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J. Phys. Chem. B 2002, 106, 610-616
Hydrogen-Induced Chemical Erosion of Amorphous Hydrogenated Carbon Thin Films: Structure and Reactivity Thomas Zecho,† Birgit D. Brandner,† Ju1 rgen Biener,‡ and Ju1 rgen Ku1 ppers*,†,‡ Experimentalphysik III, UniVersita¨ t Bayreuth, 95440 Bayreuth, Germany, and Max-Planck-Institut fu¨ r Plasmaphysik (EURATOM Association), 85748 Garching, Germany ReceiVed: June 6, 2001; In Final Form: October 1, 2001
In the present study, we investigated the effects of annealing of amorphous hydrogenated carbon (a-C:H) films, particularly with respect to structural changes of the carbon network and their impact on the hydrogenatom-induced erosion. The a-C:H films were deposited at 300 K on a Pt(111) single crystal using the ionbeam deposition (IBD) method. Electron energy loss spectroscopy (EELS) and high-resolution electron energy loss spectroscopy (HREELS) were employed to monitor structural changes of the carbon network as a function of the annealing temperature. A transition from an sp3-rich carbon network toward an sp2 dominated, “graphitic” structure was observed around 900 K. The hydrogen-induced erosion of as-deposited and annealed a-C:H films was investigated by in situ mass spectrometry. The postdeposition annealing did not change the overall erosion rates of a-C:H films; however, the product distribution indicates the growth of existing graphitic structures as a consequence of annealing at temperatures above 900 K.
1. Introduction Amorphous hydrogenated carbon (a-C:H) exhibits excellent mechanical and optical properties such as high hardness, high wear resistance, chemical inertness, and optical transparency. Because of these attractive properties, a-C:H films are utilized as wear- and corrosion-resistant coatings on lenses and magnetic hard disk drives.1 Another application arises from fusion technology, in which a-C:H films are used as protective coatings on plasma facing components to improve the plasma performance by reducing metallic impurities.2-4 The structure of a-C:H consists of small clusters of 3-fold coordinated, sp2-hybridized carbon embedded into a matrix of tetrahedrally coordinated, sp3-hybridized carbon. The proportion between sp3 and sp2 sites as well as their distribution controls the electrical and mechanical properties.5 The hydrogen content of a-C:H, up to 50 atom %,6 stabilizes the sp3 hybridization of the carbon network.7 The thermal stability of a-C:H is limited to approximately 600 K, at which sp3 CHx groups (x ) 1-3) start to decompose via hydrogen split-off, thus being transformed into sp2-hybridized CHx-1 groups: sp3-CHx f sp2 CHx-1 + 1/ H (g).8-10 In the following, this process will be termed 2 2 “graphitization” of the carbon network. In this context, the term “graphitization” does not necessarily imply the growth of extended graphitic structures but describes the development of sp2 near-range order of the atoms in the carbon network. a-C:H is a metastable material and is usually deposited by plasma-enhanced chemical vapor deposition (PECVD). The growth of an a-C:H film is controlled by the hydrogen/carbon surface chemistry:5,6,11 On one hand, hydrogen atoms promote the growth of a-C:H via creating adsorption sites for growth precursors through hydrogen abstractions. On the other hand, they simultaneously reduce the growth rate by the formation of volatile hydrocarbon species, the so-called hydrogen-induced chemical erosion of carbon. The latter reaction limits the * To whom correspondence should be addressed. E-mail: Kueppers@ uni-bayreuth.de. Tel: +49 921-553800. Fax: +49 921-553802. † Universita ¨ t Bayreuth. ‡ Max-Planck-Institut fu ¨ r Plasmaphysik.
