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Damage formation in InP due to high electronic excitation by swift heavy ions
Nuclear Instruments and Methods in Physics Research B 146 (1998) 341±349
Damage formation in InP due to high electronic excitation by swift heavy ions W. Wesch a
a,* ,
O. Herre b, P.I. Gaiduk b, E. Wendler a, S. Klaum unzer c, P. Meier
c
Institut f ur Festk orperphysik, Friedrich-Schiller-Universit at Jena, Max-Wien-Platz 1, D-07743 Jena, Germany b Institute of Applied Physics Problems, State University of Belarus, Kurchatova 7, 220062 Minsk, Belarus c Hahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany
Abstract Damage production in crystalline InP by swift Kr- and Xe-ions at room temperature was studied by cross section TEM and RBS. The type and concentration of defects varies with depth due to the dierent dominating interaction processes of the ions with the solid. A ¯uence-dependent damage production is observed in the region of dominating electronic excitation for an electronic energy deposition above 13 keV/(ion nm). Depending on the ion ¯uence point defect complexes, discontinuous tracks or amorphous layers are formed. The observed ®ndings are interpreted as the eect of a thermal spike in combination with damage accumulation resulting from imperfect recrystallization of the molten tracks. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 81.05.Ea; 61.16.Bg
1. Introduction For various insulating materials as well as some intermetallic compounds and pure metals latent track formation due to high electronic excitation by swift heavy ion irradiation has been reported. (see e.g. [1±3] and references therein). In the case of semiconductors the physical situation is much more confused. Crystalline tracks were found in amorphous Si and Ge [4], and in high-resistivity crystalline semiconductors track formation has also been reported [1]. On the other hand, it has
* Corresponding author. Tel.: +49 3641 947330; fax: +49 3641 947302; e-mail: [email protected]
been shown that high electronic excitation seems to be inecient for damage production in Si [5] and 6H-SiC [6]. However, in several semiconductors as Si, Ge, GaAs and GaP an in situ damage annealing occurs during high energy ion irradiation which is obviously correlated with the high electronic energy deposition [5,7±10]. The observed lateral shift of the bombarded surface area after 30 MeV Se irradiation into amorphized InP at low temperatures has been ascribed to the generation of a molten zone around the ion trajectory [11,12]. Recently, we could demonstrate for the ®rst time that 250 MeV Xe-ions can produce amorphous latent tracks in crystalline InP at room temperature [13]. In GaAs under the same irradiation conditions no indication for track formation was found.
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 3 5 - 2
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W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 341±349
In the present paper we report results of a more systematic study of the in¯uence of the electronic energy deposition on damage formation in InP due to high-energy ion irradiation at room temperature. 2. Experimental á1 0 0ñ InP samples were irradiated with 2.5 and 150 MeV Kr-ions and with 250 and 340 MeV Xeions at room temperature with ion ¯uences ranging from 5 ´ 1012 to 6 ´ 1014 cmÿ2 . Additionally, InP samples predamaged at )190°C by 2.5 ´ 1013 cmÿ2 Si ions with an energy of 1 MeV were irradiated with 250 MeV Xe-ions to an ion ¯uence of 7 ´ 1011 cmÿ2 . The maximum of nuclear energy deposition in the predamaged samples lies around 1 lm. To prevent sample heating especially in the case of the very high energies the ¯ux was kept constant below 1.3 ´ 1010 cmÿ2 sÿ1 . In all cases the InP pieces were mounted to the sample holder with a silver paste giving a good thermal conductivity between them. The temperature variation during the irradiations which was controlled by means of a thermocouple near the sample was less than 2°C. The implanted layers were analysed by means of Rutherford backscattering spectrometry using the channeling technique (RBS/C) and transmission electron microscopy (TEM) in combination with cross section preparation. The RBS/C measurements were carried out with 1.4 MeV He ions at a backscattering angle of 170°. Under these conditions the depth which can be analysed in a quantitative manner in InP is approximately 0.6 lm. That means that for the samples irradiated with the high-energy ions one obtains information about the depth region in which the interaction is dominated by electronic stopping. Assuming a random distribution of the displaced lattice atoms within the lattice cell, from the measured RBS spectra the depth distribution of the relative concentration of displaced lattice atoms, nda , was calculated in the framework of the discontinuous model of the dechanneling [14]. The TEM investigations were performed with an electron microscope EM-125 operating at 100 keV. By means of an ion beam sputtering equip-
Fig. 1. Number of primary displacements, Ndispl , and electronic energy loss, eel , per ion and unit depth versus the depth z, calculated by TRIM95, for 340 and 250 MeV Xe- and 150 and 2.5 MeV Kr-irradiation in InP.
