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Damage evolution in Xe-ion irradiated rutile (TiO2) single crystals
Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 314±321
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Damage evolution in Xe-ion irradiated rutile (TiO2 ) single crystals Fuxin Li a
a,b
, Manabu Ishimaru b,c, Ping Lu a, Ivan V. Afanasyev-Charkin b, Kurt E. Sickafus b,*
Department of Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA b Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Department of Materials Science and Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
Abstract Rutile (TiO2 ) single crystals with (1 1 0) and (1 0 0) orientations were irradiated with 360 keV Xe2 ions at 300 K to ¯uences ranging from 1 1017 to 5 1020 Xe/m2 . Irradiated samples were analyzed using Rutherford backscattering spectroscopy combined with ion channeling analysis (RBS/C) and transmission electron microscopy (TEM). RBS/C results showed that much of the instantaneous displacement damage produced under ion irradiation is recovered under ambient temperature irradiation conditions. Upon irradiation to a ¯uence of 2 1019 Xe/m2 , the radiation damage-induced microstructure was observed by TEM to consist of three distinct layers: (1) a layer near surface (thickness about 12 nm) exhibiting relatively homogeneous TEM contrast; (2) a second layer with a low density of relatively large-sized defects; and (3) a third layer consisting of a high concentration of small defects. After the ¯uence was increased to 5 1019 Xe/m2 , a buried amorphous layer was observed by TEM. The thickness of the amorphous layer was found to increase with increasing Xe ion ¯uence. The uppermost damage layer, which accounts for the surface subpeak in RBS/C spectra, was found to be polygonized by ion irradiation. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Fk; 61.18.Bn; 61.85.+p; 61.16.Bg; 61.72.Ff; 61.14.Lj; 81.05.Hd; 81.05.Je; 81.40.Wx; 83.80.Pc Keywords: Ion radiation eects; Rutile (TiO2 ); Crystal defects; Amorphization; Ion scattering; Ion channeling; Transmission electron microscopy; Electron diraction
1. Introduction Rutile (TiO2 ) is a common, octahedrally-coordinated structure type with space group P4/mnm (a0 0.4593 nm, c0 0.2959 nm). In previous ion irradiation studies on rutile [1±7], Rutherford *
Corresponding author. Tel.: +1-505-665-3457; fax: +1-505667-6802. E-mail address: [email protected] (K.E. Sickafus).
backscattering spectroscopy and ion channeling (RBS/C) were used to provide information regarding the evolution of radiation damage induced by ion irradiation. Recently, transmission electron microscopy (TEM) was used to obtain a direct measurement of damage distributions in heavy ion irradiated rutile [8]. This paper reports results on radiation damage behavior in ion-irradiated rutile single crystals and builds on the work from two previous investigations [7,8].
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 7 2 - 2
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2. Experimental procedure Single crystals of rutile with (1 1 0) and (1 0 0) orientations were obtained from Princeton Scienti®c Corp. (P.O. Box 143, Princeton, NJ 08542) for use in this study. The wafers were polished on one side to a mirror ®nish and cut to dimensions of approximately 10 mm 10 mm 0:5 mm. Ion irradiation experiments were performed at room temperature (300 K) using a 200 kV ion implanter in the Ion Beam Materials Laboratory at Los Alamos National Laboratory. The implanter was operated at 180 kV in order to obtain 360 keV Xe2 ions for irradiations. Samples were tilted about 15° with respect to the incident beam direction in order to minimize ion-channeling eects during implantation. Rutile wafers were irradiated with dierent Xe2 ions ¯uences ranging from 1 107 to 5 1020 ions/m2 . The ion ¯ux during irradiation was maintained at or below 3 1016 Xe/m2 s, in order to avoid heating eects. The as-implanted samples were analyzed by RBS/C along the á1 1 0ñ axis using 2 MeV He . Samples were then prepared in cross-section for TEM observation. Radiation-induced microstructures were examined in either a Philips CM-30 or a JEOL 3000F transmission electron microscope, operating at 300 kV. Ion range and damage distribution were calculated using the Monte Carlo code SRIM 2000 [9] and using full collision cascade simulations. The density for rutile used in the calculations was 4.2 g/ cm3 . Also 40 eV was used as the displacement threshold energy for both Ti and O atoms (40 eV arbitrary). 3. Results Fig. 1 shows RBS/C spectra obtained from (1 1 0)-oriented rutile single crystals irradiated with 360 keV Xe2 ions to ¯uences ranging from 8 1017 to 1 1020 Xe/m2 . A random and an aligned spectrum from an unirradiated crystal are also shown in Fig. 1 for comparison. The irradiation-induced damage, as revealed by RBS/C, exhibits two damage peaks: one near the surface with a thickness of 10±12 nm (surface subpeak), and the
Fig. 1. 2 MeV He ion RBS/C spectra obtained from (1 1 0)oriented single crystals of rutile irradiated with 360 keV Xe2 ions at ambient temperature (300 K) to increasing ion ¯uence.
