- Email: [email protected]
Rutherford backscattering studies of strain-relaxed SiGe films grown on Si substrate with compositionally graded buffer layers
Journal of Crystal Growth 378 (2013) 205–207
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Rutherford backscattering studies of strain-relaxed SiGe films grown on Si substrate with compositionally graded buffer layers Yoshinori Watanabe a, Ryuji Oshima b,n, Isao Sakata b, Koji Matsubara b, Isao Sakamoto a a
Graduate School of Engineering, University of Hosei, 3-7-2 Kajiya, Koganei, Tokyo 184-8584, Japan Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b
a r t i c l e i n f o
abstract
Available online 8 February 2013
We investigated the structural properties of 2-mm thick Si0.58Ge0.42 thin films grown on a combined set of Si1 xGex stepwise buffer layers and a Si0.51Ge0.49 strain-inverted layer on Si substrates. Raman spectroscopy and Rutherford backscattering measurements showed smaller residual strain and superior crystalline lattice ordering compared to a sample without any buffer layer. Furthermore, a Si0.58Ge0.42 thin film with a low dislocation density of less than 105 cm 2 and a smooth surface roughness of 0.903 nm can be achieved by using a combined set of Si1 xGex stepwise buffer layers and a Si0.51Ge0.49 strain-inverted layer, because most dislocations can be confined within each Si1 xGex buffer layer. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A3. Molecular beam epitaxy B1. Germanium–silicon alloy B2. Semiconducting silicon compounds
1. Introduction Epitaxially grown Si1 xGex alloys on Si substrates have been of considerable interest because of their great potential in the development of bandgap-engineered devices such as heterojunction bipolar transistors (HBT), modulation-doped field effect transistors (MODFET) [1], and solar cells [2]. It is obvious that these fascinating applications largely depend on how the material is grown, particularly with regard to the control of both surface morphology and formation of dislocations [3,4]. Therefore, a smooth surface morphology with a minimized cross-hatch pattern amplitude is required to achieve better performance in SiGe heterojunction devices. However, the epitaxial growth of SiGe thin films on Si substrates without the generation of dislocations is especially difficult because a large lattice mismatch of 4.2% exists between Ge and Si. In general, misfit dislocations are generated at the heterointerface because of the relief of an accumulated compressive strain, if the deposition thickness exceeds their critical thickness [5]. In order to improve the properties of SiGe thin films, various types of buffer layers have been investigated, such as layers grown at low temperature [6], thin Si strained layers [7,8], ionimplanted layers [9], and compositionally graded buffer layers [10,11]. Further, there have been few reports on the characterization of SiGe films on Si substrates by Rutherford backscattering (RBS) spectroscopy. Therefore, the purpose of this study is to clarify the effect of both stepwise compositionally graded Si1 xGex buffer layers and a Si0.51Ge0.49 strain-inverted layer on the structural properties of Si0.58Ge0.42 thin films by using RBS spectroscopy, aiming to achieve
high quality strain-relaxed Si1 xGex thin films with lower threading dislocations.
2. Experiments All samples were grown using molecular beam epitaxy (MBE) with two electron guns for Si and Ge solid sources. The equilibrium back pressure in the growth chamber is on the order of 10 10 Torr. First, the p-type Si (001) substrates were cleaned by a standard RCA procedure. Thermal cleaning was performed at 850 1C for 10 min in a vacuum chamber, followed by the deposition of a 100-nm thick Si buffer layer in order to obtain an atomically flat surface without any contamination. Subsequently, Si1 xGex stepwise buffer layers were deposited at 550 1C. Ge content was increased by 7% per step up to 42%, and the thickness of each layer was 400 nm. Rapid thermal annealing (RTA) was performed at 900 1C for 3 min after the deposition of each layer. Then, a 400-nm thick Si0.51Ge0.49 strain-inverted layer was grown, followed by the growth of a 2-mm thick Si0.58Ge0.42 thin film as shown in Fig.1 (denoted by S1). For comparison, Si0.58Ge0.42 thin films without a strain-inverted layer (S2) or without any buffer layer (S3) were fabricated. The structural properties were studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The strained states of epitaxial thin films were evaluated using Raman spectroscopy. RBS was conducted to study the degree of lattice disorder.
