H atom impact induced chemical erosion reaction at C:H film surfaces

23 December 1994

ELSEVIER

CHEMICAL PHYSICS LETTERS

Chemical Physics Letters 231 (1994) 193-198

H atom impact induced chemical erosion reaction at C: H film surfaces A. Horn ‘, A. Schenk, J. Biener, B. Winter, C. Lutterloh, M. Wittmann ‘, J. Kiippers Max-Planck-Institut

ftir Plasmaphysik

(EC’RATOMAssociation),

’

D-85748 Garching, Germany

Received 10 June 1994; in final form 12 September 1994

Abstract

C:H film surfaces which are subjected to a flux of thermal H atoms erode chemically via hydrocarbon, probably methyl, production. At the present H flux the erosion reaction is effective above 400 K and below 700 K, with a maximum around 600 K. The erosion efficiency at this temperature is = 0.0 1 C atom per incoming H. A kinetic analysis of the erosion reaction and competing hydrogenation and dehydrogenation surface reactions under impact of H reveals an activation energy of z 37 kcal/ mol for the H atom impact induced erosion. As the efficiency of the erosion reaction depends on the incoming H flux, it may contribute as an important reaction in low-pressure diamond synthesis.

The surface chemistry of hydrogenated carbon, in particular concerning its interaction with hydrogen atoms is of considerable interest for understanding the elementary steps in plasma assisted hard carbon film deposition [ 1 ] and low-pressure diamonds synthesis [ 21. In the latter technique a hydrocarbon/hydrogen mixture, 1% methane/hydrogen for example, is activated either thermally or by plasma means in front of a heated substrate. At this substrate activated hydrocarbons, methyl being suggested as the important one [ 3 1, supply the growth species and hydrogen atoms preferentially etch away graphitic components of the growing film [ 41. We have shown by spectroscopic means that at the substrate temperatures commonly used for low-pressure diamond synthesis, T= 1000 K, a combination of a surface hydrogenation reaction induces by impinging H atoms and a thermally activated erosion step via methyl and methane provides effective removal of graphitic spz ’ Also at Experimentalphysik VI, Universitlt Bayreuth, D-95440 Bayreuth, Germany.

carbon species from the film surface [ 5 1. In a thorough investigation of the H/C: H surface chemistry we have identified two reactions which occur at C : H film surfaces under the impact of thermal H atoms. The first is hydrogenation of unsaturated C atoms at the surface from sp and sp2 (aromatic) hybridization states to sp3, i.e. (= ) =CH to -CH2,3 [ 6 1. The second reaction is the dehydrogenation of saturated surface sp3 CH, groups to CH,_ , via an EleyRideal mechanism to form H2,gas [ 71. In this Letter we report on a further reaction initiated by H impact at C : H film surfaces, chemical erosion via hydrocarbon release from the film. The experiments were performed at several monolayers thick C: H films grown by ion beam deposition on a suitable carrier, a Pt single crystal surface covered with a graphite monolayer [ 8 1. The films were investigated with Auger electron spectroscopy ( AES), vibrational spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Through thermal decomposition of the films their H/C ratio was determined as ~0.4. As deduced from HREEL spec-

0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIOOO9-2614(94)01233-4

A. Horn et al. /Chemical

194

Physics Letters 231 (I 994) 193-I 98

troscopy, after deposition the film surfaces exhibited about equal amounts of C in the sp’ and sp3 hybridization states with a minor contribution from sp. H atoms were exposed to these surfaces from a heated Tatubeatafluxof 1.9X10’3s-‘cm-2ascalculated from the tube temperature in in-tube hydrogen pressure. H atom exposures are given below in monolayer (ML) units, where one ML corresponds to a fluence of 3.8x lOi cmp2, equal to the atom density at the graphite (000 1) surface. The film thickness measured from the ratio of the Pt and C Auger peaks of initially six monolayers thick C: H films as a function of H atom fluence and film temperature during H exposure is shown in Fig. 1. It is seen to decrease linearly with H atom fluence, the decrease being most pronounced at 600 K and less effective towards lower and higher temperature. Auger spectra measured at different spots on the film did not show inhomogeneities outside the error bars

1 /

C:H film / AES ethane / 350 K / 160 eV txMLH”at

I 600 K

LA.