improvement of the plasma performance caused by carbonization of the fusion vessel.4 Recently, we investigated product distribution of the hydrogenatom-induced chemical erosion of a few-nanometer-thick a-C:H films.12 The films were deposited at 300 K using the ion-beam deposition technique and exhibited a hydrogen content of ∼0.4 H/C. The erosion was found to proceed predominantly via the formation of C1 (methane), C2 (ethene, ethane), and C3 (propene, propane) hydrocarbons with a maximum yield of ∼0.01 C/H at ∼750 K. Higher hydrocarbon products, C3 to C6, were detected as minority species with a maximum around 650 K. In the course of our efforts to understand the surface chemistry of carbon-based materials,13-20 we developed a model21,22 that successfully explains the experimentally observed temperature dependence21 and the product distribution12 of the hydrogeninduced erosion of a-C:H. The model is solely based on experimentally observed elementary reaction steps, such as the hydrogenation of sp2 sites,16,19 the H abstraction from sp3 CHx groups,20 and the thermally activated cleavage of C-C and C-H bonds.13,18 The idea is that radical sites created by hydrogen abstractions from sp3-CH groups facilitate the cleavage of neighboring C-C bonds, driven by the formation of CdC double bonds: *C-C-X f CdC + X*; the asterisk denotes a radical site. The “activation” of the carbon network by radical sites decreases the activation energy of the rate-limiting C-C bond cleavage, leading to a decrease of the desorption temperature of hydrocarbons. According to this model, the CdC double bond may be either contained in the desorbing hydrocarbon product, leaving a radical site on the surface, or contained in a surface hydrocarbon product, producing a desorbing radical product. The stability of the involved radical intermediates, and hence the kinetics of the erosion, should be influenced by the structure of the surrounding carbon network. Such a structural effect was indeed observed by Vietzke et al.,23-26 who investigated the erosion of graphite and different types of a-C:H films and found the surface of graphite to be the least reactive. Also, the chemical vapor deposition of diamond relies on the preferential erosion of graphitic co-deposits.27 This opens up the possibility of controlling the reactivity of carbon materials
10.1021/jp012151+ CCC: $22.00 © 2002 American Chemical Society Published on Web 12/20/2001
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by specifically changing the hybridization state of the surface atoms. For fusion technology, this would allow a better control of carbon contaminations of the hydrogen plasma. In the present work, we investigated the structure sensitivity of the hydrogen/carbon surface chemistry of a-C:H films. The possibility of changing the structure of the carbon network of a-C:H by annealing without changing other experimental parameters makes a-C:H an attractive subject for studying structural effects. A “graphitic” carbon network structure was prepared by annealing a-C:H films deposited at 300 K at temperatures above 1000 K. Electron energy loss spectroscopy (EELS) and high-resolution electron energy loss spectroscopy (HREELS) were employed to monitor the structural changes during annealing. Subsequently, the surface was rehydrogenated by hydrogen atoms, and the thermal stability of sp3 CHx groups thus prepared on top of the “graphitic” carbon network was investigated by thermal desorption spectroscopy (TDS). Finally, the temperature dependence and the product distribution of the hydrogen-induced chemical erosion was studied by in situ mass spectroscopy. 2. Experimental Section The present study was performed in an ultrahigh vacuum (UHV) system equipped with instrumentation for Auger electron spectroscopy (AES), EELS, HREELS, and mass spectrometry. A base pressure of 5 × 10-11 Torr was routinely achieved. The hydrogen atom source and the quadrupole mass spectrometer (QMS) were installed into a small, differentially pumped vacuum system (source chamber), which is connected through a small aperture of 8 mm diameter to the main chamber. The exposure to hydrogen atoms was controlled by a mechanical shutter. During erosion and temperature-programmed desorption (TPD) experiments, the sample was placed just in front of the aperture. The hydrogen atoms were generated in a resistively heated tungsten tube directed perpendicularly onto the surface. The atom flux of the hydrogen source was previously calibrated utilizing the hydrogen-atom-induced erosion of a hard a-C:H film as a test reaction.28 In the present study, an atom flux of ∼1016 H cm-2 s-1 was achieved using a hydrogen flow of 0.45 sccm and a tungsten tube temperature of 2200 K. The a-C:H films were deposited on a Pt(111) single crystal at 300 K using the ion-beam deposition (IBD) technique. The ion gun was operated at an ion energy of 180 eV in an ethane (C2H6) ambient of 5 × 10-5 Torr. The Pt crystal was mounted via 0.3 mm thick tungsten wires to the liquid-nitrogen-cooled sample holder of a precision manipulator. The sample temperature was monitored by a chromel/alumel thermocouple, spotwelded to the back of the Pt crystal. The utilization of a metallic substrate facilitates the temperature control, and sample temperatures between 80 and 1400 K were readily achieved via liquid nitrogen cooling and resistive heating, respectively. The deposition rate was determined through a quantitative analysis of hydrocarbon product signals monitored during the complete erosion of a-C:H films. The result was cross-checked using the ion current measured during deposition. Prior to each experiment, the Pt surface was cleaned following standard procedures,29 and the purity of the deposited a-C:H films was routinely checked with AES. The product distribution and the temperature dependence of the chemical erosion of a-C:H were studied by means of a multiplexed mass spectrometer, which allows monitoring of up to 80 masses simultaneously. The data sets were converted into product distributions using experimentally determined fragmen-
Figure 1. EELS spectra of a 3.8-nm-thick a-C:H film as a function of the annealing temperature. The a-C:H film was heated to the indicated temperature using a linear heating ramp of 0.5 K/s, and the spectra were subsequently recorded at 300 K. A primary electron energy of 140 eV was used for reasons of surface sensitivity. The inset shows the variation of the plasmon energies versus annealing temperature.
