Dependence of pyrocarbon microstructure on the substrate and annealing during the initial stage of chemical vapor deposition

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Dependence of pyrocarbon microstructure on the substrate and annealing during the initial stage of chemical vapor deposition V. De Pauw, J. Hawecker, R. Schneider, W. Send, X.L. Wang, D. Gerthsen* Laboratorium fu¨r Elektronenmikroskopie, Universita¨t Karlsruhe (TH), D-76128 Karlsruhe, Germany

A R T I C L E I N F O

A B S T R A C T

Article history:

Electron microscopic and electron spectroscopic techniques were applied to study interface

Received 3 April 2007

properties, microstructure and texture of pyrolytic carbon obtained after short-time chem-

Accepted 7 November 2007

ical vapor deposition (CVD) on planar Si substrates. The pyrolytic carbon was obtained in a

Available online 21 November 2007

hot-wall reactor from methane at a total pressure of 20 kPa and temperature of 1100 °C. Only short depositions between 2.5 and 240 min were performed. The carbon deposition starts with the nucleation of isolated islands. The increase of residence and deposition time leads to the formation of a continuous layer by larger island sizes and higher island densities, a transition from rough to smooth surfaces and formation of pores on smooth surfaces. An increased deposition rate during the first 15 min is observed which is correlated with a granular morphology of the carbon layer. Using BN-covered Si wafers with a surface roughness on a 100 nm scale reduces the texture degree in the vicinity of the interface and strengthens adhesion of the pyrolytic carbon compared to the smooth Si substrate. The texture of high-textured pyrolytic carbon is improved significantly by annealing at 1100 °C. Ó 2007 Elsevier Ltd. All rights reserved.

1.

Introduction

The physical properties of pyrolytic carbon are strongly determined by the texture, i.e. the preferential alignment of the graphene planes with respect to the substrate, which is correlated with the conditions during chemical vapor deposition (CVD) or chemical vapor infiltration (CVI). Numerous studies exist regarding the correlation between CVI and CVD parameters and texture of pyrolytic carbon which has led to a good understanding of the carbon formation mechanisms and control of bulk texture as outlined for example by Oberlin [1], Delhaes [2] and more recently by Dong and Hu¨ttinger [3] and Hu et al. [4]. Apart from the bulk properties of pyrolytic carbon, other interesting aspects concern the interface formation and structural properties of pyrolytic carbon close to the sub-

strate–carbon interface which is relevant e.g. for the mechanical properties composite materials with a carbon matrix. Although it is well known that the chemistry and morphology of the substrate surface dominates the adhesion of carbon layers, studies of the interface properties of pyrolytic-carbon/ substrate-heterostructures with high spatial resolution are relatively scarce. For instance, Bruneton et al. [5] performed a high-resolution transmission electron microscopy (HRTEM) study of the interface between different fibers and the matrix in carbon–fiber/carbon–matrix-composites. They observed a significant influence of the atomic structure of different fiber types on the structure of the adjacent matrix by analyzing the orientation of the graphene layers in the matrix close to the fiber–matrix interface. In another study [6], it was shown that a region with high texture is observed frequently at the

* Corresponding author: Fax: +49 721 608 3721. E-mail address: [email protected] (D. Gerthsen). 0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.11.009

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fiber–matrix interface under various CVI conditions using carbon fiber preforms which consist of fibers with different structure and surface morphology. It is remarkable that the interface region shows similar structural features within a wide range of the CVI conditions – even if the bulk matrix textures differ significantly. This indicates that the control of the gas phase conditions is not necessarily sufficient to predetermine the structural properties of the pyrolytic carbon in the vicinity of the substrate–matrix interface. These observations motivate studies of the early stages of microstructure and texture development at the beginning of the CVD process. For instance, Bouchard et al. [7] analyzed the microstructure of pyrolytic carbon deposits on planar graphite surfaces by scanning tunnelling microscopy. They observed preferential island formation at surface steps under deposition conditions where smooth laminar (medium-textured) pyrolytic carbon is formed. In contrast, conditions for the formation of rough laminar (high-textured) pyrolytic carbon favour the quick coverage of the substrate surface – most likely by the condensation of aromatic molecules from the gas phase. In the present work we focus on microstructure, texture development and growth rates during the initial stage of CVD as a function of the state of the gas phase (i.e. the residence time of CH4 in the reaction chamber) and deposition durations between 2.5 min and 240 min. The experiments were carried out in a hot-wall reactor by CVD on planar substrates (Si wafers and Si wafers covered by a thin BN film). We explore possible means to modify and control the structural properties close to the substrate–layer interface – apart from the gas phase conditions – by the morphology of the substrate surface and substrate chemistry as well as annealing treatments without increasing the temperature above deposition temperature. The dependence of the surface topography, thickness profiles, growth rates and texture of the pyrolytic carbon layers on the residence time, deposition duration, substrate and annealing were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) combined with electron spectroscopic techniques and selected area electron diffraction (SAED) with appropriate image analysis techniques.

