HREM investigation of latent tracks in GeS and mica induced byhigh energy ions

J. Vetter a,*, R. Scholz b, D. Dobrev a,1, L. Nistor a,2

a Gesellschaft f�ur Schwerionenforschung, P.O. Box 110552, 64220 Darmstadt, Germanyb MPI f�ur Mikrostrukturphysik, Weinberg 2, 06120 Halle/Saale, Germany

Abstract

Observations of heavy ion latent tracks in layered materials were carried out with a 400 kV high resolution electron

microscope as well as with conventional 100 and 200 kV microscopes. The materials were irradiated normal to the

cleavage plane at the UNILAC-accelerator at GSI Darmstadt with ions up to several MeV/u. High-resolution images

of latent tracks in muscovite mica and in GeS single crystals are presented in this paper. The micrographs show that the

track core represents a disordered zone. Depending on the di�raction conditions in the surroundings of the tracks,

strain contrast centres are visible. The boundary between the amorphous track core and the intact crystal lattice is well

de®ned. The cross sections of the tracks in mica have a nearly circular shape. In contrast to that they are elliptical in

GeS and are well oriented with respect to the crystal lattice. The sharp amorphous±crystalline transition allows the ex-

act measurement of the track dimensions. The values resulting for the track diameters show a large variation in size

depending on the ion sort. The tracks produced by heavy ions have more than twice the diameter of those produced

by light ions. Relations between the track size and the energy loss for di�erent ions are given. Ó 1998 Elsevier Science

B.V. All rights reserved.

Keywords: Heavy ion irradiation; Latent tracks; High resolution TEM

1. Introduction

In insulating or semi-conducting materials anenergetic heavy ion produces a considerable alter-

ation of the original structure along its trajectory.At high velocity, the ion loses its energy mainly byinteraction with the electrons of the solid. If the ki-netic energy exceeds 10 MeV/u, the ion is succes-sively transferring more than 10 keV per latticeplane in very short time intervals. Thus, the ionpassage creates conditions initiating dramatic, rap-idly developing primary processes which cannot beobserved directly. However, the details of size,shape and internal structure of the ion track,though representing the ®nal stage of the damagecreation, contain indirect information about the

Nuclear Instruments and Methods in Physics Research B 141 (1998) 747±752

* Corresponding author. Tel.: +49 6159 712 715; fax: +49

6159 712 179; e-mail: j.vetter@gsi.de.1 Guest Scientist from Bulgarian Academy of Sciences,

Institute of Physical Chemistry, So®a, Bulgaria.2 Guest Scientist from Institute for Atomic Physics, Bucha-

rest, Romania.

0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved.

PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 9 8 - 0

possible scenarios taking place in the medium afterthe passage of the ions.

The structural changes range from the occur-rence of point defects via separate atomic clustersto continuous damage in regions called latenttracks, in which the crystal can be fully amorphi-zed. Samples containing a large number of latenttracks have been investigated by a variety of meth-ods: scattering of X-rays and neutrons or infraredspectroscopy. To provide also access to singletracks, transmission electron microscopy (TEM)was used. Scanning tunnelling (STM) and scan-ning force microscopy (SFM) were applied as well.The above microscopic techniques are capable ofcharacterising the object under study with atomicresolution. At lower resolution TEM has been ap-plied ®rst to image ®ssion fragment tracks in mica[1,2]. Later, high-resolution TEM of latent trackswas applied successfully to a variety of crystallinematerials such as yttrium±iron-garnet [3±5], zircon[6,7], GeS [8±11] and high temperature supercon-ductors [12]. In the present work, cross sectionsof latent tracks induced by heavy ions in GeSand mica were investigated by means of HREMas a function of di�erent kinds of ions and theirenergy loss.

2. Experimental

Crystals with good cleavage properties are pre-ferred for investigations of the cross sections ofheavy ion tracks by HREM. GeS single crystalscan be easily cleaved along their (0 1 0) crystallo-graphic planes which are atomically ¯at over dis-tances of several lm. The most characteristicproperty of muscovite micaKAl2(SiAl)O10(OH,F)2. is also its perfect cleavagealong the (0 0 1) basal plane into very thin sheets.

