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Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate
Micron 68 (2015) 17–22
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Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate Krzysztof Gajewski a,∗ , Daniel Kopiec a , Magdalena Moczała a , Adam Piotrowicz a , b ´ Michał Zielony a , Grzegorz Wielgoszewski a , Teodor Gotszalk a , Włodek Strupinski a b
Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, ul. Z. Janiszewskiego 11/17, PL-50372 Wrocław, Poland Institute of Electronic Materials Technology, ul. Wolczynska 133, PL-01919 Warsaw, Poland
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Article history: Received 7 April 2014 Received in revised form 12 August 2014 Accepted 12 August 2014 Available online 20 August 2014 Keywords: Graphene SiC STM C-AFM KPFM
a b s t r a c t Sublimated graphene grown on SiC is an attractive material for scientific investigations. Nevertheless the self limiting process on the Si face and its sensitivity to the surface quality of the SiC substrates may be unfavourable for later microelectronic processes. On the other hand, chemical vapor deposited (CVD) graphene does not posses such disadvantages, so further experimental investigation is needed. In this paper CVD grown graphene on 6H-SiC (0 0 0 1) substrate was investigated using scanning probe microscopy (SPM). Electrical properties of graphene were characterized with the use of: scanning tunnelling microscopy, conductive atomic force microscopy (C-AFM) with locally performed C-AFM current–voltage measurements and Kelvin probe force microscopy (KPFM). Based on the contact potential difference data from the KPFM measurements, the work function of graphene was estimated. We observed conductance variations not only on structural edges, existing surface corrugations or accidental bilayers, but also on a flat graphene surface. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Graphene, a two-dimensional carbon material, has attracted the attention of scientists for many years (Geim and Novoselov, 2007). This is result of: its extraordinary stiffness, optical transmission and electrical properties, making graphene a prospective candidate for surface coating, membranes, nanoelectromechanical systems, chemical sensors, biochemical sensors and for future applications in micro- and nanoelectronics (Geim and Novoselov, 2007; Soldano et al., 2010). Among many graphene production methods, mechanical exfoliation was the first noted that enabled fabrication of good quality material (Novoselov et al., 2004). In this method scotch tape was used to peel off the graphene from highly oriented pyrolytic graphite (HOPG). The main drawback of mechanical exfoliation is that it is impractical for mass production due to small area of obtained material (Soldano et al., 2010). Another method enabling fabrication of good quality graphene is silicon carbide (SiC) thermal decomposition. SiC is heated to over 1200 ◦ C, at which point sublimation of Si from SiC substrate starts (Emtsev et al., 2009; Starke and Riedl, 2009). The product obtained is of very good quality but
∗ Corresponding author. Tel.: +48 71 320 36 51; fax: +48 71 328 35 04. E-mail address: [email protected] (K. Gajewski). http://dx.doi.org/10.1016/j.micron.2014.08.005 0968-4328/© 2014 Elsevier Ltd. All rights reserved.
fabrication of multilayer graphene using this technology is cumbersome. The reason is the self-limiting process of the Si-face 6H-SiC (0 0 0 1) sublimation and thus only up to three graphene layers ´ et al., 2011). Additionally, sublimated can be produced (Strupinski graphene from SiC is sensitive to the SiC substrate surface quality, and released Si atoms from SiC can also influence the already produced graphene layer. Chemical vapor deposition (CVD) is a milestone in obtaining large area graphene for mass scale production (Lee et al., 2010). Graphene was produced on many substrates, such as W, Ni, Cu, ´ Pt, Ir and also SiC (Soldano et al., 2010; Strupinski et al., 2011). It was shown that the product obtained may differ in its dependence on process parameters and the substrates used (Bhaviripudi et al., 2010). It is often worse than sublimated graphene from SiC ´ – except CVD on SiC substrate. Strupinski et al. (2011) showed that CVD graphene on a SiC substrate does not exhibit disadvantages of sublimated graphene on SiC, and the layer produced shows similar electrical parameters. In this method propane as a carbon precursor is used. Although the process occurs at high temperature (∼1600 ◦ C), Si sublimation was stopped by Ar linear gas flow ´ (Strupinski et al., 2011). In order to optimize the fabrication procedure, integration of high resolution diagnostic methods is needed, one of which is scanning probe microscopy (SPM). To our knowledge most previous works were done with the use of sublimated graphene. Hiebel
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et al. (2008) observed substrate reconstruction, mono- and fewlayer graphene and resultant moiré superlattice during the first stage of graphitization of graphene on C-face 6H-SiC. Observation of rotational disorder (angle up to 25◦ ) of multilayer graphene on Cterminated 6H-SiC substrate and resultant moiré patterns was done by Varchon et al. (2008). Starke and Riedl, 2009 noted that in its dependence on procedure conditions, the interface layer between graphene (both 6H-SiC and 4H-SiC) shows different structure peri√ √ odicity, besides 6 3 × 6 3R30◦ also 6 × 6, 5 × 5, 3 × 3 or 2 × 2. Biedermann et al. (2009) studied how sublimation of C-terminated 4H-SiC can influence the appearance of not only atomically smooth few layer graphene, but also rough graphene, grain boundaries, ridges and moiré superlattices. Kellar et al. (2010) additionally to UHV scanning tunnelling microscopy (STM) measurements used conductive atomic force microscopy (C-AFM) in ambient air to identify graphene domains. Nagase et al., 2010 observed changes in contact conductance (between sublimated graphene and AFM tip) as a function of load force. Changes in contact conductance were additionally attributed to existing voids beneath the graphene layer. Lately Nagase et al., 2013 also showed switching between “ON” and “OFF” states in contact conductance as a function of CAFM scanning direction. Held et al. (2012) compared production methods of sublimated graphene on Si-terminated 6H-SiC by the use of Kelvin probe force microscopy (KPFM). Graphene grown in ultra high vacuum (UHV) had a more expanded structure in comparison with graphene grown in argon atmosphere, but this did not influence the value of contact potential difference (CPD), which was also used for the determination of the number of graphene layers. CPD of thermally decomposed SiC as a function of the number of graphene layers was earlier observed by Filleter et al. (2008). In contrast to graphene sublimated from SiC, there are only few ´ SPM investigations done on CVD graphene. Strupinski et al. (2011) used STM for the observation of honeycomb lattice of graphene √ √ grown on 4H-SiC. In this case 6 3 × 6 3R30◦ superlattice was observed. Alaboson et al., 2011 used C-AFM for nanopatterning of CVD graphene on SiC. With the use of the same method they observed results of this patterning. Each investigations listed above were conducted using individual SPM techniques. In this paper we present a comprehensive study of CVD grown graphene on 6H-SiC done using STM, C-AFM and KPFM techniques. Complementary to these measurements, C-AFM I/V spectroscopy experiments were performed. CPD data obtained from the KPFM measurements enabled us to determine graphene work function. In this way graphene grown on 6H-SiC was characterized at a scale varying from several nanometres up to a few micrometres. We observed that electrical conductivity varies not only because of existing terrace edges or existing corrugations, but also on graphene flat surface. The possibility of control of such modifications at flat surface could be used to modify the properties and behaviour of novel types of graphene based devices for nanoelectronics and biosensing.
