Ion implantation effects in laser-deposited amorphous carbon films

Thin Solid I:ihns, 193 194 (1990) 5X8 594

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ION I M P L A N T A T I O N E F F E C T S IN L A S E R - D E P O S I T E D A M O R P H O U S CARBON FILMS A. P. MALSHE, S. M. CHAUDIIARI, S. M. KANFTKAR AND S. B. (XiAI.['I

Department O[ Physics. Unive~'it.v o/" Poonu. Punt" 41 I007 I hldia,, S. T. KSHIRSAGAR

Natianal Chemical Laboratory, Pune 411007 ¢India)

Ion implantation effects have been studied in the pulsed-laser-deposited a m o r p h o u s carbon (a-C) films having a diamond-like character. 80 keV Ar" ions were implanted at various doses, and the optical and electrical properties of the films were studied as a function of ion dose. It was observed that the electrical resistivity and optical band gap decrease as the ion dose increases. The transparency increases from 801;o (for the as-deposited a-C) to over 97'o in the wavelength range 2.5 4 lam for the sample implanted at 1× 1013 ions cm -2 and at higher doses it decreases. Raman measurements at higher doses show the loss ofsp 3 hybridized carbon atoms with no sign of microcrystallinity. The absence of the Raman contribution at 600 and 1275cm t in samples implanted at doses greater than I × 1 0 Is ions cm 2 corresponds to an implantation-induced sp3-to-sp 2 transformation without graphitization.

[. INTRODU('TION

In the past few years, diamond-like a m o r p h o u s carbon (a-C) (DLC) films have generated much interest because of their technologically important properties, i.e. extreme hardness, transparency (particularly in the IR region) and chemical inertness (even towards strong acids). To date, various conventional and nonconventional methoas have been employed to deposit DLC films and also a number of attempts have been made to unfold the complex structure-property relationship in these films ~'2. However, only a very few attempts have yet been made to impart controlled modification to such films by using non-equilibrium processing methods such as ion implantation and pulsed laser processing 3 v. In some previous attempts along these lines, ion implantation effects in sp2-coordinated a-C films 3 and hydrogenated amorphous D L C films 4 have been investigated. In the former case it has been shown that the films start to undergo modification near a dose of 1 x 1015 ions cm 2 but, in the latter case, hydrogen evolution has been shown to be a major initial step towards a further ion-generated defect-induced modification process. However, to the best of our knowledge, hardly any studies have been performed on hydrogen-free D L C films deposited by either the ion beam deposition method 8 or 0040-6090/90.'$3.50

~" Elsevier Sequoia.Printed in The Netherlands

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the recently developed laser deposition method 9. In the unhydrogenated DLC films, since the carbon atoms are presumably bonded to each other in either tetrahedral or trigonal fashion in an amorphous matrix, it is interesting to investigate the role of ion bombardment in the relative population of these bonding configurations and thereby in the film properties. Such a study can be expected to be helpful in isolating the role of hydrogen in ion-bombardment-induced property modification. In the present paper we report the inert gas ion implantation effects in pulsed-ruby-laserdeposited DLC films. 2. EXPERIMENTAL DETAILS

The a-C films used for ion implantation were deposited by pulsed ruby laser evaporation from pyrolytic graphite as reported previously by the present authors 9, under conditions which lead to maximum s p 3 c o n t e n t in the films. The a-C films of 0.12-0.15 p.m thickness deposited on fused quartz, Corning glass and n-type S i ( l l l ) were implanted by 80KeV Ar ÷ ions at various doses in the range 1 × 10~3-1 × 1016 ions cm -2. During implantation the vacuum was better than 1 × 10-6 Torr. The ion energy was selected in such a way that the projected range was 780 ~ with a straggling of about 190/~. The ion beam current was set at 0.5 laA cm - 2 to avoid heating effects during implantation. The electrical resistivity measurements were carried out by a two-probe method. IR transmission measurements were carried out on the samples deposited on silicon substrates by employing a Perkin-Elmer (model 1783) IR spectrophotometer, whereas the UV-visible transmission and reflection measurements were made on samples deposited on fused quartz substrates using a Hitachi (model 330) UV-visible spectrophotometer. The optical band gap values were found by plotting (~tE) 1/2 VS. E, where ct is the absorption coefficient calculated for the sample deposited on quartz and E is the photon energy. The Raman measurements were carried out in backscattering geometry with the help of a SPEX 1403 spectrometer using green radiation 0~ = 514.5 nm) from an argon ion laser (Spectraphysics model 164) as an excitation source. A Talystep (Rank-Taylor-Hobson) recorder was used to measure the film thickness. The as-deposited a-C films had a shining tan colour with thickness uniformity variation of _+59/0. The as-deposited film had an electrical resistivity Po of 105 Q c m and a band gap Ego of about 1.3 eV. The film had an IR transparency l o of about 80~ over the wavelength range 2.5-4.0 lain. 3.

