Bonded and non-bonded hydrogen in diamond-like carbon

Journal of Non-Crystalline Solids 227–230 Ž1998. 627–631

Bonded and non-bonded hydrogen in diamond-like carbon V.I. Ivanov-Omskii ) , M.P. Korobkov, B.R. Namozov, E.A. Smorgonskaya, S.G. Yastrebov A.F. Ioffe Physico-Technical Institute, St. Petersburg 194021, Russian Federation

Abstract The techniques of infrared absorption and vacuum ultraviolet Raman spectroscopy are applied to study the states of hydrogen in diamond-like a-C:H. It is shown that, along with the state covalently bonded with carbon, there exists a quasi-free state of stretched H 2 molecules adsorbed by graphite-like structural fragments. A model of thermally-induced reversible transfer of hydrogen between the bonded and non-bonded states is suggested to describe the experimental results of thermal treatment. The techniques of infrared absorption and vacuum ultraviolet Raman spectroscopy are applied to study the states of hydrogen in diamond-like a-C:H. It is shown that, along with the state covalently bonded with carbon, there exists a quasi-free state of stretched H 2 molecules adsorbed by graphite-like structural fragments. A model of thermally-induced reversible transfer of hydrogen between the bonded and non-bonded states is suggested to describe the experimental results of thermal treatment. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-C:H; Vacuum ultraviolet Raman spectroscopy; Infrared absorption

1. Introduction The physical properties of a-C:H or diamond-like carbon ŽDLC. are in many respects controlled by hydrogen incorporated into the disordered carbon network in the process of film growth Žsee, for example, Ref. w1x.. Hydrogen can be covalently bonded with carbon or, alternatively, it can remain in a non-bonded state. Bonded hydrogen in DLC can easily be detected by infrared ŽIR. absorption spectroscopy w2–4x. From the IR spectra, it was shown in particular that the bonded states of hydrogen vanished at temperatures above ; 5008C w3,4x. Later, however, it was found that degradation of the bonded

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Corresponding author. Fax: q7-812 247 1017; e-mail: [email protected].

states at temperatures below ; 4508C was reversible, at least partially w5x. In this connection, the nature of the non-bonded state of hydrogen in DLC is of considerable interest. The first observation of ultraviolet ŽUV. Raman scattering from molecular hydrogen ŽH 2 . in DLC was reported recently w6x. In this work, the problem of bonded and non-bonded hydrogen in DLC is studied and discussed in more detail.

2. Experimental procedure The DLC films were grown on silicon substrates by sputtering of a graphite target with the assistance of a planar d.c. magnetron in an argon–hydrogen plasma Ž80% Ar and 20% H 2 . at a pressure of ; 10

0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 2 3 3 - 6

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3. Results 3.1. IR Õibrational spectra of bonded hydrogen in DLC films

Fig. 1. The C–H vibrational spectra of a DLC film before and after 1-h thermal annealing at different temperatures Žthree upper curves. and after restoration Žlower curve..

mTorr. Thermal annealing of the samples was carried out in vacuum in temperature ranging from 100 to 4508C under isothermal or isochrone conditions. The infrared ŽIR. transmission of the films in the range of stretching vibrational modes of C–H bonds Ž2800–3100 cmy1 . was measured by a two-beam spectrophotometer ŽSpecord 751R.. To study spectral characteristics of non-bonded hydrogen in the DLC films, the technique of resonant Raman scattering of vacuum UV radiation w6x was applied. Xe- and Krfilled lamps served as sources of excitation with respective wavelengths of 147 and 124 nm and an intensity of 10 3 photonsrs.

The typical IR transmission spectra, t Žhv . s IrI0 , in the range of stretching vibrations of C–H bonds is shown in Fig. 1 for DLC films subjected to thermal annealing. Here I and I0 are the amplitudes of incident and transmitted radiation, respectively. On annealing, the bands decreased in amplitude with increasing temperature, as consistent with the available data w2,3x. At the same time, the reversibility revealed in the temperature dependence of the bands was unexpected. As seen from Fig. 1 Žcurve 4., if the samples were kept in air at room temperature, the bands recovered their intensity in a time of 8 months after the initial annealing. No evidence of the band restoration was observed after an annealing during 1 h at temperatures above 5008C. 3.2. UV scattering from DLC films Fig. 2 gives a typical scattering spectrum of a DLC film illuminated by the Kr lamp at room temperature. The energy Žfrequency. shift of scattered light from the excitation line, D E s hvscat y hvexc is plotted as abscissa. It is apparent that we observe a multicomponent Raman spectrum at D E ) 0 Žthe anti-Stokes range.. In the Stokes range, D E - 0, it was possible to detect only one component of 550 cmy1 symmetric in Raman shift to the lowest-energy