2. Materials and methods 2.1. Sample preparation CVD grown graphene was fabricated in the Institute of Electronic Materials Technology (ITME Warsaw, Poland) with use of ´ procedures described elsewhere Strupinski et al. (2011). Graphene was grown on Si-face of commercially available 6H-SiC substrate (Cree, US) with a miscut of nearly 0◦ . The SiC substrate was firstly etched in H2 and after that heated in Ar. The CVD process, with propane used as a carbon precursor, was held in temperature of ∼1600 ◦ C. To stop Si sublimation from SiC substrate, Ar linear gas flow was used. In this procedure monolayer graphene, with flat
Fig. 1. Electrical measurement setup used for STM, C-AFM and KPFM modes.
shape distorted mainly on step edges, was grown on entire sub´ strate surface – rarely bilayers were also observed (Strupinski et al., 2011). A conductive electrode was placed on top of the sample to ensure good electrical contact during measurements – Fig. 1. 2.2. SPM investigations All STM, C-AFM and KPFM measurements were done using a home-made SPM microscope working at room temperature in ambient air. STM experiments were performed in constant current mode. Freshly cut STM tip was made from commercially available 80% Pt–20% Ir alloy wire (SPI Supplies, US). Scan rate depending of the scanned area varied from 1 line per second in areas of a hundred nanometers square, up to 20 lines per second in areas of few square nanometers. In C-AFM mode, for simultaneous topography and current measurements, as well as for I/V investigations, PtIr5 coated cantilevers (PPP-ContPt from Nanosensors, spring constant 0.2 N/m, resonant frequency 13 kHz) were used. During KPFM measurements PPPEFM probes from Nanosensors were applied (PtIr5 coating, force constant 2.8 N/m, resonant frequency 75 kHz). During CPD measurements single pass amplitude modulation KPFM (AM-KPFM) mode was used. Amplitude detection of cantilever vibrations was used to control the distance between micro tip and surface as well as to observe electrostatic interactions. Cantilever resonance vibrations at 75 kHz were used for topography imaging, whereas electrostatic forces were detected at the frequency of 20 kHz. Auxiliary KPFM measurements were done with the use of a commercial NanoMan VS microscope with NanoScope V controller (Veeco Instruments). KPFM images measured using this system were recorded in a two pass procedure (Serry et al., 2010). Both topography and CPD data were measured at the first resonant frequency of the SPM probe. The graphene work function was determined taking into account the known value of the PtIr5 work function (−5.3 ± 0.1 eV; Zhou et al., 1998) and the measured CPD data. Data processing was performed with the use of home-made TopoGraf software. 3. Results and discussion Fig. 2a shows a graphene surface with one terrace edge visible on the left side of the picture. Because of the monatomic thickness, graphene reproduced the 6H-SiC substrate surface (in the case of
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Fig. 2. STM images of CVD grown graphene on 6H-SiC substrate in 240 nm × 240 nm region (a), 6 nm × 6 nm area with visible moiré pattern (b). Sample bias was 10 mV in (a) and 100 mV in (b). In both images tunnelling current was set on 1 nA.
