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Nuclear Instruments and Methods in Physics Research B xxx (2011) xxx-xxx

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Nuclear Instruments and Methods in Physics Research B
journal h omepage: www.else vier.com/locate/nimb

Energy and temperature dependences of ion-induced electron emission from polycrystalline graphite
V.S. Avilkina a, N.N. Andrianova a, A.M. Borisov a, E.S. Mashkova a,, E.S. Parilis
a b

b

Institute of Nuclear Physics, Moscow State University, Leninsky Gori, 119991 Moscow, Russia California Institute of Technology, 200-306, Pasadena, CA 91125, USA

article

info

abstract
The dependences of the yield c of ion-induced kinetic electron emission from polycrystalline graphite on ion energy E were measured under high-fluence Ar+ and Nþ ion irradiation in energy range 6-30 keV from 2 room temperature to 400 œC. It has been observed that the step-like pattern of temperature dependence c(T) at the dynamic radiation damage annealing temperature Ta is gradually transformed with decreasing ion energy until c virtually ceases to depend on temperature. The experimentally obtained energy dependence of the ratio c(T > Ta)/c(T < Ta) has been analyzed using the theory of ion-induced kinetic electron emission. Some threshold values of the level of radiation damage md measured in dpa under steady state of high-fluence Ar+ and Nþ irradiation have been found. When m becomes less than a threshold value md, 2 the graphite lattice is virtually not disordered. It has been found that md for Ar+ ions is greater than md for Nþ ions. 2 Ó 2010 Elsevier B.V. All rights reserved.

Article history: Received 28 July 2010 Received in revised form 6 December 2010 Available online xxxx Keywords: High-fluence ion irradiation Ion-induced electron emission Radiation damage in carbon-based materials

1. Introduction Ion-induced kinetic electron emission, a fundamental phenomenon of ion interaction with solids, is connected to deposition of inelastic electronic energy losses of the ions [1]. This phenomenon has also many practical applications in plasma-wall interaction for monitoring the surface structure changes under different radiation damage conditions. It can be used to monitor the radiation durability and modification of spacecraft materials, and the spacecraft charging [2]. For a long time the main attention in electron emission studies was focused on the mechanism of electron excitation and projectile transport peculiarities. In recent years the secondary electron transport especially in carbon-based materials has been studied [3-7]. The studies of ion-induced electron emission of carbon-based materials, in particular, polycrystalline graphites under high-fluence 30 keV Ar+ and Nþ ion bombardment have shown that 2 the temperature dependences of secondary electron yield c manifests at a certain temperature Ta a step-like behavior typical for defect annealing curves [4,6]. This behavior has been explained by the dependence of secondary electron path length k on changes in lattice structure. Namely, a transition from strongly disordered (amorphized) surface layer under ion irradiation at T < Ta to polycrystalline one at T > Ta results in kam < kcr. The purpose of the present study is to explore the possibility of using the ion-induced electron emission to analyze the structural
Corresponding author. Tel.: +7 495 939 4167; fax: +7 495 939 0896.
E-mail address: es_mashkova@mail.ru (E.S. Mashkova). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.12.030

changes of polycrystalline graphites under high-fluence (1018- 1019 ion/cm2) 6-30 keV Ar+ and Nþ ion irradiation, and to compare 2 the obtained data with the ion-induced kinetic electron emission theory, both with and without taking into account the ion energy influence on the structure transition at Ta.

2. Experimental The experiment was carried out on the mass-monochromator of the Institute of Nuclear Physics, Moscow State University [8]. The 5-35 keV ion beam was produced in arc source with a longitudinal magnetic field. The ions were separated and the beam was focused by a Siegban-type magnetic sector field. The experimental procedure is described elsewhere [9]. The target holder allowed variation of ion incidence angles, h from 0œ to 90œ. The target's temperature could be varied from À180 œC to 1000 œC. The samples of polycrystalline graphite MPG-8 (NIIgraphite production, Moscow, Russia) were in the form of rectangular sheets (3 mm thick, 22 mm wide and 40 mm long). The irradiation was produced by 6-30 keV Ar+ and Nþ ions, ion current was $0.4 mA/cm2, and the 2 cross-section of ion beam was 0.3 cm2. The total ion fluencies ut 18 19 were 10 -10 ion/cm2, where u is the ion beam density (flux) and t is the bombardment time. Preliminary processing of the samples included vacuum annealing at T P 250 œC. The ion-induced electron emission yield c was determined as the ratio of the electron current to the primary ion current with the instrument error $2.5%.