other near the mean projected range of the implanted ions (buried damage peak). Both peaks increase with increasing ¯uence, and upon irradiation to a ¯uence of 7 1019 ions/m2 , the buried damage peak reaches the random level. An RBS/C yield at the random level suggests possible formation of a buried amorphous layer. RBS/C results are summarized in Fig. 2. Fig. 2 shows a plot of radiation damage accumulation on the Ti sublattice as a function of Xe ion dose. The damage accumulation parameter, v, in this plot is obtained by measuring the RBS/C yield at the maximum of the buried damage peak, divided by RBS/C yield at the same position (channel 508) in the random spectra. From Fig. 2, it is apparent that damage accumulation progresses in three stages: (1) a low dose regime in which the damage peak is small and increases slowly with ¯uence; (2) a transition stage in which the buried damage peak increases rapidly with ¯uence; and (3) a saturation stage in which the damage rate slows and saturates at a level v 1. A previous study of damage accumulation in cubic zirconia [10] suggests that stage 1 involves the formation of isolated defectsclusters, while stage 2 is characterized by the interlinking of extended defects such as dislocations and defect clusters. It will be demonstrated here that the observed saturation of damage in stage 3
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Fig. 2. Damage accumulation based on RBS/C measurements for the Ti sublattice in rutile. The RBS/C damage accumulation parameter v is plotted versus Xe2 ¯uence for 300 K irradiation conditions. The plot reveals that damage accumulation proceeds in three stages. The experimental data is ®tted with a nucleation and growth model for phase transformations (see Section 4).
for rutile is the result of an amorphization transformation. It is possible to obtain a crude estimate for the amount of residual damage in the rutile crystals by analysis of the buried damage peak in the aligned RBS/C spectra. Since the He ions are dechanneled predominantly by atoms displaced from lattice sites, an aligned RBS/C spectrum actually corresponds to a pro®le of the concentration of irradiation-induced interstitial atoms. An increase in the yield in an aligned spectrum indicates an increase in the number of displaced atoms. The concentration of displaced atoms, qd x, as a function of depth, x, can be estimated based on the aligned RBS/C spectrum using the following formula [11,12]: qd x qv x ÿ vR x=1 ÿ vR x;
damage region was identi®ed in an aligned RBS/C spectrum, by comparing the dechanneling rate with that from an unirradiated crystal. The straight-line approximation was then drawn from a point behind the damage peak in the aligned spectrum to the scattering height in the near-surface region from an unirradiated crystal. To avoid interference from near surface damage, the surface subpeak was substracted from each spectrum prior to analysis of the buried damage peak. Fig. 3 shows the results of an analysis of displaced atom concentrations based on RBS/C buried damage peaks, for several of the Xe-irradiated samples from Figs. 1 and 2. In Fig. 3, the integrated number of interstitial Ti atoms per incident Xe ion is plotted versus the ion ¯uence. This represents the experimentally measured ``retained damage''. Also shown in Fig. 3 is the number of Ti vacancies (or Frenkel pairs) produced per ion, based on a Monte Carlo SRIM calculation. The SRIM value is independent of ¯uence and is similar to a modi®ed Kinchin±Pease estimate for displacements [13]. It is apparent in Fig. 3 that the retained interstitial concentration rises initially and then falls with increasing ion ¯uence, and that the retained displaced atom concentration is well below the instantaneous damage level based on SRIM (the maximum number of retained dis-
1
where q is the atomic density of the element of interest in the crystal, v x is the yield ratio of the aligned spectrum to the random spectrum at depth x, and vR x is the random fraction of the aligned incident beam. In this study, vR x was estimated using a straight-line approximation. First, the
Fig. 3. Retained displacement damage versus ion ¯uence for the Ti sublattice based on RBS/C measurements. An estimate for the magnitude of the instantaneous displacement damage state, based on SRIM, is also shown for comparison.
F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321
placed atoms is only 15% of that predicted by SRIM). This suggests that signi®cant damage recovery occurs in these 300 K irradiations. Some of the as-implanted crystals were prepared in cross-section for TEM analysis to determine the nature and distribution of the irradiation-induced defects. Fig. 4(a) shows a bright-®eld (BF) image from a (1 1 0) rutile crystal irradiated to a ¯uence of 8 1017 Xe/m2 . This is a low dose sample in stage 1 of the damage accumulation curve (Fig. 2).
