Deep radiation damage in metals after ion implantation

Deep radiation damage in metals after ion implantation

710 Nuclear DEEP RADIATION E. FRIEDLAND Department DAMAGE IN METALS Instruments and Methods in Physics Research B33 (1988) 710-713 North-Holla...

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710

Nuclear

DEEP RADIATION E. FRIEDLAND Department

DAMAGE

IN METALS

Instruments

and Methods

in Physics

Research B33 (1988) 710-713 North-Holland, Amsterdam

AFTER ION IMPLANTATION

and H.W. ALBERTS

of Physics, University of Pretoria, Pretoria 0002, South Africa.

Single crystals of copper, platinum and iron were implanted offaxis at room temperature with C+, O+, Ne+, Arf, Kr+ and Xef ions having energies ranging from 50 to 150 keV. Some implantations were also performed at liquid nitrogen temperature. Damage depths were determined using a-particle channeling in a backscattering geometry. With heavy ions extremely large damage ranges were observed in copper and platinum, but not in iron. Damage ranges in samples implanted at liquid nitrogen temperature were significantly smaller than in room temperature implanted samples.

1. Introduction Energetic ions implanted into solids lose their energy by electronic excitation and atomic collision processes. Depending on the energy transferred in an atomic collision, a lattice atom can either be displaced or phonons can be excited. Whilst phonon and electronic excitations will eventually be dissipated as heat, the displacement of atoms results in radiation damage of the crystal lattice. Because of the relatively low displacement energy, primary knock-on atoms will in many cases have enough energy to displace further atoms resulting in the development of a collision cascade, creating a large number of point defects. Due to the high mobility of interstitials at room temperature, most of them will either recombine with vacancies or diffuse to the surface or into the bulk. The remaining point defects, which should mainly consist of vacancies, are expected to collapse into prismatic dislocation loops along the close-packed planes of the lattice. As these defect structures are rather immobile, the final damage depth is expected to agree more or less with the projected range of the implanted ions. This indeed is found in the case of semiconductor materials. However, damage depths exceeding projected ranges by an order of magnitude [l-5], are observed in some metals after implantation of heavy ions. In the case of copper single crystals, damage ranges were found to be practically the same for different ion species after room temperature implantations at a fixed energy, although their projected ranges differed widely [6]. Possible mechanisms responsible for this phenomenon are the following: - Thermal migration of displaced atoms and subsequent agglomeration deeper in the crystal. _ Propagation of defects under the influence of rapidly changing compressive stress field gradients induced by thermal spikes. 0168-583X/88/$03.50 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

In order to understand the processes leading to deep radiation damage, a systematic study of damage ranges in a variety of metals under different implantation conditions is currently being undertaken in this laboratory.

2. Experimental results Copper, platinum and iron single crystals were implanted with C+, O+, Net, Ar+, Kr+ and Xe+ ions having energies in the range from 50 keV to 150 keV. Total fluences of 10+i5 to 10+16 ions cm-2 were implanted at dose rates of the order of 10+12 ions cmm2 s-l. Most implantations were done at room temperature but some samples were implanted at liquid nitrogen temperature. Crystals with surfaces parallel to (111) and (110) planes were used and all implantations were performed in a direction 10” off the surface normal to minimize ion channeling. Damage profiles were investigated by a-particle channeling in a backscattering geometry using an experimental set-up as described in ref. [6]. Aligned backscattering spectra were taken for the (llO), (111) and (211) orientations at different a-particle energies and were normalized to random spectra. The position of the typical “knee” in the aligned spectra was taken as the damage depth. From the scatter of the results obtained for a given sample at different a-particle energies and orientations, experimental errors of between 10% to 15% were estimated. To avoid confusion, these errors are not indicated in the relevant figures. All the experimental damage ranges for room temperature implantations into platinum are given in fig. 1 together with the theoretical projected ranges of the different ions. The latter values were calculated by employing computer codes TRIM [7] and MARLOWE [8], which gave essentially identical results as far as

711

E. Friedland, H. W. Alberts / Deep radiation damage in metals

.

