Vertical Graphene Growth from Amorphous Carbon Films Using

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The Journal of Physical Chemistry

Vertical Graphene Growth from Amorphous Carbon Films using Oxidizing Gases Alicja Bachmatiuk,†, ‡ John Boeckl,¥ Howard Smith, ¥,# Imad Ibrahim,‡, § Thomas Gemming,‡, § Steffen Oswald, ‡ Wojciech Kazmierczak, ± Denys Makarov,‡ Oliver G. Schmidt, ‡ Juergen Eckert,‡, § Lei Fu,$ and Mark H. Rummeli*, ǁ, ▲ † Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M. Curie-Sklodowskiej 34, Zabrze 41-819, Poland, ‡ IFW Dresden, PO Box 270116, D-01171 Dresden, Germany, ¥ Air Force Research Laboratory, Materials & Manufacturing Directorate, Wright Patterson AFB, OH 45433 USA # University of Dayton Research Institute, Dayton OH 45469 USA § Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany, ± Labsoft Krzysztof Herman, PL-02828 Warsaw, Poland, & Technische Universität Dresden, Institute of Materials Science, 01062 Dresden, Germany, $ College of Chemistry and Molecular Science, Wuhan University, 430072 Wuhan, China, ǁ IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Daejon 305701, Republic of Korea, ▲ Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon

ACS Paragon Plus Environment

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440-746, Republic of Korea,

KEYWORDS: amorphous carbon, graphene, crystallization, carbothermal reduction

ABSTRACT Amorphous carbon thin films are technologically important materials that range in use from the semiconductor industry to corrosion resistant films. Their conversion to crystalline graphene layers has long been pursued, however typically this requires excessively high temperatures. Thus, crystallization routes which require reduced temperatures are important. Moreover, the ability to crystallize amorphous carbon at reduced temperatures without a catalyst could pave the way for practical graphene synthesis for device fabrication without the need for transfer or post-transfer gate deposition. To this end we demonstrate a practical and facile method to crystallize deposited amorphous carbon films to high quality graphene layers at reduced annealing temperatures by introducing oxidizing gases during the process. The reactive gases react with regions of higher strain (energy) in the system and accelerate the graphitization process by minimizing criss-cross linkages and accelerating C-C bond rearrangement at defects. In other words, the movement of crystallite boundaries are accelerated along the carbon hexagon planes by removing obstacles for crystallite coalescence.

Interest in graphene and other 2D materials (e.g. MoSe2) continues to quicken. In the case of graphene, tremendous advances have been made in its large scale catalyst assisted synthesis,1-5 however, there remain some serious drawbacks, in that for the most part, the as-produced material needs to be removed from its metal catalyst/support (typically Cu) and then transferred


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to a substrate for further processing, for example, for device fabrication. The processing steps often lead to damage of the graphene and leaves contamination material. In addition, the deposition of gate oxides when using graphene in devices can also damage the graphene. Thus, routes which can directly grow graphene on the desired substrate, for example Si/SiO2 wafers are attractive. The direct chemical vapor deposition (CVD) growth of graphene over oxides is possible but issues in terms of the graphene quality and scalability remain.6-10 Plasma enhanced CVD, PECVD, can be used to grow (vertical) graphene on almost any substrate, and like thermal CVD graphene growth over oxides, this is for the most part also a non-catalytic growth process.11-14 The catalyst-free growth of graphene from amorphous carbon has also been explored. This process usually requires high temperatures. Theoretically this has been shown to occur at temperatures of 1500 °C.15 Experimentally lower temperatures of even 1000 °C can start to graphitize amorphous carbon, however at this low temperature the graphitization is very limited. Generally, temperatures of well over 1000 °C are required to graphitize amorphous carbon. The annealing of amorphous carbon to obtain graphene can be achieved through conventional heating in a furnace in an inert atmosphere, e.g. Ar, or in vacuum 16-20 or through so called current annealing.21,22,23 Electron beam induced graphitization of amorphous carbon is also possible.24,25 In addition, the amount and distribution of residual elements such as hydrogen, nitrogen, oxygen, chlorine etc. are known to affect the graphitization efficiency and hence graphitization temperature.15,16, 23 In particular, the presence of gases of an oxidizing character, e.g. oxygen, water vapor etc., are argued to enhance graphitization by the preferential oxidation of cross links as first suggested by Noda and Inagaki.24, 25


