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Nuclear Instruments and Methods in Physics Research B 212 (2003) 164­168 www.elsevier.com/locate/nimb

Modification of graphite surface layers by nitrogen ion irradiation
L.D. Bogomolova a, A.M. Borisov a, V.A. Kurnaev b, E.S. Mashkova
a

a,*

Institute of Nuclear Physics, Moscow State University, 119992 Moscow, Russia b Moscow Engineering and Physics Institute, 115409 Moscow, Russia

Abstract To study the modified surface layer of polycrystalline graphite MPG-8 (produced by NIIGraphite, Moscow, Russia) under high-fluence irradiation by 30 keV N× ions, electron paramagnetic resonance (EPR) spectra of surface layer and 2 the dependence of the ion­electron emission yield c on the target temperature have been measured. The dependence of cÏT ÷ manifests a step-like behaviour at the temperature Ta , which is typical of the radiation-induced phase transition. The EPR analysis shows that below Ta , which corresponds to annealing of radiation damage created by ion bombardment, paramagnetic defects are typical of carbon as well as defects connected with bonding of the carbon atom with three 14 N nuclei. Above Ta , the defects are typical of graphite-like structures. The results for MPG-8 are compared with those for MPG-LT and POCO-AXF-5Q graphites published earlier. ñ 2003 Elsevier B.V. All rights reserved.
Keywords: High-dose ion bombardment; Ion­electron emission; Electron paramagnetic resonance

1. Introduction The last few years, CNx compounds have received considerable attention connected with attempts to synthesize the crystalline b-C3 N4 , which, as has been shown theoretically, would have mechanical properties, better than those of diamond [1]. The incorporation of nitrogen into carbon materials has been performed by various methods; and ion implantation is one of them. The nature of base carbon materials, the temperature during implantation, and the post-implantation annealing influence the formation of a modified surface layer [2­6]. Previously, we have performed numerous
Corresponding author. Tel.: +7-95-9393-904; fax: +7-959390-896. E-mail address: esm@anna19.npi.msu.su (E.S. Mashkova).
*

experiments and computer simulations on nitrogen ion irradiation of graphites. The results devoted to sputtering, ion-induced electron emission, surface topography, elemental composition and crystalline structure of the modified surface layer obtained in steady-state conditions are described in [7­12] for high-fluence irradiation. Among various techniques giving information on the damage produced by ion bombardment, the electron paramagnetic resonance (EPR) is one used to identify the nature of defects on the atomic scale, their local environment, and chemical bonds with the nearest atoms. The EPR is a powerful tool for identification of point electron defects in single crystal, polycrystalline and amorphous materials. Extensive EPR studies being performed for last two decades allowed identifying the point defects responsible for physical properties of a-SiNx films

0168-583X/$ - see front matter ñ 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01730-0


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[13­16]. This method is expected to give useful information also for carbon nitrides. The EPR has been applied to carbon nitrides not much. The only defects found in a-CNx films, were those typical for a-C films containing no nitrogen [17­ 19]. EPR was used in our previous works [9,12] to obtain information on modified surface layers in fine-grain polycrystalline graphites (MPG-LT and POCO-AXF-5Q) at high-fluence 30 keV N× ion 2 bombardment. Also, deposited films were investigated. Hyperfine structure (HFS) in EPR spectra, which was formed due to interaction of magnetic moment of an unpaired electron localized on carbon with nucleus magnetic moments of nitrogen, has been found. This HFS has given the direct evidence that one carbon atom is bounded with one, two and three nitrogen atoms. The aim of this work is to investigate the modified surface layer of irradiated graphite MPG-8 (NIIGraphite, Moscow) subjected to highfluence 30 keV N× ion bombardment at various 2 target temperatures by using various techniques: EPR, reflection high energy electron diffraction (RHEED), RBS and ion­electron emission.