RESULTS AND DISCUSSION

Figure 1 shows the resistivity of ion-implanted a-C film. It can be seen that the resistivity decreases very slowly for doses in the range I x 1013-1 × 1014 ions cm-2 but for higher doses (greater than I x 10 ~4 ions cm -2) there is very fast fall in the resistivity. Figure 2 shows the optical transparency behaviour as a function of ion dose in the IR region (in particular, in the 2.5-4.01.tm range). For a dose of l x l 0 1 3 ions c m - 2 the IR transparency of the ion-implanted a-C increases dramatically from 80~o to 97~o which is comparable with the best recorded transparency for DLC

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films deposited by the ion beam deposition technique TM. Figure 2 shows that the transparency again starts to decrease as the ion dose increases further. F o r doses greater than I x 10 t4 i o n s c m -2 the IR transmission decreases rapidly. N o IRsensitive graphitic b o n d stretching modes at 1430 and 1570 cm i were observed for the as-deposited as well as ion implanted a-C films. Figure 3 shows the optical band gap variation as a function of Ar ÷ ion dose. The nature of the curve is similar to Fig. 1 but the band gap curve has a knee for the dose of 5 × 1014 ions c m 2 and does not show a saturation trend like the electrical resistivity curve for doses above 5 x 10~s i o n s c m 2 Figure 4 shows the R a m a n measurements for as-deposited and ion-implanted a-C films deposited on silicon. The R a m a n spectrum for pulsed-laser-deposited a-C shows the G line at 1550 cm - 1 characteristic of a-C. Apart from the G line it also shows a b r o a d h u m p at a r o u n d 600 cm - ' and also a small h u m p at 1275 cm i. We can attribute these features to the response of sp3-bonded species embedded in the a m o r p h o u s sp 2 matrix (the details of R a m a n measurement of pulsed-laser-deposited a-C have been discussed previouslyg). The R a m a n spectra for a-C implanted at a low dose (1 × 1013-5 X 1014 i o n s c m -2) show that for a dose of I x 1013 ions cm 2 the position of the G line shifts d o w n w a r d from 1550 to 1516 cm i but again for a dose of I x 1014 ions c m - 2 the G line shifts to 1520 cm i. At these two dose values, humps at a r o u n d 600 and 1275 cm " 1 are also observed. The presence of two humps, at 600 and 1 2 7 5 c m 1 implies that the s p 3 concentration remains after low dose ion implantation. Figure 4 shows that the oosition of the G peak shifted to higher

591

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wavenumbers, at 1545 c m - 1, for higher doses (more than 1 x l0 t 5 ions cm 2). Also at the higher doses the humps at 600 and 1275 cm - t vanish, implying the absence of tetrahedral bonding of carbon species at the higher doses. Now, the above results show that the ion-implanted a-C films have broadly two different trends of optical and electrical behaviour in the lower (between I x 1013 and 5 × 1014 ions c m - 2) and higher (greater than I x 1015 ions c m - 2) dose range. In the lower dose range the optical and electrical properties vary slowly but in the higher range the electrical resistivity, optical transmission peak and band gap decrease rapidly. In the lower dose range the a-C G peak shifts to lower wavenumbers in particular for the l x l 0 1 3 and l x l 0 1 4 ionscm -2 doses. Beeman e t al. i t have attributed the downward shift of the G line to a higher proportion of sp3-bonded carbon atoms in the a-C. Thus the downward shift of the G line and presence of the two humps at 600 and 1275cm-1 can be attributed to an increase in the s p 3 character of the film at a dose of 1 x l0 t 3 ions cm - 2. This should also be responsible for the observed initial increase in the IR transparency. In the higher dose range, Raman spectroscopy measurements show the absence of humps at 600 and 1275 cm-1 with the G peak at 1545cm-1. There seems therefore to be an ionimplantation-induced transformation from the tetrahedrally bonded state to the planar trigonally bonded state. The position of the G peak at higher wavenumbers and the absence of the microcrystalline-carbon-sensitive peak at 1350 cm-1 have

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confirmed the highly a m o r p h o u s nature o f the ion-implanted a-C. in this context, it is important to mention that Ar t-ion-implanted a-C tilms remained s m o o t h and s c r a t c h - p r o o f even after a dose o f I × 1016 ions cm 2. Thus, unlike the laser D L C interaction, the i o n - D L C interaction time scale and deposited energy conditions may not allow the matrix to crystallize as is evident from the above results. 4. CONCLUSION We have found two different trends in the opto-electrical behaviour for the lower (1 x 1013-1 × 1014 ions cm 2j and higher (5 × 10~4-1 × 1016 ions cm 2) ion dose ranges. In the lower dose range a very slow decreasing trend in the electrical resistivity and optical band gap change was observed. In this range at a dose of 1 x 1013 ions cm - z a significant increase in the IR transparency was observed (from 80% to 97%) but the transparency again starts to decrease, although very slowly, as a function of ion dose. In the higher dose range the electrical resistivity, optical band gap and IR transparency fall rather sharply. Higher dose ion implantation seems to cause transformation oftetrahedrally bonded c a r b o n to the trigonally bonded state with no microcrystallinity even in D L C implanted at a high ion dose. Thus ion implantation even at higher doses does not appear to cause graphitization of the D L C films.

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ACKNOWLEDGMENTS

The authors wish to acknowledge the financial support by Department of Science and Technology under lndo-U.S, collaboration project and Defence Research and Development Organization. One of the authors (A.P.M.) would also like to thank the Council of Scientific and Industrial Research for their financial support. REFERENCES

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