Thin samples of these materials were irradiatedat the UNILAC with di�erent kinds of ions accel-erated to energies between 5.6 and 13 MeV/u at¯uences between 1010 and 5 ´ 1010 ions/cm2. Uti-lising the easy cleavage of GeS and mica thin ¯akeswere successively removed from the irradiated sideby using an adhesive tape or a razor blade. Theywere placed between double-grids for conventionalTEM observation. For high-resolution investigat-

ions some of the GeS ¯akes were subjected to ad-ditional short-time ion thinning. The samples wereexamined in JEOL100C and 4000EX microscopesand in a Philips CM20 microscope. These proce-dures allowed us to observe directly the cross sec-tions of latent ion tracks.

3. Results and discussion

3.1. TEM of germanium monosul®de

In Fig. 1 (a) and (b) micrographs of cross sec-tions of latent ion tracks are shown as they wereinduced in GeS by 129Xe and 197Au ions with anenergy of 11.4 MeV/u, i.e. without any additionalchemical or physical treatment. The number ofthe tracks in the observed region corresponds tothe ion ¯uence. The damaged volume of the tracksis more transparent for the electrons than the sur-rounding crystalline matrix under the di�ractioncondition used. The cross sections have an almostelliptical shape and are always oriented with theirlong axis along the closest packed á0 0 1ñ crystaldirection in the imaged plane. A tendency for fac-eting is noted. A comparison of the two images1(a) and (b) taken with the same magni®cationshows that the cross sections are obviously of dif-ferent size. The high-resolution EM image of across section shown in Fig. 2 reveals that the latention track is completely amorphized. In this casethe track was created by a uranium ion.

Four strain contrast centres are symmetricallyarranged about the elliptical cross section andare situated in a distance of 10±20 nm from thedamaged area. They are also oriented in respectto the crystal lattice.

Germanium monosul®de has an orthorhombiclattice. The (0 1 0) plane imaged in Fig. 2 is char-acterised by the lattice constants a� 0.429 nm andc� 0.3645 nm. They can be used for an exact de-termination of the real magni®cation of the micro-scope.

The boundary between amorphous and crystal-line area is extremely sharp and therefore allowedan exact measurement of the size of the damagedarea. In Fig. 3 averaged values for the cross sec-tion axes of latent tracks of di�erent ion species

748 J. Vetter et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 747±752

in GeS are plotted versus the energy loss. TheTRIM code [13] was used to determine the energydistribution along the ion path and the penetrationdepths. This diagram shows signi®cant di�erencesin cross section sizes for various ions as was al-

ready illustrated in Fig. 1 for two ion species.The number of displaced atoms in the cross sectionplane can be calculated from the data shown inFig. 3. In Fig. 4 the numbers of displaced atomsare plotted as a function of ion energy loss in GeS.

Fig. 1. TEM images of GeS with cross sections of latent tracks both with the same magni®cation. Tracks are created by the passage of

(a) Xe and (b) Au ions both with an energy of 11.4 MeV/u.

Fig. 2. HREM image of a latent track in GeS induced by a uranium ion with 5.6 MeV/u. The track is surrounded by four strain con-

trast regions.

J. Vetter et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 747±752 749

3.2. TEM of muscovite mica

High resolution images of track cross sectionsin muscovite mica have been acquired in the recentpast for a series of heavy-ion species by SFM [14±16], providing the functional dependence of trackdiameter versus ion energy loss. In our study thelatent tracks in mica were imaged by TEM. In ac-cordance with the SFM measurements, it wasfound that their cross sections appeared almostcircular.

Cross sections of Xe and U ions in mica arecompared in Fig. 5. Pb ion tracks are shown withhigh magni®cation in Fig. 6. Similar to GeS the

original structure of mica is completely destroyedwithin the ion track. The boundary of latent tracksin mica also appears sharp as in the case of GeS.However, no strain contrast regions have been ob-served in the neighbourhood of the tracks. Themean track diameters are shown as a function ofthe ion energy loss in Fig. 7.