tation patters and pumping speeds. Because a differentially pumped QMS was used to monitor the evolution of hydrocarbon products, the QMS signals are directly proportional to the rates of formation. A complication arises from the formation of reactive species, such as radicals because our setup does not allow the direct, collision-free detection of these products. Therefore, the assumption was made that radicals, such as CH3, react with the hydrogen-covered wall of the source chamber, and are subsequently detected as hydrogenated species, for example, as CH4 in the case of CH3. The erosion of a-C:H was studied either at constant temperatures (stationary erosion) or while slowly increasing the temperature at a constant heating rate (temperature-programmed erosion, TPE). 3. Results To follow the structural changes of the carbon network of a-C:H induced by annealing, EELS spectra were measured as a function of the annealing temperature (Figure 1). The two loss features at ∼5 and ∼26.5 eV can be attributed to the excitation of the π plasmon and the π + σ plasmon, respectively.30 With increasing annealing temperature, the π plasmon gains intensity and simultaneously shifts toward higher energy, from 4.9 to 5.5 eV, reaching a plateau around 900 K; above 1150 K, a further shift of the energy toward 6.5 eV was observed (Figure 1, insert). In contrast to the π plasmon, both the energy and intensity of the π + σ plasmon do not change with the annealing temperature. The plasmon energy is proportional to the square root of the effective concentration of the electrons contributing to a specific plasma oscillation31 and thus to the density of π and π + σ electrons of a-C:H. The π + σ plasmon of highly oriented pyrolytic graphite (HOPG) was observed at 26.6 eV.30 The measured value of the π + σ plasmon energy thus reveals a graphite-like electron density of the investigated
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Figure 2. HREELS spectra recorded from a 3.8-nm-thick a-C:H film in the CH stretch region around 3000 cm-1 prior to and after annealing at the indicated temperature. The spectra were measured at 300 K.