3. Results and discussion n
Corresponding author. Tel.: þ81 29 861 3621. E-mail address: [email protected] (R. Oshima).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.12.059
Fig. 2 shows Raman spectra measured for (a) S1, (b) S2, and (c) S3. The three strong first-order lines shown in Fig. 2 for all
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Y. Watanabe et al. / Journal of Crystal Growth 378 (2013) 205–207
Fig. 1. Schematic structure of Si0.58Ge0.42 thin film grown with Si1 xGex buffer layers on Si substrate.
Fig. 2. Raman spectra measured for (a) S1, (b) S2, and (c) S3.
Fig. 3. RBS spectra measured for (a) S1, (b) S2, and (c) S3, with channeling along the [001] axis; (d) spectrum measured for S1 with the incident beam aligned along a random axis.
Si0.58Ge0.42 samples were due to the atomic vibrations of Si–Si bonds at 502 cm 1, Si–Ge bonds at 405 cm 1, and Ge–Ge bonds at 290 cm 1 in the SiGe alloy. Here the value of residual strain (eSiGe) in each Si0.58Ge0.42 thin film was determined by reference to previously reported theoretical concepts [12]. As a result, the largest eSiGe of 0.0060 was calculated for (c), whereas we obtained a smaller eSiGe of 0.0004 for (a) and 0.0002 for (b). It was evident that the residual lattice strain in the Si0.58Ge0.42 active layer was effectively reduced by the insertion of stepwise buffer layers. Moreover, almost similar values of eSiGe for (a) and (b) suggested that a Si0.51Ge0.49 strain-inverted layer does not have a strong effect on the residual strain in Si0.58Ge0.42 thin films.
Fig. 3 shows RBS spectra measured for (a) S1, (b) S2, and (c) S3 with channeling along the [001] axis, whereas (d) shows a spectrum measured for S1 with the incident beam aligned along a random axis. We determined the minimum yield (wmin) in order to evaluate the degree of crystalline lattice disorder [13,14]. wmin was calculated from the integrated counts ratio of a random spectrum to an aligned [001] spectrum in a given energy region. We utilized the energy region from 320 channels to 380 channels, which was contributed by the Si0.58Ge0.42 thin films. The lowest wmin of 2.1% was obtained for (a), compared to 5.6% for (b) and 13.8% for (c). Consequently, the degree of crystalline lattice ordering can be improved by the insertion of stepwise compositionally graded Si1 xGex buffer layers as well as a Si0.51Ge0.49 strain-inverted layer. Fig. 4 shows the AFM surface images measured for (a) S1, (b) S2, and (c) S3. We observed a larger root mean square (RMS) roughness of 6.507 nm and a high density of pit structures associated with threading dislocations propagating to the surface for (c), whereas a smooth surface morphology with fewer pit structures was obtained for (a) and (b) with an RMS roughness of 0.903 nm for (a) and 1.219 nm for (b). Furthermore, it is noteworthy that the cross-hatch patterns associated with the locally concentrated dislocations in Si1 xGex stepwise buffer layers were not observed. In addition, cross-sectional TEM was conducted to examine the condition of dislocations in S1, as shown in Fig. 5. Though dislocations were generated at each interface between Si1 xGex buffer layers because of the relief of compressive strain, threading dislocations were not observed in the Si0.58Ge0.42 thin films. Further, it was obvious that most dislocations continued for a short distance and then terminated at the interface of each Si1 xGex buffer layer. This suggests that a lower density of misfit dislocations is preferable to control the motion of dislocations; i.e., stepwise compositionally graded Si1 xGex buffer layers are very advantageous for the control of dislocations. Furthermore, the RTA temperature should be sufficiently high to remove defects from the layer, ensure the propagation of misfit dislocations, and increase the probability of threading dislocation annihilation due to dislocation–dislocation interaction events. Finally, we summarize some crystal structural parameters related to each sample, as shown in Table 1. Dislocation density was determined from both AFM and cross-sectional TEM measurements. A high dislocation density of 1 109 cm 2 was observed for (c) that was reduced by two orders of magnitude to 1.2 107 cm 2 for (b). Moreover, the smallest dislocation density observed in the study, which was less than 105 cm 2, was achieved for (a). These results provide evidence that the material quality of Si0.58Ge0.42 thin films was improved by a Si0.51Ge0.49 strain-inverted layer as well as by Si1 xGex stepwise buffer layers. A Si0.51Ge0.49 strain-inverted layer, which is compressively strained, generates a tensile strain in Si0.58Ge0.42 thin films. The ensuing opposite polarity of strain condition at the interface between Si0.58Ge0.42 thin films and a Si0.51Ge0.49 straininverted layer results in further suppression of the propagation of dislocations.