visible terrace edges) – the measured terrace height was consistent with those noted by Filleter et al. (2008) and confirms this observation. In the centre part of Fig. 2a, there are visible corrugations covered with graphene. In high resolution STM image (Fig. 2b), a √ √ moiré pattern is visible. This 6 3 × 6 3R30◦ reconstruction is an effect of interactions between CVD graphene and beneath buffer layer. We did not observe distortions other than edges of terraces and corrugations. In contrast to reports (Biedermann et al., 2009; Hiebel et al., 2008; Kellar et al., 2010; Starke and Riedl, 2009; Varchon et al., 2008), no places without graphene or multilayer graphene were observed during STM experiments. No membrane-like structures were observed during scanning, which excludes the existence of any voids in SiC substrate (Nagase et al., 2010, 2013). In result, recorded STM images showed continuous and well conductive graphene surface. C-AFM topography of the graphene on 6H-SiC substrate in the area of 5.85 m × 5.85 m is presented in Fig. 3a with corresponding current image in Fig. 3b. Parts of terraces are covered by distortions, similar to the STM measurements. The average height ˚ To our knowledge, these corrugations of these pleats is about 3 A. appear in the case of an overly fast carbon source flow in comparison with the kinetic process on the SiC surface, during deposition. To prevent the appearance of these products, the growth rate has to be decreased by lowering the carbon precursor flow. Such details can be compared to ‘finger’ like structures, which were observed on sublimated graphene on 4H-SiC by Bolen et al. (2009), but whose origin in our opinion is rather different. ‘Fingers’ observed by Bolen were depressed relatively to the neighbourhood area due to Si sublimation. In dependence of the depth they were C-rich reconstructed SiC buffer layer or graphene. Corrugations observed during our experiments were mostly above neighbouring areas, had different height than noted by Bollen et al. and formed more chaotic structures. That can confirm the origin of these structures from the CVD process. From C-AFM measurements we can see, that these pleats modifies the conductivity of the graphene. In Fig. 3b current flows on almost all surfaces, but no current flow was observed on corrugated areas. As Fig. 2 shows, we did not observe any graphene discontinuities. In this case decrease of the conductivity of graphene layer can be caused by the decreased contact area of the scanning tip, graphene-corrugations interactions or the composition of the corrugations as residues from the CVD process. Similar to the STM measurements multilayer graphene was not observed.
Enlarged images show both pleats and terraces, giving more insight into the conductivity properties of the sample (Fig. 4). The topographic image (Fig. 4a) shows three terraces (numbered 1, 2 and 3) of graphene on 6H-SiC substrate. On terrace 3 there are surface deformations. Friction (Fig. 4b) and current images (Fig. 4c–f) at various sample bias correspond to the area in Fig. 4a. Current flows between conductive probe and biased sample on almost all areas. On terraces 1 and 2, where surface is flat, relatively uniform current flows from the sample. On terrace 1 on figures with negative sample bias (Fig. 4c and e) there are visible “dots” invisible on current images at positive sample bias (and also topography and friction images). This suggests adsorption of small molecules which influence carrier concentration, but only at negative bias. Arrows in Fig. 4c–f show current decrease caused by the scanning tip moving across the edge of the terrace. In the middle of the terrace 2 (above red circles in Fig. 4c–f) there are visible scratch-like current decreases, which are not visible on topography image. The direction of these scratches implies that they were not developed during measurements, but were an existing modification of electrical conductivity of the sample. The circle on terrace 3 shows bilayer graphene. It exhibits higher conductivity in comparison with neighbourhood area. Additionally on terrace 3, at corrugations the conductivity is both lower and higher than the surrounding area. Electrical conductivity on flat terraces is modified in a similar way to gas molecules influencing the local carrier concentration in graphene based gas detectors (Schedin et al., 2007). On the other hand on corrugated areas change in the conductivity can be caused by the different carrier concentration in residues from CVD process or different coverage by the graphene which was not observed during STM scans (note that conductivity on the corrugations is both lower and higher than in neighbourhood area). On scratch-like structures, visible only on current images, electrical conductivity can be more influenced by the mechanical stress. This stress can have less effect on terrace edges due to existing buffer layer under graphene. Previous works show, that in graphene on SiC substrate (both thermal and CVD grown) the compressive strain exists (Ni ´ et al., 2008; Röhrl et al., 2008; Strupinski et al., 2011). Considering earlier simulations and experiments, strain in graphene affects its electrical properties (Choi et al., 2010; Pereira et al., 2009; Wang et al., 2011). Additionally, it was calculated, that the work function of graphene increases when induced strain grows (Choi et al., 2010). As a result, strain may be both increased and decreased locally by surface modifications (corrugations or intentionally by SPM probe).
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Fig. 3. C-AFM topography (a) and current (b) images of CVD grown graphene on 6H-SiC substrate in the area of 5.85 m × 5.85 m. SiC terraces with graphene grown on it are visible. In places where graphene is corrugated, almost no current flow was observed. Load force was 30 nN, sample bias 200 mV.
It gives respectively decrease and increase of graphene conductivity, as can be seen in Fig. 4c–f). This dependence can be used in high resolution graphene based strain sensors (Wang et al., 2011). As confirmation of previous observations, C-AFM I/V spectroscopy was performed. Fig. 5a shows a spectroscopy map taken from current image (the brighter colour is, the higher current flows) – two in corrugated places (points 1 and 2) and two in flat surface (points 3 and 4). From spectroscopy data (Fig. 5b) it is seen that contact between conductive probe and sample exhibits non-linear properties. On residues current flowing from the sample was higher in comparison with places where flat terraces were present. Finally, Fig. 6 shows the results of KPFM measurements of CVD graphene on 6H-SiC substrate, while sample was grounded. There are visible four regions with locally distinct CPD value (region
A: 241 mV, region B: 303 mV, region C: 401 mV and region D: 532 mV). In region C and region D of the area in Fig. 6 b) higher CPD value was observed. Between these areas an area of lower CPD value exists (region A and region B). In regions A and B in Fig. 6a flat surface with one terrace edge and almost without corrugations (only on top and bottom parts of Fig. 6a was observed. Visible changes in measured CPD in these regions may appear due to visible terrace edge, corrugations or local carrier distribution. Regions C and D can be attributed respectively to a bi-layer graphene and a thicker graphene domain. In this case higher CPD value of region C is consistent with experiments done with multilayer graphene (Lee et al., 2009) and another experiments performed on sublimated SiC (Filleter et al., 2008; Kazakova et al., 2013). Fig. 6c shows CPD histogram of measured area, which
Fig. 4. C-AFM topography (a), friction (b), and current mapping (c–f) measured in the area of 685 nm × 685 nm. Three flat terraces and corrugations on CVD grown graphene on 6H-SiC are visible. Current images recorded at −200 mV, 200 mV, −10 mV, 10 mV are (c), (d), (e), and (f), respectively. On flat surface current flow from the sample is uniform. Non-uniform current flows on pleats area, where graphene has lower conductivity (and on few parts – the higher conductivity). Load force was 40 nN. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
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Fig. 5. C-AFM I/V spectroscopy map taken from current image (a) and C-AFM I/V spectroscopy data (b) of CVD grown graphene on 6H-SiC substrate. Numbers on the spectroscopy plot corresponds to the numbers on the map. Load force was 40 nN.