Please cite this article in press as: V.S. Avilkina et al., Energy and temperature dependences of ion-induced electron emission from polycrystalline graphite, Nucl. Instr. and Meth. B (2011), doi:10.1016/j.nimb.2010.12.030


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V.S. Avilkina et al. / Nuclear Instruments and Methods in Physics Research B xxx (2011) xxx-xxx

3. Results and discussion The analysis of the angular dependences of ion-induced electron emission yield c from polycrystalline graphites under high-fluence (1018-1019 ion/cm2) 30 keV Ar+ and Nþ ion irradiation 2 allowed us to show, that the mean path length k of secondary electrons at temperatures above the dynamic radiation damage annealing temperature Ta is larger than below it, kcr(T > Ta) > kam(T < Ta) [4,6]. The inequality c(T < Ta) < c(T > Ta) appears as a result of efficient disordering of the graphite lattice at normal ion incidence. In the present work the analogous temperature dependences c(T) studied in the temperature range from room temperature to 400 œC at 30 keV have been found also strongly nonmonotonic, see Fig. 1. A comparison of these data with the data for 10 and 20 keV Ar+ and Nþ illustrates the ion energy influence on 2 nonmonotonic behavior of c(T). The difference between c(T > Ta) and c(T < Ta) decreases with ion energy decreasing, and for 10 keV Ar+ the yields c(T > Ta) % c(T < Ta). For Nþ ion irradiation a 2 transformation of c(T)-dependences with projectile energy decreasing occurs more slowly. In Fig. 2 the experimentally obtained dependences of the ratio c(T > Ta)/c(T < Ta) on projectile energy E are shown as the dependences of cHT/cRT(E). Here cHT is the yield at high temperature T = 400 œC (T > Ta) and cRT is the yield for room temperature (T < Ta). These dependences have been compared with the dependences calculated using the ion-induced kinetic electron emission theory [1,3]. The secondary electron yield for incidence angle h according to the theory is given by:
1.20 1.15 1.10

0

20

40
+ - MPG-8

, dpa

60

80

100

a
1

Ar

HT / RT

1.05 1.00 0.95

2

0

5

10

15 20 E, keV
, dpa

25

30

35

1.25 1.20 1.15

0

20
N

40
+
2 - MPG-8

60

80

100

b

1
1.10 1.05 1.00 0 5 10

c ? nre kw= cos h?1 À k=ðRe cos hÞð1 À expðÀRe cosh=kÞÞ;

ð 1Þ

where n is the atomic density of the target, k is the secondary electron mean path length and w is the electron escape probability. The binary collision ionisation cross-section re is determined by the total energy deposited in the electron shells of both the target atoms and the projectiles and is expressed as [1]

2
15 20 25 30 35

E, keV
Fig. 2. The energy dependence of the ratio of cHT/cRT for graphite under Ar+ (a) and Nþ (b) irradiation. The calculations without (curve 1) and with (curve 2) took into 2 account the threshold in the level of radiation damage, when the graphite lattice is not disordered at RT irradiation.

re ðtÞ ? 1:16ao hJ

À1



ðZ 1 þ Z 2 Þ Z

1=2 1

þZ

1 =2 2

À1 2 ð 2Þ

 t arctan?0:6ðt À

to Þ10À7 ;

where J is the ionisation energy, t is the projectile velocity, to is the threshold velocity, ao is the first Bohr radius, Z1 and Z2 are the atomic numbers of projectile and target atoms, respectively. Re in

formula (1) is the mean path length at which the projectile is slowed down to the threshold velocity to [4]. For normal incidence the ratio cHT/cRT is given by