4. Conclusion

Our ion bombardment experiments have shownfor both GeS and mica that the crystal structure iscompletely amorphized in the latent track. The

Fig. 3. Sizes of the long and short axes of ion cross sections in

GeS for several ion species having di�erent energy loss.Fig. 4. The number of the displaced atoms in one atomic layer

of GeS as a function of the energy loss of di�erent ions.

Fig. 5. TEM images with equal magni®cation of muscovite mica samples with tracks of (a) Xe ions (b) U ions both with an energy of

11.4 MeV/u.

750 J. Vetter et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 747±752

boundary between amorphous areas and crystal-line matrix is sharp without any smooth transition.In agreement with previous SFM results, we ®ndthat the average size of the track cross sections de-pends strongly on the ion energy loss. The form ofthe ion track cross section depends on the degreeof crystal symmetry. In GeS, a crystal with a lowsymmetry, the cross sections are elongated with a

tendency for faceting, the principal axes being di-rected along the crystallographic axes. The radialextension of the amorphisation reaches its maxi-mum (minimum) along the symmetry axis withthe lattice constant c� 0.3645 nm (a� 0.429 nm).

In the case of mica which possesses a hexagonallattice and thus a higher symmetry than GeS, theobserved cross sections are nearly circular.No strain contrast was found in mica, giving riseto the assumption that the strain contrast in GeScan also be attributed to the lower crystal symme-try.

HREM technique used in this investigation al-lowed us to obtain images of the ion track crosssections with high-quality contrast, that is essentialfor the exact measurement of the track sizes.

References

[1] E.C.H. Silk, R.S. Barnes, Phil. Mag. 4 (1959) 970±972.

[2] P.B. Price, R.M. Walker, J. Appl. Phys. 33 (1962) 3400±

3406.

[3] G. Fuchs, F. Studer, E. Balanzat, D. Groult, M. Toule-

monde, J.C. Jousset, Europhys. Lett. 3 (1987) 321±326.

Fig. 6. Cross sections of Pb ion tracks in muscovite mica, imaged by HREM; lattice constant a� 0.518 nm.

Fig. 7. Average diameters of latent tracks in muscovite mica as

a function of energy loss.

J. Vetter et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 747±752 751

[4] C. Houpert, F. Studer, D. Groult, M. Toulemonde, Nucl.

Instr. and Meth. B 39 (1989) 720±723.

[5] F. Studer, C. Houpert, D. Groult, M. Toulemonde,

Radiat. E�. Def. Sol. 110 (1989) 55±59; 189±191.

[6] G. Braunshausen, L.A. Bursill, J. Vetter, R. Spohr, GSI

Report 1990-1, pp. 219±220.

[7] L.A. Bursill, G. Braunshausen, Phil. Mag. A 62 (1990)

395±420.

[8] R. Scholz, J. Vetter, S. Hopfe, Radiat. E�. Def. Sol. 126

(1993) 275±278.

[9] J. Vetter, R.Scholz, S. Hopfe: GSI Report 1992-1, p. 248;

GSI Report 1993-1, p. 283.

[10] J. Vetter, R. Scholz, N. Angert, Nucl. Instr. and Meth. B

91 (1994) 129±133.

[11] J. Vetter, Rad. Measur. 25 (1995) 33±38.

[12] J. Wiesner, J.G. Wen, H.W. Zandbergen, G. Wirth, H.

Fuess, Physica C 268 (1996) 161±172.

[13] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping and

Range of Ions in Solids, Pergamon Press, NY, 1985.

[14] S. Bou�ard, J. Cousty, Y. Pennec, F. Thibaudau, Radiat.

E�. Def. Sol. 126 (1993) 225±228.

[15] T. Hagen, S. Grafstr�om, J. Ackermann, R. Neumann, C.

Trautmann, J. Vetter, N. Angert, J. Vac. Sci. Technol. B

12 (1994) 1555±1558.

[16] J. Ackermann, N. Angert, R. Neumann, C. Trautmann,

M. Dischner, T. Hagen, M. Sedlacek, Nucl. Instr. and

Meth. B 107 (1996) 181±184.

752 J. Vetter et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 747±752