317
Extended defects are not apparent in this BF image. The inset microdiraction pattern from the irradiated layer is identical to the pattern obtained from the unirradiated substrate (not shown). Fig. 4(b) shows a BF image from a sample irradiated to a ¯uence of 2 1019 Xe/m2 . This ¯uence is located in stage 2 of damage accumulation (Fig. 2). Three damage layers with dierent defect characteristics are apparent in Fig. 4(b), combining to produce a damage region with a total thickness of
Fig. 4. Cross-sectional TEM-BF images and microdiraction patterns for (1 1 0) rutile crystals irradiated to successively higher Xe2 ion ¯uences: (a) 8 1017 Xe/m2 , (b) 2 1019 Xe/m2 , (c) 5 1019 Xe/m2 , (d) 1 1020 Xe/m2 . Xe2 ions were incident from the top of each BF micrograph.
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about 140 nm. The top layer (1) has thickness of about 12 nm; this layer exhibits homogeneous BF contrast. Immediately beneath the top layer is a second layer (2), distinguished by a dark band of contrast with a thickness of 70 nm. This layer contains defects with relatively large size (average 35 nm diameter) and low density. The third layer (3) has a thickness of 70 nm (also appearing dark in BF), and consists of smaller (average 10 nm diameter) and more dense defects than in layer 2. Some planar defects are visible near the bottom of layer 3; these penetrate well into the unirradiated substrate. Microdiraction patterns obtained from each damage layer indicate that all three layers remain crystalline and epitaxial with the substrate following irradiation. Fig. 4(c) shows a TEM-BF image from a (1 1 0) rutile crystal irradiated to a ¯uence of 5 1019 Xe/m2 , near the completion of stage 2 of
the damage accumulation curve (Fig. 2). The BF image indicates evidence for the onset of formation of a buried amorphous layer (labeled 3). This layer exhibits bright contrast in the BF image, and is sandwiched between dark contrast, highly defected layers (2 and 4). The amorphous structure of this layer was veri®ed by electron diraction (microdiraction inset 3). The thickness of this amorphous layer is about 20 nm and the total thickness of the damage microstructure is about 160 nm at this irradiation dose. For 360 keV Xe ion irradiation at 300 K, the critical amorphization dose at the peak damage depth (64 nm) is about 10 displacement per atom (dpa). Fig. 4(d) shows a TEM-BF image obtained from a (1 1 0) rutile crystal irradiated to ¯uence of 1 1020 Xe/m2 , well into stage 3 of damage accumulation (Fig. 2). The TEM-BF image indicates that the buried amor-
Fig. 5. SAED pattern from the surface layer area of a (1 0 0)-oriented rutile crystal irradiated to a ¯uence of 4 1019 Xe/m2 . A TEMBF image of the region analyzed is shown in the inset. The circle in the inset indicates the size of the SAED aperture. Xe2 ions were incident from the right-hand side of the inset BF micrograph.
F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321
phous layer has extended both towards the surface and into the crystal interior, reaching a thickness of 50 nm at this ion ¯uence. The surface layer in Figs. 4(b), (c) and (d), which accounts for the surface subpeak in RBS/C spectra in Fig. 1, was analyzed both by selected area electron diraction (SAED) and by high resolution TEM. Fig. 5 shows an SAED pattern obtained from the surface damage layer in a (1 0 0) rutile crystal irradiated to a ¯uence of 4 1019 Xe/ m2 . The SAED pattern includes contributions from both the uppermost damage layer and the underlying material. The SAED aperture size is shown schematically in the inset TEM-BF image in Fig. 5. It is apparent that the re¯ections in the SAED pattern in Fig. 5 are doubled. The splitting is most obvious in the higher order re¯ections. This eect is due to the superposition of two crystal orientations in the SAED pattern: the major re¯ections correspond to zone axis [0 0 1], the minor re¯ections to zone axis [0 1 1]. A common re¯ection in both of these patterns is [1 0 0]. This direction is coincident with the irradiated crystal surface normal.
319
Fig. 6 shows a high-resolution TEM image obtained from the surface damage layer in Fig. 5. A crystallite labeled B in Fig. 6 possesses a crystallographic orientation distinct from that of neighboring crystallites (A and C). Fast Fourier Transforms of the image contrast from these regions (insets A, B and C in Fig. 6) reveal that crystallite B is an [0 1 1]-oriented crystallite, while crystallites A and C are [0 0 1]. These observations suggest that the surfaces of the rutile crystal wafers are polygonized under ion irradiation. This surface polygonization seems to be accompanied by special crystallographic relationships. Nevertheless, the polygonzied surface microstructure explains the enhanced ion yield at the surface in the RBS/C spectra in Fig. 1. 4. Discussion Damage accumulation in rutile three stages, similar to observations except that stage 3 is characterized phization transformation, rather
proceeds in in ZrO2 [10], by an amorthan corre-
Fig. 6. High-resolution TEM cross-sectional image obtained from the uppermost region of the surface layer shown in Fig. 5. Also shown as insets are Fast Fourier Transforms from the regions labeled A, B and C in the high-resolution TEM image. Xe2 ions were incident from the top of the TEM micrograph.