was used for the work function estimation. Based on CPD values listed in the table in Fig. 6c, work function of the graphene (regions A and B) varies from Eg = −5.06 eV to Eg = −5.00 eV. Work function of the bilayer (region C) is Eg = −4.90 eV and Eg = −4.77 eV for thicker graphene domain (region D). This is in the agreement with the assumption that graphene CPD goes to the graphite CPD with increasing number of atomic layers. Resulting work function values of the graphene correspond to the calculated strain (Choi et al., 2010) between 10% on corrugations up to 25% on flat
surface. It is much higher than expected maximum of 1% (at −4.55 eV) of strain in graphene on SiC substrate (Röhrl et al., 2008). In our opinion this difference appears to be due to graphene–substrate interaction and possible deviation of the PtIr5 coated tip work function. In agreement with our results, compressive strain exists in the areas of graphene on 6H-SiC substrate on flat terraces. As a result higher work function is observed on flat surface. In our case, graphene corrugations are responsible for the material relaxation.
Fig. 6. KPFM results of CVD grown graphene on 6H-SiC. Sample was grounded. Images show respectively topography (a), CPD data (b), CPD data histogram (c), CPD data overlaid the topography 3D image (d).
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4. Conclusions We have reported comprehensive study of the electrical properties of the CVD grown graphene on the 6H-SiC substrate. Various SPM methods for the characterization of electrical sample properties were used to describe the graphene/6H-SiC structure. It was shown and explained how distortions of the graphene surface affect its electrical properties, resulting in both increased and decreased graphene electrical conductivity. Additionally, the intentionally induced strain may be used for changing the electrical properties of the material, with possible practical benefits. Acknowledgements This work was partially supported by the Foundation for Polish Science TEAM Programme “High-resolution force and mass metrology using actuated MEMS/NEMS devices – FoMaMet” (Grant No. TEAM/2012-9/3), co-financed by the European Regional Development Fund resources within the framework of Operational Program Innovative Economy and Wrocław University of Technology statutory grant S40036. The research leading to this invention has also partially received funding from the European Union Seventh Framework Programme under grant agreement no. 604391 Graphene Flagship and by the National Center for Research and Development under the GRAFTECH/NCBiR/12/14/2013 “GRAFMAG”. References Alaboson, J.M.P., Wang, Q.H., Kellar, J.A., Park, J., Elam, J.W., Pellin, M.J., Hersam, M.C., 2011. Conductive atomic force microscope nanopatterning of epitaxial graphene on SiC(0 0 0 1) in ambient conditions. Adv. Mater. 23, 2181–2184, http://dx.doi.org/10.1002/adma.201100367. Bhaviripudi, S., Jia, X., Dresselhaus, M.S., Kong, J., 2010. Role of kinetic factors in chemical vapor deposition synthesis of uniform large area graphene using copper catalyst. Nano Lett. 10, 4128–4133, http://dx.doi.org/10.1021/nl102355e. Biedermann, L.B., Bolen, M.L., Capano, M.A., Zemlyanov, D., Reifenberger, R.G., 2009. Insights into few-layer epitaxial graphene growth on 4HSiC(0 0 0 1− ) substrates from STM studies. Phys. Rev. B 79, 125411, http://dx.doi.org/10.1103/PhysRevB.79.125411. Bolen, M.L., Harrison, S.E., Biedermann, L.B., Capano, M.A., 2009. Graphene formation mechanisms on 4H-SiC(0 0 0 1). Phys. Rev. B 80, 115433, http://dx.doi.org/10.1103/PhysRevB.80.115433. Choi, S.-M., Jhi, S.-H., Son, Y.-W., 2010. Effects of strain on elecproperties of graphene. Phys. Rev. B 81, 081407R, tronic http://dx.doi.org/10.1103/PhysRevB.81.081407. Emtsev, K.V., Bostwick, A., Horn, K., Jobst, J., Kellogg, G.L., Ley, L., McChesney, J.L., Ohta, T., Reshanov, S.A., Röhrl, J., Rotenberg, E., Schmid, A.K., Waldmann, D., Weber, H.B., Seyller, T., 2009. Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nat. Mater. 8, 203–207, http://dx.doi.org/10.1038/nmat2382. Filleter, T., Emtsev, K.V., Seyller, Th., Bennewitz, R., 2008. Local work function measurements of epitaxial graphene. Appl. Phys. Lett. 93, 133117, http://dx.doi.org/10.1063/1.2993341. Geim, A.K., Novoselov, K.S., 2007. The rise of graphene. Nat. Mater. 6, 183–191, http://dx.doi.org/10.1038/nmat1849.
