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Nuclear Instruments and Methods in Physics Research B 230 (2005) 443-448 www.elsevier.com/locate/nimb

Ion-induced electron emission - monitoring the structure transitions in graphite
A.M. Borisov a, E.S. Mashkova
a

a,*

, A.S. Nemov a, E.S. Parilis

b

Institute of Nuclear Physics, Moscow State University, Leninsky Gori, 119992 Moscow, Russia b California Institute of Technology, 200-36, Pasadena, CA 91125, USA

Abstract The temperature dependence of ion-induced electron emission yield c under 30 keV Ar+ ion impacts at incidence angles h = 0À80œ under dynamically steady-state conditions has been measured for polygranular graphite POCOAXF-5Q. The fluencies were 1018-1019 ion/cm2, the temperatures varied from the room temperature (RT) to 400 œC. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence c(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (h > 30œ), a broad peak was found at Tp = 100 œC. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of c(h,T) to the dependence of both secondary electron path length k and primary ion ionizing path length Re on lattice structure that drastically changes due to damage annealing. Ó 2004 Elsevier B.V. All rights reserved.
PACS: 34.50.Dy; 79.20.Rf Keywords: High-dose ion irradiation; Ion induced electron emission; Radiation damage in solids

1. Introduction Intensive ion bombardment of solids often results in creation of a modified surface layer [1,2].

Corresponding author. Tel.: +7 95 9393 904; fax: +7 95 9390 896. E-mail address: esm@anna19.npi.msu.su (E.S. Mashkova).

*

In particular, radiation damage creation in solids may result in a partial or total disordering of the surface layers of semiconductors and carbonbased materials [1-4]. It is known that many processes of ion interaction with solids are sensitive to the degree of order in the solid [1,5-8]. In our previous works [9-14], where the irradiated surface layers of some polygranular graphites were analyzed, the temperature dependencies of electron

0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.12.081


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emission yield c were measured both at normal and oblique incidence for 30 keV Nþ ion beam at 2 high fluencies (P1018 ion/cm2). It has been found that the c(T) dependencies in the temperature range from room temperatures to $500 œC show a strongly nonmonotonic (step-like) behavior. The changes of c with temperature and ion incidence angle were connected to both secondary electron path length k and the escape probabilities w dependencies on lattice structure changes. The aim of the present work was to study the dependencies of the electron emission yield from POCO-AXF-5Q graphite on irradiation temperature in wide range of ion incidence angles, ion-induced surface topography, crystalline structure of the irradiated surface layers under high-fluence (1018-1019 ion/cm2) irradiation by 30 keV Ar+ ions. The noble gas ions were used to exclude the possible chemical effects connected with nitrogen-carbon interaction [12,13].

3. Results and discussion Before temperature scans c(T) have been made at any chosen ion incidence angle, the dependence of electron yield on fluence c(F) at RT has been measured till the c-stabilization occurred. Usually the electron yield went down during irradiation until the c-stabilization at steady-state conditions at F P 1018 ion/cm2. Total fluence during temperature scan was $1019 ion/cm2. The dependence of electron emission yield on target (POCO-AXF-5Q) temperature at normal and near normal 30 keV Ar+ ion incidence manifests a step-like increase (see Fig. 1) analogous to

3.4

=0

o

30 keV Ar + POCO-AXF-5Q

3.2

, el./ion

Ta

2. Experimental The experiment was performed using the massmonochromator of the Institute of Nuclear Physics, Moscow State University [3]. The experimental procedure was described elsewhere [14,15]. The samples used in this work were cut from massive pieces of POCO-AXF-5Q graphite (USA production) with the density 1.82 g/cm3. The target irradiation was carried out with 30 keV Ar+ions. The total ion current was 0.1-0.2 mA; the cross-section of the ion beam was 0.35 cm2. Ion fluencies were 1018-1019 ion/cm2. The experiments were performed under HV conditions at the working pressure during the irradiation process better then 7 ž 10À7 mbar. The ion induced electron emission yield c was determined as the ratio of the electron current to the primary ion current. For comparison, some measurements of electron emission yield of polycrystalline Cu samples were also performed. Before and after irradiation the samples were analysed by scanning electron microscopy (SEM). Crystalline structure of the surface layers was analysed by a RHEED device EMR-102 (Russian model) operated at 50 kV and electron beam current 50 lA.

3.0
Tp

2.8 2.2 2.1 0 4.05 4.00 3.95 3.90 = 40
o

Cu

50

100

150

200

250

300

350

, el./ion

3.85 3.80 3.75 3.70 3.65 3.60 0 50 100 150 200
o

Ta

Tp

250

300

350

temperature T C

Fig. 1. The temperature dependencies of c for POCO-AXF-5Q graphite and for polycrystalline copper under 30 keV Ar+ ion irradiation at h = 0œ, 40œ.