ment both plane-view and cross-section preparations were carried out at low temperatures. For comparison with the experimentally determined damage distributions the depth distributions of the number of primarily displaced atoms Ndispl (corresponding to the nuclear energy deposition) as well as of the energy deposited into electronic processes eel per incident ion and unit length were calculated by means of the TRIM 95 computer code [15] using a displacement threshold of 8 eV. The resulting pro®les are plotted in Fig. 1. In the case of the high MeV energies the maximum of the nuclear energy deposition lies in large depth, whereas the electronic energy deposition dominates near the surface down to 10. . .17 lm. For 2.5 MeV Kr-irradiation nuclear and electronic energy deposition occur in the same depth region and, in comparison to the high energies, the electronic energy loss is much lower. 3. Results and discussion As an example, Fig. 2 represents the RBS spectra of InP samples irradiated with 340 MeV Xe-ions. The yield of He -ions backscattered in the near surface region increases with increasing ion dose and reaches the random level for ¯uences NI 4 ´ 1013 cmÿ2 indicating the formation of a
W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 341±349
Fig. 2. Energy spectra of 1.4 MeV He-ions backscattered on 340 MeV Xe-implanted InP for various ion ¯uences.
heavily damaged or amorphous layer. Fig. 3 shows the concentration of displaced atoms, nda , calculated from the spectra in Fig. 2 versus the depth z. In the depth region above 0.1 lm, nda is nearly constant with depth and increases with the ion ¯uence up to its maximum value. It is worth to mention that the damage detected by RBS is small immediately below the surface. An identical behaviour was also found for 250 MeV Xe-irradiation [13].
Fig. 3. Relative concentration of displaced lattice atoms, nda , calculated from the energy spectra in Fig. 2 as a function of depth z.
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In order to compare the results obtained for the various ion energies, the ion ¯uence was recalculated into displacements per lattice atom (dpa) according to dpa Ndispl NI /N0 (N0 is the atomic density of the target) which represents the nuclear energy deposition. Fig. 4 shows the increase of nda at depth z 0.3 lm with the number of dpa for the dierent irradiations. It is clearly visible that for 250 and 340 MeV Xe-irradiation nda reaches the maximum value already at 0.016 dpa. For such a value of dpa in the case of conventional ion energies a much smaller defect concentration of nda 60.15 results (see [10]). Also in the case of 150 MeV Kr-irradiation at 0.016 dpa only a concentration of displaced atoms of nda 0.08 is measured. A remarkable increase of nda is only observed for values?0.2 dpa. In the case of 2.5 MeV Kr-irradiation the formation of an amorphous layer requires 0.7 dpa in the maximum of nuclear energy deposition and the dependence of nda versus dpa tends to agree with that observed for 150 MeV Kr-irradiation. Further, from previous work it is known that, depending on the ion mass, the nuclear energy deposition necessary to amorphize InP corresponds to 0.2 dpa (300 and 600 keV Se [10]) to 1 dpa (100 keV B [16]). These results suggest that in the case of
Fig. 4. Relative concentration of displaced lattice atoms, nda , versus the number of primary displacements per lattice atom, dpa, for 340 and 250 MeV Xe- and 150 and 2.5 MeV Kr-irradiated crystalline InP.
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Kr-irradiation with 2.5 MeV as well as with 150 MeV, the observed damage is caused by the energy deposition into nuclear processes. For the Xe-irradiations in the depth region of z 0.3 lm regarded here, the nuclear energy loss per ion and unit depth is almost the same as for 150 MeV Kr-irradiation (see Fig. 1). Therefore, the much larger damage in the case of 250 and 340 MeV Xe-irradiation (see Fig. 4) seems to be connected with the high energy deposition into electronic excitations occurring in the corresponding depth region (see Fig. 1). The morphology and kind of defects was studied by means of TEM investigations. Fig. 5 shows, as an example, bright-®eld cross section TEM (XTEM) images at various depths of a sample implanted with 250 MeV Xe ions to a dose of 7 ´ 1012 cmÿ2 (corresponding to 3 ´ 10ÿ3 dpa and nda 0.37 at z 0.3 lm, see Fig. 4). In agreement with the RBS results (see Fig. 3) a 35 nm thick surface layer is just slightly damaged and contains clusters of point defects and, probably, small-sized amorphous inclusions (see part A in Fig. 5). Between 40 and 100 nm columnar isolated spherical or elongated cylindrical defects along the ion trajectories as well as randomly distributed clusters of point defects and, probably, small-sized amorphous inclusions of 2±5 nm size are to be seen. In the depth region from 100 nm to about 10 lm (part A±C in Fig. 5) the straight lines with dark contrast clearly show the formation of tracks with diameters between 7 and 15 nm. The track diameter was determined from both plan-view and cross-section TEM images using a special optical microscope. The density of tracks of 2 ´ 1011 cmÿ2 is much smaller than the ion ¯uence of 7 ´ 1012 cmÿ2 indicating that not each impinging ion produces a visible track. The inner structure of the tracks consists of a mixture of amorphous InP and, probably, a small amount of ®ne polycrystalline grains (inset in part B of Fig. 5). At depths between 10 and 17 lm the material remains crystalline containing clusters of point defects (part D of Fig. 5), and between 19 and 21 lm a band of heavily damaged InP (part E of Fig. 5) occurs which consists of amorphous and crystalline regions and coincides with the position of the maximum of the nuclear energy deposition (see Fig. 1).
Fig. 5. Bright-®eld XTEM images of 250 MeV Xe-irradiated InP for NI 7 ´ 1012 cmÿ2 obtained at dierent depth. In part B also the corresponding diraction pattern is given.