2.

Experimental techniques

2.1.

Chemical vapor deposition

The hot-wall reactor used for the carbon depositions consists of a graphite tube with a diameter of 20 mm, where a single crystalline silicon wafer with a (1 0 0) surface (from Crystec, Berlin, Germany) and a length of 70 mm is introduced parallel to the gas flow. The surface-area/free-volume [A/V]-ratio is 0.26 mm1. A more detailed description of the reactor and CVD procedure can be found in Ref. [8]. It has to be emphasized with respect to the surface of the Si substrates that the crystalline Si is always covered by native silicon dioxide with a thickness of typically 1–2 nm. The substrates were cleaned using an ultrasonic cleaner (1) in distilled water with a small amount of detergent to remove greasy contaminations, (2) two steps of thorough cleaning in distilled water and (3) in acetone.

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The pyrolytic carbon was deposited from methane at a total pressure of 20 kPa and temperature of 1100 °C. The purity of methane was 99.999%. Short deposition durations between 2.5 and 240 min were chosen. The residence time s of the gas was adjusted to be 2 s at the end of a substrate with 70 mm length. With this scheme, the influence of the residence time on the texture and microstructure was studied by cutting the 70 mm long Si wafers into ten pieces, each piece corresponding to a residence time increase of 0.2 s. A scheme of the sectioning of the samples is given in Ref. [8]. The reaction chamber is filled with argon during the heating-up and flushed by argon during cool-down phase to avoid carbon formation under ill-defined conditions. To study the effect of substrate on the microstructure and texture development, depositions were also performed on Si substrates covered by 100 nm boron nitride (BN) which exhibits a rough surface morphology and different surface chemistry. The BN layer was deposited by plasma-enhanced CVD at 800 °C as outlined in Ref. [9]. In a third type of experiment, the improved adhesion between a BN-covered substrate and pyrolytic carbon layer was exploited to study the influence of annealing treatments at 1100 °C on the texture of the pyrolytic carbon. In this experiment, three deposition cycles of 120 min, consisting of a 90 min deposition (20 kPa CH4, 1100 °C) followed by a 30 min annealing treatment at the same temperature under argon atmosphere were performed. After the forth 90 min deposition the sample was cooled down immediately to room temperature.

2.2.

Structural characterization

Secondary electron imaging in SEM was applied for the characterization of the surface topography. The analyses were performed with scanning electron microscopes of the type LEO Supra 55 VP Gemini with a Schottky field-emission gun and JEOL JSM-6300F with a cold-cathode field-emission gun at 10 kV. A thin Pt layer was deposited in some cases on the sample surface to avoid charging. SEM images of freshly fractured surfaces perpendicular to the substrate surface yield qualitative information on the texture of the carbon layers. Using the classification scheme of according to Ref. [10] we distinguish between low-textured (LT), medium-textured (MT) and high-textured (HT) pyrolytic carbon. It was shown by Reznik et al. [11] that freshly fractured surfaces of HT carbon are characterized by sublayers oriented parallel to the substrate surface while carbon with a lower texture is characterized by granular or flake-like grains, some 10 nm in size. The quantification of the texture degree is not possible based on SEM images of fracture surfaces, but a reasonable qualitative impression of the carbon texture is obtained. Moreover, SEM imaging of large sample regions is possible due to quick and simple specimen preparation. The most interesting samples were selected for detailed cross-section TEM characterization in a Philips CM 200 FEG/ ST electron microscope which also allows quantification of the texture with respect to the orientation of the substrate surface. TEM cross-section samples were prepared following the procedure suggested by Bravman and Sinclair [12]. The texture is evaluated from selected area electron diffraction (SAED) patterns by measuring the orientation angle (OA) as