˚ excitation. Fig. 2. Raman spectrum of DLC films observed under the 1236 A

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anti-Stokes component. It should be emphasized that 10 lines in the low-energy part of the anti-Stokes Raman spectra at D E - 2200 cmy1 form a series of nearly equidistant bands. All Raman bands have two to three orders of smaller amplitudes than the excitation line. At lower temperatures, the anti-Stokes bands decrease in amplitude. In particular, the amplitudes of the Stokes and anti-Stokes 550 cmy1 bands become nearly equal at 80 K. Under excitation with the Xe lamp, no structure in the scattering spectrum in the range to 2200 cmy1 was observed.

4. Discussion 4.1. Quasi-free hydrogen The structureless scattering of the 147 nm radiation ŽXe. in the spectral range below 2200 cmy1 suggests that the scattering observed with the 124 nm line ŽKr. is a resonant effect, which is due to the proximity of the excitation frequency to resonance conditions for the electronic excitation of the hydrogen molecule, H 2 . Actually, the energy gap between . electron-vibrathe ground Ž1 Sgq. and excited Ž1 Sq u tional terms of the free molecule H 2 corresponds to 113 nm Ž10.93 eV.. We assume that the van der Waals interaction of the molecule with the host a-C:H matrix shifts the states along the energy axis, so that the gap is ‘driven’ towards the 124 nm line to make possible vertical electronic transitions. In this case, the effect of resonant Raman scattering ŽRRS. from the H 2 molecules interacting with the a-C:H structural units can be produced with the 124 nm excitation. Stokes spectra of molecular hydrogen in the gas were given by Stoicheff w7x. A correlation between our results ŽFig. 2. and the data w7x shows that we observe rotational RRS spectra of H 2 molecules in a-C:H. At least in the range of < D E < - 2200 cmy1 , the bands in Fig. 3 can be related to virtual transitions between rotational sublevels of the ground electron-vibrational state of the molecule, with absorption Ž D E ) 0. or emission Ž D E - 0. of rotational quanta. The quantum numbers, J, of the sublevels involved in the transitions are indicated for each rotational RRS peak in Fig. 3. The selection rule for the transitions implies D J s 0, "2. The

Fig. 3. Optical density at the 2920 cmy1 vibrational peak vs. 1-h annealing temperature for DLC films with different hydrogen content. Squares are related to the present work and triangles to the data w4x. Solid lines are fit the experimental data with the function Nb ŽT . with Ea1 s 0.35 Žcurve 1. or 0.30 eV Žcurve 2. Žsee text..

broader band at 2920 cmy1 ŽFig. 2. is clearly related to the C–H vibrations seen in the IR spectra ŽFig. 1.. Starting from such interpretation of the results and using the model of a rigid rotator, we can estimate the typical extent of self-rearrangement of the H 2 molecule in the DLC matrix. In this model, the rotational constant B, which defines the rotational energy, E, of the molecule as E s BJ Ž J q 1., is related to the Raman shift by the equation < D E < s 4 B Ž J q 3r2.. According to our findings, B s 54.7 cmy1 , that is somewhat smaller than B for the free molecule in the gas, where B s 59.3 cmy1 . Since for the H 2 molecule B s h 2rMre2 , where M is the proton mass and re the equilibrium interatomic distance, the parameter re can be immediately determined. The calculation gives re ( 0.078 nm instead of the well-known value 0.074 nm for the free H 2 molecule. The lower rotational constant B and larger interatomic distance re in the H 2 molecule in a-C:H as compared to the free molecule are attributed to the effect of polarization-type interaction of the molecule with the a-C:H environment. 4.2. ReÕersible transfer of hydrogen between bonded and non-bonded states The states of covalently bonded hydrogen were studied by analyzing the temperature dependence of

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the maximum IR transmission intensity after isochronal annealing. The transmission coefficient, t , at 2920 cmy1 Žsee Fig. 1. was converted into the optical density k L s ylnt . We assumed that the absorption coefficient, k , was proportional to the density of covalent C–H bonds and the film thickness L was independent of annealing time and temperature. Then the temperature dependence of the optical density is related to that of the density of bonded hydrogen, Nb ŽT .. The optical density at 2920 cmy1 vs. 1-h annealing temperature of a DLC sample is shown in Fig. 3. The earlier data w4x are also given in Fig. 3 for comparison. Evidently, our results for k L are in satisfactory agreement with the available data. Assuming that the decrease in Nb ŽT . detected from the IR spectra is due to transfer of bonded hydrogen to a non-bonded state, we can write

Fig. 4. Configurational diagram for a hydrogen atom in a diamond-like fragment of DLC.