4. Conclusion We fabricated strain-relaxed Si0.58Ge0.42 thin films on Si (001) by MBE. In order to improve the material quality of Si0.58Ge0.42 films, we employed a combined set of Si1 xGex stepwise buffer layers and a Si0.51Ge0.49 strain-inverted layer, as well as rapid thermal annealing after the deposition of each buffer layer. As a result, we observed minimized residual strain and superior crystalline lattice ordering. Furthermore, a low dislocation density
Y. Watanabe et al. / Journal of Crystal Growth 378 (2013) 205–207
207
Fig. 4. AFM images measured for (a) S1, (b) S2, and (c) S3. Scan size is 5 mm 5 mm.
of less than 105 cm 2 and surface roughness of 0.903 nm were achieved.
Acknowledgments This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI). References [1] [2] [3] [4] [5]
Fig. 5. Cross-sectional TEM images measured for S1.
[6] [7] [8]
Table 1 Crystal structural parameters related to each Si0.58Ge0.42 sample for S1, S2, and S3. Graded Strain buffer layers inverted layers S1 Yes S2 Yes S3 No
Yes No No
Residual strain eSiGe
wmin RMS (%)
(nm)
Dislocation density (cm 2)
0.0004 0.0002 0.006
2.1 5.6 13.8
0.903 1.219 6.507
o 105 1.2 107 1.0 109
[9] [10] [11] [12] [13] [14]
Y. Shiraki, A. Sakaki, Surface Science Reports 59 (2005) 153. S.A. Hadi, P. Hahemi, A. Nayfeh, J. Hoyt, ECS Transcations 41 (2011) 3. P.M. Mooney, Material Science and Engineering: R: Reports 17 (1996) 105. E.A. Fitzgerald, M.T. Currie, S.B. Samavedam, T.A. Langdo, G. Taraschi, V. Yang, C.W. Leitz, M.T. Bulsara, Physica Status Solidi A 171 (1999) 227. R. Hull, J.C. Bean, D.J. Eaglesham, J.M. Bonar, C. Buescher, Thin Solid Films 183 (1989) 117. E.A. Fitzgerald, S.B. Samavedam, Thin Solid Films 294 (1997) 3. N. Usami, R. Nihei, Y. Azuma, I. Yonenaga, K. Nakajima, K. Sawano, Y. Shiraki, Thin Solid Films 517 (2008) 14. T.-S. Yoon, J. Liu, A.M. Noori, M.S. Goorsky, Y.-H. Xie, Applied Physics Letter 87 (2005) 012104. ¨ ¨ C. Rosenblad, J. Stangl, E. Muller, G. Bauer, H. von Kanel, Materials Science and Engineering B 71 (2000) 20. P.I. Gaiduk, A.N. Larsen, J.L. Hansen, Thin Solid Films 367 (2000) 120. Z. Jiang, W. Wang, H. Gao, L. Liu, H. Chen, J. Zhou, Applied Surface Science 254 (2008) 5241. T.S. Perova, J. Wasyluk, K. Lyutovich, E. Kasper, M. Oehme, K. Rode, A. Waldron, Journal of Applied Physics 109 (2011) 033502. C. Chen, B. Yu, J. Liu, J. Cao, D. Zhu, Z. Liu, Nuclear Instruments and Methods in Physics Research B 239 (2005) 433. ¨ ¨ B. Hollander, L. Vescan, S. Mesters, S. Wickenhauser, Thin Solid Films 292 (1997) 213.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Rutherford backscattering studies of strain-relaxed SiGe films grown on Si substrate with compositionally graded buffer layers Yoshinori Watanabe a, Ryuji Oshima b,n, Isao Sakata b, Koji Matsubara b, Isao Sakamoto a a
Graduate School of Engineering, University of Hosei, 3-7-2 Kajiya, Koganei, Tokyo 184-8584, Japan Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b
a r t i c l e i n f o
abstract
Available online 8 February 2013
We investigated the structural properties of 2-mm thick Si0.58Ge0.42 thin films grown on a combined set of Si1 xGex stepwise buffer layers and a Si0.51Ge0.49 strain-inverted layer on Si substrates. Raman spectroscopy and Rutherford backscattering measurements showed smaller residual strain and superior crystalline lattice ordering compared to a sample without any buffer layer. Furthermore, a Si0.58Ge0.42 thin film with a low dislocation density of less than 105 cm 2 and a smooth surface roughness of 0.903 nm can be achieved by using a combined set of Si1 xGex stepwise buffer layers and a Si0.51Ge0.49 strain-inverted layer, because most dislocations can be confined within each Si1 xGex buffer layer. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Crystal structure A3. Molecular beam epitaxy B1. Germanium–silicon alloy B2. Semiconducting silicon compounds
