Nuclear Instruments and Methods in Physics Research B13 (1986) 439-442

North-Holland, Amsterdam

439

EFFECTS OF TARGET CONSTITUENT MASS DIFFERENCE ON COLLISION CASCADE INDUCED COMPOSITION CHANGES IN BINARY ALLOYS

M. ROSEN’, G.P. MUELLER’, M.L. ROUSH’, T.D. ANDREADIS’.’ and O.F. GOKTEPE”

~,~~“~~ Research Labo~aror~f, Washington. DC X375-50U0. USA

‘Chemical and Nucleur Engineering Department, University of Maryland, College Park, MD 20142, USA

‘White Oak Laboratory, Naval Surface Weapons Center, Silver Sprirtg? MD 20903-6lWO. USA

The cascade simulation code MARLOWE has been used to examine low dose composition changes caused by 5 keV Xe ions

normally incident on a homogeneous polycrystallinc target composed of SOS-50 % Lu and Fe. The depth profile of the collision cascade induced atomic concentration of Lu is calculated and yields an excess of Lu in the surface, in agreement with the published

results using the code TRIDYN. Calculation of the distributions of recoil Lu and Fe ions originating in particular layers indicate,

however, that the heavier mass ion is driven deeper into the target, on the average, than the lighter mass ion -contrary to general

perceptions

1. Intr~uction 2. The model

The important mechanisms leading to surface com- position changes in alloys and compounds which are subjected to ion bombardment are not clearly under- stood. In this paper we look at the contribution of collision cascades to this process and in particular the role played by mass differences among the alloy con- stituents. Earlier theoretical work [l-7] predicted an enrichment of the surface in the heavier element of a binary alloy for low incident fluences. This was thought to come about as the result of the lighter component of the alloy being knocked deeper into the target by the incident ion beam than the heavy atom component, leaving the surface enriched in the heavy component. Recently two groups have independently examined this problem - Roush et al. [S] and Moller and Eckstein [9] using the codes EVOLVE and TRIDYN, respectively, which have the capability of tracking dynamic compos- ition changes as a function of incident fluence. They came to opposite conclusions. They both considered, among others, a model amorphous target consisting of a 50% mixture of Lu and Fe. Moller and Eckstein found a surface enrichment of Lu at both low and high fluences and attributed their result to the above men- tioned “recoil implantation” mechanism. In contrast Roush et al. found surface Fe enrichment at low fluences and Lu enrichment for higher equilibrium fluence and showed that because the lighter mass component bounced around more during the develop- ment of the collision cascade, it was the heavier component that was more efficiently knocked deeper into the target. At this point we thought it would be useful to look at this situation using the cascade simulation code MARLOWE.

We considered the case of a 5 keV beam of Xe ions normally incident on a homogeneous polycrystalline target consisting of a 50%-500/c atomic mixture of Lu and Fe. This “theoretical” system was analyzed using version 11 of the code MARLOWE with Moliere potentials (and LSS screening lengths). We assumed a 500/o-50% local and nonlocal electronic loss. We took all displacement and surface binding energies to be 17 eV and 4.3 eV, respectively, and set all bulk binding energies to zero. It was thus possible to focus on the effect of atomic number and mass differences between the target constituents. We used a bee model for the target and used a lattice spacing of 0.352 nm, which gives a target density equal to that used by Roush et al. We wrote an auxiliary code to analyze the results of the MARLOWE calculation. We divided the target into slabs, each of thickness 0.704nm and calculated the mean signed projected path lengths of the Lu and Fe knock-ons from each slab - ions which stopped deeper into the target than their sites of origin were assigned positive path lengths and those that stopped closer to the surface were given negative projected path lengths. Sputtered atoms were not counted. By normalizing to the number of vacancies and recoils produced per incident particle it is possible in effect to calculate the distributions produced by a minimum fluence of 8 X 10’J cm-.2, and to calculate the depth profile of the Lu and Fe atomic concentrations at low incident fluence. It may happen that in the first slab the number of vacancies produced is a significant fraction of the number of atoms there. This would not be a low fluence point. As a result, points for the first slab should be given less weight.

