Mechanisms of ion beam induced atomic mixing in solids

Materials Science and Engineering A253 (1998) 194 – 201

Mechanisms of ion beam induced atomic mixing in solids Wolfgang Bolse * II Physikalisches Institut and Sonderforschungsbereich 345, Georg-August-Uni6ersita¨t Go¨ttingen, Bunsenstrasse 7 -9, D-37073 Go¨ttingen, Germany

Abstract In the present paper, selected typical studies on the ion-beam mixing of bi- and multi-layer systems are reviewed. It is shown that by proper variation of the materials (atomic number, chemical affinity between top and bottom layer) and of the irradiation conditions (ion species, energy, fluence, target temperature), significant conclusions can be drawn concerning the relevant mixing mechanisms. It is found that low temperature ion-beam mixing of light systems (Z 5 18) is of pure ballistic character, without any influence of chemical driving forces. For higher atomic numbers and low or medium mass ions, mixing occurs by chemically biased diffusion in spatially separated local thermal-spikes. For very heavy ions mixing effects, which are non-linear with respect to the deposited energy density, point to the formation of coherent global spikes along the ion path by the overlapping of local spikes. Very heavy ions might also be able to initiate thermal-spikes in otherwise ballistic systems (Z 5 18) at their end of range (end-of-range spikes) by a high density of subcascades. Chemically guided motion of residual defects from the collision cascade seems to play a role for ion beam induced mixing only at elevated temperatures. © 1998 Published by Elsevier Science S.A. All rights reserved. Keywords: Ion beam mixing; Ballistic systems; Mass ions; Thermal spikes

1. Introduction The ability of ion-beams to initiate atomic transport processes in solids even at very low temperatures was demonstrated in 1972, when Lee et al. [1] and van der Weg et al. [2] independently recognized atomic intermixing and silicide formation during P- and Ar-ion bombardment of Pd-coated Si. In contrast to ion implantation of atom A into matrix B, where only one single atom is added to B per incident ion, the ion bombardment of solid A – B interfaces was found to result in thousands of intermixed atoms per single ion impact. Ion beam mixing (IBM) quickly gained a widespread interest because of its technological potential for preparation and processing of new materials with novel properties, and because it is well-suited for the investigation of the mechanisms of atomic transport processes within collision cascades. Reviews on IBM on metallic, semiconducting and also insulating materials have been given by Cheng [3], Nastasi and Mayer [4], Bolse [5] and Kelly and Miotello [6].

* Present address: Institut fu¨r Strahlenphysik, Universita¨t Stuttgart, Allmandring 3, D-70569 Stuttgart, Germany. Tel.: +49 711 6853875; fax: +49 711 6853866; e-mail: [email protected]

Initially IBM was assumed to be controlled exclusively by elastic two-body collisions, and it was believed that thermodynamic constraints of interdiffusion and solid-state reactions could be easily overcome by bombarding multilayer systems with energetic heavy ions. Sigmund and Gras-Marti [7] developed a model of the ballistic mass transport in binary collisions, where the amount of mixing depends only on the atomic numbers of ion and target and the respective energy density deposited in the recoil cascade by elastic collisions, but not on the chemical properties of the involved materials. However, as was nicely demonstrated by D’Heurle et al. [8], IBM may be strongly influenced by chemical driving forces. The influence of thermochemical properties was also recognized by Johnson et al. [3,9]. These experiments clearly showed that mixing was the larger the higher was the chemical affinity between the top and the bottom layer. The influence of the chemical affinity can not be explained by a simple ballistic transport mechanism and diffusion in thermal-spikes was considered to be the main source of the observed IBM effects. A thermal-spike is characterized by a small volume along the path of either the ion (‘global’ thermal-spike) or of an energetic recoil atom (‘local’ thermal-spike) where the majority of atoms are in motion

0921-5093/98/$ - see front matter © 1998 Published by Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00727-8