a-C:H films and, neglecting the contribution of the hydrogen atoms to the total electron density, a graphite-like density of ∼2.3 g/cm3. The π plasmon of HOPG was observed at 6.5 eV; the carbon atoms of graphite are sp2-hybridized and thus contribute one π electron per carbon atom to the π electron density. Through the use of the linear approximation of Steffen et al.,32 a π plasmon energy of 4.9 eV indicates an sp2 carbon concentration of ∼50%. The shift of the π plasmon toward higher energies with increasing annealing temperature reveals the commencing “graphitization” of a-C:H. The sp2 carbon concentration increases to approximately 65% at 900 K and further increases at temperatures above 1150 K. The “graphitization” of a-C:H as a function of the annealing temperature was also studied by HREELS, monitoring the spectral range of CH stretch modes around 3000 cm-1. The sp2hybridized CH groups exhibit a characteristic CH stretch mode at ∼3065 cm-1, whereas the presence of sp3-hybridized CHx groups (x ) 1-3) is indicated by the observation of CH stretch modes below 3000 cm-1; sp-hybridized CH groups give rise to a loss feature at ∼3325 cm-1.9,15,17,33,34 Figure 2 shows the HREELS spectra collected from a 3.8-nm-thick a-C:H film in the CH stretch mode region prior to and after annealing at temperatures between 500 and 1000 K. Assuming equal oscillator strengths for sp-, sp2-, and sp3-hybridized CH groups, the spectrum of the as-deposited film indicates the presence of approximately equal amounts of sp3- and sp2-hybridized CH groups at the surface of the a-C:H film and a minority of sphybridized CH groups. Heating the a-C:H film to 600 K leads to the depletion of sp-CH groups. Upon further annealing, the intensity of graphitic CH groups increases and the signal intensity below 3000 cm-1, reflecting the surface concentration of sp3-CH sites, decreases. Above 900 K, sp3-CH-related features disappear and the concentration of sp2-CH groups starts to decrease. Previous studies revealed that the decay of sp3-CH groups around 900 K is accompanied by the evolution of hydrogen and hydrocarbons from the a-C:H film.8-10,13,17,18 The QMS signals of hydrogen and various hydrocarbon species monitored during the thermal decomposition of an 8-nm-thick a-C:H film
Figure 3. Thermal decomposition spectra of an 8-nm-thick a-C:H film (full lines, left panels). The signals displayed are representative of the evolution of hydrogen and C1 to C6 hydrocarbon species. The sample was heated from 300 to 1000 K using a heating rate of 0.5 K/s. The right panels show the TD spectra measured after reexposing the annealed a-C:H film to a fluence of ∼3 × 1017 H cm-2 at the indicated temperature. The heating rate was 0.5 K/s. For comparison, the most intense signal of every channel is reproduced in the left panel (dashed lines).
are displayed in the left panel of Figure 3; for clarity, only the main fragments reflecting the formation of C1 to C6 hydrocarbon species are included in Figure 3. The product distribution, determined from the complete set of QMS signals, ranging from 2 to 80 amu, is shown in Figure 4 (top). As previously reported,13,18 the main product of the thermal decomposition of a-C:H is molecular hydrogen. Approximately 90% of the hydrogen bound to the carbon network was detected in the hydrogen channel; the remaining 10% was found in the hydrocarbon channel leading to the erosion of a-C:H. The maximum of the hydrogen evolution was observed at 900 K, coinciding with the decay of sp3-CHx groups as monitored by HREELS. Approximately 1% of the carbon atoms of the a-C:H film were removed from the carbon network through the formation of volatile hydrocarbons. The main products are methane (16 amu), ethene/ethane (28 amu), and propene/propane (41 amu), contributing to approximately 98% of the thermally activated erosion. With increasing molecular weight of the hydrocarbon products, the maximum was observed to shift toward lower temperatures, from 880 K (methane) toward 780 K (C3 to C5 species), except for benzene, which desorbs around 930 K. According to the EELS and HREELS spectra shown in Figures 1 and 2, respectively, the film assumes a “graphitic” structure after being annealed at temperatures above 1000 K with the residual hydrogen bound to sp2 sites. By independent experiments, it was proven that the diffusion of carbon into the Pt bulk is a slow process even at 1100 K and does not jeopardize the experiments, provided that the annealing intervals selected
Chemical Erosion of a-C:H Thin Films
Figure 4. Product distributions of the thermal decomposition of an 8-nm-thick a-C:H film, as-deposited (top) and after rehydrogenation at 300 K (bottom).