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outlined in detail in Ref. [13] and the use of an extension of the DALI-software package [14]. The OA is determined from intensity scans by measuring the full width at half maximum of the azimuthal opening of the arc of the (0 0 2) reflections. A small SAED aperture with a diameter corresponding to 225 nm was chosen. OA values of less than 50° are assigned to HT carbon, OA values between 50° and 80° to MT carbon, and OA > 80° to LT carbon according to Ref. [10]. The systematical error of the OA value determination is about ±1°. To analyze the chemical composition in the interface region at high spatial resolution (electron probe of 1.5 nm in diameter) energy-dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS) were performed with a 200 keV Philips CM 20 FEG scanning transmission microscope with a Tracor Northern X-ray spectrometer and Gatan imaging filter. In particular, the bonding state of silicon across the substrate–layer interface was determined by analyzing the Si–L23 electron energy near edge structure (ELNES). To avoid contamination during small-probe analysis the specimen was kept at about 178 °C by a Gatan cooling holder. Moreover, electron spectroscopic imaging (ESI) [15] in a 200 keV energy-filtering LEO 922 electron microscope equipped with an in-column Omega energy filter was applied to distinguish BN and pyrolytic carbon in the interface region. Images were recorded with a 2048x2048 CCD camera (ProScan) with a collection angle of 12.2 mrad, an energy-selecting slit of about 21 eV, and an exposure time of 5 s. The three-window method [15] was applied to extract

the carbon distribution in the interfacial zone between substrate, BN coverage and pyrolytic carbon. The carbon distribution was imaged by displaying the local intensity of the C-K edge after background fitting and subtraction by a power-law model.

3.

Experimental results

3.1. Influence of the CVD parameters on the surface topography, texture and growth The SEM images displayed in Fig. 1 visualize the formation of pyrolytic carbon layers as a function of residence time s and deposition duration t. Figs. 1a–d show the layer morphology at s values between 0.3 and 1.9 s after short-time (5 min) deposition. The fracture surfaces and surface topography for the same s values after t = 120 min are presented in Figs. 1e–h and Figs. 1i–l, respectively. Based on these images we can deduce the following deposition stages: (1) For very short deposition durations the carbon deposition starts with the formation of islands. A representative image is shown for t = 5 min and s = 0.3 s (Fig. 1a) where islands with lateral sizes of up to 30 nm and bright contrast are observed. Isolated islands can be observed up to s = 1.2 s at t = 2.5 min. For higher s values, high deposition rates lead to a complete coverage of the substrate even for t = 2.5 min.

Fig. 1 – (a–d) SEM micrographs of the sample with t = 5 min at a residence time of (a) 0.3 s, (b) 0.9 s, (c) 1.5 s and (d) 1.9 s. (e–h) SEM images of cross-section fracture surfaces of the sample with t = 120 min at a residence time of (e) 0.3 s, (f) 0.9 s, (g) 1.5 s and (h) 1.9 s. (i–l) SEM images of the surface topography of the sample with t = 120 min at a residence time of (i) 0.3 s, (j) 0.9 s, (k) 1.4 s and (l) 2.0 s. Note, that the images are not displayed at the same magnification.

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(2) The size and density of the islands increases for longer deposition times and a continuous carbon layer is formed. The deposition time at which island coalescence occurs, depends on the residence time. The coalescence stage is visualized by Fig. 1b (s = 0.9 s, t = 5 min) whereas a 55 nm thin continuous layer exists already for s = 1.5 s and t = 5 min (Fig. 1c). (3) With further increase of the deposition and residence time, smoothing of the layer surface occurs which is demonstrated by Figs. 1i–l. We observe the formation of pores on smooth surfaces, e.g. Fig. 1l, with densities and sizes that tend to vary without a pronounced trend with respect to t and s. We point out that all layers at t = 5 min are characterized by a granular structure even at high residence times (Fig. 1d at s = 1.9 s). The granular structure is maintained at longer deposition durations at low residence times (s 6 0.9 s, Figs. 1e and f). For s = 1.5 s a flake-like fracture surface is indicative for an improvement of the texture (Fig. 1g, t = 120 min). The transition to a layered fracture surface at s = 1.9 s and t = 120 min (Fig. 1h) is associated with the formation of HT pyrolytic carbon. Growth cones are observed in Figs. 1g and h which are nucleated typically by a disturbance at the substrate surface, e.g. by small particles. The resolution of the cross-section SEM image Fig. 1h (t = 120 min, s = 1.9 s) is not sufficient to analyze the morphology of the fracture surface close to the interface to decide if the granular morphology detected at t = 5 min is maintained. To compare the structure and texture of the layer–substrate interface region at s = 1.9 s in more detail, HRTEM cross-section studies were performed for the samples with 5 min 6 t 6 120 min. HRTEM cross-section images (not shown here) reveal indeed a larger degree of disorder of the turbostratic domains for the sample with t = 5 min compared to t = 120 min. OA values were obtained from SAED patterns taken close to the substrate–layer interface and at larger distances from the interface which are given in Table 1 for different deposition durations between 5 and 120 min. The results in Fig. 1d and h and Table 1 demonstrate for large residence times that (1) The morphology of the layer is dependent on the deposition time. It changes from a granular-one (t = 5 min) to a layered-one (t = 120 min) and (2) the extension of the deposition time yields a texture improvement of the layer, as shown by the reduction of the OA-value.