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Micron journal homepage: www.elsevier.com/locate/micron
Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate Krzysztof Gajewski a,∗ , Daniel Kopiec a , Magdalena Moczała a , Adam Piotrowicz a , b ´ Michał Zielony a , Grzegorz Wielgoszewski a , Teodor Gotszalk a , Włodek Strupinski a b
Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, ul. Z. Janiszewskiego 11/17, PL-50372 Wrocław, Poland Institute of Electronic Materials Technology, ul. Wolczynska 133, PL-01919 Warsaw, Poland
a r t i c l e
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Article history: Received 7 April 2014 Received in revised form 12 August 2014 Accepted 12 August 2014 Available online 20 August 2014 Keywords: Graphene SiC STM C-AFM KPFM
a b s t r a c t Sublimated graphene grown on SiC is an attractive material for scientific investigations. Nevertheless the self limiting process on the Si face and its sensitivity to the surface quality of the SiC substrates may be unfavourable for later microelectronic processes. On the other hand, chemical vapor deposited (CVD) graphene does not posses such disadvantages, so further experimental investigation is needed. In this paper CVD grown graphene on 6H-SiC (0 0 0 1) substrate was investigated using scanning probe microscopy (SPM). Electrical properties of graphene were characterized with the use of: scanning tunnelling microscopy, conductive atomic force microscopy (C-AFM) with locally performed C-AFM current–voltage measurements and Kelvin probe force microscopy (KPFM). Based on the contact potential difference data from the KPFM measurements, the work function of graphene was estimated. We observed conductance variations not only on structural edges, existing surface corrugations or accidental bilayers, but also on a flat graphene surface. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Graphene, a two-dimensional carbon material, has attracted the attention of scientists for many years (Geim and Novoselov, 2007). This is result of: its extraordinary stiffness, optical transmission and electrical properties, making graphene a prospective candidate for surface coating, membranes, nanoelectromechanical systems, chemical sensors, biochemical sensors and for future applications in micro- and nanoelectronics (Geim and Novoselov, 2007; Soldano et al., 2010). Among many graphene production methods, mechanical exfoliation was the first noted that enabled fabrication of good quality material (Novoselov et al., 2004). In this method scotch tape was used to peel off the graphene from highly oriented pyrolytic graphite (HOPG). The main drawback of mechanical exfoliation is that it is impractical for mass production due to small area of obtained material (Soldano et al., 2010). Another method enabling fabrication of good quality graphene is silicon carbide (SiC) thermal decomposition. SiC is heated to over 1200 ◦ C, at which point sublimation of Si from SiC substrate starts (Emtsev et al., 2009; Starke and Riedl, 2009). The product obtained is of very good quality but
∗ Corresponding author. Tel.: +48 71 320 36 51; fax: +48 71 328 35 04. E-mail address: [email protected] (K. Gajewski). http://dx.doi.org/10.1016/j.micron.2014.08.005 0968-4328/© 2014 Elsevier Ltd. All rights reserved.
fabrication of multilayer graphene using this technology is cumbersome. The reason is the self-limiting process of the Si-face 6H-SiC (0 0 0 1) sublimation and thus only up to three graphene layers ´ et al., 2011). Additionally, sublimated can be produced (Strupinski graphene from SiC is sensitive to the SiC substrate surface quality, and released Si atoms from SiC can also influence the already produced graphene layer. Chemical vapor deposition (CVD) is a milestone in obtaining large area graphene for mass scale production (Lee et al., 2010). Graphene was produced on many substrates, such as W, Ni, Cu, ´ Pt, Ir and also SiC (Soldano et al., 2010; Strupinski et al., 2011). It was shown that the product obtained may differ in its dependence on process parameters and the substrates used (Bhaviripudi et al., 2010). It is often worse than sublimated graphene from SiC ´ – except CVD on SiC substrate. Strupinski et al. (2011) showed that CVD graphene on a SiC substrate does not exhibit disadvantages of sublimated graphene on SiC, and the layer produced shows similar electrical parameters. In this method propane as a carbon precursor is used. Although the process occurs at high temperature (∼1600 ◦ C), Si sublimation was stopped by Ar linear gas flow ´ (Strupinski et al., 2011). In order to optimize the fabrication procedure, integration of high resolution diagnostic methods is needed, one of which is scanning probe microscopy (SPM). To our knowledge most previous works were done with the use of sublimated graphene. Hiebel