were sufficiently short (∼1-2 min). Annealing of a-C:H at temperatures above 1000 K thus allows the transformation of an sp3-rich carbon network into an sp2-dominated structure without changing other experimental parameters. This makes a-C:H an attractive subject for studying the influence of the atomic structure of the carbon network on the hydrogen/carbon chemistry. To investigate the formation and stability of CH groups on the surface of a “graphitic” a-C:H film, annealed samples were exposed to a hydrogen fluence of ∼3 × 1017 H cm-2 s-1 at various temperatures between 300 and 900 K. Subsequently, the thermal decomposition of CH groups thus produced was monitored by mass spectrometry (Figure 3, right panel). The product distribution monitored after rehydrogenation at 300 K is displayed in Figure 4 (bottom). The initial thickness of the a-C:H film, 8 nm, allowed collection of the complete data set shown in Figure 3 from one a-C:H sample. The thermal desorption (TD) spectra, monitored after exposing the “graphitic” film to hydrogen atoms at temperatures between 300 and 800 K, reveal the desorption of hydrogen and hydrocarbon species around 800 K and thus demonstrate the formation of stable sp3-CHx surface groups (Figure 3, right panel). The intensity of the molecular hydrogen signal reaches a maximum for exposure temperatures between 700 and 800 K, whereas the hydrocarbon formation is most effective between 300 and 500 K. Above 800 K, the surface concentration of hydrogen approaches zero, at least after stopping the hydrogen exposure, and decomposition products were thus not detected in these postexposure experiments. Molecular hydrogen and the hydrocarbon products were released around 780 K, independent of the sample temperature maintained throughout the hydrogen exposure. Astonishingly, decomposition products were detected even if the hydrogen exposure was performed at 800 K, that is, at the maximum of the decomposition reaction. Compared to the TD spectra collected from the as-deposited, sp3-rich a-C:H films (Figure 3, left panel), the maxima of product signals are shifted toward lower temperatures. This clearly demonstrates the influence of the film structure, “graphitic” versus sp3-rich, on the kinetics of the hydrogen/carbon surface chemistry.
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Figure 5. Temperature dependence and product distribution of the hydrogen-induced chemical erosion of 16-nm-thick a-C:H films, asdeposited (dashed lines) and after annealing at 1100 K (full lines) using an atom flux of ∼1016 H cm-2 s-1. Only the main fragments corresponding to C1-C6 hydrocarbons are displayed. The heating rate was 0.5 K/s.
The influence of the film structure of a-C:H on both the product distribution and the temperature dependence of the hydrogen-induced erosion was further explored by TPE experiments: 16-nm-thick a-C:H films, preannealed at 1100 K or asdeposited, were exposed to a flux of ∼1016 H cm-2 s-1 while increasing the sample temperature from 300 to 1300 K using a linear temperature ramp of 0.5 K s-1. The evolution of volatile reaction products was simultaneously monitored by in situ mass spectrometry. Figure 5 displays selected QMS channels, reflecting the evolution of C1 to C6 hydrocarbon species from preannealed (full lines) and as-deposited (dashed lines) a-C:H films, and Figure 6 shows the product distributions extracted from Figure 5. The erosion data of the preannealed and the asdeposited a-C:H film exhibit some common features: (1) the main products of the erosion reaction are C1 (methane) and C2 (ethene, ethane) hydrocarbon species which exhibit maxima around 800 K; (2) higher hydrocarbons, C3 to C6 species, were detected as minority products; (3) the maxima of C3 to C5 hydrocarbons were observed around 600 K, whereas aromatic hydrocarbon products such as C6 exhibit maxima around 800 K. Changes induced by the heat pretreatment of a-C:H are (1) an increasing C2/C1 product ratio, (2) a decreasing rate of formation of C3 to C5 species around 800 K, and (3) the almost complete suppression of aromatic products. However, when comparing the two experiments, one must bear in mind that the as-deposited a-C:H film changed its structure in the course of the experiment, leading to the simultaneous detection of erosion and thermal decomposition of the a-C:H film. The stationary erosion of a 3.8-nm-thick a-C:H film, preannealed at 1100 K, was investigated at 700 K, which corresponds to the maximum of the total erosion yield.12 Figure 7 shows selected QMS channels representative of the formation of methane (16 amu), ethene/ethane (28 amu), and propene/propane (41 amu), monitored during the complete erosion of the a-C:H film. According to the product distribution shown in Figure 6, these species contribute to 98% of the overall erosion rate. After
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Zecho et al. signals exhibit a steplike increase at t ) 0 s, although sp3-CHx precursor groups were removed in the course of annealing. 4. Discussion
Figure 6. Product distribution of the hydrogen-induced chemical erosion of 16-nm-thick a-C:H films, as-deposited (hatched) and after annealing at 1100 K (filled) deduced from the experiments shown in Figure 5.
