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The energy dependence of lattice damage in graphite induced by low energy He-ion irradiation
journal of nuell?ar
Journal of Nuclear Materials 187 (1902) 294-2Y7 North-Holland
Mab!rl~ls Letter to the Editors
The energy dependence of lattice damage in graphite induced by low energy He-ion irradiation K.G. Nakamura, E. Asari * and M. Kitajima Tsukuba
Laboratories.
Nutional
Research
Institute
for Met&
I-2- I Sen,yen, &rkuhtr-shi,
Iburuki-km
.W.i. .lupcrr~
T. Kawabe Institute
qf
Physics,
Unir~ersity of Tsukuba,
1-I-I
Tennodai.
Tsukuba-shi.
Ibaruki-ken
305. .Iuparz
Received 13 November IYYI: accepted 6 February lYY2
Ion-irradiation damage of graphite has been studied extensively by Raman spectroscopy in connection with the study of the plasma-wall interaction in a fusion device [l-3], since ion implantation usually takes place with an optical skin depth and Raman scattering of graphite is very sensitive to its lattice damage. In spite of a large number of Raman studies for characterizing ion-irradiated graphite, the lattice disordering kinetics in the early stage of irradiation has not been well understood. Recently WC have developed an experimental apparatus for real-time in-situ measurements of the lattice damage caused by ion irradiation to study the lattice damage in the early stage of irradiation [4,5]. In this paper, we report the real-time Raman measurements of graphite under He’-ion irradiation with an energy range of 1-S keV. The sample used was a highly oriented pyrolytic graphite (HOPG, grade ZYA from Union Carbide), with its size being 12 X 12 X 1 mm3. The sample was cleaved using the adhesive tape technique for each measurement. The sample was attached to the mount of the manipulator and covered by mica and tantalum plates (20 mm x 30 mm) with a hole (5 mm). Ion irradiation was performed in an ultrahigh vacuum chamber (base pressure < 10~~’ Pa), and the incident angle of the ion beam was 45” normal to the c-face of HOPG to avoid the ion channeling effect. The ion
* Visiting researcher from University of Tsukuba 0022.3115/92/$05.00
cncrgy varied between 1 and 5 keV. The sample current was monitored with a digital multimeter (ADVANTEST TR6848), and set to approximately 4.1 x IO--’ A mm’. Through a correction by secondary clcctron emission [6], the ion flux was estimated to he 1.8X lOI ions m ’ s-- ‘. The incident radiation of 514.5 nm and 500 mW was provided using a cw argonion laser (coherent radiation model INNOVA 70). The scattered radiation was collected through the sapphire window of the vacuum chamber in backscattering configuration, analyzed by a double monochrometer (Japan Spectroscopic Company Ltd. TRS-660), and detected by a spectrometric multichannel analyzer (Princeton Instruments Inc. D/SIDA700). The multichannel analyzer has 700 channels and detects to a width of about 400 x IO’ m ‘_ The time resolution for the present experiments was about 6 s. Details of the experimental setup are described elsewhere [4]. Raman spectra wlere analyzed with numerical decomposition by assuming the Lorentzian line shape for the peaks. Fig. 1 shows a typical example of the time dependence of the first-order Raman spectra of HOPG under 5 keV He+ irradiation. Only the peak of Raman active EZg, mode vibration peak is observed at around as shown 1580 X 10’ m ’ (G) before He+ irradiation in fig. la. Figs. lb- Ic show the Raman spectra ohtained at 120, 240, 360, and 4X0 s after the beginning of irradiation. The disorder-induced peak at around 1360 x IO* m ’ (D), which corresponds to a maximum ol the density of states of graphite phonons. was induced
0 1992 - Elsevier Science Publishers B.V. All rights reserved
K.G. ‘~a~a~i~ra et al. / The energy dependence
ofIat&
295
damage in gruphite
Irradiation
Time
(s”?
Fig. 2. Time dependence of the observed relative intensity ratio of the disorder-induced line with respect to the Raman active line. Ion irradiation at (a) 1 keV. (h) 2 keV, (c) 3 keV, and (d) 5 keV.
L-i
I
1600
1400 Raman
shift
(lo*
mm’)
Fig. 1. Raman spectra of HOPG (a) before ion irradiation. and obtained during irradiation after (bf 120, icf 240, fd) 360, and (e) 480 s from the beginning of irradiation of 5 keV He+ at a flux of 1.8~10’” ions me2 SC’. Solids curves are the results of curve fitting.
by He+ irradiation.