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Here we explore the use of gaseous CO and SiO, produced by the carbothermal reduction of SiO2, as oxidizing gases to accelerate the formation of vertical graphene from amorphous carbon films deposited over Si/SiO2 wafers at an elevated temperature of 1000 °C in high vacuum. In the carbothermal process solid carbon reduces SiO2 to SiC (using solid carbon i.e. amorphous carbon) as: 3C + SiO2(s) → SiC(s)+ 2CO(g) in a two-step reaction: 1.

C + SiO2(s)→SiO(g)+ CO(g)

2.

2C + SiO(g)→SiC(s)+ CO(g)

The reaction is strongly endothermic.26 The production of oxidizing gases (CO and SiO) occurs in the first reaction step of the carbothermal reduction cycle. To set up the experiment a thin, 10 nm, layer of amorphous carbon is sputtered onto a Si/SiO2 wafer with an oxide layer of 100 nm. Later, a thin SiO film, 10 nm, is deposited on the surface so as to trap the oxidizing gases, SiO and CO, during annealing. For comparison samples without a SiO top film were also used as shown in figure 1.


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Figure 1. Schematic of starting samples structures. Left panel: 10 nm amorphous carbon deposited on Si wafer with 100 nm SiO2 layer, Right panel: 10 nm SiO layer deposited on 10 nm of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer. The graphitization of the amorphous carbon is clear for samples with a SiO top film which traps the produced CO and SiO gases upon annealing at 1000 °C in a furnace in high vacuum (< 7 x 10-7 mbar). For samples in which no SiO capping layer is used to trap the produced gases, only a slight graphitization is observed, clearly establishing the importance of oxidizing gases in the graphitization process of amorphous carbon at 1000 °C. RESULTS We begin by examining the Raman spectra for the starting samples and those after annealing at 1000 °C in pure Ar. Raman spectroscopy is a very powerful tool with which to study amorphous carbon and sp2 crystalline carbon (graphene/graphite) through various modes at ca. 1350 cm-1 (D mode), ca. 1600 cm-1 (G mode) and ca. 2750 cm-1 (2D mode).27 The left panel of figure 2 shows spectra for the samples prior to annealing for the Si/SiO2 wafers with only an amorphous carbon film and with an amorphous carbon film coated with a SiO topping layer. They both show a strong broad asymmetrical peak between 1100 cm-1 and 1800 cm-1 in which the broadened D and


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G modes have merged, which is typical for amorphous carbon. The broadening arises from various effects, namely, the cluster size of sp2 carbon planes, cluster distribution, the influence of stress in the films, broadening due to chemical bonding and defects.28 A smaller peak at ca. 1000 cm-1 can also be observed and this is attributed to Si from the wafer.29 As expected there is no obvious difference between the two starting samples. After annealing changes can clearly be observed, most notably for the sample with an SiO capping layer. We first examine spectrum (a) in the right panel of figure 2 which corresponds to the amorphous C film over the wafer with no capping layer and compare that of the sample before annealing (left panel) . The G and D modes have become narrower but remain merged. This leads to a clear shoulder on the left side of the broad asymmetric peak. This indicates only minor graphitization of the amorphous carbon film has occurred. The Si peak at ca. 1000 cm-1 remains visible.

