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Optical properties in the Cu-fused silica system irradiated with swift heavy ions
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 245 (2006) 219–221 www.elsevier.com/locate/nimb
Optical properties in the Cu-fused silica system irradiated with swift heavy ions Ranjana C. Gupta a, D.C. Kothari a,*, R.J. Choudhari b, Ravi Kumar b, P.K. Sahoo c, K.P. Lieb c, S. Klaumu¨nzer d a
c
Department of Physics, University of Mumbai, Vidyanagari, Santacruz East, Mumbai 400 098, India b Nuclear Science Centre, P.O. Box 10502, Aruna Asaf Ali Marg, New Delhi 110 067, India II. Physikalisches Institut, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany d Hahn-Meitner Institut, Glienicker Str. 100, D-14109 Berlin, Germany Available online 5 January 2006
Abstract Swift heavy ions are used to study the effects of electronic energy loss on Cu cluster formation in fused silica after post-irradiation annealing. Fused silica substrates covered with 10 nm thin Cu-films were irradiated using beams of either 120 MeV Ag9+ ions or 350 MeV Au26+ ions at fluences ranging from 2 · 1013 to 1 · 1014 cm2. After irradiation, the samples were annealed for 30 min in argon, at temperatures of 773–1200 K and characterized by UV–VIS absorption spectroscopy. The swift ion irradiations created E 0 and B2 defects in silica, which were partially eliminated during annealing. In addition, Cu cluster formation in silica was observed after annealing. Irradiation fluences exceeding 4 · 1013 cm2 and annealing temperatures above 1100 K are more effective in forming larger nanoclusters. 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 61.72.y; 61.80; 78.20.e Keywords: Swift ion irradiation of fused silica; Defects; Nanoparticles; Optical absorption
1. Introduction The energy deposited by swift heavy ions in solids is mainly due to inelastic collisions and is characterized by the electronic stopping power, Se. Recent experiments have shown that Se can play an important role in producing nanoclusters embedded in insulators, in a controllable manner [1–4]. In the past, low-energy ion implantation along with post-implantation annealing was used to fabricate nanocomposite glasses [5–7]. Recently, it has been shown that defects created by the nuclear energy loss component, Sn, can be used as nucleation sites for the formation of nanoparticles and the nanoparticles grow without thermal annealing during low-energy ion irradiation [8]. *
Corresponding author. Tel.: +91 22 2652 6250; fax: +91 22 2652 9780. E-mail address: [email protected] (D.C. Kothari).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.104
Valentin et al. [1] demonstrated that the nucleation of nanoclusters can take place if Se exceeds a certain threshold value and the clusters grow via Ostwald ripening during post-irradiation annealing. The cluster density has been shown to correlate with Se and nanoclusters with quite narrow size distribution have been produced [1]. For optical switching and photonic band-gap applications one should be able to produce, in a controlled way, metallic nanoparticles embedded in glasses having a narrow size distribution and the required particle density. In the present work, we study the role of Se in producing defects and nanoclusters. It is believed that, Se induced defects act as nucleation sites for nanoclusters. The role of Se induced defects in nanocluster formation and their annealing behaviour have not been studied in the past [1–4]. Substrates of fused silica covered with 10 nm Cufilms were irradiated using beams of 120 MeV Ag9+ ions
R.C. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 219–221