W. Bolse / Materials Science and Engineering A253 (1998) 194–201

and have approximately reached equipartition of kinetic energy [10,11]. Within these volumes, a high local temperature may be achieved and transient diffusion processes may take place in the molten material, which give rise to the observed atomic intermixing. Molecular dynamics (MD) simulations strongly support such an assumption [12–14]. Basing upon the Vineyard model of the energy dissipation in thermal-spikes [11], Johnson et al. developed a phenomenological model of the interface mixing in thermal-spikes by assuming a cylindrical thermal-spike volume around the ion path [3,9]. Chemical biasing of IBM was treated in analogy to Darken’s analysis of thermally activated chemical interdiffusion. While the model nicely describes the dependence of IBM on the mixing enthalpy and the cohesive energy for the data accumulated by Johnson et al. [3,9], many bi-layer couples were found which should belong to the global spike regime, but exhibited a linear rather than the predicted square scaling of the amount of mixing with the deposited energy density [15 – 17]. To account for such observations, ‘local’ thermal-spike models were developed where mixing occurs by diffusion in spatially separated (local) thermal-spikes initiated at the end of the collision subcascades [18,19]. However, there is still a debate going on whether or not IBM can be described without invoking thermal-spikes. Kelly and Miotello [20] have developed a transport model, which attributes the observed chemical biasing of IBM to the chemically guided motion of the residual defects of linear collision cascades at the ambient temperature. In the following, some typical experiments on ionbeam induced interface mixing and phase formation in metallic and ceramic systems [15 – 17,23 – 38] will be reviewed. By comparing the experimental data with predictions of the IBM models, it will finally be shown that depending on the atomic numbers of target and ion, one can distinguish between different temperature independent (in the following labelled as athermal) IBM regimes (ballistic, local spike, global spike, end-ofrange spike).

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bombardment, usually the same analysis techniques are employed to determine the irradiation induced alterations of the concentration profiles and of the microstructure. The aim is to investigate, as a function of the irradiation conditions (ion species, ion energy, ion fluence, target temperature, target composition,…), the amount of intermixed atoms and the resulting microstructure and phase composition. The characterization of the ion-mixed samples and the determination of the concentration profiles in most cases is done using ion-beam analysis techniques (IBA) like Rutherford backscattering spectroscopy (RBS) and non-resonant or resonant nuclear reactions analysis (NRA, RNRA). Detailed descriptions of the techniques can be found in [39,40,5]. In most cases, the deduced concentration profiles can be described by an errorfunction (bi-layer) or a Gaussian-like (marker layer) distribution, and the amount of mixing is therefore usually characterized by the increase of the variance of the atomic distribution. In the overwhelming majority of IBM experiments, this quantity was found to depend linearly on the applied ion fluence: Ds 2 = s 2(F) − s 2(0)= kF

(1)

This is illustrated in Fig. 1 for a Pt marker in Al after irradiation with ions of different atomic masses [33]. The mixing rate k strongly depends on the beam and target parameters (ion species, ion energy, ion fluence, target temperature, target composition,…), and a proper evaluation of these relationships can be used to shed light on the atomic transport mechanisms in collision cascades.

2. Ion-beam mixing experiments A typical IBM experiment generally consists of the preparation of a bi-, multi- or a marker-layer system of well-defined, sharp interfaces. Typical bi-layer thickness’ are of the order of 100 nm, while the individual layer thickness of a multi- or a marker-layer system is of a few nm or even less. After determination of the initial concentration profiles utilizing well-suited, often non-destructive depth-profiling techniques, and possibly the characterization of the microstructure and phase composition, the samples are exposed to a heavy ionbeam of several tens or hundreds of keV. After ion

Fig. 1. Broadening of a Pt marker-layer in Al as a function of the applied ion fluence for various ion species [33].

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3. Theoretical models of IBM

3.1. The ballistic model In the ballistic model by Sigmund and Gras-Marti [7], only elastic two-body collisions are taken into account and hence no influence of thermochemical potentials is expected. Because of the isotropic momentum distribution of the late-generation recoils, the atomic transport is of random-walk character, which results in error-function- or Gaussian-like broadening of interfaces and markers, respectively. Since the ratio R 2c /(EdN) only slightly varies for different materials the ballistic mixing rate kb =Gj[R 2c/(EdN)]FD

(2)

is mainly determined by its linear dependence on the deposited energy density FD. Rc :1 nm is the minimum separation distance for a stable Frenckel pair, Ed = 20– 70 eV the corresponding displacement threshold and N the atomic density of the target material. j= [4M1M2/ (M1 + M2)]1/2 is a kinematic factor, depending on the masses M1,2 of the colliding particles, and G = 0.2 is a dimensionless constant. The ballistic mixing efficiency kb/FD ranges between 0.05 and 0.2 nm5/keV, depending on the less well-known parameters Rc and Ed.