--AI

100

1-e

200

300

400

500

1

600

Ho [ML]

Fig. 1. Thickness of an initially six monolayer thick ion beam deposited C:H film as a function of H atom fluence at various temperatures. I ML H atom fluence corresponds to 3.8x 10” cm-*.

in Fig. 1 which represent the scatter of different measurements. A decrease in the C: H film thickness is in accordance with the assumption that H impact on the film surface initiates the formation of volatile hydrocarbons which upon desorption carry C from the film away, thereby chemically eroding the film. At the bottom of Fig. 4 the erosion yield per incoming H as a function of temperature is shown. Apparently, at the used H flux outside the temperature range of x 500 to 650 K the mechanism which leads to erosion is less effective. Although it is implicitly clear, we emphasize that in control experiments without hydrogen supply to the H source the film thickness stays constant within experimental error. Thus it is guaranteed that the present erosion phenomenon is not the purely thermally activated erosion reaction which can be observed during non-stationary thermal decomposition of a C : H film [ 9 1. We have identified the breaking of a C-CH3 bond as the rate limiting step of this purely thermal step, i.e. no H involved, which leads to the release of methyl and methane as the dominating erosion products. Its activation energy lies in a 10 kcal/mol wide Gaussian distribution around 56 kcal/ mol, the distributed, rather than precise activation barrier originating from the heterogeneity of C-C bond strengths in the C network of hydrogenated carbon material [ 8 1. In order to investigate, which types of C hybridization states are present at a C : H film surface which was exposed to H at various temperatures, HREEL spectra were recorded during the measurements shown in Fig. 1. Recording of the spectra was performed with the H source switched off. A set of typical spectra is shown in Fig. 2. In the loss region below 1500 cm-’ different C-H deformation modes overlap and an elaborate procedure is necessary to reach an assignment [ 10 1. The C-H stretch region around 3000 cm-‘, however, which is of interest in the present context, allows easy identification of the hybridization states through the C-H stretch frequencies at sp* and sp3 hybridized carbon atoms, as indicated in Fig. 2. At 400 K the surface exhibits exclusively sp3 hybridized C atoms in CH, groups and the absence of a step around 1580 cm-’ illustrates that no aromatic C=C stretches are detected. These observations are in complete agreement with the operation of the H atom

195

A. Horn et al. I Chemical Physics Letters 231 (I 994) 193-I 98

C:H films / HREELS ethane / 350 K I 160 eV +120MLH”at ;

vCH.sp3 1vCH,sp*

.,,,,I

L.I.,..,,.,,i.,.......i,.,.,..,.I,..

0

1000

2000 wave numbers [cm-‘]

3000

4000

Fig. 2. HREEL spectra measured after exposing 6 monolayer thick C: H films to several 100 ML of thermal H atoms at various temperatures. The dashed lines indicate principal vibrations. Primary energy 7 eV, specular direction.

impact induced hydrogenation reaction at C: H film surfaces reported earlier [ 6 1. An evaluation of the data given in Ref. [ 61 via the rate equation -d[CH

sp2]/dt=

[CH sp2]crH@,

One must conclude that under impact of H the hybridization of carbon atoms at a C:H film surface change from sp3 to sp2 character in the temperature range 400 to 700 K. Again we emphasize that this phenomenon is only present with H atoms impinging at the surface. It is suggested that this change may be correlated with the H atom impact induced erosion reaction deduced above. In order to exploit this in more detail, Fig. 3 collects in a scheme the reactions already identified earlier. This scheme is intended to emphasize the reactions which are suggested to contribute to the present findings. We do not infer that all reactions which proceed at hydrogenated C substrates are included in this diagram. On the left of Fig. 3 the reaction sequence of hydrogenation of a sp2 entity via an intermediate radical ‘sp” state is shown. This radical intermediate is necessary as one H atom can only hydrogenate one C center at a time. Therefore, the neighboring C center must assume this spX state. Repetitive application of hydrogenation with a final C-C bond-breaking which may involve rearrangements will lead to, for example, a methyl group as indicated in the figure. On the right of Fig. 3 the H atom impact induced dehydrogenation reaction of a sp3 entity via molecular hydrogen release is drawn. Clearly, this reaction also has an H-atom induced chemical

erosion

(1)

with the bracketed items as concentrations, an as hydrogenation cross-section and 0 as the flux of H atoms revealed OH= 4.5 AZ. As seen in Fig. 2, spectra measured after exposing the film surfaces above 400 IS to H atoms exhibit less sp3 C-H stretch intensity but growing sp2 C-H stretch vibration intensity. At 700 K the C-H stretch loss peak is clearly dominated by sp2-type C-H vibrations. In parallel with the growing sp2 C-H stretch mode the C=C stretch mode at 1580 cm-’ and sp2 C-H deformation modes around 700 cm-’ develop, further stressing the fact that at the surface of the C: H film (aromatic) sp2 related groups get the most prominent species.