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sponding to a stable, extended defect network (as in zirconia). The critical dose for amorphization of rutile at 300 K and using 360 keV Xe ions corresponds to a damage level of about 10 dpa. Damage accumulation at ambient temperature (300 K) proceeds in accordance with a nucleation and growth model for phase transformations developed by Johnson and Mehl [14] and Avrami [15] (heretofore referred to as the J-M-A model). This is demonstrated in Fig. 2 by the good agreement between the RBS/C data for Xe-ion irradiated rutile and a ®t to the J-M-A formula, v n 1 ÿ exp ÿ K/ , for ®tting parameters K 6:32 10ÿ20 and n 1.33. The incubation period (stage 1) prior to signi®cant damage accumulation is likely to be strongly aected by recovery mechanisms. Recovery is signi®cant in rutile even at ambient temperature. The experimentally measured retained damage, as expressed by the number of interstitial atoms produced per incident ion (Fig. 3), is well below a SRIM-simulated damage level. The maximum retained damage according to RBS/C is only 15% of that predicted by SRIM. Polygonization of the surface layer explains the surface subpeak observed in previous RBS/C investigations of rutile [1,2,8]. Polycrystallization has also been observed in UO2 [16] and in metals [17] exposed to ion irradiation. However, the narrow, polygonized surface layer observed in irradiated rutile crystals is unique. A mechanism for this surface recrystallization phenomenon is not apparent at this time. Factors which probably contribute to this eect include surface strains, defect pinning at the surface, long-range atomic migration, and compositional changes due to preferential surface sputtering. 5. Conclusions Microstructural damage evolution in rutile crystals irradiated with 360 keV Xe2 ions at 300 K, was investigated using RBS/C and TEM. It was determined that: 1. Damage accumulation proceeds in three stages, in accordance with a nucleation and growth model for phase transformations.
2. Damage recovery in rutile under ambient temperature irradiation conditions is signi®cant. Retained damage is estimated to be less than 15% of the calculated instantaneous displacement damage level. 3. The critical dose for amorphization of rutile under the irradiation conditions described above was determined to be 10 dpa at a depth corresponding to the peak damage. 4. For ion doses below the critical dose for amorphization, the damage microstructure consists of three layers: a thin surface layer with thickness 12 nm; a second layer with a low density of relatively large defects; and a third layer with a high density of small defects. Upon irradiation beyond the critical amorphization dose, an amorphous layer appears between two highly defected layers. The thickness of this amorphous layer increases with increasing ion ¯uence. 5. The uppermost layer of damage in the irradiated rutile crystals was found to consist of a polygonized microstructure. This explains the RBS/C surface subpeaks observed in this investigation and in previous studies. Acknowledgements The authors wish to acknowledge the support provided by the Department of Energy, Oce of Basic Energy Sciences, Division of Materials Sciences. References [1] I. Khubeis, R. Fromknecht, O. Meyer, Phys. Rev. B 55 (1997) 1. [2] I. Khubeis, O. Meyer, Nucl. Instr. and Meth. B 120 (1996) 257. [3] O. Meyer, I. Khubeis, R. Fromknecht, S. Massing, Nucl. Instr. and Meth. B 136/138 (1998) 436. [4] O. Meyer, I. Khubeis, R. Fromknecht, S. Massing, Nucl. Instr. and Meth. B 127/128 (1997) 624. [5] R. Fromknecht, R. Auer, I. Khubeis, O. Meyer, Nucl. Instr. and Meth. B 120 (1996) 252. [6] R. Fromknecht, O. Meyer, Mater. Chem. Phys. 45 (1996) 50. [7] T. Hartmann, L.M. Wang, W.J. Weber, N. Yu, K.E. Sickafus, J.N. Mitchell, C.J. Wetteland, M.A. Nastasi, M.G. Hollander, N.P. Baker, C.R. Evans, J.R. Tesmer, C.J. Maggiore, Nucl. Instr. and Meth. B 141 (1998) 398.
F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321 [8] F. Li, P. Lu, K.E. Sickafus, C.R. Evans, M. Nastasi, Mat. Res. Soc. Symp. Proc. 540 (1999) 311. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [10] K. Yasuda, M. Nastasi, K.E. Sickafus, C.J. Maggiore, N. Yu, Nucl. Instr. and Meth. B 136±138 (1998) 499. [11] O.W. Holland, D. Fathy, J. Narayan, O.S. Oen, Radiat. E. 90 (1985) 127. [12] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978, Chapter 8.