3.2. Diffusion in local thermal-spikes Collision cascades in target materials with atomic numbers Zt \20 may become space-filling as soon as the kinetic energy transferred to the recoil atoms falls below the spike-initiation threshold Ec =0.039 eV × Z 2.23 [3]. As this will occur at the end of the subcast cades, spatially separated local thermal-spikes will evolve far off the ion path. Within this hot and probably molten zone, fast transient atomic transport processes may take place. Such transient diffusion processes may be chemically driven, and in the case of high chemical affinity between the top and the bottom layer, may be strongly enhanced as compared to ballistic mixing. Considering Vineyard’s estimate of the spike temperature [11], Børgesen et al. have developed an IBM model which describes the interdiffusion of bi-layers in spherical local spikes [18]. Bolse has estimated the overlap probability for such spherical spikes and found that most likely cylindrical local spikes will form along the subcascades by overlapping spherical local spikes [19]. In both cases a similar expression was derived for the mixing rate: kls = l1{FDZ xt /(N y DH zcoh)}{1 +l2 DHmix/Hcoh}

(3)

l1 and l2 are phenomenological parameters to be fitted to experimental data. The term containing l2 describes the enhancement of the diffusivity by chemical driving forces in analogy to Darken’s analysis of chemical

interdiffusion [41]. DHcoh is the average cohesive energy of the interface forming materials and DHmix their heat of mixing. The exponents become x= 1.5, y=4/3, z= 5/3 for the spherical local-spike model, and x=1.77, y= 2/3, z =2 for the cylindrical local spike model. According to Conrad et al. [37], best fits to experimental data yield the coefficients l1 = 8.6(9 0.4) 10 − 5 nm keV2/3 and l2 = 33(9 2) for the spherical and l1 = 2.7(9 0.3) 10 − 5 nm3 keV and l2 = 50(9 4) for the cylindrical local-spike model. Like in the ballistic model, the mixing rate scales linearly with the deposited energy density. However, a strong enhancement of the atomic intermixing is predicted by the local thermal-spike models for materials of high chemical affinity, which is not expected for ballistic mixing. In addition, an explicit dependence on Zt appears in the local spike models.

3.3. Mixing in global thermal-spikes For very heavy ion mass, the collision cascade may become so dense that the local spikes overlap and form a coherent cylindrical or ellipsoidal thermal-spike (global thermal-spike) along the ion path [5,19]. The first analytical theoretical description of IBM in thermal-spikes by Johnson et al. [3,9] was based upon such a global-spike assumption. Considering Darken’s analysis of chemical diffusion, Johnson et al. derived a mixing rate kgs = l %1{F 2D/(N 5/3 DH zcoh)}{1+ l %2 DHmix/DHcoh},

(4)

where l %1 and l %2 are again fitting parameters. In contrast to the local-spike models mixing now scales with F 2D, and it does not explicitly depend on Zt. The phenomenological constants l %1 = 0.00175 nm and l %2 = 27 were determined by evaluating the mixing effect in Xe-irradiated 5d–3d and 5d–4d bi-layer couples. These experiments clearly illustrated the influence of the thermochemical properties DHmix and DHcoh on the amount of mixing. However, the predicted F 2D-scaling could not be checked because of the small variation of the deposited energy density due to the restriction of the experiments to a single ion species.

3.4. Residual defect motion at ambient temperature In order to describe chemical biasing of mixing without invoking thermal-spikes, Kelly and Miotello [20] suggested an ion-beam induced diffusion flux, which originates from the residual defects being mobile at the ambient irradiation temperature during the relaxation stage of the cascade (i.e. on a nanosecond or even larger time scale as observed in [21,22]): J ti = − D ti {1−ai (1−ai )(2hmixp/(RT(1+ p)))}dai /dx (5)

W. Bolse / Materials Science and Engineering A253 (1998) 194–201 g D ti =D bi +D ng i +D i is the total diffusion coefficient, ai is the atomic fraction of component i, and DHmix = ai (1-ai ) hmix is the heat of mixing for a regular solution. D bi is the ballistic diffusivity already introduced before, g D ng i and D i are the diffusion coefficients for non-guided and chemically guided defect motion, respectively. D ng i and D gi both should depend on the target temperature [42] and hence, also the dimensionless parameter p= g D gi /(D ng i + D i ) must become temperature dependent. In fact, when evaluating a series of sputter profiling and IBM experiments on both miscible and immiscible systems, Miotello and Kelly [43] found that the deduced values of p exhibit only a small scatter for fixed irradiation temperature, but significantly increase when increasing the target temperature. According to the model of radiation enhanced diffusion (RED) [44], the interdiffusion coefficients related to thermally activated defect motion at a given target temperature should scale with F 1/2 D .