A.M. Borisov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 443-448

445

the c(T) dependencies measured previously for MPG-LT, MPG-8 and POCO-AXF-5Q under 30 keV Nþ ion bombardment [9-14]. This temper2 ature c behaviour, as we pointed earlier, is similar to the typical defect annealing dependencies, cf. [16]. The analysis of c(T) dependencies show that they are transformed with ion incident angle increase. It has been found that in contrary to the normal ion incidence, when c is virtually constant at T > Ta %175 œC and c(T > Ta) > c(T < Ta), an increase of h results in essential changes of c(T), see Fig. 2. Namely, a relative rise of c is observed in the middle temperature (MT) range (RT < T < Ta), as the target temperature rises, c passes through a maximum (a hump) at Tp = 100 œC. The hump height increases as h rises. After T P 190 œC the yield c increases till h % 70œ having a saturation tendency with temperature rise, and the electron yield is still higher than at RT. After this, the value of c at elevated temperatures gradually approaches to the one at room temperatures. At grazing incidence the difference in electron yield outside the ``hump'' decreases. One may suppose that this transformation of the temperature dependencies reflects a complex annealing of the different kinds of radiation defects in graphite, a decreasing of the number of displacements per lattice atom (taking into account the target sputtering) as ion incident angle increases, and an alteration of the steady-state surface topography as ion incidence angle and irradiation temperature are varied. The RHEED analysis of the samples showed a difference of surface layer structure at different target temperatures. Before irradiation the RHEED pattern demonstrates some rings typical for polycrystalline graphite. Ion irradiation at RT and MT results in an appearance of a diffuse halo typical for disordered surfaces. The diffraction patterns after ion irradiation at elevated temperatures (T > Ta) are different both from the RHEED pattern before irradiation and from the ones taken when ion irradiation has been produced at T < Ta Namely, some slightly smeared polycrystalline diffraction rings are observed, cf. [17]. The SEM studies have shown that after high fluence irradiation the initial surface topography

4.70 4.65 4.60 4.55

50

o

, el./ion

4.50 4.45 4.40 4.35 4.30 4.25 5.60 5.55 5.50 5.45 60
o

Tp

, el./ion

5.40 5.35 5.30 5.25 5.20 5.15 7.5 7.4 7.3 7.2 70
o

Tp

Tp

, el./ion

7.1 7.0 6.9 6.8 6.2 6.0 5.8 Cu

11.8 11.6 11.4

80

o

Tp

, el./ion

11.2 11.0 10.8 10.6 0 50 100 150 200 250
o

300

350

temperature T C

Fig. 2. The temperature dependencies of c for POCO-AXF-5Q graphite and under 30 keV Ar+ ion irradiation at angles of incidence h = 50œ, 60œ, 70œ and 80œ. For comparison at h = 70œ the data for polycrystalline copper are presented.


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(some flakes and pores between them) is transformed. At near room temperatures some craters (the lm-dimensions) arise with rather smooth walls. Ion irradiation at middle temperatures (RT < T < Ta) induces some pits with cliff-like walls and relative flat ripples. Ion irradiation at elevated temperatures (T > Ta) results in a corrugated relief with repetitive ridges of nanometric scale. According to the Parilis-Kishinevsky theory of kinetic electron emission [5] the secondary electron yield for not too grazing incidence c ? qre kw= cos h; ð 1Þ where q is the atomic density of the target, k is the secondary electron path length, 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 equal to re ðtÞ ? 1:16ao hJ À1 ?ðZ 1 þ Z 2 ÞðZ
1=2 1 À7

the target atoms. The decrease of the ion velocity follows a simple law t2 À t2 ? kl, where tl is the l velocity at the distance l and k ? 2:48pqao e2 Z 1 Z 2 = 1=2 1=2 2=3 ?ðM 1 þ M 2 ÞðZ 1 þ Z 2 Þ [5]. Then c may be expressed as Z Re ðt0 Þ c? qre ðtl Þw expðÀl cos h=kÞ dl: ð 3Þ
0

The analysis of the angular dependence of this integral shows that at oblique incidence, above a certain angle hc determined by the inequality Recoshc 6 k, the electron yield c(h) virtually reaches its limiting value clim = q rewRe that does not depend on h. To demonstrate this behavior using some simple formulas, can take an approximation for the velocity dependence of the cross-section as re ðtÞ $ t2 À t2 ; o then its reduction along the path l is ð 4Þ