Figure 2. Raman spectra (λ=442 nm) of Left panel: starting samples (a) 10 nm amorphous carbon deposited on Si wafer with 100 nm SiO2 layer, (b) 10 nm SiO layer deposited on 10 nm


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of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer. Right panel: vacuum annealed samples at 1000 °C (a) 10 nm amorphous carbon deposited on Si wafer with 100 nm SiO2 layer, (b) 10 nm SiO layer deposited on 10 nm of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer. Inset: zoomed region showing formation of silicon carbide. The Raman spectrum collected from the sample with the capping layer over the amorphous carbon (spectrum b) from the right panel of figure 2 looks markedly different. The G mode is very narrow, the D mode is well defined and weak indicating few defects are present 26 and the 2D mode is now clearly present. These well-defined modes indicate well crystallized sp2 carbon (graphene layers) now exists. The silicon mode is also still present, however now a weak additional mode at ca. 800 cm-1 is present (see inset). This new mode is attributed to the presence of SiC which results from the carbothermal reduction of SiO2.30 To better understand the carbon films after the annealing process in some samples the carbon film was mechanically cleaved and then subjected to examination in TEM. Typical examples are provided in figure 3. The top row shows the sample with no capping layer and no real order in the material can be seen in the micrographs in essence showing amorphous carbon. Diffraction data extracted from the Fourier domain confirms the presence of amorphous carbon in agreement with the Raman data. The lower panel shows clean crystalline graphene layers in which the honeycomb lattice for graphene is clearly observed. Moreover the diffraction data show clear reflexes corresponding to graphene. This is in perfect agreement with the Raman spectroscopy data.


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Figure 3. TEM images of transferred material from vacuum annealed substrates at 1000 °C, Upper panel: 10 nm amorphous carbon deposited on Si wafer with 100 nm SiO2 layer, Lower panel: 10 nm SiO layer deposited on 10 nm of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer. We also conducted further TEM studies in which cross-section lamellas of the samples were investigated as shown in figure 4. The different layers and interfaces of the sample are easily observed and are also labeled in the micrographs. As with the mechanically cleaved samples the


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C film in the uncapped sample is amorphous (top row) while for the capped sample (lower row) dark contrast lines normal to the substrate surface are easily observed. Close examination of the spacing between these lines show an average distance of 0.35 nm concomitant with the spacing between graphene layers in few layer graphene and graphite (see figure S1 in the supporting information). The data indicates vertical graphene layer formation from amorphous carbon having occurred.

Figure 4. TEM images of cross-sectional lamellas prepared after vacuum annealing of the samples at 1000 °C, Upper panel: 10 nm amorphous carbon deposited on Si wafer with 100 nm


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SiO2 layer, Lower panel: 10 nm SiO layer deposited on 10 nm of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer. In the SiO capped samples typically a dark region of contrast could be observed (bottom right panel of figure 4 and figure S2 in the supplementary information). To better comprehend the layers we conducted electron energy loss spectroscopy line scans across the different layers of the lamella samples. The data are summarized in figure 5.

Figure 5. Left panel: EEL spectra (Si-L and C-K) of the positions marked in figure 4 from 10 nm of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer after vacuum annealing at 1000 °C. Right panel: EEL spectra (Si-L and C-K) of the positions marked in figure 4 from 10 nm SiO layer deposited on 10 nm of amorphous carbon, deposited on Si wafer with 100 nm SiO2 layer after vacuum annealing at 1000 °C. Insets: Si-L regions, showing silicon carbide formation (left inset). The Si (2p) edge begins close to 100 eV while that for Carbon (1s) begins at ca. 290 eV. The line scan spectra confirm the elemental layers of the samples. To check for SiC, the differences in the