Fig. 3. Scheme of reactions which are initiated by H impact and temperature at C:H films or other hydrogenated carbon substrates. The elementary reactions are illustrated at an individual two-carbon entity of the C: H surface.

196

A. Horn et al. /Chemical Physics Letters 231 (I 994) 193-l 98

intermediate radical spXproduct. This radical intermediate may saturate to sp3 through hydrogenation. A central point for the following discussion is the assumption that the radical centers as specified above can de-excite via split-off of a neighboring methyl group as illustrated in the reaction scheme in Fig. 3. In the present context, we refer to two studies in which methyl was observed upon interaction of H with C or hydrogenated C surfaces, a mass spectroscopy-molecular beam study [ 113, and a REMPI/MS study [ 121. Vietzke et al. [ 111 reported that thermal H atoms directed at graphite or C : H film surfaces release methyl and other radicals in the temperature range 400 to 750 K, peaking at z 550 K. Villa et al. [ 121 observed methyl as the most prominent radical species effusing from a tube which was covered with hydrogenated C and subjected to a flux of H atoms. The production of methyl was found to be connected with an activation energy of 39 kcal/mol. The assumption that in the present experiments methyl is the eroding agent, however, is not only based on the cited works [ 11,121. Alternatives would be molecular species such as methane, ethane, etc., and other radical species. We dismiss molecular species as carriers of the eroded C atoms as molecules would require almost simultaneous bond breaking of a radical and H atom addition to the radical, which is, at our H flux, not probable. Furthermore, the attack of an H atom at the methyl group attached to the C center to form methane is not confirmed in organic reactions [ 131. The C-methyl dissociation energy is z 90 kcal/mol and the energy gain from the formation of a C=C double bond is ~45 kcal/mol. Thus, the erosion step via methyl is endothermic by about 45 kcal/mol, not too high to be improbable in the temperature range addressed here. The conjecture that the radicalic spXcenters introduced by H impact can release methyl via a thermally activated step is in accordance with the following rate equations, with the bracketed quantities as concentrations, 0 as cross-sections, 0 as H atom flux, k as frequency factors, and E as activation energies. Dehydrogenation of sp3 centers to sp*, -d[CH

sp3]/dt= [CH sp3]o,@.

(2)

Hydrogenation of sp2 centers to sp”, -d[CHsp2]/df=

[CHsp2]~,@.

(3)

Hydrogenation of sp” centers to sp3, -d[CHsp”]/dt=[CH~p”]O~@.

(4)

Thermal decomposition of spXcenters to sp2 (via methyl production), -d[CH

sp”]/dt=

[CHsp”]k,

exp( -E,,./kt)

. (5)

Thermal decomposition of spX centers to sp2 (via split-off of an H atom, i.e. the reverse of reaction (4) )Y -d[CH

sp”]/dt

= [CH spX]k-n exp( -E_,,/kT)

.

(6)

Thermal decomposition of sp 3 centers to sp2 (7) Note that in this reaction scheme the overall hydrogenation reaction rate expression ( 1) is replaced by the more correct equations (2) and (3) in which the necessary intermediate explicitly occurs. From our earlier studies [6,7,9] we take r&=4.5 A2, 0,=0.05 A2, kT= 10L3,Ea,T=56+ 10 G. For the thermal decomposition reaction of sp” centers via H split-off we use the gas phase value, E_-H,= 40 kcal/ mol [ 131. In Eq. (4) it is assumed that the hydrogenation cross-section of sp” centers is identical to that of sp2 centers. This is reasonable as the microscopic process, addition of an H atom to an unsaturated C center is the same in each of the reactions. The above set of differential equations has been solved numerically using the Rosenbrock method [ 141 with k, and E, as adjustable parameters aimed at reproducing the change from sp3 to sp2 character as shown in Fig. 2 and to reveal the chemical erosion rate via methyl production as displayed in Fig. 4. This latter rate, obtained by counting the number of events in Eq. (5), is transferred to the erosion yield. The result of this computation is shown in Fig. 4 with E,= 37 kcal/mol and k,= lOI s-i as best fit results. It is seen that the experimental data concerning the change of the hybridization character at the surface and the erosion yield, depicted in the lower part of Fig. 4, are reproduced sufficiently as to warrant that the underlying mode, Eqs. (2)-(6) is correct. The contribution of step (7 ) to the result is negligible

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