As irradiation time increased, the peak hight of the D peak increased and that of the G
peak decreased. The in-plane phonon correlation length can be deduced from the peak intensity ratio (Rob) of the D peak with respect to the G peak, by use of the formula L, = 4.4/R,, (nm) [7,8], and the peak intensity ratio is a measure of the disorder caused by the lattice damage. Figs. 2a-2d show the time dependence of the observed peak intensity ratio (Robs) for ion energies of 1, 2, 3, 5 keV, respectively. The solid curves are results of curve fitting. R,, is found to be proportional to the square root of ion irradiation time for all ion energies. The slope increases as the ion energy increases for the 1-3 keV range, and the slope for 5 keV irradiation is almost the same as that for 3 keV irradiation. This difference may be due to the difference in the ion damage depth. Since the optical skin depth of 514.5 nm radiation is about 40 nm and the ion damage depth is smaller than the optical skin depth, the observed Raman spectra of ion-irradiated graphite consists of Raman scattering from both nondamaged and damaged regions. The
damage profile caused by ion irradiation can be obtained by Monte Carlo calculation and can be well approximated by the Gaussian function. The peak position and the standard deviation of the Gaussian function calculated by the Monte Carlo calculation (TRIM85 code [9]) are listed in table 1. The damage depth increases as the ion energy increases. Using the calculated damage profile, the actual damage caused by the momentum transfer from energetic incident particles and successive cascades are estimated with the mean value of the relative intensity ratio (R,) of the D peak with respect to the G peak in the damaged region f5]. Then the relation between R,, and R, is explained by
/i%exp(
- thrk/A)
d x,
(1)
Table 1 Parameters of the damaged profile approximated by the Gaussian function. D is the standard deviation, .ru is the mean depth, and P is the whole range of deposition Ion energy (keV)
(r
x0
(nm)
(nm)
1
4
2 3 5
8 11 16
2 5 10 19
15 30 42 63
296
K.G. Nakamura
et al. / The energy dependence
where P is the whole range of energy deposition, k is the optical parameter (k = 0.9 for carbon material). A is the wavelength of the exciting light, x is the depth and F(x) is the normalized damage layer depth profile calculated by the Monte Carlo calculation. The time dependence of the calculated R,, is shown in fig. 3, and its slope is almost the same for all ion energies t l-5 keV). The time dependence of R,, can be explained in terms of the reduction of the in-plane phonon correlation length due to defects caused by irradiation [S]. The number of defects (N,) per unit arca in graphite plane induced by ion irradiation is given by N<,= Ngdpvt.
damuge in graphite
Table 2 The >lope ot the time dependence of the Raman intensity ratio. the damage function and the displacement cross section (‘i.,iC is the coefficient calculated by eq. (3) in the text, C,,,, i:, the slope of the time dependence of the actual Raman intensity ratio, I’ is the damage function. and CT~,is the displaccment cross section
(2)
where N (atoms mm’) is the number of atoms in a unit area of a graphite layer, o;, (mm*) is the displacement cross section, cp (ions rn-’ s- ‘) is the incident ion flux, 11 is the mean number of total displaced atoms in the cascade per primary knock-on (damage function), and t (s) is the irradiation time [1,12]. Since graphite has a layer structure (quasi-two-dimensional crystal) and the phonon mode mentioned here is the vibration in the graphite plane, the mean distance between defects is obtained by the inverse of the square root of Nd. If we assume that the mean distance between defects in a graphite layer corresponds to an in-plane phonon correlation length L,, the mean relative intensity ratio of the Raman peaks within the ion penetration depth is obtained by R,, = 4.4 x 10~7~~~:
=Cfi, I
a
of Iattrw
h
with the experimentally obtained relation between I_,, and the relative intensity ratio. The coefficient C corresponds to the lattice disordering rate constant. The displacement cross section and the number of total displaced atoms per incident ion can be calculated by the Sigmund theory [lo] and the Kinchin-Pease theory [II], respectively. Table 2 shows the results of the calculation for cr, u. and C along with the coefficient C obtained experimentally from the slope of the time dependence of R,, in fig. 3. The damage function increases and the displacement cross section decreases as the ion energy increases. The coefficient C calculated by this model agrees well with that obtained experimentally and is almost the same for all ion energies (l-5 keW. The difference in the slope of the time dependence of the observed intensity ratio R,,, is due to the difference of the damage depth in the present ion energy range. In conclusion, the lattice disordering rate by low energy He-ion irradiation was studied by real-time Raman measurements. The difference in the observed Raman spectra for different ion energies is due to the difference of the damage depth.