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[13] M.J. Norgett, M.T. Robinson, I.M. Torrens, Nucl. Engrg. Design 33 (1975) 50. [14] W.A. Johnson, R.F. Mehl, Trans. Am. Inst. Min. Metall. Pet. Eng. 135 (1938) 416. [15] M. Avrami, J. Chem. Phys. 9 (1941) 177. [16] Hj. Matzke, A. Turos, G. Linker, Nucl. Instr. and Meth. B 91 (1994) 294. [17] A. Azzman, O. Meyer, Nucl. Instr. and Meth. B 12 (1985) 369.
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Damage evolution in Xe-ion irradiated rutile (TiO2 ) single crystals Fuxin Li a
a,b
, Manabu Ishimaru b,c, Ping Lu a, Ivan V. Afanasyev-Charkin b, Kurt E. Sickafus b,*
Department of Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA b Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Department of Materials Science and Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
Abstract Rutile (TiO2 ) single crystals with (1 1 0) and (1 0 0) orientations were irradiated with 360 keV Xe2 ions at 300 K to ¯uences ranging from 1 1017 to 5 1020 Xe/m2 . Irradiated samples were analyzed using Rutherford backscattering spectroscopy combined with ion channeling analysis (RBS/C) and transmission electron microscopy (TEM). RBS/C results showed that much of the instantaneous displacement damage produced under ion irradiation is recovered under ambient temperature irradiation conditions. Upon irradiation to a ¯uence of 2 1019 Xe/m2 , the radiation damage-induced microstructure was observed by TEM to consist of three distinct layers: (1) a layer near surface (thickness about 12 nm) exhibiting relatively homogeneous TEM contrast; (2) a second layer with a low density of relatively large-sized defects; and (3) a third layer consisting of a high concentration of small defects. After the ¯uence was increased to 5 1019 Xe/m2 , a buried amorphous layer was observed by TEM. The thickness of the amorphous layer was found to increase with increasing Xe ion ¯uence. The uppermost damage layer, which accounts for the surface subpeak in RBS/C spectra, was found to be polygonized by ion irradiation. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.82.Fk; 61.18.Bn; 61.85.+p; 61.16.Bg; 61.72.Ff; 61.14.Lj; 81.05.Hd; 81.05.Je; 81.40.Wx; 83.80.Pc Keywords: Ion radiation eects; Rutile (TiO2 ); Crystal defects; Amorphization; Ion scattering; Ion channeling; Transmission electron microscopy; Electron diraction
1. Introduction Rutile (TiO2 ) is a common, octahedrally-coordinated structure type with space group P4/mnm (a0 0.4593 nm, c0 0.2959 nm). In previous ion irradiation studies on rutile [1±7], Rutherford *
Corresponding author. Tel.: +1-505-665-3457; fax: +1-505667-6802. E-mail address: [email protected] (K.E. Sickafus).
backscattering spectroscopy and ion channeling (RBS/C) were used to provide information regarding the evolution of radiation damage induced by ion irradiation. Recently, transmission electron microscopy (TEM) was used to obtain a direct measurement of damage distributions in heavy ion irradiated rutile [8]. This paper reports results on radiation damage behavior in ion-irradiated rutile single crystals and builds on the work from two previous investigations [7,8].
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 7 2 - 2
F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321
315
2. Experimental procedure Single crystals of rutile with (1 1 0) and (1 0 0) orientations were obtained from Princeton Scienti®c Corp. (P.O. Box 143, Princeton, NJ 08542) for use in this study. The wafers were polished on one side to a mirror ®nish and cut to dimensions of approximately 10 mm 10 mm 0:5 mm. Ion irradiation experiments were performed at room temperature (300 K) using a 200 kV ion implanter in the Ion Beam Materials Laboratory at Los Alamos National Laboratory. The implanter was operated at 180 kV in order to obtain 360 keV Xe2 ions for irradiations. Samples were tilted about 15° with respect to the incident beam direction in order to minimize ion-channeling eects during implantation. Rutile wafers were irradiated with dierent Xe2 ions ¯uences ranging from 1 107 to 5 1020 ions/m2 . The ion ¯ux during irradiation was maintained at or below 3 1016 Xe/m2 s, in order to avoid heating eects. The as-implanted samples were analyzed by RBS/C along the á1 1 0ñ axis using 2 MeV He . Samples were then prepared in cross-section for TEM observation. Radiation-induced microstructures were examined in either a Philips CM-30 or a JEOL 3000F transmission electron microscope, operating at 300 kV. Ion range and damage distribution were calculated using the Monte Carlo code SRIM 2000 [9] and using full collision cascade simulations. The density for rutile used in the calculations was 4.2 g/ cm3 . Also 40 eV was used as the displacement threshold energy for both Ti and O atoms (40 eV arbitrary). 3. Results Fig. 1 shows RBS/C spectra obtained from (1 1 0)-oriented rutile single crystals irradiated with 360 keV Xe2 ions to ¯uences ranging from 8 1017 to 1 1020 Xe/m2 . A random and an aligned spectrum from an unirradiated crystal are also shown in Fig. 1 for comparison. The irradiation-induced damage, as revealed by RBS/C, exhibits two damage peaks: one near the surface with a thickness of 10±12 nm (surface subpeak), and the
Fig. 1. 2 MeV He ion RBS/C spectra obtained from (1 1 0)oriented single crystals of rutile irradiated with 360 keV Xe2 ions at ambient temperature (300 K) to increasing ion ¯uence.