1. Introduction Epitaxially grown Si1 xGex alloys on Si substrates have been of considerable interest because of their great potential in the development of bandgap-engineered devices such as heterojunction bipolar transistors (HBT), modulation-doped field effect transistors (MODFET) [1], and solar cells [2]. It is obvious that these fascinating applications largely depend on how the material is grown, particularly with regard to the control of both surface morphology and formation of dislocations [3,4]. Therefore, a smooth surface morphology with a minimized cross-hatch pattern amplitude is required to achieve better performance in SiGe heterojunction devices. However, the epitaxial growth of SiGe thin films on Si substrates without the generation of dislocations is especially difficult because a large lattice mismatch of 4.2% exists between Ge and Si. In general, misfit dislocations are generated at the heterointerface because of the relief of an accumulated compressive strain, if the deposition thickness exceeds their critical thickness [5]. In order to improve the properties of SiGe thin films, various types of buffer layers have been investigated, such as layers grown at low temperature [6], thin Si strained layers [7,8], ionimplanted layers [9], and compositionally graded buffer layers [10,11]. Further, there have been few reports on the characterization of SiGe films on Si substrates by Rutherford backscattering (RBS) spectroscopy. Therefore, the purpose of this study is to clarify the effect of both stepwise compositionally graded Si1 xGex buffer layers and a Si0.51Ge0.49 strain-inverted layer on the structural properties of Si0.58Ge0.42 thin films by using RBS spectroscopy, aiming to achieve
high quality strain-relaxed Si1 xGex thin films with lower threading dislocations.
2. Experiments All samples were grown using molecular beam epitaxy (MBE) with two electron guns for Si and Ge solid sources. The equilibrium back pressure in the growth chamber is on the order of 10 10 Torr. First, the p-type Si (001) substrates were cleaned by a standard RCA procedure. Thermal cleaning was performed at 850 1C for 10 min in a vacuum chamber, followed by the deposition of a 100-nm thick Si buffer layer in order to obtain an atomically flat surface without any contamination. Subsequently, Si1 xGex stepwise buffer layers were deposited at 550 1C. Ge content was increased by 7% per step up to 42%, and the thickness of each layer was 400 nm. Rapid thermal annealing (RTA) was performed at 900 1C for 3 min after the deposition of each layer. Then, a 400-nm thick Si0.51Ge0.49 strain-inverted layer was grown, followed by the growth of a 2-mm thick Si0.58Ge0.42 thin film as shown in Fig.1 (denoted by S1). For comparison, Si0.58Ge0.42 thin films without a strain-inverted layer (S2) or without any buffer layer (S3) were fabricated. The structural properties were studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The strained states of epitaxial thin films were evaluated using Raman spectroscopy. RBS was conducted to study the degree of lattice disorder.