The information about the complete depth distribution of damage was obtained by panoramic cross section pictures (not shown), which is schematically depicted in Fig. 6. The existence of the intermediate only slightly damaged crystalline region between 10 and 17 lm yields two conclusions. First, the damage within the ®rst 10 lm is produced by the electronic energy loss eel of the impinging ions and not by the small amount of nuclear energy deposition, because otherwise the intermediate crystalline region, where the nuclear energy deposition is higher than near the surface, should not exist. And, second, a critical value of eel must be exceeded in order to damage the crystalline InP. The critical value seems to be reached at z 10 lm, thus yielding 13 keV/(ion nm) (see Fig.
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Fig. 6. Schematic illustration of the damage structure over the whole irradiated depth for 250 MeV Xe-ions implanted into InP with an ion ¯uence of NI 7 ´ 1012 cmÿ2 .
1). The fact that for 150 MeV Kr-irradiation of InP eel is less than this value (see Fig. 1) and no in¯uence of the electronic energy deposition on the damage formation is found (see Fig. 4) and no tracks are detected by TEM (not shown), supports this value. At an ion ¯uence of 5 ´ 1013 cmÿ2 the surface region down to 10 lm is amorphous (see part A in Fig. 7 as an example for this depth region) and a second amorphous layer exists extending from 19 to 21 lm (not shown). The ion ¯uence of 5 ´ 1013 cmÿ2 corresponds to 0.55 dpa in the maximum of the nuclear energy distribution, and for this value amorphization of InP occurs also for conventional ion energies [10,17]. Again, as for 7 ´ 1012 cmÿ2 (see Fig. 6), an intermediate damaged but crystalline region occurs from about 15 to 19 lm. Between the amorphous surface layer and this crystalline region a transition region exists with spherical or buried elongated cylindrical zones consisting of amorphous and defective crystalline InP (part B of Fig. 7). A comparable behaviour is found for 340 MeV Xe-irradiation. This is illustrated in Fig. 8 for the region of high electronic energy deposition after
implantation of three ion ¯uences. For 8.5 ´ 1012 cmÿ2 point defect clusters arranged like a string of pearls are to be seen (Fig. 8a). At 4 ´ 1013 cmÿ2 more coherent columnar defects are produced, which is presented in Fig. 8b. Part A of this ®gure shows a dark ®eld cross section image, in which the amorphous regions appear dark and the elongated crystalline regions appear bright. In the corresponding bright ®eld image (part B of Fig. 8b) the crystalline regions appear dark indicating that they are damaged and not perfect. Part C of Fig. 8b shows the diraction pattern. Both diuse rings resulting from the amorphous material and re¯exes of crystalline InP are clearly to be seen. The latter suggests that the elongated crystalline regions are not polycrystalline but remained from the substrate between the latent tracks. After irradiation with an ion ¯uence of 8.5 ´ 1013 cmÿ2 the surface layer is polycrystalline, in larger depth it becomes amorphous (see Fig. 8c) followed by an intermediate crystalline region and a buried amorphous layer (not shown). A possible explanation for the occurrence of polycrystalline material is that at this ¯uence most of the ions already hit amorphous InP, which may cause the
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Fig. 7. Bright-®eld XTEM images of 250 MeV Xe-irradiated InP for NI 5 ´ 1013 cmÿ2 obtained at dierent depth.
production of crystalline tracks (as it was found in amorphous Si and Ge [4]). However, single tracks cannot be seen because of the large ¯uence increase with respect to the previous sample in which the surface layer was already amorphized to a large amount (Fig. 8b). The reason why a damage structure identical to that in 250 MeV Xe-irradiated InP is produced at slightly higher ion ¯uences for 340 MeV Xe-ions, is not completely understood. From the results presented it seems to be obvious that high electronic excitation may produce latent amorphous tracks in crystalline InP at room temperature, if the energy deposited into electronic
processes exceeds a value of about 13 keV/(ion nm). However, in dierence to the ®ndings in other materials where an energy [1±3] and velocity dependence [18,19] is known, a pronounced ¯uence dependence of concentration and structure of the damage is observed. At small ¯uences (NI 65 ´ 1012 cmÿ2 for 250 MeV Xe) only point defects and point defect complexes are detected. With increasing ion ¯uence discontinuous amorphous tracks are formed, the concentration of which is much less than the concentration of impinging ions. Obviously a single ion generates only point defect complexes in virgin crystalline InP, and amorphous tracks appear only if a critical
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Fig. 8. XTEM images of the surface region of InP irradiated with 340 MeV Xe-ions for ion ¯uences (a) 8.5 ´ 1012 cmÿ2 (bright-®eld images of two magni®cations), (b) 4 ´ 1013 cmÿ2 (A ± dark-®eld obtained by using the (2 2 0) diraction re¯ection, B ± bright-®eld image and C ± diraction pattern) and (c) 8.5 ´ 1013 cmÿ2 (bright-®eld image and diraction pattern).