Figure 7. QMS signals, representative for methane (16 amu), ethene/ ethane (28 amu), and propene/propane (41 amu) products, monitored during the erosion of an annealed, initially 3.8-nm-thick a-C:H film at 700 K. The a-C:H film was deposited at 300 K and heated to 1100 K before starting the hydrogen exposure at 700 K.
starting the hydrogen exposure at t ) 0 s, the signals exhibit a steplike increase, followed by a period of a constant erosion, until the decaying signals indicate the complete erosion of the a-C:H film. The methane channel exhibits an additional transient contribution near t ) 0 s. From the scaling of the signals, it is obvious that C2 hydrocarbon species are the main products, in accordance with the TPE experiment shown in Figure 5. Considering the sensitivity of the mass spectrometer and the pumping speed of the detected species, the overall erosion rate was determined to be ∼0.01 C/H. Astonishingly, the erosion
The temperature-induced modifications of both the electronic (Figure 1) and the vibronic (Figure 2) properties of a-C:H indicate the commencing “graphitization” above 700 K. HREELS only provides information about the hybridization of CH groups at the surface, but the corresponding bulk transition from an sp3-dominated, hydrogen-rich carbon network toward an sp2dominated structure has also been observed by means of IR spectroscopy.9 The transition is a consequence of the thermally activated release of hydrogen from sp3-hybridized CHx groups, thereby changing the hybridization state of the involved carbon atoms from sp3 to sp2: >CH-HC< f >CdC< + H2(g). Above 800 K, the structure of the carbon network is thus dominated by 3-fold-coordinated, sp2-hybridized carbon. The following discussion tries to elucidate the influence of the atomic structure of the carbon network on the hydrogen/carbon chemistry. From the data shown in Figure 3, it is evident that the heat treatment influences the kinetics of the thermal decomposition of a-C:H. After annealing and rehydrogenation, the release of hydrogen, C1 and C2 hydrocarbons, and benzene from the carbon network was observed at lower temperatures, at ∼800 K instead of ∼900 K. On the other hand, the formation of C3 to C5 species was only marginally affected by the previous history of the a-C:H film, and the release of these species from both the asdeposited and the preannealed a-C:H film was observed at 780 K. In the course of previous studies investigating the decomposition of a-C:H films,13,18 the cleavage of H-Cnetwork and C-Cnetwork bonds was identified as the rate-limiting step toward the formation of molecular hydrogen and hydrocarbon products, respectively. The cleavage of both C-H and C-C bonds generates two radical sites per bond scission, one localized on the leaving product and another embedded in the carbon matrix. The activation energy of the bond scission and thus the thermal stability of the corresponding C-H and C-C bonds is influenced by the stability of the radical products. For example, the low desorption temperature of higher hydrocarbons, C3 to C5 species, from the as-deposited a-C:H film is a consequence of the additional hyperconjungative stabilization of these radicals.35 Similarly, the atomic structure of the carbon network should affect the kinetics of the thermal decomposition of a-C:H via stabilization of the radical sites embedded in the carbon matrix. According to the EELS data shown in Figure 1, an asdeposited a-C:H film contains about equal quantities of sp2and sp3-hybridized carbon atoms. The sp2 sites form small aromatic ring clusters embedded in an sp3-type matrix.5 Annealing increases the sp2 fraction, and the graphitic clusters grow in size. Hydrogenation of the annealed a-C:H films backtransforms a thin surface layer into the original sp3-type structure and thus leads to the formation of a sandwich structure, consisting of a thin layer of a hydrogen-rich material on top of a “graphitic” carbon network. Thus, the bonding environment is quite different in both materials: sp3-CH groups in an asdeposited a-C:H film are cross-linked to an sp3-rich matrix, whereas sp3-CH groups on the surface of an annealed a-C:H film are back-bonded to a “graphitic” structure. The π-bonded systems, like aromatic rings, are capable of stabilizing radicals through delocalization. Radicals back-bonded to the “graphitic” structure of annealed a-C:H should thus exhibit an increased stability, leading to a lower value of the bond dissociation energy of the corresponding sp3-CH precursor groups. The extra