3.5

Ar - MPG-8

+

a
30 keV

+ N 2 - M P G -8
4.5 4.0

cHT kHT ?1 À kHT =Re ð1 À expðÀRe =kHT ÞÞ ? : cRT kRT ?1 À kRT =Re ð1 À expðÀRe =kRT ÞÞ

ð3Þ

b
30 keV

3.0

T
, el./ion
2.5

a

20 keV 15 keV 10 keV
0 1 00 200 300 4 00

3.5 3.0

T
20 keV

a

2.0

2.5 2.0 1.5

10 keV

1.5

temperature, T C

?

0

temperature, T ?C

1 00

200

3 00

400

Fig. 1. The temperature dependences of c for polycrystalline graphite MPG-8 under 10-30 keV Ar+ (a) and Nþ (b) ion irradiation. 2

The energy dependence of cHT/cRT is due to the energy dependence of Re / E [4]. When k/Re ( 1 the ratio cHT/cRT % kHT/kRT. When k/Re ) 1, cHT/cRT % 1, cf. [5]. Calculations of the ratios cHT/ cRT were first made using some fixed path lengths k in graphite, namely kHT = kcr = 4.6 nm, kRT = kam = 3.8 nm [6]. One can see that the calculated energy dependences of the cHT/cRT(E) presented in Fig. 2 (curves 1) reflect the experimental data only qualitatively. One may suppose that the agreement between experimental data and theory may be obtained supposing that projectile energy decreasing influences on graphite lattice structure transition under irradiation. Namely, as projectile energy decreases lattice disordering becomes not to strong and the path length of secondary electrons tends to k for polycrystalline graphite. Really, the level of the radiation damage decreases with projectile energy decrease. It is known that the level of radiation damage m is determined as the number of displacements per atom (dpa) m = u Á t Á rdam, where rdam is the radiation damage cross section [10]. The level of radiation damage is used as a universal characteristic of radiation

Please cite this article in press as: V.S. Avilkina et al., Energy and temperature dependences of ion-induced electron emission from polycrystalline graphite, Nucl. Instr. and Meth. B (2011), doi:10.1016/j.nimb.2010.12.030


V.S. Avilkina et al. / Nuclear Instruments and Methods in Physics Research B xxx (2011) xxx-xxx

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damage, allowing one to compare the data obtained under different conditions of irradiation (type of radiation, energy, and fluence). This characteristic of radiation damage is used, for example, to estimate radiation resistance of carbon-based materials widely applied in nuclear reactors, and thermonuclear facilities [11-14]. In our estimates we have supposed, following to Sigmund [15], that the radiation damage cross section is determined as rdam % 0.4Sn/Ed where Sn is the nuclear stopping cross section and Ed is the radiation damage threshold energy. The inverse square distance law for the potential of interatomic interaction is a good approximation for the keV ion energy range, while the magnitude of Sn does not depend on the ion energy E. Hence, the magnitude of rdam may be assumed constant along whole depth Rd of radiation damage. The surface erosion (sputtering) is an essential factor determining the concentration defects under high-fluence ion bombardment of materials. In many cases the dynamically equilibrium (steady-state) conditions, at which the profiles of implanted particles and radiation damage become stationary, are established as a result of motion the surface boundary due to sputtering [8,16]. At normal ion incidence the surface boundary moves with the speed Y Á u/n, where Y is the sputtering yield. After a certain time td sputtering of a layer of thickness Rd (the depth of defect production) is reached, that is determined by the equation

1.0

MPG-8

amorfous fraction, f

0.8 0.6
N Ar

a

0.4 0.2 0.0

0

10

20

30

, dpa

40

50

60

70

Fig. 3. Erf-like dependence of the amorphous fraction obtained by fitting to experimental data for graphite irradiation under Ar+ and Nþ ions. 2

where a ? 0:8853a0 Z

1=2 1

þZ

1=2 2

À2=3

is the Firsov screening con-

Rd ?

