- cal density of the F-band], is about 0.5 F-cenne per mn of track length, corresponding to - 2 X IO’* F/cm3 in each

individual track. This is jn a rather good agreement with

the previous results in LjF bombarded with rare gas ions (Ne to Xe) [20] giving a saturated concentration of F-centres in each individual tracks of the order of 4 X 10” F/cm3.

4. Point defect creation in LiF bombarded with carbon

clusters

Optical absorption spectra containing the same bands as those observed in C+-ion irradiated crystals (see Fig. 1) are obtained for carbon cluster (C3 and C5) irradiated samples. Moreover, one has KI remark that no sigvificant difference is observed in the defect production by C, C3 and C5 when the measurements integrate the complete coloured depth (2.06 p,rn for I.74 MeV carbon ions). On

the contrary, in the cluster zone (0.33 Frn, see Section 2), near the surface, an increase of the defect production is observed as a function of the size of the incident clusters [16]. In the particular case of L# crystals inadiated with 8.7 MeV C,-clusters (1.74 MeV/C) in the dose range from 2 X 10” up to L X iOL3 Cs/cm2, we have deter- mined the F-cenee production in the cluster zone using tie differential optical aborptjon technique described in Sec- tion 2 [OD, (cluster zone) = OD, (cluster Cs-1.74 MeV/C) - OD, (5 C-ions- 1.3 h4eV>]. These results are reported in Fig. 3.

Using the simple model of cylindrical ion tracks [21] which was successfully applied to estimate the radius of the tracks in LiF bombarded with various high energy heavy ions [20,21], it is possible to investigate the electronic energy loss confinement effect in the volume of the track and the consequences on the defect production. This model is based on the hypothesis that each individual track is saturated with primary point defects (F-centres) because of the very high local density of energy dissipated, characteristic of heavy ions. Consequently, when two tracks overlap, no more isolated F-<en&es are created in the overlapping region. In this case the F-centre creation law obtained from this model is in the form [21]: C, = c,[l -

exp( - n r2+)], where C, is the total F-centre concentration obtained for an irradiation with an ion dase 4, cF is the saturated concentration of F;-cenaes in each individual track, and r is the radius of the track. This relation has been applied to fit the F-centre growth curve measured in the cluster zone for 8.7 MeV C,-cluster irradiations (see inset in Fig. 3). However, only the low dose region of the curve (from 2 X lOi up to I X 1Ol2 C,/rmz) in wtih

16 A. Perez et al./Nucl. Instr. and Meth. in Phys. Res. 3 116 (J996) 13-17

101 “(,(,,I ‘,..- ‘(,(,.,’ ,“‘/ 1o’O 10” 10” 1o13 lOI

fluence (CS+- clusters/cm2)

Fig. 3. F-centre growth curve measured in tbe near surface region (“cluster zone”, thickness = 0.33 km> of 8.7 MeV C,-cluster irradiated LF crystals in the dose range from 2X 10” up to 1 X lOI C: /cm’. The best fit obtained in tbe limited low dose range (2X 10” to 1 X 10” Cz /cm*>, using ‘tie relation given in Section 4 from a simple model of cylindrical ion tracks [21], is represented by tbe solid line in the inset.

the overlaping effects between tracks are not too dominant, has been used to deduce a radius of the &,-tracks of the order of 10 nm with a saturated concentration C, = 8.2 X

10” cmP3. In fact, in the high dose regime when the overlapping effects become dominant, more sophisticated model than the previously described one has to be considered. The radius r,-, = 10 nm of thti C,-tracks in the near surface region is comparable to the radius of the tracks for single carbon ions in the same region (I~ = 8 to 9 nm), previously reported in Section 3. This result confirms the confinement of all carbon atoms of the cluster which propagate in the same track, in a region close to the sample surface. This confinement effect leads to a very high energy density in the track. Roughly, if we consider that the energy dissipated in the cluster zone, by the cluster C,, is five times [22] the energy dissipated in the same zone by a single carbon ion (1.37 keV/nm, see Section 31, we obtain an energy density in the C,-track of the order of 22 eV/nm3. This value is four times higher than the value previously calculated in Section 3 for (?-ions. As a consequence of such a high energy density, the saturated concentration of F-centres in the C,-cluster track (C, =

8.2 X 10” cm- 3, is about twice the concentration measured in single rare gas ion tracks [20], and four times the one measured in single carbon ion tracks (see Section 3). Also, the aggregation laws which govern the formation of complex defects (F-aggregate centres) from pairing of randomly close primary defects are modified. In fact, such effect was previously observed in the case of LiF bombarded with heavy ions [20]; however, it is significatly amplified in cluster irradiated crystals. In particular, a large enhancement of the F,-centre formation as a function of the cluster size was previously reported [16] and is confirmed in the present study. In this case, direct formation of complex centres inside the core of the cluster track due to the very high density of electronic excitations could be considered. Consequently, different creation laws than those previously established from experiments with crystals irradiated with ionizing radiations must be proposed.

5. Conclusion

High energy cluster bombardments is an attractive method to produce very high local densities of electronic excitations in materials. By controling the nature and the number of atoms composing the cluster as well as their energy, it is possible to investigate a wide range of energy density dissipated unattainable with any other single heavy ion bombardment. Moreover, using energetic clusters it is possible to control separately the amount of energy (from the cluster size) and the volume in which this energy is deposited (from the radius of the track which depends on the nature of the atoms composing the cluster). However, this effect is possible only in the near surface region of the bombarded materials in which the confinement of atoms of the incident cluster in the same track takes place. After this zone close to the surface, the individual ions originating from the cluster are sufficiently separated to produce an irradiation effect comparable to classical single ion bombardments. In this context, for a first approach of the damaging process characteristic of the “cluster effect”, the studies of the point defect production in cluster bombarded alkali halides are well suited. In the particular case of 8.7 MeV C,-cluster irradiated LiF crystals we have clearly observed an enhancement of the primary point defect (F-centres) creation and aggregation (F,-centres). This effect has been measured in a near surface region as deep as about 0.3 pm corresponding to an energy per carbon atom of the incident cluster decreasing from 1.74 MeV at the impact of the surface down to 1.3 MeV. In this region, the confinement of carbon atoms of the cluster in a track, the radius of which (- 10 nm) is comparable to those of single carbon ion track leads to a very high local energy density deposited which is directly responsible of the enhanced defect production measured. Future experiments with a systematic evolution of the local energy density from the nature, the size and the energy of the

A. Perez et al./Nucl. Instr. and Meth. in Phys. Res. B 116 (1996) 13-17 17

incident clusters will be of great interest to investigate the specific energy confinement effect characteristic of cluster impacts as well as the behaviour of materials under such large instantaneous energy depositions.

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MeV-cluster impacts and related phenomena