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et al. (2008) observed substrate reconstruction, mono- and fewlayer graphene and resultant moiré superlattice during the first stage of graphitization of graphene on C-face 6H-SiC. Observation of rotational disorder (angle up to 25◦ ) of multilayer graphene on Cterminated 6H-SiC substrate and resultant moiré patterns was done by Varchon et al. (2008). Starke and Riedl, 2009 noted that in its dependence on procedure conditions, the interface layer between graphene (both 6H-SiC and 4H-SiC) shows different structure peri√ √ odicity, besides 6 3 × 6 3R30◦ also 6 × 6, 5 × 5, 3 × 3 or 2 × 2. Biedermann et al. (2009) studied how sublimation of C-terminated 4H-SiC can influence the appearance of not only atomically smooth few layer graphene, but also rough graphene, grain boundaries, ridges and moiré superlattices. Kellar et al. (2010) additionally to UHV scanning tunnelling microscopy (STM) measurements used conductive atomic force microscopy (C-AFM) in ambient air to identify graphene domains. Nagase et al., 2010 observed changes in contact conductance (between sublimated graphene and AFM tip) as a function of load force. Changes in contact conductance were additionally attributed to existing voids beneath the graphene layer. Lately Nagase et al., 2013 also showed switching between “ON” and “OFF” states in contact conductance as a function of CAFM scanning direction. Held et al. (2012) compared production methods of sublimated graphene on Si-terminated 6H-SiC by the use of Kelvin probe force microscopy (KPFM). Graphene grown in ultra high vacuum (UHV) had a more expanded structure in comparison with graphene grown in argon atmosphere, but this did not influence the value of contact potential difference (CPD), which was also used for the determination of the number of graphene layers. CPD of thermally decomposed SiC as a function of the number of graphene layers was earlier observed by Filleter et al. (2008). In contrast to graphene sublimated from SiC, there are only few ´ SPM investigations done on CVD graphene. Strupinski et al. (2011) used STM for the observation of honeycomb lattice of graphene √ √ grown on 4H-SiC. In this case 6 3 × 6 3R30◦ superlattice was observed. Alaboson et al., 2011 used C-AFM for nanopatterning of CVD graphene on SiC. With the use of the same method they observed results of this patterning. Each investigations listed above were conducted using individual SPM techniques. In this paper we present a comprehensive study of CVD grown graphene on 6H-SiC done using STM, C-AFM and KPFM techniques. Complementary to these measurements, C-AFM I/V spectroscopy experiments were performed. CPD data obtained from the KPFM measurements enabled us to determine graphene work function. In this way graphene grown on 6H-SiC was characterized at a scale varying from several nanometres up to a few micrometres. We observed that electrical conductivity varies not only because of existing terrace edges or existing corrugations, but also on graphene flat surface. The possibility of control of such modifications at flat surface could be used to modify the properties and behaviour of novel types of graphene based devices for nanoelectronics and biosensing.
2. Materials and methods 2.1. Sample preparation CVD grown graphene was fabricated in the Institute of Electronic Materials Technology (ITME Warsaw, Poland) with use of ´ procedures described elsewhere Strupinski et al. (2011). Graphene was grown on Si-face of commercially available 6H-SiC substrate (Cree, US) with a miscut of nearly 0◦ . The SiC substrate was firstly etched in H2 and after that heated in Ar. The CVD process, with propane used as a carbon precursor, was held in temperature of ∼1600 ◦ C. To stop Si sublimation from SiC substrate, Ar linear gas flow was used. In this procedure monolayer graphene, with flat
Fig. 1. Electrical measurement setup used for STM, C-AFM and KPFM modes.
shape distorted mainly on step edges, was grown on entire sub´ strate surface – rarely bilayers were also observed (Strupinski et al., 2011). A conductive electrode was placed on top of the sample to ensure good electrical contact during measurements – Fig. 1. 2.2. SPM investigations All STM, C-AFM and KPFM measurements were done using a home-made SPM microscope working at room temperature in ambient air. STM experiments were performed in constant current mode. Freshly cut STM tip was made from commercially available 80% Pt–20% Ir alloy wire (SPI Supplies, US). Scan rate depending of the scanned area varied from 1 line per second in areas of a hundred nanometers square, up to 20 lines per second in areas of few square nanometers. In C-AFM mode, for simultaneous topography and current measurements, as well as for I/V investigations, PtIr5 coated cantilevers (PPP-ContPt from Nanosensors, spring constant 0.2 N/m, resonant frequency 13 kHz) were used. During KPFM measurements PPPEFM probes from Nanosensors were applied (PtIr5 coating, force constant 2.8 N/m, resonant frequency 75 kHz). During CPD measurements single pass amplitude modulation KPFM (AM-KPFM) mode was used. Amplitude detection of cantilever vibrations was used to control the distance between micro tip and surface as well as to observe electrostatic interactions. Cantilever resonance vibrations at 75 kHz were used for topography imaging, whereas electrostatic forces were detected at the frequency of 20 kHz. Auxiliary KPFM measurements were done with the use of a commercial NanoMan VS microscope with NanoScope V controller (Veeco Instruments). KPFM images measured using this system were recorded in a two pass procedure (Serry et al., 2010). Both topography and CPD data were measured at the first resonant frequency of the SPM probe. The graphene work function was determined taking into account the known value of the PtIr5 work function (−5.3 ± 0.1 eV; Zhou et al., 1998) and the measured CPD data. Data processing was performed with the use of home-made TopoGraf software. 3. Results and discussion Fig. 2a shows a graphene surface with one terrace edge visible on the left side of the picture. Because of the monatomic thickness, graphene reproduced the 6H-SiC substrate surface (in the case of
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Fig. 2. STM images of CVD grown graphene on 6H-SiC substrate in 240 nm × 240 nm region (a), 6 nm × 6 nm area with visible moiré pattern (b). Sample bias was 10 mV in (a) and 100 mV in (b). In both images tunnelling current was set on 1 nA.