4. Experimental results

4.1. Temperature dependence of IBM Although many data on IBM have been accumulated in the last 15 years, there is still a debate going on whether thermal-spikes are needed to understand the chemical biasing of IBM or if mixing can be described by thermally activated residual defect migration, as was suggested by Kelly et al. [20,42,43]. As was mentioned in Section 3, both the temperature dependence of ion mixing and its dependence on the deposited energy density can be used to investigate this question in more detail, and to separate between athermal and thermally activated processes at the ambient temperature. We have recently investigated IBM of Sb/Ni bi-layers at 77 and 300 K as a function of the deposited energy density FD [34,36], varying over more than two orders of magnitude by using a wide range of ion species. In Fig. 2, the mixing rate k, divided by F 1/2 D is plotted versus F 1/2 for the two irradiation temperatures. At low temD perature, k was found to scale linearly with FD. At 300 K an additional contribution to the mixing effect appears, and the mixing rate can be described by 1/2 . k= aFD +b(T)F D

(6)

b(T) is related to thermally activated diffusion of irradiation induced defects, as discussed in Section 3.4, and vanishes at 77 K. Similar observations were made in many other experiments [45 – 48], where mixing was found to become temperature independent below a critical temperature Tc. Mixing can thus be subdivided into an athermal contribution, which is present at any target temperature, and an additional thermal contribution, which is strongly temperature dependent and is

197

1/2 Fig. 2. k/F 1/2 D plotted vs. F D for Sb/Ni bi-layers irradiated with various ions at 77 and 300 K. The linear curves fitted to the data have the same slope, with the 77 K-curve crossing the origin [34,36].

present only above the critical temperature Tc. In metals Tc was found to scale with the average cohesive energy of the materials involved [49]. It should be also pointed out, that the temperature dependent contribution in Fig. 2 is in agreement with the F 1/2 D -scaling predicted by the RED model [44]. Usually, IBM experiments are performed such that during irradiation the sample is kept at low temperature, but depth profiling is carried out at room temperature. Residual defects generated during irradiation and frozen at the low temperature thus might become mobile during warming up of the sample and cause the observed chemical enhancement of the atomic intermixing. We have therefore performed IBM experiments on a system with negative DHmix (Sb/Ni) and an immiscible (DHmix \ 0) system (Ag/Fe) [38] in the following manner: The samples were cooled down to 20 K, which in both cases is significantly below annealing stage IE (35 K in Ag, 60 K in Ni, 160 K in Fe), which is attributed to freely migrating interstitials, and very far below annealing stage III (200–300 K in Ag), at which single and double vacancies become mobile [50]. After taking an RBS spectrum to characterize the initial interfacial concentration profiles, the samples were irradiated with Xe-ions at 20 K, and, without changing the temperature, were again depth profiled by means of RBS. After warming up to room temperature an additional RBS spectrum was taken. As can be seen in Fig. 3, which shows the RBS spectra taken with the Sb/Ni bi-layer, the irradiation has resulted in a strong mixing effect. However, no difference can be detected between the RBS spectra of the irradiated sample taken at the low irradiation temperature and taken at 300 K. Although RED is present in the Sb/Ni system at room temperature, thermally activated migration of defects, which are immobile near 20 K, obviously does not occur when warming up the sample from 20 to 300 K.

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Hence, at least in the Sb/Ni and in the Ag/Fe systems, residual defect migration does not significantly contribute to IBM below the critical temperature Tc. The main source of the athermal mixing contribution must be ballistic processes or diffusion in thermal-spikes. Especially the latter now seems to be indispensable to explain the chemical biasing of IBM at low temperatures.

