Doping of fullerenes by ion implantation

Nuclear Instruments and Methods in Physics Research B80/81 tt993) 1456-1459 North-Holland

HM 13

tlasm Intaractlona with Materials &Atoms

Doping of fullerenes by ion implantation J. Kastner and H. Kuzmany

/nstiutt für Festkörperphysik, Unirersitüt Wien and Ludwig Boltzmann histitnt für Festkörperphysik, Strutllhofgasse 4, A-1090 Wien, Austria '

L. Palmetshofer and P. Bauer

Institut für Experimentalphysik, Johannes Kepler Universität, A-4040 Linz. Austria

G. Stingeder lnstitrr'

('Innei'. 7i~elm(sclr<~ Universität Wien, Getreidemarkt 9, A-1060 Wien, Austria

- ` at target temperatures up to C cm (wj films have been implanted with 30 keV K' ions with doses from I x 1014 to 1 x 10 1 35û"C . Cunduciiviiy measurements performed in situ show a decrease in sheet resistivity with increasing dose for fullerene samples implanted at elevated temperatures, whereas implantation a6 room temperature results in no change of resistivity . After exposing the samples to air the resistivity remains constant over weeks. Ramian scattering showed that an amorphous surface layer is formed by the implantation process and that the fullerene molecules beyond this layer remain undestroyed. Secondary ion mass spectrometry and Ruthertord backscattering indicate that at elevated implant temperatures K diffuses into deeper regions of the film whereas oxygen is present only in a surface layer about 10 nm thick . 1 . Introducîïon The discovery of superconductivity at 18 K of K,C,, has stimulated many investigations on doping of fullerenes (1). There are several reports on the formation of fullerides with alkali and alkali earth metals i2-4] . A critical issue related to the application of the alkali metal fullerides is the instability when exposed to the atmosphere [3] . Up to not doping from lcc gas phase and annealing of fullerene powder with a certain content of a dopant in a closed tube were the only methods employed for the doping of fullerenes . In this paper we present for the first time alkali metal doping of fullerenes by ion implantation . Inplanted samples were characterized by in situ conductivity measurements, Raman scattering, secondary ion mass spectrometry (SIMS) and Rutherford backscattering spectrometry (RBS). 2. Experimental Thin films of C . were prepared by evaporation of ufified noaterial on undopcd silicon and quartz substrates in a vacuum of 2 x 10 -4 Pa for several hours. A typical film thickness was 300 nm. Ion implantation of ° K and 4OAr was performed at room temperature and 0168-583X/93/$06.00 © 1993 - Elsevier Science

at elevated temperatures up to 350°C; in a vacuuni better than 10 -4 Pa . The ion energy of 30 keV was chosen to keep the thickness of the damaged region rather low . Doses were in the range of 1 x 10 14 to I x 10 1° cm -z. samples for in situ conductivity measurements had a geometry of 2 x 10 mm with four evaporated gold contacts for four-point measurements. A Dfo; XY spectrometer was employed for Raman scattering. The 514 nm spectral line of an Ar+ laser was used to excite the sample, the spectrum was measured with a liquid nitrogen cooled CCD detector. SIMS depth profiles were recorded on a Camcca IMS 3f with 5.5 keV O~ as the primary ions . The crater depth was measured with an optical profilometer to transform the sputter time into a depth scale . Different sputter rates caused by the variation of the K concentration or the density and possible matrix effects were not taken into consideration. The RBS investigations were carried out widt an analyzing beam of 400 keV He' using a Standard RBS arrangenwai. 3. Results and discussion The sheet resistivity (ps) of samples implanted with K + at 30 keV and at different temperatures (room temperature and 30(rC) is shown. in fig. 1 as a function

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!. Kasrner et al. / Doping of fallerems

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Ê

N

in W uj W W z N

(Si0?)

0

10" n. i.

104

15

10

.

1'0

I 16

-2 ) DOSE (CM Fig. 1 . Sheet resistivity :is a function of the dose for C,, lilms implanted with 30 keV K' at room temperature (upper curve) and at 31H1'C (lower curve).

of the dose . Room temperature implantation results in no change of the sheet resistivity . The resistance of the nonimplamcd film on silicon is entirely due to the silicon substrate, which had a resistivity of about 2000 il cm. Since the resistivity of Si at 300°C is low, we used films on quartz suhsuatcs for in situ resistivity measurements at elevated temperatures. If the implan-

tation is perform-d at 30t)°C the sheet resistivity decreases monotonically with the dose . At a dose of 1 x 1(1 16 cm -2 p, is about 600 12/0 . The temperature dependence of the sheet resistivity is small ; p, increases about 10% during cooling of the sample from 300'C to room temperature. The sheet resistivity measured at room temperature

after implantation of 30 keV K+ in CM, on Si with a constant dose (if- I x 10 16 cm -` is plotted in fig. 2 as a function of the implantation temperature. The sheet