visible terrace edges) – the measured terrace height was consistent with those noted by Filleter et al. (2008) and confirms this observation. In the centre part of Fig. 2a, there are visible corrugations covered with graphene. In high resolution STM image (Fig. 2b), a √ √ moiré pattern is visible. This 6 3 × 6 3R30◦ reconstruction is an effect of interactions between CVD graphene and beneath buffer layer. We did not observe distortions other than edges of terraces and corrugations. In contrast to reports (Biedermann et al., 2009; Hiebel et al., 2008; Kellar et al., 2010; Starke and Riedl, 2009; Varchon et al., 2008), no places without graphene or multilayer graphene were observed during STM experiments. No membrane-like structures were observed during scanning, which excludes the existence of any voids in SiC substrate (Nagase et al., 2010, 2013). In result, recorded STM images showed continuous and well conductive graphene surface. C-AFM topography of the graphene on 6H-SiC substrate in the area of 5.85 m × 5.85 m is presented in Fig. 3a with corresponding current image in Fig. 3b. Parts of terraces are covered by distortions, similar to the STM measurements. The average height ˚ To our knowledge, these corrugations of these pleats is about 3 A. appear in the case of an overly fast carbon source flow in comparison with the kinetic process on the SiC surface, during deposition. To prevent the appearance of these products, the growth rate has to be decreased by lowering the carbon precursor flow. Such details can be compared to ‘finger’ like structures, which were observed on sublimated graphene on 4H-SiC by Bolen et al. (2009), but whose origin in our opinion is rather different. ‘Fingers’ observed by Bolen were depressed relatively to the neighbourhood area due to Si sublimation. In dependence of the depth they were C-rich reconstructed SiC buffer layer or graphene. Corrugations observed during our experiments were mostly above neighbouring areas, had different height than noted by Bollen et al. and formed more chaotic structures. That can confirm the origin of these structures from the CVD process. From C-AFM measurements we can see, that these pleats modifies the conductivity of the graphene. In Fig. 3b current flows on almost all surfaces, but no current flow was observed on corrugated areas. As Fig. 2 shows, we did not observe any graphene discontinuities. In this case decrease of the conductivity of graphene layer can be caused by the decreased contact area of the scanning tip, graphene-corrugations interactions or the composition of the corrugations as residues from the CVD process. Similar to the STM measurements multilayer graphene was not observed.
Enlarged images show both pleats and terraces, giving more insight into the conductivity properties of the sample (Fig. 4). The topographic image (Fig. 4a) shows three terraces (numbered 1, 2 and 3) of graphene on 6H-SiC substrate. On terrace 3 there are surface deformations. Friction (Fig. 4b) and current images (Fig. 4c–f) at various sample bias correspond to the area in Fig. 4a. Current flows between conductive probe and biased sample on almost all areas. On terraces 1 and 2, where surface is flat, relatively uniform current flows from the sample. On terrace 1 on figures with negative sample bias (Fig. 4c and e) there are visible “dots” invisible on current images at positive sample bias (and also topography and friction images). This suggests adsorption of small molecules which influence carrier concentration, but only at negative bias. Arrows in Fig. 4c–f show current decrease caused by the scanning tip moving across the edge of the terrace. In the middle of the terrace 2 (above red circles in Fig. 4c–f) there are visible scratch-like current decreases, which are not visible on topography image. The direction of these scratches implies that they were not developed during measurements, but were an existing modification of electrical conductivity of the sample. The circle on terrace 3 shows bilayer graphene. It exhibits higher conductivity in comparison with neighbourhood area. Additionally on terrace 3, at corrugations the conductivity is both lower and higher than the surrounding area. Electrical conductivity on flat terraces is modified in a similar way to gas molecules influencing the local carrier concentration in graphene based gas detectors (Schedin et al., 2007). On the other hand on corrugated areas change in the conductivity can be caused by the different carrier concentration in residues from CVD process or different coverage by the graphene which was not observed during STM scans (note that conductivity on the corrugations is both lower and higher than in neighbourhood area). On scratch-like structures, visible only on current images, electrical conductivity can be more influenced by the mechanical stress. This stress can have less effect on terrace edges due to existing buffer layer under graphene. Previous works show, that in graphene on SiC substrate (both thermal and CVD grown) the compressive strain exists (Ni ´ et al., 2008; Röhrl et al., 2008; Strupinski et al., 2011). Considering earlier simulations and experiments, strain in graphene affects its electrical properties (Choi et al., 2010; Pereira et al., 2009; Wang et al., 2011). Additionally, it was calculated, that the work function of graphene increases when induced strain grows (Choi et al., 2010). As a result, strain may be both increased and decreased locally by surface modifications (corrugations or intentionally by SPM probe).
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Fig. 3. C-AFM topography (a) and current (b) images of CVD grown graphene on 6H-SiC substrate in the area of 5.85 m × 5.85 m. SiC terraces with graphene grown on it are visible. In places where graphene is corrugated, almost no current flow was observed. Load force was 30 nN, sample bias 200 mV.
It gives respectively decrease and increase of graphene conductivity, as can be seen in Fig. 4c–f). This dependence can be used in high resolution graphene based strain sensors (Wang et al., 2011). As confirmation of previous observations, C-AFM I/V spectroscopy was performed. Fig. 5a shows a spectroscopy map taken from current image (the brighter colour is, the higher current flows) – two in corrugated places (points 1 and 2) and two in flat surface (points 3 and 4). From spectroscopy data (Fig. 5b) it is seen that contact between conductive probe and sample exhibits non-linear properties. On residues current flowing from the sample was higher in comparison with places where flat terraces were present. Finally, Fig. 6 shows the results of KPFM measurements of CVD graphene on 6H-SiC substrate, while sample was grounded. There are visible four regions with locally distinct CPD value (region
A: 241 mV, region B: 303 mV, region C: 401 mV and region D: 532 mV). In region C and region D of the area in Fig. 6 b) higher CPD value was observed. Between these areas an area of lower CPD value exists (region A and region B). In regions A and B in Fig. 6a flat surface with one terrace edge and almost without corrugations (only on top and bottom parts of Fig. 6a was observed. Visible changes in measured CPD in these regions may appear due to visible terrace edge, corrugations or local carrier distribution. Regions C and D can be attributed respectively to a bi-layer graphene and a thicker graphene domain. In this case higher CPD value of region C is consistent with experiments done with multilayer graphene (Lee et al., 2009) and another experiments performed on sublimated SiC (Filleter et al., 2008; Kazakova et al., 2013). Fig. 6c shows CPD histogram of measured area, which
Fig. 4. C-AFM topography (a), friction (b), and current mapping (c–f) measured in the area of 685 nm × 685 nm. Three flat terraces and corrugations on CVD grown graphene on 6H-SiC are visible. Current images recorded at −200 mV, 200 mV, −10 mV, 10 mV are (c), (d), (e), and (f), respectively. On flat surface current flow from the sample is uniform. Non-uniform current flows on pleats area, where graphene has lower conductivity (and on few parts – the higher conductivity). Load force was 40 nN. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
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Fig. 5. C-AFM I/V spectroscopy map taken from current image (a) and C-AFM I/V spectroscopy data (b) of CVD grown graphene on 6H-SiC substrate. Numbers on the spectroscopy plot corresponds to the numbers on the map. Load force was 40 nN.