0168-583X/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

VIII. MIXING

440 M. Rosen et al. I Collision cascade induced composition changes

3. Results

In fig. 1 we show the mean signed projected path lengths of Lu and Fe recoil atoms as a function of the depth of the slab from which they originated. The error bars indicate 1 standard deviation statistical errors, The points are the results of 4200 cascades. It is clear that for each slab, and especially near the surface where the statistics are better, the mean of the distribution of the heavier component, Lu, lies deeper into the target than the mean of the distribution of the Fe recoils. The differences are not large-a fraction of a lattice spacing-but lie significantly outside the errors in the projected path lengths themselves. These calculations were also done during a binding energy of 17 eV, which is what the EVOLVE calculation used, and indepen- dently also using 100% nonlocal electronic loss. No significant differences were seen. This result is in agreement with the claim of Roush et al.; but it does not necessarily follow that surface enrichment of the lighter element obtains! In order to calculate the atomic concentration of, e.g., Lu in a particular volume one must not only add the number of stopped Lu recoils to the original number of Lu atoms in that volume but aiso

1.2

1.1

0.8

subtract the number of vacancies. It is the number of uncompensated vacancies to which the concentration is sensitive. In fig. 2 we show the depth profile - at low fluence-of the relative atomic concentration of Lu. The curve through the points is the result of a cubic spline fit and has no further significance. The surface enrichment of Lu is unmistakable and agrees with the TRIDYN results. We do not want to overemphasize this agreement because the models we used are quite different.

The reason for the qualitative disagreement with the EVOLVE calculations however were not clear and led to a closer examination of some aspects of that calcul- ation. Some of the results of that study are presented in ref. [lo]. Both the TRIDYN and EVOLVE calculations were done for the case of amorphous targets, while the present cafculatiofls were for polycrystalline targets. For that reason alone one should allow the present results to stand alone and be wary of making comparis- ions. These MARLOWE calculations show that the reason for the large Lu excess in the surface is the much larger number of uncompensated Fe vacancies that occur near the surface. As one goes deeper into the target and approaches equilibrium, this surface excess

E I

Mean Signed Projected Path Length of Lu and Fe Knock-on Atoms

Lu Fe

0 5 10 15

Depth (x7.04 w )

20 25

Fig. 1. The mean signed projected path length of Lu and Fe recoil atoms as a function of the originated. The errors are statistical.

depth of the slab from which they

Relative Atomic Concentration of Lu

0.55

z? +

3 0.50

\

3

0.45

0.40 0

M. Rosen et al. ! Collision cascade induced composition changes 441

5 10 15

Depth (x7.04 A )

20

Fig. 2. The depth profiie at low fluence of the relative atomic concentration of Lu.

uXs-Fe

“Xe-Lu

25

MOLIERE TOTAL CROSS SECTION RATIO FOR INCIDENT Xe

~~

1.8 -

1.6 -

1.4 -

1.2 -

1.0 -

0.8 -

0.6 -

INVERSE SQUARE

Fig. 3. The ratio of the total Molitre cross section for Xe incident on Fe to that of Xe incident on Lu.

VIII. MIXING

442 M. Rosen et al. I Collision cascade induced composition changes

drops off. In fig. 3 we show that the Xe-Fe cross section is significantly greater than the Xe-Lu cross section in the interesting energy range.

4. Conclusions

We have examined the atomic number and mass effect on collision cascade induced composition changes in the surface of ion bombarded binary alloys. At least for the polycrystalline Lu-Fe we have looked at, we have seen that on the average the heavier component is knocked deeper into the target than the lighter component. This agrees with the results of the Maryland group’s EVOLVE calculation and is contrary to the generally held perception. (A calculation for an amorphous target was also done using version 12 of MARLOWE, with, substantially the same results.) However, this result by itself may not be important or even relevant for determining cascade induced compos- ition changes. These are sensitive rather to the distrib- ution of uncompensated vacancies. For this calculation we found that for low incident Xe fluence the surface

was substantially enriched in the heavier element. For

the amorphous run mentioned above, the depth pro- files were substantially changed. These results are discussed in greater detail in ref. [lo].

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