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concentration of these defect centres exists, i.e. formation of amorphous material requires predamaging. To prove this, 250 MeV Xe ions were implanted to an ion ¯uence of 7 ´ 1011 cmÿ2 into predamaged InP (1 MeV Si at )190°C, NI 2.5 ´ 1013 cmÿ2 ). The Xe ion ¯uence used is far below the value where in single crystalline InP visible tracks are formed. The XTEM image in Fig. 9 shows tracks around z 1 lm which is the maximum of nuclear energy deposition for 1 MeV Si ions, i.e. Xe-irradiation leads to track formation within the predamaged region. To describe these results we have used the thermal spike model [13] which assumes that the material melts around the ion trajectory up to depths for which the electronic energy loss exceeds the critical value. This melting is followed by fast cooling and resolidi®cation so that an amorphous track is formed within a crystalline surrounding
[20,21]. The critical energy loss depends on the target material and re¯ects the eciency of the mechanism, which converts electronic excitation energy into atomic motion, in comparison with concurrent mechanisms, which dissipate the excitation energy into the bulk. Assuming that in InP the same mechanisms of lattice instability are working for high electronic excitation as they are known from laser experiments on Si and GaAs [22,23] it was estimated that for eel 13 keV/(ion nm) the energy density converted into atomic motion is sucient to make the lattice unstable and to melt the material (for details see [13]). In a virgin sample the molten cores are surrounded by a perfect crystal and, during quenching of the thermal spike, epitaxial recrystallization must occur to be in accordance with the experimental ®ndings of this work. Obviously, the recrystallization is not perfect and leaves high concentrations of point defects and point defect complexes as it was observed after laser induced melting of GaAs [24]. When further ions impinge these imperfectly recrystallized regions recrystallization of the molten core may be hindered because the surrounding crystal is now less perfect. With ongoing irradiation the recrystallization speed may become smaller than the resolidi®cation velocity leading to freezing in of a rather continuous amorphous track. This may explain the observed dierence between the number of tracks and the ion ¯uence. It is also in accordance with the result of the RBS analysis that, to produce amorphous regions by electronic excitation, several ions must impinge the same area of the crystal [13]. 4. Summary
Fig. 9. Bright-®eld XTEM image of an InP-sample predamaged by 1 MeV Si-ions (¯uence 2.5 ´ 1013 cmÿ2 implanted at )190°C) and post-irradiated with 250 MeV Xe-ions to NI 7 ´ 1011 cmÿ2 obtained at depth around 1 lm (corresponding to the maximum nuclear energy loss of 1 MeV Siions).
The formation of amorphous tracks and completely amorphous layers in single crystalline InP after swift heavy ion irradiation with 250 and 340 MeV Xe has been demonstrated in depth regions where the impinging ions lose their energy preferentially via electronic excitation of the target atoms. This ionization induced damaging occurs if the electronic energy deposition per ion and unit length exceeds a threshold value eel 13 keV/(ion nm) which is e.g. not reached for Kr irradiation
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with an ion energy6150 MeV. Further, the formation of visible tracks requires predamaging of the crystalline InP. The experimental ®ndings may be understood in the framework of the thermal spike model in combination with damage accumulation resulting from imperfect recrystallization of the molten tracks. References [1] R.L. Fleischer, P.B. Price, R.M. Walker, Nuclear Tracks in Solids, University of California Press, Berkeley, CA, 1975. [2] Proceedings of the Second International Symposium On Swift Heavy Ions in Matter, Bensheim/Darmstadt, 1992, Rad. E. 126 (1±4) (1993). [3] Proceedings of the Third International Symposium On Swift Heavy Ions in Matter, Caen, 1995, Nucl. Instr. and Meth. B 107 (1995). [4] K. Izui, S. Furuno, Proceedings of the XIth International Congress on Elec. Micr., 1299, Kyoto 1986. [5] M. Levalois, P. Bogdanski, M. Toulemonde, Nucl. Instr. and Meth. B 63 (1992) 14. [6] M. Levalois, I. Lhermitte-Sebire, P. Marie, E. Paumier, J. Vicens, Nucl. Instr. and Meth. B 107 (1996) 239. [7] M. Mikou, R. Carin, P. Bogdanski, R. Madelon, Nucl. Instr. and Meth. B 107 (1996) 246. [8] S.A. Karamyan, Nucl. Instr. and Meth. B 51 (1990) 354. [9] S.A. Karamyan, V.N. Bugrov, C. Ascheron, G. Otto, S.Yu. Platonov, O.A. Yuminov, Rad. E. 126 (1993) 265.