References
20
Irradiation
0
20
Time
(s”*)
0
Fig. 3. Time dependence of the actual relative intensity ratio of the disorder-induced line with respect to the Raman active line. Ion irradiation of (a) 1 keV, (b) 2 keV, Cc) 3 keV, and (d) 5 keV.
[l] B.S. Elman, M. Shayegan, MS. Dresselhaus, H. Mazurek and G. Dresselhaus, Phys. Rev. B25 (1982) 4142. (21 M. Kitajima, K. Aoki and M. Okada, J. Nucl. Mater. 149 (1987) 269. [3] T. Tanabe, S. Muto, Y. Gotoh and K. Niwase, J. Nucl. Mater. 175 (1990) 258. [4] K. Nakamura and M. Kitajima. Appl. Phys. Lett. 59 (1991) 1550. [5] K. Nakamura and M. Kitajima, Phys. Rev. B45 (1992) 78.
K.G. Nakamura et al. / The energy dependence of lattice damage in graphite [6] Secondary electron emission mentally obtained to be 0.26, 3, and 5 keV ion irradiation, [7] D.S. Knight and W.B. White, [8] F. Tuinstra and J.L. Koenig, 1126. [9] J.F. Ziegler, J.P. Biersack
coefficients were experi0.35, 0.50, and 0.60 for 1, 2, respectively. J. Mater. Res. 4 (1989) 385. J. Chem. Phys. 53 (1970) and
U. Littmark,
in: The
297
Stopping and Range of Ions in Solids (Pergamon, New York. 1985) p. 109. [lo] P. Sigmund, in: Sputtering by Particle Bombardment I, ed. R. Behrisch (Springer, Berlin, 1981) p. 9. [II] G.H. Kinchin and R.S. Pease, Rep. Prog. Phys. 18 (1955) I.
Journal of Nuclear Materials 187 (1902) 294-2Y7 North-Holland
Mab!rl~ls Letter to the Editors
The energy dependence of lattice damage in graphite induced by low energy He-ion irradiation K.G. Nakamura, E. Asari * and M. Kitajima Tsukuba
Laboratories.
Nutional
Research
Institute
for Met&
I-2- I Sen,yen, &rkuhtr-shi,
Iburuki-km
.W.i. .lupcrr~
T. Kawabe Institute
qf
Physics,
Unir~ersity of Tsukuba,
1-I-I
Tennodai.
Tsukuba-shi.
Ibaruki-ken
305. .Iuparz
Received 13 November IYYI: accepted 6 February lYY2
Ion-irradiation damage of graphite has been studied extensively by Raman spectroscopy in connection with the study of the plasma-wall interaction in a fusion device [l-3], since ion implantation usually takes place with an optical skin depth and Raman scattering of graphite is very sensitive to its lattice damage. In spite of a large number of Raman studies for characterizing ion-irradiated graphite, the lattice disordering kinetics in the early stage of irradiation has not been well understood. Recently WC have developed an experimental apparatus for real-time in-situ measurements of the lattice damage caused by ion irradiation to study the lattice damage in the early stage of irradiation [4,5]. In this paper, we report the real-time Raman measurements of graphite under He’-ion irradiation with an energy range of 1-S keV. The sample used was a highly oriented pyrolytic graphite (HOPG, grade ZYA from Union Carbide), with its size being 12 X 12 X 1 mm3. The sample was cleaved using the adhesive tape technique for each measurement. The sample was attached to the mount of the manipulator and covered by mica and tantalum plates (20 mm x 30 mm) with a hole (5 mm). Ion irradiation was performed in an ultrahigh vacuum chamber (base pressure < 10~~’ Pa), and the incident angle of the ion beam was 45” normal to the c-face of HOPG to avoid the ion channeling effect. The ion
* Visiting researcher from University of Tsukuba 0022.3115/92/$05.00
cncrgy varied between 1 and 5 keV. The sample current was monitored with a digital multimeter (ADVANTEST TR6848), and set to approximately 4.1 x IO--’ A mm’. Through a correction by secondary clcctron emission [6], the ion flux was estimated to he 1.8X lOI ions m ’ s-- ‘. The incident radiation of 514.5 nm and 500 mW was provided using a cw argonion laser (coherent radiation model INNOVA 70). The scattered radiation was collected through the sapphire window of the vacuum chamber in backscattering configuration, analyzed by a double monochrometer (Japan Spectroscopic Company Ltd. TRS-660), and detected by a spectrometric multichannel analyzer (Princeton Instruments Inc. D/SIDA700). The multichannel analyzer has 700 channels and detects to a width of about 400 x IO’ m ‘_ The time resolution for the present experiments was about 6 s. Details of the experimental setup are