Table 1 – Texture degree of carbon close to the substratelayer interface surface and for the whole layer (average) at a residence time of 1.9 s as a function of the deposition duration Deposition duration (min)

OA (deg), whole layer OA (deg), near the substrate

5

15

30

120

40 ± 5 40 ± 5

33 ± 5 32 ± 6

36 ± 7 35 ± 6

30 ± 8 30 ± 10

The OA values are given with 2r values.

Fig. 2 – Deposition rate as a function of deposition time at different residence times s on Si substrates.

Fig. 2 displays the deposition rate as a function of the deposition duration at different residence times. The deposition rates are deduced from the measured layer thicknesses in units of layer thickness per deposition time because the substrate surface and [A/V]-ratio was identical in all cases. Only continuous layers were considered which are observed for t P 5 min at s P 0.9 s. For lower s values, continuous layers require deposition durations t P 15 min. The growth rates increase generally with the residence time. However, we observe consistently increased deposition rates at t = 5 and 15 min compared to longer t values. The deposition rate drops at t = 30 min. Only small variations of the deposition rates are observed for t P 30 min which is assigned to the achievement of steady-state deposition conditions.

3.2. The effect of substrate morphology and surface chemistry on the carbon structure The influence of the substrate roughness and surface chemistry on the carbon texture and structure was assessed by comparing two layers deposited under the same conditions with a deposition time t = 30 min on a bare and BN-covered Si substrate. In contrast to the smooth Si wafers, the SEM image of the BN surface (Fig. 3a) shows a porous structure with a roughness on a 50–100 nm scale. Fig. 3b and c display crosssection TEM images at s = 1.9 s. Fig. 3b is an ESI image of the carbon layer on the BN-covered substrate which was taken with the C-K edge which shows infiltration of the porous BN (dark contrast) by pyrolytic carbon (bright contrast). The bright-field image Fig. 3c, which was taken at the same position as Fig. 3b, shows a MT layer with a thickness of 100 nm on top of the BN. The inserted diffractogram (Fourier transformation of a HRTEM image of the interface region) yields an OA value of 69°. The texture improves to HT – as expected at s = 1.9 s – about 100 nm above the substrate surface. In contrast, the texture of the carbon layer on the flat Si substrate does not depend on the distance from the substrate surface (see Table 1, with OA = 35° close to the interface and OA = 36° for the whole layer). Fig. 4 presents HRTEM cross-section images of the interface region between the carbon and the silicon (Fig. 4a) and

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Fig. 3 – (a) SEM image of the BN-covered Si substrate before carbon deposition. (b and c) Cross-section TEM micrographs of the interface region between the BN-covered substrate and the pyrolytic carbon layer deposited at s = 1.9 s for 30 min. (b) Micrograph taken in the electron spectroscopic imaging (ESI) mode using the C-K edge. Carbon is displayed in light gray and BN/Si in black contrast. (c) Bright-field TEM image of the same region as (b) with a diffractogram (i.e. a local Fourier transform of a HRTEM image) which was taken in the carbon region directly above the BN.