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[10] W. Wesch, E. Wendler, T. Bachmann, O. Herre, Nucl. Instr. and Meth. B 96 (1995) 290. [11] L. Cliche, S. Roorda, M. Chicoine, R.A. Masut, Phys. Rev. Lett. 75 (1995) 2348. [12] A. Gutzmann, S. Klaum unzer, Nucl. Instr. and Meth. B 127/128 (1997) 12. [13] O. Herre, W. Wesch, P.J. Gaiduk, S. Klaum unzer, E. Wendler, P. Meier, F.F. Komarov, Phys. Rev. B, in press. [14] K. G artner, Nucl. Instr. and Meth. B 132 (1997) 147. [15] J.P. Biersack, J.F. Ziegler, The Stopping and Ranges of Ions in Matter, vol. 1, Pergamon Press, Oxford, 1985. [16] E. Wendler, T. Opfermann, P.I. Gaiduk, J. Appl. Phys. 82 (1997) 5965. [17] E. Wendler, T. Opfermann, P. M uller, W. Wesch, Nucl. Instr. and Meth. B 106 (1995) 303. [18] A. Meftah, F. Brisard, J.M. Constantini, M. Hage-Ali, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 48 (1993) 920. [19] Z.G. Wang, Ch. Dufour, B. Cabeau, J. Dural, G. Fuchs, E. Paumier, F. Pawlak, M. Toulemonde, Nucl. Instr. and Meth. B 107 (1996) 175. [20] M. Toulemonde, Ch. Dufour, E. Paumier, Phys. Rev. B 46 (1992) 14362. [21] Z.G. Wang, Ch. Dufour, E. Paumier, M. Toulemonde, J. Phys. Condens. Matter 6 (1994) 6733. [22] P. Stamp¯i, K.H. Bennemann, Appl. Phys. A 60 (1995) 191. [23] P. Stamp¯i, K.H. Bennemann, Phys. Rev. B 49 (1994) 7299. [24] W. Wesch, E. Wendler, G. G otz, K. Unger, H. R oppischer, Chr. Resagk, Phys. Stat. Sol. (b) 130 (1985) 539.
Damage formation in InP due to high electronic excitation by swift heavy ions W. Wesch a
a,* ,
O. Herre b, P.I. Gaiduk b, E. Wendler a, S. Klaum unzer c, P. Meier
c
Institut f ur Festk orperphysik, Friedrich-Schiller-Universit at Jena, Max-Wien-Platz 1, D-07743 Jena, Germany b Institute of Applied Physics Problems, State University of Belarus, Kurchatova 7, 220062 Minsk, Belarus c Hahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany
Abstract Damage production in crystalline InP by swift Kr- and Xe-ions at room temperature was studied by cross section TEM and RBS. The type and concentration of defects varies with depth due to the dierent dominating interaction processes of the ions with the solid. A ¯uence-dependent damage production is observed in the region of dominating electronic excitation for an electronic energy deposition above 13 keV/(ion nm). Depending on the ion ¯uence point defect complexes, discontinuous tracks or amorphous layers are formed. The observed ®ndings are interpreted as the eect of a thermal spike in combination with damage accumulation resulting from imperfect recrystallization of the molten tracks. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 81.05.Ea; 61.16.Bg
1. Introduction For various insulating materials as well as some intermetallic compounds and pure metals latent track formation due to high electronic excitation by swift heavy ion irradiation has been reported. (see e.g. [1±3] and references therein). In the case of semiconductors the physical situation is much more confused. Crystalline tracks were found in amorphous Si and Ge [4], and in high-resistivity crystalline semiconductors track formation has also been reported [1]. On the other hand, it has
* Corresponding author. Tel.: +49 3641 947330; fax: +49 3641 947302; e-mail: [email protected]
been shown that high electronic excitation seems to be inecient for damage production in Si [5] and 6H-SiC [6]. However, in several semiconductors as Si, Ge, GaAs and GaP an in situ damage annealing occurs during high energy ion irradiation which is obviously correlated with the high electronic energy deposition [5,7±10]. The observed lateral shift of the bombarded surface area after 30 MeV Se irradiation into amorphized InP at low temperatures has been ascribed to the generation of a molten zone around the ion trajectory [11,12]. Recently, we could demonstrate for the ®rst time that 250 MeV Xe-ions can produce amorphous latent tracks in crystalline InP at room temperature [13]. In GaAs under the same irradiation conditions no indication for track formation was found.
0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 3 5 - 2
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W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 341±349
In the present paper we report results of a more systematic study of the in¯uence of the electronic energy deposition on damage formation in InP due to high-energy ion irradiation at room temperature. 2. Experimental á1 0 0ñ InP samples were irradiated with 2.5 and 150 MeV Kr-ions and with 250 and 340 MeV Xeions at room temperature with ion ¯uences ranging from 5 ´ 1012 to 6 ´ 1014 cmÿ2 . Additionally, InP samples predamaged at )190°C by 2.5 ´ 1013 cmÿ2 Si ions with an energy of 1 MeV were irradiated with 250 MeV Xe-ions to an ion ¯uence of 7 ´ 1011 cmÿ2 . The maximum of nuclear energy deposition in the predamaged samples lies around 1 lm. To prevent sample heating especially in the case of the very high energies the ¯ux was kept constant below 1.3 ´ 1010 cmÿ2 sÿ1 . In all cases the InP pieces were mounted to the sample holder with a silver paste giving a good thermal conductivity between them. The temperature variation during the irradiations which was controlled by means of a thermocouple near the sample was less than 2°C. The implanted layers were analysed by means of Rutherford backscattering spectrometry using the channeling technique (RBS/C) and transmission electron microscopy (TEM) in combination with cross section preparation. The RBS/C measurements were carried out with 1.4 MeV He ions at a backscattering angle of 170°. Under these conditions the depth which can be analysed in a quantitative manner in InP is approximately 0.6 lm. That means that for the samples irradiated with the high-energy ions one obtains information about the depth region in which the interaction is dominated by electronic stopping. Assuming a random distribution of the displaced lattice atoms within the lattice cell, from the measured RBS spectra the depth distribution of the relative concentration of displaced lattice atoms, nda , was calculated in the framework of the discontinuous model of the dechanneling [14]. The TEM investigations were performed with an electron microscope EM-125 operating at 100 keV. By means of an ion beam sputtering equip-
Fig. 1. Number of primary displacements, Ndispl , and electronic energy loss, eel , per ion and unit depth versus the depth z, calculated by TRIM95, for 340 and 250 MeV Xe- and 150 and 2.5 MeV Kr-irradiation in InP.