Y u Á td : n

ð 4Þ

At this point a steady state is achieved where the radiation damage level mst decreases linearly with the depth x from a maximal value m0 at x = 0 down to zero at x = Rd:

mst ðxÞ ? m0 1 À
where



x ; Rd

x 6 Rd ;

ð 5Þ

stant, EL is the Lindhard energy, K = 4M1M2/(M1 + M2)2. The linear dependence is valid for the energy interval 0.1 6 e 6 0.5 that covers the energy range in hand. The mean value m = ?m0 is used as the level of radiation damage at high fluence irradiation conditions and is depicted on the top of Fig. 2. One may suppose, that in the studied temperature range T < Ta, when m is less than a certain threshold value md, the graphite lattice is virtually not disordered. When m is larger than md the graphite is fully amorphized. The step-like dependence of the amorphous fraction fa on m is well known for many radiation induced phase transitions [19]. In our calculation the dependence of the amorphous fraction on m has been presented by an erf-function with scaling factor k, see Fig. 3

m0

0:4 Á n Á Sn Á Rd ? : Ed Á Y ðhÞ

ð 6Þ

fa ?

1 f1 þ erf ?ðm À 2

md Þ=kg:

ð9Þ

The cascade sputtering yield can be estimated using the Sigmund formula [17]

The mean path length of secondary electrons in the case of ion irradiation of graphite at room temperature (T < Ta) depends on the amorphous fraction as

Y?

0:076 Á a Á Sn ; C o Ec
2

kRT ðmÞ ? kcr À fa ðkcr À kam Þ:

ð10Þ

where Co = 1.807 Å is the cross section of elastic interaction of low energy cascade atoms, a is a factor depending on the target atom and incident ion mass ratio M2/M1; and Ec is the binding energy of surface atoms. Then, the maximal value m0 at x = 0 is determined by the formula

m0 ?

Rd 5:26 Ec Á Á; Dx aðM 2 =M1 Þ Ed

ð 7Þ

where Dx =1/nCo is the characteristic escape depth of the sputtered atoms (%5 Å). From Eqs. (5) and (7) one can see that mst(x) does not depend on the nuclear stopping cross section. This is no surprise, since the nuclear stopping cross section Sn determines both the sputtering and radiation damage processes. The magnitude of mst(x) is determined by the ratios of the characteristic depths Rd/D[ and energies Ec/Ed. Only the depth Rd in Eq. (7) depends on the projectile energy E. It can be estimated using a linear approximation for dependence of the dimensionless depth qd = npa2KRd versus the dimensionless energy e ? E=EL [18]

When m P md +2k the amorphous fraction fa % 1, and k % kam. As the amorphous fraction decreases, k increases so, k % kcr when the amorphous fraction fa % 0. At elevated temperature T > Ta the amorphous graphite fraction is absent and kHT = kcr. Our calculations of the ratios cHT/cRT taking into account the radiation damage level, show that there are suitable parameters of erf-like function for kRT(m) giving a good agreement of the calculations with the experiment, see Fig. 2. The values of md are 40 and 30 for Ar+ and Nþ irradiation respectively. In both cases the scaling 2 factor k = 14. The corresponding erf-like dependence of amorphous fraction fa versus m is depicted in Fig. 3. A lower radiation resistance of graphite under nitrogen ion irradiation is possibly caused by creation of chemical bounds C-N blocking the diffusion processes at defect annealing. 4. Conclusion The ion-induced electron emission from the isotropic graphite MPG-8 under 6-30 keV Ar+ and Nþ ion irradiation at high fluencies 2 (1018-1019 ion/cm2) at normal ion incidence has been studied from room temperature to $400 œC. The experimentally obtained electron yield temperature dependences show a transition from step-like behavior with a jump at

qd ? 1:92e;

ð 8Þ

Please cite this article in press as: V.S. Avilkina et al., Energy and temperature dependences of ion-induced electron emission from polycrystalline graphite, Nucl. Instr. and Meth. B (2011), doi:10.1016/j.nimb.2010.12.030