Chemical Erosion of a-C:H Thin Films stabilization of the radicals created in the course of the thermal decomposition seems thus to be responsible for lower decomposition temperature of annealed/rehydrogenated a-C:H films. Furthermore, the heat treatment of a-C:H affects both the quantity and the distribution of the decomposition products (Figure 4). Both as-deposited and annealed/rehydrogenated a-C:H films release molecular hydrogen and various hydrocarbon species during thermal decomposition. However, the total amount of hydrogen detected after rehydrogenation of an annealed sample is far smaller than that detected from an asdeposited a-C:H film. This is consistent with the assumption made above that the rehydrogenation of a “graphitic” a-C:H film is limited to a thin surface layer, leaving the bulk of the “graphitic” film unaffected. The main decomposition products of the as-deposited a-C:H film were molecular hydrogen (2 amu), methane (16 amu), and ethene/ethane (28 amu); C3 to C6 hydrocarbons were detected as minority species. During the thermal decomposition of the annealed and rehydrogenated a-C:H film, only small quantities of molecular hydrogen and methane were detected, in contrast to the nearly unchanged quantities of higher hydrocarbon products, C2 to C5 species, except for benzene (78 amu), of which the formation was strongly reduced. This suggests that hydrocarbon products originate predominantly from the surface, whereas hydrogen is formed throughout the bulk of the a-C:H film. The formation of higher hydrocarbon species implies the existence of hydrogenrich regions of the carbon network. The films investigated in the present study exhibit a comparatively low hydrogen content of ∼0.4 H/C;12 the formation of higher hydrocarbon species in the bulk of the film seems thus to be an unlikely process. However, the high concentration of CH and CH2 groups on a hydrogen-terminated surface should facilitate the formation of these products. Another striking feature of the heat treatment is the suppression of the benzene formation (Figure 3, right panel). This seems to be a consequence of the increasing size of the graphitic clusters in the course of the heat treatment. The benzene signal of the as-deposited a-C:H film probably originates from isolated benzene precursors embedded in the carbon network. The desorption temperature of ∼900 K coincides with the transformation of sp3-CHx groups to sp2-CHx-1 groups and thus indicates that these benzene precursors were generated in the course of the thermal decomposition. The product distribution detected during the thermal decomposition of an annealed and rehydrogenated a-C:H film was also affected by the sample temperature maintained during hydrogen atom exposure. The maximum of the hydrocarbon formation was observed between 300 and 500 K, whereas the detected amount of molecular hydrogen reached a maximum between 700 and 800 K. The different development of hydrogen and hydrocarbon signals are caused by the commencing decomposition of sp3-CHx groups above 600 K. The hydrocarbon products originate from the surface of the rehydrogenated a-C:H film, the location of the highest concentration of sp3-CHx groups; the rate of the thermally activated decomposition of these groups increases with increasing hydrogenation temperature, and the hydrocarbon yield thus decreases. Simultaneously, atomic hydrogen starts to diffuse into the bulk of the film via consecutive C-H bond breaking and C-H bond formation, leading to the formation of a hydrogen inventory. The retention of hydrogen at 800 K reveals that the formation and the thermally activated decomposition of sp3-CHx groups proceed with similar rates. The discussion made above suggests a correlation between atomic structure and reactivity toward erosion. Indeed, such a
J. Phys. Chem. B, Vol. 106, No. 3, 2002 615 correlation was observed by Vietzke et al.,25 who studied the hydrogen-induced erosion of different types of a-C:H films and found erosion yields of 0.014 and 0.16 C/H, respectively, for the erosion of hard and polymeric a-C:H films. Vietzke et al.26 also investigated the erosion of graphite and observed a very low erosion yield of ∼10-3 C/H. In contrast to these findings, the TPE data shown in Figure 5 indicate similar reactivities of the as-deposited and the preannealed a-C:H films. The stationary erosion yields of as-deposited12 and annealed a-C:H (Figure 7) are almost identical and exhibit a value of ∼0.01 C/H at 700 K. This result suggests that both films develop similar surface structures, and thus similar reactivities, when being exposed to hydrogen atoms. Previous experiments15,19 revealed that hydrogen atoms are capable of reversing the graphitization of the a-C:H via hydrogenation of sp2 sites, 2H(g) + >CdC< f >CH-HC