3. Results and discussion n
Corresponding author. Tel.: þ81 29 861 3621. E-mail address: [email protected] (R. Oshima).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.12.059
Fig. 2 shows Raman spectra measured for (a) S1, (b) S2, and (c) S3. The three strong first-order lines shown in Fig. 2 for all
206
Y. Watanabe et al. / Journal of Crystal Growth 378 (2013) 205–207
Fig. 1. Schematic structure of Si0.58Ge0.42 thin film grown with Si1 xGex buffer layers on Si substrate.
Fig. 2. Raman spectra measured for (a) S1, (b) S2, and (c) S3.
Fig. 3. RBS spectra measured for (a) S1, (b) S2, and (c) S3, with channeling along the [001] axis; (d) spectrum measured for S1 with the incident beam aligned along a random axis.
Si0.58Ge0.42 samples were due to the atomic vibrations of Si–Si bonds at 502 cm 1, Si–Ge bonds at 405 cm 1, and Ge–Ge bonds at 290 cm 1 in the SiGe alloy. Here the value of residual strain (eSiGe) in each Si0.58Ge0.42 thin film was determined by reference to previously reported theoretical concepts [12]. As a result, the largest eSiGe of 0.0060 was calculated for (c), whereas we obtained a smaller eSiGe of 0.0004 for (a) and 0.0002 for (b). It was evident that the residual lattice strain in the Si0.58Ge0.42 active layer was effectively reduced by the insertion of stepwise buffer layers. Moreover, almost similar values of eSiGe for (a) and (b) suggested that a Si0.51Ge0.49 strain-inverted layer does not have a strong effect on the residual strain in Si0.58Ge0.42 thin films.
Fig. 3 shows RBS spectra measured for (a) S1, (b) S2, and (c) S3 with channeling along the [001] axis, whereas (d) shows a spectrum measured for S1 with the incident beam aligned along a random axis. We determined the minimum yield (wmin) in order to evaluate the degree of crystalline lattice disorder [13,14]. wmin was calculated from the integrated counts ratio of a random spectrum to an aligned [001] spectrum in a given energy region. We utilized the energy region from 320 channels to 380 channels, which was contributed by the Si0.58Ge0.42 thin films. The lowest wmin of 2.1% was obtained for (a), compared to 5.6% for (b) and 13.8% for (c). Consequently, the degree of crystalline lattice ordering can be improved by the insertion of stepwise compositionally graded Si1 xGex buffer layers as well as a Si0.51Ge0.49 strain-inverted layer. Fig. 4 shows the AFM surface images measured for (a) S1, (b) S2, and (c) S3. We observed a larger root mean square (RMS) roughness of 6.507 nm and a high density of pit structures associated with threading dislocations propagating to the surface for (c), whereas a smooth surface morphology with fewer pit structures was obtained for (a) and (b) with an RMS roughness of 0.903 nm for (a) and 1.219 nm for (b). Furthermore, it is noteworthy that the cross-hatch patterns associated with the locally concentrated dislocations in Si1 xGex stepwise buffer layers were not observed. In addition, cross-sectional TEM was conducted to examine the condition of dislocations in S1, as shown in Fig. 5. Though dislocations were generated at each interface between Si1 xGex buffer layers because of the relief of compressive strain, threading dislocations were not observed in the Si0.58Ge0.42 thin films. Further, it was obvious that most dislocations continued for a short distance and then terminated at the interface of each Si1 xGex buffer layer. This suggests that a lower density of misfit dislocations is preferable to control the motion of dislocations; i.e., stepwise compositionally graded Si1 xGex buffer layers are very advantageous for the control of dislocations. Furthermore, the RTA temperature should be sufficiently high to remove defects from the layer, ensure the propagation of misfit dislocations, and increase the probability of threading dislocation annihilation due to dislocation–dislocation interaction events. Finally, we summarize some crystal structural parameters related to each sample, as shown in Table 1. Dislocation density was determined from both AFM and cross-sectional TEM measurements. A high dislocation density of 1 109 cm 2 was observed for (c) that was reduced by two orders of magnitude to 1.2 107 cm 2 for (b). Moreover, the smallest dislocation density observed in the study, which was less than 105 cm 2, was achieved for (a). These results provide evidence that the material quality of Si0.58Ge0.42 thin films was improved by a Si0.51Ge0.49 strain-inverted layer as well as by Si1 xGex stepwise buffer layers. A Si0.51Ge0.49 strain-inverted layer, which is compressively strained, generates a tensile strain in Si0.58Ge0.42 thin films. The ensuing opposite polarity of strain condition at the interface between Si0.58Ge0.42 thin films and a Si0.51Ge0.49 straininverted layer results in further suppression of the propagation of dislocations.