2. Experiments High-purity silica glass plates, 10 mm · 10 mm · 1.5 mm in size, were chosen as substrates. Copper films of 10 nm thickness were deposited on the substrates using vacuum evaporation at a pressure of 3 · 105 mbar. The coated samples were then irradiated using either a 120 MeV Ag9+ beam, provided by the tandem accelerator of the Nuclear Science Center, Delhi, or the 350 MeV Au26+ beam of the ECR/RFQ/cyclotron facility of the Hahn-Meitner-Institut, Berlin. An electronic energy loss of Se = 11 keV/nm and 21 keV/nm in SiO2 (at the Cu/SiO2 interface) for the two ion species was estimated with the SRIM code. These Se values clearly exceeded the 1 keV/nm threshold value for track formation in fused silica; tracks of 3 nm radius were formed for Se = 8 keV/nm [11]. Thus both ion energies were sufficient to produce tracks of radii greater than 3 nm in fused silica. Please note that the Sn values for 120 MeV Ag9+ ions and 350 MeV Au26+ ions in SiO2 near the Cu–SiO2 interface are 52 and 92 eV/nm, respectively, which are two orders of magnitude lower than the corresponding Se values. The Ag9+ ion fluences ranged from 4 · 1013 to 1 · 1014 cm2, while the Au26+ ion irradiation was carried out for a fluence of 2 · 1013 cm2. After irradiation, 30 min annealings in an Ar atmosphere were performed at temperatures ranging from 500 to 1200 K. Rutherford backscattering spectroscopy (RBS) with 900 keV a-particles was used to check the average Cudepth profiles after Cu-film deposition, ion irradiation and annealing and gave access to information concerning the ion-beam-induced interface mixing. Optical absorption spectra were taken at room temperature in the wavelength range from 190 to 800 nm before and after irradiation and after each annealing step, using a UV2401PC Shimadzu spectrophotometer. 3. Results and discussion The RBS spectra (not shown here) taken before and after irradiation revealed no measurable interface mixing at the small ion fluences chosen. In Fig. 1, UV–Vis spectra taken before and after 120 MeV Ag9+ irradiation are shown as a function of the ion fluence. The spectra of the irradiated samples show two peaks at 215 and 245 nm arising from the E 0 and B2 defect centres created in the SiO2 substrates [12]. These types of defects were also observed after the 350 MeV Au26+ irradiations, as can be seen from curve 1 in Fig. 4. The E 0 centre is due to an oxygen vacancy opposite an electron in a dangling Si-sp3 orbital, while the B2 centre is due to the Si–vacancy–Si configuration. Figs. 2
2.5
2.0
Absorbance
or 350 MeV Au26+ ions at various fluences. The dual purpose was to form tracks in silica, which could act as nucleation sites and to introduce Cu into silica by ion beam mixing induced by electronic stopping [9,10]. UV–VIS absorption spectroscopy was used mainly to characterize the samples.
1.5
1.0
Non-irradiated 4e13 cm-2
3
6e13 cm-2
4
1e14 cm-2
4
1
2
0.0 100
1 2
3
0.5
200
300
400
500
600
700
800
Wavelength (nm) Fig. 1. UV–Vis spectra of non-irradiated and irradiated Cu-fused silica samples, for 120 MeV Ag9+ ion irradiations at the fluences indicated.
Annealed at 1200 K 2.0
3
4
1.5
Absorbance
220
1.0
1
Non-irradiated
2
4e13 cm-2
3
6e13 cm-2
4
1e14 cm-2
2
0.5 1
0.0
100
200
300
400
500
600
700
800
Wavelength (nm) Fig. 2. Same as Fig. 1, but taken after annealing in an Ar atmosphere at 1200 K.
and 3, which illustrate the effects of thermal annealing on the UV–Vis spectra, indicate that after annealing the 215 and 245 nm peaks disappeared. Thus, the defects are annealed out. Fig. 2 also showed a broad peak at 425 nm, which developed at 1100 K or above as well as possibly another broad peak at 580 nm. This latter peak is known to correspond to Cu nanoparticles [8]. Based on the molecular orbital (MO) calculations by Moskovits and Hulse [13], the 425 nm peak may be attributed to small Cu4 clusters. The cluster formation is considered to be due to the diffusion of Cu in the tracks formed by the swift heavy ions in silica. We recall that for both ion species, the electronic energy loss in SiO2 is higher than the threshold required for continuous track formation of radii larger than 3 nm [11]. At all the
R.C. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 219–221
cles (as inferred from the 580 nm band). With increasing temperature, the cluster size is expected to increase, which is confirmed in Fig. 3. However, in Fig. 4 no similar behavior was observed, which is probably due to a low irradiation fluence. In fact, after the 873 K annealing, the 335 nm peak shifts towards 285 nm, possibly corresponding to Cu2 clusters. After annealing at 1073 K, no peaks were observed and the absorbance value increased indicating dispersed Cu atoms in the matrix.