ment both plane-view and cross-section preparations were carried out at low temperatures. For comparison with the experimentally determined damage distributions the depth distributions of the number of primarily displaced atoms Ndispl (corresponding to the nuclear energy deposition) as well as of the energy deposited into electronic processes eel per incident ion and unit length were calculated by means of the TRIM 95 computer code [15] using a displacement threshold of 8 eV. The resulting pro®les are plotted in Fig. 1. In the case of the high MeV energies the maximum of the nuclear energy deposition lies in large depth, whereas the electronic energy deposition dominates near the surface down to 10. . .17 lm. For 2.5 MeV Kr-irradiation nuclear and electronic energy deposition occur in the same depth region and, in comparison to the high energies, the electronic energy loss is much lower. 3. Results and discussion As an example, Fig. 2 represents the RBS spectra of InP samples irradiated with 340 MeV Xe-ions. The yield of He -ions backscattered in the near surface region increases with increasing ion dose and reaches the random level for ¯uences NI 4 ´ 1013 cmÿ2 indicating the formation of a
W. Wesch et al. / Nucl. Instr. and Meth. in Phys. Res. B 146 (1998) 341±349
Fig. 2. Energy spectra of 1.4 MeV He-ions backscattered on 340 MeV Xe-implanted InP for various ion ¯uences.
heavily damaged or amorphous layer. Fig. 3 shows the concentration of displaced atoms, nda , calculated from the spectra in Fig. 2 versus the depth z. In the depth region above 0.1 lm, nda is nearly constant with depth and increases with the ion ¯uence up to its maximum value. It is worth to mention that the damage detected by RBS is small immediately below the surface. An identical behaviour was also found for 250 MeV Xe-irradiation [13].
Fig. 3. Relative concentration of displaced lattice atoms, nda , calculated from the energy spectra in Fig. 2 as a function of depth z.
343
In order to compare the results obtained for the various ion energies, the ion ¯uence was recalculated into displacements per lattice atom (dpa) according to dpa Ndispl NI /N0 (N0 is the atomic density of the target) which represents the nuclear energy deposition. Fig. 4 shows the increase of nda at depth z 0.3 lm with the number of dpa for the dierent irradiations. It is clearly visible that for 250 and 340 MeV Xe-irradiation nda reaches the maximum value already at 0.016 dpa. For such a value of dpa in the case of conventional ion energies a much smaller defect concentration of nda 60.15 results (see [10]). Also in the case of 150 MeV Kr-irradiation at 0.016 dpa only a concentration of displaced atoms of nda 0.08 is measured. A remarkable increase of nda is only observed for values?0.2 dpa. In the case of 2.5 MeV Kr-irradiation the formation of an amorphous layer requires 0.7 dpa in the maximum of nuclear energy deposition and the dependence of nda versus dpa tends to agree with that observed for 150 MeV Kr-irradiation. Further, from previous work it is known that, depending on the ion mass, the nuclear energy deposition necessary to amorphize InP corresponds to 0.2 dpa (300 and 600 keV Se [10]) to 1 dpa (100 keV B [16]). These results suggest that in the case of
Fig. 4. Relative concentration of displaced lattice atoms, nda , versus the number of primary displacements per lattice atom, dpa, for 340 and 250 MeV Xe- and 150 and 2.5 MeV Kr-irradiated crystalline InP.
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Kr-irradiation with 2.5 MeV as well as with 150 MeV, the observed damage is caused by the energy deposition into nuclear processes. For the Xe-irradiations in the depth region of z 0.3 lm regarded here, the nuclear energy loss per ion and unit depth is almost the same as for 150 MeV Kr-irradiation (see Fig. 1). Therefore, the much larger damage in the case of 250 and 340 MeV Xe-irradiation (see Fig. 4) seems to be connected with the high energy deposition into electronic excitations occurring in the corresponding depth region (see Fig. 1). The morphology and kind of defects was studied by means of TEM investigations. Fig. 5 shows, as an example, bright-®eld cross section TEM (XTEM) images at various depths of a sample implanted with 250 MeV Xe ions to a dose of 7 ´ 1012 cmÿ2 (corresponding to 3 ´ 10ÿ3 dpa and nda 0.37 at z 0.3 lm, see Fig. 4). In agreement with the RBS results (see Fig. 3) a 35 nm thick surface layer is just slightly damaged and contains clusters of point defects and, probably, small-sized amorphous inclusions (see part A in Fig. 5). Between 40 and 100 nm columnar isolated spherical or elongated cylindrical defects along the ion trajectories as well as randomly distributed clusters of point defects and, probably, small-sized amorphous inclusions of 2±5 nm size are to be seen. In the depth region from 100 nm to about 10 lm (part A±C in Fig. 5) the straight lines with dark contrast clearly show the formation of tracks with diameters between 7 and 15 nm. The track diameter was determined from both plan-view and cross-section TEM images using a special optical microscope. The density of tracks of 2 ´ 1011 cmÿ2 is much smaller than the ion ¯uence of 7 ´ 1012 cmÿ2 indicating that not each impinging ion produces a visible track. The inner structure of the tracks consists of a mixture of amorphous InP and, probably, a small amount of ®ne polycrystalline grains (inset in part B of Fig. 5). At depths between 10 and 17 lm the material remains crystalline containing clusters of point defects (part D of Fig. 5), and between 19 and 21 lm a band of heavily damaged InP (part E of Fig. 5) occurs which consists of amorphous and crystalline regions and coincides with the position of the maximum of the nuclear energy deposition (see Fig. 1).