described elsewhere [4]. Raman spectra wlere analyzed with numerical decomposition by assuming the Lorentzian line shape for the peaks. Fig. 1 shows a typical example of the time dependence of the first-order Raman spectra of HOPG under 5 keV He+ irradiation. Only the peak of Raman active EZg, mode vibration peak is observed at around as shown 1580 X 10’ m ’ (G) before He+ irradiation in fig. la. Figs. lb- Ic show the Raman spectra ohtained at 120, 240, 360, and 4X0 s after the beginning of irradiation. The disorder-induced peak at around 1360 x IO* m ’ (D), which corresponds to a maximum ol the density of states of graphite phonons. was induced
0 1992 - Elsevier Science Publishers B.V. All rights reserved
K.G. ‘~a~a~i~ra et al. / The energy dependence
ofIat&
295
damage in gruphite
Irradiation
Time
(s”?
Fig. 2. Time dependence of the observed relative intensity ratio of the disorder-induced line with respect to the Raman active line. Ion irradiation at (a) 1 keV. (h) 2 keV, (c) 3 keV, and (d) 5 keV.
L-i
I
1600
1400 Raman
shift
(lo*
mm’)
Fig. 1. Raman spectra of HOPG (a) before ion irradiation. and obtained during irradiation after (bf 120, icf 240, fd) 360, and (e) 480 s from the beginning of irradiation of 5 keV He+ at a flux of 1.8~10’” ions me2 SC’. Solids curves are the results of curve fitting.
by He+ irradiation.
As irradiation time increased, the peak hight of the D peak increased and that of the G
peak decreased. The in-plane phonon correlation length can be deduced from the peak intensity ratio (Rob) of the D peak with respect to the G peak, by use of the formula L, = 4.4/R,, (nm) [7,8], and the peak intensity ratio is a measure of the disorder caused by the lattice damage. Figs. 2a-2d show the time dependence of the observed peak intensity ratio (Robs) for ion energies of 1, 2, 3, 5 keV, respectively. The solid curves are results of curve fitting. R,, is found to be proportional to the square root of ion irradiation time for all ion energies. The slope increases as the ion energy increases for the 1-3 keV range, and the slope for 5 keV irradiation is almost the same as that for 3 keV irradiation. This difference may be due to the difference in the ion damage depth. Since the optical skin depth of 514.5 nm radiation is about 40 nm and the ion damage depth is smaller than the optical skin depth, the observed Raman spectra of ion-irradiated graphite consists of Raman scattering from both nondamaged and damaged regions. The
damage profile caused by ion irradiation can be obtained by Monte Carlo calculation and can be well approximated by the Gaussian function. The peak position and the standard deviation of the Gaussian function calculated by the Monte Carlo calculation (TRIM85 code [9]) are listed in table 1. The damage depth increases as the ion energy increases. Using the calculated damage profile, the actual damage caused by the momentum transfer from energetic incident particles and successive cascades are estimated with the mean value of the relative intensity ratio (R,) of the D peak with respect to the G peak in the damaged region f5]. Then the relation between R,, and R, is explained by
/i%exp(
- thrk/A)
d x,
(1)
Table 1 Parameters of the damaged profile approximated by the Gaussian function. D is the standard deviation, .ru is the mean depth, and P is the whole range of deposition Ion energy (keV)
(r
x0
(nm)
(nm)
1
4
2 3 5
8 11 16
2 5 10 19
15 30 42 63
296
K.G. Nakamura
et al. / The energy dependence
where P is the whole range of energy deposition, k is the optical parameter (k = 0.9 for carbon material). A is the wavelength of the exciting light, x is the depth and F(x) is the normalized damage layer depth profile calculated by the Monte Carlo calculation. The time dependence of the calculated R,, is shown in fig. 3, and its slope is almost the same for all ion energies t l-5 keV). The time dependence of R,, can be explained in terms of the reduction of the in-plane phonon correlation length due to defects caused by irradiation [S]. The number of defects (N,) per unit arca in graphite plane induced by ion irradiation is given by N<,= Ngdpvt.