BN-covered Si substrate (Fig. 4b). An amorphous SiO2 layer on top of the crystalline silicon with a thickness of about 10 nm is found in both cases. The structure of the carbon layer in Fig. 4a is characterized by nm-sized turbostratic domains (two examples are marked) which are typical for HT pyrolytic carbon (OA = 35° in this sample region). The nanostructure of interface region for the BN-covered substrate (Fig. 4b) is different. The fringes close to the amorphous layer are attributed to the BN. The regions with fringes decrease with increasing distance from the interface which is consistent with the porous nature of the BN layer shown in Fig. 3a. The material between the fringes does not show a pronounced contrast (two regions are encircled in Fig. 4b) which is indicative of material with a low degree of order. Although the distinction between BN and carbon by HRTEM is not straightforward, the disordered mate-

rial is attributed to the infiltrated carbon which is clearly seen in the ESI image Fig. 3b. We note that the BN surface is not necessarily oriented parallel to the electron beam and the local lack of fringes may be also due to the misalignment between the electron-beam direction and graphene planes. To analyze possible chemical reactions of the gaseous hydrocarbon molecules with the Si substrate EELS, ELNES and EDX analyses were performed with high spatial resolution across the interface between the Si wafer and the carbon layer. Fig. 5a shows a cross-section HRTEM image of the interface region between the crystalline Si substrate (with an orientation along the [1 1 0]-zone axis) and the pyrolytic carbon layer. EEL spectra were taken across the interface region along the line in Fig. 5a with a probe diameter of about 1.5 nm and a

Fig. 4 – Cross-section HRTEM images of the interface region between the pyrolytic carbon and the (a) bare SiO2/Si substrate and (b) BN-covered substrate for t = 30 min and s = 1.9 s.

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Fig. 5 – (a) Cross-section HRTEM image of the interface region. The EEL spectra in the Si substrate and SiOxCy layer displayed in (b) were taken along the line marked in (a). (b) shows the local change of the Si-L2,3 ELNES.

step width of 1.5 nm. Fig. 5b shows the corresponding local variation of the Si-L2,3 edge (edge onset at E  99 eV) with the background subtracted. Two effects can be observed, namely that the fine structure of the Si-L2,3 edge changes in a characteristic way and the edge onset as well. In more detail, the Si-L2,3 edge of pure silicon is characterized by a threshold value of approximately 99 eV and its ELNES by a pronounced maximum at about 100.5 eV. These peculiarities are visible in case of the six posterior spectra of the series drawn in Fig. 5b as expected for the substrate. The interface between Si substrate and oxide layer is indicated by a drastic drop of the peak intensity at 100.5 eV. The next two following spectra show fine-structure features being typical for SiO2, i.e. two distinctly split relatively sharp peaks at about 108 eV and 115 eV, where additionally the edge onset is chemically shifted to approximately 105 eV. Moreover, in these spectra indications for a small signal starting at 100 eV are observed that can be attributed elemental Si – possibly due to Si which was redeposited on the TEM sample surface by ion milling during TEM sample preparation. In the last three spectra in the foreground the net Si-L2,3 intensity decreases step by step. In addition, a strong broadening of the peak at 108 eV together with an onset shift towards about 101 eV occurs, where the originally second oxidic peak (115 eV) vanishes gradually. This behavior is a clear indication of supplementary Si–C bonds [16]. Thus, the outermost zone of the layer between the Si substrate and the carbon layer consists of silicon oxycarbide. The presence of an oxycarbide is also verified by the detection of carbon and oxygen in addition to Si in respective EEL and EDX spectra. A carbon concentration between 5 at.% and close to 50 at.% was estimated to be contained in the silicon oxycarbide layer by quantitative EELS. Despite quantification errors, the large scatter of the data on the C concentration is related presumably to different local compositions. Other elements apart from Si, C, and O were not

detected indicating that the concentration of impurities must be below the detection limit in the order of 0.1–1 at.%.

3.3.

Development of the carbon texture on annealing

A practical motivation for using rough substrate surfaces is the improvement of the adhesion between the pyrolytic carbon and the substrate. Applying the repeated deposition/ annealing cycle described in Section 2.1 we observe a stronger adhesion on BN compared to depositions on Si substrates with a native SiO2 layer where the pyrolytic carbon tends to exfoliate. The SEM image of the cross-sectional fracture surface Fig. 6a taken at a position corresponding to s = 1.7 s shows the formation of a high density of growth cones induced by the roughness of the BN film. The cones are the origin of slight intensity variations in the overview cross-section TEM image (Fig. 6b) within the lower part (1 lm) of the carbon layer. A pronounced straightening of the sublayers parallel to the substrate surface and intensive fragmentation occurs in the center and upper part of the carbon layer which is indicative of a high texture degree. SAED patterns (Fig. 6b) taken in the lower part of the layer containing growth cones yield OA = 34° and an exceptional low value of OA = 24° ± 1° in the region without growth cones. Such a high texture was never observed by us without annealing.