E( + 30 keV 16 cm -2

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i5 Oû

RAMAN SHIFT (CMFig. 3. Raman spectra of an unimplanted C,, film and of Films implanted with K' (30 keV, 1 X10" cm -2 ) at room temperature and at 3110°C. resistivity decreases strongly when the implant temperature is increased from room temperature to 300°C. A further increase of the implantation temperature to 350'C results in considerable eväpo, ::tion of the fullerene film during the implantation process. Up to an implant temperature of 300°C no material lo; s was detected. In contrast to fullcrides doped by conventional methods[3] the resistivity of our films remained almost unchanged when exposed to air. After several weeks the resistivity increased only a few percent. Conventionally doped fullerenes lose their metallic conductivity immediately when exposed to air since oxygen with-

draws potassium atoms from the fulleride to form KO, or K,O, on the surface [3J. In order to elucidate this surprising behaviour, various experimental methods were employed to study the

structure, the K distribution and the oxygen content of our films. The structure of the implanted films was studied by Raman scattering, which is very sensitive to changes in the microscopic structure . Fig. 3 shows the Raman spectra of an unimplanted C~ film and of films implanted at room temperature and at 300°C with K+ (30 keV, 1 x 14 16 em -2 ) . The Raman spectrum of the unimpianted sample shows two strong lines at 495 and 1467 cm -1 and eight weaker lines [2?.

w z N

5002

t 0

100

. 200

300

IMPLANT TEMPERATURE ('C) Fig. 2. Sheet resistivity measured at room temperature of C,, films after implantation of K' in C6 on silicon (30 keV, 1 x10 16 cm -2 ) as a function of the implantation temperature.

Implantation results in a decrease of the Raman lines of Ca and introduces a broad, asymmetric line between 1000 and 1700 cm - t. Thin films with a thickness below 80 nm exhibit only this broad asymmetric ieature

and no signal from the fullerene . The broad line is well known for amorphous carbon (a-C). Similar Raman spectra have been reported for different forms of a-C Vc. NOVEL TECHNIQUES (c)

1458 [5-71. Therefore, the Raman data lead to the conclusion that our implanted films consist of two layers. Within the range of the implanted atoms we have transformed the C 6o to a form of amorphous carbon, beyond this layer the fullerdne molecules remain undestroyed. A similar transformation of C 6 into amorphous carbon has been reported for proton irradiation tß1- . The a-C layer does not increase in thickness when the implantation temperature is raised . As can be seen from fig. 3 the ratio of the Raman intensities of C r, to a-C remain unaffected when the implant temperature is varied . The distribution of the implanted potassium atoms was investigated by SIMS measurements . Fig . 4 shows the depth profile of "K + for samples implanted with K (30 keV, 1 X 10' 6 cm -Z ) at different temperatures . For room temperature implantation, the experimental profile is in fairly good agreement with the theoretical profile calculated with TRIM [41 (except the tail at greater depths). An increase in the implant temperature leads to a broadening of the K profile. At 200°C the concentration in the tail region is almost as high as at the projected range . At 300°C the second peak is already larger than the concentration at the projected range . A comparison of the sheet concentrations (integra,ed profiles) shows that at high implantation temperatures the sheet concentration is smaller (200°C: 15%, 30(rC: 20% loss) as compared to the value obtained at room temperature implantation. Obviously, at high implant temperatures a certain amount of K dffusc out of the s?:rfacc during implantation . However, the diffusion into the depth is much more pronounced. At 300°C more than half of the implanted atoms diffuse out of the implanted region and accumulate at a depth of about 100 nm . The outdiffusion of K from the implanted region into the depth is strongly

Fig. 4. SIMS potassium profiles for Cm, films implanted with K' (30 keV, 1 X 10'6 cm -1) at different implantation temper atures. R, designates the projected range calculated will, TRIM .

CHANNEL NUMBER Fig. 5 . RBS spectrum of a C 6 film implanted with K' (30 x keV, 1 10 6 cm - =) at room temperature.