Fig. 5. Bright-®eld XTEM images of 250 MeV Xe-irradiated InP for NI 7 ´ 1012 cmÿ2 obtained at dierent depth. In part B also the corresponding diraction pattern is given.
The information about the complete depth distribution of damage was obtained by panoramic cross section pictures (not shown), which is schematically depicted in Fig. 6. The existence of the intermediate only slightly damaged crystalline region between 10 and 17 lm yields two conclusions. First, the damage within the ®rst 10 lm is produced by the electronic energy loss eel of the impinging ions and not by the small amount of nuclear energy deposition, because otherwise the intermediate crystalline region, where the nuclear energy deposition is higher than near the surface, should not exist. And, second, a critical value of eel must be exceeded in order to damage the crystalline InP. The critical value seems to be reached at z 10 lm, thus yielding 13 keV/(ion nm) (see Fig.
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Fig. 6. Schematic illustration of the damage structure over the whole irradiated depth for 250 MeV Xe-ions implanted into InP with an ion ¯uence of NI 7 ´ 1012 cmÿ2 .
1). The fact that for 150 MeV Kr-irradiation of InP eel is less than this value (see Fig. 1) and no in¯uence of the electronic energy deposition on the damage formation is found (see Fig. 4) and no tracks are detected by TEM (not shown), supports this value. At an ion ¯uence of 5 ´ 1013 cmÿ2 the surface region down to 10 lm is amorphous (see part A in Fig. 7 as an example for this depth region) and a second amorphous layer exists extending from 19 to 21 lm (not shown). The ion ¯uence of 5 ´ 1013 cmÿ2 corresponds to 0.55 dpa in the maximum of the nuclear energy distribution, and for this value amorphization of InP occurs also for conventional ion energies [10,17]. Again, as for 7 ´ 1012 cmÿ2 (see Fig. 6), an intermediate damaged but crystalline region occurs from about 15 to 19 lm. Between the amorphous surface layer and this crystalline region a transition region exists with spherical or buried elongated cylindrical zones consisting of amorphous and defective crystalline InP (part B of Fig. 7). A comparable behaviour is found for 340 MeV Xe-irradiation. This is illustrated in Fig. 8 for the region of high electronic energy deposition after
implantation of three ion ¯uences. For 8.5 ´ 1012 cmÿ2 point defect clusters arranged like a string of pearls are to be seen (Fig. 8a). At 4 ´ 1013 cmÿ2 more coherent columnar defects are produced, which is presented in Fig. 8b. Part A of this ®gure shows a dark ®eld cross section image, in which the amorphous regions appear dark and the elongated crystalline regions appear bright. In the corresponding bright ®eld image (part B of Fig. 8b) the crystalline regions appear dark indicating that they are damaged and not perfect. Part C of Fig. 8b shows the diraction pattern. Both diuse rings resulting from the amorphous material and re¯exes of crystalline InP are clearly to be seen. The latter suggests that the elongated crystalline regions are not polycrystalline but remained from the substrate between the latent tracks. After irradiation with an ion ¯uence of 8.5 ´ 1013 cmÿ2 the surface layer is polycrystalline, in larger depth it becomes amorphous (see Fig. 8c) followed by an intermediate crystalline region and a buried amorphous layer (not shown). A possible explanation for the occurrence of polycrystalline material is that at this ¯uence most of the ions already hit amorphous InP, which may cause the
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Fig. 7. Bright-®eld XTEM images of 250 MeV Xe-irradiated InP for NI 5 ´ 1013 cmÿ2 obtained at dierent depth.
production of crystalline tracks (as it was found in amorphous Si and Ge [4]). However, single tracks cannot be seen because of the large ¯uence increase with respect to the previous sample in which the surface layer was already amorphized to a large amount (Fig. 8b). The reason why a damage structure identical to that in 250 MeV Xe-irradiated InP is produced at slightly higher ion ¯uences for 340 MeV Xe-ions, is not completely understood. From the results presented it seems to be obvious that high electronic excitation may produce latent amorphous tracks in crystalline InP at room temperature, if the energy deposited into electronic
processes exceeds a value of about 13 keV/(ion nm). However, in dierence to the ®ndings in other materials where an energy [1±3] and velocity dependence [18,19] is known, a pronounced ¯uence dependence of concentration and structure of the damage is observed. At small ¯uences (NI 65 ´ 1012 cmÿ2 for 250 MeV Xe) only point defects and point defect complexes are detected. With increasing ion ¯uence discontinuous amorphous tracks are formed, the concentration of which is much less than the concentration of impinging ions. Obviously a single ion generates only point defect complexes in virgin crystalline InP, and amorphous tracks appear only if a critical
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Fig. 8. XTEM images of the surface region of InP irradiated with 340 MeV Xe-ions for ion ¯uences (a) 8.5 ´ 1012 cmÿ2 (bright-®eld images of two magni®cations), (b) 4 ´ 1013 cmÿ2 (A ± dark-®eld obtained by using the (2 2 0) diraction re¯ection, B ± bright-®eld image and C ± diraction pattern) and (c) 8.5 ´ 1013 cmÿ2 (bright-®eld image and diraction pattern).