4.

Discussion

4.1. Influence of the CVD parameters and annealing on the carbon structure The initial stage of carbon CVD under the applied conditions is characterized by the formation of nano-sized islands which

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Fig. 6 – (a) SEM micrograph of the cross-section fracture surface of the carbon layer on a BN-covered Si–substrate obtained by four depositions of 90 min interrupted by 30 min annealing treatments at 1100 °C and (b) overview cross-section TEM micrograph. The SAED patterns inserted in (b) are taken close to the surface and close to the BN/ carbon interface.

can be detected up to s = 1.2 s at t = 2.5 min. Coalescence of the islands leads to the formation of a continuous layer for increasing deposition and residence times. At higher s values, where isolated islands cannot be found even at t = 2.5 min, the granular morphology of the layers can be considered as an indicator that islands are also formed initially. The texture degree and growth rate increases, as expected, with the residence time. These results are consistent with the results of previous work which is extensively discussed in Ref. [8] where the same deposition temperature but a gas mixture of 10% CH4/90% Ar at a total pressure of 100 kPa was used. For short, the texture degree is closely related to the gas phase composition. MT pyrolytic carbon is obtained if small linear hydrocarbon species dominate at low s values. Maturation of the gas phase at higher s leads to the formation of HT carbon due to the presence of larger hydrocarbon species and polyaromatic hydrocarbon (PAH) molecules. An optimum ratio between small linear and large PAH molecules is necessary for the formation of HT pyrocarbon according to the particle-filler model proposed by Dong and Hu¨ttinger [3].

An unexpected observation is the improvement of the texture with increasing deposition time at high residence times (s = 1.9 s) close to the carbon–substrate interface. The carbon layer exhibits a granular morphology for t = 5 min (Fig. 1b–d) which can be correlated with the nucleation and coalescence of islands at the beginning of the CVD process. The granular structure is even observed at s = 1.9 s (Fig. 1d for t = 5 min) where HT pyrolytic carbon is formed for longer depositions (Fig. 1h for t = 120 min, carbon with layered fracture surface). Cross-section TEM and SAED analyses for s = 1.9 s reveal a reduced texture degree for t = 5 min compared to carbon formed during longer depositions (see Table 1 values close to the substrate: OA = 40° for t = 5 min vs. 36° 6 OA 6 30° for t P 15 min). These findings indicate that the deposited carbon can change during the ongoing deposition. It is well known that an improvement of the alignment of graphene planes – in the sense of graphitization – requires temperatures significantly above 1100 °C. However, it was also shown by Lavenac et al. [17] that the deposited pyrolytic carbon contains a significant amount of hydrogen which can be released partially during the deposition process itself and during cooldown from deposition to room temperature. Dehydrogenation was shown to be correlated with the formation of pores according to Ref. [18]. Pores are indeed observed at extended deposition durations (e.g. Fig. 1l, and even more pronounced for t = 240 min) which can be taken as an indication for hydrogen release. We therefore propose that the texture improvement of the initially deposited carbon is at least partially related to the release of hydrogen which may promote straightening and ordering of the graphene planes. Annealing at 1100 °C during 30 min deposition interruptions was found to result in an amazing improvement of the carbon texture (OA = 24° with respect to 30° without annealing). The texture improvement was achieved despite the rough surface of the BN-covered Si wafer and a high density of growth cones which are nucleated at the substrate–carbon interface. We speculate that the origin of the texture improvement is again related to dehydrogenation like the texture improvement close to the substrate interface during longer depositions at s = 1.9 s. Approximately 1 lm pyrolytic carbon is formed during each 90 min deposition sequence. Interrupting the growth for 30 min at 1100 °C leaves plenty of time for dehydrogenation and structural rearrangement on the nanoscale. The result could be a significantly improved alignment of turbostratic domains due to the vacancies left by the released hydrogen.