3.0 -2
6e13 cm
2.5
Absorbance
2.0
6
1
Non-irradiated
2
Irradiated
3
500 K
4
700 K
5
900 K
6
1100 K
1.5 2
1.0
4. Conclusions 3
0.5
4 1
0.0 100
200
5
300
400
500
600
700
800
Wavelength (nm) Fig. 3. UV–Vis spectra of Cu-fused silica after 120 MeV Ag9+ irradiation at a fluence of 6 · 1013 cm2 and annealing in Ar at the temperatures given.
fluences considered, the tracks overlap. With increasing fluence, the defect density also increases and therefore more nucleation sites are available for cluster formation. However, for a fluence of more than 6 · 1013 cm2, there could be a saturation effect, which may lead to the slight decrease in the intensity of the 425 nm peak for the sample irradiated with 1 · 1014 Ag ions/cm2. The annealing behavior at a fixed Ag ion fluence of 6 · 1013 cm2 is illustrated in Fig. 3. Annealing at 500, 700 and 900 K produces a band centered around 335 nm, which, according to MO calculations [13], may be attributed to Cu3 clusters. At 1100 K, the Cu3 clusters get converted to Cu4 clusters (as inferred from the rising intensity of the 425 nm peak) or even to larger nanoparti2.0
1.5
Absorbance
221
1
irridiated
2
773 K
3
873 K
4
1073 K
1.0
4
1
0.5
2 3
0.0 100
200
300
400
500
600
700
800
900
1000
Wavelength (nm) Fig. 4. UV–Vis spectra of Cu-fused silica after 350 MeV Au26+ irradiation at a fluence of 2 · 1013 cm2 and annealing in Ar at the temperatures shown.
Swift heavy ion irradiations create E 0 and B2 defects in Cu-coated, fused silica. Post-irradiation annealing helps to eliminate these defects and form Cu clusters. At the highest annealing temperature of 1200 K, Cu4 clusters and larger-size particles are formed. Irradiation fluences of more than 4 · 1013 cm2 and annealing temperatures exceeding 1100 K appear to be more effective in forming larger nanoclusters. Acknowledgements This work was supported by Deutsche Forschungsgemeinschaft (DFG) and the Nuclear Science Centre, New Delhi. The authors are indebted to Dr. K. Zhang and V. Milinovic (Go¨ttingen) for assistance during the Au-ion irradiations and RBS analyses and to S.S. Patil, R.P. Fernandes and Trupti N. Warang (Mumbai) for assistance during the experiments in India. References [1] E. Valentin, H. Bernas, C. Ricolleau, F. Creuzet, Phys. Rev. Lett. 86 (2001) 99. [2] T. Mohanty, A. Pradhan, S. Gupta, D. Kanjilal, Nanotechnology 15 (2004) 1620. [3] A. Barbu, P. Pareige, V. Jacquet, Nucl. Instr. and Meth. B 146 (1998) 278. [4] A. Iwase, T. Hasegawa, Y. Chimi, T. Tobita, N. Ishikawa, M. Suzuki, T. Kambara, S. Ishino, Nucl. Instr. and Meth. B 195 (2002) 309. [5] G. Battaglin, P. Calveli, E. Cattaruza, F. Gonella, R. Polloni, Appl. Phys. Lett. 78 (2001) 3953. [6] R.H. Magruder III, R.F. Haglund Jr., L. Yang, J.E. Witting, R.A. Zuhr, J. Appl. Phys. 76 (1994) 708. [7] D. Ila, E.K. Williams, S. Surkisov, C.C. Smith, D.B. Poker, D.K Hensley, Nucl. Instr. and Meth. B 141 (1998) 289. [8] M.K. Patel, B.J. Nagare, D.M. Bagul, S.K. Haram, D.C. Kothari, Surf. Coat. Technol. 196 (2005) 96. [9] S.K. Sinha, D.C. Kothari, A.K. Balmuragan, A.K. Tyagi, D. Kanjilal, Surf. Coat. Technol. 158–159 (2002) 609. [10] W. Bolse, B. Schattat, A. Feyh, T. Renz, Nucl. Instr. and Meth. B 218 (2004) 80. [11] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. B 216 (2004) 1. [12] A. Oliver, J.C. Cheang-Wong, A. Crespo, L. Rodriguez-Fernandez, J.M. Hernandez, E. Munoz, R. Espejel-Morales, Mater. Sci. Eng. 78 (2000) 32; A. Oliver, J.C. Cheang-Wong, A. Crespo, L. Rodriguez-Fernandez, J.M. Hernandez, E. Munoz, R. Espejel-Morales, Nucl. Instr. and Meth. B 175 (2001) 495. [13] M. Moskovits, J.E. Hulse, J. Chem. Phys. 67 (1977) 4271.