Nb Ž T . s Nb0 1 y exp Ž yEa1rkT . , where Ea1 is the transfer activation energy and Nb0 the maximal density of bonded hydrogen. The function Nb ŽT . ŽFig. 3, solid curves. gives a good fit to the experimental data with the adjustable parameter Ea1 s 0.35 Žcurve 1. or 0.30 eV Žcurve 2. for the two different samples. The estimate of Ea1 appears to be due to a non-exponential function of the experimental dependences typically expected for disordered structures. As mentioned above, the bonded state of hydrogen is restored with a rate increasing with temperature. The restoration process can be approximately described by an estimated activation energy, Ea2 ; 0.2 eV, for the density Nb ŽT .. The reversible transfer of hydrogen between bonded and non-bonded states under thermal excitation is illustrated by a configuration diagram for a proton ŽFig. 4.. The reversibility of the transfer implies that a hydrogen atom escaping from the bonded state remains in the sample, and moreover, moves not far away from the initial position. In fact, the minimal distance, x 1 min , required of escaping a proton from the bonded state can be estimated from the uncertainty relation: x 1 min ; hr62 MEa1 . Taking Ea1 from the experiment, we obtain x 1 min ; 0.008 nm. A similar estimation for the reverse replacement of the proton from the initial non-bonded position, with Ea2 instead of Ea1 , gives x 2 min ; 0.014 nm. Thus, the positions of bonded and

non-bonded hydrogen are separated by a distance of the order of Ž x 1 min q x 2 min .r2 ; 0.01 nm ŽFig. 4., which is much shorter than the covalent bond lengths in the system. We assume that the non-bonded state of hydrogen is a thermally excited state of hydrogen. It is worthwhile to note here that the corresponding activation energy, Ea1 , is comparable to the 2900 cmy1 vibrational quantum for the C–H bond. In addition, the barrier height for the reverse transition, Ea2 , is close to the 3rd rotational level of the quasi-free molecule H 2 . Therefore, we conclude that both states of hydrogen are coupled. In this case, the excited state is represented as quasi-molecular hydrogen resulting from dehydrogenation of neighboring carbons in the sp 3-states, with the transition to the sp 2-states and p-bonding. The quasi-molecule H 2 will be located close to the two carbons due to the van der Waals forces related to rising from the polarizability of the p-bonds. The above model of reversible transfer of hydrogen between bonded and quasi-molecular states involves reversible conversion from the sp 3 bonds of the related carbon atoms to the sp 2 bonds, or reversible local transformation of a diamond-like structural fragment into a graphite-like one. In this way, the presence of hydrogen leads to a certain structural flexibility of the material.

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5. Conclusions

Acknowledgements

Ži. In DLC films, the quasi-free molecular states of hydrogen co-exist with states covalently bonded with carbon. The two atomic molecules H 2 are adsorbed by graphite-like carbon fragments based on the sp 2-hybridized valence bonds. Žii. The van der Waals forces responsible for the adsorption of H 2 molecules by graphite-like fragments efficiently stretch the molecules, resulting in a decrease in their rotational and presumably vibrational frequencies. Žiii. In the diamond-like fragments, two states of hydrogen are separated by an asymmetric energy barrier ; 0.1 nm in width and with an energy of several tenths of electron Volt. This type of barrier allows for reversible transfer of hydrogen atoms between the bonded and quasi-molecular states on heating the sample to not-too-high temperatures ŽT - 4508C..

The work was supported by the Russian Foundation for Basic Research ŽGrant 96-0216851-a. and in part by the US Defense Department. References w1x J.C. Angus, C.C. Hayman, Science 241 Ž1988. 913. w2x A. Grill, in: K. Spear, J.P. Dismukes ŽEds.., Synthetic Diamond: Emerging CVD Science and Technology, Wiley, New York, 1994, p. 130. w3x J. Fink, T. Muller-Heinzering, J. Peluger, B. Scheerer, B. Dischler, P. Koidl, A. Bubenzer, R.E. Sah, Phys. Rev. B 30 Ž1984. 4713. w4x P.B. Lukins, D.R. McKenzie, A.M. Vassallo, J.V. Hanna, Carbon 31 Ž4. Ž1993. 569. w5x V.I. Ivanov-Omskii, G.S. Frolova, S.G. Yastrebov, Tech. Phys. Lett. 23 Ž4. Ž1997. 251. w6x V.I. Ivanov-Omskii, M.P. Korobkov, B.R. Namozov, E.A. Smorgonskaya, Tech. Phys. Lett., 1997, in press. w7x B.P. Stoicheff, Can. J. Phys. 35 Ž1957. 730.