4.2. The effect of surface morphology and chemistry of the substrate on the carbon structure and growth rate Apart from the granular morphology we observe an enhanced carbon formation rate during the very early stage of CVD. A possible explanation for this finding can be provided by the effect of the substrate chemistry. Hashishin et al. [19] suggested that SiC, CO and H2O are formed if SiO2 and CH4 react at temperatures above 1000 °C. It is also well known that a thin layer of native SiO2 on a Si wafer decomposes at temperatures above 900 °C at low oxygen partial pressure according to Lander and Morrison [20] and Gelain et al. [21] by the following reaction:

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Si(s) + SiO2 (s) ! 2SiO(g) The presence of SiO molecules in the gas phase will affect the gas phase reactions. Moreover, SiC may be formed by the following reaction [19]: SiO(g) + C(g) + 2H2 (g) ! SiC(s) + 2H2 O(g) The incorporation of carbon in the SiO2 coverage of the substrate and Si–C bonds could be indeed detected by EDXS and EELS measurements (Fig. 5). SiO molecules and related reaction products in the gas phase will also change the maturation of the gas phase and carbon formation processes as compared to a pure carbon surface during the more advanced stages of the CVD process. Moreover, impurities (e.g. Si) may be catalytically active. The transition from initial to steady-state deposition condition would then be related with the disappearance of the chemical effects of the substrate. The comparison of the carbon structure close to the substrate–layer interface deposited under identical conditions on a silicon and BN-covered Si substrate clearly demonstrates the strong influence of the morphology and chemistry of the substrate surface. This is particularly obvious at high residence time (s = 1.9 s) where HT carbon is directly formed on a Si wafer (OA values Table 1 and Fig. 4a). In contrast MT carbon is formed within 100 nm on top of the porous BN layer. Fig. 4b shows regions with disordered structure within the BN layer which are attributed to the infiltrated carbon. The carbon properties close to the BN layer are connected with an improved adhesion compared to carbon on a Si wafer. Several effects must be taken into account to contribute to the observed changes of the carbon structure in the vicinity of the BN–carbon interface: (1) During the infiltration of the BN layer, the [A/V]-ratio within the pores of the BN is significantly higher than on a planar substrate. This will alter the local composition of the gas phase and the structure of the deposited carbon. (2) The chemical effect of the SiO2 on the gas phase reaction will be diminished due to the BN coverage. (3) The rough surface of the (infiltrated) BN impedes the formation of straight graphene layers and leads to a lower-textured material (within 100 nm above the BN layer) with a high density of growth cones close to the interface compared to smooth substrate surfaces.

5.

Summary

The microstructure of pyrolytic carbon layers and the interface between Si wafers and carbon was analyzed by electron microscopic and electron spectroscopic techniques after short-time chemical vapor deposition using 20 kPa CH4 at 1100 °C. A distinction can be made between initial and steady-state carbon formation. The initial deposition stage is characterized island formation or a layer with granular morphology and an increased carbon formation rate. The granular layer structure changes to a layered-one with improved texture at high residence times where high-textured pyrolytic

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carbon is obtained under steady-state conditions. The transition between initial and steady-state conditions is assigned to the effects of the substrate chemistry. The native SiO2 layer on Si wafers changes the gas phase reactions compared to a pure carbon surface and possible catalytic reactions may result in an enhanced carbon formation rate. The carbon structure is strongly modified if a BN-covered Si substrate is used. The chemical effects of the substrate are reduced due to the BN coverage and the porous BN morphology leads to the formation of medium-textured carbon with a significantly improved adhesion. We observe an improvement of the texture under conditions where high-textured pyrolytic carbon is formed (a) during the deposition process itself after the transition from initial to steady-state CVD conditions and (b) in a significant way by interrupting the growth for 30 min at deposition temperature (1100 °C). We suggest that the texture improvement is at least partially related to hydrogen release from the deposited material resulting in an improved alignment of turbostratic domains on the nanoscale. However, further studies are necessary to clarify this point. Our study shows that the morphology and chemistry of the substrate surface as well as annealing at a comparably low temperature can be exploited to control – to a certain degree – the pyrolytic carbon properties close to the substrate–carbon interface.

Acknowledgements This research was performed in the Center of Excellence in Research (Sonderforschungsbereich) 551 at the University of Karlsruhe funded by the German Research Foundation (Deutsche Forschungsgemeinschaft). The authors are grateful to Prof. W.A. Goedel and T. Thamm (Department of Physical Chemistry, Institute of Chemistry, Chemnitz University of Technology, Germany) for the deposition on BN films on Si wafers. We also thank Prof. U. Go¨sele for giving us access to the microscopy facility of Max Planck Institute for Microstructure Physics (Halle, Germany).

R E F E R E N C E S

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