4.2. The athermal mixing regime As was discussed in the last paragraph, mixing at low temperatures is of athermal nature. In the following we will show that, depending on the atomic numbers of target and ion, different mixing regimes can be identified, which show either pure ballistic transport or can be attributed to chemically biased interdiffusion in thermal-spikes of different shapes and origin. The main characteristics which allow one to distinguish between the respective mixing mechanisms are the dependencies of the mixing rate k on the deposited energy density FD, on the average atomic number Zt of the target material, and on the chemical parameters, namely the cohesive energy DHcoh and the mixing enthalpy DHmix. In a recent study on the IBM of metallic and ceramic–metal interfaces, a great variety of light, medium-mass, and also heavy materials and a wide range of binding energies and chemical affinities was covered. In most cases the FD-dependence of the mixing rate was carefully determined covering a wide range of FD-values by systematically varying the ion species (ranging from 4He to 208Pb). Except for a few samples, where the irradiation was carried out with very heavy ions and which will be discussed later, the investigated systems all exhibit a linear relationship between k and FD. Fig. 4 shows the results for Pt/Ni bi-layers reported

Fig. 4. Mixing rates for various ions found after irradiation of Pt/Ni bi-layers at 77 K as a function of the deposited energy density FD [37].

by Conrad et al. [37]. Although, according to Cheng et al. [3], this system should belong to the global spike regime and was used for the assessment of the globalspike model, it obeys a linear scaling of the mixing rate with the deposited energy density, rather than the quadratic scaling predicted by the global spike model. Similar observations were made in all other systems investigated by the Go¨ttingen group, and consequently it was concluded that diffusion in global thermal-spikes can be excluded as a source for the observed mixing effects in these cases. In order to differentiate between ballistic and local thermal-spike mixing, which both exhibit a linear relationship between k and FD, one has to investigate the mixing effect as a function of the chemical affinities between the bottom and the top layer. In order to eliminate the influence of other parameters like N, Zt and especially FD, reduced mixing rates were defined, which of course depend on the model and in case of the ballistic approach reads as hbal = (k/FD)N,

(7)

For systems which are governed by ballistic mixing hbal should be almost constant and independent of the heat of mixing of the involved materials. If, on the other hand, chemically driven diffusion in local thermalspikes is the active mixing mechanism, the reduced cylindrical local spike mixing rate hcls = (k/FD)N 2/3 DH 2coh/Z 1.77

Fig. 3. RBS spectrum of an Sb/Ni bi-layer before and after irradiation with 400 keV Xe-ions (2× 1015 cm − 2) at a temperature of 20 K. RBS was performed at 20 K immediately before and after ion irradiation and after warming up the sample to 300 K.

(8)

should depend linearly on the ratio DHmix/DHcoh. In Fig. 5i, the reduced ballistic mixing rate is plotted versus DHmix/DHcoh for a number of metal– metal and metal–ceramic interfaces. While the hbal-values for the pure metal systems (closed symbols) scatter and on the average seem to increase with increasing chemical affinity, the nitride coated samples (open symbols),

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Fig. 5. Reduced ballistic mixing rate bal for low- and medium-Zt metal – nitride (open symbols) and metal – metal (closed symbols) bi-layers vs. DHmix/Dcoh [5].

except for Ni3N/Al, show an almost constant reduced mixing rate, which furthermore compares with the prediction of the ballistic model. If, on the other hand, hcls is plotted versus DHmix/DHcoh, as done in Fig. 6, the metallic systems are fairly well-described by a common curve, increasing with increasing DHmix/DHcoh, while the nitride-coated systems do not show any correlation with this parameter. This behavior is in good agreement with the fractal approach of the collision cascade by Cheng et al. [3]: In the strongly bound nitrides spike initiation is not possible because of the low average Zt B 18 and mixing is completely governed by ballistic transport processes without any chemical guidance. In the metallic systems and also in Ni3N/Al, which have a higher Zt diffusion in local thermal-spikes seems to be the dominant mixing mechanism, which is biased by chemical driving forces. However, in contrast to the model, the reduced local-spike mixing rate hcls increases faster than linear with DHmix/DHcoh, which is not yet understood and needs further theoretical investigation. Additional striking evidence that thermal-spikes play a significant role for IBM is given by the results on the thermally immiscible Ag/Fe bi-layer system recently reported by Crespo-Sosa et al. [38]. Although the low energy edge corresponding to the interfacial side of the Ag-layer in the RBS spectrum broadened during ion irradiation, these authors clearly showed by means of scanning tunneling microscopy that this effect could be almost completely attributed to surface roughening. The remaining interface mixing was found to be much smaller than predicted by the ballistic model. This finding was strongly supported by the results of a