correlated with the decrease in the sheet resistivity (see fig. 2). As already mentioned, the strong affinity of alkali metals to oxygen usually destroys alkali metal fullerides immediately when exposed to air [31 . Since our implanted samples are very stable, the role of oxygen must be completely different . This has been studied by Rutherford backscattering spectrometry . The RBS spectrum of a C 6 film implanted with K' (30 keV, 1 X loll cm - `) at room temperature is shown in fig . 5 . With increasing channel number the strong signal of carbon, a very weak signal of oxygen and a broad ..+ignal of the implanted K atoms can be seer. . RBS measurements on samples implanted at elevated temperatures give similar results, only the K signal is broader . TI: ;; K signal of samples implanted at 200 and 300°C shows a double peak structure, which is in qualitative agreement with the SIMS results . The analysis of the oxygen peak shows that oxygen is present only in a very thin surface layer . The thickness of the layer is about 10 run, the oxygen concentration about 2 at.%. These results strongly indicate that the a-C layer built during implantation protests the underlying K fulleride effectively from oxidation . A critical issue for the doping of fullerides by ion implantation is the correct interpretation of the decrease in sheet resistivity with implant dose and temperature (figs. 1 and 2). It is well known that a-C shows resistivity values ranging from about 10"' to lo - ' fl cm depending on the preparation, impurity content and disorder [101. Ion implantation of various forms of a-C r:,suits in similar saturation values of the resistivity (p z 10 - ' 11 cm) [7). Only for McV implantation of Ar in polymers and various forms of a-C resistivity values as low as 5 X 10 - ° f cm have been reported [111. Therefore, it cannot be definitely excluded that the sheet conductivity of our implanted C 6 films is partly caused by the a-C surface layer.

J. Kasiner et al. / Doping of fullerenes

In order to obtain resistivity values of a-C layers prepared by ion implantation of fullerenes, Ar + implantation at different temperatures has been performed . Implantation (30 kcV, I x 10 11 cm -2 ) at room temperature led to a sheet resistivity of 200 kil/O, whereas for the implantation performed at 300°C a value of 1400 fl/0 was obtained. The latter value corresponds to a resistivity of 10 -2 IZ cm assuming a thickness of 80 nm for the amorphous layer. These results show that the a-C surface layer cannot be neglected for the determination of the resistivity of K implanted fullerenes . For samples implanted at 300°C the sheet resistivity of the buried fullerene layer alone is about 1100 fl/ 0 . This corresponds to a resistivity of 6 x 10 - ' f cm assuming a layer thickness of 50 nm . This resistivity value is surprisingly close to published values foi the resistivity of K,C (A) 13], although we K cencentration is about a factor 3 lower than the stG ichioinetric composition of K,1C., One possible explanation is that K IC6, ) is formed at elevated temperatures and precipitates to dispersed KsCnn clusters at room temperature. Details of the diffusion and thermodynamics of the doping process by ion implanation will be published elsewhere [121. 4. Conclusion We have shown that ion implantation is a very useful technique for the doping of fullerenes . Using proper implantation conditions, it is possible to obtain d highly conductive structure consisting of two layers. The surface layer is a-C, underneath is a doped fullcrene layer . The a-C layer protects the fullerene layer from oxidation . Therefore, the samples are very stable on air and can be studied easily by a v^riety of experimental methods .

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Acknowledgements This work was supported by the Osteuropa F6rderung des BMtWF project GZ 45 .212/2-27b91 . We thank K .H. Ablinger for performing the implantation, K . Piplits for assistance in SIMS measurements and Z. riastl for useful discussions .

References [1] A.F . ",-bard, M.J. Rosseinsky, R .C. Haddon, D.W . Murphy, S .H. Glarun,, T.M . Palstra, A .P. Ramirez and A .R. Kortan, Nature 350 `,1991) 600. [2] T. Pichler, M . Matus, J. Kürti and H. Kuzmany, Phys . Rev . B45 (1992) 13841 . 13] P .J . Benning, D.M . Poirier, T .R. Oho, Y . Chen, M .B . Jost, F. Stepniak, G .H . Kroll, J .H . Weaver, J . Hure and R .E . Smalley, ibid ., p. 6899. (41 Y . Chen, F. ; tepniak, J.H. Weaver, L .P. Chibante and R .E. Smalley, bid ., 8845. [51 B.S . Ellman, M.S . Lresselhaus, G . Dres~elhaus, E .W. Maby and M . Mazurek, Pbys . Rev. B24 (1981) 1027. [6] L. Calcagno at,d G . Foti, Nucl. Instr . and Meth . B59/60 (1991) 1153 . (71 S. Prawer, R . Kalish, M. Adel and V. Richter, J. Appl. Phys . 61 (1987 4492 . [8] R.G. Musket, R.A . Hawley-Fedder and W .L . Bell, Radiat. Eff. 118 (1991) 225 . [9] J .F. Ziegler, J.P . Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol . 1 (Pergamon, New York, 1985). [10] Th . Frauenheim, U. Stephan, K. Bewilogua, F. Junguk:kei, P . Blaudeck and E. Fromm, Thin Solid Films 182 (1989) 63 . III] T. Venkatesan, Nucl. Instr. and Meth . B7/8 (1985) 461 . [12] J. Kastner, L . Palmetshofer and H. Kuzmany, in preparation .

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