2. Experimental The ion irradiation was performed using the Mass-monochromator of the Institute for Nuclear Physics of Moscow State University [20]. An experimental set-up is described in details in [21]. The samples of MPG-8 with q ¼ 1:62 g/cm3 were mechanically polished, cleaned and washed in an ultrasonic bath in acetone and ethanol, and finally annealed in vacuum. The irradiation was performed mainly with 30 keV N× ions at normal 2 incidence. Ion current density was 0.1­0.3 mA/ cm2 . All the measurements were made under dynamically steady-state conditions. The same conditions were kept in our previous investigations of N× ion interactions with graphites (angular and 2 temperature dependences of the sputtering yield Y , ion­electron emission coefficient c, RBS profiles, and surface topography). The steady state is characterized by the stabilization of all the parameters mentioned. It occurs at fluencies of about

1019 N/cm2 . The SEM studies have shown that the initial surface topography (the pores and the flakes between them are typical) is transformed after the high-fluence irradiation. Namely, the surface roughness increases, pore dimensions increase, protuberances between the pores (the column structures with cone-like tops) rise, and column axes align along the ion beam direction as the erosion theory predicts. Typical examples were given in Fig. 2 in [11]. The ion-induced electron emission coefficient c was determined as the ratio of the secondary electron current to the incident ion current. The apparatus error was 2%. Both before and after ion irradiation the samples were analysed using RHEED, RBS (1.5 MeV 4 He× ions) and EPR. In the EPR measurements we investigated fine powders (named below ``scraps'') peeled by scratching the surface of the graphite sample with a steel pencil. The EPR spectra were recorded at room temperature (RT) and at 77 K on the modified RE-1306 spectrometer working at X-band frequency (9.5 GHz) with 100 kHz modulations. The g-values for defects in the studied samples were measured with respect to the signal of Mn2× ions in MgO powder, which was preliminary calibrated at the Russian Institute of Standards. The reference MgO samples were pressed in capsules and were placed in the spectrometer together with a powder sample under study. The EPR spectral parameters were determined by comparing the experimental and calculated spectra. The method of calculations of the spectra was described in [12].

3. Results and discussion 3.1. RBS, RHEED and ion-induced electron emission data The elemental composition of the modified layer of the irradiated sample was obtained from RBS spectra analysis. Typical spectra are shown in Fig. 1. One can see that nitrogen and oxygen are found in the surface layer of the sample irradiated near RT. The simulation of the RBS spectra has shown that the spectra can be reasonably described under the assumption that the composition


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4.5

4000

non- irradiated ion- irradiated at ~RT ion- irradiated at ~HT

2000

(electrons/mol.ion)

Yield

4.0

MPG-LT POCO

N O
0 50 75 100 125

Ta

MPG-8

Channel
Fig. 1. RBS spectra of 1.5 MeV He× ions for MPG-8 before and after irradiation by 30 keV N× ions at 60 and 370 °C. 2
3.5 100 200 300
O

400

500

temperature T ( C)
Fig. 2. The temperature dependence of c for MPG-LT [11], POCO-AXF-5Q [12] and MPG-8 under 30 keV N× ion irradi2 ation.

of the modified layer is C:N:O ¼ 76:20:4 and the width is 54 nm. The latter corresponds to width of 15 keV N× stopping profile in carbon according to the TRIM-SP simulation [11]. For the samples irradiated at elevated temperatures of 250­400 °C, the surface O-peak practically disappears. Nitrogen concentration becomes 2 times lower and penetration depth 2 times larger. Oxygen exists in graphites as a volume impurity, and its concentration in MPG-8 is 1.5 at.%. At high temperature (HT) implantation, O-concentration is practically the same. The studies of the surface layers by RHEED have shown that ion irradiation at RT leads to disordering of the initial graphite lattice, while the RHEED patterns at HT are close to those of parent graphite. The same effect was also observed at high-fluence Ar× ion bombardment. It is known that ion­solid interaction processes are sensitive to phase transitions in solids [20,22]. In particular, ion­electron emission yield c changes if the semiconductor surface suffers the phase transition from the crystal to the amorphous state, as well as if polymorphous transformations in solids go under ion bombardment [22]. The dependence of c on the target temperature was used to trace the radiation-induced transformations in the surface layer. It was found that c increases stepwise in a relatively narrow temperature interval around Ta , where Ta corresponds to the annealing temperature of the ion-induced radiation damage (see Fig. 2). Similar measurements were

performed also with 30 keV Ar× ions. It has been found that cÏT ÷ dependence is analogous to that at 30 keV N× bombardment [11,12]. This proves that 2 the temperature behaviour of c is not only due to release of the implanted nitrogen and inherent oxygen with increase of the target temperature. It should be noted that bombardment at elevated temperatures gives similar c-values for various graphites (MPG-LT, MPG-8 and POCO-AXF5Q), see Fig. 2; whereas at T < Ta they are different being in the following relation: cÏMPG-8÷ < cÏPOCO÷ < cÏMPG-LT÷. This is possibly connected both with different O-concentrations and with presence of various C­N-compounds created in different graphites. Taking these results into account, the EPR studies were fulfilled at the temperatures both below and above the c-jump temperature Ta . 3.2. EPR data Fig. 3 shows the EPR spectra of non-irradiated MPG-8 graphite and scraps obtained after highfluence 30 keV N× ion irradiation at RT and HT. 2 The EPR spectra of non-irradiated MPG-8 (and MPG-LT and POCO studied previously [9,12]) are similar and contain a slightly asymmetric single line with the base-crossing g ¼ 2:010 ± 0:001 and peak-to-peak line width DHpp ¼ 2:01 ± 0:02 mT.