other near the mean projected range of the implanted ions (buried damage peak). Both peaks increase with increasing ¯uence, and upon irradiation to a ¯uence of 7 1019 ions/m2 , the buried damage peak reaches the random level. An RBS/C yield at the random level suggests possible formation of a buried amorphous layer. RBS/C results are summarized in Fig. 2. Fig. 2 shows a plot of radiation damage accumulation on the Ti sublattice as a function of Xe ion dose. The damage accumulation parameter, v, in this plot is obtained by measuring the RBS/C yield at the maximum of the buried damage peak, divided by RBS/C yield at the same position (channel 508) in the random spectra. From Fig. 2, it is apparent that damage accumulation progresses in three stages: (1) a low dose regime in which the damage peak is small and increases slowly with ¯uence; (2) a transition stage in which the buried damage peak increases rapidly with ¯uence; and (3) a saturation stage in which the damage rate slows and saturates at a level v 1. A previous study of damage accumulation in cubic zirconia [10] suggests that stage 1 involves the formation of isolated defectsclusters, while stage 2 is characterized by the interlinking of extended defects such as dislocations and defect clusters. It will be demonstrated here that the observed saturation of damage in stage 3
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F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321
Fig. 2. Damage accumulation based on RBS/C measurements for the Ti sublattice in rutile. The RBS/C damage accumulation parameter v is plotted versus Xe2 ¯uence for 300 K irradiation conditions. The plot reveals that damage accumulation proceeds in three stages. The experimental data is ®tted with a nucleation and growth model for phase transformations (see Section 4).
for rutile is the result of an amorphization transformation. It is possible to obtain a crude estimate for the amount of residual damage in the rutile crystals by analysis of the buried damage peak in the aligned RBS/C spectra. Since the He ions are dechanneled predominantly by atoms displaced from lattice sites, an aligned RBS/C spectrum actually corresponds to a pro®le of the concentration of irradiation-induced interstitial atoms. An increase in the yield in an aligned spectrum indicates an increase in the number of displaced atoms. The concentration of displaced atoms, qd x, as a function of depth, x, can be estimated based on the aligned RBS/C spectrum using the following formula [11,12]: qd x qv x ÿ vR x=1 ÿ vR x;
damage region was identi®ed in an aligned RBS/C spectrum, by comparing the dechanneling rate with that from an unirradiated crystal. The straight-line approximation was then drawn from a point behind the damage peak in the aligned spectrum to the scattering height in the near-surface region from an unirradiated crystal. To avoid interference from near surface damage, the surface subpeak was substracted from each spectrum prior to analysis of the buried damage peak. Fig. 3 shows the results of an analysis of displaced atom concentrations based on RBS/C buried damage peaks, for several of the Xe-irradiated samples from Figs. 1 and 2. In Fig. 3, the integrated number of interstitial Ti atoms per incident Xe ion is plotted versus the ion ¯uence. This represents the experimentally measured ``retained damage''. Also shown in Fig. 3 is the number of Ti vacancies (or Frenkel pairs) produced per ion, based on a Monte Carlo SRIM calculation. The SRIM value is independent of ¯uence and is similar to a modi®ed Kinchin±Pease estimate for displacements [13]. It is apparent in Fig. 3 that the retained interstitial concentration rises initially and then falls with increasing ion ¯uence, and that the retained displaced atom concentration is well below the instantaneous damage level based on SRIM (the maximum number of retained dis-
1
where q is the atomic density of the element of interest in the crystal, v x is the yield ratio of the aligned spectrum to the random spectrum at depth x, and vR x is the random fraction of the aligned incident beam. In this study, vR x was estimated using a straight-line approximation. First, the
Fig. 3. Retained displacement damage versus ion ¯uence for the Ti sublattice based on RBS/C measurements. An estimate for the magnitude of the instantaneous displacement damage state, based on SRIM, is also shown for comparison.
F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321
placed atoms is only 15% of that predicted by SRIM). This suggests that signi®cant damage recovery occurs in these 300 K irradiations. Some of the as-implanted crystals were prepared in cross-section for TEM analysis to determine the nature and distribution of the irradiation-induced defects. Fig. 4(a) shows a bright-®eld (BF) image from a (1 1 0) rutile crystal irradiated to a ¯uence of 8 1017 Xe/m2 . This is a low dose sample in stage 1 of the damage accumulation curve (Fig. 2).