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V.S. Avilkina et al. / Nuclear Instruments and Methods in Physics Research B xxx (2011) xxx-xxx [4] A.M. Borisov, E.S. Mashkova, A.S. Nemov, E.S. Parilis, Nucl. Instr. and Meth. B 230 (2005) 443. [5] S. Cernusca, M. Fursatz, H.P. Winter, F. Aumayer, Europhys. Lett. 70 (6) (2005) 768. [6] A.M. Borisov, E.S. Mashkova, Nucl. Instr. and Meth. B 258 (2007) 109. [7] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, E.S. Parilis, E.A. Pitirimova, M.A. Timofeev, Vacuum 84 (2010) 1033. [8] E.S. Mashkova, V.A. Molchanov, Medium-energy Ion Reflection from Solids, North-Holland, Amsterdam, 1985. p. 444. [9] N.N. Andrianova, A.M. Borisov, E.S. Mashkova, E.S. Parilis, S. Yu, Nucl. Instr. and Meth. B 267 (2009) 2761. [10] P. Ehrhart, W. Schilling, H. Ullmaier, Radiation damage in crystals, Encyclopedia Applied Physics, vol. 15, VCH Publishers, Inc., 1996. p. 429. [11] Yu S. Virgil'ev, I.P. Kalyagina, Inorg. Mater. 40 (Suppl. 1) (2004) 33. [12] G.J. Dienes, G.B. Vineyard, Radiation Effects in Solids, Interscience Publishers Inc., New York, 1957. p. 243. [13] K. Niwase, T. Tanabe, J. Nucl. Mater. 179-181 (1991) 218. [14] A.E. Gorodetsky, A.V. Markin, V.N. Chernikov, A.P. Zakharov, T.A. Burtseva, I.V. Mazul, N.N. Shirkov, G.D. Tolstolutskaya, V.F. Rybalko, Fusion Eng. Des. 43 (1998) 129. [15] P. Sigmund, Appl. Phys. Lett. 14 (1969) 114. [16] N.N. Andrianova, A.M. Borisov, J. Surf. Invest. X-ray Synchrotron Neutron Tech. 2 (2) (2008) 189. [17] R. Behrish, Sputtering by Particle Bombardment I, Springer, Berlin, 1981. p. 336. [18] Y. Kido, J. Kawamoto, Appl. Phys. Lett. 48 (3) (1986) 257. [19] W. Schilling, H. Ullmaier, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Material Science and Technology, VCH Verlagesgesellschaft mbH, 1994, p. 241 (Chapter 9).

the damage annealing temperature Ta for 30 keV ion energy to temperature independent curves as ion energy decreases. The calculations using erf-like dependence of amorphous fraction versus the level of radiation damage m (dpa) made it possible to find the threshold value md when the graphite lattice is virtually not disordered under Ar+ and N þ ion irradiation at room tempera2 ture. It results in the disappearance of step-like behavior of electron yield temperature dependence with ion energy decreasing. Molecular nitrogen ion irradiation causes a stronger graphite lattice disordering than argon ion irradiation does. Acknowledgement This work is supported by the Ministry of Education and Science of the Russian Federation (Contract No. 02.740.11.0389). References
[1] E.S. Parilis, L.M. Kishinevsky, N.Yu. Turaev, B.E. Baklitzky, F.F. Umarov, V.Kh. Verleger, S.L. Nizhnaya, I.S. Bitensky, Atomic Collisions on Solid Surfaces, Amsterdam, North-Holland, 1993. p. 663 (Chapter 11). [2] L.S. Novikov, M.I. Panasyuk, E.N. Voronina, AIP Conf. Proc. 1087 (2009) 637. [3] A.M. Borisov, E.S. Mashkova, A.S. Nemov, E.S. Parilis, Vacuum 80 (2005) 295.

Please cite this article in press as: V.S. Avilkina et al., Energy and temperature dependences of ion-induced electron emission from polycrystalline graphite, Nucl. Instr. and Meth. B (2011), doi:10.1016/j.nimb.2010.12.030