Nuclear Instruments and Methods in Physics Research B 245 (2006) 219–221 www.elsevier.com/locate/nimb
Optical properties in the Cu-fused silica system irradiated with swift heavy ions Ranjana C. Gupta a, D.C. Kothari a,*, R.J. Choudhari b, Ravi Kumar b, P.K. Sahoo c, K.P. Lieb c, S. Klaumu¨nzer d a
c
Department of Physics, University of Mumbai, Vidyanagari, Santacruz East, Mumbai 400 098, India b Nuclear Science Centre, P.O. Box 10502, Aruna Asaf Ali Marg, New Delhi 110 067, India II. Physikalisches Institut, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany d Hahn-Meitner Institut, Glienicker Str. 100, D-14109 Berlin, Germany Available online 5 January 2006
Abstract Swift heavy ions are used to study the effects of electronic energy loss on Cu cluster formation in fused silica after post-irradiation annealing. Fused silica substrates covered with 10 nm thin Cu-films were irradiated using beams of either 120 MeV Ag9+ ions or 350 MeV Au26+ ions at fluences ranging from 2 · 1013 to 1 · 1014 cm2. After irradiation, the samples were annealed for 30 min in argon, at temperatures of 773–1200 K and characterized by UV–VIS absorption spectroscopy. The swift ion irradiations created E 0 and B2 defects in silica, which were partially eliminated during annealing. In addition, Cu cluster formation in silica was observed after annealing. Irradiation fluences exceeding 4 · 1013 cm2 and annealing temperatures above 1100 K are more effective in forming larger nanoclusters. 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 61.72.y; 61.80; 78.20.e Keywords: Swift ion irradiation of fused silica; Defects; Nanoparticles; Optical absorption
1. Introduction The energy deposited by swift heavy ions in solids is mainly due to inelastic collisions and is characterized by the electronic stopping power, Se. Recent experiments have shown that Se can play an important role in producing nanoclusters embedded in insulators, in a controllable manner [1–4]. In the past, low-energy ion implantation along with post-implantation annealing was used to fabricate nanocomposite glasses [5–7]. Recently, it has been shown that defects created by the nuclear energy loss component, Sn, can be used as nucleation sites for the formation of nanoparticles and the nanoparticles grow without thermal annealing during low-energy ion irradiation [8]. *
Corresponding author. Tel.: +91 22 2652 6250; fax: +91 22 2652 9780. E-mail address: [email protected] (D.C. Kothari).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.104
Valentin et al. [1] demonstrated that the nucleation of nanoclusters can take place if Se exceeds a certain threshold value and the clusters grow via Ostwald ripening during post-irradiation annealing. The cluster density has been shown to correlate with Se and nanoclusters with quite narrow size distribution have been produced [1]. For optical switching and photonic band-gap applications one should be able to produce, in a controlled way, metallic nanoparticles embedded in glasses having a narrow size distribution and the required particle density. In the present work, we study the role of Se in producing defects and nanoclusters. It is believed that, Se induced defects act as nucleation sites for nanoclusters. The role of Se induced defects in nanocluster formation and their annealing behaviour have not been studied in the past [1–4]. Substrates of fused silica covered with 10 nm Cufilms were irradiated using beams of 120 MeV Ag9+ ions
R.C. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 219–221