317
Extended defects are not apparent in this BF image. The inset microdiraction pattern from the irradiated layer is identical to the pattern obtained from the unirradiated substrate (not shown). Fig. 4(b) shows a BF image from a sample irradiated to a ¯uence of 2 1019 Xe/m2 . This ¯uence is located in stage 2 of damage accumulation (Fig. 2). Three damage layers with dierent defect characteristics are apparent in Fig. 4(b), combining to produce a damage region with a total thickness of
Fig. 4. Cross-sectional TEM-BF images and microdiraction patterns for (1 1 0) rutile crystals irradiated to successively higher Xe2 ion ¯uences: (a) 8 1017 Xe/m2 , (b) 2 1019 Xe/m2 , (c) 5 1019 Xe/m2 , (d) 1 1020 Xe/m2 . Xe2 ions were incident from the top of each BF micrograph.
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about 140 nm. The top layer (1) has thickness of about 12 nm; this layer exhibits homogeneous BF contrast. Immediately beneath the top layer is a second layer (2), distinguished by a dark band of contrast with a thickness of 70 nm. This layer contains defects with relatively large size (average 35 nm diameter) and low density. The third layer (3) has a thickness of 70 nm (also appearing dark in BF), and consists of smaller (average 10 nm diameter) and more dense defects than in layer 2. Some planar defects are visible near the bottom of layer 3; these penetrate well into the unirradiated substrate. Microdiraction patterns obtained from each damage layer indicate that all three layers remain crystalline and epitaxial with the substrate following irradiation. Fig. 4(c) shows a TEM-BF image from a (1 1 0) rutile crystal irradiated to a ¯uence of 5 1019 Xe/m2 , near the completion of stage 2 of
the damage accumulation curve (Fig. 2). The BF image indicates evidence for the onset of formation of a buried amorphous layer (labeled 3). This layer exhibits bright contrast in the BF image, and is sandwiched between dark contrast, highly defected layers (2 and 4). The amorphous structure of this layer was veri®ed by electron diraction (microdiraction inset 3). The thickness of this amorphous layer is about 20 nm and the total thickness of the damage microstructure is about 160 nm at this irradiation dose. For 360 keV Xe ion irradiation at 300 K, the critical amorphization dose at the peak damage depth (64 nm) is about 10 displacement per atom (dpa). Fig. 4(d) shows a TEM-BF image obtained from a (1 1 0) rutile crystal irradiated to ¯uence of 1 1020 Xe/m2 , well into stage 3 of damage accumulation (Fig. 2). The TEM-BF image indicates that the buried amor-
Fig. 5. SAED pattern from the surface layer area of a (1 0 0)-oriented rutile crystal irradiated to a ¯uence of 4 1019 Xe/m2 . A TEMBF image of the region analyzed is shown in the inset. The circle in the inset indicates the size of the SAED aperture. Xe2 ions were incident from the right-hand side of the inset BF micrograph.
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phous layer has extended both towards the surface and into the crystal interior, reaching a thickness of 50 nm at this ion ¯uence. The surface layer in Figs. 4(b), (c) and (d), which accounts for the surface subpeak in RBS/C spectra in Fig. 1, was analyzed both by selected area electron diraction (SAED) and by high resolution TEM. Fig. 5 shows an SAED pattern obtained from the surface damage layer in a (1 0 0) rutile crystal irradiated to a ¯uence of 4 1019 Xe/ m2 . The SAED pattern includes contributions from both the uppermost damage layer and the underlying material. The SAED aperture size is shown schematically in the inset TEM-BF image in Fig. 5. It is apparent that the re¯ections in the SAED pattern in Fig. 5 are doubled. The splitting is most obvious in the higher order re¯ections. This eect is due to the superposition of two crystal orientations in the SAED pattern: the major re¯ections correspond to zone axis [0 0 1], the minor re¯ections to zone axis [0 1 1]. A common re¯ection in both of these patterns is [1 0 0]. This direction is coincident with the irradiated crystal surface normal.
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Fig. 6 shows a high-resolution TEM image obtained from the surface damage layer in Fig. 5. A crystallite labeled B in Fig. 6 possesses a crystallographic orientation distinct from that of neighboring crystallites (A and C). Fast Fourier Transforms of the image contrast from these regions (insets A, B and C in Fig. 6) reveal that crystallite B is an [0 1 1]-oriented crystallite, while crystallites A and C are [0 0 1]. These observations suggest that the surfaces of the rutile crystal wafers are polygonized under ion irradiation. This surface polygonization seems to be accompanied by special crystallographic relationships. Nevertheless, the polygonzied surface microstructure explains the enhanced ion yield at the surface in the RBS/C spectra in Fig. 1. 4. Discussion Damage accumulation in rutile three stages, similar to observations except that stage 3 is characterized phization transformation, rather
proceeds in in ZrO2 [10], by an amorthan corre-
Fig. 6. High-resolution TEM cross-sectional image obtained from the uppermost region of the surface layer shown in Fig. 5. Also shown as insets are Fast Fourier Transforms from the regions labeled A, B and C in the high-resolution TEM image. Xe2 ions were incident from the top of the TEM micrograph.