damuge in graphite
Table 2 The >lope ot the time dependence of the Raman intensity ratio. the damage function and the displacement cross section (‘i.,iC is the coefficient calculated by eq. (3) in the text, C,,,, i:, the slope of the time dependence of the actual Raman intensity ratio, I’ is the damage function. and CT~,is the displaccment cross section
(2)
where N (atoms mm’) is the number of atoms in a unit area of a graphite layer, o;, (mm*) is the displacement cross section, cp (ions rn-’ s- ‘) is the incident ion flux, 11 is the mean number of total displaced atoms in the cascade per primary knock-on (damage function), and t (s) is the irradiation time [1,12]. Since graphite has a layer structure (quasi-two-dimensional crystal) and the phonon mode mentioned here is the vibration in the graphite plane, the mean distance between defects is obtained by the inverse of the square root of Nd. If we assume that the mean distance between defects in a graphite layer corresponds to an in-plane phonon correlation length L,, the mean relative intensity ratio of the Raman peaks within the ion penetration depth is obtained by R,, = 4.4 x 10~7~~~:
=Cfi, I
a
of Iattrw
h
with the experimentally obtained relation between I_,, and the relative intensity ratio. The coefficient C corresponds to the lattice disordering rate constant. The displacement cross section and the number of total displaced atoms per incident ion can be calculated by the Sigmund theory [lo] and the Kinchin-Pease theory [II], respectively. Table 2 shows the results of the calculation for cr, u. and C along with the coefficient C obtained experimentally from the slope of the time dependence of R,, in fig. 3. The damage function increases and the displacement cross section decreases as the ion energy increases. The coefficient C calculated by this model agrees well with that obtained experimentally and is almost the same for all ion energies (l-5 keW. The difference in the slope of the time dependence of the observed intensity ratio R,,, is due to the difference of the damage depth in the present ion energy range. In conclusion, the lattice disordering rate by low energy He-ion irradiation was studied by real-time Raman measurements. The difference in the observed Raman spectra for different ion energies is due to the difference of the damage depth.
References
20
Irradiation
0
20
Time
(s”*)
0
Fig. 3. Time dependence of the actual relative intensity ratio of the disorder-induced line with respect to the Raman active line. Ion irradiation of (a) 1 keV, (b) 2 keV, Cc) 3 keV, and (d) 5 keV.
[l] B.S. Elman, M. Shayegan, MS. Dresselhaus, H. Mazurek and G. Dresselhaus, Phys. Rev. B25 (1982) 4142. (21 M. Kitajima, K. Aoki and M. Okada, J. Nucl. Mater. 149 (1987) 269. [3] T. Tanabe, S. Muto, Y. Gotoh and K. Niwase, J. Nucl. Mater. 175 (1990) 258. [4] K. Nakamura and M. Kitajima. Appl. Phys. Lett. 59 (1991) 1550. [5] K. Nakamura and M. Kitajima, Phys. Rev. B45 (1992) 78.
K.G. Nakamura et al. / The energy dependence of lattice damage in graphite [6] Secondary electron emission mentally obtained to be 0.26, 3, and 5 keV ion irradiation, [7] D.S. Knight and W.B. White, [8] F. Tuinstra and J.L. Koenig, 1126. [9] J.F. Ziegler, J.P. Biersack
coefficients were experi0.35, 0.50, and 0.60 for 1, 2, respectively. J. Mater. Res. 4 (1989) 385. J. Chem. Phys. 53 (1970) and
U. Littmark,
in: The
297
Stopping and Range of Ions in Solids (Pergamon, New York. 1985) p. 109. [lo] P. Sigmund, in: Sputtering by Particle Bombardment I, ed. R. Behrisch (Springer, Berlin, 1981) p. 9. [II] G.H. Kinchin and R.S. Pease, Rep. Prog. Phys. 18 (1955) I.