Fig. 1. Experimental damage ranges in platinum single crystals after room temperature implantations with different ions. The solid lines are projected ranges as predicted by TRIM and MARLOWE.

projected ranges are concerned. The largest damage ranges at a fixed energy were obtained with Ar+ and Krf ions, whilst those for Xe+ and the lighter ions were significantly lower. This differs from the results found after implantations into copper, where similar damage ranges were found for all ions with the exception of Xe + [6]. Experimental damage ranges after argon implantations into iron, copper and platinum are compared in fig. 2. Whilst these ranges are very similar for copper and platinum, much smaller damage ranges are found in iron. In the case of copper and platinum the damage ranges exceed the projected ranges by about a factor of

Fig. 3. Ratios of damage range to projected range as function of the atomic mass of the implanted ions for iron, copper and platinum single crystals. Straight lines are drawn to guide the eye. A logarithmic scale is used for the abscissa. five, whereas in iron this difference is only approximately 50%. In all cases the damage ranges are linearly dependent on implantation energy. Ratios of the damage range to the projected range for implantations of different ions into these three metals are depicted in fig. 3. For platinum and copper these relative damage ranges exhibit roughly a logarithmic dependence on ion mass. Whilst for light ions the ratios are comparable for the two metals, they are appreciably larger for platinum than for copper with heavy ions. Damage ranges are significantly reduced when the implantations are performed at low temperatures. In table 1 the results for argon implantations at liquid nitrogen temperature are compared with the corresponding data for room temperature implantations. In the case of iron, the damage range at 77 K is, within experimental error, in agreement with the expected projected range of argon ions. In copper, however, damage ranges do still exceed the projected ranges by almost a factor of three at this temperature, although this is significantly smaller than observed at room temperature. Table 1 Comparison of the relative damage ranges R,/R, implantations at 77 K and 300 K Metal

Fig. 2. Experimental damage ranges in iron, copper and platinum single crystals after room temperature implantations with argon ions. Straight solid lines are drawn to indicate the linear dependence on energy. The broken lines represent projected ranges.

cu cu Fe

for argon

Implantation energy (keV)

RD/RP

(77 K)

(300 K)

100 150 150

2.8kO.2 2.7 f 0.4 1.1*0.1

4.3 + 0.4 4.0 + 0.4 1.5+0.1

R,/R,

X. RADIATION

DAMAGE

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E. Friedland, H. W. Alberts / Deep radiation damage in metals

3. Discussion of resuks

As the factor by which damage depths exceed the projected ranges of implanted ions increases with ion mass, the primary point defect density seems to be an important parameter. The average number of point defects produced by a single ion mainly depends on its initial energy. From fig. 4, where results are plotted for Monte Carlo calculations using the codes TRIM and MARLOWE, it is obvious that the influence on the atomic numbers of the ion and target atom is less important. Also shown in this figure is the estimate of the simple Z-independent Kin&in-Pease model, which is appreciably higher, especially for the lower Z values. In the TRIM calculations a displacement energy of 25 eV was assumed together with a binding energy of 2 eV and a surface energy of 1 eV. The code TRIM does not take into account the crystal structure and therefore neglects channeling effects. This is probably one of the reasons for the lower results obtained with MARLOWE. The same binding energy of 2 eV was assumed in the MARLOWE calculations and all recoils were followed until their energy dropped below 5 eV. Although the total number of point defects predicted by these two computations differ by up to 30%, both calculations show that the total number of vacancies increases by less than a factor of three from the very light to the very heavy ions and stays more or less constant above argon. However, the defect density increases appreciably with increasing atomic number of the ion due to the decreasing implantation depth. The calculations furthermore predict that for carbon ions approximately 15% of the initial energy is converted into primary damage, which increases to about

4

I

___-_______~~__-__K,NCH,N-PE~SE

/

Fig. 4. Number of vacancies created by some 100 keV ions in copper and platinum as predicted by computer codes TRIM and MARLOWE. Parameters used are given in the text. Also shown in the simple Kin&in-Pease estimate.