2. Experiments High-purity silica glass plates, 10 mm · 10 mm · 1.5 mm in size, were chosen as substrates. Copper films of 10 nm thickness were deposited on the substrates using vacuum evaporation at a pressure of 3 · 105 mbar. The coated samples were then irradiated using either a 120 MeV Ag9+ beam, provided by the tandem accelerator of the Nuclear Science Center, Delhi, or the 350 MeV Au26+ beam of the ECR/RFQ/cyclotron facility of the Hahn-Meitner-Institut, Berlin. An electronic energy loss of Se = 11 keV/nm and 21 keV/nm in SiO2 (at the Cu/SiO2 interface) for the two ion species was estimated with the SRIM code. These Se values clearly exceeded the 1 keV/nm threshold value for track formation in fused silica; tracks of 3 nm radius were formed for Se = 8 keV/nm [11]. Thus both ion energies were sufficient to produce tracks of radii greater than 3 nm in fused silica. Please note that the Sn values for 120 MeV Ag9+ ions and 350 MeV Au26+ ions in SiO2 near the Cu–SiO2 interface are 52 and 92 eV/nm, respectively, which are two orders of magnitude lower than the corresponding Se values. The Ag9+ ion fluences ranged from 4 · 1013 to 1 · 1014 cm2, while the Au26+ ion irradiation was carried out for a fluence of 2 · 1013 cm2. After irradiation, 30 min annealings in an Ar atmosphere were performed at temperatures ranging from 500 to 1200 K. Rutherford backscattering spectroscopy (RBS) with 900 keV a-particles was used to check the average Cudepth profiles after Cu-film deposition, ion irradiation and annealing and gave access to information concerning the ion-beam-induced interface mixing. Optical absorption spectra were taken at room temperature in the wavelength range from 190 to 800 nm before and after irradiation and after each annealing step, using a UV2401PC Shimadzu spectrophotometer. 3. Results and discussion The RBS spectra (not shown here) taken before and after irradiation revealed no measurable interface mixing at the small ion fluences chosen. In Fig. 1, UV–Vis spectra taken before and after 120 MeV Ag9+ irradiation are shown as a function of the ion fluence. The spectra of the irradiated samples show two peaks at 215 and 245 nm arising from the E 0 and B2 defect centres created in the SiO2 substrates [12]. These types of defects were also observed after the 350 MeV Au26+ irradiations, as can be seen from curve 1 in Fig. 4. The E 0 centre is due to an oxygen vacancy opposite an electron in a dangling Si-sp3 orbital, while the B2 centre is due to the Si–vacancy–Si configuration. Figs. 2
2.5
2.0
Absorbance
or 350 MeV Au26+ ions at various fluences. The dual purpose was to form tracks in silica, which could act as nucleation sites and to introduce Cu into silica by ion beam mixing induced by electronic stopping [9,10]. UV–VIS absorption spectroscopy was used mainly to characterize the samples.
1.5
1.0
Non-irradiated 4e13 cm-2
3
6e13 cm-2
4
1e14 cm-2
4
1
2
0.0 100
1 2
3
0.5
200
300
400
500
600
700
800
Wavelength (nm) Fig. 1. UV–Vis spectra of non-irradiated and irradiated Cu-fused silica samples, for 120 MeV Ag9+ ion irradiations at the fluences indicated.
Annealed at 1200 K 2.0
3
4
1.5
Absorbance
220
1.0
1
Non-irradiated
2
4e13 cm-2
3
6e13 cm-2
4
1e14 cm-2
2
0.5 1
0.0
100
200
300
400
500
600
700
800
Wavelength (nm) Fig. 2. Same as Fig. 1, but taken after annealing in an Ar atmosphere at 1200 K.
and 3, which illustrate the effects of thermal annealing on the UV–Vis spectra, indicate that after annealing the 215 and 245 nm peaks disappeared. Thus, the defects are annealed out. Fig. 2 also showed a broad peak at 425 nm, which developed at 1100 K or above as well as possibly another broad peak at 580 nm. This latter peak is known to correspond to Cu nanoparticles [8]. Based on the molecular orbital (MO) calculations by Moskovits and Hulse [13], the 425 nm peak may be attributed to small Cu4 clusters. The cluster formation is considered to be due to the diffusion of Cu in the tracks formed by the swift heavy ions in silica. We recall that for both ion species, the electronic energy loss in SiO2 is higher than the threshold required for continuous track formation of radii larger than 3 nm [11]. At all the
R.C. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 219–221
cles (as inferred from the 580 nm band). With increasing temperature, the cluster size is expected to increase, which is confirmed in Fig. 3. However, in Fig. 4 no similar behavior was observed, which is probably due to a low irradiation fluence. In fact, after the 873 K annealing, the 335 nm peak shifts towards 285 nm, possibly corresponding to Cu2 clusters. After annealing at 1073 K, no peaks were observed and the absorbance value increased indicating dispersed Cu atoms in the matrix.