was used for the work function estimation. Based on CPD values listed in the table in Fig. 6c, work function of the graphene (regions A and B) varies from Eg = −5.06 eV to Eg = −5.00 eV. Work function of the bilayer (region C) is Eg = −4.90 eV and Eg = −4.77 eV for thicker graphene domain (region D). This is in the agreement with the assumption that graphene CPD goes to the graphite CPD with increasing number of atomic layers. Resulting work function values of the graphene correspond to the calculated strain (Choi et al., 2010) between 10% on corrugations up to 25% on flat
surface. It is much higher than expected maximum of 1% (at −4.55 eV) of strain in graphene on SiC substrate (Röhrl et al., 2008). In our opinion this difference appears to be due to graphene–substrate interaction and possible deviation of the PtIr5 coated tip work function. In agreement with our results, compressive strain exists in the areas of graphene on 6H-SiC substrate on flat terraces. As a result higher work function is observed on flat surface. In our case, graphene corrugations are responsible for the material relaxation.
Fig. 6. KPFM results of CVD grown graphene on 6H-SiC. Sample was grounded. Images show respectively topography (a), CPD data (b), CPD data histogram (c), CPD data overlaid the topography 3D image (d).
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4. Conclusions We have reported comprehensive study of the electrical properties of the CVD grown graphene on the 6H-SiC substrate. Various SPM methods for the characterization of electrical sample properties were used to describe the graphene/6H-SiC structure. It was shown and explained how distortions of the graphene surface affect its electrical properties, resulting in both increased and decreased graphene electrical conductivity. Additionally, the intentionally induced strain may be used for changing the electrical properties of the material, with possible practical benefits. Acknowledgements This work was partially supported by the Foundation for Polish Science TEAM Programme “High-resolution force and mass metrology using actuated MEMS/NEMS devices – FoMaMet” (Grant No. TEAM/2012-9/3), co-financed by the European Regional Development Fund resources within the framework of Operational Program Innovative Economy and Wrocław University of Technology statutory grant S40036. The research leading to this invention has also partially received funding from the European Union Seventh Framework Programme under grant agreement no. 604391 Graphene Flagship and by the National Center for Research and Development under the GRAFTECH/NCBiR/12/14/2013 “GRAFMAG”. References Alaboson, J.M.P., Wang, Q.H., Kellar, J.A., Park, J., Elam, J.W., Pellin, M.J., Hersam, M.C., 2011. Conductive atomic force microscope nanopatterning of epitaxial graphene on SiC(0 0 0 1) in ambient conditions. Adv. Mater. 23, 2181–2184, http://dx.doi.org/10.1002/adma.201100367. Bhaviripudi, S., Jia, X., Dresselhaus, M.S., Kong, J., 2010. Role of kinetic factors in chemical vapor deposition synthesis of uniform large area graphene using copper catalyst. Nano Lett. 10, 4128–4133, http://dx.doi.org/10.1021/nl102355e. Biedermann, L.B., Bolen, M.L., Capano, M.A., Zemlyanov, D., Reifenberger, R.G., 2009. Insights into few-layer epitaxial graphene growth on 4HSiC(0 0 0 1− ) substrates from STM studies. Phys. Rev. B 79, 125411, http://dx.doi.org/10.1103/PhysRevB.79.125411. Bolen, M.L., Harrison, S.E., Biedermann, L.B., Capano, M.A., 2009. Graphene formation mechanisms on 4H-SiC(0 0 0 1). Phys. Rev. B 80, 115433, http://dx.doi.org/10.1103/PhysRevB.80.115433. Choi, S.-M., Jhi, S.-H., Son, Y.-W., 2010. Effects of strain on elecproperties of graphene. Phys. Rev. B 81, 081407R, tronic http://dx.doi.org/10.1103/PhysRevB.81.081407. Emtsev, K.V., Bostwick, A., Horn, K., Jobst, J., Kellogg, G.L., Ley, L., McChesney, J.L., Ohta, T., Reshanov, S.A., Röhrl, J., Rotenberg, E., Schmid, A.K., Waldmann, D., Weber, H.B., Seyller, T., 2009. Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nat. Mater. 8, 203–207, http://dx.doi.org/10.1038/nmat2382. Filleter, T., Emtsev, K.V., Seyller, Th., Bennewitz, R., 2008. Local work function measurements of epitaxial graphene. Appl. Phys. Lett. 93, 133117, http://dx.doi.org/10.1063/1.2993341. Geim, A.K., Novoselov, K.S., 2007. The rise of graphene. Nat. Mater. 6, 183–191, http://dx.doi.org/10.1038/nmat1849.
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