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concentration of these defect centres exists, i.e. formation of amorphous material requires predamaging. To prove this, 250 MeV Xe ions were implanted to an ion ¯uence of 7 ´ 1011 cmÿ2 into predamaged InP (1 MeV Si at )190°C, NI 2.5 ´ 1013 cmÿ2 ). The Xe ion ¯uence used is far below the value where in single crystalline InP visible tracks are formed. The XTEM image in Fig. 9 shows tracks around z 1 lm which is the maximum of nuclear energy deposition for 1 MeV Si ions, i.e. Xe-irradiation leads to track formation within the predamaged region. To describe these results we have used the thermal spike model [13] which assumes that the material melts around the ion trajectory up to depths for which the electronic energy loss exceeds the critical value. This melting is followed by fast cooling and resolidi®cation so that an amorphous track is formed within a crystalline surrounding
[20,21]. The critical energy loss depends on the target material and re¯ects the eciency of the mechanism, which converts electronic excitation energy into atomic motion, in comparison with concurrent mechanisms, which dissipate the excitation energy into the bulk. Assuming that in InP the same mechanisms of lattice instability are working for high electronic excitation as they are known from laser experiments on Si and GaAs [22,23] it was estimated that for eel 13 keV/(ion nm) the energy density converted into atomic motion is sucient to make the lattice unstable and to melt the material (for details see [13]). In a virgin sample the molten cores are surrounded by a perfect crystal and, during quenching of the thermal spike, epitaxial recrystallization must occur to be in accordance with the experimental ®ndings of this work. Obviously, the recrystallization is not perfect and leaves high concentrations of point defects and point defect complexes as it was observed after laser induced melting of GaAs [24]. When further ions impinge these imperfectly recrystallized regions recrystallization of the molten core may be hindered because the surrounding crystal is now less perfect. With ongoing irradiation the recrystallization speed may become smaller than the resolidi®cation velocity leading to freezing in of a rather continuous amorphous track. This may explain the observed dierence between the number of tracks and the ion ¯uence. It is also in accordance with the result of the RBS analysis that, to produce amorphous regions by electronic excitation, several ions must impinge the same area of the crystal [13]. 4. Summary
Fig. 9. Bright-®eld XTEM image of an InP-sample predamaged by 1 MeV Si-ions (¯uence 2.5 ´ 1013 cmÿ2 implanted at )190°C) and post-irradiated with 250 MeV Xe-ions to NI 7 ´ 1011 cmÿ2 obtained at depth around 1 lm (corresponding to the maximum nuclear energy loss of 1 MeV Siions).
The formation of amorphous tracks and completely amorphous layers in single crystalline InP after swift heavy ion irradiation with 250 and 340 MeV Xe has been demonstrated in depth regions where the impinging ions lose their energy preferentially via electronic excitation of the target atoms. This ionization induced damaging occurs if the electronic energy deposition per ion and unit length exceeds a threshold value eel 13 keV/(ion nm) which is e.g. not reached for Kr irradiation
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with an ion energy6150 MeV. Further, the formation of visible tracks requires predamaging of the crystalline InP. The experimental ®ndings may be understood in the framework of the thermal spike model in combination with damage accumulation resulting from imperfect recrystallization of the molten tracks. References [1] R.L. Fleischer, P.B. Price, R.M. Walker, Nuclear Tracks in Solids, University of California Press, Berkeley, CA, 1975. [2] Proceedings of the Second International Symposium On Swift Heavy Ions in Matter, Bensheim/Darmstadt, 1992, Rad. E. 126 (1±4) (1993). [3] Proceedings of the Third International Symposium On Swift Heavy Ions in Matter, Caen, 1995, Nucl. Instr. and Meth. B 107 (1995). [4] K. Izui, S. Furuno, Proceedings of the XIth International Congress on Elec. Micr., 1299, Kyoto 1986. [5] M. Levalois, P. Bogdanski, M. Toulemonde, Nucl. Instr. and Meth. B 63 (1992) 14. [6] M. Levalois, I. Lhermitte-Sebire, P. Marie, E. Paumier, J. Vicens, Nucl. Instr. and Meth. B 107 (1996) 239. [7] M. Mikou, R. Carin, P. Bogdanski, R. Madelon, Nucl. Instr. and Meth. B 107 (1996) 246. [8] S.A. Karamyan, Nucl. Instr. and Meth. B 51 (1990) 354. [9] S.A. Karamyan, V.N. Bugrov, C. Ascheron, G. Otto, S.Yu. Platonov, O.A. Yuminov, Rad. E. 126 (1993) 265.
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