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sponding to a stable, extended defect network (as in zirconia). The critical dose for amorphization of rutile at 300 K and using 360 keV Xe ions corresponds to a damage level of about 10 dpa. Damage accumulation at ambient temperature (300 K) proceeds in accordance with a nucleation and growth model for phase transformations developed by Johnson and Mehl [14] and Avrami [15] (heretofore referred to as the J-M-A model). This is demonstrated in Fig. 2 by the good agreement between the RBS/C data for Xe-ion irradiated rutile and a ®t to the J-M-A formula, v n 1 ÿ exp ÿ K/ , for ®tting parameters K 6:32 10ÿ20 and n 1.33. The incubation period (stage 1) prior to signi®cant damage accumulation is likely to be strongly aected by recovery mechanisms. Recovery is signi®cant in rutile even at ambient temperature. The experimentally measured retained damage, as expressed by the number of interstitial atoms produced per incident ion (Fig. 3), is well below a SRIM-simulated damage level. The maximum retained damage according to RBS/C is only 15% of that predicted by SRIM. Polygonization of the surface layer explains the surface subpeak observed in previous RBS/C investigations of rutile [1,2,8]. Polycrystallization has also been observed in UO2 [16] and in metals [17] exposed to ion irradiation. However, the narrow, polygonized surface layer observed in irradiated rutile crystals is unique. A mechanism for this surface recrystallization phenomenon is not apparent at this time. Factors which probably contribute to this eect include surface strains, defect pinning at the surface, long-range atomic migration, and compositional changes due to preferential surface sputtering. 5. Conclusions Microstructural damage evolution in rutile crystals irradiated with 360 keV Xe2 ions at 300 K, was investigated using RBS/C and TEM. It was determined that: 1. Damage accumulation proceeds in three stages, in accordance with a nucleation and growth model for phase transformations.
2. Damage recovery in rutile under ambient temperature irradiation conditions is signi®cant. Retained damage is estimated to be less than 15% of the calculated instantaneous displacement damage level. 3. The critical dose for amorphization of rutile under the irradiation conditions described above was determined to be 10 dpa at a depth corresponding to the peak damage. 4. For ion doses below the critical dose for amorphization, the damage microstructure consists of three layers: a thin surface layer with thickness 12 nm; a second layer with a low density of relatively large defects; and a third layer with a high density of small defects. Upon irradiation beyond the critical amorphization dose, an amorphous layer appears between two highly defected layers. The thickness of this amorphous layer increases with increasing ion ¯uence. 5. The uppermost layer of damage in the irradiated rutile crystals was found to consist of a polygonized microstructure. This explains the RBS/C surface subpeaks observed in this investigation and in previous studies. Acknowledgements The authors wish to acknowledge the support provided by the Department of Energy, Oce of Basic Energy Sciences, Division of Materials Sciences. References [1] I. Khubeis, R. Fromknecht, O. Meyer, Phys. Rev. B 55 (1997) 1. [2] I. Khubeis, O. Meyer, Nucl. Instr. and Meth. B 120 (1996) 257. [3] O. Meyer, I. Khubeis, R. Fromknecht, S. Massing, Nucl. Instr. and Meth. B 136/138 (1998) 436. [4] O. Meyer, I. Khubeis, R. Fromknecht, S. Massing, Nucl. Instr. and Meth. B 127/128 (1997) 624. [5] R. Fromknecht, R. Auer, I. Khubeis, O. Meyer, Nucl. Instr. and Meth. B 120 (1996) 252. [6] R. Fromknecht, O. Meyer, Mater. Chem. Phys. 45 (1996) 50. [7] T. Hartmann, L.M. Wang, W.J. Weber, N. Yu, K.E. Sickafus, J.N. Mitchell, C.J. Wetteland, M.A. Nastasi, M.G. Hollander, N.P. Baker, C.R. Evans, J.R. Tesmer, C.J. Maggiore, Nucl. Instr. and Meth. B 141 (1998) 398.
F. Li et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 314±321 [8] F. Li, P. Lu, K.E. Sickafus, C.R. Evans, M. Nastasi, Mat. Res. Soc. Symp. Proc. 540 (1999) 311. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [10] K. Yasuda, M. Nastasi, K.E. Sickafus, C.J. Maggiore, N. Yu, Nucl. Instr. and Meth. B 136±138 (1998) 499. [11] O.W. Holland, D. Fathy, J. Narayan, O.S. Oen, Radiat. E. 90 (1985) 127. [12] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978, Chapter 8.
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