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fine structure of the C1s edge between pure carbon and SiC are not easily discerned. They are more easily discerned from the Si2P edge. To do so we employ a subtraction technique 32 in which we subtract the SiOx signal from the pure SiOx layer from the interface region between the carbon and wafer silicon oxide layer, and also the SiO layer in the capped sample, as shown in the insets of the panels in figure 5. In the uncapped sample no SiC is detected while in the capped sample SiC is detected at the carbon-silicon oxide interface in the wafer. X-ray photoemission spectroscopy (XPS) studies were also conducted. For samples with no capping layer no SiC was detected regardless of measurement angle. In the case of capped samples, weak SiC peaks were detected when measuring at low angles. However, given XPS is a highly surface sensitive technique probing typically only a few nm in depth for silicon oxides it seems unlikely that the SiC can be unequivocally attributed to the SiC layer forming over the underlying SiO2 layer. A more likely scenario is that local areas of the SiO capping layer were oxidized during air exposure which later in the reaction undergo a carbo-thermal reaction to form SiC, see figure S3 in the supplementary information. The core level spectroscopy investigations are in excellent agreement with the Raman spectroscopic data which also indicates the presence of SiC from capped samples and confirms that in these samples the carbothermal reduction process has been active and hence SiO and CO have been produced during the reaction. DISCUSSION We now turn to explaining the experimental observations. In the uncapped sample no capping film resides over the amorphous carbon film so that once the carbothermal reaction begins the gaseous products from the first part of the reaction (SiO and CO) diffuse out of the amorphous carbon film and are removed since the system sits in high vacuum. For this reason the second part of the reduction process cannot occur and no SiC forms at the amorphous carbon/SiO2


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interface. Any graphitization that occurs is then predominantly due to annealing and at 1000 °C this is negligible as confirmed by our experimental data. With the SiO capping layer in place, the gaseous products, i.e. SiO and CO, from the first step of the carbothermal reaction are trapped by the capping layer thus enabling the second reaction step to occur and a thin SiC layer forms at the carbon/SiO2 interface. The SiC layer in essence informs us that CO and SiO gases were trapped and given that annealing at 1000 °C hardly graphitizes the amorphous carbon it is clear the SiO and CO gases play a role in accelerating the graphitization process so as to form high quality vertical graphene layers. It is not clear why vertical graphene layers form since previous studies annealing amorphous carbon on a substrate at sufficiently high temperatures to graphitize the material tend to form graphene layers parallel to the substrate surface 19,23 and this is argued to occur due to the van der Waals interactions between the C atoms and the substrate. We postulate that in our case, we form SiC, through a carbothermal reaction, and it is well known that that SiC produced by this method has a high degree of stacking faults and step sites.32 Thus, these steps, which are high reactivity sites, provide locations were carbon binding is more favorable enabling the edges of a developing graphene edge to anchor at the SiC-C interface. This is also in keeping with graphene edge stabilization at SiC steps formed from the decomposition of SiC.33 In terms of the role of SiO and CO gases in accelerating the graphitization of the amorphous carbon, this probably occurs through a process first conceived by Noda and Inagaki.24,25 In this process chemically reactive gases can accelerate the graphitization process. Initially heat provides a means for tiny graphene platelets to form. Large internal stresses will exist due to the anisotropy in thermal expansion of the graphene/graphite crystallites. The stresses are likely concentrated at the criss-cross linkages between platelets and these regions under strain will be


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more reactive to an oxidizing gas. This is in agreement with studies showing preferential oxidation of graphene and graphite at edges and defects 34 and narrow diameter carbon nanotubes being more reactive than larger diameter tubes due to their additional strain.35 Thus, as the reactive gas reacts with regions of higher strain, energy, the system can accelerate the graphitization process by minimizing criss-cross linkages and accelerating C-C bond rearrangement at defects, viz. the movement of crystallite boundaries are accelerated along the carbon hexagon planes by removing obstacles for crystallite coalescence.25 CONCLUSION

In this work the crystallization of amorphous carbon films on silicon wafers at relatively low temperature using oxidizing gases is demonstrated. The oxidizing gases are produced as part of a carbothermal chemical reaction and in this case the gases need to be trapped by a capping oxide layer. The demonstrated technique shows how, in principle, amorphous carbon trapped in a gate oxide could be converted to high quality graphene. This removes the need for damaging steps, namely, to transfer graphene or to deposit a gate material on graphene. The role of the oxidizing gases (SiO and CO in this case) help accelerate the graphitization progression by minimizing criss-cross linkages and accelerating C-C bond rearrangement at defects. In effect, this allows the movement of crystallite boundaries along the carbon hexagon planes by removing obstacles for crystallite coalescence as described by Noda et al in references 24 and 25.