þZ

1=2 À1 2 2Þ

t ð 2Þ

 arctan?0:6ðt À to Þ10 ;

re ðtl Þ ? re ðtÞð1 À l=Re Þ; where Re ? ðt2 À t2 Þ=k : o

ð 5Þ

where J is the ionisation energy, t is the projectile velocity, to is the threshold velocity, ao is the Bohr radius, Z1 and Z2 are the atomic numbers of the projectile and target atom respectively. It should be noted that the theory was developed for polycrystalline metals that were usually considered as randomly packed solids and the problem of radiation damage influence has not been discussed. As far as we know, the effects of radiation damage on electron emission under ion bombardment were discussed for the semiconductor crystals in connection with disappearing of the electron emission anisotropy under high dose ion bombardment [5-8,18]. As to graphite it is known that the physical properties are drastically altered by radiation damage, see for example [4,16]. We suppose that the changes in crystal perfection and size of the crystallites in graphite may influence the electron path length k and Re. Let Re(to) be the mean path length at which an ion is slowed down to the threshold velocity to due to energy loss along its path l, and Re cos h is the depth at which it still retains the power to ionise

ð 6Þ

By substituting Eq. (6) into the Eq. (3), we get c ? clim k=ðRe cos hÞ Â?1 À k=ðRe cosÞð1 À expðÀRe cos h=kÞÞ;

ð 7Þ

which at normal and near normal incidence depends mainly on k. At oblique and grazing ion incidence c is determined by k=ðRe cos hÞ. When approaching the sliding incidence, the grazing angle cannot become exactly zero due to roughness of the surface. There is a distribution of the surface areas over the roughness angles. The effect of roughness is more pronounced at large angels, than at small ones, where the functions of h are very slow. As a first approximation, it could be taken into account by introducing a mean roughness angle hr that has to be subtracted from the nominal angle of incidence h. Then c cos h=c
lim

? k=Re ?1 À k=ðRe cosðh À hr ÞÞ Âð1 À expðÀRe cosðh À hr Þ=kÞÞ: ð8Þ


A.M. Borisov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 443-448
30 keV Ar+ - POCO-AXF-5Q
3.6 3.2 2.8
/R e = 0.39 /R e = 0.36 /R e = 0.35 calculations for rough surface

447

cos

2.4 2.0 1.6 1.2 0.8 0

experiment 25 C 100 C 300 C
o o o

calculation for smooth surface /Re = 0.35

15

30

45

60

75

90

angle of incidence (deg.)
Fig. 3. The angular dependencies of c cos h: the experimental data and the calculated curves.

Fig. 3 depicts the curves ccos h versus h calculated using the Eqs. (7) and (8), i.e. without and with taking into account the surface roughness. It can be seen that the latter with hr = 12œ gives a much better fit to the experimental points. The curves for three temperatures 25, 100 and 300 œC correspond to the values of the ratios k/Re = 0.35, 0.36 and 0.39, and hc = 69.5œ, 68.9œ and 67œ, respectively, which are rather close. They agree with the value of k for graphite [8] and the estimates of Re made using SRIM code. One can see that at relatively large h the curves for RT and HT virtually merge. The MT experimental curve passes between the RT and HT curves, then intersects them and at grazing ion incidence cMT > cRT > cHT cf. Fig. 2. Comparing the experimentally obtained angular dependencies with those calculated one may conclude that the jump of at T = Ta (see Fig. 1) is due to the increase of secondary electron path length k, if Re is assumed constant. The secondary electrons before their escape from solid experience flux attenuation due to multiple scattering on both the lattice and impurity atoms, the implanted particles, and the radiation defects. The electron path length k = (qr)À1 is determined by the electron flux attenuation cross section r. The polycrystalline metals, as well as the polygranular graphite with relatively large-size grains may be considered as

some sets of single crystals. The cross-section rcr for the single crystal is known to be smaller than the cross-section ram of electron flux attenuation in randomly packed amorphous solids [19]. It should be noted that a similar situation was discussed in connection with the interpretation of track formation in amorphous metals under swift heavy ion bombardment [20,21]. As it was pointed above the radiation damage annealing in graphite results in transition of disordered surface layer at near RT ion irradiation into relatively ordered structure at T > Ta. It results in turn in an increase of c at T = Ta due to a decrease of electron flux attenuation cross-section in the graphite lattice. If not only k but Re changes as well, the situation becomes more complicated. In particular, if at MT k increases only slightly, whereas Re rises essentially, the features at oblique and grazing angles (the hump at Tp = 100 œC) appear more and more clearly. Besides, as the angle of incidence increases, the reduction of the number of displacements per atom (dpa) influences the temperature dependence of c. Indeed, some simple estimates of the function dpa(h), taking into account the angular dependence of the sputtering yield [15], show a decrease in dpa by an order of magnitude within the studied range of h. This leads to a shift of the borders on the temperature scale between different structure transitions with the angle of incidence changing. A change in the defect annealing conditions in the basal planes and between them is also possible [22].