4. Conclusion We fabricated strain-relaxed Si0.58Ge0.42 thin films on Si (001) by MBE. In order to improve the material quality of Si0.58Ge0.42 films, we employed a combined set of Si1 xGex stepwise buffer layers and a Si0.51Ge0.49 strain-inverted layer, as well as rapid thermal annealing after the deposition of each buffer layer. As a result, we observed minimized residual strain and superior crystalline lattice ordering. Furthermore, a low dislocation density
Y. Watanabe et al. / Journal of Crystal Growth 378 (2013) 205–207
207
Fig. 4. AFM images measured for (a) S1, (b) S2, and (c) S3. Scan size is 5 mm 5 mm.
of less than 105 cm 2 and surface roughness of 0.903 nm were achieved.
Acknowledgments This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI). References [1] [2] [3] [4] [5]
Fig. 5. Cross-sectional TEM images measured for S1.
[6] [7] [8]
Table 1 Crystal structural parameters related to each Si0.58Ge0.42 sample for S1, S2, and S3. Graded Strain buffer layers inverted layers S1 Yes S2 Yes S3 No
Yes No No
Residual strain eSiGe
wmin RMS (%)
(nm)
Dislocation density (cm 2)
0.0004 0.0002 0.006
2.1 5.6 13.8
0.903 1.219 6.507
o 105 1.2 107 1.0 109
[9] [10] [11] [12] [13] [14]
Y. Shiraki, A. Sakaki, Surface Science Reports 59 (2005) 153. S.A. Hadi, P. Hahemi, A. Nayfeh, J. Hoyt, ECS Transcations 41 (2011) 3. P.M. Mooney, Material Science and Engineering: R: Reports 17 (1996) 105. E.A. Fitzgerald, M.T. Currie, S.B. Samavedam, T.A. Langdo, G. Taraschi, V. Yang, C.W. Leitz, M.T. Bulsara, Physica Status Solidi A 171 (1999) 227. R. Hull, J.C. Bean, D.J. Eaglesham, J.M. Bonar, C. Buescher, Thin Solid Films 183 (1989) 117. E.A. Fitzgerald, S.B. Samavedam, Thin Solid Films 294 (1997) 3. N. Usami, R. Nihei, Y. Azuma, I. Yonenaga, K. Nakajima, K. Sawano, Y. Shiraki, Thin Solid Films 517 (2008) 14. T.-S. Yoon, J. Liu, A.M. Noori, M.S. Goorsky, Y.-H. Xie, Applied Physics Letter 87 (2005) 012104. ¨ ¨ C. Rosenblad, J. Stangl, E. Muller, G. Bauer, H. von Kanel, Materials Science and Engineering B 71 (2000) 20. P.I. Gaiduk, A.N. Larsen, J.L. Hansen, Thin Solid Films 367 (2000) 120. Z. Jiang, W. Wang, H. Gao, L. Liu, H. Chen, J. Zhou, Applied Surface Science 254 (2008) 5241. T.S. Perova, J. Wasyluk, K. Lyutovich, E. Kasper, M. Oehme, K. Rode, A. Waldron, Journal of Applied Physics 109 (2011) 033502. C. Chen, B. Yu, J. Liu, J. Cao, D. Zhu, Z. Liu, Nuclear Instruments and Methods in Physics Research B 239 (2005) 433. ¨ ¨ B. Hollander, L. Vescan, S. Mesters, S. Wickenhauser, Thin Solid Films 292 (1997) 213.