combined Perturbed Angular Correlation and Mo¨ßbauer study by Neubauer et al. [51,52]. In addition Krebs et al. [53] have found that the interfaces of Ag/Fe multi-layers became even sharper after Ar ion bombardment at low fluences. Obviously, the ballistic mixing effect, which should be present in any case was canceled by another transport mechanism, which is sensitive to the high positive heat of mixing and causes demixing of the ballistically intermixed atoms. Chemically guided migration of residual defects can be excluded because after 20 K ion irradiation no difference was observed when performing depth profiling at 20 K and after warming up to 300 K (see Section 4.1). Hence, diffusion in thermal-spikes remains as the only explanation for the observed demixing effect. When irradiating Ni-markers in Sb and vice versa Shi et al. [35] observed a strong increase of the mixing rate when the deposited energy density exceeded a critical value F cr D, i.e. when using ions of very high atomic mass. This non-linear increase of k at high FD-values was interpreted being due to the transition to the global thermal-spike regime by overlapping local spikes, and the respective transition from linear to quadratic FDscaling of the mixing rate. Such a transition was predicted by Bolse [5,19] to occur at 0.23 F cr + Z 0.23 ))(Mi /(Mi + Mt)). D = aN(Zi Zt/(Z i t

(9)

The phenomenological constant a: 3.4× 10 keV per nm2 was estimated from the above measurements and the values for the individual systems agree well with each other. Although the quadratic scaling of the mixing rate could not be verified, these result strongly −4

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Fig. 6. Reduced cylindrical local-spike mixing rate hcls for various metal – nitride (open symbols) and metal – metal (closed symbols) bi-layers vs. DHmix/Dcoh [5].

support the idea that, for very heavy ions, and correspondingly small mean free path between sequential primary collisions, overlapping subcascades result in the formation of a coherent global spike along the ion path. Recently, Bolse and Weber [55,5] claimed that even in ‘ballistic’ systems with low Zt, i.e. which for low and medium mass ions exhibit pure ballistic mixing, thermal-spikes can be initiated at the end-of-range of sufficiently heavy ions. These so-called ‘end-of-range’ (eor) spikes are related to the decreasing mean free path of a heavy ion at low energies, i.e. when approaching its end-of-range. This increasing density of primary collisions at the end-of-range of a very heavy ion may result in subcascade overlap and possibly the initiation of an eor-spike. The critical energy for eor-spike initiation was estimated as [55,5]

center of the implantation profile, it will not only result in an enhanced mobility of the target atoms, but also of the implanted species. The segregation behavior of rare gas ions implanted into ceramic–metal interfaces recently reported by Weber et al. [54] and Bolse et al. [5,55] strongly supports this model. The eor-spike model also explains the deviation of the mixing rate from the ballistic model in Xe-irradiated TiN/Al [16,25]. While mixing with Ar ions is fully described by the ballistic approach, an additional contribution must be assumed for Xe ions, when the ion range distribution overlaps with the interface. The observed enhancement of the mixing effect in this case can be easily attributed to the enhanced mobility of the interface forming atoms in the eor-spike volume.

E %c :0.028 eV((Mi +Mt)/Mt)(Zi Zt(Z 0.23 +Z 0.23 )) i t

5. Summary

(10)

For ions of similar or lower atomic number as compared with the average atomic number of the target, Zi 5Zt, also the eor-spike threshold is equal or lower than the local-spike threshold, E %c 5Ec. Since in a ballistic system the Ec falls below the displacement threshold Ed, in that case also the eor-spike threshold E %c is smaller than Ed, and eor-spikes cannot be initiated. However, with increasing atomic number also E %c increases and for ions of sufficiently large atomic numbers, Zi Zt, the eor-spike threshold may become significantly larger than the displacement threshold, E %c \Ed \ Ec, and an eor-spike will form even in a ballistic system. Since the eor-spike appears near the

By reviewing selected typical IBM studies, it was possible to separate between different ion-beam induced atomic transport mechanisms, depending on the irradiation conditions. Mixing of low mass materials (Zt below about 18) seems to be fully controlled by pure ballistic processes, without the influence of any chemical driving forces. Low and medium mass ion bombardment of materials with higher average atomic number results in the initiation of local thermal spikes, and mixing as a result of transient diffusion at the high spike temperature. This diffusion process is strongly biased by the chemical affinity of the involved materials. There is an indication that global thermal-spikes

W. Bolse / Materials Science and Engineering A253 (1998) 194–201

form by the overlapping of local spikes when using ions of very high mass. Very heavy ions may also initiate so-called end-of-range spikes in systems which otherwise are governed by ballistic transport. Residual defect motion at the ambient temperature becomes important only at higher temperatures.

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