4. Conclusion The temperature dependence of ion-induced electron emission yield c under 30 keV Ar+ ion impacts at incidence angles h = 0œ-80œ under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), the patterns have been found similar to those for the initial graphite surfaces.


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A.M. Borisov et al. / Nucl. Instr. and Meth. in Phys. Res. B 230 (2005) 443-448 [4] T.D. Burshell, MRS Bull. 22 (4) (1997) 29. [5] 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, Elsevier, Amsterdam, 1993 (Chapter 11). [6] I.N. Evdokimov, E.S. Mashkova, V.A. Molchanov, Phys. Lett. 8 (1967) 619. [7] I.N. Evdokimov, I.M. Fayazov, E.S. Mashkova, V.A. Molchanov, V.A. Snisar, Radiat. Eff. Def. Solids 112 (1990) 221. [8] B.A. Brusilovsky, Appl. Phys. A 50 (1990) 111. [9] A.M. Borisov, V.S. Kulikauskas, E.S. Mashkova, A.V. Safronov, Poverkhnost 8 (2001) 59 (in Russian). [10] L.D. Bogomolova, A.M. Borisov, N.A. KrasilÕnikova, V.S. Kulikauskas, E.S. Mashkova, W. Eckstein, Izvestija Akad. Nauk, Ser. Fis. 66 (2002) 551 (in Russian). [11] A.M. Borisov, W. Eckstein, E.S. Mashkova, J. Nucl. Mater. 304 (2002) 15. [12] L.D. Bogomolova, A.M. Borisov, N.A. KrasilÕnikova, E.S. Mashkova, A.S. Nemov, V.V. Tarasova, Radiat. Eff. Def. Solids 157 (2002) 493. [13] L.D. Bogomolova, A.M. Borisov, V.A. Kurnaev, E.S. Mashkova, Nucl. Instr. and Meth. B 212 (2003) 164. [14] A.M. Borisov, E.S. Mashkova, A.S. Nemov, Vacuum 73 (2004) 65. [15] E.S. Mashkova, B.A. Molchanov, I.M. Fayazov, W. Eckstein, Poverkhnost 2 (1994) 33 (in Russian). [16] P. Ehrhart, W. Schilling, H. Ullmaier, Radiation damage in crystals, Encyclopedia of Applied Physics, Vol. 15, VCH Publishers, 1996, p. 429. [17] V.N. Chernikov, A.E. Gorodetsky, S.L. Kanashenko, A.P. Sakharov, W.R. Wampler, B.L. Doyle, J. Nucl. Mater. 220-222 (1995) 912. [18] Yu.V. Martynenko, Phys. Stat. Solidi 15 (1996) 767. [19] J.M. Ziman, Principles of the Theory of Solids, At the University Press, Cambridge, 1964. [20] Yu.N. Yavlinskii, Nucl. Instr. and Meth. B 146 (1998) 142. [21] Yu.N. Yavlinskii, Radiat. Eff. Def. Solids 153 (2000) 75. [22] K. Niwase, T. Tanabe, J. Nucl. Mater. 179-181 (1991) 218.

The dependence c(T) has been found to be nonmonotonic and for normal and near normal ion incidence manifests a step-like increase (at Ta % 175 œC) typical for a radiation induced phase transition. At oblique and grazing incidence, a broad peak was found at Tp = 100 œC. An analysis based on the theory of kinetic ioninduced electron emission connects the behavior of c(h,T) to the dependence of both secondary electron path length k and primary ion ionizing path length Re on lattice structure that changes due to damage annealing. Therefore, recording the angular and temperature dependencies of the ion-induced electron emission yields gives a possibility to monitor in situ the temperature-depending radiation induced structure changes and phase transitions in graphite. Acknowledgements The authors are grateful to E.A. Pitirimova for RHEED and to A.B. Pavolotzky for SEManalysis. References
[1] R. Behrisch (Ed.), Sputtering by Particle Bombardment I, Springer, Berlin, 1981. [2] J.S. Williams, J.M. Poate (Eds.), Ion Implantation and Beam Processing, Academic Press, New York, 1984. [3] E.S. Mashkova, V.A. Molchanov, Medium-energy Ion Reflection from Solids, North-Holland, Amsterdam, 1985.