35% for xenon ions. However, as a large proportion of the vacancies will recombine with the highly mobile interstitials, it is safe to assume that independent of the atomic number of the implanted ion, most of the ion energy is dissipated as heat. This should lead to extremely high temperatures in the vicinity of the ion track, especially in the case of heavy ions, where this heat is released in a much smaller volume. Some experimental results [9] seem to indicate that for ultra-short time intervals temperatures might exceed lo4 K. Even if one assumes that, due to the relatively weak electron-phonon coupling, most of this energy will be quickly dispersed by the electron gas, it is feasible to expect strong time-dependent stress field gradients near the ion track. In this field dislocation loops could act as sources for dislocation multiplication [lo]. Such dislocations may be propagated into the bulk resulting in deep radiation damage. In order to explain deep radiation damage, one has to understand why this phenomenon mainly occurs in metals and not in semiconductors, and why it is much more prominent in some metals than in others. It seems to be difficult to explain these differences by interstitial migration and subsequent agglomeration deeper in the bulk, as the migration energy for interstitials is of the same order of magnitude for different crystal structures. On the other hand it is well known that the Peierls force, resisting dislocation motion, depends sensitively on the crystal structure [ll]. Although the magnitude of this force can only be estimated crudely, it is known to be large for diamond structures, whilst in the closepacked cubic metals the Peierls force is small. This could explain not only the absence of deep radiation damage in semiconductors, but also the observed difference of the relative damage ranges in the fee metals copper and platinum compared to the bee metal iron. Due to the higher packing density in the (111) plane of the fee structure than in the (110) plane of the bee structure, the Peierls force opposing glide along these planes of highest density should be appreciably larger in iron. It is also interesting to note that the relative damage range in the bee metal vanadium [l] is found to be considerably smaller than in copper and platinum. Finally, the observed temperature dependence of the relative damage range is also consistent with the expected behaviour of dislocation motion, although similar arguments may be put forward for point defect migration.

The authors would like to thank Prof. J.H. van der Merwe for valuable discussions and Dr J.F. Prins for the implantations. We are also grateful to the Foundation of Research and Development of the Council for Scientific and Industrial Research for funding this work.

E. Friedland, H. W. Alberts / Deep radiation damage in metals

References [l] G. Linker, M. Gettings and 0. Meyer, in: Ion Implantations in Semiconductors and other Materials, ed. B.L. Crowder (Plenum, New York, 1973) p. 465. [2] M. Gettings K.G. Langguth and G. Linker, in: Applications of Ion beams to Metals, eds. ST. Picraux, E.P. EerNisse and F.L. Vook (Plenum, New York, 1974) p. 241. [3] D.K. Sood and G. Deamaley, J. Vat. Sci. Technol. 12 (1975) 463. [4] M. Vos and D.O. Boerma, Nucl. Instr. and Meth. B15 (1986) 337. [5) E. Friedland, J.B. Malherbe, H.W. Alberts, R.E. Vorster and J.F. Prim, S. Afr. J. Phys. 9 (1986) 135.

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(61 E. Friedland, H. le Roux and J.B. Malherbe, Radiat. Eff. Lett. 87 (1986) 281. [7] J.F. Ziegler, J.B. Biersack and U. Littmark, The Stopping and Range of Ions in Solids (Pergamon Press, New York, 1985). [8] M.T. Robinson and I.M. Torrens, Phys. Rev. B9 (1974) 5008. [9] L.M. Gratton, A. Miotello and C. Tosello, Phys. Status Solidi 100 (1987) 53. [lo] D.A. Jones and J.W. Mitchell, Philos. Mag. 3 (1958) 1. [ll] H.G. van Bueren, Imperfections in Crystals (North-Holland, Amsterdam, 1961) p. 62.

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DAMAGE