3.0 -2
6e13 cm
2.5
Absorbance
2.0
6
1
Non-irradiated
2
Irradiated
3
500 K
4
700 K
5
900 K
6
1100 K
1.5 2
1.0
4. Conclusions 3
0.5
4 1
0.0 100
200
5
300
400
500
600
700
800
Wavelength (nm) Fig. 3. UV–Vis spectra of Cu-fused silica after 120 MeV Ag9+ irradiation at a fluence of 6 · 1013 cm2 and annealing in Ar at the temperatures given.
fluences considered, the tracks overlap. With increasing fluence, the defect density also increases and therefore more nucleation sites are available for cluster formation. However, for a fluence of more than 6 · 1013 cm2, there could be a saturation effect, which may lead to the slight decrease in the intensity of the 425 nm peak for the sample irradiated with 1 · 1014 Ag ions/cm2. The annealing behavior at a fixed Ag ion fluence of 6 · 1013 cm2 is illustrated in Fig. 3. Annealing at 500, 700 and 900 K produces a band centered around 335 nm, which, according to MO calculations [13], may be attributed to Cu3 clusters. At 1100 K, the Cu3 clusters get converted to Cu4 clusters (as inferred from the rising intensity of the 425 nm peak) or even to larger nanoparti2.0
1.5
Absorbance
221
1
irridiated
2
773 K
3
873 K
4
1073 K
1.0
4
1
0.5
2 3
0.0 100
200
300
400
500
600
700
800
900
1000
Wavelength (nm) Fig. 4. UV–Vis spectra of Cu-fused silica after 350 MeV Au26+ irradiation at a fluence of 2 · 1013 cm2 and annealing in Ar at the temperatures shown.
Swift heavy ion irradiations create E 0 and B2 defects in Cu-coated, fused silica. Post-irradiation annealing helps to eliminate these defects and form Cu clusters. At the highest annealing temperature of 1200 K, Cu4 clusters and larger-size particles are formed. Irradiation fluences of more than 4 · 1013 cm2 and annealing temperatures exceeding 1100 K appear to be more effective in forming larger nanoclusters. Acknowledgements This work was supported by Deutsche Forschungsgemeinschaft (DFG) and the Nuclear Science Centre, New Delhi. The authors are indebted to Dr. K. Zhang and V. Milinovic (Go¨ttingen) for assistance during the Au-ion irradiations and RBS analyses and to S.S. Patil, R.P. Fernandes and Trupti N. Warang (Mumbai) for assistance during the experiments in India. References [1] E. Valentin, H. Bernas, C. Ricolleau, F. Creuzet, Phys. Rev. Lett. 86 (2001) 99. [2] T. Mohanty, A. Pradhan, S. Gupta, D. Kanjilal, Nanotechnology 15 (2004) 1620. [3] A. Barbu, P. Pareige, V. Jacquet, Nucl. Instr. and Meth. B 146 (1998) 278. [4] A. Iwase, T. Hasegawa, Y. Chimi, T. Tobita, N. Ishikawa, M. Suzuki, T. Kambara, S. Ishino, Nucl. Instr. and Meth. B 195 (2002) 309. [5] G. Battaglin, P. Calveli, E. Cattaruza, F. Gonella, R. Polloni, Appl. Phys. Lett. 78 (2001) 3953. [6] R.H. Magruder III, R.F. Haglund Jr., L. Yang, J.E. Witting, R.A. Zuhr, J. Appl. Phys. 76 (1994) 708. [7] D. Ila, E.K. Williams, S. Surkisov, C.C. Smith, D.B. Poker, D.K Hensley, Nucl. Instr. and Meth. B 141 (1998) 289. [8] M.K. Patel, B.J. Nagare, D.M. Bagul, S.K. Haram, D.C. Kothari, Surf. Coat. Technol. 196 (2005) 96. [9] S.K. Sinha, D.C. Kothari, A.K. Balmuragan, A.K. Tyagi, D. Kanjilal, Surf. Coat. Technol. 158–159 (2002) 609. [10] W. Bolse, B. Schattat, A. Feyh, T. Renz, Nucl. Instr. and Meth. B 218 (2004) 80. [11] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. B 216 (2004) 1. [12] A. Oliver, J.C. Cheang-Wong, A. Crespo, L. Rodriguez-Fernandez, J.M. Hernandez, E. Munoz, R. Espejel-Morales, Mater. Sci. Eng. 78 (2000) 32; A. Oliver, J.C. Cheang-Wong, A. Crespo, L. Rodriguez-Fernandez, J.M. Hernandez, E. Munoz, R. Espejel-Morales, Nucl. Instr. and Meth. B 175 (2001) 495. [13] M. Moskovits, J.E. Hulse, J. Chem. Phys. 67 (1977) 4271.