The presented approach could be modified to use oxidizing gases directly further enhancing the versatility of the approach. The direct introducing of an oxidizing gas would globally graphitize any amorphous contact it comes in to contact with, while capped systems based on the carbothermal process could be more local and avoid the need for a post synthesis gate deposition that


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might affect the graphene. Moreover, the use of amorphous carbon films and deposited oxides are both fully compatible with existing techniques in the semiconductor industry. Supporting Information Available. Supporting TEM images of cross-sectional lamellas and XPS measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

METHODS Thin films of amorphous carbon and silicon monoxide were prepared over P-doped silicon wafers with 100 nm SiO2 layer. The growth of amorphous carbon was carried out using RFmagnetron sputter deposition at room temperature in a high vacuum chamber using argon as the sputter gas. The base pressure in the chamber was 8 x 10-8 mbar and partial pressure of argon during the deposition process was 7.5 x 10-4 mbar. The deposition rate for the amorphous carbon was 0.35 Å/s. The silicon monoxide capping layers were deposited using a commercial target in electron beam evaporation in a high vacuum chamber with a base pressure of 6 x 10-6 mbar. The deposition of the SiO capping was performed at room temperature at the rate of 2 Å/s. Two types films on Si wafers were prepared; one with only a 10 nm amorphous carbon layer and the other with a 10 nm of amorphous carbon layer followed by capping layer 10 nm of silicon monoxide. Both of the substrates were transferred into a quartz tube reactor and annealed under high vacuum conditions (< 7 x 10-7 mbar) at 1000 °C for 72 hours.


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Raman spectroscopy measurements of the substrates before and after annealing were performed using an inVia Raman microscope spectrometer from Renishaw utilizing a 442 nm laser and approximately 2 mW power. Aberration-corrected high-resolution transmission electron microscopy (HR-TEM) studies were accomplished using two microscopes; A Jeol JEM-2010F retrofitted with two CEOS third-order spherical aberration correctors for the objective lens (CETCOR) and the condenser system (CESCOR) and an image corrected (CEOS) FEI Titan3 80-300 equipped with electron energy loss spectrometer (Gatan Tridem 865 ER). Both TEM instruments were operated at 80 kV. Specimen preparation for HRTEM investigations were accomplished by two methods. In the first, by mechanical cleaving and the second using a focused ion beam (FIB) lift-out technique. For the mechanical cleaving sample preparation, a standard Cu TEM grid was sandwiched between the sample and a clean Si wafer. They were then compressed by hand and twisted slightly. After parting the Si wafer off the sample, the TEM grid was removed and loaded for examination in the TEM. Two FIB machines were used for the FIB prepared samples; a FEI Strata DB235 and FEI Helios 600i. Photoemission X-ray spectroscopy was measured using a PHI5600LS spectrometer equipped with a mono-chromatic Al K α source (1486.6 eV) with an overall spectral resolution of 0.5 eV and operating with a base pressure of 5 x 10-10 mbar). Conflict of Interest: The authors declare no competing financial interest. Corresponding Author


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* Correspondence should be addressed to Mark H. Rummeli, email: [email protected], tel: +8201099844055 ACKNOWLEDGMENT The research was supported by the Institute of Basic Sciences, Korea and the Sino-German Center for Research Promotion (Grants GZ 871). All are gratefully acknowledged. A.B. thanks the Foundation for Polish Science for the financial support within the frames of the Homing Plus Programme (Grant agreement HOMING PLUS/2013-7/2). I.I. thanks the German Excellence Initiative via the Cluster of Excellence EXC1056 “Center for Advancing Electronics Dresden” (CfAED). We also thank S. Avdoshenko for discussion and input to the underlying mechanisms.

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The catalyst-free conversion of amorphous carbon to highly crystalline graphene by simple annealing at 1000 oC in oxidizing gas is demonstrated.


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Vertical Graphene Growth from Amorphous Carbon Films Using

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