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Fig. 3. EPR spectra of non-irradiated MPG-8 (1), ion-irradiated one at T ¼ 370 °C (2) and at T ¼ 60 °C experimental (3) and simulated (4) spectra. The inset shows the components, from which the simulated spectrum has been constructed.

The EPR spectrum of MPG-8 irradiated at T ¼ 380 °C is identical to the spectra of non-irradiated graphites. EPR spectrum for MPG-8 irradiated at RT is essentially different from that obtained at the elevated temperature. Really, there are additional lines that indicate the presence of HFS arising because of the interaction of the unpaired electron spin with nuclear magnetic moments. As it is known, 14 N have the spin equal to 1. In this case, interaction of the unpaired electron even with one nitrogen nucleus can lead to splitting of the line into three components. If unpaired electron interacts with two or three nitrogen atoms the number of lines increases depending on the number of neighbour nitrogen atoms and their positions. This gives an opportunity to estimate the number of nitrogen atoms bonded to carbon atoms after ion irradiation. The simulation of EPR spectrum for MPG-8 is presented in Fig. 3. One can see that in this case HFS is determined by interaction of the unpaired electron located in the carbon atom with three 14 N nuclei. Comparison of the experimental data with simulated ones shows their good agreement. The inset in Fig. 3 demonstrates the components of the simulated spectrum. One of them is a wide single line with g ¼ 2:0029. The second calculated spectrum is a seven component HFS at g ¼ 2:0032 and HFS constant A ¼ 0:49 mT. The intensity ratios of the compo-

nent is 1:3:6:7:6:3:1. This indicates the interaction of the unpaired carbon electron with three equivalent 14 N nuclei. Similar HFS was formerly observed for POCO-AXF-5Q irradiated at RT [12]. At RT implantation both MPG-8 and POCOAXF-5Q contain carbon bonds with three nitrogen atoms in the modified surface layers. Similar HFS with g ¼ 2:0033 was observed, for example, å for the radical C(CN)3 in some compounds [23]. å The value of HFS indicates direct C­N3 bonding, i.e. formation of a K-like centre similar to that in Si3 N4 [13­16] for which A ¼ 0:47 mT. The ratio of the number of paramagnetic centres contributing to HFS to the number of defects responsible for the line with g ¼ 2:0029 is about 3% for MPG-8. It should be noted that the EPR data were obtained with powder collected after scratching the surface. It should also be noted that we observed an average effect and we do not know the distribution of defects in the surface layer. One can assume that the difference of points defects created by nitrogen irradiation at room and elevated temperatures is due to different conditions of formation of paramagnetic defects (a disordered surface layer near RT and a crystalline graphite phases, which is similar to the structure of unirradiated graphites, at HTs).

4. Conclusion Surface layers of MPG-8 graphite modified by high-fluence 30 keV N× ion irradiation have been 2 studied. The implanted N concentration has been found to be 20 at.% at RT and approximately twice less at elevated temperatures (>250 °C). The dependence of the ion-induced electron emission coefficient c on the target temperature exhibits a step-like behaviour at T ¼ Ta typical of damage annealing that agrees with RHEED data. The EPR spectra analysis has shown that defects connected with C­N bonds are formed due to irradiation at T < Ta in addition to the defects typical of carbon. It is possible that the defects analogous to K-type centres in Si3 N4 appear. The HFS observed for MPG-8 and POCO-AXF-5Q graphites have seven components; and this is associated with the interaction of the unpaired


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electron located in carbon atom with three identical 14 N nuclei. At elevated temperatures (T > Ta ) the defects appertained mainly to graphite-like structures are formed.

Acknowledgements The work was sponsored by the Ministry of Education and the Ministry of Science and Technologies of the Russian Federation. The authors are grateful to Dr. V.S. Kulikauskas for RBS analysis and to Dr. E.A. Pitirimova for RHEED.

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